Hot Horizontal Branch Stars Research Articles

Evidence for a Sizable Age Spread among Galaxies from the Ultraviolet‐Upturn Phenomenon in Early‐Type Systems

The suggestion of Lee that the age spread among galaxies is responsible for the systematic variation of the ultraviolet upturn among early-type systems is examined here with detailed population synthesis models. Our models differ from previous ones by including (1) the effect of metallicity spreads and (2) detailed modeling of the variations in H-R diagram morphology (including the helium-burning phase) with age and metallicity. Our models suggest that the far-UV radiation of these systems is dominated by a minority population of metal-poor, hot horizontal-branch (HB) stars and their post-HB progeny, with some contribution from metal-rich post-asymptotic giant branch stars, while the optical radiation is dominated by a metal-rich population. The systematic variation of the UV upturn depends on the contribution from metal-poor, hot HB stars and their post-HB progeny, which in turn depends on the ages of old stellar populations in galaxies. Our result implies a prolonged epoch of galaxy formation, in the sense that more massive galaxies (in denser environments) formed first. With the assumption that the UV-upturn phenomenon is solely due to the age variations among galaxies, we estimate the difference in age between the giant elliptical galaxies and the spiral bulges of the Local Group to be ~3 Gyr. This suggests that the best estimate for the lower limit of the age of the universe is ~19 Gyr, which of course would be in conflict with the current estimate of H0, together with the standard cosmological models with zero cosmological constant.

Far-Ultraviolet Imaging of the Globular Cluster NGC 7099 with the Second Wide-Field and Planetary Camera

We have imaged the globular cluster NGC 7099 in the far-UV and visible with WFPC2 on the Hubble Space Telescope. Our far-UV images show a sparsely populated and fully resolved central region. The far-UV to visible color-magnitude diagram of these stars shows a well-defined horizontal branch with no evidence for hot, more evolved descendants. We find one hot horizontal branch star; the center of the cluster harbors four luminous blue stragglers. Far-UV images do not show any singularly steep density gradient at small radii among the horizontal branch stars of this post-core-collapse cluster.

Evidence for a Sizable Age Spread among Galaxies from the UV Upturn Phenomenon in Early-Type Systems

The suggestion of Lee (1994, ApJ, 430, L113) that the age spread among galaxies is responsible for the systematic variation of ultraviolet (UV) upturn among the early-type systems is confirmed here with the detailed population synthesis models. Our models suggest that the far-UV spectra of these systems are composite of hot metal-poor horizontal-branch (HB) stars (and their post-HB progeny) and metal-rich post-asymptotic giant-branch (PAGB) stars. The systematic variation of UV upturn, however, depends on the contribution from hot metal-poor HB stars and their progeny, which in turn depends on the ages of oldest stellar populations in galaxies. Consequently, our models predict that the strength of absorption features, such as C IV and Si IV, is anticorrelated with the strength of UV upturn. This is consistent with the far-UV spectra for NGC1399 and M31 obtained by the Hopkins Ultraviolet Telescope aboard the Astro-1 space shuttle mission. We note that the opposite trend is expected in other's models that favor metal-rich solution for the UV upturn phenomenon without age spread among galaxies. Our result implies a prolonged epoch of galaxy formation, in the sense that more massive galaxies (in denser environments) formed first. With the assumption that the UV upturn phenomenon is solely due to the age variations among galaxies, we tentatively estimate the difference in age between the giant ellipticals and our Galaxy to be 4 billion years or more. This suggests that the best estimate for the lower limit of the age of the Universe is close to 20 Gyrs, which of course would be in conflict with the current estimate of the Ho together with the standard cosmological models with zero cosmological constant. The reader is referred to Park & Lee (1995, ApJL, in press) for details of this work.

The Hot End of Evolutionary Horizontal Branches

view Abstract Citations (6) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Hot End of Evolutionary Horizontal Branches Castellani, V. ; degl'Innocenti, S. ; Pulone, L. Abstract In this paper we investigate the hot end of the horizontal branch (HB), presenting evolutionary constraints concerning the C-M diagram location and the gravity of hot HB stars. According to the adopted evolutionary scenario, we predict an upper limit for HB temperatures of about log Te = 4A5, remarkably cooler than previous estimates. We find that such a theoretical prescription appears in good agreement with available observational data concerning both stellar temperatures and gravities. Finally, the problem of gaps in the HB distribution is discussed, suggesting that, at least in the case of the cluster NGC 6752, evolutionary gaps, a bimodal distribution of CNO, and stellar rotation should all play a negligible role. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175780 arXiv: arXiv:astro-ph/9407057 Bibcode: 1995ApJ...446..228C Keywords: GALAXY: GLOBULAR CLUSTERS: INDIVIDUAL NGC NUMBER: NGC 6752; STARS: AGB AND POST-AGB; STARS: EVOLUTION; STARS: HERTZSPRUNG-RUSSELL DIAGRAM; STARS: INTERIORS; STARS: POPULATION II; Astrophysics E-Print: postscript file of 10 pages plus 1 tables,rep.1 5 figures will be added later as postscript file The tex file and the other two not postscript figures are available upon request at scilla@vaxfe.fe.infn.it, rep.1 full text sources arXiv | ADS | data products SIMBAD (4)

The Age Spread Among Galaxies and the Origin of the UV Radiation from Elliptical Galaxies

Recent UV observations of elliptical galaxies are interpreted as evidence for the global second parameter phenomenon of horizontal-branch (HB) morphology within, as well as between, these galaxies. In this picture, the origin of the UV radiation is mostly due to hot HB stars and their post-HB progeny produced by the metal-poor tail of the wide metallicity distribution expected to be present in these systems. The attractive feature of this model is that the bimodal temperature distributions of HB stars (and their progeny), required to generate the 2000 Å dip of the spectral energy distribution (SED), can naturally be reproduced from the standard HB population models with large range of metal abundance (see Lee 1994, ApJ, 430, L113). Detailed population synthesis models are presented, which reproduce the systematic variation of UV upturn among elliptical galaxies (Fig 1). If age is the major second parameter, as suggested by the fossil record in our Galaxy, the observed UV color gradient and the UV upturn-total mass (mean metallicity) correlation, within and between the early-type systems, would imply, respectively, (1) that most galaxies formed from the inside out, and (2) that there is age spread among galaxies, in the sense that more massive galaxies are older (and more metal-rich in the mean) than less massive galaxies as a result of more efficient star formation (and metal enrichment) in denser environments.

Far-ultraviolet imaging of the globular cluster NGC 6681 with WFPC2

We have imaged the globular cluster NGC 6681 in the far-UV and visible with WFPC2 on the HST. Our far-UV images show a sparsely populated and fully resolved central region, and we detect 122 stars. The far-UV to visible color-magnitude diagram shows a well-defined horizontal branch with no evidence for hot, more evolved descendants. We find one hot horizontal-branch star significantly below the model zero-age horizontal branch, but the rest are consistent with evolutionary models within uncertainties in calibration, distance, and reddening. The center of the cluster harbors two luminous blue stragglers. Our far-UV images graphically confirm that there is no steep density gradient at small radii among the horizontal-branch stars of this post-core-collapse cluster and show no evidence for significant color gradients.

Spectrophotometric evolution of elliptical galaxies. 1: Ultraviolet excess and color-magnitude-redshift relations

Spectrophotometric evolution of elliptical galaxies. 1: Ultraviolet excess and color-magnitude-redshift relations

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

This series of papers comprises a systematic exploration of the hypothesis that the far-ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the zero-age horizontal branch (ZAHB) through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short-wavelength (900-3000 A) observations

The ultraviolet-bright stars of Omega Centauri, M3, and M13

Two new UV-bright stars detected within 2 arcmin of the center of Omega Cen are spectroscopically investigated with the short-wavelength spectrograph of the IUE. The IUE spectra of the UV-bright stars UIT-1 and UIT-2 in the core of Omega Cen superficially resemble those of Population I mid-B stars. The absorption lines of the core UV-bright stars are significantly weaker than in Population I stars, consistent with their membership in the cluster. Synthetic spectra calculated from low-metallicity Kurucz model stellar atmospheres are compared with the spectra. These objects are insufficiently luminous to be classical hydrogen-burning post-AGB stars. They may be evolved hot horizontal branch stars which have been brightened by more than 3 mag since leaving the zero-age horizontal branch. It is inferred from the spectra and luminosity of the core UV-bright stars that similar objects could provide the source of the UV light in elliptical galaxies.

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

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)

Horizontal-branch stars in NGC 6752 with strong helium lines

view Abstract Citations (7) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Horizontal-Branch Stars in NGC 6752 with Strong Helium Lines Crocker, Deborah A. ; Rood, Robert T. ; Oconnell, Robert W. Abstract Two hot horizontal-branch (HB) stars have been observed in the globular cluster NGC 6752. Both stars have strong He I lines at 4026A which indicate an abundance which is similar to that of Population I stars. These are the only observations of globular cluster HB stars showing apparently normal helium abundance. How these observations may be related to some unsolved problems of horizontal-branch morphology is considered. Publication: The Astrophysical Journal Pub Date: October 1986 DOI: 10.1086/184753 Bibcode: 1986ApJ...309L..23C Keywords: Abundance; Globular Clusters; Helium; Horizontal Branch Stars; Line Spectra; Stellar Spectrophotometry; Stellar Rotation; Astrophysics; CLUSTERS: GLOBULAR; STARS: ABUNDANCES; STARS: HORIZONTAL-BRANCH; STARS: ROTATION full text sources ADS | data products SIMBAD (5)

Particle Transport in Non-Magnetic Stars

AbstractThe observations of AmFm, λ Booti, HgMn and He rich stars that are explained without any arbitrary parameter by diffusion are briefly reviewed, followed by those observations that are not explained by this simple model. Mass loss is then shown to explain a large fraction of the observations that are not explained in the parameter free model. It seems to play a role in the λ Booti, AmFm, He rich and the hot horizontal branch stars. It is only of about 10−15 to 10−13 Mo/yr. Abundance anomalies then help to determine stellar hydrodynamics. It is finally suggested that recent observations of Li underabundances in F stars of the Hyades represent an extension of the AmFm star phenomenon.

Lte-Blanketed and Unblanketed Models for SDO Stars

Sd0 stars have particular importance in astrophysics due to their position in the HR diagram between the hot horizontal-branch stars and the precursors of the white dwarfs. A careful determination of their parameters is therefore highly desirable.

Advanced Evolution in Globular Clusters. the Ultraviolet-Bright Stars in Eight Globular Clusters

view Abstract Citations (66) References (64) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Advanced Evolution in Globular Clusters. the Ultraviolet-Bright Stars in Eight Globular Clusters Zinn, Robert Abstract Thirty-eight ultraviolet-bright stars-i.e., stars that are brighter than the horizontal branch (HB) stars and bluer than the giant branch stars-in the globular clusters M3, M10, M13, M15, M53, M92, NGC 4147, and NGC 5466 have been observed spectroscopically at 130 A mm -`. Radial-velocity measurements indicate that 15 of these stars are cluster members. The radial velocity of NGC 5466 (+115 t 4 km ) was measured from the spectrograms of seven red giants. It is suggested that Mayall's radial velocity for M53 should be revised to -89 km s '. The effective temperatures of the ultraviolet-bright stars and the Cepheid variables with periods longer than 1 day have been derived from their broad-band colors or taken from the few published analyses. Their luminosities have been derived from the distance moduli adopted for the clusters and from bolometric corrections taken from the literature. A detailed comparison is made between post-HB evolutionary tracks and the positions of the ultraviolet- bright stars (including the Cepheid variables) in the H-R diagram. For the most part, the theoretical tracks provide adequate explanations for the ultraviolet-bright stars. This comparison has led to the following conclusions: (i) As predicted by the theory, a strong correlation exists between the presence of supra-horizontalbranch stars in a cluster and the presence of HB stars hotter than log = 4.1. (ii) The theoretical relationship between the blueness of the HB and the luminosities of the ultraviolet-bright stars with log LIL0 > 2.3 is supported by observation. (iii) The rare Population II Cepheid variables that have periods between 4 and 12 days are most likely stars that have evolved from the blue end of the HB (log Teff > 4.1). Therefore, their paucity is probably caused by the paucity of hot HB stars. Subject headings: Cepheids and W Virginis stars - globular clusters - stellar evolution Publication: The Astrophysical Journal Pub Date: November 1974 DOI: 10.1086/153197 Bibcode: 1974ApJ...193..593Z full text sources ADS | data products SIMBAD (38)

The Gaseous Component of the Peculiar Galaxy NGC 5253

view Abstract Citations (58) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Gaseous Component of the Peculiar Galaxy NGC 5253 Welch, Gary A. Abstract Results are reported of an extensive investigation of the galaxy NGC 5253'. The absolute magnitude and mass are estimated and are found to compare closely with those of NGC 205 and M32 of the Local Group. The distance modulus is approximately 26.8. NGC 5253 does not closely resemble a Seyfert galaxy as was suggested in a preliminary study. The distribution of stellar surface brightness and the lack of resolution appear to mimic an elliptical system. However, complex structure near the center argues against the existence of a relaxed stellar component there. Furthermore, ionized gas is found throughout the central region out to distances of 15"-20"-a considerable fraction of the entire galaxy is enhanced by emission. The condition of the gas more closely resembles that of galactic H ii regions than of planetary nebulae or Seyfert nuclei. About 1O BO V stars or 1O hot horizontal-branch stars are needed to produce the nebular emission. An abnormally high N/O abundance ratio in the gas suggests that stellar evolutionary processes have enriched it. A model of the emission nebula indicates that the mass ratio (H ii)/(total) is roughly 0.003, and the ratio (H i)/(total) may be 10 times as large. Thus the gas content far exceeds that in ordinary ellipticals, and in fact is on the same order as that of the Iagellanic Clouds. Publication: The Astrophysical Journal Pub Date: September 1970 DOI: 10.1086/150586 Bibcode: 1970ApJ...161..821W full text sources ADS | data products NED (7)