Ultraviolet emission from main-sequence companions of AGB stars

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.

A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars

“…We indicate 1σ errors with dark shaded regions and indicate the locations of a few spectral features. Chemical subtype symbols that appear to the left of the object name are the same as in Figure 2. UV emission from AGB stars has been suggested as a potential tool to detect binary companions from AGB stars (Sahai et al 2008;Ortiz & Guerrero 2016). Over most of the electromagnetic spectrum luminous AGB stars will outshine main sequence and/or post-AGB companions.…”

“…This study includes the samples of Sahai et al (2008) and "confirmed" binary AGB stars derived from a radial velocity (RV) study by Famaey et al (2009) 5 . Ortiz & Guerrero (2016) esti-5 Sahai et al (2008 and Famaey et al (2009) remark on the complications that pulsations of AGB stars pose for RV measurements of such stars. Nevertheless, Famaey et al (2009) conclude that some of the Mira and semi-regular variables they studied are mated the NUV and FUV flux in excess of photospheric emission based on stellar spectral templates and proposed that if an AGB star had a NUV excess ≥ 20 and/or if an AGB star is detected in FUV, then its UV emission indicates a binary companion.…”

“…Nevertheless, Famaey et al (2009) conclude that some of the Mira and semi-regular variables they studied are mated the NUV and FUV flux in excess of photospheric emission based on stellar spectral templates and proposed that if an AGB star had a NUV excess ≥ 20 and/or if an AGB star is detected in FUV, then its UV emission indicates a binary companion. Applying the UV-based criteria, Ortiz & Guerrero (2016) inferred a binary fraction of ∼ 60% for their sample of AGB stars. However, for a given putative companion, Ortiz & Guerrero (2016) also found discrepancies among the effective temperatures obtained from their three temperature estimation methods (i.e., NUV excess, FUV excess, and the ratio of these excesses).…”

“…Indeed, IUE observations reveal the prevalence of chromospheric-like UV emission from cool giants and supergiants (Luttermoser 2000). The photospheric models adopted by Sahai et al (2008) and Ortiz & Guerrero (2016) do not include radiation from such overlying chromospheric emission, and it would appear likely such a process is responsible for AGB UV emission in many, if not most, cases.…”

H12CN and H13CN excitation analysis in the circumstellar outflow of R Sculptoris

“…In binary systems, active binary companions can emit UV-radiation from the inside as well (e.g. Sahai et al 2008;Ortiz & Guerrero 2016). A recent search for UV emission from AGB stars has revealed that about 180 AGB stars, ∼ 50 % of the AGB stars observed with Galaxy Evolution Explorer (GALEX), have detectable near-or far-UV emission (Montez et al, in prep), supporting the possible existence of internal sources of UV-radiation.…”