arXiv subjects

Leo Girardi

Publications and source records attributed to Leo Girardi.

112 records · Page 7Linked to original sources

arXiv1999

A secondary clump of red giant stars: why and where

Based on the results of detailed population synthesis models, it was recently claimed that the clump of red giants in the colour-magnitude diagram (CMD) of composite stellar populations should present an extension to lower luminosities, which goes down to about 0.4 mag below the main clump. This feature is made of stars just massive enough for having ignited helium in non-degenerate conditions. In this paper, we go into more details about the origin and properties of this feature. We first compare the clump theoretical models with data for clusters of different ages and metallicities, basically confirming the predicted behaviours. We then refine the previous models in order to show that: (i) The faint extension is expected to be clearly separated from the main clump in the CMD of metal-rich populations, defining a secondary clump by itself. (ii) It should be present in all galactic fields containing ~1 Gyr old stars and with mean metallicities higher than about Z=0.004. (iii) It should be particularly strong, if compared to the main red clump, in galaxies which have increased their star formation rate in the last Gyr or so of their evolution. In fact, secondary clumps similar to the model predictions are observed in the CMD of nearby stars from Hipparcos data, and in those of some LMC fields observed to date. There are also several reasons why this secondary clump may be missing or hidden in other observed CMDs of galaxy fields (e.g. photometric errors or differential absorption larger than 0.2 mag). Nonetheless, this structure may provide important constraints to the star formation history of Local Group galaxies. We comment also on the intrinsic luminosity variation and dispersion of clump stars, which may limit their use as either absolute or relative distance indicators.

astro-ph↗

arXiv1999

The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds

We investigate the formation of carbon stars as a function of the stellar mass and parent metallicity. Theoretical modelling is based on an improved scheme for treating the third dredge-up in synthetic calculations of thermally pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual criterion (based on a constant minimum core mass for the occurrence of dredge-up, M_c^min) is replaced by one on the minimum temperature at the base of the convective envelope, T_b^dred, at the stage of the post-flash luminosity maximum. Envelope integrations then allow determination of M_c^min as a function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus inferring if and when dredge-up first occurs. Moreover, the final possible shut down of the process is predicted. Extensive grids of TP-AGB models were computed using this scheme. We present and discuss the calibration of the two dredge-up parameters (lambda and T_b^dred) aimed at reproducing the carbon star luminosity function (CSLF) in the LMC. It turns out that the faint tail is almost insensitive to the history of star formation rate (SFR) in the parent galaxy (it is essentially determined by T_b^dred), in contrast to the bright wing which may be more affected by the details of the recent SFR. Once the faint end is reproduced, the peak location is a stringent calibrator of lambda. The best fit to the observed CSLF in the LMC is obtained with Z=0.008, lambda=0.50, log(T_b^dred)=6.4, and a constant SFR up to 5x10^8 yr ago. A good fit to the CSLF in the SMC is then easily derived from the Z=0.004 models, with a single choice of parameters, and a constant SFR over the entire significant age interval. The results are consistent with the theoretical expectation that the third dredge-up is more efficient at lower Zs.

astro-ph↗

arXiv1998

Near IR photometry of the old open clusters Berkeley 17 and Berkeley 18. Probing the age of the galactic disc

We report on near IR ($J$ and $K$ bands) observations of two $8 \times 8 \~(arcmin)^{2}$ regions centered on the old open clusters Berkeley~17 and Berkeley~18, for which only optical photometry (in $B$,$V$ and $I$ bands) exists. $J$ and $K$ photometry allows us to obtain an independent estimate of cluster metallicity by means of the relationship between the spectroscopic metallicity and the Red Giant Branch (RGB) slope calibrated by Tiede et al (1997). From the analysis of the colour magnitude diagram (CMD) and luminosity function (LF), Berkeley~17 turns out to have a metal content $[Fe/H]~\sim~-0.35$. It is $9~Gyr$ old, suffers from a reddening $E(B-V) = 0.58 mag$ and has an heliocentric distance of $2.5 kpc$. Berkeley~17 comes out to be substantially younger than in previous work (age $\approx 13~Gyr$). On the other hand Berkeley~18 is found to have solar metal abundance, and to be younger than Berkeley~17, with an age of about $4~Gyr$. While we confirm Kaluzny (1997) reddening estimate, we significantly revise the distance of the cluster, which lies $4.5 kpc$ from the Sun. These results on two open clusters believed to be between the oldest put constraints on the age and the evolution of the Galactic Disc. The absence of clusters older than $8-9~Gyr$ suggests the possibility that the Galaxy underwent a star formation minimum between 13 and $10~Gyr$ ago.

astro-ph↗

arXiv1998

The evolution of the V-K colours of single stellar populations

We discuss how the integrated V-K colours of single stellar populations evolve with age and metallicity. Some of the uncertainties associated to the properties of the underlying stellar models are throughfully discussed. First, we present the effects that different formulations for the mass-loss rates produce on the initial-final mass relation, and hence on the integrated colours. Second, we describe the colour changes occurring at the onset of the AGB and RGB stars. The onset of the AGB is marked by a colour jump to the red, whose amplitude is highly dependent on the metallicity and mass-loss rates adopted in the models. The overluminosity with respect to the standard Mc-L relation, that occurs in the most massive AGB stars, causes a smoothing of this colour jump. On the other hand, we find that a transient red phase takes place ~1.5x10^8 yr after the RGB develops. We then discuss the possible reasons for the transition of V-K colours (from ~1.5 to 3) that takes place in LMC clusters of SWB type IV. A revision of the ages attributed to the clusters reveals that the transition may not be as fast as originally suggested. A gradual (or delayed) transition of the colours, as predicted by models which include the overluminosity of the most massive AGB stars, describes better the data than the sudden colour-jump predicted by classical models.

astro-ph↗