Ultraviolet emission from main-sequence companions of AGB stars

TL;DR

This study uses UV observations and models to identify main-sequence companions of AGB stars, revealing that about 60% of nearby AGB stars likely have such companions detectable via UV excess.

Contribution

It introduces UV-based criteria for detecting main-sequence companions of AGB stars and applies these to a local sample, estimating the companion fraction.

Findings

Approximately 60% of nearby AGB stars show UV excess indicative of MS companions.

UV excess detection suggests companions are earlier than K0 spectral type.

UV emission may be absorbed or produced in accretion flows, complicating interpretation.

Abstract

Although the majority of known binary Asymptotic Giant Branch (AGB) stars are symbiotic systems (i.e. with a WD as a secondary star), main-sequence companions of AGB stars can be more numerous, even though they are more difficult to find because the primary high luminosity hampers the detection of the companion at visual wavelengths. However, in the ultraviolet the flux emitted by a secondary with T eff > 5500 ~ 6000 K may prevail over that of the primary, and then it can be used to search for candidates to binary AGB stars. In this work, theoretical atmosphere models are used to calculate the UV excess in the GALEX near- and far-UV bands due to a main-sequence companion. After analysing a sample of confirmed binary AGB stars, we propose as a criterium for binarity: (1) the detection of the AGB star in the GALEX far-UV band and/or (2) a GALEX near-UV observed-to-predicted flux ratio > 20. These criteria have been applied to a volume-limited sample of AGB stars within 500 pc of the Sun; 34 out of the sample of 58 AGB stars ( ~ 60%) fulfill them, implying to have a MS companion of spectral type earlier than K0. The excess in the GALEX near- and far-UV bands cannot be attributed to a single temperature companion star, thus suggesting that the UV emission of the secondary might be absorbed by the extended atmosphere and circumstellar envelope of the primary or that UV emission is produced in accretion flows.