Neutral hydrogen towards the Gum Nebula

We have analyzed the distribution of the 21 cm neutral hydrogen emission in an extended area towards the center of the Gum Nebula. The observational data were in part acquired with the 30 m radiotelescope of the Instituto Argentino de Radioastronomía and complemented with a set of data taken from the southern HY survey made by Stron et al. (1982). Both data sets were fitted with appropriate software to produce HI maps of the region 250̣° ≤ l ≤ 274°, -10° ≤ b ≤ 5̣° with a sampling of 0:5 in both coordinates. We find that the gas at the distance of the Gum Nebula (about 400 pc away) shows a very complex distribution. The integrated HI in the velocity interval (-2,+10) km s⁻¹ appears roughly distributed along a broad ring, approximately 12° in diameter, centered at about l=266°, b=-2°. This same ring-shaped structure has been also detected in the λ2.6 mm ¹²CO line (May et al., 1988). In addition, by studying the HI emission of the region at higher velocities (i.e., at larger kinematical distances), we have identified several signatures indicative of energetic events. Briefly, the neutral gas at about 1700 pc (Vₗₛᵣ = 12 km s⁻¹) exhibits a large minimum, surrounded by an incomplete higher density HI ring. Remarkably, three open clusters: Pi 6, Pi 8 and Wat 6, are located in a perfect alignment with the compressed peripherical HI. These three stellar aggregates have similar ages (3x10⁷ years) and distances (d=1700 pc). The observed morphology and the distance coincidence between stars and gas both strongly suggest a genetic link among them. Besides, at d=4000 pc (vₗₛᵣ = +34 km s⁻¹), a neutral hydrogen bubble is observed around WR stars (Niemela and Cappa de Nicolau, 1990, and references therein), and is probably created by the strong stellar wind. This would be the first case of a wind blown neutral gas bubble detection around a star so close to the galactic plane (b=0.77°).Asociación Argentina de Astronomí