Aaron J. Grocholski

WIYN Open Cluster Study. X. The K-Band Magnitude of the Red Clump as a Distance Indicator

In an effort to improve the utility of the helium-burning red clump luminosity as a distance indicator, we explore the sensitivity of the K-band red clump absolute magnitude [MK(RC)] to metallicity and age. We rely upon JK photometry for 14 open clusters and two globular clusters from the Second Incremental Data Release of the Two Micron All Sky Survey (2MASS) Point Source Catalog. The distances, metallicities, and ages of the open clusters are all on an internally consistent system, while the K(RC) values are measured from the 2MASS data. For clusters younger than ~2 Gyr, MK(RC) is insensitive to metallicity but shows a dependence on age. In contrast, for clusters older than ~2 Gyr, MK(RC) is influenced primarily by the metallicity of the population and shows little or no dependence on the age. Theoretical red clump models based on the formalism of Girardi et al. reinforce this finding. Over comparable metallicity and age ranges, our average MK(RC) value is in accord with that based on solar neighborhood red clump stars with Hipparcos parallaxes. Lastly, we compute the distance to the open cluster NGC 2158 using our red clump calibration. Adopting an age of 1.6 ± 0.2 Gyr and [Fe/H] = -0.24 ± 0.06, our calibration yields a distance of (m - M)V = 14.38 ± 0.09.

Ca II Triplet Spectroscopy of Large Magellanic Cloud Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters

Using the FORS2 instrument on the Very Large Telescope, 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 Ca II triplet are used to derive cluster radial velocities and abundances. We determine mean cluster velocities to typically 1.6 km s-1 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. Combining our data with archival Hubble Space Telescope WFPC2 photometry, we find that the newly measured cluster, NGC 1718, is one of the most metal-poor ([Fe/H] ~ -0.80) intermediate-age (~2 Gyr) inner disk clusters in the LMC. Similar to what was found by previous authors, this cluster sample has radial velocities consistent with that of a single rotating disk system, with no indication that the newly reported clusters exhibit halo kinematics. In addition, our findings confirm previous results that show that the LMC lacks the metallicity gradient typically seen in nonbarred spiral galaxies, suggesting that the bar is driving the mixing of stellar populations in the LMC. However, in contrast to previous work, we find that the higher metallicity clusters (-1.0 dex) in our sample show a very tight distribution (mean [Fe/H] = -0.48, σ = 0.09), with no tail toward solar metallicities. The cluster distribution is similar to what has been found for red giant stars in the bar, which indicates that the bar and the intermediate-age clusters have similar star formation histories. This is in good agreement with recent theoretical models that suggest the bar and intermediate-age clusters formed as a result of a close encounter with the SMC ~4 Gyr ago.

A New Hubble Space Telescope Distance to NGC 1569: Starburst Properties and IC 342 Group Membership

We present deep HST ACS/WFC photometry of the dwarf irregular galaxy NGC 1569, one of the closest and strongest nearby starburst galaxies. These data allow us, for the first time, to unequivocally detect the tip of the red giant branch and thereby determine the distance to NGC 1569. We find that this galaxy is 3.36 ± 0.20 Mpc away, considerably farther away than the typically assumed distance of 2.2 ± 0.6 Mpc. Previously thought to be an isolated galaxy due to its shorter distance, our new distance firmly establishes NGC 1569 as a member of the IC 342 group of galaxies. The higher density environment may help explain the starburst nature of NGC 1569, since starbursts are often triggered by galaxy interactions. On the other hand, the longer distance implies that NGC 1569 is an even more extreme starburst galaxy than previously believed. Previous estimates of the rate of star formation for stars younger than 1 Gyr become stronger by more than a factor of 2. Stars older than this were not constrained by previous studies. The dynamical masses of NGC 1569s three super star clusters, which are already known as some of the most massive ever discovered, increase by ~53% to (6-7) × 105 M☉.

Distances to Populous Clusters in the Large Magellanic Cloud 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 (LMC). Using deep near-infrared photometry obtained with ISPI on the CTIO 4 m, 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 and Sarajedini to predict each clusters 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 LMCs 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)0 = 18.40 ± 0.04 ± 0.08 , or D0 = 47.9 ± 0.9 ± 1.8 kpc.

WIYN Open Cluster Study – XVI. Optical/infrared photometry and comparisons with theoretical isochrones

We present combined optical/near-infrared 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] = -0.27 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 [M V , (B - V) 0 ], [M V , (V - I) 0 ] and [M V , (V - K) 0 ] colour-magnitude diagrams as well as the distance-independent [(V - K) 0 , (B - V) 0 ] and [(V - K) 0 , (V - I) 0 ] two-colour diagrams. We conclude that none of the theoretical models reproduces the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence. In particular, there are significant zero-point 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.

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 allowing 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)0] 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.

LMC Cluster Abundances and Kinematics

We present results from a project aimed at better understanding the kinematics and metallicities of populous clusters in the LMC. We have utilized FORS2 on the VLT to obtain infrared spectra for more than 200 stars in 28 LMC clusters spanning a large range of ages (~ 1–13 Gyr) and metallicities (-0.3 > [Fe/H] > -2.0). The absorption lines of the calcium II triplet were then used to calculate radial velocities and [Fe/H]. We determine mean cluster velocities to typically 1.6 km s-1 and mean metallicities to 0.04 dex (random error). Similar to what was found by previous authors, this cluster sample has motions consistent with that of a single rotating disk system, with no indication of halo kinematics. However, in contrast to previous work, we find that the higher metallicity clusters in our sample show a very tight [Fe/H] distribution with no tail toward solar metallicities. The cluster distribution is similar to what has been found for red giant stars in the bar, which indicates that the bar and the intermediate age clusters have similar star formation histories. This is in good agreement with recent theoretical models that suggest the bar and intermediate age clusters formed as a result of a close encounter with the SMC. Our findings also confirm previous results which show that the LMC lacks the metallicity gradient typically seen in non-barred spiral galaxies, suggesting that the bar is driving the mixing of stellar populations in the LMC.

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.

Metallicity and kinematics of a large sample of LMC and SMC clusters

We have carried out a large-scale investigation of the metallicity and kinematics for a number of LMC and SMC star clusters using Ca ii triplet spectra obtained at the VLT. Our sample includes 28 LMC and 16 SMC clusters, covering a wide range of ages and spatial extent of the host galaxy. We determine mean cluster velocities to about 2 km s-1 and metallicities to 0.05 dex (random error), from about 7 members per cluster. Herein we present the main results for this study for the cluster metallicity distributions, metallicity gradients, age-metallicity relations and kinematics.

Detailed Properties of Populous Clusters in the Large Magellanic Cloud

We present results from a series of projects aimed at better understanding the ages, velocities, metallicities, and distances of populous clusters in the LMC. Using FORS2 on the VLT, we obtained near-infrared spectra for more than 200 stars in 28 LMC clusters, which span a large range of ages and metallicities. The strong absorption lines of the Ca II triplet are used to calculate cluster velocities and abundances. We determine cluster ages through main sequence fitting of theoretical isochrones to deep optical photometry using a combination of published photometry, VLT FORS2 images and archival HST WFPC2 images. As shown by Grocholski & Sarajedini, a cluster’s age and [Fe/H] can be used to predict the K-band luminosity of core helium burning red clump (RC) stars in that cluster. Using ISPI on the CTIO 4m telescope, we imaged 17 LMC clusters in the J− and K−bands, with the resulting photometry reaching …0.5 mag below the RC. We combine the measured apparent RC magnitudes with the predicted absolute values to determine absolute distances to each of these clusters and thereby probe the structure of the LMC as traced by its intermediate-age clusters.