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Chemical enrichment of stars due to accretion from the ISM during the Galaxy's assembly


Shen, Sijing; Kulkarni, Girish; Madau, Piero; Mayer, Lucio (2017). Chemical enrichment of stars due to accretion from the ISM during the Galaxy's assembly. Monthly Notices of the Royal Astronomical Society, 469(4):4012-4021.

Abstract

Using the Eris zoom-in cosmological simulation of assembly of a Milky Way analogue, we study the chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar medium (ISM) during the Galaxy's development. We consider metal-poor and old stars in the Galactic halo and bulge through the use of stellar orbits, gas density and metallicity distributions in Eris. Assuming spherically symmetric Bondi–Hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about $10^{−24}$ and $10^{−22} M\odot yr^{−1}$, respectively, at redshifts $z \lesssim 3$, but this accretion rate increases roughly a hundred-fold to about $10^{−20} M\odot yr^{−1}$ at higher redshifts due to increased gas density. Bulge and halo stars accrete similar amounts of metals at high redshifts when kinematically distinct bulge and halo have not yet developed, and both sets of stars encounter a similar metal distribution in the ISM. Accretion alone can enrich main-sequence stars up to [Fe/H] $\sim$ −2 in extreme cases, with the median enrichment level due to accretion of about [Fe/H] $\sim$ −6 to −5. Because accretion mostly takes place at high redshifts, it is $\alpha$-enriched to [$\alpha$/Fe] $\sim$ 0.5. We find that accretive metal enrichment is sufficient to affect the predicted metallicity distribution function of halo stars at [Fe/H] < −5. This can hinder attempts to infer natal chemical environment of metal-poor stars from their observed enrichment. Peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling the natal and accreted metal content of stars.

Abstract

Using the Eris zoom-in cosmological simulation of assembly of a Milky Way analogue, we study the chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar medium (ISM) during the Galaxy's development. We consider metal-poor and old stars in the Galactic halo and bulge through the use of stellar orbits, gas density and metallicity distributions in Eris. Assuming spherically symmetric Bondi–Hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about $10^{−24}$ and $10^{−22} M\odot yr^{−1}$, respectively, at redshifts $z \lesssim 3$, but this accretion rate increases roughly a hundred-fold to about $10^{−20} M\odot yr^{−1}$ at higher redshifts due to increased gas density. Bulge and halo stars accrete similar amounts of metals at high redshifts when kinematically distinct bulge and halo have not yet developed, and both sets of stars encounter a similar metal distribution in the ISM. Accretion alone can enrich main-sequence stars up to [Fe/H] $\sim$ −2 in extreme cases, with the median enrichment level due to accretion of about [Fe/H] $\sim$ −6 to −5. Because accretion mostly takes place at high redshifts, it is $\alpha$-enriched to [$\alpha$/Fe] $\sim$ 0.5. We find that accretive metal enrichment is sufficient to affect the predicted metallicity distribution function of halo stars at [Fe/H] < −5. This can hinder attempts to infer natal chemical environment of metal-poor stars from their observed enrichment. Peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling the natal and accreted metal content of stars.

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Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Institute for Computational Science
Dewey Decimal Classification:530 Physics
Language:English
Date:2017
Deposited On:22 Feb 2018 15:34
Last Modified:19 Aug 2018 14:10
Publisher:Oxford University Press
ISSN:0035-8711
Additional Information:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017 The Authors Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.
OA Status:Green
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1093/mnras/stx1094
Project Information:
  • : FunderFP7
  • : Grant ID320596
  • : Project TitleEMERGENCE - The Emergence of Structure during the Epoch of Reionization
  • : FunderH2020
  • : Grant ID638707
  • : Project TitleCOEVOLUTION - Black holes and their host galaxies: coevolution across cosmic time

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