arXiv subjects

L. Federici

Publications and source records attributed to L. Federici.

43 records · Page 3Linked to original sources

arXiv2003

Correlations of Globular Cluster Properties: Their Interpretations and Uses

Correlations among the independently measured physical properties of globular clusters (GCs) can provide powerful tests for theoretical models and new insights into their dynamics, formation, and evolution. We review briefly some of the previous work, and present preliminary results from a comparative study of GC correlations in the Local Group galaxies. The results so far indicate that these diverse GC systems follow the same fundamental correlations, suggesting a commonality of formative and evolutionary processes which produce them.

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arXiv2002

The Andromeda Project. I. Deep HST-WFPC2 V,I photometry of 16 fields toward the disk and the halo of the M31 galaxy. Probing the stellar content and metallicity distribution

HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the luminous nearby spiral galaxy M31, sampling the stellar content of the disk and the halo at different distances from the center, from ~ 20 to ~ 150 arcmin (i.e. ~ 4.5 to 35 kpc), down to limiting V and I magnitudes of ~ 27. The Color-Magnitude diagrams (CMD) show the presence of complex stellar populations, including an intermediate age/young population and older populations with a wide range of metallicity. Those fields superposed on the disk of M31 generally show a blue plume of stars which we identify with main sequence members. Accordingly, the star formation rate over the last 0.5 Gyr appears to have varied dramatically with location in the disk. All the CMDs show a prominent Red Giant Branch (RGB) with a descending tip in the V band, characteristic of metallicity higher than 1/10 Solar. A red clump is detected in all of the fields, and a weak blue horizontal branch is frequently present. The metallicity distributions (MDs), obtained by comparison of the RGB stars with globular cluster templates, are basically similar in all the sampled fields: they all show a long, albeit scantly populated metal-poor tail and a main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas the fraction of metal-poor stars seems to be approximately constant in all fields, the fraction of very-metal-rich stars varies with position and seems to be more prominent in those fields superposed on the disk and/or with the presence of streams or substructures. This might indicate and possibly trace interaction effects with some companion, e.g. M32.

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arXiv2001

RR Lyrae Variables in the Globular Clusters of M31: a First Detection of Likely Candidates

The purpose of this paper is to show that RR Lyrae variables exist and can be detected in M31 globular clusters. We report on the first tentative identification of RR Lyrae candidates in four globular clusters of the Andromeda galaxy, i.e. G11, G33, G64 and G322. Based on HST-WFPC2 archive observations in the F555W and F814W filters spanning a total interval of about 5 consecutive hours we find evidence for 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322, respectively. Several more candidates can be found by relaxing slightly the selection criteria. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters, starting from the very blue HB in G11, and progressively moving to redder HBs in G64, G33 and G322.

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arXiv2001

A Survey of M31 Globular Clusters using WFPC2 on board HST

We report new, as yet unpublished HST/WFPC2 V, I photometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters, including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=--0.7 with a tail toward low metallicity, resembling the halo of NGC 5128; metal rich giants appear to be centrally concentrated.

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arXiv1996

The \mv vs \fe Calibration: I. HST Color-Magnitude Diagrams of 8 Globular Clusters in M 31

Color Magnitude Diagrams (CMDs) of individual stars in 8 Globular Clusters in M31 down to about 1 mag fainter (V$\sim 26.5$) than the Horizontal Branch have been obtained with the Hubble Space Telescope. In particular, we observed G280 and G351 with the FOC (f/96+F430W, f/96+F480LP) while the WFPC2 (F555W,F814W) frames for G1,G58,G105,G108,G219+Bo468 were retrieved from the HST archive. The cluster metallicities -\fe- range from -1.8 to -0.4. Coupled with sufficiently accurate (to $\sim\pm0.1$ mag) measures of the mean brightness of the HB --\vhb--, appropriate estimates of reddening for each cluster, and the adoption of a distance modulus to M31 of \dmo = 24.43, this has allowed us to yield a direct calibration for the mean absolute magnitude of the HB at the instability strip --\mv-- with varying \fe: M_V^{HB} = (0.13 \pm 0.07)\fe + (0.95 \pm 0.09), where the associated errors result from the adopted global errors in the measure and best fitting procedures. The slope of the derived relation is fully consistent with that predicted by the standard and canonical models ($\sim0.15$) and obtained by various ground-based observations, while it is only marginally compatible with higher values ($\sim0.30$), also obtained in the past. The zero-point, which is crucial to absolute age determinations, depends on the adopted distance to M31 and is moreover affected by an additional error due to the residual uncertainties in the \hst photometric zero-points ($\sim0.05$ mag, at least). If confirmed, such a calibration of the \mv \vs \fe relationship would imply old absolute ages ($> 16$Gyr) for the oldest Galactic globulars and fairly small age spread among those having a constant magnitude difference between the Main-Sequence Turnoff and the HB.

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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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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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