Clues on the hot star content and the ultraviolet output of elliptical galaxies

Abstract

view Abstract Citations (414) References (146) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Clues on the Hot Star Content and the Ultraviolet Output of Elliptical Galaxies Greggio, Laura ; Renzini, Alvio Abstract The relatively strong ultraviolet radiation flux emitted by elliptical galaxies shortward of ~ 2000 A is certainly due to the presence of hot stars. Yet, so far it has not been firmly established whether such hot stars are young, massive objects being currently formed in these galaxies, or whether old, low-mass stars suffice to produce the observed UV radiation, as well as the observed correlation with the galaxy metallicity. In this paper we investigate in great detail under which conditions the old-star option can give acceptable solutions. Several kinds of hot star candidates have been considered, including post-red giants, hot horizontal-branch stars and their AGB- manque progeny, post-early-AGB stars, and post-AGB stars of the helium- and hydrogen-burning varieties. Among binary star candidates we have considered post-red giants produced by a Roche-lobe overflow, and accreting white dwarfs. We present numerical simulations showing how sensitive is the production of the various kinds of hot stars to small differences in the assumptions concerning the helium enrichment and the mass-loss trends with increasing metallicity. These numerical simulations and additional arguments tend to disfavor classical post- AGB stars as the most important UV producers, at least among the galaxies with the strongest UV upturns. Also binary stars seems insufficient, unless stable mass transfer onto white dwarf primaries will turn out to be possible for a wider range of initial binary parameters than generally accepted. Rather, this study tends to favor hot HB stars, AGB-manque, and post-EAGB stars, whose production, however, is extremely sensitive to ill-known parameters. For this reason we conclude that stellar evolution theory alone cannot currently solve the problem of the origin of the UV output of elliptical galaxies. We therefore list in detail for each of all the above candidates the major observable characteristics which can help future space observations to discriminate among the various options, with particular reference to resolvable stellar systems such as galactic globular clusters, M32, and the bulges of the Milky Way and M31. Finally, we also discuss the predicted evolution with lookback time (redshift) and argue that the UV upturn of ellipticals is likely to be the most rapidly evolving feature in the spectrum of these galaxies, possibly allowing the detection of evolutionary effects already at rather modest redshifts (z <~ 0.2). Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169384 Bibcode: 1990ApJ...364...35G Keywords: Elliptical Galaxies; Galactic Evolution; Hot Stars; Stellar Evolution; Ultraviolet Radiation; Asymptotic Giant Branch Stars; Binary Stars; Galactic Bulge; Metallicity; Stellar Luminosity; Stellar Models; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: STELLAR CONTENT; STARS: EVOLUTION; ULTRAVIOLET: GENERAL full text sources ADS | data products SIMBAD (2) MAST (1)