CO emission survey of asymptotic giant branch stars with ultraviolet excesses

¹ Centro de Astrobiologia (CAB), CSIC-INTA. Postal address: ESAC, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
e-mail: jalonso@cab.inta-csic.es
² Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 29 June 2023
Accepted: 19 December 2023

Abstract

Context. The transition from the spherically symmetric envelopes around asymptotic giant branch (AGB) stars to the asymmetric morphologies observed in planetary nebulae is still not well understood, and the shaping mechanisms are a subject of debate. Even though binarity is widely accepted as a promising option, it is limited by the complication of identifying binary AGB stars observationally. Recently, the presence of ultraviolet excesses in AGB stars has been suggested as a potential indicator of binarity.

Aims. Our main goals are to characterise the properties of the circumstellar envelopes (CSEs) around candidate AGB binary stars, specifically those selected based on their UV excess emission, and to compare these properties with those derived from previous CO-based studies of AGB stars.

Methods. We observed the ¹²CO (J=1–0) and ¹²CO (J=2–1) millimetre-wavelength emission in a sample of 29 AGB binary candidates with the IRAM-30 m antenna. We measured the systemic velocities and the terminal expansion velocities from their line profiles. Population diagrams were used to interpret the results, enabling the estimation of excitation temperatures (T_ex), mass-loss rates (Ṁ), and the characteristic sizes of the envelope layers where the CO millimetre emission originates (R_s). We explored different trends between the envelope parameters deduced, multiwavelength flux measurements, and other properties of our sample, and compared them with those previously derived from larger samples of AGB stars found in the literature.

Results. We detected ¹²CO emission in 15 sources, of which 5 are first detections. We found relatively low expansion velocities (3 km s⁻¹ ≲ V_exp ≲ 20 km s⁻¹) in our sample. We derived the average excitation temperature and column density of the CO-emitting layers, which we used to estimate self-consistently the average mass-loss rate (10⁻⁸ M_⊙ yr⁻¹ ≲ Ṁ ≲ 10⁻⁵ M_⊙ yr⁻¹) and the CO pho-todissociation radius (5 × 10¹⁵ cm ≲ R_CO ≲ 2 × 10¹⁷ cm) of our targets. We find a correlation between CO intensity and IRAS 60 µm fluxes, revealing a CO-to-IRAS 60 µm ratio lower than for AGB stars and closer to that found for pre-planetary nebulae (pPNe). An anti-correlation is observed between ¹²CO (and IRAS 60 µm) and the near-ultraviolet (NUV), but no such correlation is observed with the far-ultraviolet (FUV). It is also worth noting that there is no correlation between bolometric luminosity and NUV or FUV.

Conclusions. For the first time we have studied the mass-loss properties of UV-excess AGB binary candidates and estimated their main CSE parameters. Our sample of uvAGB stars shows similarities with the broader category of AGB stars, except for a distinct CO-to-IRAS 60 µm trend suggesting enhanced CO photodissociation. Our findings, based on single-dish low-J CO line emission observations, support the dust-driven wind scenario and indicate that alternative mass-loss mechanisms are not necessary (in principle) to explain the ~200–2000 yr old mass-loss ejecta in uvAGBs. The different relationships between ¹²CO and IRAS 60 µm, with NUV and FUV are consistent with an intrinsic origin of NUV emission, but potential dominance of an extrinsic process (e.g. presence of a binary companion) in FUV emission.