To see or not to see a bow shock. Identifying bow shocks with Hα allsky surveys

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, AJ, 110, 2614). 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. Hα, are most welcome. However due to their low velocity these bow shocks have a size of ∼ 1°, and could only be observed as a whole with great difficulties. In the light of the new Hα 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 Hα-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, AJ, 110, 2614). 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 (∼ 10⁴ K), that fit well to the up-to-date picture of the Warm Ionised Medium.