arXiv subjects

M. Catelan

Publications and source records attributed to M. Catelan.

At least 199 records · Page 11Linked to original sources

arXiv2002

M75, a Globular Cluster with a Trimodal Horizontal Branch. I. Color-Magnitude Diagram

Deep UBVI photometry for a large field covering the distant globular cluster M75 (NGC 6864) is presented. We confirm a previous suggestion (Catelan et al. 1998a) that M75 possesses a bimodal horizontal branch (HB) bearing striking resemblance to the well-known case of NGC 1851. In addition, we detect a third, smaller grouping of stars on the M75 blue tail, separated from the bulk of the blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may correspond to the upper part of a very extended, though thinly populated, blue tail. Thus M75 appears to have a trimodal HB. The presence of the "Grundahl jump" is verified using the broadband U filter. We explore the color-magnitude diagram of M75 with the purpose of deriving the cluster's fundamental parameters, and find a metallicity of [Fe/H] = -1.03 +/- 0.17 dex and -1.24 +/- 0.21 dex in the Carretta & Gratton (1997) and Zinn & West (1984) scales, respectively. We discuss earlier suggestions that the cluster has an anomalously low ratio of bright red giants to HB stars. A differential age analysis with respect to NGC 1851 suggests that the two clusters are essentially coeval.

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arXiv2001

Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

We revisit the "second parameter" pair of globular clusters NGC 288/NGC 362 on the basis of theoretical models for red giant branch (RGB) and horizontal branch (HB) stars. The results of the most extensive set of RGB/HB simulations computed so far for these clusters are presented for two different metallicities. Using several different analytical mass loss formulae for RGB stars, we derive relative "HB morphology ages." We compare them with the relative main-sequence turnoff ages derived by application of the "bridge test" by Bellazzini et al. (2001), who found that NGC 288 is 2+/-1 Gyr older than NGC 362. We find that adoption of a higher metallicity ([Fe/H] ~ -1.2), as favored by the Carretta & Gratton metallicity scale, makes age a much more plausible second parameter candidate for this pair than is the case when a lower metallicity ([Fe/H] ~ -1.5), closer to the Zinn & West scale, is adopted. However, while the different HB morphology of these two clusters can be reproduced by canonical HB models with [Fe/H] ~ -1.2 and an age difference of 2 Gyr, this explanation is not without difficulty. In particular, we confirm previous suggestions that canonical models are unable to reproduce the detailed HB morphology of NGC 288 at its red end, for as yet unknown reasons. Finally, we show that the mass dispersion on the HB of NGC 362 is substantially larger than for NGC 288, and suggest that there is a correlation between the mass dispersion on the HB phase and the central density of globular clusters. This is presumably related to the way environmental effects affect RGB mass loss. [Abridged]

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arXiv2001

Horizontal-Branch Models and the Second-Parameter Effect. IV. The Case of M3 and Palomar 3

We present a detailed analysis of the "second-parameter pair" of globular clusters M3 (NGC 5272) and Palomar 3. Our main results can be summarized as follows: i) The horizontal-branch (HB) morphology of M3 is significantly bluer in its inner regions (observed with the Hubble Space Telescope) than in the cluster outskirts (observed from the ground), i.e., M3 has an internal second parameter. Most plausibly the mass loss on the red giant branch (RGB) has been more efficient in the inner than in the outer regions of the cluster. ii) The dispersion in mass of the Pal 3 HB is found to be very small -- consistent with zero -- and we argue that this is unlikely to be due to a statistical fluctuation. It is this small mass dispersion that leads to the most apparent difference in the HB morphologies of M3 and Pal 3. iii) The relative HB types of M3 and Pal 3, as measured by mean colors or parameters involving the number of blue, variable, and red HB stars, can easily be accounted for by a fairly small difference in age between these clusters, of order 0.5-1 Gyr -- which is in good agreement with the relative age measurement, based on the clusters' turnoffs, by VandenBerg (2000).

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arXiv2001

Second Parameter Effects in and between M3 and Palomar 3

We study the globular clusters M3 and Palomar 3 as a "second parameter (2ndP) pair," showing that: i) M3 has a surprisingly strong *internal* 2ndP; ii) The dispersion in mass on the Pal 3 horizontal branch (HB) is intrinsically very small, leading to the most apparent differences in HB morphology between M3 and Pal 3; iii) Ignoring the difference in HB mass dispersion between M3 and Pal 3, their relative HB types can be accounted for by a fairly small difference in age, of order 0.5-1 Gyr.

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arXiv2000

The outer-halo globular cluster NGC 6229. IV. Variable stars

We present time-series B, V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the Borissova et al. (1997) candidate list located very close to the cluster center. Two variable stars suspected by Carney, Fullton, & Trammell (1991) are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates from Borissova et al. (1997) are confirmed variables -- three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one eclipsing binary (most likely an Algol system) and one bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard (1999) has been employed which, together with the Welch & Stetson (1993) technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRab's, whereas the other two are RRc's. As originally suggested by Mayer (1961), NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are compared against those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.

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arXiv1999

Horizontal-Branch Models and the Second-Parameter Effect. III. The Impact of Mass Loss on the Red Giant Branch, and the Case of M5 and Palomar 4/Eridanus

Deep HST photometry has recently been presented for the outer-halo globular clusters Palomar 4 and Eridanus. The new high-precision color-magnitude diagrams obtained for these globulars have allowed a measurement of their ages relative to M5 (NGC 5904), which is a well-observed, much closer cluster. Assuming that the globular clusters share the same chemical composition, Pal 4/Eridanus have been reported to be younger than M5 by ~1-2 Gyr, based on both the magnitude difference between the horizontal branch (HB) and the turnoff and the difference in color between the turnoff and the lower subgiant branch. In the present article, we address the following question: What age difference would be required to account for the difference in HB types between M5 and Pal 4/Eridanus, assuming age to be the "second parameter"? We find that, unless all these clusters (including M5) are younger than 10 Gyr, such an age difference is substantially larger than that based on an analysis of the cluster turnoffs. To reach such a conclusion, six different analytical mass loss rate formulae (reported in an Appendix), all implying a dependence of mass loss on the red giant branch on age, were employed. Our results appear to be in conflict with claims that age can be the only second parameter in the Galactic globular cluster system.

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arXiv1999

Blue Horizontal--Branch Stars: The "Jump" in Stromgren u, Low Gravities, and Radiative Levitation of Metals

We study the ``jump'' in the blue horizontal--branch (BHB) distribution first detected by Grundahl et al. (1998) in the Galactic globular cluster (GC) M13. On the basis of Stromgren photometry for a sample of fourteen GC's we show that: 1) The jump is best characterized as a systematic shift, on a (u, u-y) color-magnitude diagram, from canonical zero-age HB (ZAHB) models, in the sense that the stars appear brighter and/or hotter than the models; 2) the jump is a ubiquitous phenomenon, ocurring over the temperature range 11,500 < Teff < 20,000K; 3) An analogous feature is present in (log g, log Teff) diagrams -- indicating a common physical origin for the two phenomena; 4) The physical mechanism responsible for the jump phenomenon is most likely radiative levitation of iron and other

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arXiv1999

Mass Loss on the Red Giant Branch and the Second-Parameter Phenomenon

The "second-parameter effect" is characterized by the existence of globular clusters (GCs) with similar metallicity [Fe/H] (the "first parameter") but different horizontal-branch (HB) morphologies. One of the primary second-parameter candidates is cluster age. In the present paper, we address the following issue: "Are the age differences between second-parameter GCs, as derived from their main-sequence turnoff properties, consistent with their relative HB types?" In order to provide an answer to this question, several analytical formulae for the mass loss rate on the red giant branch are analyzed and employed. The case of M5 vs. Palomar 4/Eridanus is specifically discussed. Our results show that, irrespective of the mass loss formula employed, the relative turnoff ages of GCs are insufficient to explain the second-parameter phenomenon, unless GCs are younger than 10 Gyr.

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arXiv1999

Blue horizontal branch stars in metal-rich globular clusters. I. NGC 6388 and NGC 6441

We report the first results of an ongoing spectroscopic survey of blue horizontal branch stars in the metal-rich ([Fe/H] ~ -0.5) globular clusters NGC 6388 and NGC 6441. Based on data obtained with the ESO-New Technology Telescope (NTT), we provide gravities and temperatures for four stars in NGC 6388 and three stars in NGC 6441. These results are marginally inconsistent with the predictions of canonical evolutionary theory, but disagree strongly with all non-canonical scenarios that explain the sloped horizontal branches seen in the colour-magnitude diagrams of these clusters.

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arXiv1999

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 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.

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arXiv1999

Physical Parameters of Hot Horizontal-Branch Stars in NGC 6752: Deep Mixing and Radiative Levitation

Atmospheric parameters (Teff, log g and log(He/H)) are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars MgII and FeII lines are detected indicating an iron enrichment by a factor 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster metallicity. This finding adds to the growing evidence that radiative levitation plays a significant role in determining the physical parameters of blue HB stars. Indeed, we find that iron enrichment can explain part, but not all, of the problem of anomalously low gravities along the blue HB. Thus the physical parameters of horizontal branch stars hotter than about 11,500K in NGC 6752, as derived in this paper, are best explained by a combination of helium mixing and radiative levitation effects.

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arXiv1998

The outer-halo globular cluster NGC 6229. III. Deep CCD photometry

Deep BV photometry for a large field covering the outer-halo globular cluster NGC 6229 is presented. For the first time, a colour-magnitude diagram (CMD) reaching below the main sequence turnoff has been obtained for this cluster. Previous results regarding the overall morphology of the horizontal and giant branches are confirmed. In addition, nine possible extreme horizontal-branch stars have been identified in our deep images, as well as thirty-three candidate blue stragglers. We also find the latter to be more centrally concentrated than subgiant branch stars covering the same range in V magnitude. A comparison of the cluster CMD with the M5 (NGC 5904) ridgeline from Sandquist et al. (1996) reveals that: i) NGC 6229 and M5 have essentially identical metallicities; ii) NGC 6229 and M5 have the same ages within the errors in spite of their different horizontal-branch morphologies.

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arXiv1998

The Second-Parameter Effect in Metal-Rich Globular Clusters

Recent Hubble Space Telescope observations have found that the horizontal branches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope upward with decreasing B-V. Such a slope is not predicted by canonical HB models and cannot be produced by either a greater cluster age or enhanced mass loss along the red-giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to explore three non-canonical scenarios for explaining the sloped HB's in NGC 6388 and NGC 6441: i) A high cluster helium abundance scenario, where the HB evolution is characterized by long blue loops; ii) A rotation scenario, where internal rotation during the RGB phase increases the HB core mass; iii) A helium-mixing scenario, where deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HB's with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388. Other possible observational tests are suggested.

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arXiv1998

Is There a Difference in Luminosity between Field and Cluster RR Lyrae Variables?

Recent Hipparcos results have lent support to the idea that RR Lyrae variables in the halo field and in globular clusters differ in luminosity by ~0.2mag. In this Letter, we study the pulsation properties of RR Lyraes in clusters with distances determined via main-sequence fitting to Hipparcos parallaxes for field subdwarfs, and compare them with the properties of field variables also analyzed with Hipparcos. We show that the period-temperature distributions for field and cluster variables are essentially indistinguishable, thus suggesting that there is no significant difference in luminosity between them.

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arXiv1997

Three contributions to the "A Half Century of Stellar Pulsation Interpretations" proceedings

Paper I: "RR Lyrae Variables in Globular Clusters and the Second-Parameter Phenomenon" Authors: M. Catelan, A. V. Sweigart, J. Borissova Abstract: We propose to use RR Lyrae variables in globular clusters to probe the origin of the second-parameter phenomenon. Paper II: "New Possible Variables in the Outer-Halo Globular Cluster Palomar 3" Authors: J. Borissova, N. Spassova, M. Catelan, V. D. Ivanov Abstract: We present a list of ten possible variable stars in the globular cluster Pal 3. Seven are new suspected variables. The variability of the RR Lyr candidate reported by Burbidge & Sandage (1958), as well as of the Pop. II Cepheid and RR Lyr from Gratton & Ortolani (1984), are confirmed. Paper III: "On the Production of Bright RR Lyrae Variables in Metal-Rich Globular Clusters" Authors: A. V. Sweigart, M. Catelan Abstract: See astro-ph/9708174.

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arXiv1997

On the Production of Bright RR Lyrae Variables in Metal-Rich Globular Clusters

Recent HST observations of the metal-rich bulge clusters NGC 6388 and NGC 6441 by Rich et al. (1997) have found that the horizontal branches (HBs) in these clusters slope upward with decreasing B-V. Such an upward slope in the HB morphology is not predicted by canonical HB models. Moreover, it cannot be produced by either a greater cluster age or enhanced mass loss along the red-giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to understand the cause of the sloped HBs in NGC 6388 and NGC 6441. Three scenarios have been investigated: i) A high cluster helium abundance scenario, where the HB morphology is determined by long blue loops; ii) A rotation scenario, where the core mass in the HB models is increased by internal rotation during the HB phase; iii) A helium-mixing scenario, where deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HBs with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388 as well as with the properties of the metal-rich field RR Lyrae variables and V9 in 47 Tuc.

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arXiv1997

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 (1971) and Newell (1973) chi-squared test, we find that the NGC 6229 gap is significant at the 99.7% level. However, in a set of 1,000 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.

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arXiv1997

Color-Magnitude Diagrams of Merged Globular Clusters: Metallicity Effects

Mergers of globular clusters (GCs) once associated with dwarf spheroidal (dSph) galaxies have recently been suggested as an explanation for the bimodal horizontal branches (HBs) of some Galactic GCs, most notably NGC 1851, NGC 2808, and NGC 6229. Through analysis of the available color-magnitude diagrams for the GCs in the Fornax and Sagittarius dSph satellites of the Milky Way, as well as their metallicity distributions, we argue that the merger of two GCs would most likely produce a bimodal distribution in red giant branch (RGB) colors, or at least a significant broadening of the RGB, due to the expected difference in metallicity between the two merging globulars. No GC with a bimodal RGB is currently known, and the tightness of the RGB sequences in the above bimodal-HB GCs implies that a merger origin for their HB bimodality is unlikely.

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