A Search for Mass Loss on the Cepheid Instability Strip using HI 21-cm Line Observations

We present the results of a search for HI 21-cm line emission from the circumstellar environments of four Galactic Cepheids (RS Pup, X Cyg, $ζ$ Gem, and T Mon) based on observations with the Karl G. Jansky Very Large Array. The observations were aimed at detecting gas associated with previous or ongoing mass loss. Near the long-period Cepheid T Mon, we report the detection of a partial shell-like structure whose properties appear consistent with originating from an earlier epoch of Cepheid mass loss. At the distance of T Mon, the nebula would have a mass (HI+He) of $\sim0.5M_{\odot}$, or $\sim$6\% of the stellar mass. Assuming that one-third of the nebular mass comprises swept-up interstellar gas, we estimate an implied mass-loss rate of ${\dot M}\sim (0.6-2)\times10^{-5} M_{\odot}$ yr$^{-1}$. No clear signatures of circumstellar emission were found toward $ζ$ Gem, RS Pup, or X Cyg, although in each case, line-of-sight confusion compromised portions of the spectral band. For the undetected stars, we derive model-dependent $3σ$ upper limits on the mass-loss rates, averaged over their lifetimes on the instability strip, of $<(0.3-6)\times10^{-6} M_{\odot}$ yr$^{-1}$ and estimate the total amount of mass lost to be less than a few per cent of the stellar mass.