text:
\section{Introduction}

Luminous blue variables (LBVs) represent a transitional state in the
evolution of massive stars between main sequence and hydrogen depleted
Wolf-Rayets and are characterised by significant photometric and
spectroscopic variability (e.g. Lamers \cite{lamers}, Humphreys \&
Davidson \cite{HD}). Two characteristic modes have historically been
identified: (i) 1-2 magnitude excursions on $\sim$year timescales at
constant bolometric luminosity (L$_{\rm bol}$) and (ii) giant ($\geq$2mag)
eruptions, during which the luminosity of the star increases, but for
which the timescales are currently uncertain due to their rarity.

Recently, LBVs have been the subject of renewed interest. With the likely
downwards revision of Main Sequence mass loss rates it has been
suggested that \object{$\eta$ Car}-like giant eruptions play a key role
in stripping the H rich mantle from post-MS stars prior to the WR phase
(Smith \& Owocki \cite{smith06}). Moreover, some anomalously faint Type
IIn SNe have been hypothesised to be LBVs in an eruptive phase rather than
core-collapse events (e.g. Humphreys et al. \cite{h99}, Goodrich et al.
\cite{goodrich}). Finally, several lines of evidence have been advanced to
suggest that LBVs may be the immediate progenitors of a subset of Type II
SN, including some of the most luminous Type IIn events ever observed
(e.g. Smith et al. \cite{smith07}, Gal-Yam et al. \cite{galyam}, Trundle
et al. \cite{trundle}, Pastorello et al. \cite{pastorello}, Kotak \& Vink
\cite{kotak}), in turn
leading to the suggestion that the occurrence of \object{$\eta$ Car}-like
eruptive events may warn of an imminent SN (e.g. Smith et al.
\cite{LBVSN}, Woosley et al. \cite{woosley}).

Given these possibilities, an understanding of the physics leading to both
modes of LBV variability is urgently required. Unfortunately, such a goal
is hampered by the evident rarity of LBVs and their attendant outbursts.
Only two examples of non L$_{\rm bol}$ conserving eruptions have been
spectroscopically studied -  HD~5980 (Koenigsberger
\cite{koenigsberger})
and NGC2363-V1 (Drissen et al. \cite{drissen}) - both of which occurred in
low metallicity environments. However, with the expansion of IR astronomy
over the past decade, a number of new candidate LBVs have been
identified within the Galaxy via both imaging and spectroscopy (e.g. Clark
et al. \cite{clark05} and references therein). Of these, the presence of
high mass circumstellar nebulae surrounding a subset of B hypergiants is
particularly suggestive of enhanced mass loss in the past, possibly
associated with giant eruptions (Ueta et al. \cite{ueta}, Clark et al.
\cite{clark03b}, Smith \& Owocki \cite{smith06}).

\begin{figure*}
\resizebox{\hsize}{!}{\includegraphics[angle=0]{12358fg1.ps}}
\caption{JHK band lightcurves and (H-K) colour index for \object{AFGL 2298} spanning $\sim$two decades. Data are from
Jimenez-Esteban et al. (\cite{jimenez}; filled triangles),
Ueta et al. (\cite{ueta}; filled squares), Pasquali \& Comeron (\cite{pasquali};
open circles) and Clark et al. (\cite{clark05}) and this work (open squares).
Where not shown, error-bars are smaller than the size of the respective symbols.
The arrows in the K band  panel indicate the times at which the spectroscopic observations were made,
with the low, medium and high  resolution observations made with the
AZT-24 1.1m, UKIRT/CGS4 and the VLT/ISAAC respectively (see Sect. 2).}
\label{light_curve}
\end{figure*}

\object{AFGL 2298} (=\object{IRAS  18576+0341}), observed as part of a
long term spectroscopic and photometric monitoring  campaign of
(candidate) LBVs, has been found to be highly variable (Clark et
al. \cite{clark03a}).  Spectroscopic analysis of near-IR observations
suggested significant changes in stellar radius and temperature
between 2001-2002, indicative of an LBV excursion. In this paper we
update the results of this effort, encompassing an extended lightcurve
and additional medium resolution spectroscopy and an accompanying
analysis with the non-LTE model atmosphere code CMFGEN (Hillier \& Miller
\cite{hil98},  \cite{hil99}).  Finally, in light of these results and
other recent  results, we discuss the observational properties of LBV
variability and  the implications for the formation of ejection nebulae.

\section{Data Reduction}

Near-IR JHK broadband photometric observations of \object{AFGL 2298} were
obtained at the AZT-24 1.1m telescope in Campo Imperatore (Italy) from
2001 March - 2008 October.  The SWIRCAM 256x256 HgCdTe detector was
employed, yielding a scale of 1.04~arcsec/pix, resulting in a
$\sim4^{\prime} \times 4^{\prime}$ field of view. Standard techniques of
sky subtraction and flat-fielding were applied. Between two and four
standards were employed for the calibration of the photometry. All
standards were located within the target frames, had near-IR magnitudes
between 9.9 and 11.5~mag. and were found to be constant to within
$\pm$0.02mag over the course of the observations. The resultant lightcurve
is presented in Fig.~\ref{light_curve}.

Spectra of \object{AFGL 2298} have been obtained from a number of
different telescopes and are summarised in Table~\ref{log}, and presented
in Figs.~2--\ref{hires}. A description of the UKIRT/CGS4 observations and reductions may be
found  in Clark et al. (\cite{clark03a}). Long term, low resolution
($R\sim$270)
spectroscopic monitoring was undertaken with the AZT-24 1.1m telescope and
the IR imaging camera SWIRCAM+HK band grism - providing spectral coverage
between 1.45 -- 2.38$\mu m$. Higher resolution (R$\sim$8900) follow up
observations were subsequently made with the VLT in 2006 and 2007.

The VLT observations were made with ISAAC in the  short-wavelength
(SW) medium resolution (MR) mode with a narrow 0.3'' slit. To
achieve
spectral coverage from 2.04 -- 2.22$\mu$m two exposures were
obtained, centred at 2.10$\mu$m and 2.16$\mu$m. All data were taken with a
count rate of below 10,000 ADU, therefore no correction for non-linearity
was necessary. Data reduction was accomplished using the {\sc FIGARO} and
{\sc KAPPA} software packages. Note that the telluric correction was poor
for the 2007 spectrum due to rapidly varying sky conditions.

\section{Results}

\subsection{Photometry}

With the addition of the data presented here and in Jimenez Esteban et al.
(\cite{jimenez}) the lightcurve for \object{AFGL 2298} extends for $\sim$2
decades, revealing significant near-IR variability throughout this time
(Fig. 1). Our well sampled lightcurve from 2001-8 reveals two minima. The
first, deeper minimum occurred in the second half of 2002 and the second in
the first half of 2007, with the corresponding maximum in mid 2006. Low
amplitude ($\sim$0.2mag) variability over timescales of $\sim$days is
superimposed on the long term trends in all 3 wavebands.

In addition to these minima, the comparatively poorly sampled data set of
Jimenez Esteban et al. (\cite{jimenez}) provides evidence for a further
photometric minimum between 1993-5 and a maximum between 1996--1999. The 3
minima and 2 maxima appear to have differing intensities, with a maximum
peak to trough variation of $\geq$1.6 mag observed between the 1996--1999
maximum and 2002 minimum.

\begin{table}
\begin{center}
\caption[]{Log of the spectroscopic observations
between 2001-8.}\label{log}
\begin{tabular}{cccr}
\hline
\hline
Date &  Telescope & $\lambda$ & Resolution \\
     &                 & ($\mu$m) & \\
\hline
27/06/01  & UKIRT/CGS4 & 2.0-2.5 & 800\\
22/07/02  & AZT-24 1.1m  & 1.45-2.38 & 270 \\
06/08/02  & UKIRT/CGS4 & 2.0-2.5 & 800\\
17/05/06  &  AZT-24 1.1m  & 1.45-2.38 & 270 \\
30/06/06  & VLT/ISAAC & 2.04-2.22 & 8900 \\
15/05/07 &  AZT-24 1.1m  & 1.45-2.38 & 270 \\
09/05/07 & VLT/ISAAC & 2.04-2.22 & 8900 \\
16/06/08 & AZT-24 1.1m  & 1.45-2.38 & 270 \\
04/09/08 & AZT-24 1.1m  & 1.45-2.38 & 270 \\
\hline
\end{tabular}
\end{center}
\end{table}

\begin{figure}
\includegraphics[width=8cm]{12358fg2.ps}\\
\caption{Selected low resolution AZT-24  spectra of \object{AFGL 2298} from 2002-2008. Note
the significant reduction  in the strengths of all emission lines between 2006-9 in comparison
to 2001-2 (also see Fig. 3).}
\end{figure}

 \begin{figure*}
\includegraphics[width=12cm]{12358fg3.eps}\\
\caption{Montage of 4 epochs of medium (UKIRT/CGS4: R$\sim$800) and high
(VLT/ISAAC; R$\sim$8900) resolution  K band
spectroscopy  of \object{AFGL 2298}  from 2001 and 2007 (solid lines) with the major transitions indicated (note that telluric subtraction was poor for the 2007 observation). The resultant spectra from the non-LTE model atmosphere
analysis in Sect. 4.2 are overplotted (dotted lines). Note the different scales on the y-axis.}\label{hires}
\end{figure*}

In the bottom panel of Fig.~\ref{light_curve} we plot the (H-K) colour
index, finding no
systematic variation with either the long term secular changes or short
timescale low amplitude variability during these observations. As such
this behaviour differs from that demonstrated by the candidate LBVs
\object{G24.73+0.69} and \object{G26.47+0.02} (Clark et al.
\cite{clark05}, Clark et al. in prep.), where both stars became
significantly redder as they brightened.  Indeed the lack of colour
variability associated with either rising or falling branches of the
lightcurve of \object{AFGL 2298} argues against the changes being due to
episodes of enhanced mass loss\footnote{Where a brightening accompanied by
an increase in reddening may reflect increased emission from circumstellar
material (and vice versa e.g. \object{WR137} and \object{WR19}; Williams
et al. \cite{williams} \& Veen et al. \cite{veen}), while a reduction
(increase) in brightness accompanied by an increase (decrease) in
reddening may be attributed to increased (decreased) extinction due to
very dense circumstellar material (e.g. \object{IRAS 16029-3041};
Jimenez-Esteban et al. \cite{jimenez}).}. Instead, we attribute it to
changes in the properties of the underlying star, although the
identical near-IR colours of early OB stars preclude a determination
of stellar temperature from these data alone.

 For our spectroscopic analysis, we adopt $E(B-V)$=9 mag from Ueta et
al.
(\cite{ueta}), equating to an infrared extinction of $A_{\rm J}=8.25$,
$A_{\rm H}=4.65$ and $A_{\rm K}=2.7$, plus a distance of 10 kpc (distance
modulus 15.0 mag), as in Clark et al. (\cite{clark03a}). With this choice
of infrared extinction, absolute magnitudes in the J,  H and K bands
differ by at most 0.1 mag, in agreement with predictions from atmospheric
models (Section 4). No  evidence was found for hot dust in any of
the 4 epochs of spectroscopic observations -- JHK magnitudes were
well reproduced by the reddened spectral energy distributions in all
cases.

\begin{figure} \includegraphics[angle=-90,width=7cm]{12358fg4.ps}\\
\caption{Long slit spectrum of \object{AFGL2298} centred on the Br$\gamma$
line with a North(uppermost)-South orientation. Extended, nebular emission
is present up to 4~arcsec northwards of  the star (0.2~pc for a distance of
10~kpc), with the elliptical morphology consistent with material expanding
at up to $\pm\sim$70~kms$^{-1}$. Note that the physical extent and the
pronounced asymmetrical nature of the emission is consistent with the
morphology of the ionised component of the nebula found from the radio
continuum observations of Umana et al. (\cite{umana}). }\label{nebula}
\end{figure}

\subsection{Spectroscopy}

Figure~2 presents our AZT-24 low resolution (R$\sim$270) H-
and  K-band spectroscopy of AFGL 22298, while low-medium resolution
(R$\sim$800-8900) UKIRT and VLT K-band spectroscopy is
presented in Figure~\ref{hires}. These illustrate the significant
change in spectral morphology between 2001 - 2002 reported by Clark et al.
(\cite{clark03a}), as well as a subsequent dramatic reduction in strength
of the emission lines of all species by 2006 May, a state which has
persisted for the past $\sim$2.5~yr. Unfortunately, a lack of observations
between 2002 August and 2006 May prevent us localising the transition - if
indeed it were a sudden event - over the course of the secular brightening
of \object{AFGL 2298} during this period (recall Fig~\ref{light_curve}).
However,
following the 2006 maximum, no pronounced changes are observed in the low
resolution spectra despite the subsequent photometric variability.

This is emphasised by the  medium resolution VLT/ISAAC spectrum of
2006
June 30 (Fig.~\ref{hires}) - fortuitously obtained close to infrared
maximum (Fig.~\ref{light_curve}) - which
reveals a  narrow emission line spectrum (FWHM$_{Br \gamma}$=70kms$^{-1}$)
dominated
by Br$\gamma$ and low excitation metallic species. Comparison to the 2002
UKIRT/CGS4 spectrum obtained during the photometric minimum confirms the
pronounced weakening in line emission for all species; He\,{\sc i} emission
- prominent in the 2001--2002 spectra - is entirely absent, with the
He\,{\sc i} 2.112$\mu$m transition seen in absorption.

A second  VLT/ISAAC medium resolution spectrum obtained on 2007 May 9
-
this time  during the subsequent local photometric minimum - shows a
moderate
recovery in the strength of Fe\,{\sc ii} emission lines, but no significant
changes in the strength of Br$\gamma$ or the Mg\,{\sc ii} and He\,{\sc i}
lines. These changes - in particular the absence of He\,{\sc i} emission -
suggest a modest reduction in temperature after 2002 (Sect. 4.2;
Table~\ref{summary_afgl}),
although {\em all the spectra are entirely consistent with the
classification of AFGL 2298 as a cool highly luminous B supergiant
throughout the 2001-9 period.}

Finally, these observations reveal spatial extended emission in the
Br$\gamma$ line at distances of up to 4~arcsec from the star, allowing us
to infer an expansion velocity of $\pm \sim$70kms$^{-1}$ for the detached
ejection nebula associated with \object{AFGL 2298} (Fig.~\ref{nebula}).
Adopting the
radial extent and mass given by Ueta et al. (\cite{ueta})  implies a
kinematic age of 8300~yr and consequently a {\em time averaged} mass loss
rate of 1.2$\times10^{-3}$ M$_{\odot}$yr$^{-1}$ during nebular formation.

\section{Physical properties of \object{AFGL 2298}:}

\subsection{1989--2001}

In the absence of spectroscopic data, the interpretation of the
photometric lightcurve of \object{AFGL 2298} prior to 2001 is subject to
uncertainties due to the fact that near-IR colours of hot OB stars are
$\sim$constant. Nevertheless, given the similar stellar luminosities,
magnitude of the excursions ($\Delta$J$\sim$2.5; Fig.~\ref{light_curve} \&
Groh, priv.
comm.)  and timescale of variability ($\sim$2-4yrs;
Fig.~\ref{light_curve}, Spoon et al.
\cite{spoon}), it would appear that \object{AG Car} should serve as a
suitable template for comparison to \object{AFGL 2298}. Groh
(\cite{thesis}) studied \object{AG Car} through two full photometric
cycles from 1986-2005 and found a transition from T$_{\rm eff} \sim$24 to
$\sim$8 kK, anti-correlated with both V and J band magnitudes, as expected
for LBV excursions.

\begin{table}
\begin{center}
\caption{Summary of physical and wind properties of AFGL 2298 for
2001--2007.}\label{summary_afgl}
\begin{tabular}{lccccc}
\hline
\hline
Epoch &  2001 Jun &  2002 Aug &    2006 Jun &     2007 May \\
\hline
J     &    13.4   &   13.7    &      12.7   &      13.2    \\
H     &    9.8    &   10.0    &       9.2   &       9.6    \\
K     &    7.9    &    7.9    &       7.2   &       7.6    \\
M$_{\rm H}$   & -9.85 & -9.65  &    -10.45  &     -10.05 \\
\hline
T$_{\ast}$ (kK)& 12.5  & 15.5   &     11.0   &      11.5  \\
R$_{\ast}$ (R$_{\odot}$)& 262   &     158  &    385 &   312 \\
T$_{\rm eff}$  (kK)  &  11.7 &  10.9 &   10.3 &    10.9  \\
R$_{2/3}$  (R$_{\odot}$)&  300  &   320 &    444 &  353 \\
BC$_{\rm H}$  &  -0.85  &       -0.90  &          -0.55  &        -0.65 \\
L$_{\ast}$/L$_{\odot}$ &   1.5$\times$10$^6$ & 1.3$\times$10$^6$ & 2.0$\times$10$^6$ & 1.5$\times$10$^6$ \\
\.{M} (M$_{\odot}$yr$^{-1}$) & 4.5$\times 10^{-5}$ & 1.2$\times 10^{-4}$ & 5.2$\times 10^{-5}$  & 4$\times 10^{-5}$ \\
v$_{\infty}$ kms$^{-1}$  & {\em 200} & {\em 200} & 125  & 125 \\
H/He   &   {\em 5} & {\em 5} & {\em 5} & {\em 5} \\
\hline
\end{tabular}
\note{For all epochs we have assumed DM=15,
E(B-V)=9, $A_{\rm J}=8.25$, $A_{\rm H}=4.64$, $A_{\rm K}=2.69$ (from
Ueta et al. \cite{ueta})
and H/He=5.0, while
v$_{\infty} \sim$200kms$^{-1}$ was assumed for 2001--2002; these
latter two parameters are given in italics. Mass-loss rates presented here are
{\it not}  corrected for wind clumping. We find errors of $\pm$2 kK
($\pm$0.2 mag in BC),  $\pm$1.5 kK ($\pm$0.2mag) and  $\pm$0.5 kK
($\pm$0.1mag) for 2001, 2002 and 2006-2007 respectively.}
\end{center}
\end{table}

Somewhat unexpectedly, in light of the prevailing orthodoxy,  Groh et
al.
(\cite{groh}) found that the L$_{\rm bol}$ of AG Car was {\em not}
conserved over the period of these cycles, with a reduction of $\sim$50\%
during photometric maximum/temperature minimum when compared to
photometric minimum/temperature maximum. Similar behaviour has been
observed for \object{S Dor} (e.g. van Genderen \cite{vG}), with Lamers
(\cite{lamers95}) interpreting the resultant deficit in radiative energy
during photometric maximum as being due to the increased mechanical energy
required to move and subsequently support the outer layers of the star
against gravity at a greatly expanded radius.

However, \object{AFGL 2298} was significantly cooler (T$_{eff} \sim$10.8 kK) in the
2002--2003 photometric minimum than \object{AG Car} in a comparable phase
($\sim$24 kK). Such a low temperature implies a (H-band) bolometric
correction of $<$1 mag (Sect. 4.2). If \object{AFGL 2298} were to have
mirrored the behaviour of AG Car between 1989-2001 - i.e. with periods of
increasing IR brightness corresponding to a decrease in temperature (and
vice versa) - the magnitude of near IR variability observed ($>$1.6 mag)
would preclude such changes occuring at reduced or even constant
L$_{\rm bol}$
and instead would indicate that the increase in IR brightness that
accompanied a reduction in stellar temperature {\em would at least in part
be due to a genuine increase in luminosity.} Trivially, such a conclusion
would also be drawn if the variability occurred at constant or increasing
temperature. We will return to this below, upon consideration of the
results of non-LTE modeling of the post 2001 data.

 \begin{figure}
\includegraphics[width=0.625\columnwidth,angle=270]{12358fg5.eps}\\
\includegraphics[width=0.625\columnwidth,angle=270]{12358fg6.eps}\\
\caption{VLT/ISAAC spectroscopy of AFGL  2298 for (upper panel) Jun
2006
together with synthetic spectra for 11$\pm$0.5 kK; (lower panel) May 2007
together with synthetic spectra of 11.5$\pm$0.5 kK, demonstrating the
temperature sensitivity of K-band He\,{\sc i} lines.}\label{fits}
\end{figure}

\subsection{2001--2007}

While we may not unambiguously interpret the 1989--2001 behaviour of
\object{AFGL 2298} from photometric data alone, the contemporaneous
spectroscopy in the period
2001--2007 allows us to accomplish this via a non-LTE  spectroscopic
analysis using
CMFGEN (Hillier \& Miller \cite{hil98}, \cite{hil99}).
We {\em explicitly} follow an identical
methodology to that employed in Clark et al. (\cite{clark03a}), in order
to permit a direct comparison of all four epochs of spectroscopic
observations.

\subsubsection{Atmospheric code}

 CMFGEN solves the
radiative transfer equation in the co-moving
frame, under the additional constraints of statistical and radiative
equilibrium. It does not solve the momentum equation, so a density or
velocity structure is required. For the  supersonic part, the velocity is
parameterized with a classic $\beta$-type  law. CMFGEN incorporates line
blanketing through a super-level  approximation, in which atomic levels of
similar energies are grouped into
a single super-level which is used to compute the atmospheric structure.
Our atomic model is similar to that employed by Drissen et al.
(\cite{drissen}), including ions from H\,{\sc i}, He\,{\sc i-ii}, N\,{\sc
i-ii}, O\,{\sc i-ii}, Mg\,{\sc i-ii}, Al\,{\sc ii-iii}, Si\,{\sc ii-iii},
S\,{\sc ii-iii}, Ti\,{\sc ii-iii} and Fe\,{\sc ii-iii}. By number, the
main contributors to line blanketing are Fe\,{\sc ii-iii}. H/He$\sim$5 by
number was adopted for all models, since solely K-band datasets do not
readily allow the determination of H/He contents without a temperature
diagnostic independent of helium. Solar abundances are taken for metals,
with the exception of 5 times solar for nitrogen, as expected for
chemically evolved LBVs.

Stellar temperatures, $T_{\ast}$, correspond to a
Rosseland  optical depth of 20, which are typically (up to) a few hundred
degrees  higher than effective temperatures, $T_{2/3}$, relating to
a Rosseland optical depth of 2/3.

 We have assumed a depth-independent Doppler profile for all lines
when
solving for the atmospheric structure in the co-moving frame, while in the
final calculation of the emergent spectrum in the observer's frame, we
have adopted a uniform turbulence of 20 km\,s$^{-1}$. Incoherent electron
scattering and Stark broadening for hydrogen and helium lines are
adopted.

One improvement afforded by the ISAAC datasets was the means of
observationally constraining the terminal wind velocity in 2006-7, which
had to be estimated for the 2001--2002 data.  As in Clark et al.
(\cite{clark03a}), a $\beta$=2 velocity law was adopted for all
subsequent epochs.  The form of the velocity law is not expected to differ
from epoch to epoch, especially between the crucial 2006 and 2007
datasets. With regard to wind clumping, this was {\em not} included for
consistency  with Clark et al. (\cite{clark03a}), although volume filling
factors of $f\sim$0.1 typically reduce mass-loss rates by a factor of
$\sqrt{1/f}\sim$3.

\subsubsection{Analysis}

We derive the stellar temperature of AFGL 2298 primarily using the
K-band He\,{\sc i} line diagnostics. Specifically, the higher resolution
2006-7 VLT/ISAAC datasets
allowed the He\,{\sc i} 2.058$\mu$m emission line and 2.112$\mu$m
absorption line to be resolved, thereby allowing significantly more
robust physical parameters (e.g. He\,{\sc i} 2.058$\mu$m and Fe\,{\sc ii}
2.060$\mu$m were blended in 2001--2002 UKIRT/CGS4 datasets).

For Jun 2006, the simultaneous presence of strong 2.112$\mu$m
absorption
plus negligible 2.058$\mu$m emission occurs for a narrow range in
temperatures,  namely 11$\pm$0.5 kK (equivalent to $\pm$0.1 mag in
Bolometric Correction;  BC), as shown in the upper panel of
Fig.~\ref{fits}. Similar conclusions
were drawn for two LBVs in the Quintuplet cluster by Najarro et al.
(\cite{paco08}).
The mass-loss rate is  fixed from Br$\gamma$ with other
K-band  features either reproduced  satisfactorily (Mg\,{\sc ii} doublet),
or predicted somewhat too strong (Fe\,{\sc ii} lines).

For May 2007, weak 2.112$\mu$m absorption plus weak 2.058$\mu$m P
Cygni
emission implies a slightly higher stellar temperature of 11.5$\pm$0.5 kK
(again reliable to $\pm$0.1 mag in BC) -- see lower panel of
Fig.~\ref{fits} -- plus a reduced mass-loss rate from  Br$\gamma$. For
this epoch, both Mg\,{\sc ii}  and Fe\,{\sc ii} lines are now
satisfactorily  reproduced, including the  Fe\,{\sc ii} feature partially
blended with 2.058$\mu$m.

For reference, we have also investigated the potential effect of
a varying
$\beta$ velocity law, for the 2007 epoch. Use of a $\beta$ = 1
law would favour a 0.5~kK higher temperature, yet maintain the H-band
bolometric correction to within 0.03 mag. Similarly, a $\beta$ = 3 law
favours a 0.5~kK lower temperature, albeit with a H-band bolometric
correction consistent to within 0.05 mag. These differences lie within the
formal uncertainties quoted above, and so do not affect our
main conclusions. Alas, no diagnostics
are available in the K--band from which a determination of the form of the
velocity law can be uniquely obtained.

For the 2001 and 2002 epochs, we have retained models from Clark et
al. (\cite{clark03a}), but in view
of the unresolved He\,{\sc i} + Fe\,{\sc ii} 2.06$\mu$m blend and weak
He\,{\sc i} 2.112$\mu$m feature we admit substantially larger
uncertainties in temperatures - 12.5$\pm$1.5 kK for Jun 2001 and 15$\pm$2 kK
for Aug 2002 ($\pm$0.2 mag in BC). In view of the limitations outlined
below, it is  apparent that  the comparison
between physical parameters for AFGL 2298  for 2006 and 2007  epochs is
more significant than that between 2001 and 2002 or between UKIRT/CGS4
and VLT/ISAAC datasets.

 Specifically for Jun 2001, the blended emission feature at 2.06$\mu$m
is significantly stronger than the Fe\,{\sc ii} 2.04$\mu$m line,
and indeed comparable  in strength to Fe\,{\sc ii} 2.089$\mu$m, suggesting
that P Cygni
He\,{\sc i} 2.058$\mu$m emission makes a significant contribution to the
feature. In contrast He\,{\sc i} 2.112$\mu$m remains weakly in absorption
(albeit blended with Fe\,{\sc ii} 2.12$\mu$m), from which we estimate
a temperature of 12.5$\pm$1.5 kK (recall Fig.~\ref{fits}). Errors are
calculated on the basis that 2.06$\mu$m is dominated by Fe\,{\sc ii} at
significantly lower temperatures,  while negligible 2.112$\mu$m He\,{\sc
i} absorption is predicted at higher temperatures.

 For Aug 2002, a higher stellar
temperature of 15.5$\pm$2  kK is suggested on the basis that the
2.06$\mu$m blend is now stronger in emission than 2.089$\mu$m Fe\,{\sc
ii}, such that it is presumably dominated by He\,{\sc  i} 2.058$\mu$m. In
addition, He\,{\sc i}
2.112$\mu$m is now observed to be weakly in emission.  Similar issues
apply regarding errors  for the 2002 dataset, although a slightly
higher temperature (by $\sim$0.5~kK) is potentially  favoured by the
weakness of  the Mg\,{\sc ii} 2.13--2.14$\mu$m doublet.

\subsection{ Results}

 A summary of spectroscopic results is presented in
Table~\ref{summary_afgl}. We find that the large changes in photometric
magnitude
between 2001--2007 (e.g. $\Delta$J=1.0 mag) were driven by significant
variation in  stellar radius (from 160-385 R$_{\odot}$ between
2002--2006), albeit with only
moderate changes in temperature  ($\Delta$T$_{\ast}\leq$4.5 kK);
significantly smaller than observed in the S Dor excursions of \object{AG
Car} (Groh et al. \cite{groh}). During this period, the mean
photospheric expansion velocity was $v_{exp}$=1.25~kms$^{-1}$.

In particular, the photometric fading between 2006--2007 appears to result
almost entirely  from a reduction in stellar radius from 385 to 310
R$_{\odot}$
($v_{exp}$=-1.6~kms$^{-1}$) at near-constant temperature
($\Delta$T$_{\ast}\sim0.5$ kK). {\em This combination of parameters
results
in a decrease in L$_{\rm bol}$ for \object{AFGL 2298} from 2 to
1.5$\times10^6$L$_{\odot}$ in less than one year.} Such a
variation differs from that inferred for both \object{S Dor} (Lamers
\cite{lamers95}) and \object{AG Car} (Groh et al. \cite{groh}) in
the
sense  that \object{AFGL 2298} is most luminous while at maximum radius,
reversing the behaviour of the other LBVs.

Both terminal wind velocity and (unclumped) mass loss rates determined for
\object{AFGL 2298} (Table~\ref{summary_afgl}) are comparable to those
found for
other highly luminous (candidate) LBVs such as the \object{Pistol Star}
and \object{FMM~362} (Najarro et al. \cite{paco08}). The maximum mass-loss
rate observed for \object{AFGL 2298} is significantly lower than that
found for \object{$\eta$ Car} and, somewhat surprisingly, the cool B
supergiant \object{HDE 316285}, which is of a similar temperature but is a
factor of $\sim$5 less luminous (Hillier et al. \cite{hillier},
\cite{hillier98}). Both these stars support winds with very high
performance numbers ($\eta$ (=c \.{M} v$_{\infty}$/L) of 18 and 4.5,
respectively;  Hillier et al. \cite{hillier98}, \cite{hillier}); the lower
value for \object{AFGL 2298} ($\eta\sim$unity) demonstrating that its
current wind could likely be driven through radiation pressure.
The
results of our modelling -- in particular the comparatively modest mass
loss
rates --  indicate that the changes in physical properties of \object{AFGL
2298} reflect real changes in the stellar temperature and radius.

Finally, our spectroscopic modelling enables us to determine BCs
for \object{AFGL 2298} as a function of stellar temperature and hence
interpret the photometric lightcurve prior to 2001. In doing so, we find
that assuming a BC$\sim$0 mag (corresponding to T$_{\ast}\sim8$ kK;
appropriate for an LBV in the cool state)  during the 1996--1999 maximum
results in log(L$_{\ast}$/L$_{\odot}$)$\sim$6.4, rising to
log(L$_{\ast}$/L$_{\odot}$)$\sim$6.6 for an assumed T$_{\ast}\sim11$ kK,
with the former estimate representing a robust lower limit to
luminosity at  this time.{\em In comparison to the 2007 results, this
demonstrates at a $>$5$\sigma$ level of significance that the L$_{\rm bol}$
of  \object{AFGL~2298} was not conserved between 1996--2007 }. We
present an updated HR diagram in Figure~\ref{hrd}. At its highest
luminosity,
\object{AFGL 2298} was one of the brightest stars in the Galaxy, being
rivalled and surpassed only by \object{WR 102ka} (Barniske et al.
\cite{barniske}) and $\eta$ Car (Hillier et al. \cite{hillier98}).

\begin{figure}
\resizebox{\hsize}{!}{\includegraphics[angle=0]{12358fg7.ps}}
\caption{H-R diagram showing the positions of AFGL 2298 between 1996--2007
in comparison to both
confirmed  and candidate  Galactic LBVs (squares), narrow emission line sources from the Galactic Centre cluster
(triangles) and Galactic YHGs (circles). We present  two possible
locations for AFGL 2298 between 1996--1998 based on
temperatures of 8 kK and 11.5 kK (see Sect. 4 for justification).
References for luminosity and temperature for
individual systems are given in  Clark et al. (\cite{clark05}), with the exception of the Galactic Centre
narrow emission line sources (Martins et al. \cite{martins}),
the \object{Pistol Star} and FMM362 (Najarro et al. \cite{paco08}),
\object{SGR 1806-20} (Bibby et al. \cite{bibby}),
\object{HD 326823} (Marcolino et al. \cite{marcolino}),
\object{AG Car} (Groh et al. \cite{groh}),
\object{HR Car} (Szeifert et al. in prep.)
\object{W243} (Ritchie et al. submitted),
\object{WR 102ka} (Barniske et al. \cite{barniske}),
\object{[OMN2000] LS1} (Clark et al. \cite{clark09}) and
\object{G24.73+0.69} (Clark et al. in prep.).}\label{hrd}
\end{figure}

\begin{table*}
\begin{center}
\caption{Summary of the stellar properties of \object{AFGL 2998} and other
LBVs observed to undergo giant eruptions.}\label{lbv_summary}
\begin{tabular}{lcccccccl}
\hline
\hline
Star & Epoch & $R_{\ast}$     & $T_{\rm eff}$ & log($L_{\ast}/L_{\odot}$)
&
H/He & $v_{\infty}$
& log $\dot M$ /$\sqrt f$ & Reference\\
     &      & ($R_{\odot}$)  & (kK)      &                           &
& (kms$^{-1}$) & $M_{\odot}$ yr$^{-1}$ & \\
\hline
AFGL 2298 & 1996--9   &  -   & -   & $>$6.4 &  - &  - & - & a\\
              & 2001 Jun & 262 &  11.7 & 6.2 & {\em 5.0} & {\em 200} &
-4.3 & a\\
              & 2002 Aug & 158 &  10.9 & 6.1 & {\em 5.0} & {\em 200} &
-3.9 & a \\
              & 2006 Jun & 385 &  10.3 & 6.3 & {\em 5.0} & 125       &
-4.3 & a \\
              & 2007 May & 312 &  10.9 & 6.2 & {\em 5.0} & 125       &
-4.4 & a \\
\hline
NGC 2363-V1          &$<$1993    &  -   &  -    & $\leq${\em 5.9} &   - &
-    &  -   &b \\
                    &1997 Nov & 356 & 13.5   & 6.58& {\em 4.0} & 325
& -3.3 & b,c\\
                    &1999 Jul & 201 & 18.5   & 6.63& {\em 4.0} & 290
& -3.2 & b,c\\
                    &2000 Oct & 225 & 17.5   & 6.63& {\em 4.0} &  -
& -3.3 & c, d\\
                    &2001 Dec & 223 & 18.5   & 6.72& {\em 4.0} &  -
& -3.5 & c, d\\
                    &2003 May & 194 & 20.5   & 6.78& {\em 4.0} &  -
& -3.8 & c, d\\
                    &2004 May & - &{\em 26.0}& {\em 6.80}&  -   &    -
& {\em $<$ -3.4} &c, d \\
\hline
HD 5980            &1978-80  &  -   &{\em 60.5}  & {\em 6.4}  &  - & - &  -
& e\\
                    &1993 Nov &  -   &{\em 52.0}  & {\em 6.5}   & - & - &
- & e\\
                    &1994 Sep & 280 & 23   & 7.05& 0.7       & 500       &
-3.1 & b\\
                    &1994 Dec & 130 & 35   & 6.50& 2.5       & 600       &
-3.0 & e\\
\hline
$\eta$ Car & 1840's       &  -   & -     & {\em 7.5} &   -
& {\em 650 - 3000}     & {\em $\ga$-0.3} & f, g\\
                      & 1890's       &  -   & -     &  -        &   -   &
{\em 200}               &  {\em -1.8}    & h, i \\
                      & 1998.2      & 60-881  & 35.3-9.2 & 6.74 & 5.0 &
500 & -3.0 & j\\
\hline
P Cygni       & 1600's       &  -   & -     & -  &   -   &
{\em 137}           &{\em -1.7}    &k  \\
                        & 1989-92     &   76 & 18.7 &  5.8       &   3.5 &
185 & -4.5 &l \\
\hline
\end{tabular}
\note{ Those
parameters that are not directly determined from non-LTE model atmosphere analysis - for example wind parameters derived
from present day nebular properties - are given in italics. Due to the extreme mass loss rate for
\object{$\eta$ Car} a degeneracy is present for the stellar radius and temperature (Hillier et al. \cite{hillier}).
References used in the construction of this table are
$^a$ Clark et al. (\cite{clark03a}) and this study;
$^b$ Drissen et al. (\cite{drissen});
$^c$ Petit et al. (\cite{petit05});
$^d$ Petit et al. (\cite{petit});
$^e$ Koenigsberger et al. (\cite{koen})
$^f$ Humphreys et al (\cite{h99});
$^g$ Smith et al. (\cite{smith03});
$^h$ Smith  (\cite{smith05});
$^i$ Smith  (\cite{nature}); $^j$ Hillier et al. (\cite{hillier});
$^k$ Smith \& Hartigan (\cite{smith06b}); $^l$ Najarro (\cite{pacopcyg}). Volume filling factors, $f$, range from
0.1 (\object{HD 5980} and \object{$\eta$ Car}),  through 0.3 (\object{NGC
2363--V1}) to 0.5  (\object{P Cygni}). Time averaged mass
loss  rates for \object{$\eta$ Car} and \object{P Cygni} derived from
nebular masses are absolute (i.e. $f$=1). Finally, note that the values of $V_{\infty}$  presented  for
NGC 2363-V1 in 1997 \& 1999 are from Drissen et al. (\cite{drissen}), while R$_*$, T$_{eff}$, log(L$_{*}$) and $\dot {M}$
are from the updated models presented by
Petit et al. (\cite{petit05}).}
\end{center}
\end{table*}

\section{AFGL 2298 in the context of LBV eruptions}

Our results strongly support the hypothesis that
the  bolometric luminosity of \object{AFGL 2298} has varied by at
least a
factor of  $\sim$2 over the past 2 decades (1996--1999 compared to
2002)\footnote{In the following discussion we shall assume that the
properties
of \object{AFGL 2298} in 2001--2002 are those of it in quiescence, despite
this being significantly cooler than e.g. \object{AG Car} in such a state
(24 kK;  Groh et al. \cite{groh}); if this is incorrect then the
duration
of the
non-luminosity conserving behaviour and the magnitude of such changes
will both be lower limits.}. Consequently, these variations differ from both
the canonical excursions of LBVs to lower temperatures at near constant
luminosity, {\em and} also the reduction in luminosities inferred for
both
\object{AG Car} and \object{S Dor} during such transitions to cooler
states, for which the deficit in radiative energy is thought to be due to
the requirement to support the expansion of the outer layers of the star.

Given that increases in L$_{\rm bol}$ for other LBVs have previously only
been
associated with \object{$\eta$ Car}-like giant eruptions, these
observations are of considerable interest; more so given the twin
suggestions that such behaviour is accompanied by significant mass loss -
permitting the transition from Main Sequence to hydrogen depleted
Wolf-Rayet and yielding the characteristic LBV ejection nebulae - and may
presage the occurrence of a SN (Sect. 1). As such, does the `outburst' of
\object{AFGL 2298} permit it to serve as a template system to calibrate
other less well observationally constrained events, or are the known
non-luminosity  conserving eruptions too heterogeneous to allow this?

\subsection{\object{HD~5980} and \object{NGC2363--V1}}

Prior to \object{AFGL 2298}, two LBVs have been the subject of
detailed spectroscopic analysis during the course of a giant eruption:
\object{HD 5980} in the SMC and NGC2363--V1  within NGC 2366,
both low
metallicity
environments. Results of a synthesis of the long-term lightcurves,
optical spectroscopy and, for a subset of the data, non-LTE model
atmospheric analysis (Drissen et al.  \cite{drissen}, Koenigsberger et al.
\cite{koen}) are presented in Table~\ref{lbv_summary}. A comparison
between \object{AFGL 2298}, HD~5980 and NGC~2363--V1
reveals that the properties of the three outbursts differ
significantly from one another in terms of the duration of the event -
$\sim$6 months for \object{HD 5980} (e.g. Koenigsberger et al.
\cite{koen})
to $>$7 yrs for \object{NGC 2363-V1} (Petit et al. \cite{petit}) and
apparently \object{AFGL 2298}. Unless fortuitously located when
\object{AFGL 2298} was inaccessible, the sampling frequency of the
lightcurve would appear to exclude such a rapid outburst as that of
\object{HD 5980}, although one could have been missed in \object{NGC
2363--V1} (Petit et al. \cite{petit}).

Additionally, significant variance is found between peak luminosity and
the evolution of stellar temperature during the outbursts
(Table~\ref{lbv_summary}). In  particular, while the temperature of
\object{AFGL 2298} varied by only  $\sim$4.5 kK between 2001--2007, the
initial, pre-1997, phase of the  outburst of \object{NGC 2363--V1} must
have been accompanied by a  significant reduction in temperature (for any
reasonable progenitor;  Drissen et al. \cite{drissen}), a trend which has
since reversed, with  temperature increasing by $\sim$13 kK over the
following seven years,
during which time L$_{\rm bol}$ has continued to increase.  Conversely,
the increase in L$_{\rm bol}$ found for \object{HD 5980} appears to have
been solely associated with a rapid and dramatic cooling
($\Delta$T$\sim$30 kK), with the subsequent increase in
temperature associated with a corresponding reduction in luminosity
(noting that the long term secular changes in brightness prior to this
event appear to have occurred at $\sim$constant L$_{\rm
bol}$)\footnote{Our estimates of the stellar temperature of \object{HD
5980}  prior to the 1994 June-October outburst were made using the
photometric
and spectroscopic summary present by Moffat et al. (\cite{moffat}) under
the assumption that components B and C were of constant brightness
(utilising values from Foellmi et al. \cite{foellmi}) and adopting
temperatures and bolometric corrections appropriate for WN3h and WN6h
stars in the Small Magellanic Cloud from Martins et al.
(\cite{martins09}).}.

Finally, despite the lower metallicity of their host galaxies, the mass
loss rates for both \object{HD 5980} and \object{NGC 2363-V1} are
significantly higher than those observed for \object{AFGL 2298}, although
for all three stars they appear to be orders of magnitude lower than those
associated with \object{$\eta$ Car} and \object{P Cygni} in outburst
(Table~\ref{lbv_summary}).

\subsection{The historic eruptions of \object{$\eta$ Car} and \object{P Cygni}}

Constraints on the outbursts of both stars may be inferred from their
historical lightcurves and current properties of their circumstellar
ejecta and are summarised in Table~\ref{lbv_summary}, supplemented with
the results of non-LTE spectroscopic analyses of their current
physical  properties. P  Cygni was observed to undergo two outbursts of
$\sim$6yr
and $\sim$3yr
duration
which commenced in 1600\,AD and 1655\,AD respectively. In both events it
was
inferred to brighten visually by 2-3 mag, with the colour information
associated with the latter outburst consistent with a (non-unique)
interpretation that the L$_{\rm bol}$ was also higher at this time (Lamers
\&
de Groot \cite{lamers92}). Smith \& Hartigan (\cite{smith06b}) report the
discovery of a dynamically young 0.1 M$_{\odot}$ nebula associated with P
Cygni which they attribute to the 1600\,AD event; if correct, this would
imply a mass loss rate of $\sim$0.02 M$_{\odot}$yr$^{-1}$, $>$2 orders of
magnitude larger than those found for \object{AFGL 2298}, \object{HD 5980}
or \object{NGC 2363-V1}.

In a similar manner, Smith (\cite{smith05}) estimated a comparable time
averaged mass loss rate for the 1890s outburst of \object{$\eta$ Car}
(Table~\ref{lbv_summary}). The properties of the star during this
outburst are difficult
to define due to the uncertain circumstellar extinction, but Humphreys et
al. (\cite{h99}) concluded that it is not necessary to postulate an
increase in L$_{\rm bol}$ during this event. However, this was clearly not
the case for the famous and well documented 1840s eruption
(Table~\ref{lbv_summary} and
references therein). While the $\sim$20 yr duration of the giant eruption of
$\eta$ Car appears comparable to the ongoing eruption of \object{NGC
2363--V1}
(Humphreys et al.  \cite{h99}), the stellar luminosity and mass loss rate
were both extreme; the latter exceeding those of \object{AFGL 2298},
\object{NGC 2363--V1} and \object{HD 5980} by over 3 orders of magnitude.

 While the wind properties inferred for \object{AFGL 2298} are
relatively modest and in principle consistent with line driving, those
inferred for the 1840s and 1890s outbursts (and
the 1600s outburst of \object{P Cygni}) are high enough that an
alternative mechanism is required to power the outflows.  Moreover, based
on the difference in properties between the two Homunculi and further
supported by the recent discovery of a high velocity component of the
1840s ejecta, Smith (\cite{smith05}, \cite{nature}) suggest that two
different physical mechanisms may have initiated and driven the 1840s and
1890s outbursts of \object{$\eta$ Car}.

\begin{table}
\begin{center}
\caption[]{Summary of the progenitor and peak
bolometric luminosities of the extragalactic
SNe imposters (top panel) and the dust enshrouded optical transients
 (lower panel).}\label{sn}
\begin{tabular}{lccl}
\hline
\hline
Transient &\multicolumn{2}{c}{ log(L$_{\ast}$/L$_{\odot}$)} & Reference
\\
          &  Progenitor & Outburst \\
\hline
SN 1961V & 6.4 & 8.7 &a, b\\
SN 1954J  & 5.7 & 6.5 &b\\
SN 1997bs & $<6.2$ & 7.4 &c \\
SN 2000ch  & 6.2  & 6.9 & d \\
\hline
NGC300-OT1  &4.5 & 6.6 & e\\
            & 4.7 & 7.1 & f\\
SN 2008S      &4.5 &  7.5 & g\\

\hline
\multicolumn{4}{l}{Luminosities assume a bolometric correction
of $\sim0$}\\
\multicolumn{4}{l}{as appropiate for an F supergiant. References
are}\\
\multicolumn{4}{l}{$^a$ Goodrich et al. (\cite{goodrich});
$^b$ Humphreys et al. (\cite{h99})}\\
\multicolumn{4}{l}{$^c$ Van Dyk et al. \cite{1997bs});
$^d$ Wagner et al. (\cite{wagner})}\\
\multicolumn{4}{l}{$^e$ Bond et al. (\cite{bond});
$^f$ Berger et al. (\cite{berger})}\\
\multicolumn{4}{l}{$^g$ Smith et al. (\cite{smith08b})}
\end{tabular}
\end{center}
\end{table}

\subsection{The extragalactic SNe imposters}

Finally, we consider the so-called SNe imposters or \object{$\eta$ Car}
analogues. Humphreys et al. (\cite{h99}) and Van Dyk (\cite{VD}) review
the nine examples identified prior to 2005; of these 4 have been little
studied, while a fifth, \object{SN 2008kg} appears to be an LBV undergoing
a normal LBV excursion and so is not discussed further (e.g. Maund et al.
\cite{maund}). We summarise the limited information on the progenitor and
peak bolometric luminosities of the remaining objects in Table~\ref{sn}.
For  completeness, we also include two recently  discovered optical
transients
for which a connection with SNe imposters has been proposed (e.g. Bond et
al. \cite{bond}, Smith et al.  \cite{smith08b}), although their dust
enshrouded progenitors appear to be of lower mass than known LBVs.

As with \object{P Cygni} and \object{$\eta$ Car}, the lack of
spectroscopic observations for the stars complicates determination of
their quiescent and outburst properties; the values derived from the
literature in Table~\ref{sn} typically assume an A/F spectral type in {\em
outburst} (with the natures of the precursors summarised in Van Dyk et al.
\cite{VD}, and references therein). However, we note the similarity of the
outburst spectrum of \object{SN 1997bs} to those of \object{NGC2363--V1}
circa 1997 (van Dyk et al. \cite{1997bs}, Petit et al. \cite{petit}),
potentially implying an upwards revision for the outburst luminosity to
log(L$_{\ast}$/L$_{\odot}$)$\sim$7.9.

If the dusty optical transients \object{SN2008S} and \object{NGC300-OT1}
are super-Eddington outbursts (e.g. Smith et al. \cite{smith08}) rather
than electron capture SNe (Thompson et al. \cite{thompson}), the mass
range for such events will be extended downwards into the
10--20M$_{\odot}$
regime (e.g. Berger et al. \cite{berger}). However, even without the
inclusion of these objects, the progenitor and outburst luminosities of
the SNe imposters emphasise the diversity of  non-luminosity
conserving eruptions.

Conclusions over the timescale for such events are difficult to draw due to
limited sampling of the lightcurves and possible observational biases
introduced by surveys optimised to identify SNe. Nevertheless, the
durations of the outbursts of \object{SN1961v}, \object{SN1954j} and
\object{SN1997bs} ($<$1yr; Humphreys et al. \cite{h99} and refs. therein,
van Dyk et al. \cite{1997bs})  all appear significantly shorter than those
of \object{$\eta$ Car}, \object{NGC 2363--V1} and the behaviour of
\object{AFGL 2298} at any stage since 1989. However the timescales of
these outbursts, if not their magnitudes, are comparable to that of
\object{HD 5980}. The variability of \object{SN2000ch} is characterised by
it's extreme rapidity ($\sim$2mag. on timescales of a $\sim$week; Wagner et
al. \cite{wagner}), with similar fluctuations also present during the
1840s giant outburst of \object{$\eta$ Car} (Frew \cite{frew});
comparable
behaviour appears to be absent for \object{AFGL 2298}, at least since
2001.

\begin{table}
\begin{center}
\caption[]{Summary of the total mass and nebular expansion velocity of LBV (top panel) and YHG
(bottom panel)  circumstellar ejecta.}\label{lbv_nebulae}
\begin{tabular}{lccl}
\hline
\hline
Star & v$_{neb}$ & M & Reference\\
     &  (km s$^{-1}$)& (M$_{\odot}$) \\
\hline
$\eta$ Car (Homunculus) & 650 &$>$12 &a \\
\hspace{0.7cm} (Little Homunculus) & 200 & 0.1 & b\\
P Cygni & 136 & 0.1  & c\\
Pistol Star & 60-95 & 11 & d, e  \\
AFGL 2298 & 70 & 10 & f  \\
AG Car & 70 & 8.9 & g, h\\
G79.29+0.46 & 44 & 15 & i, j, k\\
Wra 751 & 26 & 1.7 & g, l \\
HD 168625 & 19 & 2.1 & m, n\\
\hline
IRC +10 420  &  27 & 0.24 & o   \\
             &  35 & 0.46 & o   \\
HD 179821   &  35 & 4.0 & o   \\
\hline
\multicolumn{4}{l}{$^a$Smith et al. (\cite{smith03});
 $^b$Smith (\cite{smith05})}\\
\multicolumn{4}{l}{$^c$Smith \& Hartigan (\cite{smith06b});
$^d$Figer et al. (\cite{figer99})}\\
\multicolumn{4}{l}{ $^e$Figer et al. (\cite{figer98});
 $^f$Ueta et al. (\cite{ueta})}\\
\multicolumn{4}{l}{ $^g$Voors et al. (\cite{voors});
 $^h$Smith et al. (\cite{smith91})}\\
\multicolumn{4}{l}{ $^i$Higgs et al. (\cite{higgs});
$^j$Waters et al. (\cite{waters})}\\
\multicolumn{4}{l}{ $^k$Voors et al. (\cite{voorsb});
 $^l$Hutsemekers \& van Drom (\cite{hut91})}\\
\multicolumn{4}{l}{ $^m$Pasquali et al. (\cite{anna});
 $^n$O'Hara et al. (\cite{ohara})}\\
\multicolumn{4}{l}{ $^o$Castro-Carrizo et al. (\cite{castro})}
\end{tabular}
\end{center}
\end{table}

\section{Outbursts and nebular formation}

Given the apparently diverse nature of giant eruptions described in
Sect.~5, it is of interest to determine whether the
physical properties of LBV ejection nebulae are similarly heterogeneous.
In Table~\ref{lbv_nebulae} we summarise the properties of those nebulae
surrounding {\em  Galactic} LBVs for which both current expansion velocity
and mass have
been determined (with an expanded summary presented in Clark et al.
\cite{clark03b}). In compiling this we note that the nebulae associated
with \object{P Cygni} and \object{$\eta$ Car} are relatively youthful
($<400$yrs) compared to the dynamical ages of the remaining LBVs
($>$10$^3$yr; references given in caption, Table~\ref{lbv_nebulae}); this
will be returned to shortly.

One must also be careful to distinguish those stars which might have
ejected their nebulae during a RSG phase, an issue highlighted by Voors
et al. (\cite{voors}), who found that the dust composition of RSG ejecta
is comparable to that of LBV nebulae, suggesting similar conditions during
their formation. Assuming evolution at constant $L_{\rm bol}$ once stars
enter
the supergiant phase, both observational constraints and theoretical
predictions suggest that stars with log(L$_{\ast}$/L$_{\odot}$)$\leq$5.8
will pass through a Red Supergiant(RSG) /Yellow Hypergiant (YHG) phase
prior to evolving to higher temperatures (Humphreys \& Davidson \cite{HD},
Meynet \& Maeder \cite{meynet}). Thus, from the stars listed in
Table~\ref{lbv_nebulae},  \object{G79.29+0.46}, \object{Wra 751} and
\object{HD 168625} could all have passed through such a phase.

Given such an hypothesis, it is notable that the mass and velocities of
the ejecta associated with the YHGs \object{IRC +10 420} and \object{HD
179821}, which are thought to be post-RSG objects, are comparable to those
of \object{Wra 751} and \object{HD 168625} (although the mass of
\object{G79.29+0.46} is significantly higher; Table~\ref{lbv_nebulae}).
Assuming that the  YHG nebulae continue to expand at their current
velocities, in
$\sim$5000~yr they would be the same size (and age) as the \object{Wra
751} nebula is today.  Additionally, \object{IRC +10 420} currently
appears to be evolving to higher temperatures and an early Blue
Supergiant/WNL phase (Oudmaijer \cite{rene}). At such a point the combined
nebulae+stellar system would effectively be indistinguishable from
\object{Wra 751}. Indeed observations of early Blue Supergiant/WNL stars
in Wd1 - which have passed through a YHG phase - show them to be
significantly variable as well (Clark et al. in prep.), thus replicating
all observational properties of an LBV.
Therefore, it is at least possible that the ejecta around (a subset of)
the low luminosity LBVs was formed in a previous cool hypergiant
phase\footnote{See Smith (\cite{smith07}) for a counterargument
for
\object{HD 168625}.}. In this respect the 2001 outburst of \object{$\rho$
Cas} which resulted in a brief phase of extreme mass loss with
\.{M}$\sim$10$^{-3}$M$_{\odot}$yr$^{-1}$ - directly comparable to the time
averaged mass loss rates inferred for \object{IRC +10 420} and \object{HD
179821} (Castro-Carrizo et al. \cite{castro}) - may form a workable
paradigm.

Of the stars that are too luminous to have evolved through a RSG phase,
there is a notable similarity in the properties of the nebulae of
\object{AFGL 2298}, \object{AG Car} and the \object{Pistol Star} (Table~
\ref{lbv_nebulae}), while the stars themselves are all of comparable
luminosity; highly suggestive of a uniform formation history.
In comparison to \object{P Cygni} and \object{$\eta$ Car} (during the
production of the Little Homunculus), it is  unclear whether the
differences in the nebular expansion velocities observed for \object{AFGL 2298},
\object{AG Car} and the \object{Pistol Star}
and, for the first two stars, the {\em time
averaged} mass loss rates (Sect. 3.2, Voors et al. \cite{voors}) are
solely due to observing the nebulae at different evolutionary stages.
However, it does appear difficult to reconcile the extreme physical
properties present during the formation of the Homunculus - in particular
the velocity and mass loss rate - with those of \object{AFGL 2298},
\object{AG Car} and the \object{Pistol Star}.

For example, Langer (2008,
priv. comm.) suggested that after several thousand years the rapid
expansion of the polar lobes of the Homunculus would reduce their surface
brightness below our current detection threshold, leaving the slower
moving equatorial material as the sole detectable result of the 1840s
outburst, which by that time would resemble the nebulae around
\object{AFGL 2298} et al. However, Smith et al. (\cite{smith03}) found
that only 10-20\% ($\leq$2.5M$_{\odot}$) of the total mass of the
Homunculus is located in the equatorial region, whereas the masses of the
nebulae in question are $\sim$8.9-11M$_{\odot}$. Thus, at the very least
it would appear that the latitudinal distribution of nebular mass would
have to differ between these objects; applying the `equatorial' to `polar'
mass ratio found for the Homunculus to \object{AFGL 2298}, \object{AG Car}
and the \object{Pistol Star} would imply total nebular masses in excess of
$\sim$50M$_{\odot}$.

\section{Concluding Remarks}

The continued spectroscopic and photometric monitoring of \object{AFGL
2298} clearly indicates that it has been highly variable over the last
$\sim$20~yr. The magnitude of variability ($>$1.6 mag in the near-IR) is
typical of that observed for L$_{\rm bol}$ conserving outbursts in LBVs.
However, the results of non-LTE model atmosphere analysis suggest that
unlike normal S Dor-like LBV excursions, which occur at constant
luminosity, the L$_{\rm bol}$ of \object{AFGL 2298} varied during this
time, being a factor of $\geq$2 greater during 1996--1999 in comparison to the
photometric minimum of 2001--2002 (indeed, during its 1996--1999 peak it
was one of the most luminous stars known in the Galaxy). The changes in luminosity
appear to be driven by expansion and contraction of the photosphere at
$\sim$constant temperature. As such they differ from those observed for
\object{S Dor} and \object{AG Car}, where an expansion of the star is
accompanied by a significant cooling, resulting in an overall reduction in
L$_{\rm bol}$. Throughout these changes both the wind velocity and mass
loss
rate of \object{AFGL 2298} were moderate, directly comparable with other
highly luminous (candidate) LBVs such as the \object{Pistol Star} and
\object{FMM~362} (Najarro et al. \cite{paco08}) and entirely consistent
with expectations for a line driven wind.

By comparison to the 2001--2002 minimum, the excess energy radiated during
the
1996--1999 photometric maximum requires the star to generate an additional
$\sim$10$^{47}$ergs. We note that with the exception of the Homunculus, this is
 $\sim$comparable to the current kinetic energies of LBV ejecta (determined from
the nebular masses and expansion velocities given in
Table~\ref{lbv_nebulae}). This value does not include the extra requirement of supporting the
extended outer layers of the  star against gravity during this period. Without
a measurement of the mass of material involved, it is difficult to infer
the energy budget for this, but simply assuming a similar mass to that
suggested for \object{S Dor} and \object{AG Car} (0.2--0.6M$_{\odot}$;
Lamers \cite{lamers95}, Groh et al. \cite{groh}) would require the
star
to find
a further $\sim$10$^{47}$ergs over this period.

Due to the renewed interest in the role that non L$_{\rm bol}$ conserving
'Giant' LBV eruptions play in the evolution of massive stars and also
their possible role in signalling incipient SNe, we compared the
properties of \object{AFGL 2298} to those of other LBV eruptions. From the
limited data available we found that the properties of such eruptions
appear to be highly heterogeneous, to the extent that it was not possible
to identify a characteristic template for such events. Timescale varied
from $<$0.5 - $\sim$20~yr, with peak luminosities and mass loss rates
spanning $>$2 and $>$3 orders of magnitude respectively.  Placed in this
context, we found that the behaviour of \object{AFGL 2298} defined a lower
bound to the properties of LBV eruptions in terms of increase in
L$_{\rm bol}$
and mass loss rate.

Likewise, significant variation was found between the {\em current}
properties of the ejection nebulae associated with LBVs. While this likely
partly reflects observational selection effects introduced by observing
nebulae at different stages in their life cycle, we suspect that real
differences in the physics of the ejection event may also be involved. For
example, a subset of the lower luminosity
(log(L$_{\ast}$/L$_{\odot}$)$<$5.8) stars could eject their nebulae in
a prior cool hypergiant, rather than the current LBV,  phase (e.g. Voors et al. \cite{voors}).
We note that a comparison of the disparate properties of the Little and Big Homunculi
of \object{$\eta$ Car} also led Smith (\cite{smith05}) to suggest that the
underlying physical cause of both events differed from one another.
Indeed, the mass loss rate of \object{$\eta$ Car} during the production of
the Homunculus is unrivalled by that of any other LBV, with the exception
of the mass loss inferred for the progenitors of \object{SN2006gy}
(Agnoletto et al. \cite{ag}) and \object{SN2006tf} (Smith et al.
\cite{smith08}).

As such, given the diverse nature of the outbursts, nebulae and, where
quantifiable, progenitors, one might consider the possibility that more
than one physical process may result in non L$_{\rm bol}$ conserving
eruptions
(e.g. Smith \cite{smith05}), depending on the properties of the underlying
(binary?) star. Indeed, a number of different mechanisms have already been
proposed to explain Giant eruptions, including hydrodynamical
instabilities (Smith \& Owocki \cite{smith06}), the pulsational pair
instability (Woosley et al. \cite{woosley}), tidal interaction in a binary
(Koenigsberger \cite{k08}) and binary mergers (Morris \& Podsialowski
\cite{morris}). If this is the case, then the term Luminous Blue Variable
would be more applicable as a phenomenological description of the
observational properties of a star, rather than as a description of a
unique evolutionary phase.

\begin{acknowledgements}

JSC acknowledges support from an RCUK fellowship. AZT-24 observations are
made within an agreement between Pulkovo, Rome and Teramo observatories.

\end{acknowledgements}

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