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

A Semi-Empirical Study of the Mass Distribution of Horizontal Branch Stars in M3 (NGC 5272)

Horizontal branch (HB) stars in globular clusters offer us a probe of the mass loss mechanisms taking place in red giants. For M3 (NGC 5272), different shapes for the HB mass distribution have been suggested, including Gaussian and sharply bimodal alternatives. Here we study the mass distribution of HB stars in M3 by comparing evolutionary tracks and photometric observations. Our approach is thus of a semi-empirical nature, describing as it does the mass distribution that is favored from the standpoint of canonical stellar evolutionary predictions for the distribution of stars across the CMD. We locate, for each individual HB star, the evolutionary track whose distance from the star's observed color and magnitude is a minimum. Artificial tests reveal that our method would be able to detect a bimodal mass distribution, if present. We study the impact of different procedures for taking into account the evolutionary speed, and conclude that they have but a small effect upon the inferred mass distribution. We find that a Gaussian shape, though providing a reasonable first approximation, fails to account for the detailed shape of M3's HB mass distribution: the latter may have skewness and kurtosis that deviate slightly from a perfectly Gaussian solution. Alternatively, the excess of stars towards the wings of the distribution may also be accounted for in terms of a bimodal distribution in which both the low- and the high-mass modes are normal, the former being significantly wider than the latter. However, we also show that the inferred distribution of evolutionary times is inconsistent with theoretical expectations. This result is confirmed on the basis of three independent sets of HB models, suggesting that the latter underestimate the effects of evolution away from the zero-age HB. (abridged)Comment: 12 pages, 13 figures. A&A, in pres

Discovery of a Tight Brown Dwarf Companion to the Low Mass Star LHS 2397a

Using the adaptive optics system, Hokupa'a, at Gemini-North, we have directly imaged a companion around the UKIRT faint standard M8 star, LHS 2397a (FS 129) at a separation of 2.96 AU. Near-Infrared photometry of the companion has shown it to be an L7.5 brown dwarf and confirmed the spectral type of the primary to be M8. We also derive a substellar mass of the companion of 0.068 Msun, although masses in the range (0.061-0.069) are possible, and the primary mass as 0.090 Msun (0.089-0.094). Reanalysis of archival imaging from HST has confirmed the secondary as a common proper motion object. This binary represents the first clear example of a brown dwarf companion within 4 AU of a low mass star and should be one of the first late L dwarfs to have a dynamical mass. As part of a larger survey of M8-L0 stars, this object may indicate that there is no ``brown dwarf desert'' around low mass primaries.Comment: 17 pages, 3 figures, Accepted for publication in the Astrophysical Journa

Hot Horizontal-Branch Stars: The Ubiquitous Nature of the "Jump" in Stromgren u, Low Gravities, and the Role of Radiative Levitation of Metals

A "jump" in the BHB distribution in the V, u-y CMD was recently detected in the GC M13. It is morphologically best characterized as a discontinuity in u, u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematically from (in the sense of appearing brighter and/or hotter than) canonical ZAHBs. We present u, y photometry of 14 GCs obtained with 3 different telescopes (Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whose HB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB, and other GC parameters. We suggest that the u-jump is a ubiquitous feature, intrinsic to all HB stars hotter than 11,500K. We draw a parallel between the ubiquitous nature of the u-jump and the problem of low measured gravities among BHB stars. We note that the "logg-jump" occurs over the same temperature range as the u-jump, and that it occurs in every metal-poor GC for which gravities have been determined--irrespective of [Fe/H], mixing history on the RGB, or any other GC parameters. Furthermore, the u-jump and the logg-jump are connected on a star-by-star basis. The two are likely different manifestations of the same physical phenomenon. We present a framework which may simultaneously account for the u-jump and the logg-jump. Reviewing spectroscopic data for several field BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidence that radiative levitation of heavy elements takes place at Teff>11,500 K, dramatically enhancing their abundances in the atmospheres of BHB stars in the "critical" temperature region. Model atmospheres taking diffusion effects into account are badly needed, and will likely lead to better overall agreement between canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update

Young Clusters in the Magellanic Clouds II

We present the results of a quantitative study of the degree of extension to the boundary of the classical convective core within intermediate mass stars. The basis of our empirical study is the stellar population of four young populous clusters in the Magellanic Clouds which has been detailed in Keller, Bessell & Da Costa (2000). The sample affords a meaningful comparison with theoretical scenarios with varying degrees of convective core overshoot and binary star fraction. Two critical properties of the population, the main-sequence luminosity function and the number of evolved stars, form the basis of our comparison between the observed data set and that simulated from the stellar evolutionary models. On the basis of this comparison we conclude that the case of no convective core overshoot is excluded at a 2 sigma level.Comment: 27 pages, 12 figures, AJ accepte

The early evolution of Globular Clusters: the case of NGC 2808

Enhancement and spread of helium among globular cluster stars have been recently suggested as a way to explain the horizontal branch blue tails, in those clusters which show a primordial spread in the abundances of CNO and other elements involved in advanced CNO burning (D'Antona et al. 2002). In this paper we examine the implications of the hypothesis that, in many globular clusters, stars were born in two separate events: an initial burst (first generation), which gives origin to probably all high and intermediate mass stars and to a fraction of the cluster stars observed today, and a second, prolonged star formation phase (second generation) in which stars form directly from the ejecta of the intermediate mass stars of the first generation. In particular, we consider in detail the morphology of the horizontal branch in NGC 2808 and argue that it unveils the early cluster evolution, from the birth of the first star generation to the end of the second phase of star formation. This framework provides a feasible interpretation for the still unexplained dichotomy of NGC 2808 horizontal branch, attributing the lack of stars in the RR Lyr region to the gap in the helium content between the red clump, whose stars are considered to belong to the first stellar generation and have primordial helium, and the blue side of the horizontal branch, whose minimum helium content reflects the helium abundance in the smallest mass (~4Msun)contributing to the second stellar generation. This scenario provides constraints on the required Initial Mass Function, in a way that a great deal of remnant neutron stars and stellar mass black holes might have been produced.Comment: 23 pages, 7 figures, in press on The Astrophysical Journa

Ultraviolet Imagery of NGC 6752: A Test of Extreme Horizontal Branch Models

We present a 1620 A image of the nearby globular cluster NGC 6752 obtained with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 mission of the Space Shuttle Endeavour in 1995 March. An ultraviolet-visible color-magnitude diagram (CMD) is derived for 216 stars matched with the visible photometry of Buonanno et al. (1986). This CMD provides a nearly complete census of the hot horizontal branch (HB) population with good temperature and luminosity discrimination for comparison with theoretical tracks. The observed data show good agreement with the theoretical zero-age horizontal branch (ZAHB) of Sweigart (1996) for an assumed reddening of E(B-V) = 0.05 and a distance modulus of 13.05. The observed HB luminosity width is in excellent agreement with the theoretical models and supports the single star scenario for the origin of extreme horizontal branch (EHB) stars. However, only four stars can be identified as post-EHB stars, whereas almost three times this many are expected from the HB number counts. If this effect is not a statistical anomaly, then some non-canonical effect may be decreasing the post-EHB lifetime. The recent non-canonical models of Sweigart (1996), which have helium-enriched envelopes due to mixing along the red giant branch, cannot explain the deficit of post-EHB stars, but might be better able to explain their luminosity distribution.Comment: 14 pages, AASTeX, includes 4 EPS figures ApJ Letters accepte

Discovery of > 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue HB

We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which places it among the top three most RR Lyrae-rich globular clusters known. Likely members of the cluster in our studied field, from our preliminary number counts, include about 130 fundamental-mode (RRab) pulsators, with = 0.548 d, and about 75 first-overtone (RRc) pulsators, with = 0.300 d. The average periods and the position of the RRab variables with well-defined light curves in the Bailey diagram both suggest that the cluster is of Oosterhoff type I. However, the morphology of the cluster's horizontal branch (HB) is strikingly similar to that of the Oosterhoff type II globular cluster M15 (NGC 7078), with a dominant blue HB component and a very extended blue tail. Since M15 and M62 differ in metallicity by about one dex, we conclude that metallicity, at a fixed HB type, is a key parameter determining the Oosterhoff status of a globular cluster and the position of its variables in the Bailey diagram.Comment: 5 pages, 4 figures. ApJ Letters, in pres

Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components

We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen. The central regions of the cluster were covered with a mosaic of F435W, F625W, and F658N-band data collected with ACS/HST. The outer reaches were covered with a large set of U,B,V,I-band data collected with the [email protected] ESO/MPI telescope. The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB stars and ~12,500 stars brighter than the subgiant branch and fainter than the RGB bumps. The HB morphology changes with the radial distance. The relative number of extreme HB stars decreases from ~30% to ~21% when moving from the center toward the outer regions of the cluster, while the fraction of less hot HB stars increases from ~62% to ~72%. We performed a detailed comparison between observed ratios of different stellar tracers and predictions based on canonical evolutionary models with a primordial helium (Y=0.23) content and metal abundances (Z=0.0002,0.001) that bracket the observed spread in metallicity of Omega Cen stars. We found that the empirical star counts of HB stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB compared with the rate of RG and MSTO stars supports the evidence that we are facing a true excess of HB stars. The same comparison was performed by assuming a mix of stellar populations made with 70% of canonical stars and 30% of He-enhanced stars. The discrepancy between theory and observations decreases by a factor of two when compared with rates predicted by canonical He content models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed. Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and 30% (Y=0.33) larger than predicted lifetime ratios.Comment: 54 pages, 17 figures,to be published in ApJ, see link at http://stellari.wiki.zoho.co

NGC 2419: a large and extreme second generation in a currently undisturbed cluster

We analyse complementary HST and SUBARU data for the globular cluster NGC 2419. We make a detailed analysis of the horizontal branch (HB), that appears composed by two main groups of stars: the luminous blue HB stars ---that extend by evolution into the RR Lyrae and red HB region--- and a fainter, extremely blue population. We examine the possible models for this latter group and conclude that a plausible explanation is that they correspond to a significant (~30 %) extreme second generation with a strong helium enhancement (Y~0.4). We also show that the color dispersion of the red giant branch is consistent with this hypothesis, while the main sequence data are compatible with it, although the large observational error blurs the possible underlying splitting. While it is common to find an even larger (50 -- 80) percentage of second generation in a globular cluster, the presence of a substantial and extreme fraction of these stars in NGC 2419 might be surprising, as the cluster is at present well inside the radius beyond which the galactic tidal field would be dominant. If a similar situation had been present in the first stages of the cluster life, the cluster would have retained its initial mass, and the percentage of second generation stars should have been quite small (up to ~10 %). Such a large fraction of extreme second generation stars implies that the system must have been initially much more massive and in different dynamical conditions than today. We discuss this issue in the light of existing models of the formation of multiple populations in globular clusters.Comment: 14 pages, 14 figures (5 in low resolution format), 3 tables, accepted for publication in MNRA