Stellar mass loss, rotation and the chemical enrichment of early-type galaxies

We present a comparison between the [Ca, C, N/Fe]-mass relations observed in local spheroids and the results of a chemical evolution model which already successfully reproduces the [Mg/Fe]-mass and the [Fe/H]-mass relations in these systems. We find that the [Ca/Fe]-mass relation is naturally explained by such a model without any additional assumption. In particular, the observed underabundance of Ca with respect to Mg can be attributed to the different contributions from Type Ia and Type II supernovae to the nucleosynthesis of these two elements. For C and N, we consider new stellar yields that take into account stellar mass loss and rotation. These yields have been shown to successfully reproduce the C and N abundances in Milky Way metal-poor stars. The use of these new stellar yields produces a good agreement between the chemical evolution model predictions and the integrated stellar population observations for C. In the case of N, the inclusion of fast rotators and stellar mass-loss nucleosynthesis prescriptions improves our predictions for the slope of the [N/Fe] versus sigma relation, but a zero-point discrepancy of 0.3 dex remains. This discrepancy cannot be removed, either by increasing the N yields or by assuming a larger amount of fast rotators in spheroids, because in both cases this leads to an overproduction of the N abundances in the gas phase in these galaxies at high redshift (e.g. the Lyman break galaxy MS 1512-cB58). This work demonstrates that current stellar yields are unable to simultaneously reproduce the large mean stellar [< N/Fe >] ratios inferred from integrated spectra of elliptical galaxies in Sloan Digital Sky Survey and the low N abundance measured in the gas of high-redshift spheroids from absorption lines. However, since chemical evolution models for the Milky Way computed with the Geneva stellar yields constitute at present the only way to account for the N/O, C/O and (12)C/(13)C abundance ratios observed in very metal-poor halo stars, it seems reasonable to suggest that there may be uncertainties in either the inferred stellar or gas-phase N abundances at the level of similar to 0.3 dex.