Circumstellar CO J = 3→2 detected around the evolving metal-poor ([Fe/H] ≈ -1.15 dex) AGB star RU Vulpeculae

Abstract

We report the first detection of CO J = 3→2 around a truly metal-poor evolved star. RU Vulpeculae is modelled to have T_eff ≈ 3620 K, L ≈ 3128 ± 516 L_⊙, log(g) = 0.0 ± 0.2 dex and [Fe/H] = -1.3 to -1.0 dex, and is modelled to have recently undergone a thermal pulse. Its infrared flux has approximately doubled over 35 yr. ALMA observations show the 3→2 line is narrow (half-width ∼1.8-3.5 km s^-1). The 2→1 line is much weaker: it is not confidently detected. Spectral-energy-distribution fitting indicates very little circumstellar absorption, despite its substantial mid-infrared emission. A VISIR mid-infrared spectrum shows features typical of previously observed metal-poor stars, dominated by a substantial infrared excess but with weak silicate and (possibly) Al₂O₃ emission. A lack of resolved emission, combined with weak 2→1 emission, indicates the dense circumstellar material is truncated at large radii. We suggest that rapid dust condensation is occurring, but with an aspherical geometry (e.g. a disc or clumps) that does not obscure the star. We compare with T UMi, a similar star which is currently losing its dust.