Ultraviolet and optical spectroscopy of AGB stars showing UV excess
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 signi...
| Autores: | , , |
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| Tipo de recurso: | artículo |
| Estado: | Versión publicada |
| Fecha de publicación: | 2019 |
| País: | España |
| Institución: | Consejo Superior de Investigaciones Científicas (CSIC) |
| Repositorio: | DIGITAL.CSIC. Repositorio Institucional del CSIC |
| OAI Identifier: | oai:digital.csic.es:10261/179354 |
| Acceso en línea: | http://hdl.handle.net/10261/179354 |
| Access Level: | acceso abierto |
| Palabra clave: | Circumstellar matter Ultraviolet: stars Post-AGB Binaries: general Stars: chromospheres Stars: AGB |
| Sumario: | 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.© 2018 The Author(s). |
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