3D NLTE analysis of the most iron-deficient star, SMSS0313-6708

Abstract

Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1−10 M ), to massive (10−100 M ), or even supermassive (100−1000 M ). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE) are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 by means of the most realistic methods available, and compare the results to predicted supernova yields. Methods. A 3D hydrodynamical Stagger model atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < −6.53 (3σ), than previous estimates based on 3D NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D− 3D NLTE differences, all of which lead to a higher abundance determination. Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20−60 M exploding with low energies (1−2 B, where 1 B = 10 51 erg), we also find good fits for progenitors of 10 M , with very low explosion energies (<1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae.

Ultra-metal-poor Stars: Spectroscopic Determination of Stellar Atmospheric Parameters Using Iron Non-LTE Line Abundances

“…Our 3859.911Å result agrees with that of Bessell et al (2015), [Fe/H](LTE) < −7.80, who used the same line. Our 3608.859Å LTE value agrees with that of Nordlander et al (2017) within 0.1 dex (they report [Fe/H](1D,LTE) < −7.34). In NLTE, we determine an upper limit of [Fe/H](NLTE) < −6.52 from the 3608.859Å line, and [Fe/H](NLTE) < −6.72 from the resonance 3859.911Å.…”

“…Our 1D, NLTE result is, however, 0.19 dex lower than their 3D, NLTE value. Overall, Nordlander et al (2017) report agreement between their 1D and 3D NLTE results which adds confidence to our result and the use of our NLTE method. As no Fe lines could be detected, we did not compute any NLTE stellar parameters but instead adopt the temperature, gravity and microturbulent velocity from Bessell et al (2015).…”

“…However, a full 3D, NLTE analysis would likely decrease the scatter even more, but it is still challenging and computationally expensive to employ. In this context, it is encouraging that for SMSS J0313−6708, the only UMP star for which a full 3D, NLTE analysis has been performed, Nordlander et al (2017) report fairly similar 1D NLTE and 3D NLTE results (within 0.2 dex), whereas much larger differences were obtained between LTE and NLTE models. This highlights that accurate abundances can presently be most efficiently obtained with 1D, NLTE models, such as the present study, whenever reliable atomic data are included.…”

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

“…SMSS J031300.36-670839.3 (hereafter SMSS J0313-6708, Keller et al 2014), is the most iron-poor star identified at present, with [Fe/H] ≤ −6.53, (Nordlander et al 2017). Li, C, Mg and Ca have been measured and there are upper limits on several other elements.…”

The Universality of the Rapid Neutron-capture Process Revealed by a Possible Disrupted Dwarf Galaxy Star*

“…We present the inferred orbital parameters from both analyses in Table 1. 4. DISCUSSION Our measurement of [Sr, Ba/H] ≈ −6 for 2MASS J151113.24-213003.0 is surpassed by only three ultrametal poor halo stars: BPS CS 22968-0014, BPS CS 22885-0096, and SMSS J031300.36-670839.3 Keller et al 2014;Bessell et al 2015;Nordlander et al 2017). The [Sr, Ba/Fe] ≈ −3 abundances we measured in 2MASS J151113.24-213003.0 are more than a factor of two smaller than any previous measurement for an extremely metal-poor star in the comprehensive Stellar Abundances for Galactic Archaeology (SAGA) Database (Suda et al 2008).…”