arXiv subjects

M. Catelan

Publications and source records attributed to M. Catelan.

At least 163 records · Page 9Linked to original sources

arXiv2009

Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272): The Horizontal Branch Test

It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium-enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of > 0.02 in virtually all blue HB stars. Using high-precision Stroemgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the cluster's blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.

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arXiv2009

Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review 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 (GCs) 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. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. 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 Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]

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arXiv2008

The Ages of Stars: The Horizontal Branch

Horizontal branch (HB) stars play a particularly important role in the "age debate," since they are at the very center of the long-standing "second parameter" problem. In this review, I discuss some recent progress in our understanding of the nature and origin of HB stars.

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arXiv2008

The Globular Cluster NGC 5286. I. A New CCD BV Color-Magnitude Diagram

We present BV photometry of the Galactic globular cluster NGC 5286, based on 128 V frames and 133 B frames, and covering the entire face of the cluster. Our photometry reaches almost two magnitudes below the turn-off level, and is accordingly suitable for an age analysis. Field stars were removed statistically from the cluster's color-magnitude diagram (CMD), and a differential reddening correction applied, thus allowing a precise ridgeline to be calculated. Using the latter, a metallicity of [Fe/H] = -1.70 +/- 0.10 in the Zinn & West scale, and [Fe/H] = -1.47 +/- 0.02 in the Carretta & Gratton scale, was derived on the basis of several parameters measured from the red giant branch, in good agreement with the value provided in the Harris catalog. Comparing the NGC 5286 CMD with the latest photometry for M3 by P. B. Stetson (2008, priv. comm.), and using VandenBerg isochrones for a suitable chemical composition, we find evidence that NGC 5286 is around 1.7 +/- 0.9 Gyr older than M3. This goes in the right sense to help account for the blue horizontal branch of NGC 5286, for which we provide a measurement of several morphological indicators. If NGC 5286 is a bona fide member of the Canis Major dwarf spheroidal galaxy, as previously suggested, our results imply that the latter's oldest components may be at least as old as the oldest Milky Way globular clusters.

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arXiv2008

First Results from the Catalina Real-time Transient Survey

We report on the results from the first six months of the Catalina Real-time Transient Survey (CRTS). In order to search for optical transients with timescales of minutes to years, the CRTS analyses data from the Catalina Sky Survey which repeatedly covers twenty six thousand of square degrees on the sky. The CRTS provides a public stream of transients that are bright enough to be followed up using small telescopes. Since the beginning of the survey, all CRTS transients have been made available to astronomers around the world in real-time using HTML tables, RSS feeds and VOEvents. As part of our public outreach program the detections are now also available in KML through Google Sky. The initial discoveries include over 350 unique optical transients rising more than two magnitudes from past measurements. Sixty two of these are classified as supernovae, based on light curves, prior deep imaging and spectroscopic data. Seventy seven are due to cataclysmic variables (only 13 previously known), while an additional 100 transients were too infrequently sampled to distinguish between faint CVs and SNe. The remaining optical transients include AGN, Blazars, high proper motions stars, highly variable stars (such as UV Ceti stars) and transients of an unknown nature. Our results suggest that there is a large population of SNe missed by many current supernova surveys because of selection biases. These objects appear to be associated with faint host galaxies. We also discuss the unexpected discovery of white dwarf binary systems through dramatic eclipses.

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arXiv2008

Stellar Evolution and Variability in the Pre-ZAHB Phase

One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the RGB and is followed by a series of secondary flashes before the star settles on the zero-age HB (ZAHB). Yet, no stars have ever been positively identified in this key phase in the life of a low-mass star (hereafter the "pre-ZAHB" phase). We investigate the possibility that some pre-ZAHB stars may cross the instability strip (IS), thus becoming variable. In particular, it has been suggested that some RR Lyrae stars (RRL) with high period change rates (P-dot) may in fact be pre-ZAHB stars. We present the first theoretical effort devoted to interpreting some of the high-P-dot stars as pre-ZAHB pulsators. We constructed a grid of evolutionary tracks using the Garching Stellar Evolution Code for a chemical composition appropriate to the case of the globular cluster M3, where a number of stars with high P-dot values are found. We follow each star's pre-ZAHB evolution in detail, compute periods and P-dot values for the stars lying inside the IS, and produce pre-ZAHB Monte Carlo simulations for the case of M3. Our results indicate that one should expect of order 1 pre-ZAHB star for every 60 or so bona-fide HB stars in M3. Among the pre-ZAHB stars, ~22% are expected to fall within the IS boundaries, presenting RRL-like pulsations. On average, the pre-ZAHB pulsators are expected to have longer periods than the bona-fide HB pulsators, and 76% of them are predicted to show negative P-dot values. The most likely P-dot value for the pre-ZAHB variables is ~-0.3 d/Myr, but more extreme P-dot values are also possible: 38% of the variables are predicted to have P-dot < -0.8 d/Myr. It appears likely, therefore, that some RRL stars in M3 with high (absolute) P-dot values are pre-ZAHB pulsators in disguise. (abridged)

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arXiv2008

The Period-Luminosity Relation of RR Lyrae Stars in the SDSS Photometric System

We provide the first detailed study of the RR Lyrae period-luminosity (PL) relation in the ugriz bandpasses of the Sloan Digital Sky Survey (SDSS) filter system. We argue that tight, simple PL relations are not present in the SDSS filters, except for the redder bandpasses i and (especially) z. However, for all bandpasses, we show that, by incorporating terms involving a (fairly reddening-independent) "pseudo-color" C_0 = (u-g)_0 - (g-r)_0, tight (non-linear) relations do obtain. We provide theoretically calibrated such relations in the present paper, which should be useful to derive precise absolute magnitudes (hence distances) and intrinsic colors (hence reddening values) to even {\em individual} field RR Lyrae stars. For applications to cases where photometry in all five passbands may not be available, we also provide simple (though less precise) average PL relations for the i and z bandpasses, which read as follows: M_z = 0.839 - 1.295 log P + 0.211 log Z, M_i = 0.908 - 1.035 log P + 0.220 log Z. Similarly, simple period-color relations for (r-i)_0, (g-r)_0, and (u-z)_0 are also provided.

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arXiv2008

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)

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arXiv2008

The RR Lyrae Period-Luminosity-(Pseudo-)Color and Period-Color-(Pseudo-)Color Relations in the Strömgren Photometric System: Theoretical Calibration

We present a theoretical calibration of the RR Lyrae period-luminosity-color and period-color-color relations in the multiband uvby Stroemgren photometric system. Our theoretical work is based on calculations of synthetic horizontal branches (HBs) for four different metallicities, fully taking into account evolutionary effects for a wide range in metallicities and HB morphologies. While our results show that "pure" period-luminosity and period-color relations do not exist in the Stroemgren system, which is due to the large scatter that is brought about by evolutionary effects when the uvby bandpasses are used, they also reveal that such scatter can be almost completely taken into account by incorporating Stroemgren pseudo-color [C_0 = (u-v)_0 - (v-b)_0] terms into those equations, thus leading to tight period-luminosity-{\em pseudo}-color (PLpsC) and period-color-{\em pseudo}-color (PCpsC) relations. We provide the latter in the form of analytical fits, so that they can be applied with high precision even in the case of field stars. In view of the very small sensitivity of C_0 to interstellar reddening, our PLpsC and PCpsC relations should be especially useful for the derivation of high-precision distance and reddening values. In this sense, we carry out a first application of our relations to field RR Lyrae stars, finding evidence that the stars RR Lyr, SU Dra, and SS Leo -- but not SV Hya -- are somewhat overluminous (by amounts ranging from ~0.05 to 0.20 mag in y, and thus V) with respect to the average for other RR Lyrae stars of similar metallicity.

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arXiv2008

Evidence for an Overluminosity of the Variable Star RR Lyr, and a Revised Distance to the LMC

We use theoretical models to establish a tight relationship for the absolute magnitudes of RR Lyrae stars as a function of their periods and Stroemgren pseudo-color c_0 = (u-v)_0 - (v-b)_0. Applying this to RR Lyr, and comparing the result with the predicted average absolute magnitude for stars of similar metallicity from the same models, yields an overluminosity of 0.064 +/- 0.013 mag in Stroemgren y (and thus similarly in V) for RR Lyr. Based on a revised value for RR Lyr's trigonometric parallax, and on a newly derived reddening value of E(B-V) = 0.015 +/- 0.020, we provide a corrected relationship between average absolute magnitude and metallicity for RR Lyrae stars that takes RR Lyr's evolutionary status fully into account for the first time. Applying this relationship to the LMC, we derive a revised true distance modulus of (m-M)_0 = 18.44 +/- 0.11.

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arXiv2008

The Globular Cluster M15. I. Identification, Discovery, and Period Determination of Variable Stars

We present new $BVI$ CCD photometry for variables in the globular cluster M15. Our photometry was obtained using both the image subtraction package ISIS and DAOPHOT/ALLFRAME. The data were acquired in 2001 on two observing runs on 11 observing nights using the 2-m telescope of the Bulgarian National Astronomical Observatory ``Rozhen'' with a Photometrics CCD camera. For 40 previously known variables, we present a period for the first time, and improved periods were obtained for many previously known variables. Fourteen new variables are reported. We present updated Bailey diagrams for the cluster and discuss its Oosterhoff classification. Although many of M15's RRab pulsators fall at an intermediate locus between Oosterhoff types I and II in the Bailey diagram, we argue that M15 is indeed a bona-fide Oosterhoff type II

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arXiv2008

Is a binary fraction-age relation responsible for the lack of EHB binaries in globular clusters?

The recently-discovered lack of close binaries, among extreme horizontal branch (EHB) stars in Galactic globular clusters, has thus far constituted a major puzzle, in view of the fact that blue subdwarf stars - the field counterparts of cluster EHB stars - are well-known to present a high binary fraction. In this Letter, we provide new results that confirm the lack of close EHB binaries in globular clusters, and present a first scenario to explain the difference between field and cluster EHB stars. First, in order to confirm that the lack of EHB binaries in globular clusters is a statistically robust result, we undertook a new analysis of 145 horizontal branch stars in NGC6752, out of which forty-one belong to the EHB. To search for radial-velocity variations as a function of time, we repeated high-resolution (R=18500) spectroscopy of all stars, four times during a single night of observations. We detected a single, hot (25000 K), radial-velocity variable star as a close-binary candidate. From these results, we estimate an upper-limit for the close (period P < 5 day) binary fraction f among NGC6752 EHB stars of 16% (95% confidence level), with the most probable value being f=4%. Thus our results clearly confirm the lack of close binaries among the hot HB stars in this cluster. We suggest that the confirmed discrepancy between the binary fractions for field and cluster EHB stars is the consequence of an f-age relation, with close binaries being more likely in the case of younger systems. We analyze theoretical and observational results available in the literature, which support this scenario. If so, an age difference between the EHB progenitors in the field and in clusters, the former being younger (on average) by up to several Gyr, would naturally account for the startling differences in binary fraction between the two populations.

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arXiv2007

On the newly discovered Canes Venatici II dSph galaxy

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, B-V color-magnitude diagram that reaches V = 25.5 mag, showing the galaxy's main sequence turn off at V = 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II,a fundamental-mode variable (RRab) with period P_ab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with P_c = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, = 21.48 +/- 0.02 mag, is used to obtain a precision distance modulus of mu_0 = 21.02 +/- 0.06 mag and a corresponding distance of 160(+4,-5} kpc, for an adopted reddening E(B-V) = 0.015 mag.

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arXiv2007

Multimodal Horizontal Branches: Empirical Evidence and Possible Evolutionary Scenarios

We review the available empirical evidence for the presence of "gaps" and multimodal distributions among horizontal branch (HB) stars, along with some of the theoretical scenarios that have been proposed to explain these features. While gaps along the HB have become increasingly less prominent and frequent as more and better color-magnitude diagram data have been obtained for Galactic globular clusters, the evidence for multimodal HBs has instead become stronger. In addition, different HB modes have recently started to be traced down to multiple components that have been detected among subgiant branch and main sequence stars, thus suggesting that their origin lies in the complex physical processes that took place at the earliest stages in the history of massive stellar clusters.

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arXiv2007

Binaries among Extreme Horizontal Branch Stars in Globular Clusters

Following a review of our present knowledge about blue subdwarf stars in globular clusters, we present an overview of the results of searches for close binaries among these stars, including results previously published in the literature and reporting recent and preliminary results of new data. Previous investigations revealed a lack of close systems in NGC 6752, which we confirm with new, more extensive observations. Our estimate of the close binary fraction in this cluster is only 4%. From a review of the relevant literature, there are indications that a low close binary fraction among extreme horizontal branch (EHB) stars is a common feature in globular star clusters. On the other hand, the field EHB population shows evidence of a remarkably high binary fraction. Such a difference among globular cluster and field populations, although not yet explained in detail on the basis of theoretical models, must necessarily be related to different formation histories for EHB stars in the field and in clusters. In this framework, preliminary results indicate that close systems could be relatively common in the peculiar globular cluster NGC 2808, although the sample of studied stars is still small. This would imply that not all clusters share the same behavior, as far as EHB star formation is concerned. We briefly explore possible explanations for these results.

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arXiv2007

A Search for EHB Pulsators in the Globular Cluster NGC 6752

We describe the status of a project whose main goal is to detect variability along the extreme horizontal branch of the globular cluster NGC 6752. Based on Magellan 6.5m data, preliminary light curves are presented for some candidate variables. By combining our time-series data, we also produce a deep CMD of unprecedented quality for the cluster which reveals a remarkable lack of main sequence binaries, possibly pointing to a low primordial binary fraction.

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arXiv2007

Horizontal Branch Stars and the Ultraviolet Universe

Extremely hot horizontal branch (HB) stars and their progeny are widely considered to be responsible for the "ultraviolet upturn" (or UVX) phenomenon observed in elliptical galaxies and the bulges of spirals. Yet, the precise evolutionary channels that lead to the production of these stars remain the source of much debate. In this review, we discuss two key physical ingredients that are required in order for reliable quantitative models of the UV output of stellar populations to be computed, namely, the mass loss rates of red giant branch stars and the helium enrichment "law" at high metallicities. In particular, the recent evidence pointing towards a strong enhancement in the abundances of the alpha-elements in the Galactic bulge (compared to the disk), and also the available indications of a similar overabundance in (massive) elliptical galaxies, strongly suggest that the helium abundance Y may be higher in ellipticals and bulges than it is in spiral disks by an amount that may reach up to 0.15 at [Fe/H] ~ +0.5. If so, this would strongly favor the production of hot HB stars at high metallicity in galactic spheroids. We also discuss the existence of mass loss recipes beyond the commonly adopted Reimers "law" that are not only more consistent with the available empirical data, but also much more favorable to the production of extended HB stars at high metallicity. Finally, we discuss new empirical evidence that suggests that different evolutionary channels may be responsible for the production of EHB stars in the field and in clusters.

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arXiv2007

The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations

In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441. We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot horizontal branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and Omega Centauri. We briefly review the theoretical scenarios that have been suggested for interpreting this observational feature. We also investigate the tilted horizontal branch morphology and provide further evidence that supports early suggestions that this feature cannot be interpreted as an effect of differential reddening. We show that a possible solution of the puzzle is to assume that a small fraction - ranging between 10-20% - of the stellar population in the two clusters is strongly helium enriched (Y=0.40 in NGC 6388 and Y=0.35 in NGC 6441). The occurrence of a spread in the He abundance between the canonical value (Y=0.26) and the quoted upper limits can significantly help in explaining the whole morphology of the horizontal branch and the pulsational properties of the variable stars in the target clusters.

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