TOPoS

Abstract

Context. Extremely metal-poor (EMP) stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible. Aims. Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine their chemical composition. Methods. We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres. Results. We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] <−5.2). We were also able to measure Li in three stars at [Fe/H] ~−4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low α-to-iron ratios. Conclusions. The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H] ~−4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in α-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low α-to-iron ratios is supported by our observations.

The Pristine survey – VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates

“…Discovered by Starkenburg et al (2018) and included in this work, Pristine 221.8781 + 9.7844 is the second most metal-poor star also in the silicate dominated region with [C/Fe] < +1.76 and could also potentially be a carbon-normal UMP star. All 11 other stars from the literature with [Fe/H] < -4.5 show a clear enhancement in carbon (see, e.g., Bonifacio et al 2018b;Yoon et al 2019, and references therein). Larger samples of extremely metal-poor stars, especially those with robust carbon measurements, are important in order to better understand these trends.…”

Primordial to extremely metal-poor AGB and Super-AGB stars: White dwarf or supernova progenitors?

“…First, the possibility of comparing with observations is limited because of the relatively small sample of detected objects in the most metal-poor regime. At present only ∼ 10 stars are known to have metallicity [Fe/H] 2 < −4.5 (Starkenburg et al 2017, Aguado et al 2018, and Bonifacio et al 2018 and references therein). The record is held by the star detected by Keller et al (2014), with [Fe/H] < −7.1.…”

Chemical analysis of very metal-poor turn-off stars from SDSS-DR12

“…The SDSS-based metallicities found by the method of Ludwig et al (2008) are essentially confirmed by the analysis at higher resolution. This method has been very successful and has led to a series of papers that have Article number, page 2 of 14 already been published (Caffau et al 2011a(Caffau et al ,b, 2012(Caffau et al , 2014Bonifacio et al 2015;Caffau et al 2016;Bonifacio et al 2018). The most metal-poor stars are formed out of gas that has been very likely enriched by the ejecta of a single or a few supernovae.…”