Search

The non-detection of zero-metallicity stars in ultra-faint dwarf galaxies (UFDs) can be used to constrain the initial mass function (IMF) of the first (PopIII) stars by means of a statistical comparison between available data and predictions from chemical evolution models. To this end we develop a model that follows the formation of isolated UFDs, calibrated to best reproduce the available data for the best studied system: Boötes I. Our statistical approach shows that UFDs are the best suitable systems to study the implications of the persisting non-detection of zero-metallicity stars on the PopIII IMF, i.e. its shape, the minimum mass (m min), and the characteristic mass (m ch). We show that accounting for the incomplete sampling of the IMF is essential to compute the expected number of long-lived PopIII stars in inefficiently star-forming UFDs. By simulating the colour–magnitude diagram of Boötes I, and thus take into account the mass-range of the observed stars, we can obtain even tighter constrains on m min. By exploiting the 96 stars with measured metallicities (⁠|$\rm i \lt 19$|⁠) in the UFDs represented by our model, we demonstrate that |$m_{\mathrm{ ch}} \gt 1\: \rm {M_{\odot }}$| or |$m_{\mathrm{ min}} \gt 0.8 \:\rm {M_{\odot }}$| at |$99\%$| confidence level. This means that a present-day IMF for PopIII stars is excluded by our model, and a top-heavy PopIII IMF is strongly favoured. We can limit |$m_{\mathrm{ min}} \gt 0.8\: \rm {M_{\odot }}$| independent of the PopIII IMF shape by targeting the four UFDs Boötes I, Hercules, Leo IV, and Eridanus II with future generation instruments, such as ELT/MOSAIC (⁠|$\rm i \lt 25$|⁠), which can provide samples of >10 000 stars. [ABSTRACT FROM AUTHOR]

We investigate the frequency and origin of carbon-enhanced metal-poor (CEMP) stars in Local Group dwarf galaxies by means of a statistical, data-calibrated cosmological model for the hierarchical build-up of the Milky Way and its dwarf satellites. The model self-consistently explains the variation with dwarf galaxy luminosity of the observed: (i) frequency and [Fe/H] range of CEMP stars; (ii) metallicity distribution functions; (iii) star formation histories. We show that if primordial faint supernovae dominated the early metal-enrichment, then CEMPno stars enriched by the first stellar generations should be present in all dwarf galaxies, with similar number of stars and CEMP fractions at [Fe/H] < -4. We demonstrate that the probability to observe a star that is carbon-enhanced within a given [Fe/H] range strongly depends on the luminosity of the dwarf galaxy and, on average, it is an order of magnitude lower in 'classical' Sculptor-like dwarf spheroidal (dSph) galaxies (P ≤ 0.02) than in the least luminous ultra-faint dwarfs (P ≈ 0.1). In addition, we explain why it may be easier to find CEMP-no stars at [Fe/H] ≈ -2 in classical dSph galaxies than in ultra-faint dwarfs. These are consequences of the dramatic variation in the fraction of stars at [Fe/H] < -3 with galaxy luminosity: ≥40 per cent for galaxies with L < 105 L⊙, and ≤0.2 per cent for L > 107 L⊙. We present model predictions for the low-Fe tail and CEMP fraction of stars in dwarf galaxies, with particular emphasis on the Sculptor dSph, that can be used to shed light on the properties of the first stars. [ABSTRACT FROM AUTHOR]