The evolution of CNO isotopes: the impact of massive stellar rotators

Abstract

Chemical abundances and abundance ratios measured in galaxies provide precious information about the mechanisms, modes and time scales of the assembly of cosmic structures. Yet, the nucleogenesis and chemical evolution of elements heavier than helium are dictated mostly by the physics of the stars and the shape of the stellar mass spectrum. In particular, estimates of CNO isotopic abundances in the hot, dusty media of high-redshift starburst galaxies offer a unique glimpse into the shape of the stellar initial mass function (IMF) in extreme environments that can not be accessed with direct observations (star counts). Underlying uncertainties in stellar evolution and nucleosynthesis theory, however, may hurt our chances of getting a firm grasp of the IMF in these galaxies. In this work, we adopt new yields for massive stars, covering different initial rotational velocities. First, we implement the new yield set in a well-tested chemical evolution model for the Milky Way. The calibrated model is then adapted to the specific case of a prototype submillimeter galaxy (SMG). We show that, if the formation of fast-rotating stars is favoured in the turbulent medium of violently star-forming galaxies irrespective of metallicity, the IMF needs to be skewed towards high-mass stars in order to explain the CNO isotope ratios observed in SMGs. If, instead, stellar rotation becomes negligible beyond a given metallicity threshold, as is the case for our own Galaxy, there is no need to invoke a top-heavy IMF in starbursts.

Carbon, oxygen, and iron abundances in disk and halo stars. Implications of 3D non-LTE spectral line formation

“…5.1) appears to be consistent with the Milky Way having had two main infall episodes, the latter responsible for forming the thin disk (Chiappini et al 1997(Chiappini et al , 2001. The location of this underdensity corresponds to the discontinuity in several of the two-infall Galactic chemical evolution models recently presented in Romano et al (2019), that marks the onset of the second infall episode (see their Fig. 1) and thus separates the younger thin disk from the older thick disk (e.g.…”

High-redshift star formation in the ALMA era

“…However, based on the multiple transitions from 13 CO detected in their stacked spectrum, Spilker et al (2014) estimate that ALMA will be able to detect (and even resolve) these faint lines in a (admittedly bright) L IR = 5 × 10 13 L galaxy in only 30 min per line. This could open up a new window into the cosmic isotope enrichment history, including providing a dust-insensitive probe of the stellar initial mass function (IMF; as proposed by Romano et al 2017Romano et al , 2019. Indeed, Zhang et al (2018b) find low 13 CO/C 18 O abundance ratios for a sample of four strongly lensed SMGs at z 2 − 3 observed with ALMA and, based on the models of Romano et al (2017Romano et al ( , 2019, argue that these ratios imply top-heavy IMFs in high-redshift SMGs.…”

“…This could open up a new window into the cosmic isotope enrichment history, including providing a dust-insensitive probe of the stellar initial mass function (IMF; as proposed by Romano et al 2017Romano et al , 2019. Indeed, Zhang et al (2018b) find low 13 CO/C 18 O abundance ratios for a sample of four strongly lensed SMGs at z 2 − 3 observed with ALMA and, based on the models of Romano et al (2017Romano et al ( , 2019, argue that these ratios imply top-heavy IMFs in high-redshift SMGs. This would be consistent with the results from the earliest attempts at modelling SMGs in the cosmological context (Baugh et al 2005), though more recent models can reproduce submillimetre number counts without the need to invoke a top-heavy IMF (Lagos et al 2019).…”

Nitrogen isotopes in the interstellar medium: A chemical journey across the Galaxy

“…They predict a linear positive trend up to 8 kpc because of novae outbursts as the main way to produce the 15 N, and a flattening trend above this distance because of the low-level star formation and gas infall on long timescales (inside-out galaxy formation) that reduce the nova e↵ect (see e.g. [19,20]). GCE models can reproduce the observational trend but not the absolute values.…”