Ultraviolet and optical spectroscopy of AGB stars showing UV excess

DOI: 
10.1093/mnras/sty3076
Publication date: 
01/02/2019
Main author: 
Ortiz, Roberto
IAA authors: 
Guerrero, Martín A.
Authors: 
Ortiz, Roberto;Guerrero, Martín A.;Costa, Roberto D. D.
Journal: 
Monthly Notices of the Royal Astronomical Society
Refereed: 
Yes
Publication type: 
Article
Volume: 
482
Pages: 
4697-4712
Abstract: 
We have examined ultraviolet (UV) and optical UB spectra of 20 UV-emitting Asymptotic Giant Branch (AGB) stars of various variability classes to study the intensity of the continuum and emission lines as a function of stellar visual magnitude to shed light on the origin of their UV emission. A significant fraction (60 per cent) of these stars show Fe I and Fe II emission lines and ̃1/4 show Balmer lines in emission. The emission in the GALEX [FUV] and [NUV] bands is dominated by continuum emission, with a limited (≤36 per cent) contribution from emission lines. The UV spectra of sources with multiple GALEX or IUE observations reveal short-term (of a few days or less) variability, which does not follow the pulsation cycle. The intensity of the Mg II λ2800 doublet, a classical diagnostic of chromospheric activity, is anticorrelated with the spectral slope in the near-UV that could be partially attributed to temperature variations in a stellar chromosphere. We observed that the intensity of Mg II λ2800 in o Cet has a sharp maximum at the phase ϕ ≃ 0.35 after the light curve maximum. Other LPV stars (T Cet and R Com) show strong UV Fe II emission lines near this same phase and, like the Mg II doublet, their excitation can be driven by pulsation. Our results suggest that far-UV emission from AGB stars might be external (hot companion, accretion disc), but contemporary photometric and spectral UV observations covering the whole pulsation period are required to ascertain its true origin.
Database: 
ADS
SCOPUS
URL: 
https://www.scopus.com/inward/record.uri?eid=2-s2.0-85061105090&doi=10.1093%2fmnras%2fsty3076&partnerID=40&md5=80039125dd763c6659b35894030a63ee
ADS Bibcode: 
2019MNRAS.482.4697O
Keywords: 
binaries: general;stars: chromospheres;circumstellar matter;ultraviolet: stars;stars: AGB and post-AGB