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

TL;DR

This study reports the first detection of CO J=3-2 emission around a metal-poor evolved star, RU Vulpeculae, revealing insights into its circumstellar environment and recent thermal pulse activity.

Contribution

It provides the first observation of CO J=3-2 around a truly metal-poor AGB star, highlighting its circumstellar properties and dust condensation processes.

Findings

Detection of narrow CO J=3-2 line with ALMA.

Weak or absent CO J=2-1 emission suggests truncated dense circumstellar material.

Infrared features indicate rapid dust condensation with aspherical geometry.

Abstract

We report the first detection of CO J = 3-2 around a truly metal-poor evolved star. RU Vulpeculae is modelled to have Teff ~ 3620 K, L ~ 3128 +/- 516 L_Sun, 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 years. ALMA observations show the 3-2 line is narrow (half-width ~1.8-3.5 km/s). 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.