| 1. |
- D’Orazi, Valentina, et al.
(författare)
-
The GALAH survey : tracing the Milky Way’s formation and evolution through RR Lyrae stars
- 2024
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - 0035-8711. ; 531:1, s. 137-162
-
Tidskriftsartikel (refereegranskat)abstract
- Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterized using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t ≥ 10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H] ≈ −1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from −1 to approximately solar metallicity. This sub-group includes almost 1/4 of the sample, it is characterized by thin disc kinematics and displays sub-solar α-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia–Sausage–Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams.
|
|
| 2. |
- Skuladottir, Asa, et al.
(författare)
-
Zero-metallicity Hypernova Uncovered by an Ultra-metal-poor Star in the Sculptor Dwarf Spheroidal Galaxy
- 2021
-
Ingår i: Astrophysical Journal Letters. - : Institute of Physics Publishing (IOPP). - 2041-8205 .- 2041-8213. ; 915:2
-
Tidskriftsartikel (refereegranskat)abstract
- Although true metal-free "Population III" stars have so far escaped discovery, their nature, and that of their supernovae, is revealed in the chemical products left behind in the next generations of stars. Here we report the detection of an ultra-metal-poor star in the Sculptor dwarf spheroidal galaxy AS0039. With [Fe/H](LTE) = -4.11, it is the most metal-poor star discovered in any external galaxy thus far. Contrary to the majority of Milky Way stars at this metallicity, AS0039 is clearly not enhanced in carbon, with [C/Fe](LTE) = -0.75, and A(C) = +3.60, making it the lowest detected carbon abundance in any star to date. Furthermore, it lacks alpha-element uniformity, having extremely low [Mg/Ca](NLTE) = -0.60 and [Mg/Ti](NLTE) = -0.86, in stark contrast with the near solar ratios observed in C-normal stars within the Milky Way halo. The unique abundance pattern indicates that AS0039 formed out of material that was predominantly enriched by a similar to 20 M (circle dot) progenitor star with an unusually high explosion energy E = 10 x 10(51) erg. Therefore, star AS0039 represents some of the first observational evidence for zero-metallicity hypernovae and provides a unique opportunity to investigate the diverse nature of Population III stars.
|
|
