  OB-stars have the highest luminosities and strongest stellar winds of all
stars, which enables them to interact strongly with their surrounding ISM, thus
creating bow shocks. These offer us an ideal opportunity to learn more about
the ISM. They were first detected and analysed around runaway OB-stars using
the IRAS allsky survey by van Buren et al. (1995). Using the geometry of such
bow shocks information concerning the ISM density and its fluctuations can be
gained from such infrared observations. As to help to improve the bow shock
models, additional observations at other wavelengths, e.g. H-Alpha, are most
welcome. However due to their low velocity these bow shocks have a size of ~1
degrees, and could only be observed as a whole with great difficulties. In the
light of the new H-Alpha allsky surveys (SHASSA/VTSS) this is no problem any
more. We developed different methods to detect bow shocks, e.g. the improved
determination of their symmetry axis with radial distance profiles. Using two
H-Alpha-allsky surveys (SHASSA/VTSS), we searched for bow shocks and compared
the different methods. From our sample we conclude, that the correlation
between the direction of both proper motion and the symmetry axis determined
with radial distance profile is the most promising detection method. We found
eight bow shocks around HD 17505, HD 24430, HD 48099, HD 57061, HD 92206, HD
135240, HD 149757, and HD 158186 from 37 candidates taken from van Buren et al.
(1995). Additionally to the traditional determination of ISM parameters using
the standoff distance of the bow shock, another approach was chosen, using the
thickness of the bow-shock layer. Both methods lead to the same results,
yielding densities (~1 cm^{-3}) and the maximal temperatures (~10^4 K), that
fit well to the up-to-date picture of the Warm Ionised Medium.

Astrophysics