The Interstellar Medium in the Environs of O-Type Stars

Abstract

We analyze the distribution of the neutral gas in the neighborhood of four southern O-type stars based on observations of the 21 cm H i line. These data disclose H i bubbles probably associated with the stars HD 112244 [O8.5Iab(f)], HD 155913 [O5Vn((f))], HD 175754 [O8II((f))], and HD 175876 [O6.5III(n)(f)]. The dimensions of these bubbles are in the range 90 to 170 pc, and their expansion velocities are low, about 10 km s^-1, implying dynamical ages of (3-6) x 10^6 yr. Only a few percent of the mechanical energy of the stellar wind is transformed into kinetic energy of the shells. Their origin and the contribution of other early-type stars to the formation of these bubbles are discussed. Particularly, it is suggested that HD 155913 belongs to an unknown OB association that could also have contributed in the formation of the H i bubble surrounding this star. These new H i bubbles resemble H i bubbles surrounding Wolf-Rayet (W-R) stars. The presence of stellar wind bubbles associated with O-type stars supports an interpretation where the massive progenitors of the W-R stars are also responsible for the formation of the H i bubbles detected around W-R stars.