arXiv subjects

M. Castellani

Publications and source records attributed to M. Castellani.

41 records · Page 3Linked to original sources

arXiv1999

A new investigation on the Antlia Dwarf Galaxy

We present deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT). The CMDs present two key features: a well-defined RGB, and a sample of bright blue stars, belonging to a young stellar component. The comparison between theory and observations confirms that this sample of bright stars is consistent with the occurrence of a star formation episode $\approx0.1$ Gyr ago. In agreement with previous investigations (Sarajedini et al. 1997; Aparicio et al. 1997), we also find that this young stellar population is more centrally concentrated than the old one. By adopting the new calibration of the Tip of the RGB (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is $(m-M)_0=25.89\pm0.10$ mag, and therefore a distance $D=1.51\pm 0.07$ Mpc. This distance determination is $\approx13%$ larger than the values suggested in previous investigations. By adopting the calibration of the RGB $(V-I)$ color index as a function of the metallicity, we estimated that the mean metallicity of Antlia stellar population is of the order of $[Fe/H]\approx-1.3$. The differential RGB luminosity function shows an excess in the observed counts -at the $2σ$ level- when compared with theoretical predictions. Plain arguments on the dependence of the TRGB luminosity on stellar age suggest that this discrepancy might be due to a stellar component with an age approximately equal to 0.7 Gyr.

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arXiv1999

HST Photometry of the Fornax dSph Galaxy: cluster 4 and its field

Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dSph galaxy and its surrounding field. These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations. The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal branch (HB) stars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the positions of the HBs of the five globular clusters in Fornax, which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main-sequence suggests that star formation began in the field about 12 Gyrs ago and continued to about 0.5 Gyr ago. There are separate subgiant branches in the CMD, which indicate that the star formation was not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H] around -2.0. This is considerably lower than estimates from the integrated light of the cluster, and the origins of this discrepancy are discussed.

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arXiv1998

Stellar populations in the dwarf spheroidal galaxy Leo I

We present a detailed study of the color magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, allow us to obtain an accurate sampling of the stellar populations also at the faint magnitudes corresponding to the Main Sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium-burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimations for both the distance modulus and the age(s) for the main stellar components of Leo I are derived. More in details, from the upper luminosity of the Red Giant Branch and the lower luminosity of the Subgiant Branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m-M)_V=22.00$\pm$0.15 mag and an age of 10--15 Gyr or 9--13 Gyr, adopting a metallicity Z=0.0001 and 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip, as given by convective pulsating models.

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arXiv1997

The stellar content of the Sagittarius Dwarf Galaxy

We present V,I deep CCD photometry for three fields of the dwarf galaxy in Sagittarius (Sgr). One of the fields is centered on the globular cluster NGC 6715 (M54), which lies in one of the dense clumps of the Sgr galaxy. Comparing the CMD of Sgr with those of globular clusters which are believed to be kinematically associated with the dwarf galaxy, we conclude that the stellar population of Sgr presents a spread in metallicity, and that the dominant population (about 10 Gyr old) is extremely similar to the star content of the associated globular cluster Terzan 7. The estimated distance to Sgr is about d=24.55 Kpc.

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arXiv1996

"Not too old" metal deficient stellar populations: the case of metallicity Z=0.00001

We investigate the evolution of metal deficient stellar structures, presenting H-burning isochrones covering cluster ages from 800 Myr to 7 Gyr. Evolutionary evidences for selection effects in the metallicity distribution of very metal poor H-burning red giants are reported. The evolution of stars during central and shell He burning is further investigated, discussing the occurrence of He burning pulsators as a function of cluster age.

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