Search for: All records

ABSTRACT Using a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age versus metallicity or abundance ratios versus age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] versus metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary versus secondary or odd elements) and on the lifetimes of their sources (short-lived versus long-lived ones). The latter property is critical in determining the single- or double- branch behaviour of an elementary abundance ratio in the Solar neighbourhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different time-scales and help determining the site of e.g. the r-process(es). We define the ‘abundance distance’ between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behaviour in other disc regions. 

ABSTRACT Carbon-enhanced metal-poor (CEMP) stars are the living fossils holding records of chemical enrichment from early generations of stars. In this work, we perform a set of numerical simulations of the enrichment from a supernova (SN) of a first generation of metal-free (Pop III) star and the gravitational collapse of the enriched cloud, considering all relevant cooling/heating processes and chemical reactions as well as the growth of dust grains. We adopt faint SN models for the first time with progenitor masses MPopIII = 13–$$80 \ {\rm M_{\bigodot }}$$, which yield C-enhanced abundance patterns ([C/Fe] = 4.57–4.75) through mixing and fallback of innermost layers of the ejecta. This model also considers the formation and destruction of dust grains. We find that the metals ejected by the SN can be partly re-accreted by the same dark matter minihalo, and carbon abundance of the enriched cloud A(C) = 3.80–5.06 is lower than the abundance range of observed CEMP stars (A(C) ≳ 6) because the mass of the metals ejected by faint SNe is smaller than normal core-collapse SNe due to extensive fallback. We also find that cloud fragmentation is induced by gas cooling from carbonaceous grains for $$M_{\rm Pop III}= 13 \ {\rm M_{\bigodot }}$$ even with the lowest iron abundance [Fe/H] ∼ −9. This leads to the formation of low-mass stars, and these ‘giga metal-poor’ stars can survive until the present-day Universe and may be found by future observations.