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Research article | 22 May 2014
Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory
G.-Y. Niu et al.
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Carlotta Scudeler, Luke Pangle, Damiano Pasetto, Guo-Yue Niu, Till Volkmann, Claudio Paniconi, Mario Putti, and Peter Troch
Hydrol. Earth Syst. Sci., 20, 4061–4078, https://doi.org/10.5194/hess-20-4061-2016,https://doi.org/10.5194/hess-20-4061-2016, 2016
Multimodel assessment of climate change-induced hydrologic impacts for a Mediterranean catchment
Enrica Perra, Monica Piras, Roberto Deidda, Claudio Paniconi, Giuseppe Mascaro, Enrique R. Vivoni, Pierluigi Cau, Pier Andrea Marras, Ralf Ludwig, and Swen Meyer
Flow dynamics in hyper-saline aquifers: hydro-geophysical monitoring and modeling
Klaus Haaken, Gian Piero Deidda, Giorgio Cassiani, Rita Deiana, Mario Putti, Claudio Paniconi, Carlotta Scudeler, and Andreas Kemna
The paper presents a general methodology that will help understand how freshwater and saltwater may interact in natural porous media, with a particular view at practical applications such as the storage of freshwater underground in critical areas, e.g., semi-arid zones around the Mediterranean sea. The methodology is applied to a case study in Sardinia and shows how a mix of advanced monitoring and mathematical modeling tremendously advance our understanding of these systems.
A process-based diagnosis of catchment coevolution in volcanic landscapes: synthesis of Newtonian and Darwinian approaches
Takeo Yoshida and Peter A. Troch
Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2016-263,https://doi.org/10.5194/hess-2016-263, 2016
Revised manuscript not accepted
Studies in volcanic catchments have revealed two hydrological signatures significantly correlated with the age of the bedrock. To understand why such a simple relations have emerged, we used a hydrological model and conducted a numerical experiment. We found younger catchments require longer for the transmission zone storage to fill and empty, take longer to release water from deep aquifers, and have greater recharge to deep aquifers, supporting the hypotheses formulated by the empirical study.
Coevolution of volcanic catchments in Japan
We derived indices of landscape properties as well as hydrological response and examined their relation with catchment age and climate. We found significant correlation between drainage density and baseflow index with age, but not with climate. We compared our data with data from volcanic catchments in Oregon and could confirm that baseflow index decreases with time, but also discovered that drainage density seems to stabilize after 2M years, after an initial increase due to landscape incision.
Influence of climate variability on water partitioning and effective energy and mass transfer in a semi-arid critical zone
Xavier Zapata-Rios, Paul D. Brooks, Peter A. Troch, Jennifer McIntosh, and Craig Rasmussen
In this study, we quantify how climate variability in the last 3 decades (1984–2012) has affected water availability and the temporal trends in effective energy and mass transfer (EEMT). This study takes place in the Jemez River basin in northern New Mexico. Results from this study indicated a decreasing trend in water availability, a reduction in forest productivity (4 g C m−2 per 10 mm of reduction in precipitation), and decreasing EEMT (1.2–1.3 MJ m2 decade−1).
Hillslope-scale experiment demonstrates the role of convergence during two-step saturation
A. I. Gevaert, A. J. Teuling, R. Uijlenhoet, S. B. DeLong, T. E. Huxman, L. A. Pangle, D. D. Breshears, J. Chorover, J. D. Pelletier, S. R. Saleska, X. Zeng, and P. A. Troch
More articles (2)
Subject: Hillslope hydrology | Techniques and Approaches: Modelling approaches
Recession discharge from compartmentalized bedrock hillslopes
Clément Roques, David E. Rupp, Jean-Raynald de Dreuzy, Laurent Longuevergne, Elizabeth R. Jachens, Gordon Grant, Luc Aquilina, and John S. Selker
Streamflow dynamics are directly dependent on contributions from groundwater, with hillslope heterogeneity being a major driver in controlling both spatial and temporal variabilities in recession discharge behaviors. By analysing new model results, this paper identifies the major structural features of aquifers driving streamflow dynamics. It provides important guidance to inform catchment-to-regional-scale models, with key geological knowledge influencing groundwater–surface water interactions.
Frozen soil hydrological modeling for a mountainous catchment northeast of the Qinghai–Tibet Plateau
Hongkai Gao, Chuntan Han, Rensheng Chen, Zijing Feng, Kang Wang, Fabrizio Fenicia, and Hubert Savenije
Frozen soil hydrology is one of the 23 unsolved problems in hydrology (UPH). In this study, we developed a novel conceptual frozen soil hydrological model, FLEX-Topo-FS. The model successfully reproduced the soil freeze–thaw process, and its impacts on hydrologic connectivity, runoff generation, and groundwater. We believe this study is a breakthrough for the 23 UPH, giving us new insights on frozen soil hydrology, with broad implications for predicting cold region hydrology in future.
On the similarity of hillslope hydrologic function: a clustering approach based on groundwater changes
Fadji Z. Maina, Haruko M. Wainwright, Peter James Dennedy-Frank, and Erica R. Siirila-Woodburn
We propose a hillslope clustering approach based on the seasonal changes in groundwater levels and test its performance by comparing it to several common clustering approaches (aridity index, topographic wetness index, elevation, land cover, and machine-learning clustering). The proposed approach is robust as it reasonably categorizes hillslopes with similar elevation, land cover, hydroclimate, land surface processes, and subsurface hydrodynamics, hence a similar hydrologic function.
Spatiotemporal changes in flow hydraulic characteristics and soil loss during gully headcut erosion under controlled conditions
Mingming Guo, Zhuoxin Chen, Wenlong Wang, Tianchao Wang, Qianhua Shi, Hongliang Kang, Man Zhao, and Lanqian Feng
Gully headcut erosion is always a difficult issue in soil erosion, which hinders the revelation of gully erosion mechanisms and the establishment of a gully erosion model. This study clarified the spatiotemporal changes in flow properties, energy consumption, and soil loss, confirming that gully head consumed the most of flow energy (78 %) and can contribute 89 % of total soil loss. Critical energy consumption initiating soil erosion of the upstream area, gully head, and gully bed is confirmed.
Estimation of rainfall erosivity based on WRF-derived raindrop size distributions
Qiang Dai, Jingxuan Zhu, Shuliang Zhang, Shaonan Zhu, Dawei Han, and Guonian Lv
Rainfall is a driving force that accounts for a large proportion of soil loss around the world. Most previous studies used a fixed rainfall–energy relationship to estimate rainfall energy, ignoring the spatial and temporal changes of raindrop microphysical processes. This study proposes a novel method for large-scale and long-term rainfall energy and rainfall erosivity investigations based on rainfall microphysical parameterization schemes in the Weather Research and Forecasting (WRF) model.
Physically based model for gully simulation: application to the Brazilian semiarid region
Pedro Henrique Lima Alencar, José Carlos de Araújo, and Adunias dos Santos Teixeira
Soil erosion by water has been emphasized as a key problem to be faced in the 21st century. Thus, it is critical to understand land degradation and to answer fundamental questions regarding how and why such processes occur. Here, we present a model for gully erosion (channels carved by rainwater) based on existing equations, and we identify some major variables that influence the initiation and evolution of this process. The successful model can help in planning soil conservation practices.
Assessing the perturbations of the hydrogeological regime in sloping fens due to roads
Fabien Cochand, Daniel Käser, Philippe Grosvernier, Daniel Hunkeler, and Philip Brunner
Hydrol. Earth Syst. Sci., 24, 213–226, https://doi.org/10.5194/hess-24-213-2020,https://doi.org/10.5194/hess-24-213-2020, 2020
Roads in sloping fens constitute a hydraulic barrier for surface and subsurface flow. This can lead to the drying out of downslope areas of the fen as well as gully erosion. By combining fieldwork and numerical models, this study presents an assessment of the hydrogeological impact of three road structures especially designed to limit their impact. The study shows that the impact of roads on the hydrological regime in fens can be significantly reduced by using appropriate engineering measures.
A review of the (Revised) Universal Soil Loss Equation ((R)USLE): with a view to increasing its global applicability and improving soil loss estimates
Rubianca Benavidez, Bethanna Jackson, Deborah Maxwell, and Kevin Norton
Soil erosion is a global problem and models identify vulnerable areas for management. One such model is the Revised Universal Soil Loss Equation. We review its different sub-factors and compile studies and equations that modified it for local conditions. The limitations of RUSLE include its data requirements and exclusion of gullying and landslides. Future directions include accounting for these erosion types. This paper serves as a reference for others working with RUSLE and related approaches.
Hybridizing Bayesian and variational data assimilation for high-resolution hydrologic forecasting
Felipe Hernández and Xu Liang
Predicting floods requires first knowing the amount of water in the valleys, which is complicated because we cannot know for sure how much water there is in the soil. We created a unique system that combines the best methods to estimate these conditions accurately based on the observed water flow in the rivers and on detailed simulations of the valleys. Comparisons with popular methods show that our system can produce realistic predictions efficiently, even for very detailed river networks.
Multi-source data assimilation for physically based hydrological modeling of an experimental hillslope
Anna Botto, Enrica Belluco, and Matteo Camporese
We present a multivariate application of the ensemble Kalman filter (EnKF) in hydrological modeling of a real-world hillslope test case with dominant unsaturated dynamics and strong nonlinearities. Overall, the EnKF is able to correctly update system state and soil parameters. However, multivariate data assimilation may lead to significant tradeoffs between model predictions of different variables, if the observation data are not high quality or representative.
A new method, with application, for analysis of the impacts on flood risk of widely distributed enhanced hillslope storage
Peter Metcalfe, Keith Beven, Barry Hankin, and Rob Lamb
Flooding is a significant hazard and extreme events in recent years have focused attention on effective means of reducing its risk. An approach known as natural flood management (NFM) seeks to increase flood resilience by a range of measures that work with natural processes. The paper develops a modelling approach to assess one type NFM of intervention – distributed additional hillslope storage features – and demonstrates that more strategic placement is required than has hitherto been applied.
Towards improved parameterization of a macroscale hydrologic model in a discontinuous permafrost boreal forest ecosystem
Abraham Endalamaw, W. Robert Bolton, Jessica M. Young-Robertson, Don Morton, Larry Hinzman, and Bart Nijssen
This study applies plot-scale and hill-slope knowledge to a process-based mesoscale model to improve the skill of distributed hydrological models to simulate the spatially and basin-integrated hydrological processes of complex ecosystems in the sub-arctic boreal forest. We developed a sub-grid parameterization method to parameterize the surface heterogeneity of interior Alaskan discontinuous permafrost watersheds.
Reconstructing long-term gully dynamics in Mediterranean agricultural areas
Antonio Hayas, Tom Vanwalleghem, Ana Laguna, Adolfo Peña, and Juan V. Giráldez
Gully erosion is one of the most important erosion processes. In this study, we provide new data on gully dynamics over long timescales with an unprecedented temporal resolution. We apply a new Monte Carlo based method for calculating gully volumes based on orthophotos and, especially, for constraining uncertainties of these estimations. Our results show that gully erosion rates are highly variable from year to year and significantly higher than other erosion processes.
Evaluating performance of simplified physically based models for shallow landslide susceptibility
Giuseppe Formetta, Giovanna Capparelli, and Pasquale Versace
This paper focuses on performance evaluation of simplified, physically based landslide susceptibility models. It presents a new methodology to systemically and objectively calibrate, verify, and compare different models and models performances indicators in order to individuate and select the models whose behavior is more reliable for a certain case study. The procedure was implemented in a package for landslide susceptibility analysis and integrated the open-source hydrological model NewAge.
Determinants of modelling choices for 1-D free-surface flow and morphodynamics in hydrology and hydraulics: a review
Bruno Cheviron and Roger Moussa
This review paper investigates the determinants of modelling choices for numerous applications of 1-D free-surface flow and morphodynamics in hydrology and hydraulics. Each case study has a signature composed of given contexts (spatiotemporal scales, flow typology, and phenomenology) and chosen concepts (refinement and subscales of the flow model). This review proposes a normative procedure possibly enriched by the community for a larger, comprehensive and updated image of modelling strategies.
Use of satellite and modeled soil moisture data for predicting event soil loss at plot scale
F. Todisco, L. Brocca, L. F. Termite, and W. Wagner
We developed a new formulation of USLE, named Soil Moisture for Erosion (SM4E), that directly incorporates soil moisture information. SM4E is applied here by using modeled data and satellite observations obtained from the Advanced SCATterometer (ASCAT). SM4E is found to outperform USLE and USLE-MM models in silty–clay soil in central Italy. Through satellite data, there is the potential of applying SM4E for large-scale monitoring and quantification of the soil erosion process.
Quantification of the influence of preferential flow on slope stability using a numerical modelling approach
W. Shao, T. A. Bogaard, M. Bakker, and R. Greco
The effect of preferential flow on the stability of landslides is studied through numerical simulation of two types of rainfall events on a hypothetical hillslope. A model is developed that consists of two parts. The first part is a model for combined saturated/unsaturated subsurface flow and is used to compute the spatial and temporal water pressure response to rainfall. Preferential flow is simulated with a dual-permeability continuum model consisting of a matrix/preferential flow domain.
Hydrological hysteresis and its value for assessing process consistency in catchment conceptual models
O. Fovet, L. Ruiz, M. Hrachowitz, M. Faucheux, and C. Gascuel-Odoux
We studied the annual hysteretic patterns observed between stream flow and water storage in the saturated and unsaturated zones of a hillslope and a riparian zone. We described these signatures using a hysteresis index and then used this to assess conceptual hydrological models. This led us to identify four hydrological periods and a clearly distinct behaviour between riparian and hillslope groundwaters and to provide new information about the model performances.
Derivation and evaluation of landslide-triggering thresholds by a Monte Carlo approach
D. J. Peres and A. Cancelliere
A Monte Carlo approach, combining rainfall-stochastic models and hydrological and slope stability physically based models, is used to derive rainfall thresholds of landslide triggering. The uncertainty in threshold assessment related to variability of rainfall intensity within events and to past rainfall (antecedent rainfall) is analyzed and measured via ROC-based indexes, with a specific focus dedicated to the widely used power-law rainfall intensity-duration (I-D) thresholds.
Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale
D. Windhorst, P. Kraft, E. Timbe, H.-G. Frede, and L. Breuer
Analysis of landslide triggering conditions in the Sarno area using a physically based model
G. Capparelli and P. Versace
The influence of grid resolution on the prediction of natural and road-related shallow landslides
D. Penna, M. Borga, G. T. Aronica, G. Brigandì, and P. Tarolli
Coupled prediction of flood response and debris flow initiation during warm- and cold-season events in the Southern Appalachians, USA
J. Tao and A. P. Barros
Predicting subsurface stormflow response of a forested hillslope – the role of connected flow paths
J. Wienhöfer and E. Zehe
Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin)
A. Richard, S. Galle, M. Descloitres, J.-M. Cohard, J.-P. Vandervaere, L. Séguis, and C. Peugeot
A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution
S. R. Lutz, H. J. van Meerveld, M. J. Waterloo, H. P. Broers, and B. M. van Breukelen
A paradigm shift in stormflow predictions for active tectonic regions with large-magnitude storms: generalisation of catchment observations by hydraulic sensitivity analysis and insight into soil-layer evolution
Makoto Tani
Derivation of critical rainfall thresholds for shallow landslides as a tool for debris flow early warning systems
M. N. Papa, V. Medina, F. Ciervo, and A. Bateman
Statistical analysis and modelling of surface runoff from arable fields in central Europe
P. Fiener, K. Auerswald, F. Winter, and M. Disse
Hydrological modelling of a slope covered with shallow pyroclastic deposits from field monitoring data
R. Greco, L. Comegna, E. Damiano, A. Guida, L. Olivares, and L. Picarelli
Physically based modeling of rainfall-triggered landslides: a case study in the Luquillo forest, Puerto Rico
C. Lepore, E. Arnone, L. V. Noto, G. Sivandran, and R. L. Bras
Characterization of groundwater dynamics in landslides in varved clays
J. E. van der Spek, T. A. Bogaard, and M. Bakker
A critical assessment of simple recharge models: application to the UK Chalk
A. M. Ireson and A. P. Butler
The effect of spatial throughfall patterns on soil moisture patterns at the hillslope scale
A. M. J. Coenders-Gerrits, L. Hopp, H. H. G. Savenije, and L. Pfister
Snow accumulation/melting model (SAMM) for integrated use in regional scale landslide early warning systems
G. Martelloni, S. Segoni, D. Lagomarsino, R. Fanti, and F. Catani
Suspended sediment concentration–discharge relationships in the (sub-) humid Ethiopian highlands
C. D. Guzman, S. A. Tilahun, A. D. Zegeye, and T. S. Steenhuis
A model of hydrological and mechanical feedbacks of preferential fissure flow in a slow-moving landslide
D. M. Krzeminska, T. A. Bogaard, J.-P. Malet, and L. P. H. van Beek
Scale effect on overland flow connectivity at the plot scale
A. Peñuela, M. Javaux, and C. L. Bielders
Hydrol. Earth Syst. Sci., 17, 87–101, https://doi.org/10.5194/hess-17-87-2013,https://doi.org/10.5194/hess-17-87-2013, 2013
Physical models for classroom teaching in hydrology
A. Rodhe
Coupling the modified SCS-CN and RUSLE models to simulate hydrological effects of restoring vegetation in the Loess Plateau of China
G. Y. Gao, B. J. Fu, Y. H. Lü, Y. Liu, S. Wang, and J. Zhou
Effects of peatland drainage management on peak flows
C. E. Ballard, N. McIntyre, and H. S. Wheater
A conceptual model of the hydrological influence of fissures on landslide activity
D. M. Krzeminska, T. A. Bogaard, Th. W. J. van Asch, and L. P. H. van Beek
A structure generator for modelling the initial sediment distribution of an artificial hydrologic catchment
T. Maurer, A. Schneider, and H. H. Gerke
A novel explicit approach to model bromide and pesticide transport in connected soil structures
J. Klaus and E. Zehe
Quantifying spatial and temporal discharge dynamics of an event in a first order stream, using distributed temperature sensing
M. C. Westhoff, T. A. Bogaard, and H. H. G. Savenije
Effect of high-resolution spatial soil moisture variability on simulated runoff response using a distributed hydrologic model
J. Minet, E. Laloy, S. Lambot, and M. Vanclooster
A steady-state saturation model to determine the subsurface travel time (STT) in complex hillslopes
T. Sabzevari, A. Talebi, R. Ardakanian, and A. Shamsai
Comparison of algorithms and parameterisations for infiltration into organic-covered permafrost soils
Y. Zhang, S. K. Carey, W. L. Quinton, J. R. Janowicz, J. W. Pomeroy, and G. N. Flerchinger
Anderson, S. P., Bales, R. C., and Duffy, C. J.: Critical Zone Observatories: Building a network to advance interdisciplinary study of Earth surface processes, Mineralog. Mag., 72, 7–10, 2008.
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\section{Introduction}
The short Kelvin-Helmholtz timescales of massive young stellar
objects (YSOs) allow these objects to begin emitting ionizing
photons and to excite nearby material while still embedded in their
natal clouds \citep{lad99}. One of the earliest observable
manifestations of newly formed OB stars is the ultracompact HII
(UCHII) region. These embedded HII regions are initially small ($D
{\leq} 0.1~pc$) and dense ($n_{e} {\sim} 10^4~cm^{-3}$) and have a
typical emission measure (EM) $\sim$ 10$^7$~pc~cm$^{-6}$
\citep{chu02}. As HII regions evolve their sizes increase, but they
are still considered to be UCHII regions as long as their emission
measures (or surface brightnesses) remain high.

Since most stars with M $\ge$ 8~M$_{\odot}$ should form UCHII
regions shortly after their birth, studies of UCHII regions can help
us learn about physical conditions in high-mass star forming regions
and the evolution of the physical conditions as the massive stars
turn on. A large percentage, $\ge$ 50$\%$, of UCHII regions have
cometary, core-halo or irregular morphologies
\citep{wooC89b,kurCW94,walBHR98,depWDMD05}, arguing against the
classical expanding sphere model of HII regions \citep{savG55}. In
addition, statistical arguments based on the Galactic population of
UCHII regions lead to estimates of the lifetime of these regions
that are much longer than expected from the expanding sphere model
\citep{wooC89a,hoaKLKH07}. Many models have been proposed for UCHII
region formation \citep[see the review by][and references
therein]{chu99}, with solving the lifetime problem a major goal.
However, the age of an individual UCHII region is hard to determine
through observations. A number of theoretical models have been
geared toward explaining both these long apparent lifetimes and the
observed morphologies of UCHII regions
\citep{holJLS94,dysWR95,redWD96,wilDR96,redWD98}.

Observations of the morphology and gas kinematics of UCHII regions
can help us to constrain models for the structure and evolution of
the regions much more definitively than observations of the
morphology alone. The morphological type is not a unique indicator
of the UCHII region physics because many models can produce similar
morphologies \citep{ten79, yorTB83, macVW91, vanM92, artH06}. While
it is difficult to choose between different models based on the
morphologies of HII regions, their kinematic signatures are very
distinct. To understand the structure and evolution of the regions
around the youngest massive stars, it is therefore necessary to
observe gas kinematics inside UCHII regions with the highest
spectral and spatial resolution. The large extinction associated
with molecular clouds around UCHII regions, however, prevents direct
observation at optical wavelengths and even at near-infrared
wavelengths \citep{dopFCSCT06}. Thus, observations at longer
wavelengths become crucial to UCHII region studies.

Radio interferometric observations can provide data with high
angular and spectral resolution. At high enough frequencies, where
non-LTE effects become less important, hydrogen radio recombination
line (RRL) emission has the same dependence on the electron density
as thermal free-free radio continuum emission, and the emissivities
of these lines are well known. RRL observations can provide
information on gas density and kinematics, and, to a limited degree,
on gas temperature. However, thermal broadening of hydrogen RRLs due
to the low mass of hydrogen atoms degrades the spectral resolution
of radio spectroscopy. The thermal line width of the RRLs (or any
hydrogen recombination lines like Br$\alpha$ and Br$\gamma$, for
that matter) in an ionized region (T$_e {\simeq} $10$^4$~K) is
$\simeq$ 20~km~s$^{-1}$, comparable to the velocities of bulk flows
in many sources. Furthermore, the incomplete uv sampling of radio
interferometric survey can both overemphasize compact structures
and, more generally limit the spatial dynamic range of the
observations.

Mid-IR fine structure line observations offer an alternative to
radio and IR recombination line observations. Fine structure lines
have small thermal line widths because they are emitted by heavy
elements. The importance of this feature of the fine structure lines
has been illustrated previously by high spectral resolution maps of
[Ne~II] emission in Mon R2 and G29.96-0.02 which show single lines
or multiple components of lines with widths significantly smaller
than the thermal widths of the hydrogen recombination lines
\citep{jafZLR03,zhuLJRG05}. In some cases, the broad, largely
Gaussian hydrogen recombination lines are, in fact, superpositions
of several much narrower kinematic components (see especially Fig. 3
of \cite{jafZLR03}). In addition, although the emitting species are
several orders of magnitude less abundant than hydrogen, the fine
structure lines are brighter than recombination lines because
collisional excitation cross-sections are much larger than the
relevant recombination cross-sections. Typical electron densities
over all compact and most ultracompact H~II regions are lower than
the critical density of the [Ne~II] 12.8~$\mu$m transition (${\sim}
5 \times 10^5~cm^{-3}$), so this line shares the property of the
recombination lines of having a line strength proportional to
emission measure. We should mention that turbulence can also have
significant effects on line widths. Moreover, turbulence affects
both RRL and fine structure line observations and prevents
detections of velocity variations (due to gas bulk motions) with
scales smaller than the intrinsic line width combining thermal and
turbulent effects along any line of sight direction.

Our previous [Ne~II] line observations of a few UCHII regions have
demonstrated the effectiveness of using high spectral and spatial
resolution maps of fine structure lines to study such regions
\citep{jafZLR03,zhuLJRG05}. In these studies, we demonstrated that
the kinematic patterns in the ionized gas in Mon R2 and G29.96-0.02
are consistent with tangential flows along the surfaces of thin,
more-or-less parabolic shells. The goal of the current paper is to
use [Ne~II] mapping as a tool to examine a larger sample of compact
and ultracompact HII regions with a range of shapes and sizes in
order to look for a common thread in the kinematics that might lead
to a better understanding of the physics and evolution of these
sources. We present scan-mapped spectroscopic observations of
fifteen Galactic compact and ultracompact HII regions. This survey
is the first mid-infrared high-spectral resolution study of gas
kinematics in a large sample of UCHII regions.

In \S~{\bf \ref{secobservation}}, we describe our observational
method. We introduce our data reduction procedure in \S~{\bf
\ref{secdatareduction}}. In \S~{\bf \ref{secresult}}, we investigate
the morphology and kinematics of individual objects. Finally, the
conclusions drawn from our observations are presented in \S~{\bf
\ref{secdiscussion}}.

\section{Observations}
\label{secobservation} Candidates in our survey were selected based
on their radio continuum flux density levels \citep{wooC89b},
biasing our list toward radio-bright compact and ultracompact HII
regions. Observations were performed with the high spectral
resolution, mid-IR (5-25~$\mu$m) cross dispersed spectrograph TEXES
\citep[Texas Echelon Cross Echelle Spectrograph,][]{lacRGJZ02} on
the 3.0 meter NASA IRTF on Mauna Kea in Hawaii.

We list our observational parameters in Table~\ref{obspara}. Our
spatial and spectral resolution were approximately 1.8$''$ and
4~km~s$^{-1}$. For each object, a north-south oriented
slit was used to scan the object from west to east. We chose the
step size along the scans so that there were at least two steps per
slit width and chose the scan lengths to cover the emission seen in
radio continuum maps available in the literature. Before each scan,
we offset the telescope to the west several arcseconds beyond the
edge of the maps shown in the figures. Several extra steps were
taken at the beginning and the end of each scan in order to measure
the sky background. The lengths of the scans and the initial offsets
were adjusted after examining a preliminary scan with our quick-look
program \citep{lacRGJZ02}. We made multiple overlapping scans,
offset north-south, to cover the sources. TEXES takes ambient
temperature blackbody and sky flats at the beginning of each scan
for flux calibration.

\section{Data Reduction}
\label{secdatareduction} We use a custom Fortran reduction program
\citep{lacRGJZ02} to correct optical distortions, flat field, remove
cosmic ray spikes, and fix bad pixels. The same program also does
wavelength and flux calibrations. We use atmospheric lines for
wavelength calibration, with wavelengths obtained from HITRAN
\citep{rotRGMEFP98}. We use an IDL script to subtract sky background
and combine multiple scans. The sky emission at each slit position
is linearly interpolated from the sky frames at the beginning and
the end of each scan. Sometimes, we start scans too late or end
scans too early. Extended line emission is present in our sky frames
and is subtracted off from other frames. This will result in an
underestimate of line flux from targets. Multiple scans are
cross-correlated, shifted and added to create the datacubes.
Normally, no shift is needed along the spectral direction. One
exception was the source W3, whose datacube was obtained by
combining data from two nights and where the maps were registered
using the compact continuum source IRS5 \citep{wynBN72}. Both the
Earth's motion and the spectral settings of the instrument on two
different nights affect the wavelength calibration. The continuum
flux level at each pixel in the map is obtained from spectral points
off the line and is subtracted from the spectra. Finally, we
resample the datacubes so that each pixel has a size of
$0.3''{\times}0.3''$ on the sky. The coordinates of the resulting
continuum maps and [Ne~II] line maps are relative to the peak
positions in the integrated [Ne~II] line maps. When radio continuum
maps are available, they are cross-correlated with the [Ne~II] maps
so that both maps can be plotted on the same scale. The coordinates
in Table 1 were taken from the literature and served as reference
points during our line observations.

\section{Results}
\label{secresult}

In this section we discuss highlights of individual objects and
features common to several sources. [Ne~II] line emission datacubes
for all objects are available online. {\bf Note: the data cubes will
be available through the ApJ web site.}

\subsection{G29.96~-0.02}

G29.96~-0.02 (G29.96 hereafter) is a cometary UCHII region at a
distance of $\sim7.4$~kpc, is $\sim$20$''$$\times$15$''$ in size,
and is one of the most luminous UCHII regions in the sky at 2 cm
wavelength. The ionized region is considered as an archetype of the
UCHII regions caused by bow shocks \citep{macVW91}, although some
past observations of the gas velocities along the symmetry axis of
this source argue against this interpretation, implying that the
source is a blister flow \citep{lumH96} or a blister flow modified
by a stellar wind \citep{lumH99}. We presented [Ne~II] line mapping
observations and compared gas kinematics in the region with a bow
shock model in \cite{zhuLJRG05}. We include here a summary of the
model and results for G29.96 from the earlier paper as a template
for understanding the kinematics of some of the other sources in the
sample. The bow shock model predicts that an approximately
paraboloidal shell of swept-up material will form in front of a star
moving supersonically inside a molecular cloud. The shell is
supported by the ram pressure of the stellar wind and the ambient
medium. Swept-up material, including ionized and neutral gas, is
forced to flow along the surface of the shell. With different
viewing angles for the shell, the model can reproduce many of the
observed features in ionic line observations of G29.96 as well as
several other UCHII regions in our sample, including morphologies
and the distinctive shapes that parabolic flows have in
position-velocity (p-v) diagrams.

Figure~\ref{g2996mapneii} shows a 2~cm radio continuum map from
\cite{feyGCV95} and our spectrally integrated [Ne~II] line map of
G29.96. The two maps closely resemble each other, indicating that
most of the [Ne~II] emission arises from gas with n$<$n$_{crit}$ and
that the extinction to the gas is either rather uniform or small at
12.8~$\mu$m. In fact, \cite{morSMPDBCR02} observed that the electron
density in the region is sub-critical (N$_e$$\sim$680 cm$^{-3}$ -
5.7$\times$10$^4$ cm$^{-3}$) and \cite{marPMTCR02} reported low
infrared extinction (A$_K$$\simeq$1.6) toward the region. Our
findings are consistent with these observations. Both maps show an
emission arc open to the east with extended emission in the eastern
part of the region. A filament extends from the southern edge of the
nebula to the east, forming a ``tail'' of the cometary HII region.
Channel maps at nine equally spaced velocities are shown in
Figure~\ref{g2996nechanmapneii}. These channel maps show a
morphology change from a thin crescent in the most blue-shifted
channels to a fan-like region in red-shifted channels.
Figures~\ref{g2996pv1} and \ref{g2996pv2} show position-velocity
(p-v) diagrams of several representative cross cuts made through the
observed and model data cubes. The observations and the model
display similar p-v structures (as shown in our previous paper, a
characteristic ``$\Lambda$'' shape for cuts perpendicular to the
nebular symmetry axis and a ``7'' shape for cuts parallel to the
axis). Similar changes in structure from cut to cut can also be
seen. The similarities in line emission morphology and gas
kinematics between the observations and the model indicate that the
ionized gas in G29.96 flows along a thin shell at
$\sim20$~km~s$^{-1}$, as it would if it were produced by a bow
shock. We use these distinctive patterns in the p-v diagrams as
indicators of the presence of surface flows in other sources where
we have not carried out detailed model fits. However, one clear
discrepancy between the bow shock model that gives the correct shape
to the contours in the model p-v diagrams in Figures~\ref{g2996pv1}
and \ref{g2996pv2} and the observed sources is the
$\sim$15~km~s$^{-1}$ difference between the observed and modeled
velocity of the ambient neutral material. We discuss in
\S~\ref{secdiscussion} modifications of the bow-shock model to
explain this discrepancy.

\subsection{G5.89~-0.39}

G5.89~-0.39, also called W28 A2(1), is 2.6~kpc from the Sun
\citep{dowWBW80,kimK03}. \cite{wooC89b} measured continuum images of
the source at both 2~cm (Figure~\ref{g589map}, left) and 6~cm. A
shell-like morphology with significantly limb-brightened walls
surrounding the slightly elongated central cavity can be recognized.
The G5.89 region also hosts a source of extremely broad (FWHM
$\sim$140~km~s$^{-1}$) CO J=1-0 emission \citep{sheC96,harF88}.
\cite{acoCW98} have used multi-epoch radio continuum observations to
argue that the ionized shell is expanding at $\sim$35~km~s$^{-1}$.
Radio recombination line observations of \cite{rodRG02} show a
complex velocity structure across the region, which cannot be
explained with a consistent dynamical model. Mid-IR dust continuum
observations show that G5.89 deviates from spherical symmetry and
resembles a shell partially open to the south, with significant SW
and SE protrusions from the geometric center, implying an
anisotropic expansion of the ionization front due to density
variations in the surrounding molecular medium \citep{balAJKM92}.

Our [Ne~II]~12.8~$\mu$m line map (Figure~\ref{g589map}, right)
shows a morphology similar to that of the mid-infrared continuum,
with a bright arc along the north side of the source.
With our $\sim 1.5''$ resolution, the kinematic signature of an
expanding shell is not apparent, although there is some evidence
that the line is broadest toward the center of the source
(Figure~\ref{g589chanmapneii}).
The majority of the emission is blue-shifted
by $\sim$9~km~s$^{-1}$ with respect to the molecular cloud
\citep{hatTM98}, and the extent of the
region becomes relatively smaller toward the red end of the
spectrum.

\subsection{G11.94~-0.62}
Located at a distance of 4.2~kpc \citep{chuWC90}, G11.94~-0.62
was categorized by \cite{wooC89b} as a cometary
UCHII region. In their radio continuum map (Figure~\ref{g1194map},
left), the region shows an emission arc with its apex pointing to
the west and diffuse emission extending to the east and north. A
broad $^{12}$CO(J=1-0) line was observed toward the object
\citep{sheC96}. The CO line width (full width at zero intensity) of
over 28~km~s$^{-1}$ suggests the presence of high velocity outflows.
\cite{debRTP03} imaged the region at 11.7~$\mu$m and 20.8~$\mu$m.
The region does not show a cometary shape in their mid-IR images
and, although their pointing has an astrometric accuracy about
1$''$, the radio peak does not match any structure in the mid-IR
images. The overall appearance of the region at mid-IR and radio
continuum wavelengths does not match well.

Our [Ne~II]~12.8~$\mu$m line map (Figure~\ref{g1194map}, right)
matches the mid-IR continuum observations \citep{debRTP03} very
well. By registering our map with the continuum map in
\cite{debRTP03}, we determine the position of the radio continuum
peak in our map, marked with an `X' at (-1.17$''$, 1.67$''$). Five
separate emission peaks (designated as A-E) can be seen in our
integrated [Ne~II] map. They all have corresponding 11.7$\mu$m and
20.8$\mu$m emission peaks in \cite{debRTP03}. Linear sizes of these
clumps range from $\sim$2$''$ in C to over 7$''$ in A and E. Like
the mid-IR continuum, the [Ne~II] morphology is a poor match to that
of the radio continuum.

The spectra of the clumps C, D and E each can be fit with a single
Gaussian profile with a center velocity very close to the molecular
cloud velocity \citep[V$_{LSR} \simeq $39~km~s$^{-1}$,][]{chuWC90}.
Figure~\ref{g1194ABCspec} shows line profiles in the region of A, B
and C. Over this part of the source, the lines are broad, with most
of the additional emission to the red of the cloud velocity. The
profiles in Figure~\ref{g1194ABCspec} show that most of the [Ne~II]
emission occurs over the range V$_{LSR}$ = 30-60 km s$^{-1}$.
Complex morphology, probably induced by dust extinction, prevents a
detailed analysis of the kinematics in the region.

\subsection{G30.54~+0.02}
G30.54~+0.02 has a limb-brightened, or ''horseshoe'' shape in both the radio
continuum map \citep{wooC89b} and our [Ne~II] line
map (Figure~\ref{g3054map}).
The more concentrated emission with a bigger opening in the
radio continuum map probably is the result of the higher resolution
of the interferometer observations and of the uneven sensitivity of the
snapshot mode aperture synthesis to compact and extended emission.
Both radio and [Ne~II] line observations show that
the emission arc breaks into at least three components.

Figure~\ref{g3054profile} shows [Ne~II] profiles across the region
while Figure~\ref{g3054pv} displays position-velocity diagrams on
selected cuts along and across the symmetry axis. Large line widths
(20-30~km~s$^{-1}$ FWHM), together with smooth profiles imply that
turbulence or substantial small-scale motions are present and may
mask the bulk motion of the gas. Nevertheless, a change of velocity
pattern within the structure is still apparent.

The velocity pattern along the bright arc is consistent with
significant tangential flows of the type seen in G29.96, but here
mostly in the plane of the sky. At the vertex of the horseshoe,
there is a single broad line with a velocity centroid close to the
molecular cloud velocity (Figure~\ref{g3054pv}). On the arm of the
horseshoe to the east of the vertex the line peak shifts $\sim$
7~km~s$^{-1}$ to the blue of the molecular cloud velocity while on
the western arm the molecular line and [Ne~II] peak velocities
agree. [Ne~II] emission is present over at least 40-50~km~s$^{-1}$
at all positions (Figures~\ref{g3054profile} and \ref{g3054pv}).

Along the symmetry axis and away from the bright arc, the match to a
tangential flow is less clear. Rather than breaking up into two
narrow lines, the emission on the axis and behind the vertex is at a
single velocity close to that of the molecular cloud. A small
velocity gradient across the apex may indicate an inclination to the
plane of sky.

\subsection{G33.92~+0.11}
G33.92~+0.11 is particularly remarkable because
of the apparent disconnect between its nondescript morphology and
its distinctive position-velocity structure. It was first
categorized as a core-halo UCHII region by \cite{wooC89b} and later
as a shell UCHII region by \cite{feyCGNJ92}. \cite{feyCGNJ92} showed
that the 20 cm extended emission is roughly shell-like, although the
cavity in their 20~cm image is offset to the southeast of the
emission peak, and the rest of the region has a morphology close to
a core-halo region with cometary extended emission. C$^{18}$O
observations showed two emission cores, and one of them is
associated with the peak of the UCHII region \citep{watM99}.

The integrated [Ne~II] line map shows an irregular peak surrounded
by extended emission (Figure~\ref{g3392map}). Mid-infrared continuum
observations show similar structure to that seen in the [Ne~II] line
observations \citep{givRBW07}. The major structure is a resolved
ellipsoid oriented northeast-southwest with an additional extension
to the east along the minor axis. The [Ne~II] and radio continuum
maps look very similar, but the central peak splits into two
components in the neon line, possibly as a result of foreground
extinction.

Position-velocity diagrams along a few cross cuts through G33.92
(Figure~\ref{g3392pv}) show kinematics identifiable with the
kinematics typical of a flow along a surface similar to that seen in
G29.96 including the characteristic ``7'' and ``$\Lambda$''
patterns, despite its more nondescript morphology. Profiles at
individual positions are broad and frequently asymmetric
(Figure~\ref{g3392multispec}). Channel maps
(Figure~\ref{g3392chanmapneii}) show that three prominent components
form an arc at blue velocities and persist to the ambient material
velocity, V$_{LSR} \simeq $108~km~s$^{-1}$. This arc has a sharp
falloff to the west. A fourth emission peak and extended emission
can be seen to the east of the arc. The positions of these emission
peaks shift from channel to channel. This position shift is
especially prominent for the peak to the east of the arc. It moves
almost 5$''$ southeast from a position close to the arc in the
channel at V$_{LSR} {\simeq} $107~km~s$^{-1}$ to (+6,-3) in the
channel at V$_{LSR} {\simeq} $89~km~s$^{-1}$. The three components
along the arc move a smaller distance to the northwest. Towards the
red end of the spectrum, the emission components merge together and
fade away. We note that the overall shape of the region changes from
a thin crescent at blue-shifted velocities to a fan at red-shifted
velocities. Bow shock models, like that for G29.96 predict a similar
morphological change.

\subsection{G43.89~-0.78}
G43.89~-0.78 was classified as a cometary UCHII
region by \cite{wooC89b}.
\cite{macVW91} used G43.89 as one of their bow shock
model tests. They concluded, based on the continuum maps
that they needed a viewing angle between 45$^{\circ}$ and 60$^{\circ}$
from head-on in order to fit the morphology well.
\cite{sheC96} found
evidence for high-velocity molecular material in the region with a CO
line width over 39~km~s$^{-1}$. Low-resolution 21~$\mu$m imaging
observations showed the region as a point source with extended
emission to the southeast \citep{croC03}, while the radio continuum
observations showed extended emission northwest of an emission peak.

The cometary morphology and kinematics of the [Ne~II] line emission
(Figure~\ref{g4389map}-\ref{g4389pv}) are consistent with flows
along the surface of the region like those seen in G29.96. We fitted
the p-v diagrams with a model like that used for G29.96
\citep{zhuLJRG05}. The best fit was with a stellar motion of
15~km~s$^{-1}$ and 140$^{\circ}$ away from the direction to the
observer. However, as with G29.96, there is a discrepancy between
the model, which predicts that the ionic emission should be
redshifted relative to the molecular emission, and the observed
emission, which is predominantly blueshifted.

\subsection{G45.07~+0.13}
G45.07~+0.13 was classified as an UCHII region with a shell
morphology by \citet{turM84}. At a distance of $\sim$6.0~kpc
\citep{chuWC90}, it has a diameter $\sim$0.9$''$ at $\lambda$=2~cm
\citep{wooC89b}. The slight velocity gradient in the H76$\alpha$
line was interpreted as the result of either an expanding ring
\citep{garRV86} or ionized bipolar outflows \citep{limW99}. A group
of water masers lie 2$''$ to the north of the radio continuum peak
\citep{hofC96,debRTP03}.

G45.07 is unresolved in our 12.8~$\mu$m [Ne~II] map at 2$''$ spatial
resolution (Figure~\ref{g4507mapneii}, right). The [Ne~II] line
toward G45.07 is much narrower (V$_{FWHM} \simeq$ 20~km~s$^{-1}$)
than is the H76$\alpha$ radio recombination line
\citep[$\sim$48~km~s$^{-1}$,][]{garRV86}. The difference in widths
can arise either from pressure broadening of the H76$\alpha$ line or
from the presence of high velocity material at very high density
where the recombination line emissivity is proportional to n$_e^2$
while the [Ne~II] emissivity only scales with n$_e$. Dust continuum
emission is observed at G45.07 and at a second source $\sim$2$''$
north of G45.07 (Figure~\ref{g4507mapneii}, left). The location of
the second source matches the positions of the H$_2$O masers. We did
not detect any line emission from the second source, supporting the
suggestion that this source is at an early evolutionary stage
\citep{hofC96,debRTP03}.

\subsection{G45.12~+0.13}
G45.12~+0.13 (G45.12 hereafter) is located at a distance of
6.9~kpc \citep{chuWC90}.
It was observed by \cite{wooC89b} with the VLA at 2 and 6~cm and revealed
at least 3 peaks with a roughly arc-like overall morphology.

Our [Ne~II] line observations do not completely resolve the three
components along the radio emission arc, which is only $\sim$3''
long (referred to as Source ``N'' in Figure~\ref{g4512map}). The
channel maps (Figure~\ref{g4512chanmap}) show that the ``horns'' of
the arc-like structure are more prominent at lower redshifts while
the bright spot at the middle of the arc dominates at higher
velocities. The p-v diagrams of a cut parallel to the symmetry axis
of the source N do show ``7'' patterns of cometary H~II regions
(Figure~\ref{g4512Npv}, top panels), but a cut perpendicular to the
axis does not show the expected ``$\Lambda$'' shaped pattern
(Figure~\ref{g4512Npv}, bottom panels). It is not clear how much the
kinematics in G45.12N resembles that in a cometary HII region,
because its ``7'' type p-v diagram is not dramatic compared to that
in G29.96 (Figure~\ref{g2996pv1}).

We also find two fainter sources to the south of the arc which were
not seen in the sparsely sampled VLA snapshot image. To show these
two faint sources clearly, we extend contour levels down to 1$\%$ of
the peak value in our line map. The southeastern source (SE) is
extended and almost round-shaped. It is hard to determine its
symmetry axes. Its p-v diagrams along RA and DEC cuts
(Figure~\ref{g4512SEpv}) show evidence of gas bulk motion and most
of emission is blue-shifted relative to the ambient molecular gas.
Tentative ``7'' shaped emission distribution can be seen in these
diagrams, suggesting that surface flows may be present in the
source. The southwestern source (SW) is irregular and contains
multiple peaks. These emission peaks surround an emission minimum at
the same location $\sim$(2.$''$7, -8$''$) in almost all channels
(Figure~\ref{g4512chanmap}). A broken shell-like morphology with an
opening to the west suggests an expanding shell structure for SW,
but the ring of the line emission does not collapse at the ends of
the Doppler shift-range as would be expected for an expanding shell.
The velocity distributions in both N and SW are more or less
symmetric about the ambient material velocity, which is
59~km~s$^{-1}$. Line emission from SE is relatively blue-shifted,
and the width of the [Ne~II] line is narrower in SE than in the
other two sources.

\subsection{G45.45~+0.06}
G45.45~+0.06 was categorized by \cite{wooC89b} as a cometary UCHII
region. In the 6~cm radio continuum image, the source extends
$\sim9''$~EW and $6''$~NS and has a sharp and bright ionization
front to the north and extended emission to the south
(Figure~\ref{g4545map}, left panel). Multiple emission peaks
arranged along the ionization front form a distorted ring around a
cavity. At a distance 6.6~kpc, the integrated continuum flux density
implies an O7.5 main sequence star as the ionizing star
\citep{chuWC90}. \cite{felSHH98} acquired images at H and K$'$ bands
and three mid-infrared bands centered at 3.8$\mu$m, 10.5$\mu$m and
11.7$\mu$m and found that the region contained a young OB cluster.

The overall morphology of the [Ne~II] line emission
(Figure~\ref{g4545map}, right) does not look like that of the radio
continuum map. Most of the line emission comes from an elongated
structure aligned northeast to southwest with the extended emission
to the southwest roughly matching that seen in Br$\gamma$ by
\cite{felSHH98}. The intermediate velocity (43-53~km~s$^{-1}$)
channel maps (Figure~\ref{g4545chanmapneii}) most nearly match the
radio emission. The NE-SW elongated structure is primarily present
in red-shifted velocity channels. Extended emission north of the
radio arc and a overlying compact 11.7~$\mu$m source
\citep{felSHH98} dominate the blue-shifted channels. The differences
between the [Ne~II] and radio distributions could be due to
incomplete uv coverage of the snap shot radio observations or/and to
greater sensitivity of [Ne II] line observations.

\subsection{W51 IRS2}
W51 IRS2 (W51d) is a luminous \citep[L = 2-4 $\times $10$^6$
L$_{\odot}$,][]{eriT80,jafHG87} ultracompact HII region/molecular
cloud core within a more extensive star forming complex (Carpenter
\& Sanders 1998). Proper motion studies of H$_2$O maser features
place W51 IRS2 at a distance of $\sim$ 7~kpc
\citep{genDSR81,schMG81,imaW02}. Elsewhere, we have presented
0.5$''$ resolution maps of the [S~IV] 10.5~$\mu$m and [Ne~II] 12.8
$\mu$m lines toward this source \citep{lacJZRBG07}. These maps
reveal a number of embedded UCHII regions, which dominate the
[Ne~II] emission, a more extensive surface flow, seen mostly in
[S~IV], and a prominent $\sim$ 100~km~s$^{-1}$ jet, which we
attribute to ionization of one lobe of a protostellar jet. The
surface flow has a position-velocity pattern indicating that the gas
is moving along a shell with its vertex pointed away from the
observer. At the shell edges, the emission is blue shifted by
10-15~km~s$^{-1}$ with respect to the ambient cloud. The flow is
less evident in [Ne~II] where several compact sources are much
brighter, either because of the lower extinction at 12.8~$\mu$m or
because of the lower ionization potential of neon than of S$^{++}$.

We include our somewhat more extensive 2$''$ resolution IRTF map of
W51 IRS2 in the [Ne~II] line here for completeness.
Figure~\ref{w51dmapneii} compares the distribution of radio
continuum and [Ne~II] emission. The channel maps in
Figure~\ref{w51dchanmapneii}, as well as the top p-v diagram in
Figure~\ref{w51dpv} show that the two components of the eastern
source d1 have velocities that differ by $\sim$ 10~km~s$^{-1}$, with
the southern component blueshifted by $\sim$ 12~km~s$^{-1}$ from the
molecular cloud velocity. The molecular emission spectra of CS show
a second feature close to the [Ne~II] velocity in the southern
component (49.5~km~s$^{-1}$, Plume et al. 1997). The northern
component of W51d1 and W51d have [Ne~II] velocities close to the
velocity of the main component of the molecular cloud. We also
notice that the ionized gas toward the center of the region is
somewhat redshifted with respect to ambient material, which is
likely because the ionized gas is pushed into the cloud by
stellar/cluster winds.

\subsection{G61.48~+0.09B}
G61.48~+0.09B  belongs to the emission nebula complex Sh2-88B at a
distance of 5.4~kpc \citep{chuWC90}. \cite{garLG94} compared
kinematics from hydrogen recombination line and molecular line
observations \citep{macLBRMF81,chuWC90} and concluded that the
region was undergoing a blister flow to the southwest. They argued
that the hydrogen recombination line kinematics and the radio
continuum morphology of the region were best explained by expansion
of ionized gas in a non-uniform medium. Radio continuum observations
show that the region consists of two components, designated as B1
and B2 in Figure~\ref{g6148mapradio}. The more compact and spherical
B2 is located $\sim$11$''$ east of the peak of B1, which is extended
and has slightly curved contours that hint toward a cometary
morphology. Our [Ne~II] line emission distribution
(Figure~\ref{g6148mapneii}), which is very similar to the
distribution of Br$\gamma$ emission \citep{pugAFHW04}, shows
significantly different structure than the radio continuum map. The
morphological difference between the [Ne~II] line emission and the
radio continuum emission is likely due to dust extinction. The
[Ne~II] emission has complex morphology and kinematics. Multiple
components are present over a 35$''$ by 20$''$ area. A bright ridge
of emitting gas runs diagonally from the northeast to south central
in the map. A less bright irregular structure extends to the
northwest from the south.

The p-v diagrams (Figure~\ref{g6148pv}) show that the gas observable
in the [Ne~II] line has a steep velocity gradient. The [Ne~II]
emission near B2 is blueshifted by a few km~s$^{-1}$ with respect to
the neutral cloud velocity. Over much of B1, the bulk of the neon
emission arises in redshifted material with the westernmost gas
ranging from 0-20 km~s$^{-1}$ to the red of the molecular line
center. The morphological differences between the radio and [Ne~II]
maps make it difficult to associate the kinematic and morphological
structures. The RRL observations of \cite{garLG94}, which are not
affected by extinction, show that the bulk of the ionized material
is redshifted with respect to the molecular cloud.

\subsection{K3-50A (G70.3~+1.6)}
K3-50 is a four-component \citep[A-D,][]{wyn69} HII region complex
at a distance of 8.7~kpc \citep{har75}. The kinematic distance was
computed using the galactic rotational curve of \cite{sch65} with
the assumed Sun-to-GC distance of 10~kpc and a 250~km~s$^{-1}$
circular velocity at the position of the Sun \citep{rubT69,rub65}.
More recent estimates of the distance to the Galactic Center
($sim$7.94~kpc) imply that this distance is too large
\citep{eisSGOT03}.
The UCHII region K3-50A appears to
be the youngest and dominates the emission at infrared wavelengths.
\cite{hofBPW04} presented K$'$ band bispectrum speckle
interferometric data of K3-50A and their observations suggested that
K3-50A is excited by a small cluster of massive to intermediate-mass
stars. Radio continuum and hydrogen recombination line observations
found that the ionized gas in K3-50A was undergoing a high-velocity
bipolar outflow \citep{depGPR94}. The left panel of
Figure~\ref{k350map} shows the radio continuum image
\citep{depGPR94}. There is extended emission both north and south of
the dashed box that shows the extent of the [Ne~II] map.

The [Ne~II] line emission in K3-50A peaks at the radio peak and has
a remarkably similar distribution to that of the radio continuum
within the dashed box (Figure~\ref{k350map}). This is consistent
with its relatively low extinction \citep[$A_K=1.6$,][]{marPMTCR02}.
Most of the line emission comes from the core of the region. The
integrated line map shows that the core of K3-50A consists of two
sources, source 1 (S1) at the line emission peak and source 2 (S2)
$\sim 1.5''$ southeast of S1. Line emission extends north and south
from the central region. In the 12.8~$\mu$m continuum
(Figure~\ref{k350map}), a more compact emission peak is slightly
offset ($\sim$1$''$) to the north of S1.

The [Ne~II] emission has a very broad velocity range, from
-67~km~s$^{-1}$ to -8~km~s$^{-1}$ (Figure~\ref{k350chanmapneii}).
S1, at (0,0), is present in all velocity channels. S2 is bright in
the channels at the red end of the spectrum. A third component (S3)
$\sim2''$ to the south of S1 is present in the channels at central
and blue-shifted velocities. All three components lie significantly
to the blue of the molecular cloud which has V$_{LSR}$ =
-24~km~s$^{-1}$ \citep{sheC96}. S3 has the biggest velocity shift
with a line center at $\sim$-39~km~s$^{-1}$, with S1 and S2 at
$\sim$-35~km~s$^{-1}$. Position-velocity diagrams of K3-50A show the
velocity differences between different sources
(Figure~\ref{k350pv}). The position-velocity diagram for a
north-south cut (top panel of Figure~\ref{k350pv}) shows a velocity
gradient while the region right around S1 shows evidence for a
barely resolved bipolar structure with a total velocity extent of 60
km s$^{-1}$ (middle panel of Figure~\ref{k350pv}). Because the
region is very extended north-south, line emission from the region
fills our slit from end to end and emission from the northern and
southern lobes are very faint. The emission indicated by the last
four contours in Figure~\ref{k350map} is at least 100 times fainter
than emission at the peak. Our map did not cover enough area to show
the two more extended emission lobes.

\subsection{S106}
S106 (G76.4~-0.6) is a well-known bipolar HII region consisting of
two lobes with the symmetry axis oriented at a position angle of
$\sim$30$^{\circ}$.
An equatorial gap is seen in all wavelengths from optical to radio.
Earlier investigators suggest that a dense, circumstellar disk surrounds a
star and blocks the ionizing photons in the disk plane
forming a biconical nebula \citep{balS82}.
A point source, presumably the excitation source of the nebula,
is located at the center of the equatorial gap
\citep{pipSKKSM76,felMSRE84}.
\cite{felMSRE84} noticed that the bright radio continuum peaks in
S106 tended to be distributed at the edges of the radio lobes and
inferred that the inner volume was filled with gas with lower density.
A radio continuum map from their observations is shown in
Figure~\ref{s106mapradioneii}.
Dense molecular clumps were identified on both sides of the central
source of the HII region \citep{schSKSB02} but there is
no evidence of a smooth molecular disk.
\cite{smiJGK01} imaged the region at 3 to 20~$\mu$m.
Some of their images (11.7~$\mu$m) show two dark lanes.
One crosses the center of the region from the east to the west and
divides the region into the north and south lobes.
The other originates east of the region,
crosses the south lobe and forms a small angle ($\sim$30$^{\circ}$)
with the first dark lane.

The integrated [Ne~II] line map and the radio continuum map are
stunningly similar (Figure~\ref{s106mapradioneii}).
The bipolar morphology and individual emission concentrations are
clearly seen in both maps.
The south lobe is characterized by
a few bright clumps along the edge, while the north lobe contains
more diffuse emission.
The axis of the bipolar structure, which is indicated by the sharp edges of
the south lobe, runs NNE-SSW.
The main difference between the radio and [Ne~II] maps is that
the second dark lane seen in the infrared continuum is also seen in
[Ne~II].

The velocity structure shown in the p-v diagrams
(Figure~\ref{s106pv}) and the channel maps
(Figure~\ref{s106chanmapneii}) is consistent with a picture of S106
as a nearly edge-on bipolar nebula. Both figures indicate that the
south lobe is slightly blue shifted ($\sim$3 km s$^{-1}$) with
respect to the molecular cloud velocity while the material in the
northern lobe is on-average red-shifted by $\sim$4 km s$^{-1}$. At
the same time, individual intensity peaks show moderately narrow
($\sim$10 km s$^{-1}$ lines. Projection effects may lead to velocity
shifts from one emission peak to its neighbors.

\subsection{NGC7538A}
NGC7538A belongs to an HII region-molecular cloud complex at a
distance of ${\sim}3.5$~kpc \citep{hanLR02}. In the 6 cm radio
continuum image, it is a compact, spherical HII region with a radius
$\sim5''$ \citep{wooC89b} (Figure~\ref{n7538map}, left),
corresponding to $\sim$19,000~AU. Molecular line observations show
that molecular gas toward NGC7538 has a LSR velocity
$-56.9$~km~s$^{-1}$ \citep{dicDW81}. \cite{bloWPFAG98} suggested
that NGC7538A is formed by a stellar wind bow shock with a star
moving from the northwest to the southeast away from the Earth into
the molecular cloud.

Since dust extinction toward the region is not huge
\citep[$A_K\sim1.3$,][]{werBGMNW79,herHFMHWP81,camT84}, our [Ne~II]
line map of NGC7538A (Figure~\ref{n7538map}, right) resembles the
radio continuum map closely except that the radio component which
should be at the southwestern corner of the source A, $\sim$(-5,-2),
is not present in our [Ne~II] map. This is probably due to our poor
spatial resolution. [Ne~II] line channel maps, spanning a velocity
range from -82 to -51~km~s$^{-1}$, are shown in
Figure~\ref{n7538chanmapneii}. A cavity can be seen in the most
blue-shifted channels and in the integrated line map. The shape of
the region changes from ring-like at the blue-shifted channels to
core-like at the red-shifted channels. The channel maps in
Figure~\ref{n7538chanmapneii} show that most of the ionized gas in
NGC7538A is blue-shifted with respect to the molecular cloud. The
limb-brightened appearance of the source in the integrated line
strength and radio continuum maps (Figure~\ref{n7538map}) might lead
to the conclusion that the source is a limb-brightened spherical
shell. However, the pattern of a small central source at the
molecular cloud velocity and a larger, shell-like structure at bluer
velocities, combined with the ``$\Lambda$'' and ``7'' structures
seen in the pv cuts through the source (Figure~\ref{n7538apv}) argue
against this picture. The kinematics and morphology are more
consistent with a tangential flow along a shell with the symmetry
axis close to the line of sight and with the vertex pointing away
from the observer.

\subsection{W3A+B}
W3 belongs to an extensive HII region-molecular cloud complex
located in the Perseus spiral arm \citep[D = 2.0~kpc,][]{hacBMRI06}.
Aperture synthesis observations identified four compact/ultracompact
radio continuum sources in a 3 arc minute diameter region
\cite[A-D,][]{wyn71}. Infrared imaging at 1.65 to 20$\mu$m revealed
nine objects (IRS~1-9) in W3 \citep{wynBN72}, four of which are
associated with the compact radio continuum sources: IRS1 and IRS2
with the circular source W3A; IRS3 with W3B and IRS4 with W3C.
Earlier authors have used the ``broken-shell'' morphology and the
small velocity gradients seen in hydrogen recombination lines toward
W3A to argue for a picture of this region as a blister flow
\citep{dicHG83,tieGCWJ97}. W3B has also been characterized as an
emerging blister flow \citep{tieGCWJ97}.

W3A has the morphology of a limb-brightened broken shell in our
[Ne~II] map (Figure~\ref{w3map}), in close agreement with the
distribution of the radio continuum emission. Most of the line
emission forms a crescent open to the south with a horizontal
structure crossing the center of the incomplete shell. An irregular
emission cavity is present inside the crescent. Two finger-like
structures extend from the southeast corner of the ring to the
south. If we ignore these finger-like structures and another small
lobe of emission on the northwest edge of the shell, the region
closely resembles a half-ring. Such an image becomes clear in the
channel maps of the region (Figure~\ref{w3achanmapneii}). The
half-ring morphology is present in channels over a broad velocity
range. The sharp outer edge of the ring is evident. The emission
level drops rapidly to the background level within a few arcseconds.

The shell-like morphology of W3A might lead one to expect the source
to have the kinematics of an expanding shell. However, p-v diagrams
(Figure~\ref{w3apv}) of the region do not support this picture. We
do not see the two components toward the center of the source which,
in an expanding shell, would arise from the front and back sides of
the shell and would gradually merge into one component at an
intermediate velocity at the edge of the shell. Instead, we see a
very broad line ($\geq$25 km~s$^{-1}$) along the bright arc-like rim
of the region becoming narrow blue-shifted and red-shifted lines
toward the center of the region. The change of the line width and
the center velocities of the lines are abrupt. A surface flow,
rather than an expanding shell, can better explain the observed p-v
structure. The fact that both p-v cuts show the same pattern of a
broad feature at the bright limb followed by a double line within
the cavity implies that the vertex of the flow lies between the two
cuts. An examination of the -53.5 km s$^{-1}$ and -26.8 km s$^{-1}$
channel maps in Figure~\ref{w3achanmapneii} supports this picture.
The emission in the southern part of W3A comes from a narrower
velocity range and therefore is not part of the surface flow seen in
the north.

The compact HII regions W3B and W3A have similar central velocities
and line widths. The overall [Ne~II] line emission distribution in
W3B (see left panels in Figure~\ref{w3bpv}) looks like an irregular
shell. The line profile at most positions in the region can be fit
by a single Gaussian. W3B contains a low surface brightness cavity,
which can be seen in both the integrated line map and the channel
maps (Figure~\ref{w3bchanmapneii}) of the region. The total shift in
velocity centroid in W3B is relatively modest, from $\sim$-42 km
s$^{-1}$ to -34 km s$^{-1}$ and there is an apparently random
element to the velocity variations at the 2-3 km s$^{-1}$ level.
Nonetheless, both the channel maps (Figure~\ref{w3bchanmapneii}) and
the p-v cuts (Figure~\ref{w3bpv}) are consistent with tangential
flow along the ionized walls of a cavity. The channel maps show
emission from the central region primarily at the least negative
velocities, while the shell is most prominent in the bluer channels.
The p-v diagrams also show the reddest velocities at positions
crossing the center of the cavity and the bluest velocities along
the edge.

\section{Discussion}
\label{secdiscussion}

We have presented high spatial and spectral resolution mapping of
the 12.8 $\mu$m [Ne~II] line toward a sample of 18 compact and
ultracompact HII regions, where we count distinct HII regions within
clusters as separate sources. In our discussion, we make use of
previously published results for two of the 18 regions: G29.96-0.02
\citep{zhuLJRG05} and W51 IRS2 \citep{lacJZRBG07}. We also include
in the discussion one source \citep[Mon~R2,][]{jafZLR03}, for which
no results are presented in the current paper, bringing our sample
to 19 sources.

In order to discuss possible interpretations of the observed source
morphologies and kinematics, we first sketch p-v diagrams of several
types of models that have been suggested for UCHII regions. We then
compare the observed morphologies and kinematics to these sketches.
We emphasize that the sketches are only meant to be qualitative
descriptions of generic models. They are not derived from
hydrodynamic simulations.

\subsection{Qualitative Models}

To facilitate the discussion of the observed gas motions, we first
consider qualitative kinematic models of UCHII regions.
Sketches of several types of models of UCHII regions are shown in
Figures~\ref{ssspv}-\ref{sshpv}.

A. (Figure~\ref{ssspv})  An uniformly expanding shell source appears
as a limb-brightened disk. Its p-v diagram is ring-like, centered on
the molecular cloud velocity, V$_{amb}$.

B. (Figure~\ref{ssspv})  An expanding filled sphere appears as a
limb-darkened disk. Its p-v diagram is a filled ellipse centered at
V$_{amb}$.

C. (Figure~\ref{sbspv})  The appearance of a bow-shock source, in
which a wind from a star moving supersonically through a molecular
cloud sweeps up a paraboloidal shell, with the motion of the gas in
the shell resulting from the combined momenta of the stellar wind
and the swept-up material \citep{macVW91,vanM92,wil00,zhuLJRG05,
artH06} depends on the viewing angle. If viewed from the side
($\theta = 90^\circ$, left panel), two velocities are seen at each
offset, on opposite sides of V$_{amb}$. The line is broad in the
head, and the two velocities approach each other in the tail. If
viewed from the tail ($\theta = 180^\circ$, right panel), the source
would not be obviously cometary in appearance, and the velocity
would vary from V$_{\star}$ at the center to V$_{amb}$ at the edges.
From an intermediate angle ($\theta = 135^\circ$, middle panel), we
see a p-v pattern like those seen in G29.96, but offset in velocity.
A broad line, centered between V$_{amb}$ and V$_{\star}$ is seen at
the head of the comet. Of the two velocities seen at $\theta =
90^\circ$, the V $<$ V$_{amb}$ branch is foreshortened and may blend
into the head (see Figure~\ref{g2996pv2}). The V $>$ V$_{amb}$
branch rises toward V$_{\star}$ then falls back toward V =
V$_{amb}$. The line centroid is at V $>$ V$_{amb}$. The p-v diagrams
would be mirrored about V$_{amb}$ if the star were moving toward the
observer ($\theta < 90^{\circ}$).

D. (Figure~\ref{sppv})  A paraboloidal shell can also occur in which
the motion of the gas in the shell is predominantly due to the
pressure gradient along the shell, which causes an acceleration of
the gas toward the tail \citep{zhuLJRG05,artH06}. In this case, the
paraboloidal shape of the shell could result either from a
relatively slow motion of the star through the molecular cloud or
from a density gradient in the cloud. The resulting appearance and
p-v diagrams are similar to those of the bow-shock model, but the
p-v diagrams are offset toward negative velocities, with the
velocity centroid at V $<$ V$_{amb}$ (for $\theta > 90^{\circ}$).
This is the offset seen in G29.96, indicating that it belongs to
this category.

E. (Figure~\ref{sshpv})  A filled blister flow could result if the wind
from the ionizing star is too weak to prevent the ionized gas from
flowing away from the surface of the molecular cloud, past the star
\citep{yorTB83,com97,henAG05,artH06}. Such a region is formed when a
density gradient is present in the natal molecular cloud. In this
case, the source morphology and p-v diagrams would depend on the
shape of the blister cavity in the molecular cloud, but blister
flows are largely axial and a broad line would be likely to be seen
at all positions. For $\theta > 90^{\circ}$, the line centroid is at
V $<$ V$_{amb}$. The largest velocities would be seen toward the
tail, or at the center for $\theta \approx 180^{\circ}$.

A number of sources in our sample either have confused kinematics or
have kinematics for which we have no simple qualitative models:
Several sources show ring, horseshoe, or bipolar morphologies,
without the kinematic signatures of shells or paraboloidal flows.
Several regions appear to be clusters of overlapping sources, making
the interpretation of their kinematics difficult. Several sources
have distinctly different morphologies in [Ne~II] and the radio
continuum, suggesting that variable extinction is distorting their
appearance in the infrared. These sources also have confusing
kinematics. And several sources are too small for their [Ne~II]
kinematics to have been resolved.

\subsection{Observed Source Morphologies}

The extraordinarily good match between the radio and [Ne~II]
morphologies in the majority (12 of 16) of the resolved sources
indicates that most neon is in Ne$^+$, that the gas density is
almost everywhere below the critical density of the 12.8 $\mu$m
line, and that the centimeter-wave radio continuum emission is for
the most part optically thin. Extinction is the most likely cause of
the strong differences between radio continuum and [Ne II]
morphologies in the other four sources. Some of the differences
could also be due to incomplete UV coverage in the radio snapshot
observations. And, differences in sensitivity between [NeII] and
radio continuum observations may also play a role.

In many cases, radio interferometer observations show that UCHII
regions are accompanied by substantial radio continuum flux from
extended, low-surface brightness ionized gas
\citep{kurWHO99,kimK01}. These results imply that UCHII regions may
exist in asymmetric regions where the ionizing radiation reaches
dense gas in one direction and more extended low density gas in the
other direction. Our scanning technique does not permit us to find
these extended haloes in [Ne~II] due to limited sky area we can
cover and the sky background subtraction method we use, but the
frequent presence of emission halos may have implications for the
interpretation of the source kinematics.

\subsection{Observed Source Kinematics}

Of the 12 sources that are resolved and not badly confused by
extinction, nine show clear evidence of bulk motions, which are
usually not symmetric about the molecular cloud velocities. These
nine sources all show strong evidence of tangential flows along the
surfaces of shells. Three of these sources, G29.96
(Figures~\ref{g2996pv1} and \ref{g2996pv2}), G33.92
(Figure~\ref{g3392pv}), and G43.89 (Figure~\ref{g4389pv}), show,
with varying degrees of clarity, the ``$\Lambda$'' and ``7''
patterns in p-v diagrams that are similar to the patterns
established in the surface flow models (types C and D). In these
three cases, the patterns indicate that the terminal velocities are
blueshifted with respect to the molecular cloud velocities. Along
the symmetry axis, the velocities cross the molecular cloud velocity
from blue to red and back to blue going from the vertex into the
source for G29.96 and G33.92. In G43.89, the velocity approaches the
cloud velocity from the blue side but does not cross it before
moving back to the blue. The kinematic behavior we observe in all
three sources is very similar to what we see in the models of G29.96
(Figures~\ref{g2996pv1} and \ref{g2996pv2}), which are tilted with
the vertex away from the observer and have an inclination of the
symmetry axis of about 45$^{\circ}$ with respect to the line of
sight. With their vertices pointed away from the observer, the
blueshift relative to V$_{amb}$ means that these sources are of
kinematic type D (pressure-driven surface flows; Figure~\ref{sppv}).

The three sources in our sample whose [Ne~II] and radio continuum
morphologies most closely resemble limb-brightened spherical shells
do not have kinematic patterns consistent with a closed-shell
geometry. The kinematic patterns for three of the sources, NGC~7538A
(Figure~\ref{n7538apv}), W3B (Figure~\ref{w3bpv}), and W51 IRS2,
\citep[see][]{lacJZRBG07}, are consistent with motion of the ionized
gas along roughly parabolic shells with vertices pointing almost
directly away from the observer. In the position-velocity plots, the
most positive velocities, usually close to the velocity of the
dense molecular gas, are toward the vertex while the velocities
along the bright rim are shifted to the blue. The presence of this
pattern is largely independent of the position angle of the
position-velocity cut through the center of the source.
These sources are of kinematic type D with $\theta \approx 180^{\circ}$.

The remaining sources with understandable kinematics also have a
strongly limb-brightened appearance. The kinematics revealed by the
[Ne~II] spectroscopy show that G30.54 (Figure~\ref{g3054pv}), W3A
(Figure~\ref{w3apv}), and Mon~R2 \citep{jafZLR03} are also likely to
be limb-brightened parabolas with significant flow of the ionized
gas along the ionized shell. For these three sources, however, the
kinematics indicate that the symmetry axes lie near the plane of the
sky. The sources show broad lines at the source vertex and then
double lines farther along the symmetry axis with the two components
appearing at the extreme velocities of the line seen at the vertex.
Along the edges of each source, the lines are single and
significantly narrower. These sources are of either kinematic type D
or type C, with $\theta \approx 90^{\circ}$.

Four sources have very similar [Ne~II] and radio morphologies, but
do not fit into any of the kinematic categories we have discussed.
S106 (Figure~\ref{s106mapradioneii}) has a clear bipolar morphology,
but confused kinematics. G45.12N (Figure~\ref{g4512Npv}) has a
cometary morphology, but does not have obvious cometary kinematics,
of either type C or type D. G45.12SW (Figure~\ref{g4512SEpv}) has a
ring or shell morphology, but does not have the kinematic signature
of a shell. K3-50A (Figure~\ref{k350map}) appears to be a cluster of
sources, which are too blended to allow us to sort out their
kinematics. The peaks in the IRTF map of [Ne~II] in W51~IRS2
(Figure~\ref{w51dmapneii}) are also blended. The Gemini map of
\cite{lacJZRBG07} separates these sources but does not resolve their
kinematics. However, the kinematics of the extended ionized gas in
W51~IRS2 shown in \cite{lacJZRBG07} look much like a type D surface
flow with $\theta \approx 180^{\circ}$.

Complex morphologies and kinematics in the four sources with
substantially different [Ne~II] and radio continuum morphologies
prevent a coherent explanation for gas motion in these sources.
However, it is clear that systematic gas motions are present. In
G11.94, there is a component of the [Ne~II] emission at the same
velocity as the ambient cloud (Figure~\ref{g1194ABCspec}), but there
is also a substantial amount of emission at many positions in a red
wing or in an identifiable red component. In G45.45
(Figure~\ref{g4545map}), the kinematic pattern is confusing, and the
most prominent feature in [Ne~II] is not apparent in the radio
continuum. G61.48 (Figure~\ref{g6148mapneii}) is the source where
the [Ne~II] morphology least resembles the distribution of the radio
continuum emission. The [Ne~II] emission we see may therefore not be
sampling the bulk of the gas, but the dominance of red-shifted gas
in the RRL observations \citep{garLG94} is consistent with the
conclusion from the [Ne~II]-radio continuum morphology difference,
namely that the source is on the back side of the molecular cloud
and the ionized material is flowing away from the neutral core.

\subsection{Comparison of Observed Sources with Qualitative Models}

The kinematic types that we have assigned to observed sources are
given in Table~\ref{kintypes}. Examination of this list shows that
none of the observed sources fits into category A or B, the
classical expanding shell and sphere models of HII regions. None has
a ring-like p-v diagram of an expanding shell, and the sources with
broad lines are either too small for their morphologies to be
apparent or have confused morphologies. In addition, none of the
sources fits into category E, the classical filled blister flow with
the ionized gas flowing away from the ionization front.

Even more remarkably, in spite of the frequency of bow-shock
morphologies, none of the observed sources clearly fits into
category C, a bow shock moving with a star through a molecular
cloud. Because they are nearly edge-on, Mon R2, G30.54, and W3A
could fit into either category C or category D. The other sources
with cometary kinematic signatures all have p-v patterns
characteristic of sources tipped away from the observer (${\theta}
> 90^{\circ}$) with their velocity centroids blueshifted relative to
the neighboring molecular material, the signature of a
pressure-driven surface flow.

The preponderance of sources with their tails tipped toward the
observer ($\theta \geq 90^{\circ}$) probably results from a
combination of extinction and selection effects. The extinction at
12.8~$\mu$m is relatively low
\citep[A$_{12.8}$/A$_V$$\sim$0.05,][]{rieL85,rosBD00}, but the
column density through the kind of massive core that harbors massive
young stars and UCHII regions can be substantial \citep[A$_V$
$\approx$ a few hundred,][]{beuKDS00}. The density in such cores
drops rather steeply, with power-law exponents of 1.6-1.8
\citep{mueSEJ02,beuSMMSW02}. If UCHII regions are not in the centers
of cores, they should therefore be visible in [Ne~II] when the
exciting stars lie between the front side and $\theta \approx
90^{\circ}$. Our survey was somewhat biased in favor of sources
lying on the front sides of molecular clouds by including sources
known from optical and near-infrared observations (NGC 7538, W3, and
Mon R2). In addition, we were forced to abandon efforts to observe
several UCHII regions (G8.67-0.36, G10.10+0.74, G10.62-0.38,
G12.208-0.10, G33.50+0.20, G42.42-0.27, G45.47+0.05, G50.23+0.33)
because they were undetectable or too faint to map.
These selection effects are likely to explain the lack of sources
with kinematics indicating that they lie on the back sides of their
clouds.

More than half of the observed sources fit into category D, a flow along
the surface of a paraboloidal shell, which accelerates relative to
molecular cloud as it moves away from the apex of the paraboloid.
Although we have not made a hydrodynamic model of such a flow, we
know of no driving mechanism for the acceleration other than the
pressure gradient in the ionized gas. \cite{zhuLJRG05} found that
the pressure gradient acceleration would be small if the ionized and
swept-up molecular gas were coupled so both had to be accelerated.
But we have estimated the acceleration if the ionized gas is allowed
to slip past the molecular shell, and we find that speeds
$\sim$20-30~km~s$^{-1}$ can occur. The hydrodynamic models of
\cite{artH06} include the essential effects present in the
combination of blister and bow shock models: density gradients in
the neutral medium, stellar winds, and stellar motion. The
\cite{artH06} models do an excellent job of matching the range of
source morphologies and reproduce the most significant kinematic
signatures using reasonable values for density gradients, mass loss
rates, and stellar velocities. One critical area where they fall
short is in explaining the width of the lines at the source vertex.
The observed effect is most evident in W3A (Figure~\ref{w3apv}) and
in Mon~R2 \citep{jafZLR03} where the symmetry axes are near the
plane of the sky. The splitting of the broad line into two narrow
lines at the velocity extremes of the broad line as one crosses the
shell to the inside implies that the gas accelerates to
$\sim~\pm$10~km~s$^{-1}$ within a small solid angle (less than a
steradian, as seen from the point of view of the ionizing star) at
the vertex. This, in turn, implies a significant pressure gradient.

The [Ne~II] kinematics of our sample unite a large and diverse group
of UCHII and compact HII regions into a single empirical picture:
The HII regions coexist with dense and massive molecular cores
\citep{beuSMMSW02,shiEYKJ03,sriBSMW02}. The ionized regions have the
form of partial, roughly parabolic shells with their open sides
usually pointing away from the centers of the dense neutral
condensations and gas flows tangentially along these shells.

The evidence that very many UCHII regions are dense shells with
tangential flows away from the parent molecular cores has
implications for the survivability of the molecular cores. Earlier
examinations of the erosion of dense cores by external O stars did
not take the compression of neutral material by stellar wind shocks
into account. These efforts led to estimates that when as little as
4\% of the core material formed into stars with a Salpeter IMF,
these stars could erode the remaining core entirely away
\citep{whi79}. It has already been pointed out, however that the
dense shells formed by stellar wind pressure would greatly inhibit
the process of erosion \citep{chu91,lumH99,hoa06}. Our results
indicate that most if not all OB stars breaking out of dense cores
form such dense shells at their boundaries.

\acknowledgments

The development of TEXES was made possible with support from the NSF
and the SOFIA program through grant USRA-8500-98-008. Observations
with TEXES are supported by NSF grant AST-0607312. Part of this
research was performed in the Rochester Imaging Detector Laboratory
with support from a NYSTAR Faculty Development Program grant. MJR
acknowledges the support of NSF grant NSF-0708074 and NASA grant
NNG04GG92G. TKG was supported by the Lunar and Planetary Institute,
which is operated by the Universities Space Research Association
under NASA CAN-NCC5-679. DTJ would like to thank the
Max-Planck-Institut fuer extraterrestrische Physik for its
hospitality. We thank Alan Fey and Ed Churchwell for letting us to
have their data on G29.96~-0.02 and other objects. We also want to
acknowledge Achim Tieftrunk, Yolanda Gomez, Marcello Felli, Chris
DePree, Ralph Gaume for their permission to use their published
figures in this work.

redpajama_set_name: RedPajamaArXiv