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

The most metal-deficient stars hold important clues about the early buildup 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 ([C/Fe] ≥ +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 ([Fe/H] < -2.0); this is more than a 10-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42⁺¹⁴_-13 per cent for stars with [Fe/H] < -3.0, and decreases to $16⁺³_-3 per cent for [Fe/H] < -2.5 and 5.7^+0.6_-0.5 per cent for [Fe/H]  < -2.0. The PIGS inner Galaxy CEMP fraction for [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.