The Origin of the Most Iron-Poor Star

Abstract

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered [Fe/H] < −7.1 star SMSS J031300. We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal-free, supernovae (SNe). Using properly calibrated faint SN explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor SN ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the SN reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15 and 0.84), resulting in dust yields in the range (0.025-2.25) M . We follow the collapse and fragmentation of a star-forming cloud enriched by the products of these faint SN explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.

Pop III i-process nucleosynthesis and the elemental abundances of SMSS J0313−6708 and the most iron-poor stars

“…We do not find such high temperatures in any of our stellar evolution models, including tests with similar high and even higher initial mass. Pop III models of Limongi & Chieffi (2012) used by Marassi et al (2014) are in better agreement with ours as they have similar H burning temperatures and do not produce appreciable Ca in quiescent burning phases.…”

“…Li, C, Mg and Ca have been measured and there are upper limits on several other elements. HE 1327-2326 (Frebel et al 2006(Frebel et al , 2008 and HE 1017-5240 (Christlieb E-mail: oclark01@uvic.ca Low-energy or faint supernovae with strong fallback would have either very little or no nucleosynthetic contribution from the supernova explosion (Keller et al 2014;Takahashi et al 2014;Marassi et al 2014). However, many the best fit models proposed for CEMP-no stars are within the mass range where Pop III and the lowest-metallicity stars are expected to collapse directly into black holes with no supernova explosion (Heger et al 2003).…”

Low-energy Population III supernovae and the origin of extremely metal-poor stars

“…This process could explain the near absence of iron group elements in J031300. Similar yields have been reported for low-energy ( 0.5 B, where 1 B = 10 51 erg) explosions of less massive progenitors (see Marassi et al 2014;Ishigaki et al 2014;Takahashi et al 2014). Such events are considered to be low-energy because they have energies that are either well below the average for CC SNe (∼ 0.9 B; Kasen & Woosley 2009) or, as in the case of the 60 M progenitor discussed above, have low energies per unit mass.…”

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

“…To reproduce the elemental abundance of the most ironpoor CE-EMP stars such as SMSS J0313−6708 (Keller et al 2014), ESN ∼ 10 51 erg and M ( 56 Ni) 10 −3 M are favored (Marassi et al 2014;Ishigaki et al 2014). In our calculations, we estimate r max C = 34.3 µm with ESN = 1.0×10 51 erg and M ( 56 Ni) = 1 × 10 −7 M .…”