ORCID Profile
0000-0002-5021-6737
Current Organisation
Scuola Normale Superiore
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Publisher: Oxford University Press (OUP)
Date: 08-08-2013
Publisher: Oxford University Press (OUP)
Date: 09-06-2015
Publisher: Oxford University Press (OUP)
Date: 24-08-2016
Publisher: American Astronomical Society
Date: 02-2021
Abstract: We present the Stromlo Stellar Tracks, a set of stellar evolutionary tracks, computed by modifying the Modules for Experiments in Stellar Astrophysics (MESA) 1D stellar evolution package, to fit the Galactic Concordance abundances for hot ( T 8000 K) massive (≥10 M ⊙ ) main-sequence (MS) stars. Until now, all stellar evolution tracks have been computed at solar, scaled-solar, or α -element-enhanced abundances, and none of these models correctly represent the Galactic Concordance abundances at different metallicities. This paper is the first implementation of Galactic Concordance abundances to the stellar evolution models. The Stromlo tracks cover massive stars (10 ≤ M / M ⊙ ≤ 300) with varying rotations ( v / v crit = 0.0, 0.2, 0.4) and a finely s led grid of metallicities (−2.0 ≤ [Z/H] ≤ +0.5 Δ[Z/H] = 0.1) evolved from the pre-main sequence to the end of 12 C burning. We find that the implementation of Galactic Concordance abundances is critical for the evolution of MS, massive hot stars in order to estimate accurate stellar outputs ( L , T eff , g ), which, in turn, have a significant impact on determining the ionizing photon luminosity budgets. We additionally support prior findings of the importance that rotation plays on the evolution of massive stars and their ionizing budget. The evolutionary tracks for our Galactic Concordance abundance scaling provide a more empirically motivated approach than simple uniform abundance scaling with metallicity for the analysis of H ii regions and have considerable implications for determining nebular emission lines and metallicity. Therefore, it is important to refine existing stellar evolutionary models for comprehensive high-redshift extragalactic studies. The Stromlo tracks are available to the astronomical community.
Publisher: Oxford University Press (OUP)
Date: 09-08-2014
Publisher: Oxford University Press (OUP)
Date: 11-02-2022
Abstract: Understanding the evolution of the N/O ratio in the interstellar medium (ISM) of galaxies is essential if we are to complete our picture of the chemical evolution of galaxies at high redshift, since most observational calibrations of O/H implicitly depend upon the intrinsic N/O ratio. The observed N/O ratio, however, shows large scatter at low O/H, and is strongly dependent on galactic environment. We show that several heretofore unexplained features of the N/O distribution at low O/H can be explained by the N seen in metal-poor galaxies being mostly primary nitrogen that is returned to the ISM via pre-supernova winds from rapidly rotating massive stars (M ≳ 10 M⊙, v/vcrit ≳ 0.4). This mechanism naturally produces the observed N/O plateau at low O/H. We show that the large scatter in N/O at low O/H also arises naturally from variations in star-formation efficiency. By contrast, models in which the N and O come primarily from supernovae provide a very poor fit to the observed abundance distribution. We propose that the peculiar abundance patterns we observe at low O/H are a signature that dwarf galaxies retain little of their SN ejecta, leaving them with abundance patterns typical of winds.
Publisher: Oxford University Press (OUP)
Date: 12-11-2022
Abstract: Star formation models predict that the metal-poor initial mass function (IMF) can be substantially different from that observed in the metal-rich Milky Way. This changeover occurs because metal-poor gas clouds cool inefficiently due to their lower abundance of metals and dust. However, predictions for the metal-poor IMF to date rely on assuming solar-scaled abundances, i.e. [X/O] = 0 at all [O/H]. There is now growing evidence that elements such as C and O that dominate metal line cooling in the ISM do not follow solar scaling at low metallicities. In this work, we extend models that predict the variation in the characteristic (or the peak) IMF mass as a function of metallicity using [C/O] ratios derived from observations of metal-poor Galactic stars and of H ii regions in dwarf galaxies. These data show [C/O] & 0 at subsolar [O/H], which leads to a substantially different metal-poor IMF in the metallicity range where C i and C ii cooling dominate ISM thermodynamics, resulting in an increase in the characteristic mass by a factor as large as 7. An important consequence of this difference is a shift in the location of the transition from a top- to a bottom-heavy IMF upwards by 0.5–1 dex in metallicity. Our findings indicate that the IMF is very sensitive to the assumptions around solar-scaled ISM compositions in metal-poor systems (e.g. dwarf galaxies, the Galactic halo, and metal-poor stars) that are a key focus of JWST.
Publisher: Oxford University Press (OUP)
Date: 02-04-2020
Abstract: The evolutionary paths taken by massive stars with M ≳ 60 M⊙ remain substantially uncertain. They begin their lives as main-sequence (MS) O stars. Depending on their masses, rotation rates, and metallicities, they can then encounter a wide range of evolutionary states with an equally broad set of possible surface compositions and spectral classifications. We present a new grid of calculations for the evolution of such stars that covers a broad range in mass, M/M⊙ = 60–150, rotation rate, $v/v_{\\rm crit} = 0$–0.6, metallicity, [Fe/H] = −4 to 0, and α-element enhancement, [α/Fe] = 0–0.4. We show that rotating stars undergo rotationally induced dredge-up of nucleosynthetic products, mostly He and N, to their surfaces while still on the MS. Non-rotating metal-rich stars also reveal the products of nucleosynthesis on their surfaces because even modest amounts of mass-loss expose their ‘fossil’ convective cores: regions that are no longer convective, but were part of the convective core at an early stage in the star’s evolution. Thus, surface enhancement of He and N is expected for rotating stars at all metallicities, and for non-rotating stars if they are relatively metal-rich. We calculate a stellar atmosphere for a representative model from our grid, properly accounting for He and N enhancement, and show that the resulting spectrum provides a good match to observed WNL stars, strongly suggesting that the physical mechanisms we have identified are the ultimate cause of the WNL phase.
Publisher: MDPI AG
Date: 19-10-2021
Abstract: We review the current scenario of long-duration Gamma-ray burst (LGRB) progenitors, and in addition, present models of massive stars for a mass range of 10–150M⊙ with ΔM=10M⊙ and rotation rate v/vcrit=0 to 0.6 with a velocity resolution Δv/vcrit=0.1. We further discuss possible metallicity and rotation rate distribution from our models that might be preferable for the creation of successful LGRB candidates given the observed LGRB rates and their metallicity evolution. In the current understanding, LGRBs are associated with Type-Ic supernovae (SNe). To establish LGRB-SN correlation, we discuss three observational paths: (i) space-time coincidence, (ii) evidence from photometric light curves of LGRB afterglows and SN Type-Ic, (iii) spectroscopic study of both LGRB afterglow and SN. Superluminous SNe are also believed to have the same origin as LGRBs. Therefore, we discuss constraints on the progenitor parameters that can possibly dissociate these two events from a theoretical perspective. We further discuss the scenario of single star versus binary star as a more probable pathway to create LGRBs. Given the limited parameter space in the mass, mass ratio and separation between the two components in a binary, binary channel is less likely to create LGRBs to match the observed LGRB rate. Despite effectively-single massive stars are fewer in number compared to interacting binaries, their chemically homogeneous evolution (CHE) might be the major channel for LGRB production.
Publisher: Oxford University Press (OUP)
Date: 09-2023
Publisher: Cambridge University Press (CUP)
Date: 11-2022
DOI: 10.1017/S1743921322000722
Abstract: Understanding the nucleosynthetic origin of nitrogen and the evolution of the N/O ratio in the interstellar medium is crucial for a comprehensive picture of galaxy chemical evolution at high-redshift because most observational metallicity (O/H) estimates are implicitly dependent on the N/O ratio. The observed N/O at high-redshift shows an overall constancy with O/H, albeit with a large scatter. We show that these heretofore unexplained features can be explained by the pre-supernova wind yields from rotating massive stars (M≳10M ⊙ ,ν/ν crit ≳0.4). Our models naturally produce the observed N/O plateau, as well as the scatter at low O/H. We find the scatter to arise from varying star formation efficiency. However, the models that have supernovae dominated yields produce a poor fit to the observed N/O at low O/H. This peculiar abundance pattern at low O/H suggests that dwarf galaxies are most likely to be devoid of SNe yields and are primarily enriched by pre-supernova wind abundances.
No related grants have been discovered for Arpita Roy.