Identification of a Class of Low-Mass Asymptotic Giant Branch Stars Struggling to Become Carbon Stars in the Magellanic Clouds

We have identified a new class of Asymptotic Giant Branch (AGB) stars in the Small and Large Magellanic Clouds (SMC/LMC) using optical to infrared photometry, light curves, and optical spectroscopy. The strong dust production and long-period pulsations of these stars indicate that they are at the very end of their AGB evolution. Period-mass-radius relations for the fundamental-mode pulsators give median current stellar masses of 1.14 M_sun in the LMC and 0.94 M_sun in the SMC (with dispersions of 0.21 and 0.18 M_sun, respectively), and models suggest initial masses of <1.5 M_sun and <1.25 M_sun, respectively. This new class of stars includes both O-rich and C-rich chemistries, placing the limit where dredge-up allows carbon star production below these masses. A high fraction of the brightest among them should show S star characteristics indicative of atmospheric C/O ~ 1, and many will form O-rich dust prior to their C-rich phase. These stars can be separated from their less-evolved counterparts by their characteristically red J-[8] colors.Comment: 16 pages, 18 figures, accepted for publication in Ap

Stellar Crowding and the Science Case for Extremely Large Telescopes

We present a study of the effect of crowding on stellar photometry. We develop an analytical model through which we are able to predict the error in magnitude and color for a given star for any combination of telescope resolution, stellar luminosity function, background surface brightness, and distance. We test our predictions with Monte Carlo simulations of the LMC globular cluster NGC 1835, for resolutions corresponding to a seeing-limited telescope, the $HST$, and an AO-corrected 30-m (near diffraction limited) telescope. Our analytically predicted magnitude errors agree with the simulation results to within $\sim$20%. The analytical model also predicts that errors in color are strongly affected by the correlation of crowding--induced photometric errors between bands as is seen in the simulations. Using additional Monte Carlo simulations and our analytical crowding model, we investigate the photometric accuracy which 30-m and 100-m Extremely Large Telescopes (ELTs) will be able to achieve at distances extending to the Virgo cluster. We argue that for stellar populations work, ELTs quickly become crowding-limited, suggesting that low--Strehl AO systems may be sufficient for this type of science.Comment: 25 pages, 19 figures in 35 separate files, Astronomical Journal, accepte

Star Formation Histories from HST Color-Magnitude Diagrams of Six Fields of the LMC

We present results on the analysis of background field stars found in HST WFPC2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce V-I,V color-magnitude diagrams of the field stars and use them to explore the LMC's star formation history. The photometry approaches V~26, well below the turnoff of an ancient ~14 Gyr LMC population of stars. We use a maximum-likelihood technique to fit model CMDs drawn from Bertelli et al. (1994) isochrones to the observed CMDs. We constrain the models by the age-metallicity relation derived from LMC clusters, test four IMF slopes, and fit for the reddening, distance modulus, and star formation rate. We find that we can just resolve structure in SFR(t) with time steps of ~0.15 in log age, implying a resolution of ~4x10^8 years at an age of 1 Gyr. For a Salpeter IMF, our derived star formation history for the NGC 1754 field is characterized by an enhanced star formation rate over the past 4 Gyr, qualitatively resembling that derived by others for a variety of LMC fields. The remaining four fields, however, appear to have had high levels of star formation activity as long as 5-8 Gyr ago; these fields lie in the LMC Bar while the NGC 1754 field lies in the disk, suggesting that the inner regions of the LMC contain significantly more older stars than the outer regions. Examining the residuals of the models and observations, we find that the old red giant branches of the models provide a poor fit to the observations, which suggests an error in the model isochrones. The effect of the disagreement appears to be to underestimate the contribution of the old population.Comment: 48 pages, 19 Postscript figures, LaTeX, accepted by the Astronomical Journa

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

The Physical Properties of Red Supergiants: Comparing Theory and Observations

Red supergiants (RSGs) are an evolved stage in the life of intermediate massive stars (than than 25 solar masses). For many years, their location in the H-R diagram was at variance with the evolutionary models. Using the MARCS stellar atmospheres, we have determined new effective temperatures and bolometric luminosities for RSGs in the Milky Way, LMC, and SMC, and our work has resulted in much better agreement with the evolutionary models. We have also found evidence of significant visual extinction due to circumstellar dust. Although in the Milky Way the RSGs contribute only a small fraction (than than 1 percent) of the dust to the interstellar medium (ISM), in starburst galaxies or galaxies at large look-back times, we expect that RSGs may be the main dust source. We are in the process of extending this work now to RSGs of higher and lower metallicities using the galaxies M31 and WLM.Comment: Invited review; to appear in Massive Stars as Cosmic Engines, IAU Symp. 250, ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University Press