arXiv subjects

Brian Chaboyer

Publications and source records attributed to Brian Chaboyer.

At least 55 records · Page 3Linked to original sources

arXiv2009

An Oosterhoff Analysis of the Galactic Bulge Field RR Lyrae stars: Implications On Their Absolute Magnitudes

We present an analysis of the period-$V$-amplitude plane for RR0 Lyrae stars (fundamental mode pulsators) with "normal" light curves in the bulge using the MACHO bulge fields. Although bulge globular clusters have RR Lyraes that divide into two reasonable distinct groups according to the average period of the RR0 Lyraes (Oosterhoff 1939), there is no evidence of a gap between Oosterhoff I and II stars in the bulge field star sample. The majority of the bulge RR0 Lyrae field star population have a difference in period compared to the Oosterhoff I cluster M3 (Delta log P) that is shifted by about 0.02 days with regard to the Milky Way Oosterhoff I population, and the sample includes stars with Delta log P > 0.06 days, a characteristic hardly seen in Milky Way globular clusters. The metal-rich RR0 Lyrae stars in the Galactic bulge sample have Delta log P values on the other side of the spectrum as those in the metal-rich globular clusters NGC 6388 and NGC 6441. We find that the $V$-amplitude for a given period is a function of Delta log P, and not of metal abundance, similar to the result found by Clement & Shelton (1999) for RR Lyrae stars in Milky Way globular clusters. A comparative study of the bulge field stars with similar metallicities but different Oosterhoff types is carried out. Bulge field RR0 Lyrae variables with Delta log P values similar to Oosterhoff II clusters are about 0.2 mag brighter than RR0 Lyrae variables with Delta log P similar to Oosterhoff I clusters. Reliance upon a single absolute magnitude-[Fe/H] relation may be inappropriate when considering populations with different Delta log P.

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arXiv2009

Distance to the Sagittarius Dwarf Galaxy using MACHO Project RR Lyrae stars

We derive the distance to the northern extension of the Sagittarius (Sgr) dwarf spheroidal galaxy from 203 Sgr RR0 Lyrae stars found in the MACHO database. Their distances are determined differentially with respect to 288 Galactic Bulge RR0 Lyrae stars also found in the MACHO data. We find a distance modulus difference of 2.41 mags at $l$ = 5$^{\circ}$ and $b$ = -8$^{\circ}$ and that the extension of the Sgr galaxy towards the galactic plane is inclined toward us. Assuming $\rm R_{GC}$ = 8 kpc, this implies the distance to these stars is $(m-M)_0$ = 16.97 $\pm$ 0.07 mags, which corresponds to D = 24.8 $\pm$ 0.8 kpc. Although this estimate is smaller than previous determinations for this galaxy and agrees with previous suggestions that Sgr's body is truly closer to us, this estimate is larger than studies at comparable galactic latitudes.

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arXiv2008

The ACS Survey of Galactic Globular Clusters. VII. Relative Ages

The ACS Survey of Galactic Globular Clusters is a Hubble Space Telescope (HST) Treasury program designed to provide a new large, deep and homogeneous photometric database. Based on observations from this program, we have measured precise relative ages for a sample of 64 Galactic globular clusters by comparing the relative position of the clusters' main sequence turn offs, using main-sequence fitting to cross-compare clusters within the sample. This method provides relative ages to a formal precision of 2-7%. We demonstrate that the calculated relative ages are independent of the choice of theoretical model. We find that the Galactic globular cluster sample can be divided into two groups -- a population of old clusters with an age dispersion of ~5% and no age-metallicity relation, and a group of younger clusters with an age-metallicity relation similar to that of the globular clusters associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases or processes. The first one would be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clusters in the old group were formed. Also these clusters could have been formed before reionization in dwarf galaxies that would later merge to build the Milky Way halo as predicted by Lambda-CDM cosmology. However, the galactocentric metallicity gradient shown by these clusters seems difficult to reconcile with the latter. As for the younger clusters, it is very tempting to argue that their origin is related to their formation within Milky Way satellite galaxies that were later accreted, but the origin of the age-metallicity relation remains unclear.

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arXiv2008

Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae stars from their Lightcurves

We present metallicities of 2690 RR0 Lyrae stars observed toward the MACHO Survey fields in the Galactic Bulge. These [Fe/H] values are based upon an empirically calibrated relationship that uses the Fourier coefficients of the light curve and are accurate to ~0.2 dex. The majority of the RR0 Lyrae stars in our sample are located in the Galactic Bulge, but 255 RR0 stars are associated with Sagittarius dwarf galaxy. We find that the RR0 Lyrae stars that belong to the Galactic Bulge have average metallicities [Fe/H] = -1.25, with a broad metallicity range from [Fe/H] = -2.26 to -0.15. The RR0 stars from Sagittarius dwarf galaxy have lower average metallicity of [Fe/H] = -1.55 \pm 0.02, with an intrinsic dispersion of 0.25 dex, similar to that in the bulge. A correlation between metallicity and galactocentric distance is found, in a sense that for the metal-poor RR0 Lyrae stars ([Fe/H]< -1.5 dex), the closer a star is to the Galactic center, on average, the more metal rich it is. However, for the metal-rich RR0 Lyrae stars ([Fe/H] > -1.2 dex), this trend is reversed. Using mean magnitudes of MACHO RR Lyrae stars, we searched for the evidence of the Galactic bar, and found marginal evidence of a bar. The absence of a strong bar indicates that the RR Lyrae in the bulge represent a different population than the majority of the bulge stars, which are metal rich and are part of a bar.

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arXiv2008

The Dartmouth Stellar Evolution Database

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from -2.5 to +0.5, [alpha/Fe] from -0.2 to +0.8 (for [Fe/H] <=0) or +0.2 (for [Fe/H] >0), and initial He mass fractions from Y=0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 Msun, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)c, Stromgren uvby, SDSS ugriz, 2MASS JHKs, and HST ACS-WFC and WFPC2. The Dartmouth Stellar Evolution Database is accessible through a website at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.

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arXiv2008

An ACS Survey of Globular Clusters V: Star Catalog for Each Cluster

The ACS Survey of Globular Clusters has used HST's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous dataset for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available PSF models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we run a standard set of ~10^5 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the world-wide web.

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arXiv2007

The Extinction Toward the Galactic Bulge from RR Lyrae Stars

We present mean reddenings toward 3525 RR0 Lyrae stars from the Galactic bulge fields of the MACHO Survey. These reddenings are determined using the color at minimum $V$-band light of the RR0 Lyrae stars themselves and are found to be in general agreement with extinction estimates at the same location obtained from other methods. Using 3256 stars located in the Galactic Bulge, we derive the selective extinction coefficient $R_{V,VR}=A_V/E(V-R) = 4.3 \pm 0.2$. This value is what is expected for a standard extinction law with $R_{V,BV} = 3.1 \pm 0.3$.

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arXiv2007

The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy

We present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular Clusters. Our deep (F606W~26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised usingboth isochrone and main-sequence fitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (0.1-0.8 Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.

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arXiv2007

The New Standard Stellar Population Models (NSSPM) -- The Prologue

We are developing a brand new stellar population models with flexible chemistry (isochrones plus stellar colors and spectra) in order to set a new standard of completeness and excellence. Here we present preliminary results to assess the effects of stellar evolution models and stellar model atmosphere to the well-known Lick indices at constant heavy element mass fraction Z that self-consistently account for varying heavy element mixtures. We have enhanced chemical elements one by one. Our ultimate goal is to demonstrate 10% absolute mean ages for a sample of local galaxies derived from an integrated light spectrum.

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arXiv2006

The Effect of Cluster Environment on Galaxy Evolution in the Pegasus I Cluster

We present HI observations of 54 galaxies in the Pegasus cluster. The observations include single dish HI measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as HI images, obtained with the VLA for 10 of these. The Arecibo profiles reveal an overall HI deficiency in the cluster, with ~40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2-3. The HI morphology of some galaxies shows that the HI disk is smaller than the optical disk and slightly offset from the stars. We find a correlation between HI deficiency and the ratio of the HI disk size to optical disk size. More HI deficient galaxies have relatively smaller HI disks, a configuration that is usually attributed to an interaction between the ISM of the galaxy and the hot ICM. Such a result is surprising since the Pegasus cluster has a low level of X-ray emission, and a low velocity dispersion. The low velocity dispersion, coupled with the lack of a dense hot ICM indicate that ram pressure stripping should not play a significant role in this environment. In addition, two of the galaxies, NGC7604 and NGC7648, are morphologically peculiar. Their peculiarities indicate contradictory scenarios of what is triggering their unusual star formation. Halpha imaging, along with long-slit spectroscopy of NGC7648 reveal morphological features which point to a recent tidal interaction. On the other hand, Halpha imaging,along with VLA HI mapping, of NGC7604 reveal morphological features suggestive of a ram pressure event. Our data indicate that ISM-ICM interactions may play a role in a wider variety of environments than suggested by simple ram pressure arguments.

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arXiv2006

Willman 1 - A Galactic Satellite at 40 kpc With Multiple Stellar Tails

SDSSJ1049+5103, commonly known as Willman 1, is an extremely low-luminosity Milky Way companion whose properties are intermediate between those of globular clusters and dwarf spheroidals. In this paper, we use deep photometry to show that this object is old and moderately metal-poor, has a distance of 38 +/- 7 kpc, has an M_V of -2.5 mag, and has a half-light radius of 21 +/- 7 pc, consistent with previous estimates. The spatial distribution of Willman 1's main sequence stars shows 1) its total spatial extent exceeds its tidal radius for a range of assumptions about its total mass and its orbit and 2) the presence of prominent multi-directional stellar tails. The tidal interactions causing these tail features may explain the large physical size of Willman 1 relative to low-luminosity globular clusters. It is the most distant Galactic object yet known to display prominent tails, and is the only distant satellite to display multi-directional tails. Although we cannot at present determine the cause of this unusual morphology, preliminary comparisons between the morphology of Willman 1 and published simulations suggest that it may be near the apocenter of its orbit, or that it may have interacted with another halo object. We find a significant difference between the luminosity functions of stars in the center and in the tails of Willman 1, strongly suggesting mass segregation much like that seen in Palomar 5. Although Willman 1 has more pronounced tidal tails than most confirmed Milky Way dwarf galaxies, because of its very low stellar mass we cannot at present rule out the possibility that it has a dark matter halo. (Abridged)

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arXiv2006

A New Color-Magnitude Diagram for Palomar 11

We present new photometry for the Galactic thick disk globular cluster Palomar 11 extending well past the main sequence turn-off in the V and I bands. This photometry shows noticeable red giant and subgiant branches. The difference in magnitude between the red horizontal branch (red clump) and the subgiant branch is used to determine that Palomar 11 has an age of 10.4+/-0.5 Gyr. The red clump is used to derive a distance d_\sun=14.3+/-0.4 kpc, and a mean cluster reddening of E(V-I)=0.40+/-0.03. There is differential reddening across the cluster, of order δE(V-I)~0.07. The colour magnitude diagram of Palomar 11 is virtually identically to that of the thick disk globular cluster NGC 5927, implying that these two clusters have a similar age and metallicity. Palomar 11 has a slightly redder red giant branch than 47 Tuc, implying that Palomar 11 is 0.15 dex more metal-rich, or 1 Gyr older than 47 Tuc. Ca II triplet observations (Rutledge et al. 1997) favour the hypothesis that Palomar 11 is the same age as 47 Tuc, but slightly more metal-rich.

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arXiv2006

From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries

Stellar models which incorporate simple diffusion or shear induced mixing are used to describe canonical extra mixing in low mass red giants of low and solar metallicity. These models are able to simultaneously explain the observed Li and CN abundance changes along upper red giant branch (RGB) in field low-metallicity stars and match photometry, rotation and carbon isotopic ratios for stars in the old open cluster M67. The shear mixing model requires that main sequence (MS) progenitors of upper RGB stars possessed rapidly rotating radiative cores and that specific angular momentum was conserved in each of their mass shells during their evolution. We surmise that solar-type stars will not experience canonical extra mixing on the RGB because their more efficient MS spin-down resulted in solid-body rotation, as revealed by helioseismological data for the Sun. Thus, RGB stars in the old, high metallicity cluster NGC 6791 should show no evidence for mixing in their carbon isotopic ratios. We develop the idea that canonical extra mixing in a giant component of a binary system may be switched to its enhanced mode with much faster and somewhat deeper mixing as a result of the giant's tidal spin-up. This scenario can explain photometric and composition peculiarities of RS CVn binaries. The tidally enforced enhanced extra mixing might contribute to the star-to-star abundance variations of O, Na and Al in globular clusters. This idea may be tested with observations of carbon isotopic ratios and CN abundances in RS CVn binaries.

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arXiv2005

Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump

A Monte Carlo simulation exploring uncertainties in standard stellar evolution theory on the red giant branch of metal-poor globular clusters has been conducted. Confidence limits are derived on the absolute V-band magnitude of the bump in the red giant branch luminosity function (M_v,b) and the excess number of stars in thebump, R_b. The analysis takes into account uncertainties in the primordial helium abundance, abundance of alpha-capture elements, radiative and conductive opacities, nuclear reaction rates, neutrino energy losses, the treatments of diffusion and convection, the surface boundary conditions, and color transformations. The uncertainty in theoretical values for the red giant bump magnitude varies with metallicity between +0.13/-0.12 mag at [Fe/H] = -2.4 and +0.23/-0.21 mag at [Fe/H] = -1.0$. The dominant sources of uncertainty are the abundance of the alpha-capture elements, the mixing length, and the low-temperature opacities. The theoretical values of M_v,b are in good agreement with observations. The uncertainty in the theoretical value of R_b is +/-0.01 at all metallicities studied. The dominant sources of uncertainty are the abundance of the alpha-capture elements, the mixing length, and the high-temperature opacities. The median value of R_b varies from 0.44 at [Fe/H] = -2.4$ to 0.50 at [Fe/H] = -1.0. These theoretical values for R_b are in agreement with observations.

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arXiv2005

UBVI CCD Photometry of the Old Open Cluster Berkeley 17

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color-magnitude diagrams (CMDs) are constructed and reach well past the main-sequence turn-off for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal-rich inner halo/thick disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791 are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger than the thick disk globular clusters. These results confirm the status of Berkeley 17 as one of the oldest known open cluster in the Milky Way, and its age provides a lower limit to the age of the Galactic disk.

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arXiv2005

The Young Open Cluster NGC 2129

The first CCD UBV(RI) photometric study in the area of the doubtful open cluster NGC 2129 is presented. Photometry of a field offset 15 arcmin northward is also provided, to probe the Galactic disk population toward the cluster. Using star counts, proper motions from the UCAC2 catalog, colour-magnitude and colour-colour diagrams we demonstrate that NGC 2129 is a young open cluster. The cluster radius is 2.5 arcmin, and across this region we find evidence of significant differential reddening, although the reddening law seems to be normal toward its direction. Updated estimates of the cluster fundamental parameters are provided. The mean reddening is found to be E(B-V)=0.80$\pm$0.08 and the distance modulus is $(m-M)_o$= 11.70$\pm0.30$. Hence, NGC 2129 is located at 2.2$\pm$0.2 kpc from the Sun inside the Local spiral arm. The age derived from 37 photometrically selected members is estimated to be approximately 10 million years. These stars are used to provide new estimates of the cluster absolute proper motion components.

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arXiv2003

The impact of pollution on stellar evolution models

An approach is introduced for incorporating the concept of stellar pollution into stellar evolution models. The approach involves enhancing the metal content of the surface layers of stellar models. In addition, the surface layers of stars in the mass range of 0.5-2.0 Solar masses are mixed to an artificial depth motivated by observations of lithium abundance. The behavior of polluted stellar evolution models is explored assuming the pollution occurs after the star has left the fully convective pre main sequence phase. Stellar models polluted with a few Earth masses of iron are significantly hotter than stars of the same mass with an equivalent bulk metallicity. Polluted stellar evolution models can successfully reproduce the metal-rich, parent star tau Bootis and suggest a slightly lower mass than standard evolution models. Finally, the possibility that stars in the Hyades open cluster have accreted an average of 0.5 Earth masses of iron is explored. The results indicate that it is not possible to rule out stellar pollution on this scale from the scatter of Hyades stars on a color-magnitude diagram. The small amount of scatter in the observational data set does rule out pollution on the order of 1.5 Earth masses of iron. Pollution effects at the low level of 0.5 Earth masses of iron do not produce substantial changes in a star's evolution.

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arXiv2003

Stellar pollution and [Fe/H] in the Hyades

The Hyades open cluster presents a unique laboratory for planet formation and stellar pollution studies because all of the stars have essentially the same age and were born from the same cloud of gas. Furthermore, with an age of roughly 650 Myr most of the intermediate and low mass stars are on the main sequence. Given these assumptions, the accretion of metal rich material onto the surface of a star during and shortly after the formation of planetary systems should be evident via the enhanced metallicity of the star. Building on previous work, stellar evolution models which include the effects of stellar pollution are applied to the Hyades. The results of several Monte Carlo simulations, in which the amount of accreted material is drawn at random from a Gaussian distribution with standard deviation equal to half the mean, are presented. An effective temperature-[Fe/H] relation is produced and compared to recent observations. The theoretical predictions presented in this letter will be useful in future searches for evidence of stellar pollution due to planet formation. It is concluded that stellar pollution effects at the mean level of >=2 Earth masses of iron are ruled out by current observational data.

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