The First Empirical Mass Loss Law for Population II Giants

Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars show an excess of mid-infrared light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using a simple model and our observations we derive mass loss rates for these stars. Finally, we obtain the first empirical mass loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass loss rate is considerably shallower than the widely used Reimers Law. The results presented here are the first from our Spitzer survey of a carefully chosen sample of 17 Galactic Globular Clusters, spanning the entire metallicity range from about one hundredth up to almost solar

Towards the absolute planes: a new calibration of the Bolometric Corrections and Temperature scales for Population II Giants

We present new determinations of bolometric corrections and effective temperature scales as a function of infrared and optical colors, using a large database of photometric observations of about 6500 Population II giants in Galactic Globular Clusters (GGCs), covering a wide range in metallicity (-2.0<[Fe/H]<0.0). New relations for BC_K vs (V-K), (J-K) and BC_V vs (B-V), (V-I), (V-J), and new calibrations for T_eff, using both an empirical relation and model atmospheres, are provided. Moreover, an empirical relation to derive the R parameter of the Infrared Flux Method as a function of the stellar temperature is also presented.Comment: 10 pages, 12 .ps figures, MN Latex, accepted by MNRA

RR Lyrae variables in the globular cluster M3 (NGC5272). I. BVI CCD photometry

New BVI CCD photometry is presented for 60 RR Lyrae variables in the globular cluster M3. Light curves have been constructed and ephemerides have been (re)-derived for all of them. Four stars (i.e. V29, V136, V155 and V209), although recognized as variables, had no previous period determinations. Also, the period derived for V129 is significantly different from the one published by Sawyer-Hogg (1973). Light curve parameters, i.e. mean magnitudes, amplitudes and rise-times, have been derived. The discussion of these results in the framework of the stellar evolution and pulsation theories will be presented in a forthcoming paper.Comment: 19 pages, latex, uses mn.sty, 12 encapsulated figures, to be published in MNRAS, text and figures also available at http://www.bo.astro.it/bap/BAPhome.html or via anonymous ftp at ftp://boas3.bo.astro.it/bap/files (bap98-12-textfig.ps

Globular Clusters in the Magellanic Clouds.I:BV CCD-Photometry for 11 Clusters

We present BV CCD-data for 11 intermediate-age LMC clusters; the main conclusions are: 1. in the (V_to, V_cl,m) and (V-to, (V_to-V_cl,m)) planes the models yield a good overall description of the data; 2. with the current sample, it is impossible to firmly choose between "classical" and "overshooting" models; 3. the separation in colour between the MS band and the Red He-burning Clump is smaller than predicted by theoretical tracks; 4. the existence of the so-called "RGB phase-transition (Renzini and Buzzoni 1986) seems to be confirmed.Comment: 62 pages, 37 figures and tables 6 to 16 available on request, uuencoded compressed postscript file with tables 1-5 and 17-18 included, BAP 08-1994-020-OA

An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters

The main aim of the present work is to derive an empirical mass-loss (ML) law for Population II stars in first and second ascent red giant branches. We used the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8 micron range of a carefully chosen sample of 15 Galactic globular clusters spanning the entire metallicity range and sampling the vast zoology of horizontal branch (HB) morphologies. We complemented the IRAC photometry with near-infrared data to build suitable color-magnitude and color-color diagrams and identify mass-losing giant stars. We find that while the majority of stars show colors typical of cool giants, some stars show an excess of mid-infrared light that is larger than expected from their photospheric emission and that is plausibly due to dust formation in mass flowing from them. For these stars, we estimate dust and total (gas + dust) ML rates and timescales. We finally calibrate an empirical ML law for Population II red and asymptotic giant branch stars with varying metallicity. We find that at a given red giant branch luminosity only a fraction of the stars are losing mass. From this, we conclude that ML is episodic and is active only a fraction of the time, which we define as the duty cycle. The fraction of mass-losing stars increases by increasing the stellar luminosity and metallicity. The ML rate, as estimated from reasonable assumptions for the gas-to-dust ratio and expansion velocity, depends on metallicity and slowly increases with decreasing metallicity. In contrast, the duty cycle increases with increasing metallicity, with the net result that total ML increases moderately with increasing metallicity, about 0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars, we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&