Branch Stars In Globular Clusters Research Articles

Correction to: Metallicity relations in LMC and SMC from the slope of red giant branch stars in globular cluster

Correction to: Metallicity relations in LMC and SMC from the slope of red giant branch stars in globular cluster

Metallicity relations in LMC and SMC from the slope of red giant branch stars in globular clusters

Metallicity relations in LMC and SMC from the slope of red giant branch stars in globular clusters

Photophilic hadronic axion from heavy magnetic monopoles

We propose a model for the QCD axion which is realized through a coupling of the Peccei-Quinn scalar field to magnetically charged fermions at high energies. We show that the axion of this model solves the strong CP problem and then integrate out heavy magnetic monopoles using the Schwinger proper time method. We find that the model discussed yields axion couplings to the Standard Model which are drastically different from the ones calculated within the KSVZ/DFSZ-type models, so that large part of the corresponding parameter space can be probed by various projected experiments. Moreover, the axion we introduce is consistent with the astrophysical hints suggested both by anomalous TeV-transparency of the Universe and by excessive cooling of horizontal branch stars in globular clusters. We argue that the leading term for the cosmic axion abundance is not changed compared to the conventional pre-inflationary QCD axion case for axion decay constant fa> 1012 GeV.

Lithium abundance in lower red giant branch stars of Omega Centauri

We present Li, Na, Al, and Fe abundances of 199 lower red giant branch star members of the stellar system Omega Centauri, using high-resolution spectra acquired with FLAMES at the Very Large Telescope. The A(Li) distribution is peaked at A(Li) ∼ 1 dex with a prominent tail towards lower values. The peak of the distribution well agrees with the lithium abundances measured in lower red giant branch stars in globular clusters and Galactic field stars. Stars with A(Li) ∼ 1 dex are found at metallicities lower than [Fe/H] ∼ –1.3 dex but they disappear at higher metallicities. On the other hand, Li-poor stars are found at all metallicities. The most metal-poor stars exhibit a clear Li–Na anti-correlation, where about 30% of the sample have A(Li) lower than ∼0.8 dex, while these stars represent a small fraction of normal globular clusters. Most of the stars with [Fe/H] > –1.6 dex are Li poor and Na rich. The Li depletion measured in these stars is not observed in globular clusters with similar metallicities and we demonstrate that it is not caused by the proposed helium enhancements and/or young ages. Hence, these stars formed from a gas already depleted in lithium. Finally, we note that Omega Centauri includes all the populations (Li-normal/Na-normal, Li-normal/Na-rich, and Li-poor/Na-rich stars) observed, to a lesser extent, in mono-metallic GCs.

Synthetic photometry of globular clusters: Uncertainties on synthetic colors

Context. Synthetic photometry is a great tool for studying globular clusters, especially for understanding the nature of their multiple populations. Aims. Our goal is to quantify the errors on synthetic photometry that are caused by uncertainties on stellar and observational/calibration parameters. These errors can be taken into account when building synthetic color-magnitude diagrams (CMDs) that are to be compared to observed CMDs. Methods. We have computed atmosphere models and synthetic spectra for two stars, Pollux and Procyon, that have stellar parameters typical of turn-off and bottom red giant branch stars in globular clusters. We then varied the effective temperature, surface gravity, microturbulence, the carbon, nitrogen, and oxygen abundances, and [Fe/H]. We quantified the effect on synthetic photometry in the following filters: Johnson UBVRI and HST F275W, F336W, F410M, F438W, F555W, F606W, and F814W. We also estimated the effects of extinction, atmospheric correction, and of the Vega reference spectrum on the resulting photometry. In addition, we tested the ability of our models to reproduce the observed spectral energy distribution and observed photometry of the two stars. Results. We show that variations are generally stronger in blue filters, especially those below 4500 Å. Dispersions on synthetic colors due to uncertainties on stellar parameters vary between less than 0.01 and to 0.04 magnitude, depending on the choice of filters. Uncertainties on the zero points, the extinction law, or the atmospheric correction affect the resulting colors at a level of a few 0.01 magnitudes in a systematic way. The models reproduce the flux-calibrated spectral energy distribution of both stars well. Comparison between synthetic and observed UBVRI photometry shows a variable degree of (dis)agreement. The observed differences indicate that different reduction and calibration processes are performed to obtain respectively observed and synthetic photometry, and they call for publication of all the details of the reduction process to produce synthetic photometry at a 0.01 mag level, which is required to interpret observed CMDs.

The treatment of mixing in core helium-burning models – III. Suppressing core breathing pulses with a new constraint on overshoot

Theoretical predictions for the core helium burning phase of stellar evolution are highly sensitive to the uncertain treatment of mixing at convective boundaries. In the last few years, interest in constraining the uncertain structure of their deep interiors has been renewed by insights from asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of growth of helium-burning convective cores based on the higher buoyancy of material ingested from outside the convective core. In this paper we test the implications of such a limit for stellar models with a range of initial mass and metallicity. We find that the constraint on mixing beyond the Schwarzschild boundary has a significant effect on the evolution late in core helium burning, when core breathing pulses occur and the ingestion rate of helium is fastest. Ordinarily, core breathing pulses prolong the core helium burning lifetime to such an extent that models are at odds with observations of globular cluster populations. Across a wide range of initial stellar masses ($0.83 \leq M/\text{M}_\odot \leq 5$), applying the Spruit constraint reduces the core helium burning lifetime because core breathing pulses are either avoided or their number and severity reduced. The constraint suggested by Spruit therefore helps to resolve significant discrepancies between observations and theoretical predictions. Specifically, we find improved agreement for $R_2$, the observed ratio of asymptotic giant branch to horizontal branch stars in globular clusters; the luminosity difference between these two groups; and in asteroseismology, the mixed-mode period spacing detected in red clump stars in the \textit{Kepler} field.

ON THE METALLICITY DISTRIBUTION OF THE PECULIAR GLOBULAR CLUSTER M22

ABSTRACT In our previous study, we showed that the peculiar globular cluster (GC) M22 contains two distinct stellar populations, namely the Ca-w and Ca-s groups, which have different physical properties, chemical compositions, spatial distributions, and kinematics. We proposed that M22 was most likely formed via a merger of two GCs with heterogeneous metallicities in a dwarf galaxy environment and then later accreted to our Galaxy. In their recent study, Mucciarelli et al. claimed that M22 is a normal monometallic globular cluster without any perceptible metallicity spread among the two groups of stars, which challenges our results and those of others. We devise new strategies for the local thermodynamic equilibrium abundance analysis of red giant branch stars in GCs and show that there exists a spread in the iron abundance distribution in M22.

A lack of close binaries among hot horizontal branch stars in globular clusters

Models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars, but the observations of their analogues in globular clusters has posed new problems, while the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars. We search for binaries of period P<200 days among a sample of blue horizontal branch stars (Teff=12000-22000 K) in NGC2808, a cluster known to host three distinct stellar populations and a multimodal horizontal branch. The final sample consists of 64 targets. The radial velocity of the targets was measured in fourteen epochs, spanning a temporal interval of about 75 days. We detect no RV variable object among stars cooler than the photometric G1 gap at 17000 K, while two close (P<10 days) and two intermediate-period (P=10-50 days) systems are found among hotter targets. The close and intermediate-period binary fraction for stars cooler than the gap are fc<5% and fip<10%, respectively, with 95% confidence. The most probable values among hotter stars are fc~20% and fip~30%, but the 90%- confidence level intervals are large (6-42% and 11-72%, respectively). The G1 gap appears as a discontinuity in the binary faction, with a higher incidence of binaries among hotter stars, but a constant increase in f with temperature rather than a discontinuity cannot be excluded from our observations. We find that intermediate-period binaries, never investigated before among cluster HB stars, could play an important role, being more than ~15-20% of the hottest stars of our sample. Our results indicate that fc among hot HB stars is most probably higher for younger clusters, confirming the recently proposed age-fc relation. However, the large observed difference in binary fraction between clusters (e.g. NGC2808 and NGC6752) is still not reproduced by binary population synthesis models.

What we would like to know about extreme horizontal branch stars in globular clusters

In this contribution, we explore some open questions about Extreme Horizontal Branch (EHB) stars in globular clusters. In particular, we present the current status of the search for rapid pulsators, He-depleted stars, and close binaries, three kind of objects very common among field EHB stars, but which first surveys failed to detect in clusters. We also analyze how the lack of cluster EHB binaries can reconcile with theoretical expectations. We give special attention to the first close EHB binary discovered in a globular cluster, whose characteristics point to a very rare or even unique object. Finally, we analyze some recent puzzling results about spectroscopically derived masses, that could point to the presence of two distinct families of EHB’s in clusters. For all these topics, we present the recent advancement in knowledge, the results requiring more investigation, and what has still to be done to fix the unsolved problems, showing the main points of our studies, the aims of our works, and what we expect to obtain from them.

The formation of hot subdwarf stars and its implications for the UV-upturn phenomenon of elliptical galaxies

In this paper, we review the formation scenario for field hot subdwarf stars and extreme horizontal branch stars in globular clusters and discuss how the scenario helps us to understand the UV-upturn phenomenon of elliptical galaxies. It is widely accepted that field hot subdwarf stars originate from binary evolution via the following three channels, common envelope evolution channel for hot subdwarf binaries with short orbital periods, stable Roche lobe overflow channel for hot subdwarf binaries with long orbital periods, and the double helium white dwarf merger channel for single hot subdwarfs. Such a scenario can also explain the lack of binarity of extreme horizontal branch stars in globular clusters. We have applied, in an a priori way, the scenario to the study of UV-upturn phenomenon of elliptical galaxies via an evolutionary population synthesis approach and found that the UV-upturn can be naturally explained. This has major implications for understanding the evolution of UV-upturn and elliptical galaxies in general. In particular, it implies that the UV-upturn is not a sign of age, as had been postulated previously, and should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals. The above a priori UV-upturn model is supported by recent GALEX observations and has been applied to naturally explain the colours of both dwarf ellipticals and giant ellipticals without the requirement of dichotomy between their stellar population properties.

A lack of close binaries among hot horizontal branch stars in globular clusters

Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. The present paper extends our study of NGC6752 to M80 and NGC5986. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC5986 and 31 in M80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations, temperatures, gravities, and helium abundances. By means of a statistical analysis, we detected one EHB close binary candidate per cluster. In M80, the best estimate of the close binary EHB fraction is f=12%, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC5986, although our best estimate is rather low (f=25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC6752, our analysis of M80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature. Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations.

Self-enrichment by asymptotic giant branch stars in globular clusters: comparison between intermediate and high metallicities

We present theoretical evolutionary sequences of intermediate-mass stars (M = 3-6.5 M ⊙ ) with metallicity Z = 0.004. Our goal is to test whether the self-enrichment scenario by massive asymptotic giant branch stars may work for the high-metallicity globular clusters, after previous works by the same group showed that the theoretical yields by this class of objects can reproduce the observed trends among the abundances of some elements, namely the O-Al and O-Na anticorrelations, at intermediate metallicities, i.e. [Fe/H] = -1.3. We find that the increase in the metallicity favours only a modest decrease of the luminosity and the temperature at the bottom of the envelope for the same core mass, and also the efficiency of the third dredge-up is scarcely altered. On the contrary, differences are found in the yields, due to the different impact that processes with the same efficiency have on the overall abundance of envelopes with different metallicities. We expect the same qualitative patterns as in the intermediate-metallicity case, but the slopes of some of the relationships among the abundances of some elements are different. We compare the sodium-oxygen anticorrelation for clusters of intermediate metallicity (Z ≈ 10 -3 ) and clusters of metallicity large as in these new models. Although the observational data are still too scarce, the models are consistent with the observed trends, provided that only stars of M ≥ 5 M ⊙ contribute to self-enrichment.

Iron abundances of B-type post-asymptotic giant branch stars in globular clusters: Barnard 29 in M 13 and ROA 5701 in ω Cen*

High-resolution optical and ultraviolet (UV) spectra of two B-type post-asymptotic giant branch (post-AGB) stars in globular clusters, Barnard 29 in M 13 and ROA 5701 in ω Cen, have been analysed using model atmosphere techniques. The optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and the 2d-Coudé spectrograph on the 2.7-m McDonald telescope, while the UV observations are from the Goddard high-resolution spectrograph on the Hubble Space Telescope (HST). Abundances of light elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical spectra, while the UV data provide additional Fe abundance estimates from Fe iii absorption lines in the 1875–1900 Å wavelength region. A general metal underabundance relative to young B-type stars is found for both Barnard 29 and ROA 5701. These results are consistent with the metallicities of the respective clusters, as well as with previous studies of the objects. The derived abundance patterns suggest that the stars have not undergone a gas–dust separation, contrary to previous suggestions, although they may have evolved from the AGB before the onset of the third dredge-up. However, the Fe abundances derived from the HST spectra are lower than those expected from the metallicities of the respective clusters, by 0.5 dex for Barnard 29 and 0.8 dex for ROA 5701. A similar systematic underabundance is also found for other B-type stars in environments of known metallicity, such as the Magellanic Clouds. These results indicate that the Fe iii UV lines may yield abundance values which are systematically too low by typically 0.6 dex and hence such estimates should be treated with caution.

TheuBVIPhotometric System. II. Standard Stars

Paper I of this series described the design of a CCD-based photometric system that is optimized for ground-based measurements of the size of the Balmer discontinuity in stellar spectra. This "uBVI" system combines the Thuan-Gunn u filter with the standard Johnson-Kron-Cousins BVI filters and can be used to discover luminous yellow supergiants in extragalactic systems and post–asymptotic giant branch stars in globular clusters and galactic halos. In the present paper we use uBVI observations obtained on 54 nights with 0.9 m telescopes at Kitt Peak and Cerro Tololo to construct a catalog of standardized u magnitudes for standard stars taken from the 1992 catalog of Landolt. We describe the selection of our 14 Landolt fields and give details of the photometric reductions, including red leak and extinction corrections, transformation of all the observations onto a common magnitude system, and the establishment of the photometric zero point. We present a catalog of u magnitudes of 103 stars suitable for use as standards. We show that data obtained with other telescopes can be transformed to our standard system with better than 1% accuracy.

The $\Delta a$ research group

We summarize of more than 25 years of research with the three filter, intermediate-band, in more detail. As an outlook, we present a new approach to search for chemically peculiar horizontal branch stars in globular clusters and to detect stellar variability of various objects observed during our photometric observations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

An Historical Overview

Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (Teff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research.

Canonical Extra Mixing in Low‐Mass Red Giants

We have used the latest observational data on the evolutionary variations of the surface chemical composition in low-mass metal-poor stars, both in the field and in globular clusters, to constrain the basic properties of extra mixing in upper red giant branch (RGB) stars. Two different models of extra mixing have been incorporated into a stellar evolution code: a parametrical diffusion model and a model with rotation-induced turbulent diffusion. Application of the first model to the interpretation of the observed variations of the surface abundances of Li, C, and N and of the isotopic ratio 12C/13C in field stars has revealed that, for the majority of upper RGB stars, the depth and rate of extra mixing do not appear to vary appreciably from star to star. Furthermore, comparisons of our calculations with the results obtained by other authors show that at least the extra mixing depth does not seem to depend strongly on metallicity. Therefore, we propose to call this universal nonconvective mixing process canonical extra mixing. We also put forward the hypothesis that some of the upper RGB stars may be experiencing enhanced extra mixing, which is much faster (by a factor of ~100) and somewhat deeper than canonical extra mixing. This could explain the phenomenon of Li-rich giants. Enhanced extra mixing could also contribute to the O-Na and Mg-Al anticorrelations that are seen in some globular cluster red giants. A possible mechanism of extra mixing in upper RGB stars may be turbulent diffusion or/and meridional circulation induced by rotation. In this case, enhanced extra mixing requires rotational velocities that are ≈10 times as fast as those that are sufficient for the occurrence of canonical extra mixing. Observations do not exclude this possibility because (1) the dispersion in the surface rotational velocities of field Li-rich giants span a range of a factor of ~10 and (2) the extremely fast rotation of blue horizontal branch stars in globular clusters may require that their RGB precursors had been spun up appreciably by an external source. Star-to-star abundance variations in globular clusters may well have been produced as the result of both evolutionary and primordial processes. In the primordial scenario, the nuclearly processed material that is accreted by low-mass main-sequence stars may have originated primarily in earlier generations of massive asymptotic giant branch stars that had undergone hot bottom burning of their envelopes and partly in mass-losing upper RGB stars that had been just a bit more massive than the present-day main-sequence turnoff stars and had experienced extra mixing in the past.

Spherical planetary nebulae

By examining their mass loss history and their distribution in the galaxy, I argue that spherical planetary nebulae (PNe) form a special group among all planetary nebulae. The smooth surface brightness of most spherical PNe suggests that their progenitors did not go through a final intensive wind (FIW, also termed superwind) phase. While ∼70% of the PNe of all other PNe groups are closer to the galactic center than the sun is, only ∼30% of spherical PNe are; ∼70% of them are farther away from the galactic center. These, plus the well-known high scale height above the galactic plane of spherical PNe, suggest that the progenitors of spherical PNe are low mass stars having low metallicity. Although many stars have these properties, only ∼10% of all PNe are spherical. By comparing the galactic distribution of spherical PNe to the metallicity evolution in the galaxy, I find that the critical metallicity above which no spherical PNe are formed is [Fe/H] ∼ -0.4. I explain this as well as other properties of spherical PNe in the context of the companion model for shaping PNe, arguing that spherical PNe are formed from stars that have no close companion, stellar or substellar, orbiting them. I discuss the connection of the proposed scenario to the recent finding of extrasolar planets and to the presence of blue horizontal branch stars in globular clusters.

On the existence of differences in luminosity between horizontal branch stars in globular clusters and in the field

The discrepancy between a LONG distance scale derived from Hipparcos based distances to globular clusters via main sequence fitting to local subdwarfs, and a SHORT distance scale derived from the absolute magnitude of field RR Lyraes via statistical parallaxes and the Baade-Wesselink method could be accounted for whether an intrinsic difference of about 0.1-0.2 mag was found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters. In this paper we discuss the possible existence of such a systematic difference comparing the period-shifts observed for field and cluster RR Lyraes. Various approaches based on different parameters and data-sets for both cluster and field variables were used in order to establish the size of such a hypothetical difference, if any. We find that on the whole very small not significant differences exist between the period-metallicity distributions of field and cluster RR Lyraes, thus confirming with a more quantitative approach, the qualitative conclusions by Catelan (1998). This observational evidence translates into a very small difference between the horizontal branch luminosity of field and cluster stars, unless RR Lyraes in Globular Clusters are about 0.06 Mo more massive than field RR Lyrae at same metallicity, which is to be proven.