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A Formation Timescale of the Galactic Halo from Mg Isotopes in Dwarf Stars

Carlos, Marília and Karakas, Amanda I. and Cohen, Judith G. and Kobayashi, Chiaki and Meléndez, Jorge (2018) A Formation Timescale of the Galactic Halo from Mg Isotopes in Dwarf Stars. Astrophysical Journal, 856 (2). Art. No. 161. ISSN 1538-4357.

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We determine magnesium isotopic abundances of metal-poor dwarf stars from the galactic halo, to shed light on the onset of asymptotic giant branch (AGB) star nucleosynthesis in the galactic halo and constrain the timescale of its formation. We observed a sample of eight new halo K dwarfs in a metallicity range of −1.9 < [Fe/H] < −0.9 and 4200 < T_(eff)(K) < 4950, using the HIRES spectrograph at the Keck Observatory (R ≈ 10^5 and 200 ≤ S/N ≤ 300). We obtain magnesium isotopic abundances by spectral synthesis on three MgH features and compare our results with galactic chemical evolution models. With the current sample, we almost double the number of metal-poor stars with Mg isotopes determined from the literature. The new data allow us to determine the metallicity when the ^(26)Mg abundances start to become important, [Fe/H] ~ −1.4 ± 0.1. The data with [Fe/H] > −1.4 are somewhat higher (1–3σ) than previous chemical evolution model predictions, indicating perhaps higher yields of the neutron-rich isotopes. Our results using only AGB star enrichment suggest a timescale for formation for the galactic halo of about 0.3 Gyr, but considering also supernova enrichment, the upper limit for the timescale formation is about 1.5 Gyr.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Carlos, Marília0000-0003-1757-6666
Karakas, Amanda I.0000-0002-3625-6951
Cohen, Judith G.0000-0002-8039-4673
Kobayashi, Chiaki0000-0002-4343-0487
Meléndez, Jorge0000-0002-4933-2239
Additional Information:© 2018. The American Astronomical Society. Received 2017 November 6; revised 2018 February 15; accepted 2018 February 27; published 2018 April 4. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. M.C. would like to acknowledge support from CAPES. This work was also conducted during a scholarship supported by Capes/PDSE (88881.135113/2016-01) at Monash University. J.M. is thankful for the support of FAPESP (2012/24392-2, 2014/18100-4) and CNPq (Bolsa de Produtividade). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Software: numpy (van der Walt et al. 2011), matplotlib (Hunter 2007), ATLAS9 (Castelli & Kurucz 2004), MOOG (Sneden 1973), MAKEE (, IRAF (Tody 1986; Tody et al. 1993).
Funding AgencyGrant Number
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)88881.135113/2016-01
Fundação de Amparo à Pesquisa do Estado de Sao Paulo (FAPESP)2012/24392-2
Fundação de Amparo à Pesquisa do Estado de Sao Paulo (FAPESP)2014/18100-4
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)UNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:Galaxy: halo; stars: abundances; stars: AGB and post-AGB
Record Number:CaltechAUTHORS:20180426-144524292
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Official Citation:Marília Carlos et al 2018 ApJ 856 161
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86068
Deposited By: George Porter
Deposited On:27 Apr 2018 16:53
Last Modified:27 Apr 2018 16:53

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