Barbara Paltrinieri

Blue Straggler Stars: The Spectacular Population in M80*

Using Hubble Space Telescope WFPC2 observations in two ultraviolet (UV) filters (F225W and F336W) of the central region of the high-density Galactic globular cluster (GGC) M80, we have identified 305 blue straggler stars (BSS), which represents the largest and most concentrated population of BSS ever observed in a GGC. We also identify the largest clean sample of evolved BSS yet found. The high stellar density alone cannot explain the BSS, and we suggest that in M80 we are witnessing a transient dynamical state, during which stellar interactions are delaying the core-collapse process leading to an exceptionally large population of collisional BSS.

Multimodal Distributions along the Horizontal Branch

We report on the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) U, V, and far-ultraviolet observations of three galactic globular clusters (GGCs), NGC 5272 = M3, NGC 6205 = M13, and NGC 6093 = M80. Two of these clusters (namely, M13 and M80) have horizontal-branch (HB) tails that extend to the helium-burning main sequence, with the hottest stars reaching theoretical effective temperatures above 35,000 K. In both clusters, groups of stars are found to be separated by narrow gaps along the blue HB sequence. These gaps appear at similar locations in the color-magnitude diagrams of the two clusters. While stochastic effects may give rise to variations in the color distribution along the HB, the coincidence of gaps in different clusters effectively rules this out as the primary cause. The comparison among the clusters strongly suggests that there are separate physical processes operating during the earlier red giant phase of evolution to produce mass loss.

Blue Straggler Stars: A Direct Comparison of Star Counts and Population Ratios in Six Galactic Globular Clusters

The central regions of six Galactic globular clusters (GGCs) (M3, M80, M10, M13, M92, and NGC 288) have been imaged using HST-WFPC2 and the ultraviolet filters (F255W and F336W). The selected sample covers a large range in both central density (log ρ0) and metallicity ([Fe/H]). In this paper, we present a direct cluster-to-cluster comparison of the blue straggler star (BSS) population as selected from (m255, m255-m336) color magnitude diagrams. We have found (1) that BSSs in three of the clusters (M3, M80, M92) are much more concentrated toward the center of the cluster than the red giants; because of the smaller BSS samples for the other clusters, we can only note that the BSS radial distributions are consistent with central concentration; and (2) that the specific frequency of BSSs varies greatly from cluster to cluster. The most interesting result is that the two clusters with largest BSS specific frequency are at the central density extremes of our sample: NGC 288 (lowest central density) and M80 (highest). This evidence, together with the comparison with theoretical collisional models, suggests that both stellar interactions in high-density cluster cores and at least one other alternate channel operating in low-density GGCs play an important role in the production of BSSs. We also note a possible connection between horizontal-branch morphology and blue straggler luminosity functions in these six clusters.

Blue Stragglers, Young White Dwarfs, and UV-Excess Stars in the Core of 47 Tucanae*

We used a set of archived Hubble Space Telescope/WFPC2 images to probe the stellar population in the core of the nearby galactic globular cluster (GGC) 47 Tuc. From the ultraviolet (UV) color magnitude diagrams (CMDs) obtained for ~4000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (1) 43 blue stragglers stars (BSSs), including 20 new candidates; (2) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; and (3) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The colors of the WD candidates identified here define a clean pattern in the CMDs, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster—if the existence of such a large population is confirmed, we have finally found the long-searched-for population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by Chandra and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions, together with the huge stellar crowding in the cluster center, prevents statistically reliable identifications based only on positional coincidences.

Hubble Space Telescope Ultraviolet Observations of the Cores of M3 and M13

We present preliminary results from Hubble Space Telescope (HST)/WFPC2 observations of the central regions of the of the Galactic globular clusters M13 and M3. The clusters are almost identical in most respects, including chemical composition, but there are dramatic differences in both the horizontal-branch (HB) and blue straggler populations. The M13 HB has a long blue tail extending 4.5 mag in V, reaching well below the level of the main-sequence turnoff. M3 has no such feature. M3 and M13 are thus an extreme case of the second-parameter problem in HB morphology. Also present in the M13 HB are two gaps similar to those seen in the clusters NGC 6752 and NGC 2808. M3 has a specific frequency of blue stragglers 3 times larger than that of M13. Our results imply that neither age nor cluster density, two popular second-parameter candidates, are likely to be responsible for the observed differences.

The Luminosity Function of M3

We present a high-precision, large-sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground-based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fitted by canonical standard stellar models from the red giant branch (RGB) tip to below the main-sequence turnoff point. Specifically, neither the RGB LF bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

VLT Observations of the Peculiar Globular Cluster NGC 6712. III. The Evolved Stellar Population

We present extensive UBVR photometry of the Galactic globular cluster (GGC) NGC 6712 obtained with the ESO Very Large Telescope (VLT) which reaches down to 2 mag below the main-sequence turn-off and allows us for the first time to determine the age of this cluster. By using the apparent luminosity of the zero age horizontal branch (ZAHB), VZAHB = 16.32 ± 0.05 and the stellar main-sequence (MS) turn-off (TO) magnitude VTO = 19.82 ± 0.10, we obtain ΔV = 3.5 ± 0.1 (a value fully compatible with that derived for other clusters), which suggests that, at an age of ~12 Gyr, NGC 6712 is coeval with other GGC of similar metallicity. We derive interstellar reddening by comparing the position and morphology of the red giant branch (RGB) with a wide variety of reference clusters and find E(B-V) = 0.33 ± 0.05, a value significantly lower than had been determined previously. Assuming this value for the reddening, we determine a true distance modulus of (m-M)0 = 14.55, corresponding to a distance of ~8 kpc. We find a population of 108 candidate blue straggler stars (BSSs), surprisingly large when compared with the typical BSS content of other low-concentration clusters. Moreover, we detect a very bright blue star in the core of NGC 6712 that might be a post-AGB star. These results, combined with those already presented in two companion papers, strongly support the hypothesis that NGC 6712 was, at some early epoch of its history, much more massive and concentrated. The continued interaction with the bulge and the disk of the Galaxy has driven it toward dissolution, and what we now observe is nothing but the remnant core of a cluster that once was probably one of the most massive in the Galaxy.

ROSAT HRI X-Ray Observations of the Open Globular Cluster NGC 288

A ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288, which is located near the South Galactic Pole. This is the first deep X-ray image of this system. We detect a Low Luminosity Globular Cluster X-ray source (LLGCX) RXJ005245.0-263449 with an X-ray luminosity of (5.5+-1.4)x10^32 ergs/s (0.1-2.0 keV), which is located very close to the cluster center. There is evidence for X-ray variability on a time scale of <~ 1 day. The presence of this LLGCX in such an open cluster suggests that dense stellar systems with high interaction rates are not needed to form LLGCXs. We also searched for diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are L_X^h < 9.5x10^32 ergs/s (0.52-2.02 keV) and L_X^s < 9.3x10^32 ergs/s (0.11-0.41 keV). These imply upper limits to the diffuse X-ray to optical light ratios in NGC 288 which are lower than the values observed for X-ray faint early-type galaxies. This indicates that the soft X-ray emission in these galaxies is due either to a component which is not present in globular clusters (e.g., interstellar gas, or a stellar component which is not found in low metallicity Population II systems), or to a relatively small number of bright Low Mass X-ray Binaries (LMXBs).