ABSTRACT We present chemical abundances for 21 elements (from Li to Eu) in 150 metal-poor Galactic stars spanning −4.1 < [Fe/H] < −2.1. The targets were selected from the SkyMapper survey and include 90 objects with [Fe/H] ≤ −3 of which some 15 have [Fe/H] ≤ −3.5. When combining the sample with our previous studies, we find that the metallicity distribution function has a power-law slope of Δ(log N)/Δ[Fe/H] = 1.51 ± 0.01 dex per dex over the range −4 ≤ [Fe/H] ≤ −3. With only seven carbon-enhanced metal-poor stars in the sample, we again find that the selection of metal-poor stars based on SkyMapper filters is biased against highly carbon-rich stars for [Fe/H] > −3.5. Of the 20 objects for which we could measure nitrogen, 11 are nitrogen-enhanced metal-poor (NEMP) stars. Within our sample, the high NEMP fraction (55 per cent ± 21 per cent) is compatible with the upper range of predicted values (between 12 per cent and 35 per cent). The chemical abundance ratios [X/Fe] versus [Fe/H] exhibit similar trends to previous studies of metal-poor stars and Galactic chemical evolution models. We report the discovery of nine new r-I stars, four new r-II stars, one of which is the most metal-poor known, nine low-α stars with [α/Fe] ≤ 0.15 as well as one unusual star with [Zn/Fe] = +1.4 and [Sr/Fe] = +1.2 but with normal [Ba/Fe]. Finally, we combine our sample with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy.
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Neutron star mergers and their impact on second generation star formation in the early universe
The exact evolution of elements in the Universe, from primordial to heavier elements produced via the r-process, is still under scrutiny. The supernova deaths of the very first stars led to the enrichment of their local environments, and can leave behind neutron stars (NSs) as remnants. These remnants can end up in binary systems with other NSs, and eventually merge, allowing for the r-process to occur. We study the scenario where a single NS merger (NSM) enriches a halo early in its evolution to understand the impact on the second generation of stars and their metal abundances. We perform a suite of high-resolution cosmological zoom-in simulations using enzo where we have implemented a new NSM model varying the explosion energy and the delay time. In general, an NSM leads to significant r-process enhancement in the second generation of stars in a galaxy with a stellar mass of ∼105 M⊙ at redshift 10. A high explosion energy leads to a Population II (Pop II) mass fraction of 72 per cent being highly enhanced with r-process elements, while a lower explosion energy leads to 80 per cent being enhanced, but only 14 per cent being highly enhanced. When the NSM has a short delay time of 10 Myr, only 5 per cent of the mass fraction of Pop II stars is highly enhanced, while 64 per cent is highly enhanced for the longest delay time of 100 Myr. This work represents a stepping stone towards understanding how NSMs impact their environments and the metal abundances of descendant generations of stars.
more » « less- NSF-PAR ID:
- 10491239
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 528
- Issue:
- 4
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 5825-5835
- Size(s):
- ["p. 5825-5835"]
- Sponsoring Org:
- National Science Foundation
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