arXiv subjects

F. Fusi Pecci

Publications and source records attributed to F. Fusi Pecci.

58 records · Page 4Linked to original sources

arXiv1994

Globular Clusters in the Magellanic Clouds.II: IR-Array Photometry for 12 Globulars and Contributions to the Integrated Cluster Light

We report the results of the observations in the JHK bands of 12 globular clusters in the LMC, and present CMDs down to K=16 for about 450 stars. We merge the data with the BV ones presented in paper I and use the merged data to study the evolution of integrated magnitudes and colours of SSPs. We find that the AGB contributes about 60 percent of integrated clusters light in K, while the contribution from RGB is correlated with the age of the cluster. The age at which the first appearance of the RGB takes place depends on the models adopted ranging from about 600 Myr in the "classical" models to about 1.5 Gyr with the "overshooting" models. Both models give the same fractional contribution of the RGB to the cluster light, in agreement with the Fuel Consumption Theorem. The overall picture is consistent with the early conclusions of Persson et al. 1983 and Frogel et al. 1990 that J-K colour is driven by AGB stars, V-K is controlled by AGB and RGB stars and B-V is partially influenced by the whole population redder than the RGB clump, but is strongly dependent on the fading and reddening of the TO stars with age.

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arXiv1994

Low Mass X-Ray Binaries in Globular Clusters: A New Metallicity Effect

Galactic Globular Clusters (GCs) containing bright X sources (L_x > 10^{36} erg/s), commonly associated with Low Mass X-Ray Binaries (LMXBs), are found to be significantly denser and more metal-rich than normal non-X-ray clusters both in the Galaxy and in M31. Within a framework where LMXBs in GCs are generated via tidal captures in high-density clusters and (2+1) encounters in low-density globulars, the higher incidence of LMXBs with increasing metallicity is shown to be {\it intrinsic} and not just a by-product of other effects. Two possible mechanisms are examined: the first one assumes a dependence of the cluster IMF on metallicity as recently published in the literature. The observed LMBXs, more frequently occurring in metal rich clusters, agrees with the predicted number of NS only if metallicity accounts for a minor contribution to the observed variation of the IMF slope. Other alternatives explored, such as the total variation of the observed IMF slopes is due to (a) just metallicity and (b) the combination of metallicity and position in the Galaxy lead to a clear-cut disagreement with the data. The second mechanism assumes that, at fixed cluster density, the rate of tidal captures depends on radius and mass of the capturing star. Based on standard stellar models, stars with higher metal content have wider radii and higher masses, hence the rate of tidal captures increases with increasing metallicity. From the order of magnitude computations made, this new effect by itself could explain the observed ratio of 4 between the frequencies of X-ray clusters in the metal-rich and metal-poor groups we observationally determined. However, there is no reason to exclude that both mechanisms can be at work.

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arXiv1994

Blue Stragglers in Galactic Globular Clusters: Playing with Specific Quantities

We present preliminary results obtained from the comparison of the specific frequencies of Blue Straggler Stars (BSS) detected so far in a sample of 26 Galactic globular clusters. The number of BSS seems to increase almost linearly with increasing the amount of sampled light in loose clusters, while it drops abruptly for clusters having intermediate-high central densities. In particular, a simple interpretative scenario where the BSS in loose clusters are produced from primordial binaries and those detected in high density globulars are due to star interactions leading to binary formation, merging. etc. seems compatible with these early results. The possibility that this observational evidence could be ascribed to systematic biases mostly related to the increasing difficulty to detect BSS candidates with increasing cluster concentration, is also discussed.

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arXiv1993

HST Observations of Globular Clusters in M31.I:Surface Photometry of 13 Objects

We present the initial results of a study of globular clusters in M31, using the Faint Object Camera (FOC) on the Hubble Space Telescope (HST). The sample of objects consists of 13 clusters spanning a range of properties. Three inde- pendent image deconvolution techniques were used in order to compensate for the optical problems of the HST, leading to mutually fully consistent results. We present detailed tests and comparisons to determine the reliability and limits of these deconvolution methods, and conclude that high-quality surface photome- try of M31 globulars is possible with the HST data. Surface brightness profiles have been extracted, and core radii, half-light radii, and central surface brightness values have been measured for all of the clusters in the sample. Their comparison with the values derived from ground-based observations indi- cates the later to be systematically and strongly biased by the seeing effects, as it may be expected. A comparison of the structural parameters with those of the Galactic globulars shows that the structural properties of the M31 globu- lars are very similar to those of their Galactic counterparts. A candidate for a post-core-collapse cluster, Bo 343 = G 105, has been already identified from these data; this is the first such detection in the M31 g. c. system.

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