Emission from B stars

Prior to the EUV sky surveys, O and B stars exhibiting strong mass-loss were expected to be a minor, but nevertheless important, group of EUV sources, the emission arising from hot, shocked gas in the stellar winds. Little thought was given to the likelihood of detecting photospheric EUV flux since photospheric helium was expected to restrict emission to the longest EUV wavelengths, most affected by interstellar attenuation. Nevertheless, the existence of the so-called β CMa tunnel of low column density, extending over distances of 200–300 pc (e.g. Welsh 1991) promoted the hope that a few such objects might be detected in this direction at wavelengths longward of 504 Å. The subsequent detection of the B2 II star ∊ CMa (Adhara, d = 188 pc) in the 500–740 Å (tin) filter during the EUVE sky search was not, therefore, particularly remarkable. However, the intensity of the flux recorded outshone all other non-solar sources of EUV radiation, including the well-known hot white dwarf HZ 43, previously believed to be the brightest EUV source, although this star remains the brightest object at the shortest EUV wavelengths (Vallerga et al. 1993).

The magnitude of the detected EUVE tin count rate (98 ± 10 counts s−1) was a strong indication that the line-of-sight column density was even lower than the upper limit of 3 × 1018 cm−2 estimated indirectly by Welsh (1991) from NaI absorption line studies.