Detection of the Red Giant Branch Stars in M82 Using the Hubble Space Telescope

We present color-magnitude diagrams and luminosity functions of stars in two halo regions of the irregular galaxy in M82, based on F555W and F814W photometry taken with the Hubble Space Telescope and Wide Field Planetary Camera 2. The I-band luminosity function shows a sudden jump at I~23.95 mag, which is identified as the tip of the red giant branch (TRGB). Adopting the Lee et al. (1993) calibration of the TRGB based on the RR Lyrae distances to Galactic globular clusters, we obtain the distance modulus of (m-M)_0=27.95 +- 0.14 (random) +- 0.16 (systematic) mag. This corresponds to a linear distance of 3.9 +- 0.3 (random) +- 0.3 (systematicf) Mpc, which agrees well with the distance of M81 deteremined from the HST observations of the Cepheid variable stars. In addition, we observe a significant number of stars apparently brighter than the TRGB. However, with the current data, we cannot rule out whether these stars are blends of fainter stars, or are indeed intermediate-age asymptotic giant branch stars.Comment: 8 figure

Impact of Rotation on the Evolution of Low-Mass Stars

High precision photometry and spectroscopy of low-mass stars reveal a variety of properties standard stellar evolution cannot predict. Rotation, an essential ingredient of stellar evolution, is a step towards resolving the discrepancy between model predictions and observations. The first rotating stellar model, continuously tracing a low-mass star from the pre-main sequence onto the horizontal branch, is presented. The predicted luminosity functions of stars of globular clusters and surface rotation velocities on the horizontal branch are discussed.Comment: 5 pages, 4 figures. Oral contrubution. Proceedings of the workshop "XXI Century challenges for stellar evolution", held in Cefalu' (Sicily, Italy), August 29 - September 2, 2007; S. Cassisi and M. Salaris Eds. To be published in Mem. SAIt Vol. 79 No.

On the primordial scenario for abundance variations within globular clusters. The isochrone test

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globulars.We have tested, with fully consistent stellar evolution calculations, if theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the Color-Magnitude-Diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations, and varying initial He mass fractions. Comparisons with 'normal' alpha-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the Red Giant Branch tip are also very weakly affected, apart from - depending on the He content of the polluting material - the Red Giant Branch bump region. We also study the distribution of stars along the Zero Age Horizontal Branch, and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.Comment: Accepted for publication in Ap

Effective temperatures of red giants in the APOKASC catalogue and the mixing length calibration in stellar models

Red giants in the updated APOGEE-Kepler catalogue, with estimates of mass, chemical composition, surface gravity and effective temperature, have recently challenged stellar models computed under the standard assumption of solar calibrated mixing length. In this work, we critically reanalyse this sample of red giants, adopting our own stellar model calculations. Contrary to previous results, we find that the disagreement between the effective temperature scale of red giants and models with solar calibrated mixing length disappears when considering our models and the APOGEE-Kepler stars with scaled solar metal distribution. However, a discrepancy shows up when alpha-enhanced stars are included in the sample. We have found that assuming mass, chemical composition and effective temperature scale of the APOGEE-Kepler catalogue, stellar models generally underpredict the change of temperature of red giants caused by alpha-element enhancements at fixed [Fe/H]. A second important conclusion is that the choice of the outer boundary conditions employed in model calculations is critical. Effective temperature differences (metallicity dependent) between models with solar calibrated mixing length and observations appear for some choices of the boundary conditions, but this is not a general resultComment: 8 pages, 10 figures, Astronomy & Astrophysics, in pres

A large stellar evolution database for population synthesis studies: VI. White dwarf cooling sequences

We present a new set of cooling models and isochrones for both H- and He-atmosphere white dwarfs, incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive - a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H- and He-burning phases, number of thermal pulses experienced by the progenitors, progenitor metallicity and the $^{12}C(\alpha,\gamma)^{16}O$ reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase, and the $^{12}C(\alpha,\gamma)^{16}O$ reaction rate. We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood, and the interpretation of the observed number ratios between H- and He-atmosphere white dwarfs. The new white dwarf sequences and an extensive set of white dwarf isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI website.Comment: 34 pages, 13 figures, The Astrophysical Journal, in pres

Galactic Globular Cluster Stars: from Theory to Observation

We use evolutionary calculations presented in a recent paper (Cassisi et al. 1998) to predict B,V,I magnitudes for stars in galactic globulars. The effect of the adopted mixing length on stellar magnitudes and colors is discussed, showing that the uncertainty on such a theoretical parameter prevents the use of MS stars as bona fide theoretical standard candles. However, comparison with Hipparcos data for field subdwarfs discloses a substantial agreement between theory and observation. Present predictions concerning the magnitude of TO and of HB stars are compared with similar results appeared in the recent literature. We present and discuss a theoretical calibration of the difference in magnitude between HB and TO as evaluated with or without element sedimentation. Finally we use theoretical HB magnitudes to best fit the CM diagram of M68 and M5, taken as representative of metal poor and intermediate metallicity galactic globulars, deriving an age of 11$\pm$1.0 Gyr and 10$\pm$1.0 Gyr, respectively, for the adopted chemical compositions, plus an additional uncertainty of $\pm$1.4 Gyr if the uncertainty on the chemical composition is taken into account. This result is discussed on the basis of current evaluations concerning cluster ages and distance moduli.Comment: 8 pages, 13 postscript figures, 6 postscript tables To be published on Astronomy & Astrophysics Supplement Serie