Mixing, Enhanced Helium and Blue Tails in Globular Clusters

We investigate the consequences of an increase in the envelope helium abundance of pre-helium flash red giants in globular clusters. Comparing predictions with the CM diagrams of a few crucial GC, one finds no evidence for a substantial increase in the surface helium content of HB members of these clusters, at least for objects in the RR Lyrae region or close to it. The possibility that the most peculiar giants belong to the asymptotic giant branch is discussed. The consequences of a delay in the helium flash are briefly examined

Helium self--enrichment and the second parameter problem in M3 and M13

Inspection of the CM diagrams of globular clusters having similar heavy element content shows that the luminosity of the red giant bump relative to the turnoff (Delta V_{TO}^{bump}) differs by more than 0.1 mag between clusters with different horizontal branch morphology. Unfortunately, careful consideration of the data leaves us with only one pair (M3 and M13) of clusters good for a quantitative discussion. For this pair we consider differences in age and helium content as possible causes for the difference in Delta V_{TO}^{bump}, and find more convincing support for the latter. A larger helium content in M13 stars (Y~0.28 vs. Y~0.24) accounts for various CM diagram features, such as the difference in the luminosity level of RR Lyr variables and of the red giant bump with respect to the turnoff luminosity, and the horizontal branch morphology. This enhanced helium can be tentatively understood in the framework of self--enrichment by massive asymptotic giant branch stars in the first ~100 Myr of the cluster life. A modest self--enrichment can be present also in M3 and can be the reason for the still unexplained presence of a not negligible number of luminous, Oosterhoff II type RR Lyr variables. The hypothesis that a larger helium content is the second parameter for clusters with very blue horizontal branch morphology could be checked by an accurate set of data for more clusters giving turnoff, RR Lyrs and bump magnitudes within a unique photometry.Comment: in press on Astronomy and Astrophysic

In search of massive single-population Globular Clusters

Most Globular Clusters so far examined host (at least) two stellar populations. This feature requires a two--step process, in which the nuclearly processed matter from a first generation (FG) of stars gives birth to a second generation (SG) bearing the fingerprint of a fully CNO-cycled matter. Since the present population of most globular clusters is made up largely of SG stars, a substantial fraction of the FG (>~90%) must be lost. Nevertheless, two types of clusters dominated by a simple stellar population (FG clusters) should exist: either clusters initially too small to be able to retain a cooling flow and form a SG (FG-only clusters), or massive clusters that could retain the CNO processed ejecta and form a SG, but were unable to lose a significant fraction of their FG (mainly-FG clusters). We attempt a classification of FG clusters, based on the morphology of their horizontal branches (HBs), as displayed in photomectric catalogues for 106 clusters. FG candidates are the clusters in which the HB can be reproduced by the evolution of an almost unique mass. <20% of clusters with [Fe/H]<-0.8 appear to be FG, but only ~10% probably had a mass sufficient to form at all an SG. This small percentage confirms on a wider database that the SG is a dominant constituent of today's clusters, suggesting that its formation is an ingredient necessary for the survival of globular clusters during their dynamical evolution in the Galactic tidal field. Pal3 turns out to be a good example of FG-only cluster. HB simulations and space distribution of its components, indicate that M 53 is a "mainly-FG" cluster. Mainly-FG candidates may be also NGC5634, NGC5694 and NGC6101. In contrast, NGC 2419 contains >30% of SG stars, and its present dynamical status bears less information on its formation process than the analysis of the chemical abundances of its stars and of its HB morphology.Comment: 11 pages, 7 figures, accepted for publication in MNRA

A Superwind from Early Post-Red Giant Stars?

We suggest that the gap observed at 20,000 K in the horizontal branches of several Galactic globular clusters is caused by a small amount of extra mass loss which occurs when stars start to "peel off" the red giant branch (RGB), i.e., when their effective temperature starts to increase, even though they may still be on the RGB. We show that the envelope structure of RGB stars which start to peel off is similar to that of late asymptotic giant branch stars known to have a super-wind phase. An analogous super-wind in the RGB peel-off stars could easily lead to the observed gap in the distribution of the hottest HB stars.Comment: 9 pages; Accepted by ApJ Letters; Available also at http://www.astro.puc.cl/~mcatelan

The puzzle of metallicity and multiple stellar populations in the Globular Clusters in Fornax

We examine the photometric data for Fornax clusters, focussing our attention on their horizontal branch color distribution and, when available, on the RR Lyr variables fraction and period distribution. Based on our understanding of the HB morphology in terms of varying helium content in the context of multiple stellar generations, we show that clusters F2, F3 and F5 must contain substantial fractions of second generation stars (~54-65%). On the basis of a simple chemical evolution model we show that the helium distribution in these clusters can be reproduced by models with cluster initial masses ranging from values equal to ~4 to ~10 times larger than the current masses. Models with a very short second generation star formation episode can also reproduce the observed helium distribution but require larger initial masses up to about twenty times the current mass. While the lower limit of this range of possible initial GC masses is consistent with those suggested by the observations of the low metallicity field stars, we also discuss the possibility that the metallicity scale of field stars (based on CaII triplet spectroscopy) and the metallicities derived for the clusters in Fornax may not be consistent with each other. The reproduction of the HB morphology in F2,F3,F5 requires two interesting hypotheses: 1) the first generation HB stars lie all at "red" colours. According to this interpretation, the low metallicity stars in the field of Fornax, populating the HB at colours bluer than the blue side ((V-I)o<=0.3 or (B-V)o<=0.2) of the RR Lyrs, should be second generation stars born in the clusters;a preliminary analysis of available colour surveys of Fornax field provides a fraction ~20% of blue HB stars, in the low metallicity range; 2) the mass loss from individual second generation red giants is a few percent of a solar mass larger than the mass loss from first generation stars.Comment: 14 pages, 8 figures. Accepted for publication in MNRA

Is mass loss along the red giant branch of globular clusters sharply peaked? The case of M3

There is a growing evidence that several globular clusters must contain multiple stellar generations, differing in helium content. This hypothesis has helped to interpret peculiar unexplained features in their horizontal branches. In this framework we model the peaked distribution of the RR Lyr periods in M3, that has defied explanation until now. At the same time, we try to reproduce the colour distribution of M3 horizontal branch stars. We find that only a very small dispersion in mass loss along the red giant branch reproduces with good accuracy the observational data. The enhanced and variable helium content among cluster stars is at the origin of the extension in colour of the horizontal branch, while the sharply peaked mass loss is necessary to reproduce the sharply peaked period distribution of RR Lyr variables. The dispersion in mass loss has to be <~ 0.003 Msun, to be compared with the usually assumed values of ~0.02 Msun. This requirement represents a substantial change in the interpretation of the physical mechanisms regulating the evolution of globular cluster stars.Comment: Accepted for publication in The Astrophysical Journa

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of eighteen blue horizontal-branch (BHB) stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10 to 30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ

Helium variation due to self-pollution among Globular Cluster stars: consequences on the horizontal branch morphology

It is becoming clear that `self--pollution' by the ejecta of massive asymptotic giant branch stars has an important role in the early chemical evolution of globular cluster stars, producing CNO abundance spreads which are observed also at the surface of unevolved stars. Considering that the ejecta which are CNO processed must also be helium enriched, we have modelled stellar evolution of globular cluster stars by taking into account this possible helium enhancement with respect to the primordial value. We show that the differences between the main evolutionary phases (main sequence, turn--off and red giants) are small enough that it would be very difficult to detect them observationally. However, the difference in the evolving mass may play a role in the morphology of the horizontal branch, and in particular in the formation of blue tails, in those globular clusters which show strong CNO abundance variations, such as M13 and NGC 6752

Infrared Photometry of Red Supergiants in Young Clusters in the Magellanic Clouds

We present broad-band infrared photometry for 52 late-type supergiants in the young Magellanic Clouds clusters NGC 330, NGC 1818, NGC 2004 and NGC 2100. Standard models are seen to differ in the temperature they predict for the red supergiant population on the order of 300K. It appears that these differences most probably due to the calibration of the mixing-length parameter, $\alpha_{P}$, in the outermost layers of the stellar envelope. Due to the apparent model dependent nature of $\alpha_{P}$ we do not quantitatively compare $\alpha_{P}$ between models. Qualitatively, we find that $\alpha_{P}$ decreases with increased stellar mass within standard models. We do not find evidence for a metallicity dependence of $\alpha_{P}$.Comment: 11 pages, 4 figures. AJ accepte