The Interstellar Medium around Galactic WN Stars: WR 2, WR 128, and WR 151

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Abstract
The neutral hydrogen distribution has been studied in the direction of three Galactic Wolf-Rayet (W-R) stars, using the 100 m Effelsberg radio telescope. Neutral hydrogen voids, regions of low H I emissivity, are observed over a 10–16 km s-1 velocity range for the stars WR 2 (=HD 6327), WR 128 (=HD 187282), and WR 151 (=CX Cep). These minima are interpreted as the observable 21 cm counterpart of the interstellar bubble created by the powerful winds of W-R stars and their progenitors. The H I cavities are elongated structures having an axial ratio ranging from 1.3 (WR 151) to 2.0 (WR 128). As pointed out in previous studies, the W-R stars are always off-center with respect to either the geometric center of the H I void or the absolute minimum inside the cavity. This offset ranges from 40% to 70% of the H I hole's minor axis. The major axes of these cavities are ~30 pc for both WR 2 and WR 128, and ~50 pc for WR 151. The missing H I mass amounts to 96 M⊙ (WR 128), 200 M⊙ (WR 2), and 480 M⊙ (WR 151). Some of the physical parameters of the ring nebula associated with WR 128, such as its expansion velocity, 80 ± 22 km s-1, radius, 6.3 ± 0.2 pc, and characteristic age, (7.4 ± 2.4) × 104 yr, are derived from a simple geometric model that accounts for the discrepancy between the radial velocity of the H I hole and the ring nebula related to WR 128.