Helium-burning Red Clump Stars Research Articles

Distances to Populous Clusters in the Large Magellanic Cloud via theK-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 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)0 = 18.40 ± 0.04 ± 0.08 , or D0 = 47.9 ± 0.9 ± 1.8 kpc.

Young star clusters immersed in intermediate-age fields in the Small Magellanic Cloud

We present CCD photometry in the Washington C and T 1 filters for six star clusters (B 34, NGC 256, NGC 265, NGC 294, ΙC 1611 and NGC 376) in the Small Magellanic Cloud (SMC) and their surrounding fields. The resultant colour-magnitude diagrams (CMDs) extend from T 1 ∼ 14 to as faint as T 1 ∼ 22 revealing the main-sequence turnoffs of the clusters. Adopting a metallicity of Z = 0.004, we compare our cluster photometry with theoretical isochrones in the Washington system in order to derive ages. To facilitate age determination of the surrounding fields, we use the magnitude difference between the helium-burning red clump stars and the main-sequence turnoff. Finally, we estimate mean metallicities for the field stars by comparing the location of the field red giant branch with standard giant branches for Galactic globular clusters of known abundance, corrected for age effects. Combining these results with our previous work, we find a clear trend of younger clusters being located closer to the centre of the SMC. In addition, there is a tendency for the mean metallicity and its dispersion to be greater inside 4° of the SMC's centre as compared to outside this radius. As far as the properties of the field stars are concerned, we find little correlation between the ages of the clusters and those of the field stars against which they are projected. Clearly, more work needs to be done to clarify these trends.

New galactic open cluster candidates from DSS and 2MASS imagery

An inspection of the DSS and 2MASS images of selected Milky Way regions has led to the discovery of 66 stellar groupings whose morphologies, color-magnitude diagrams, and stellar density distributions suggest that these objects are possible open clusters that do not yet appear to be listed in any catalogue. For 24 of these groupings, which we consider to be the most likely to be candidates, we provide extensive descriptions on the basis of 2MASS photometry and their visual impression on DSS and 2MASS. Of these cluster candidates, 9 have fundamental parameters determined by fitting the color-magnitude diagrams with solar metallicity Padova isochrones. An additional 10 cluster candidates have distance moduli and reddenings derived from K magnitudes and (J-K) color indices of helium-burning red clump stars. As an addendum, we also provide a list of a number of apparently unknown galactic and extragalactic objects that were also discovered during the survey.

The 2MASS Color-Magnitude Diagram of the Globular Cluster Lyngå 7

We have extracted JHKS photometry from the All-Sky Point Source Catalog of the Two Micron All Sky Survey (2MASS) in the vicinity of the globular cluster Lynga 7. The cluster color-magnitude diagram (CMD) extends from the tip of the red giant branch to approximately 1 mag below the core helium burning red clump stars. The reddening-independent red giant branch slope in the 2MASS photometric system is measured to be Δ(J - KS)/ΔKS = -0.104 ± 0.004, which implies a metal abundance of [Fe/H] = -0.76 ± 0.06 on the Carretta & Gratton scale. Adopting a reddening of E(B - V) = 0.73 ± 0.12 as the average of previously published values, we use the inferred absolute K-band magnitude of the red clump [MK(RC) = -1.28 ± 0.07] to calculate a distance of (m - M)0 = 14.33 ± 0.10 (7.3 ± 0.3 kpc). Finally, we have reanalyzed the previously published optical CMD of Lynga 7 from Ortolani and coworkers along with the 2MASS near-IR photometry and conclude that Lynga 7 is close in age to 47 Tucanae, if not slightly (~2 Gyr) younger.

A Warp in the Large Magellanic Cloud Disk?

We present a study of the shape of the Large Magellanic Cloud (LMC) disk. We use the brightnesses of core helium burning red clump stars identified in V-I, I color-magnitude diagrams of 50 randomly selected LMC fields observed with the Cerro Tololo Inter-American Observatory 0.9 m telescope to measure relative distances to the fields. Random photometric errors and errors in the calibration are controlled to 1%. Following correction for reddening measured through the color of the red clump, we solve for the inclination and position angle of the line of nodes of the tilted plane of the LMC, finding i = 358 ± 24 and θ = 145° ± 4°. Our solution requires that we exclude 15 fields in the southwest of the LMC that have red clump magnitudes ~0.1 mag brighter than the fitted plane. On the basis of these fields, we argue that the LMC disk is warped and twisted, containing features that extend up to 2.5 kpc out of the plane. We argue that alternative ways of producing red clump stars brighter than expected, such as variations in age and metallicity of the stars, are unlikely to explain our observations.

Deep [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] WFPC2 Photometry of M31’s Thick Disk (?)

We present deep color-magnitude diagrams (CMDs) for a field along the outer disk of M31 based on archival Hubble Space Telescope Wide Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. The CMDs, which contain a total of about 50,000 stars, feature a prominent red giant branch (RGB) along with a significant population of helium-burning red clump stars. In addition, they exhibit the rarely seen asymptotic giant branch clump, as well as a weak Population II horizontal branch. There is also the hint of a ~2 Gyr subgiant branch at the faintest levels of the CMDs. After adopting an M31 distance of (m - M)0 = 24.5 and a reddening of E(B-V) = 0.08, we draw the following conclusions. (1) The I-band absolute magnitude of the helium-burning red clump stars is MI(RC) = -0.29 ? 0.05, which is in accord with the value derived from Hipparcos parallaxes of solar neighborhood clump stars by Stanek & Garnavich. (2) The metallicity distribution function constructed from bright RGB stars shows a characteristic shape; however, a pure halo population consisting of metal-poor and intermediate-metallicity components (as advocated in the literature) is not sufficient to account for this shape. Instead, an additional Gaussian component with [Fe/H] = -0.22 ? 0.26, comprising 70% of the total number of stars, is required. (3) A comparison of our CMD with theoretical isochrones indicates that the majority of stars in our M31 field have ages that are 1.5 Gyr. (4) These points, along with the physical location of our field in M31, suggest that we are observing the thick-disk population of this galaxy.