| Summary: | We present the results of a search for H i 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 (H I+He) of ∼0.5M[subscript ⨀], or ~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 [superscript dot]M∼(0.6-2) x 10⁻⁵ M[subscript ⨀] 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) x 10⁻⁶ M[subscript ⨀] yr⁻¹ and estimate the total amount of mass lost to be less than a few percent of the stellar mass.
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