Distances to Populous Clusters in the LMC via the K-Band Luminosity of the Red Clump

We present results from a study of the distances and distribution of a sample of intermediate-age clusters in the Large Magellanic Cloud. Using deep near-infrared photometry obtained with ISPI on the CTIO 4m, we have measured the apparent K-band magnitude of the core helium burning red clump stars in 17 LMC clusters. We combine cluster ages and metallicities with the work of Grocholski & Sarajedini to predict each cluster's absolute K-band red clump magnitude, and thereby calculate absolute cluster distances. An analysis of these data shows that the cluster distribution is in good agreement with the thick, inclined disk geometry of the LMC, as defined by its field stars. We also find that the old globular clusters follow the same distribution, suggesting that the LMC's disk formed at about the same time as the globular clusters, ~ 13 Gyr ago. Finally, we have used our cluster distances in conjunction with the disk geometry to calculate the distance to the LMC center, for which we find (m-M)o = 18.40 +/- 0.04_{ran} +/- 0.08_{sys}, or Do = 47.9 +/- 0.9 +/- 1.8 kpc.Comment: 31 pages including 5 figures and 7 tables. Accepted for publication in the August 2007 issue of A

HST/ACS Photometry of Old Stars in NGC 1569: The Star Formation History of a Nearby Starburst

(abridged) We used HST/ACS to obtain deep V- and I-band images of NGC 1569, one of the closest and strongest starburst galaxies in the Universe. These data allowed us to study the underlying old stellar population, aimed at understanding NGC 1569's evolution over a full Hubble time. We focus on the less-crowded outer region of the galaxy, for which the color-magnitude diagram (CMD) shows predominantly a red giant branch (RGB) that reaches down to the red clump/horizontal branch feature (RC/HB). A simple stellar population analysis gives clear evidence for a more complicated star formation history (SFH) in the outer region. We derive the full SFH using a newly developed code, SFHMATRIX, which fits the CMD Hess diagram by solving a non-negative least squares problem. Our analysis shows that the relative brightnesses of the RGB tip and RC/HB, along with the curvature and color of the RGB, provide enough information to ameliorate the age-metallicity-extinction degeneracy. The distance/reddening combination that best fits the data is E(B-V) = 0.58 +/- 0.03 and D = 3.06 +/- 0.18 Mpc. Star formation began ~ 13 Gyr ago, and this accounts for the majority of the mass in the outer region. However, the initial burst was followed by a relatively low, but constant, rate of star formation until ~ 0.5-0.7 Gyr ago when there may have been a short, low intensity burst of star formation.Comment: 50 pages, including 17 figures. Accepted for publication in A

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.Comment: Accepted for publication in MNRAS. 12 pages, 37 figures, 4 tables. High resolution figures available from http://www.astro.ufl.edu/~aaron/opt_ir_figs

Calcium II Triplet Spectroscopy of LMC Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters

Abridged Abstract - Utilizing the FORS2 instrument on the VLT, we have obtained near infrared spectra for more than 200 stars in 28 populous LMC clusters. This cluster sample spans a large range of ages (~ 1-13 Gyr) and metallicities (-0.3 > [Fe/H] > -2.0) and has good areal coverage of the LMC disk. The strong absorption lines of the Calcium II triplet are used to derive cluster radial velocities and abundances. We determine mean cluster velocities to typically 1.6 km/s and mean metallicities to 0.04 dex (random error). For eight of these clusters, we report the first spectroscopically determined metallicities based on individual cluster stars, and six of these eight have no published radial velocity measurements. (continued in paper)Comment: 26 pages of text plus 14 figures and 6 tables. Accepted for publication in AJ. Scheduled for Vol. 132, No. 4 (October 2006

Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud - III. Horizontal branch morphology

We leverage new high-quality data from Hubble Space Telescope program GO-14164 to explore the variation in horizontal branch morphology among globular clusters in the Large Magellanic Cloud (LMC). Our new observations lead to photometry with a precision commensurate with that available for the Galactic globular cluster population. Our analysis indicates that, once metallicity is accounted for, clusters in the LMC largely share similar horizontal branch morphologies regardless of their location within the system. Furthermore, the LMC clusters possess, on average, slightly redder morphologies than most of the inner halo Galactic population; we find, instead, that their characteristics tend to be more similar to those exhibited by clusters in the outer Galactic halo. Our results are consistent with previous studies, showing a correlation between horizontal branch morphology and age.DMis grateful for support from an Australian Research Council (ARC) Future Fellowship (FT160100206). DG gratefully acknowledges support from the Chilean BASAL Centro de Excelencia en Astrofisica y Tecnologıas Afines (CATA) grant PFB-06/2007

Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud - IV. Evidence for multiple populations in Hodge 11 and NGC 2210

We present a multiple population search in two old Large Magellanic Cloud (LMC) Globular clusters, Hodge 11 and NGC 2210. This work uses data from the Advanced Camera for Surveys and Wide Field Camera 3 on the Hubble Space Telescope from programme GO- 14164 in Cycle 23. Both of these clusters exhibit a broadened main sequence with the second population representing (20±~5) per cent forNGC2210 and (30±~5) per cent forHodge 11. In both clusters, the smaller population is redder than the primary population, suggesting CNO variations. Hodge 11 also displays a bluer second population in the horizontal branch, which is evidence for helium enhancement. However, even though NGC 2210 shows similarities to Hodge 11 in the main sequence, there does not appear to be a second population on NGC 2210's horizontal branch. This is the first photometric evidence that ancient LMC Globular clusters exhibit multiple stellar populations.Fil: Gilligan, Christina K.. Dartmouth College; Estados UnidosFil: Chaboyer, Brian. Dartmouth College; Estados UnidosFil: Cummings, Jeffrey D.. University Johns Hopkins; Estados UnidosFil: Mackey, Dougal. Australian National University. Research School of Astronomy & Astrophysics; AustraliaFil: Cohen, Roger E.. Space Telescope Science Institute; Estados UnidosFil: Geisler, Douglas. Universidad de La Serena; ChileFil: Grocholski, Aaron J.. University of Washington; Estados UnidosFil: Parisi, Maria Celeste. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Sarajedini, Ata. University of Florida; Estados UnidosFil: Ventura, Paolo. Osservatorio Astronomico Di Roma; ItaliaFil: Villanova, Sandro. Universidad de Concepción; ChileFil: Yang, Soung-Chul. Korea Astronomy And Space Science Institute; Corea del SurFil: Wagner-Kaiser, Rachel. University of Florida; Estados Unido

Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud - V. Multiple populations in ancient globular clusters

We examine four ancient LargeMagellanic Cloud (LMC) globular clusters (GCs) for evidence of multiple stellar populations using the Advanced Camera for Surveys andWide Field Camera 3 on the Hubble Space Telescope Programme GO-14164. NGC 1466, NGC 1841, and NGC 2257 all show evidence for a redder, secondary population along themain sequence. Reticulum does not showevidence for the presence of a redder population, but thisGChas the least number of stars and Monte Carlo simulations indicate that the sample of main-sequence stars is too small to robustly infer whether a redder population exists in this cluster. The second, redder, population of the other three clusters constitutes ∼ 30 - 40 per cent of the total population along the main sequence. This brings the total number of ancient LMC GCs with known split or broadened main sequences to five. However, unlike for Hodge 11 and NGC 2210 (see Gilligan et al. (2019)), none of the clusters shows evidence for multiple populations in the horizontal branch. We also do not find evidence of a second population along the red giant branch.Fil: Gilligan, Christina K.. Dartmouth College; Estados UnidosFil: Chaboyer, Brian. Dartmouth College; Estados UnidosFil: Cummings, Jeffrey D.. University Johns Hopkins; Estados UnidosFil: Mackey, Dougal. Australian National University; AustraliaFil: Cohen, Roger E.. Space Telescope Science Institute; Estados UnidosFil: Geisler, Douglas. Universidad de Concepción; ChileFil: Grocholski, Aaron J.. University of Florida; Estados UnidosFil: Parisi, Maria Celeste. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomía Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomía Teórica y Experimental; ArgentinaFil: Sarajedini, Ata. University of Florida; Estados UnidosFil: Ventura, Paolo. Osservatorio Astronomico Di Roma; ItaliaFil: Villanova, Sandro. Universidad de Concepción; ChileFil: Yang, Soung-Chul. Korea Astronomy And Space Science Institute; Corea del SurFil: Wagner-Kaiser, Rachel. University of Florida; Estados Unido

Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud - IV. Evidence for multiple populations in Hodge 11 and NGC 2210

We present a multiple population search in two old Large Magellanic Cloud (LMC) Globular clusters, Hodge 11 and NGC 2210. This work uses data from the Advanced Camera for Surveys and Wide Field Camera 3 on the Hubble Space Telescope from programme GO14164 in Cycle 23. Both of these clusters exhibit a broadened main sequence with the second population representing (20 ±∼5) per cent for NGC 2210 and (30±∼5) per cent for Hodge 11. In both clusters, the smaller population is redder than the primary population, suggesting CNO variations. Hodge 11 also displays a bluer second population in the horizontal branch, which is evidence for helium enhancement. However, even though NGC 2210 shows similarities to Hodge 11 in the main sequence, there does not appear to be a second population on NGC 2210’s horizontal branch. This is the first photometric evidence that ancient LMC Globular clusters exhibit multiple stellar populations.This research has used NASA’s Astrophysics Data System. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5- 26555. This work was supported in part by STScI through a grant HST-GO-14164. DG gratefully acknowledges support from the Chilean Centro de Excelencia en Astrof´ısica y Tecnolog´ıas Afines (CATA) BASAL grant AFB-170002. DG also acknowledges financial support from the Direccion de Investigaci ´ on y Desarrollo ´ de la Universidad de La Serena through the Programa de Incentivo a la Investigacion de Acad ´ emicos (PIA-DIDULS). DM gratefully ´ acknowledges support from an Australian Research Council (ARC) Future Fellowship (FT160100206). SV gratefully acknowledges the support provided by Fondecyt reg. n. 1170518