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Title: Metal Mixing in the r-process Enhanced Ultrafaint Dwarf Galaxy Reticulum II*
Abstract The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r -process event. The r -process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find 72 − 12 + 10 % of the stars are r -process-enhanced, with a mean [ Ba / H ] = − 1.68 ± 0.07 and unresolved intrinsic dispersion σ [Ba/H] <0.20. The homogeneous r -process abundances imply that Ret II’s metals are well mixed by the time the r -enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r -process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r -enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.  more » « less
Award ID(s):
2205847 1815403 2206264
NSF-PAR ID:
10429367
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
The Astronomical Journal
Volume:
165
Issue:
3
ISSN:
0004-6256
Page Range / eLocation ID:
100
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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