The Pristine Inner Galaxy Survey (PIGS) III: carbon-enhanced metal-poor stars in the bulge

Abstract

The most metal-deficient stars hold important clues about the early build-up and chemical evolution of the Milky Way, and carbon-enhanced metal-poor (CEMP) stars are of special interest. However, little is known about CEMP stars in the Galactic bulge. In this paper, we use the large spectroscopic sample of metal-poor stars from the Pristine Inner Galaxy Survey (PIGS) to identify CEMP stars ($\rm {[C/Fe]} \geqslant +0.7$) in the bulge region and to derive a CEMP fraction. We identify 96 new CEMP stars in the inner Galaxy, of which 62 are very metal-poor ($\rm {[Fe/H]} < -2.0$); this is more than a ten-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42^{\, +14\, }_{\, -13} {{\ \rm per\ cent}}$ for stars with $\rm {[Fe/H]} < -3.0$, and decreases to $16^{\, +3\, }_{\, -3} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} < -2.5$ and $5.7^{\, +0.6\, }_{\, -0.5} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} < -2.0$. The PIGS inner Galaxy CEMP fraction for $\rm {[Fe/H]} < -3.0$ is consistent with the halo fraction found in the literature, but at higher metallicities the PIGS fraction is substantially lower. While this can partly be attributed to a photometric selection bias, such bias is unlikely to fully explain the low CEMP fraction at higher metallicities. Considering the typical carbon excesses and metallicity ranges for halo CEMP-s and CEMP-no stars, our results point to a possible deficiency of both CEMP-s and CEMP-no stars (especially the more metal-rich) in the inner Galaxy. The former is potentially related to a difference in the binary fraction, whereas the latter may be the result of a fast chemical enrichment in the early building blocks of the inner Galaxy.

The Pristine Inner Galaxy Survey (PIGS) IV: A photometric metallicity analysis of the Sagittarius dwarf spheroidal galaxy

“…The low-resolution (𝑅 ∼ 1300) optical and intermediate resolution (𝑅 ∼ 11 000) calcium triplet spectra were analysed through a full-spectrum fitting using the FERRE code 1 (Allende Prieto et al 2006), providing effective temperatures, surface gravities, metallicities and carbon abundances (see Arentsen et al 2020 for details). More spectroscopic follow-up was obtained later in 2020, which has been analysed in the same way and has already been used in Arentsen et al (2021). In total, four AAT pointings included dedicated observations of Sgr stars (see their positions in Figure 1).…”

A method for identifying metal-poor stars with Gaia BP/RP spectra

“…Additionally, the dataset has to cover a wide metallicity range, so that both metal-poor and metal-rich stars can be covered, and we can ultimately distinguish between them. Since low metallicities are involved in the process" the last important factor that we have to pay attention to is the carbon enhancement that has been observed in many metal-poor stars (Beers & Christlieb 2005;Frebel & Norris 2015;Arentsen et al 2021). The initial dataset parameters are shown in Table 1 and Figure…”

Constructing the Milky Way Stellar Halo in the Galactic Center by Direct Orbit Integration

“…The most visible part of the MW is its disk, which can be decomposed into a denser thin disk with a wide range of ages (Robin et al 2003;Sharma et al 2011Sharma et al , 2019Belokurov et al 2020;Franchini et al 2020), and an older, more diffuse, and metal-poor thick disk (Chiba & Beers 2000;Bensby et al 2003Bensby et al , 2005Reddy et al 2006;Kilic et al 2017). The bulge/ bar is the most concentrated component and is dominated by old and metal-rich stars (McWilliam & Rich 1994;Clarkson et al 2008;Koch et al 2016;Williams et al 2016;Arentsen et al 2020Arentsen et al , 2021Lucey et al 2021;Queiroz et al 2021). The halo surrounding the disk and bulge/bar, is divisible into two broadly overlapping structural components: an inner and an outer halo.…”