Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling

Abstract

Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower-limits of carbon and iron abundances of Acr(C) ~ 6 and [Fe/H]cr ~ -5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low-mass, i.e., long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius r_i^{cool} of a species i and condensation efficiency f_{ij} of a key element j as r_C^{cool}/f_{C, C} = 10 um and r_Sil^{cool}/f_{Sil, Mg} = 0.1 um to reproduce Acr(C) and [Fe/H]cr, which give a universal condition 10^{[C/H]-2.30} + 10^[Fe/H] > 10^{-5.07} for the formation of every EMP star. Instead of the conventional boundary [C/Fe] = 0.7 between CE- and CN-EMP stars, this condition suggests a physically meaningful boundary [C/Fe]_{b} = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.

6 pages, 4 figures, accepted as a Letter to Monthly Notices of the Royal Astronomical Society (MNRASL)

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• Implications of inhomogeneous metal mixing for stellar archaeology
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• Unveiling the contribution of Pop III stars in primeval galaxies at redshift $\ge 6$
• Evidence of First Stars-enriched Gas in High-redshift Absorbers
• Seeding the second star -- II. CEMP star formation enriched from faint supernovae
• Are Faint Supernovae Responsible for Carbon-Enhanced Metal-Poor Stars?
• Understanding the origin of CEMP-no stars through ultra-faint dwarfs
• The Pristine survey XIV: chemical analysis of two ultra-metal-poor stars
• Inferring the velocity of early massive stars from the abundances of extremely metal-poor stars
• Can Population III stars be major origins of both merging binary black holes and extremely metal poor stars?
• Espresso observations of HE 0107$-$5240 and other CEMP-no stars with $[Fe/H]\le -4.5$
• A Critique of the Spite Plateau, and the Astration of Primordial Lithium
• Does the structure of Pop III supernova ejecta affect the elemental abundance of extremely metal-poor stars?
• Accurate Metallicities for Very Metal-Poor Stars from the Ca~II Infrared Triplet