We report the radio detection of a shell-like HI structure in proximity to, and probably associated with, the nova V458 Vul. High spectral resolution observation with the Giant Metrewave Radio Telescope has made it possible to study the detailed kine
matics of this broken and expanding shell. Unlike the diffuse Galactic HI emission, this is a single velocity component emission with significant clumping at ~ 0.5 scales. The observed narrow line width of ~ 5 km/s suggests that the shell consists of mostly cold gas. Assuming a distance of 13 kpc to the system, as quoted in the literature, the estimated HI mass of the nebula is about 25 M_sun. However, there are some indications that the system is closer than 13 kpc. If there is a physical association of the HI structure and the nova system, the asymmetric morphology and the off-centred stellar system indicates past strong interaction of the mass loss in the asymptotic giant branch phase with the surrounding interstellar medium. So far, this is the second example, after GK Per, of a large HI structure associated with a classical nova.
The first low radio frequency (<1.4 GHz) detection of the outburst of the recurrent nova RS Ophiuchi is presented in this letter. Radio emission was detected at 0.61 GHz on day 20 with a flux density of ~48 mJy and at 0.325 GHz on day 38 with a flux
density of ~ 44 mJy. This is in contrast with the 1985 outburst when it was not detected at 0.327 GHz even on day 66. The emission at low radio frequencies is clearly non-thermal and is well-explained by a synchrotron spectrum of index alpha ~ -0.8 (S propto nu^alpha) suffering foreground absorption due to the pre-existing, ionized, warm, clumpy red giant wind. The absence of low frequency radio emission in 1985 and the earlier turn-on of the radio flux in the current outburst are interpreted as being due to higher foreground absorption in 1985 compared to that in 2006, suggesting that the overlying wind densities in 2006 are only ~30% of those in 1985.
