COLD NEUTRAL GAS ASSOCIATED WITH THE GUM NEBULA

Based on HI maps covering the 3 600 square degrees area delimited by 2 30 degrees less than or equal to l less than or equal to 290 degrees, -30 degrees less than or equal to b less than or equal to 30 degrees, we investigated the neutral hydrogen distribution in the Gum nebula an d its environments. The main result is the appearence of a huge disk o f neutral gas at a kinematical distance compatible with that of the ne bula, and whose angular dimensions coincide with the optical image of the Gum nebula. The disk, roughly centered at l similar or equal to 26 2 degrees, b similar or equal to -3 degrees, is located at similar to 500 pc and has a radius of similar to 150 pc. We propose that this HI disk is the neutral gas component of the optical nebula, hence becomin g the first time in which a structure with the dimensions of the optic al nebula is detected in other wavelengths. We analyze the fitting of observations with different theoretical models and conclude that the m ost plausible interpretation is that the Gum nebula is a supershell pr oduced by the action of repeated SN explosions, where the explosions m ay have originated in the aged association Vela OB2. In this context, the thick HI shell shown by Dubner et al. (1992) near (266 degrees, -2 .5 degrees) and the Vela SNR are the debris of two of the several outb ursts which started in the region about 6 10(6) years ago. The souther n boundary is distorted due to the presence of a similar to 28 degrees radius shell, which appears to be an interstellar bubble driven by th e stellar winds of the O-type star HD 49798.