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ABSTRACT We present a detailed chemical abundance and kinematic analysis of six extremely metal-poor (−4.2 ≤ [Fe/H] ≤−2.9) halo stars with very low neutron-capture abundances ([Sr/H] and [Ba/H]) based on high-resolution Magellan/MIKE spectra. Three of our stars have [Sr/Ba] and [Sr/H] ratios that resemble those of metal-poor stars in ultra-faint dwarf galaxies (UFDs). Since early UFDs may be the building blocks of the Milky Way, extremely metal-poor halo stars with low, UFD-like Sr and Ba abundances may thus be ancient stars from the earliest small galactic systems that were accreted by the proto-Milky Way. We label these objects as Small Accreted Stellar System (SASS) stars, and we find an additional 61 similar ones in the literature. A kinematic analysis of our sample and literature stars reveals them to be fast-moving halo objects, all with retrograde motion, indicating an accretion origin. Because SASS stars are much brighter than typical UFD stars, identifying them offers promising ways towards detailed studies of early star formation environments. From the chemical abundances of SASS stars, it appears that the earliest accreted systems were likely enriched by a few supernovae whose light element yields varied from system to system. Neutron-capture elements were sparsely produced and/or diluted, with r-process nucleosynthesis playing a role. These insights offer a glimpse into the early formation of the Galaxy. Using neutron-capture elements as a distinguishing criterion for early formation, we have access to a unique metal-poor population that consists of the oldest stars in the universe.
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Metal-poor stars observed with the Magellan Telescope – IV. Neutron-capture element signatures in 27 main-sequence stars
ABSTRACT We present a chemo-dynamical analysis for 27 near main-sequence turnoff metal-poor stars, including 20 stars analysed for the first time. The sample spans a range in [Fe/H] from −2.5 to −3.6, with 44 per cent having [Fe/H]<−2.9. We derived chemical abundances for 17 elements, including strontium and barium. We derive Li abundances for the sample, which are in good agreement with the ‘Spite Plateau’ value. The lighter elements (Z < 30) generally agree well with those of other low-metallicity halo stars. This broadly indicates chemically homogeneous gas at the earliest times. We used the [Sr/Ba] versus [Ba/Fe] diagram to classify metal-poor stars into five populations based on their observed ratios. We find HE 0232 − 3755 to be a likely main r-process star, and HE 2214 − 6127 and HE 2332 − 3039 to be limited-r stars. CS30302-145, HE 2045 − 5057, and CD −24°17504 plausibly originated in long-disrupted early dwarf galaxies. We also find that the derived [Sr/H] and [Ba/H] values for CD −24°17504 are not inconsistent with the predicted yields of the s-process in massive rotating low-metallicity stars models. Further theoretical explorations will be helpful to better understand the earliest mechanisms and time scales of heavy element production for comparison with these and other observational abundance data. Finally, we investigate the orbital histories of our stars. Most display halo-like kinematics although three stars (CS 29504-018, HE 0223 − 2814, and HE 2133 − 0421) appear to be disc-like in nature. This confirms the extragalactic origin for CS 30302-145, HE 2045 − 5057, and, in particular, CD −24°17504 which likely originated from a small accreted stellar system as one of the oldest stars.
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- PAR ID:
- 10489019
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 528
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 2912-2929
- Size(s):
- p. 2912-2929
- Sponsoring Org:
- National Science Foundation
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