Horizontal Branch In Globular Clusters Research Articles

Advancing globular cluster constraints on the axion-photon coupling

We improve the current upper bound on the axion-photon coupling derived from stellar evolution using the R 2 parameter, the ratio of stellar populations on the Asymptotic Giant Branch to Horizontal Branch in Globular Clusters. We compare this with data from simulations using the stellar evolution code MESA which include the effects of axion production. Particular attention is given to quantifying in detail the effects of uncertainties on the R and R 2 parameters due to the modelling of convective core boundaries. Using a semiconvective mixing scheme we constrain the axion-photon coupling to be gaγγ < 0.47 × 10-10 GeV-1. This rules out new regions of QCD axion and axion-like particle parameter space. Complementary evidence from asteroseismology suggests that this could improve to as much as gaγγ < 0.34 × 10-10 GeV-1 as the uncertainties surrounding mixing across convective boundaries are better understood.

The second and third parameters of the horizontal branch in globular clusters

The Horizontal Branch (HB) second parameter of Globular Clusters (GCs) is a major open issue in stellar evolution. Large photometric and spectroscopic databases allow a re-examination of this issue. We derive median and extreme (90% of the distribution) colours and magnitudes of stars along the HB for about a hundred GCs. We transform these into median and extreme masses of stars on the HB taking into account evolutionary effects, and compare these masses with those expected at the tip of the Red Giant Branch to derive the total mass lost by the stars. A simple linear dependence on metallicity of this total mass lost explains well the median colours of HB stars. Adopting this mass loss law as universal, we find that age is the main second parameter. However, at least a third parameter is clearly required. The most likely candidate is the He abundance, which might be different in GCs stars belonging to the different stellar generations whose presence was previously derived from the Na-O and Mg-Al anticorrelations. Variations in the median He abundance allow explaining the extremely blue HB of some GCs; such variations are correlated with the R-parameter. Suitable He abundances allow deriving ages from the HB which are consistent with those obtained from the Main Sequence. Small corrections to these latter ages are then proposed, producing a tight age-metallicity relation for disk and bulge GCs. Star-to-star variations in the He content explain the extension of the HB. There is a strong correlation between this extension and the interquartile of the Na-O anticorrelation. The main driver for the variations in the He-content within GCs seems the total cluster mass. 47 Tuc and M3 exhibit exceptional behaviours; however, they can be accommodated in a scenario for the formation of GCs that relates their origin to cooling flows generated after very large episodes of star formation.

Multivariate analysis of globular cluster horizontal branch morphology: searching for the second parameter

Aims. The interpretation of globular cluster horizontal branch (HB) morphology is a classical problem that can significantly blur our understanding of stellar populations. Methods. We present a new multivariate analysis connecting the effective temperature extent of the HB with other cluster parameters. The work is based on Hubble Space Telescope photometry of 54 Galactic globular clusters. Results. The present study reveals the important role of the total mass of the globular cluster on its HB morphology. More massive clusters tend to have HBs more extended to higher temperatures. For a set of three input variables including the temperature extension of the HB, [Fe/H] and M_V, the first two eigenvectors account for 90% of the total sample variance. Conclusions. Possible effects of cluster self-pollution on HB morphology, stronger in more massive clusters, could explain the results derived here.

A New Version of Reimers' Law of Mass Loss Based on a Physical Approach

We present a new semiempirical relation for the mass loss of cool stellar winds, which so far has frequently been described by Reimers' law. Originally, this relation was based solely on dimensional scaling arguments without any physical interpretation. In our approach, the wind is assumed to result from the spillover of the extended chromosphere, possibly associated with the action of waves, especially Alfven waves, which are used as guidance in the derivation of the new formula. We obtain a relation akin to the original Reimers law, but which includes two new factors. They reflect how the chromospheric height depends on gravity and how the mechanical energy flux depends, mainly, on the effective temperature. The new relation is tested and sensitively calibrated by modeling the blue end of the horizontal branch of globular clusters. The most significant difference from mass-loss rates predicted by the Reimers relation is an increase by up to a factor of 3 for luminous late-type (super)giants, in good agreement with observations.

TheuBVIPhotometric System. I. Motivation, Implementation, and Calibration

This paper describes the design principles for a CCD-based photometric system that is highly optimized for ground-based measurement of the size of the Balmer jump in stellar energy distributions. It is shown that, among ultraviolet filters in common use, the Thuan-Gunn u filter is the most efficient for this purpose. This filter is combined with the standard Johnson-Kron-Cousins B, V, and I bandpasses to constitute the uBVI photometric system. Model stellar atmospheres are used to calibrate color-color diagrams for the uBVI system in terms of the fundamental stellar parameters of effective temperature, surface gravity, and metallicity. The u-B index is very sensitive to log g, but also to [Fe/H]. It is shown that an analog of the Str\omgren c_1 index, defined as (u-B)-(B-V), is much less metallicity dependent, but still sensitive to log g. The effect of interstellar reddening on u-B is determined through synthetic photometric calculations, and practical advice is given on dealing with flat fields, atmospheric extinction, the red leak in the u filter, and photometric reductions. The uBVI system offers a wide range of applicability in detecting stars of high luminosity in both young (yellow supergiants) and old (post-AGB stars) populations, using stars of both types as standard candles to measure extragalactic distances with high efficiency, and in exploring the horizontal branch in globular clusters. In many stellar applications, it can profitably replace the classical uBVI system. Paper II in this series will present a network of well-calibrated standard stars for the uBVI system.

The accretion of brown dwarfs and planets by giant stars -- II. Solar-mass stars on the red giant branch

This paper extends our previous study of planet/brown dwarf accretion by giant stars to solar mass stars located on the red giant branch. The model assumes that the planet is dissipated at the bottom of the convective envelope of the giant star. The giant's evolution is then followed in detail. We analyze the effects of different accretion rates and different initial conditions. The computations indicate that the accretion process is accompanied by a substantial expansion of the star, and in the case of high accretion rates, hot bottom burning can be activated. The possible observational signatures that accompany the engulfing of a planet are also extensively investigated. They include : the ejection of a shell and a subsequent phase of IR emission, an increase in the 7Li surface abundance and a potential stellar metallicity enrichment, spin-up of the star due to the deposition of orbital angular momentum, the possible generation of magnetic fields and a related X-ray activity due to the development of shear at the base of the convective envelope, and the effects on the morphology of the horizontal branch in globular clusters. We propose that the IR excess and high Li abundance observed in 4-8% of the G and K giants originate from the accretion of a giant planet, a brown dwarf or a very low-mass star.

Mass loss and AGB evolution in extra-galactic stellar populations

The effect of metallicity on the AGB mass loss is reviewed. Observations have mainly been limited to the Magellanic Clouds but are observationally feasible throughout the Local Group. Expansion velocities are predicted to depend strongly on Z but the peak mass-loss rates appear not to. The Mira PL relation shows no evidence for a Z dependence, giving a powerful potential distance indicator. I derive a distance modulus to the LMC of m − M = 18.63 ± 0.09 as well as a bright calibration for the Horizontal Branch in globular clusters. Finally, the predicted initial-final mass relation at low Z is shown to give rise to higher mass remnants. This would result in an increased supernova rate in young, low-Z populations, such as found in the early Universe.

Miras and Mass Loss in the Local Group

Evolved AGB stars pass through a phase of heavy mass loss during which they become optically obscured. We describe an observational program to find and study such stars in the Magellanic Clouds. Many of the obscured stars are found to be luminous carbon stars, a population previously missing. We discuss the Mira PL relation, valid before the onset of the heaviest mass loss. We find that a number of semi-regulars fit this relation, based on Hipparcos parallaxes. No evidence is seen for a Z-dependence, giving a powerful potential distance indicator. We derive a distance modulus to the LMC of m − M = 18.63 ± 0.09 as well as a bright calibration for the Horizontal Branch in globular clusters.

Bimodality and Gaps on Globular Cluster Horizontal Branches. II. The Cases of NGC 6229, NGC 1851, and NGC 2808

The outer halo globular cluster NGC 6229 has a peculiar horizontal-branch (HB) morphology, with clear indications of a bimodal HB and a gap on the blue HB. In this paper, we present extensive synthetic HB simulations to determine whether peculiar distributions in the underlying physical parameters are needed to explain the observed HB morphology. We find that a unimodal mass distribution along the HB can satisfactorily account for the observed HB bimodality, provided the mass dispersion is substantially larger than usually inferred for the Galactic globular clusters. In this case, NGC 6229 should have a well-populated, extended blue tail. A truly bimodal distribution in HB masses can also satisfactorily account for the observed HB morphology, although in this case the existence of an extended blue tail is not necessarily implied. The other two well-known bimodal-HB clusters, NGC 1851 and NGC 2808, are briefly analyzed. While the HB morphology of NGC 1851 can also be reproduced with a unimodal mass distribution assuming a large mass dispersion, the same is not true of NGC 2808, for which a bimodal, and possibly multimodal, mass distribution seems definitely required. The problem of gaps on the blue HB is also discussed. Applying the standard Hawarden and Newell χ2 test, we find that the NGC 6229 gap is significant at the 99.7% level. However, in a set of 1000 simulations, blue-HB gaps comparable to the observed one are present in ~6%-9% of all cases. We employ a new and simple formalism, based on the binomial distribution, to explain the origin of this discrepancy, and conclude that Hawarden's method, in general, substantially overestimates the statistical significance of gaps.

Third parameter effects in globular clusters

We show that the apparent correlations between the length of the globular cluster horizontal branch and cluster luminosity and density [Fusi Pecci et al. AJ, 105, 1145 (1993)] may be affected by observational bias. This bias results from the fact that one can trace horizontal branches over greater lengths in rich clusters than in poor ones. The existence of a correlation between the length of the horizontal branch and the cluster central density is confirmed at about the 2σ level of confidence. The strength of an apparent correlation between cluster luminosity M v and the length of the horizontal branch is found to depend critically on the choice of sample used to analyze the data

The Oosterhoff Period Effect and the Age of the Galactic Globular Cluster System

AbstractThe Oosterhoff division of globular clusters into two dichotomous mean period groups is a result of the variation with metallicity of the combined effects of (1) a mean increase in period with decreasing metallicity, and (2) the change of globular cluster horizontal branch (HB) morphology from the M3 to the M13 HB type within the instability strip in the metallicity range of [Fe/H] between -1.7 and -1.9. A new representation of the Oosterhoff period effect showing this property is made from the individual cluster data in Figure 1. The relation between period and metallicity for cluster and for field RR Lyraes at the blue fundamental edge of the instability strip in the HR diagram as read from this figure islog Pab = -0.122(±0.02)([Fe/H]) - 0.500(±0.01)using the metallicity scale of Butler.The high slope coefficient is consistent with the extant models of the HB when they are read at the varing temperature of the fundamental blue edge given by equation (3) of the text. Most of the current literature treats only the constant temperature condition, which is manifestly incorrect. It is this temperature effect that reconciles the observations and the models.A new calibration of the absolute magnitudes of RR Lyrae stars as a function of metallicity, combined with new oxygen enhanced isochrones for globular clusters (Bergbusch &amp; VandenBerg 1992) reduces the age of the Galactic globular cluster system to 14.1 ± 0.3 Gyr (internal error). The resulting lower age of the universe which, when combined with a Hubble constant near 50 km s-1 Mpc-1 determined from type I supernovae, shows that the cosmological expansion has been decelerated by an amount consistent with the closure density, permitting Ω ∼ 1 now from the timing test.

HORIZONTAL BRANCH A STARS. II. THE TEMPERATURE SCALE AND A DISCUSSION OF THE SURFACE GRAVITIES

ABSTRACT Effective temperatures and surface gravities have been derived for field and globular-cluster horizontal-branch A stars. The values are obtained by fitting spectrum scans to the energy distributions calculated by ATLAS model atmospheres (Kurucz, 1979, 1981). The validity and accuracy of the results are discussed.

LIGHT VARIATIONS OF THE POPULATION II F-TYPE SUPERGIANT HD 46703.

ABSTRACT Photometric monitoring has revealed brightness variations of 0m.1 on a time scale of weeks for HD 46703, a metal-deficient F-type field analog of the stars lying above the horizontal branch in globular clusters. The authors suggest that HD 46703 belongs to the "89 Her" class of luminous F-type variables. Since HD 46703 is unquestionably a halo object, it is almost certainly a low-mass star. The authors suggest that it, and probably the other 89 Her variables, are masquerading as supergiants during their final evolution off the asymptotic giant branch.

Forced mixing in stars

It is suggested that in various stages of stellar evolution the generally neglected gravitational potential fluctuations, generated by convective density fluctuations, can induce mixing exterior to violent convection zones. Special consideration is given to stars undergoing a helium shell flash; especially in degenerate conditions. It is conjectured that the extent of some globular cluster horizontal branches is due not to stochastic mass loss on the red giant branch but, in large part, to stochastic and marginal forced mixing effects at the time of the helium flash.

Model for horizontal branch of globular clusters

Model for horizontal branch of globular clusters

Horizontal branch in globular clusters of the galaxy and the magellanic clouds in sculptor-type dwarf galaxies

Despite the sparsity of data on globular clusters in the Magellanic Clouds and Sculptor-type dwarf galaxies, a preliminary comparison of them with globular clusters of the Galaxy can be made. 1) If observational selection does not seriously distort the results, there is a higher fraction of young globular clusters among those in the Magellanic Clouds that have a horizontal branch than among the globular clusters of the Galaxy. In addition, the clusters in Lhe Large Cloud are older than those in the Small Cloud. 2) To the know similarities between globular clusters and Sculptor-type dwarf elliptic galaxies one can add the following characteristics of the latter: a) the α distribution, b) the values of r, and c) position on the α-r diagram. If these parametess reflect the history of development of the corresponding systems, this similarity suggests that the observed stage of stellar evolution is similar in Sculpto r-type globular clusters.

Color of the horizontal branch of globular clusters

Color of the horizontal branch of globular clusters

On the Blue End of the Horizontal Branch in Globular Clusters

On the Blue End of the Horizontal Branch in Globular Clusters

Surface Helium Abundance in Horizontal Branch and Halo B Stars

THE surface helium abundance of stars at the blue end of the horizontal branch in globular clusters and of halo B stars has been estimated by spectroscopists to be lower than that at the Sun's surface by factors ranging from 10 (refs. 1 and 2) to 100 (ref. 3). It is assumed that the halo B stars are the field-equivalents of horizontal branch stars, and we hereafter refer to both classes as blue horizontal branch stars.

The Horizontal Branch in Globular Clusters.

The Horizontal Branch in Globular Clusters.