arXiv subjects

Ata Sarajedini

Publications and source records attributed to Ata Sarajedini.

At least 73 records · Page 4Linked to original sources

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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arXiv2008

Evidence for metallicity spreads in three massive M31 globular clusters

We quantify the intrinsic width of the red giant branches of three massive globular clusters in M31 in a search for metallicity spreads within these objects. We present HST/ACS observations of three massive clusters in M31, G78, G213, and G280. A thorough description of the photometry extraction and calibration is presented. After derivation of the color-magnitude diagrams, we quantify the intrinsic width of the red giant branch of each cluster. This width translates into a metallicity dispersion that indicates a complex star formation history for this type of system. For G78, sigma_[Fe/H]}=0.86 \pm 0.37; for G213, 0.89 \pm 0.20; and for G280, 1.03 \pm 0.26. We find that the metallicity dispersion of the clusters does not scale with mean metallicity. We also find no trend with the cluster mass. We discuss some possible formation scenarios that would explain our results.

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arXiv2007

Wide-Field Survey of Globular Clusters in M31. II. Kinematics of the Globular Cluster System

We present a kinematic analysis of the globular cluster(GC) system in M31. Using the photometric and spectroscopic database of 504 GCs, we have investigated the kinematics of the M31 GC system. We find that the all GC system shows strong rotation, with rotation amplitude of v_rot~190km/s, and that a weak rotation persists even for the outermost samples at |Y|>5kpc. The rotation-corrected velocity dispersion for the GC system is estimated to be sigma_{p,r}~130km/s, and it increases from sigma_{p,r}~120km/s at |Y|<1kpc to sigma_{p,r}~150km/s at |Y|>5kpc. These results are very similar to those for the metal-poor GCs. This shows that there is a dynamically hot halo in M31 that is rotating but primarily pressure-supported. We have identified 50 "friendless" GCs, and they appear to rotate around the major axis of M31. For the subsamples of metal-poor and metal-rich GCs, we have found that the metal-rich GCs are more centrally concentrated than the metal-poor GCs, and both subsamples show strong rotation. For the subsamples of bright and faint GCs, it is found that the rotation for the faint GCs is stronger than that for the bright GCs. We have identified 56 GCs and GC candidates with X-ray detection. It is found that the majority of X-ray emitting GCs follow the disk rotation, and that the redder, more metal-rich, and brighter GCs are more likely to be detected as X-ray emitting GCs. We have derived a rotation curve of M31 using the GCs at |Y|<0.6kpc. We have estimated the dynamical mass of M31 using `Projected Mass Estimator(PME)' and `Tracer Mass Estimator(TME)'. We finally discuss the implication of these results and compare the kinematics of GCs with that of planetary nebulae in M31.

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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

Wide-Field Survey of Globular Clusters in M31. I. A Catalog of New Clusters

We present the result of a wide-field survey of globular clusters (GCs) in M31 covering a 3deg x 3deg field c. We have searched for GCs on CCD images taken with Washington CMT1 filters at the KPNO 0.9 m telescope using steps: (1) inspection of morphological parameters given by the SExtractor package such as stellarity, full maximum, and ellipticity; (2) consulting the spectral types and radial velocities obtained from spectra takena spectrograph at the WIYN 3.5 m telescope; and (3) visual inspection of the images of each object. We have and GC candidates, of which 605 are newly found GCs and GC candidates and 559 are previously known GCs. Amoects there are 113 genuine GCs, 258 probable GCs, and 234 possible GCs, according to our classification critee known objects there are 383 genuine GCs, 109 probable GCs, and 67 possible GCs. In total there are 496 genprobable GCs and 301 possible GCs. Most of these newly found GCs have T1 magnitudes of 17.5 - 19.5 mag, [17.9 < V < 19.9 mag assuming (C-T1) ~ 1.5], and (C-T1) colors in the range 1 - 2.

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arXiv2007

A Catalog of Star Cluster Candidates in M33

We present a new catalog of star cluster candidates in the nearby spiral galaxy M33. It is based on eight existing catalogs wherein we have cross-referenced identifications and endeavored to resolve inconsistencies between them. Our catalog contains 451 candidates of which 255 are confirmed clusters based on HST and high resolution ground-based imaging. The catalog contains precise cluster positions (RA and Dec), magnitudes and colors in the UBVRIJHKs filters, metallicities, radial velocities, masses and ages, where available, and galactocentric distances for each cluster. The color distribution of the M33 clusters appears to be similar to those in the Large Magellanic Cloud with major peaks at (B-V)o~0.15, and (B-V)o~0.65. The intrinsic colors are correlated with cluster ages, which range from 10^{7.5} to 10^{10.3} years. The age distribution of the star clusters supports the notion of rapid cluster disruption with a slope of alpha=-1.09 +/- 0.07 in the dN_{cluster}/dt ~ t^{alpha} relation. In addition, comparison to theoretical single stellar population models suggests the presence of an age-metallicity relation among these clusters with younger clusters being more metal-rich. Analysis of the radial distribution of the clusters yields some evidence that younger clusters (age <~ 1 Gyr) may be more concentrated toward the center of M33 than older ones. A similar comparison with the radial profile of the M33 field stars shows the clusters to be more centrally concentrated at the greater than 99.9% confidence level. Possible reasons for this are presented and discussed; however, the overwhelming conclusion seems to be that a more complete and thorough cluster search is needed covering at least 4 square degrees centered on M33.

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arXiv2006

The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry

We present the first results of a large ACS Survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N > ~10 for main sequence stars with masses > ~0.2Msun in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, 6779, 5053, 6144, Palomar 2, E 3, Lynga 7, Palomar 1, and NGC 6366. Our CMDs extend reliably from the horizontal branch to as much as seven magnitudes fainter than the main sequence turnoff and represent the deepest CMDs published to-date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons along with fitting the cluster main sequences to theoretical isochrones yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E 3 which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.

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arXiv2006

The Stellar Populations of M33's Outer Regions II: Deep ACS Imaging

Studying the stellar populations in the outskirts of spiral galaxies can provide important constraints on their structure, formation, and evolution. To that end, we present VI photometry obtained with the Advanced Camera for Surveys for three fields located ~ 20' - 30' in projected distance southeast of M33's nucleus (corresponding to ~ 4 - 6 visual scale lengths or ~ 9 - 13 kpc in deprojected radius). The color-magnitude diagrams reveal a mixed stellar population whose youngest constituents have ages no greater than ~ 100 Myr and whose oldest members have ages of at least several Gyr. The presence of stars as massive as 3 - 5 Msun is consistent with global star formation thresholds in disk galaxies but could argue for a threshold in M33 that is on the low end of observational and theoretical expectations. The metallicity gradient as inferred by comparing the observed red giant branch (RGB) to the Galactic globular clusters is consistent with M33's inner disk gradient traced by several other studies. The surface density of RGB stars drops off exponentially with a radial scale length of 4.7' +/- 0.1'. The scale length increases with age in a manner similar to the vertical scale height of several nearby late-type spirals. Based on the metallicity gradient, density gradient, and mixed nature of the stellar populations, we conclude these fields are dominated by a disk population although we cannot rule out the presence of a small halo component.

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arXiv2006

The "Super-Halo" of M31 and M33

Two recent observations regarding the halo of M33 seem to contradict each other. First, the star clusters in the halo of M33 exhibit an age range of 5 to 7 Gyr suggesting a formation scenario that involves the chaotic fragmentation and accretion of dwarf satellites. In contrast, deep photometric searches for the resultant tidal tails and stellar streams in the vicinity of M33 have turned up nothing significant. In this contribution, we have tried to reconcile these apparently disparate observations. We suggest that M33 is situated within a 'superhalo' which contains many other dwarf spheroidal and dwarf irregular galaxies that are satellites of M31. In such a scenario, the tidal field of M31 could have disrupted and/or diluted the leftover tails and streams leaving little to be detected in the present day.

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arXiv2006

Newly Identified Star Clusters in M33. I. Integrated Photometry and Color-Magnitude Diagrams

We present integrated photometry and color-magnitude diagrams (CMDs) for 24 star clusters in M33, of which 12 were previously uncataloged. These results are based on Advanced Camera for Surveys observations from the Hubble Space Telescope of two fields in M33. Our integrated V magnitudes and V-I colors for the previously identified objects are in good agreement with published photometry. We are able to estimate ages for 21 of these clusters using features in the CMDs, including isochrone fitting to the main sequence turnoffs for 17 of the clusters. Comparisons of these ages with the clusters' integrated colors and magnitudes suggest that simple stellar population models perform reasonably well in predicting these properties.

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arXiv2006

A Genuine Intermediate-Age Globular Cluster in M33

We present deep integrated-light spectroscopy of nine M33 globular clusters taken with the Hectospec instrument at the MMT Observatory. Based on our spectroscopy and previous deep color-magnitude diagrams obtained with HST/WFPC2, we present evidence for the presence of a genuine intermediate-age globular cluster in M33. The analysis of Lick line indices indicates that all globular clusters are metal-poor ([Z/H] <~ -1.0) and that cluster M33-C38 is about 5-8 Gyr younger than the rest of the sample M33 star clusters. We find no evidence for a population of blue horizontal branch stars in the CMD of M33-C38, which rules out the possibility of an artificially young spectroscopic age due to the presence of hot stars. We infer a total mass of 5-9 x 10^4 M_sol for M33-C38, which implies that M33-C38 has survived ~2-3 times longer than some dynamical evolution model predictions for star clusters in M33, although it is not yet clear to which dynamical component of M33 - thin disk, thick disk, halo - the cluster is associated.

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arXiv2006

RR Lyrae Variables in M33. I. Evidence For a Field Halo Population

We present observations of RR Lyrae variables in the Local Group late-type spiral galaxy M33. Using the Advanced Camera for Surveys on the Hubble Space Telescope, we have identified 64 ab-type RR Lyraes in M33. We have estimated reddenings for these stars based on their minimum light V-I colors and metallicities based on their periods. From the distributions of these properties, we conclude that the RR Lyraes belong to two populations - one associated with the halo of M33 and the other with its disk. Given that RR Lyraes are produced by populations older than ~10 Gyr, this suggests that not only does the field halo of M33 contain an old component, but so does its disk. This is one of the best pieces of evidence for the existence of a halo field component in M33. Using a relation between RR Lyrae absolute magnitude and metallicity (Mv(RR) = 0.23[Fe/H] + 0.93), we estimate a mean distance modulus of <(m-M)o> = 24.67 +/- 0.08 for M33. This places M33 approximately 70 kpc beyond M31 in line-of-sight distance.

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arXiv2005

Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars

We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3 magnitudes of the white dwarf cooling sequence, and main sequence stars eight magnitudes and more below the turn-off. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/- 0.18 mag. We couple our distance estimates with extensive photometry of the cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement. (Abridged)

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arXiv2005

The Metallicity Distribution Function of Field Stars in M31's Bulge

We have used Hubble Space Telescope Wide Field Planetary Camera 2 observations to construct a color-magnitude diagram (CMD) for the bulge of M31 at a location ~1.6 kpc from the galaxy's center. Using scaled-solar abundance theoretical red giant branches with a range of metallicities, we have translated the observed colors of the stars in the CMD to abundances and constructed a metallicity distribution function (MDF) for this region. The MDF shows a peak at [M/H]~0 with a steep decline at higher metallicities and a more gradual tail to lower metallicities. This is similar in shape to the MDF of the Milky Way bulge but shifted to higher metallicities by ~0.1 dex. As is the case with the Milky Way bulge MDF, a pure closed box model of chemical evolution, even with significant pre-enrichment, appears to be inconsistent with the M31 bulge MDF. However, a scenario in which an initial infall of gas enriched the bulge to an abundance of [M/H] ~ -1.6 with subsequent evolution proceeding as a closed box provides a better fit to the observed MDF. The similarity between the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to the significant differences in the MDFs of their halo populations. This suggests that the bulk of the stars in the bulges of both galaxies were in place before the accretion events that occurred in the halos could influence them.

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arXiv2004

The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function

We present deep CCD photometry in the VI passbands using the WIYN 3.5m telescope of a field located approximately 20' southeast of the center of M33; this field includes the region studied by Mould & Kristian in their 1986 paper. The color-magnitude diagram (CMD) extends to I~25 and shows a prominent red giant branch (RGB), along with significant numbers of asymptotic giant branch and young main sequence stars. The red clump of core helium burning stars is also discernable near the limit of our CMD. The I-band apparent magnitude of the red giant branch tip implies a distance modulus of (m-M)_I = 24.77 +/- 0.06, which combined with an adopted reddening of E(V-I)=0.06 +/- 0.02 yields an absolute modulus of (m-M)_0 = 24.69 +/- 0.07 (867 +/- 28 kpc) for M33. Over the range of deprojected radii covered by our field (~8.5 to ~12.5 kpc), we find a significant age gradient with an upper limit of ~1 Gyr (~0.25 Gyr/kpc). Comparison of the RGB photometry to empirical giant branch sequences for Galactic globulars allows us to use the dereddened color of these stars to construct a metallicity distribution function (MDF). The primary peak in the MDF is at a metallicity of [Fe/H] ~ -1.0 with a tail to lower abundances. The peak does show radial variation with a slope of d[Fe/H]/dR_{deproj} = -0.06 +/- 0.01 dex/kpc. This gradient is consistent with the variation seen in the inner disk regions of M33. As such, we conclude that the vast majority of stars in this field belong to the disk of M33, not the halo as previously thought.

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arXiv2004

Variations in Star Formation History and the Red Giant Branch Tip

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (~1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M_I ~ -4.0. If more than 30% of all stars formed over the lifetime of the Universe are formed at these ages, then the distance could be overestimated by 10-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than <[Fe/H]> = -0.3. If more than ~70% of all stars formed have [Fe/H] > -0.3, the distance could be overestimated by ~10-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V-I)_0 at M_I = -3.5 can be used to test if the aforementioned age and metallicity conditions are met.

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arXiv2003

WIYN Open Cluster Study. XIX. Main Sequence Fitting Distances to Open Clusters Using V-K Color-Magnitude Diagrams

We have combined existing optical magnitudes for stars in seven open clusters and 54 field stars with the corresponding JHKs photometry from the Two Micron All Sky Survey (2MASS). Combining optical with near-IR photometry broadens the color baseline minimizing the influence of photometric errors and allows better discrimination between cluster stars and contaminating foreground and background populations. The open clusters in this study include NGC 2516, M35, M34, NGC 3532, M37, M67, and NGC 188. The field stars we are using possess high quality Hipparcos parallaxes and well-determined metal abundances allowing us to empirically determine the dependence of (V-K) color on metal abundance in the range -0.45<=[Fe/H]<=+0.35. Using this relation along with the parallaxes of the field stars, we are able to construct an unevolved main sequence in the [Mv,(V-K)o] diagram for a specific abundance. These diagrams are then used to fit to the cluster main sequences in the [V,V-K] color-magnitude diagram in order to estimate a distance for each open cluster. We find that the resultant distances are within the range of distances found in the literature via the main sequence fitting technique. It is hoped that this will spur an expansion of the current (limited) database of star clusters with high quality (V-K) photometry down to the unevolved main sequence.

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arXiv2003

WIYN Open Cluster Study XVI: Optical/Infrared Photometry and Comparisons With Theoretical Isochrones

We present combined optical/near-IR photometry (BVIK) for six open clusters - M35, M37, NGC 1817, NGC 2477, NGC 2420, and M67. The open clusters span an age range from 150 Myr to 4 Gyr and have metal abundances from [Fe/H] = -0.3 to +0.09 dex. We have utilized these data to test the robustness of theoretical main sequences constructed by several groups as denoted by the following designations - Padova, Baraffe, Y^2, Geneva, and Siess. The comparisons of the models with the observations have been performed in the [Mv, (B-V)o], [Mv, (V-I)o], and [Mv, (V-K)o] colour-magnitude diagrams as well as the distance-independent [(V-K)o, (B-V)o] and [(V-K)o, (V-I)o] two-colour diagrams. We conclude that none of the theoretical models reproduce the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence. In particular, there are significant zeropoint and shape differences between the models and the observations. We speculate that the crux of the problem lies in the precise mismatch between theoretical and observational colour-temperature relations. These results underscore the importance of pursuing the study of stellar structure and stellar modelling with even greater intensity.

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