Chemical Yields from the First Stars

We examine the dependence of stellar yields on the metallicity Z of the stellar population. This effect may be important for the very first chemical enrichment from Population III stars, at very low Z. In the range of massive stars, mass loss rates varying with Z have remarkable effects. We also estimate chemical yields from Very Massive Objects (from 120 to 1000 Mo), which might have formed more easily in the very low-Z environment of the first stellar generations.Comment: 5 pages, 7 figures, to appear in The First Stars, Proceedings of the second MPA/ESO workshop, Eds.: Weiss, Abel, Hill, Springer, Heidelberg, 200

Star formation and chemical evolution in SPH simulations: a statistical approach

In Smoothed Particles Hydrodynamics (SPH) codes with a large number of particles, star formation as well as gas and metal restitution from dying stars can be treated statistically. This approach allows to include detailed chemical evolution and gas re-ejection with minor computational effort. Here we report on a new statistical algorithm for star formation and chemical evolution, especially conceived for SPH simulations with large numbers of particles, and for parallel SPH codes. For the sake of illustration, we present also two astrophysical simulations obtained with this algorithm, implemented into the Tree-SPH code by Lia & Carraro (2000). In the first one, we follow the formation of an individual disc-like galaxy, predict the final structure and metallicity evolution, and test resolution effects. In the second one we simulate the formation and evolution of a cluster of galaxies, to demonstrate the capabilities of the algorithm in investigating the chemo-dynamical evolution of galaxies and of the intergalactic medium in a cosmological context.Comment: 17 pages, 20 figures, accepted for publication on MNRA

Accurate fundamental parameters for Lower Main Sequence Stars

We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the InfraRed Flux Method to multi-band photometry. Our study is based on 104 stars for which we have excellent BVRIJHK photometry, excellent parallaxes and good metallicities. Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement. A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in literature shows that systematic effects still exist in the calibrations at the level of a few percent. Our InfraRed Flux Method temperature scale is 100K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3% smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology. Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices. We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well chosen set of G and K dwarfs would go a long way to addressing this problem.Comment: 34 pages, 20 figures. Accepted by MNRAS. Landscape table available online at http://users.utu.fi/luccas/IRFM

A chemical evolution model for galaxy clusters

We develop a toy-model for the chemical evolution of the intracluster medium, polluted by the galactic winds from elliptical galaxies. The model follows the "galaxy formation history" of cluster galaxies, constrained by the observed luminosity function.Comment: 6 pages, 10 figures, to appear in the Proceedings of the Workshop "Chemical Enrichment of Intracluster and Intergalactic Medium", Vulcano, Italy, 14-18 May 2001, ASP Conference Serie

The Tully-Fisher relation and its evolution with redshift in cosmological simulations of disc galaxy formation

We present predictions on the evolution of the Tully-Fisher (TF) relation with redshift, based on cosmological N-body/hydrodynamical simulations of disc galaxy formation and evolution. The simulations invoke star formation and stellar feedback, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling and effects of a meta-galactic UV field, including simplified radiative transfer. At z=0, the simulated and empirical TF relations are offset by about 0.4 magnitudes (1 sigma) in the B and I bands. The origin of these offsets is somewhat unclear, but it may not necessarily be a problem of the simulations only. As to evolution, we find a brightening of the TF relation between z=0 and z=1 of about 0.85 mag in rest-frame B band, with a non-evolving slope. The brightening we predict is intermediate between the (still quite discrepant) observational estimates. This evolution is primarily a luminosity effect, while the stellar mass TF relation shows negligible evolution. The individual galaxies do gain stellar mass between z=1 and z=0, by a 50-100%; but they also correspondingly increase their characteristic circular speed. As a consequence, individually they mainly evolve ALONG the stellar mass TF relation, while the relation as such does not show any significant evolution