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Abstract
Aims. A probable carbon enhanced metal-poor (CEMP) star, Pisces II 10694, was discovered recently in the ultra-faint (UFD) galaxy Pisces II. This galaxy is supposed to be very old, suspected to include dark matter, and likely formed the bulk of its stars before the reionisation of the Universe. Methods. New abundances have been obtained from observations of Pisces II 10694 at the Kueyen ESO VLT telescope, using the high-efficiency spectrograph: X-shooter. Results. We found that Pisces II 10694 is a CEMP-no star with [Fe/H] = −2.60 dex. Careful measurements of the CH and C2 bands confirm the enhancement of the C abundance ([C/Fe] = +1.23). This cool giant has very probably undergone extra mixing and thus its original C abundance could be even higher. Nitrogen, O, Na, and Mg are also strongly enhanced, but from Ca to Ni the ratios [X/Fe] are similar to those observed in classical very metal-poor stars. With its low Ba abundance ([Ba/Fe] = −1.10 dex) Pisces II 10694 is a CEMP-no star. No variation in the radial velocity could be detected between 2015 and 2017. The pattern of the elements has a shape similar to the pattern found in galactic CEMP-no stars like CS 22949-037 ([Fe/H] = −4.0) or SDSS J1349+1407 ([Fe/H] = −3.6). Conclusions. The existence of a CEMP-no star in the UFD galaxy Pisc II suggests that this small galaxy likely hosted zero-metallicity stars. This is consistent with theoretical predictions of cosmological models supporting the idea that UFD galaxies are the living fossils of the first star-forming systems.
Highlights
The ultra-faint dwarf (UFD) galaxies contain the largest fraction of metal-poor stars of any galaxy type (e.g. Kirby et al 2008) and they could be the direct descendants of the first generation of galaxies in the Universe (Bovill & Ricotti 2009; Salvadori & Ferrara 2009)
Since we were able to obtain an estimation of the Sr abundance in this star (Table 2), it is interesting to check whether it shares the same characteristics as the other carbon enhanced metal-poor (CEMP)-no stars
Salvadori et al (2015) investigated the frequency of CEMPno stars in dwarf galaxies with different luminosities. One of their key results is that the probability of observing CEMPno stars, imprinted by primordial faint SNe, increases with decreasing galaxy luminosity and, on average, this probability is an order of magnitude higher in UFDs than in more massive classical Sculptor-like dwarf spheroidal galaxies
Summary
The ultra-faint dwarf (UFD) galaxies contain the largest fraction of metal-poor stars of any galaxy type (e.g. Kirby et al 2008) and they could be the direct descendants of the first generation of galaxies in the Universe (Bovill & Ricotti 2009; Salvadori & Ferrara 2009). The C-enhancement made it easier for them to form from a gas depleted in other metals, and the large scatter of the carbon abundance displayed in these stars (about 1 dex) is the result of the nucleosynthesis of a limited number of faint supernovae of zero metallicity that have polluted the gas. If this scenario is correct, many CEMP-no stars should be found in dwarf galaxies, and especially in UFD galaxies, which are the faintest (L < 105 L ), the more metal-poor, and likely the oldest galactic systems (Salvadori et al 2015).
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