arXiv subjects
Publications and source records attributed to L. Carigi.
We summarise the results of recent work investigating the effects of stellar yields which take into account rotation on the predictions of chemical evolution models for early populations of galaxies.
We discuss the accuracy of the abundance determinations of H II regions in our Galaxy and other galaxies. We focus on the main observational constraints derived from abundance determinations that have implications for models of galactic chemical evolution: a) the helium to hydrogen abundance ratio, He/H; b) the oxygen to hydrogen abundance ratio, O/H; c) the carbon to oxygen abundance ratio, C/O; d) the helium to oxygen and helium to heavy elements abundance ratios, Delta Y/ Delta O and Delta Y/ Delta Z; and e) the primordial helium abundance, Yp.
We present three types of chemical evolution models for irregular galaxies: closed-box with continuous star formation rates (SFRs), closed-box with bursting SFRs, and O-rich outflow with continuous SFRs. We discuss the chemical evolution of the irregular galaxies NGC 1560 and II Zw 33, and a ``typical'' irregular galaxy. The fraction of low-mass stars needed by our models is larger than that derived for the solar vicinity, but similar to that found in globular clusters. For our typical irregular galaxy we need a mass fraction of about 40% in the form of substellar objects plus non baryonic dark matter inside the Holmberg radius, in good agreement with the results derived for NGC 1560 and II Zw 33 where we do have an independent estimate of the mass fraction in non baryonic dark matter. Closed-box models are better than O-rich outflow models in explaining the C/O and Z/O observed values for our typical irregular galaxy.
We discuss the chemical evolution of metal poor galaxies and conclude that their oxygen deficiency is not due to: the production of black holes by massive stars or a varying slope of the Initial Mass Function, IMF, at the high-mass end. A varying IMF at the low-mass end alone or in combination with: (a) an outflow of oxygen-rich material, (b) an outflow of well-mixed material, and (c) the presence of dark matter that does not participate in the chemical evolution process, is needed to explain their oxygen deficiency. Outflow of material rich in oxygen helps to account for the large helium to oxygen mass ratio values derived from these objects, but it works against explaining the carbon to oxygen mass ratio and the heavy elements minus carbon and oxygen to oxygen mass ratio values.