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

Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers

We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (rh~=1.6rc) the binary fraction (fb) lies in the range 0.08-0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable fb lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r>=1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars. Based on observations made with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal GO-6804

Deep HST-WFPC2 photometry of NGC 288. II. The Main Sequence Luminosity Function

The Main Sequence Luminosity Function (LF) of the Galactic globular cluster NGC 288 has been obtained using deep WFPC2 photometry. We have employed a new method to correct for completeness and fully account for bin-to-bin migration due to blending and/or observational scatter. The effect of the presence of binary systems in the final LF is quantified and is found to be negligible. There is a strong indication of the mass segregation of unevolved single stars and clear signs of a depletion of low mass stars in NGC 288 with respect to other clusters. The results are in good agreement with the prediction of theoretical models of the dynamical evolution of NGC 288 that take into account the extreme orbital properties of this cluster.Comment: 16 pages, 6 .ps figures. Low resolution version of fig. 1; full resolution figure soon available at http://www.bo.astro.it/bap/BAPhome.html l. Latex. emulateapj5.sty macro included. Accepted for publication by The Astronomical Journa

Dust is forming along the red giant branch of 47 TUC

We present additional evidence that dust is really forming along the red giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal branch to the RGB-tip (Origlia et al. 2007). The presence of dust had been inferred from an infrared excess in the (K-8) color, with K measured from high spatial resolution ground based near-IR photometry and "8" referring to Spitzer-IRAC 8 micron photometry. We show how (K-8) is a far more sensitive diagnostic for detecting tiny circumstellar envelopes around warm giants than colors using only the Spitzer-IRAC bands, for example the (3.6-8) color used by Boyer et al. (2010). In addition, we also show high resolution HST-ACS I band images of the giant stars which have (K-8) color excess. These images clearly demonstrate that Boyer et al (2010) statement that our detections of color excess associated with stars below the RGB-tip arise from blends and artefacts is simply not valid.Comment: 12 pages, 4 figure

ISOCAM observations of Galactic Globular Clusters: mass loss along the Red Giant Branch

Deep images in the 10 micron spectral region have been obtained for five massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (omega Cen), NGC 6388, NGC 7078 (=M15) and NGC 6715 (=M54) in the Sagittarius Dwarf Spheroidal using ISOCAM in 1997. A significant sample of bright giants have an ISOCAM counterpart but only < 20% of these have a strong mid-IR excess indicative of dusty circumstellar envelopes. From a combined physical and statistical analysis we derive mass loss rates and frequency. We find that i) significant mass loss occurs only at the very end of the Red Giant Branch evolutionary stage and is episodic, ii) the modulation timescales must be greater than a few decades and less than a million years, and iii) mass loss occurrence does not show a crucial dependence on the cluster metallicity.Comment: 26 pages, 9 figure