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Journal ArticleDOI

Metal-poor stars. III - On the evolution of horizontal-branch stars

TL;DR: Horizontal branch stars evolution based on mass distributions from comparison with giant branch, investigating evolutionary track characteristics during core helium burning was studied in this paper, where the authors proposed a model of the evolution of horizontal branch stars.
Abstract: Horizontal branch stars evolution based on mass distributions from comparison with giant branch, investigating evolutionary track characteristics during core helium burning
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Journal ArticleDOI
TL;DR: In this paper, a reassessment of the Oosterhoff dichotomy and the distribution of globular clusters in the horizontal branch (HB) morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC.
Abstract: We review and discuss horizontal branch (HB) stars in a broad astrophysical context, including both variable and non-variable stars A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics We show that the Oosterhoff dichotomy and the distribution of globular clusters in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC—an argument which, due to its strong reliance on the ancient RR Lyrae stars, is essentially independent of the chemical evolution of these systems after the very earliest epochs in the Galaxy’s history Convenient analytical fits to isochrones in the HB type–[Fe/H] plane are also provided In this sense, a rediscussion of the second-parameter problem is also presented, focusing on the cases of NGC 288/NGC 362, M13/M3, the extreme outer-halo globular clusters with predominantly red HBs, and the metal-rich globular clusters NGC 6388 and NGC 6441 The recently revived possibility that the helium abundance may play an important role as a second parameter is also addressed, and possible constraints on this scenario discussed We critically discuss the possibility that the observed properties of HB stars in NGC 6388 and NGC 6441 might be accounted for if these clusters possess a relatively minor population of helium-enriched stars A technique is proposed to estimate the HB types of extragalactic globular clusters on the basis of integrated far-UV photometry The importance of bright type II Cepheids as tracers of faint blue HB stars in distant systems is also emphasized The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyr, is also revisited Taking into due account the evolutionary status of RR Lyr, the derived relation implies a true distance modulus to the LMC of (m–M)0=1844±011 Techniques providing discrepant slopes and zero points for the MV(RRL)–[Fe/H] relation are briefly discussed We provide a convenient analytical fit to theoretical model predictions for the period change rates of RR Lyrae stars in globular clusters, and compare the model results with the available data Finally, the conductive opacities used in evolutionary calculations of low-mass stars are also investigated

320 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that far-ultraviolet radiation is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution.
Abstract: ▪ Abstract Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in elliptical galaxies and early-type spiral bulges. It is the most variable photometric feature associated with old stellar populations. Recent observational and theoretical evidence shows that it is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution. These are probably descendants of the dominant, metal rich population of the galaxies. Their lifetime UV outputs are remarkably sensitive to their physical properties and hence to the age and the helium and metal abundances of their parents. UV spectra are therefore exceptionally promising diagnostics of old stellar populations, although their calibration requires a much improved understanding of giant branch mass loss, helium enrichment, and atmospheric diffusion.

292 citations


Cites background from "Metal-poor stars. III - On the evol..."

  • ...Detailed evolutionary tracks for these phases are shown, for example, by Dorman et al (1993). is MENV, the hotter is the ZAHB location (e.g. Iben & Rood 1970, Rood 1973, Sweigart 1987, Dorman 1992)....

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Journal ArticleDOI
TL;DR: In this article, a nonlinear, time-dependent convective hydrodynamical model of RR Lyrae stars is presented, assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z = 0.0001?0.02).
Abstract: We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z = 0.0001?0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He-burning horizontal-branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental (FU) and first overtone polsators (FOs). We predict the topology of the instability strip (IS) as a function of the metal content and new analytical relations for the edges of the IS in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period?Radius?Metallicity relations for FU and FO pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (UBVRI) and near-infrared (NIR; JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and in turn new and homogenous metal-dependent (RIJHK) Period?Luminosity relations. Moreover, we compute new dual and triple-band optical, optical?NIR, and NIR Period?Wesenheit?Metallicity relations. Interestingly, we find that the optical Period-W(V, B?V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic data sets.

215 citations


Cites background from "Metal-poor stars. III - On the evol..."

  • ...This means that the computation of pulsation model does require the knowledge of the mass-luminosity relationof the stellar structures we are dealing with. van Albada & Baker in their seminal investigations adopted evolutionary prescriptions for HB stars by Iben & Rood (1970)....

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Journal ArticleDOI
TL;DR: In this paper, a model of the galaxy is constructed and evolved in which the integrated influence of stellar and supernova nucleosynthesis on the composition of the interstellar gas is traced numerically.
Abstract: A model of the galaxy is constructed and evolved in which the integrated influence of stellar and supernova nucleosynthesis on the composition of the interstellar gas is traced numerically. Our detailed assumptions concerning the character of the matter released from evolving stars and supernovae are guided by the results of recent stellar evolutionary calculations and hydrodynamic studies of supernova events. It is difficult to visualize an epoch of massive star formation in the collapsing gas cloud which formed our galaxy which would enrich the gas rapidly enough to account for the level of heavy element abundances in halo population stars; we have therefore proposed a stage of star formation which is entirely pregalactic in character. We suggest that the Jeans' length-sized initial condensations in the expanding universe discussed by Peebles and Dicke may provide the appropriate setting for this first generation of stars. Guided by these considerations, and by the need for a substantial quantity of 'unseen' mass to bind our local group of galaxies, we have constructed a model of the galaxy in which this violent early phase of massive star formation produces both (1) approximately 25% of the level of heavy elements observed in the solar system and (2) an enormous unseen mass in the form of black holes.

214 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that a moderate 12C(α,γ)16O reaction rate is sufficient to account for a high central oxygen abundance in He-burning stellar models.
Abstract: Pulsating white dwarfs provide constraints to the evolution of progenitor stars. We revise He-burning stellar models, with particular attention to core convection and to its connection with the nuclear reactions powering energy generation and chemical evolution. Theoretical results are compared to the available measurements for the variable white dwarf GD 358, which indicate a rather large abundance of central oxygen (Metcalfe and coworkers). We show that the attempt to constrain the relevant nuclear reaction rate by means of the white dwarf composition is faced with a large degree of uncertainty related to evaluating the efficiency of convection-induced mixing. By combining the uncertainty of the convection theory with the error on the relevant reaction rate, we derive that the present theoretical prediction for the central oxygen mass fraction in white dwarfs varies between 0.3 and 0.9. Unlike previous claims, we find that models taking into account semiconvection and a moderate 12C(α,γ)16O reaction rate are able to account for a high central oxygen abundance. The rate of the 12C(α,γ)16O used in these models agrees with the one recently obtained in laboratory experiments by Kunz and coworkers. On the other hand, when semiconvection is inhibited, as in the case of classical models (bare Schwarzschild criterion) or in models with mechanical overshoot, an extremely high rate of the 12C(α,γ)16O reaction is needed to account for a large oxygen production. Finally, we show that the apparent discrepancy between our result and those reported in previous studies depends on the method used to avoid the convective runaways (the so-called breathing pulses) that are usually encountered in modeling late stage of core He-burning phase.

149 citations