Sulfur in presolar silicon carbide grains from asymptotic giant branch stars

Abstract

We studied 14 presolar SiC mainstream grains for C-, Si-, and S-isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite-normalized S/Si ratios between 2x10(-5) and 2x10(-4). All grains have S-isotopic compositions compatible within 2 sigma of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low-mass asymptotic giant branch (AGB) stars. In a S-33 versus S-34 plot the data fit along a line with a slope of 1.8 +/- 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC2 into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess S-32 found in some Type AB SiC grains are the result of insitu decay of radioactive Si-32 from born-again AGB stars that condensed into AB grains

Coordinated Analysis of Two Graphite Grains From the CO3.0 Lap 031117 Meteorite: First Identification of a Co Nova Graphite and a Presolar Iron Sulfide Subgrain

“…The negative δ 33,34 S values observed in grain LAP-141 might also reflect the production of 32 S by the decay of the short-lived 32 Si (τ 1/2 = 153 years) in the O/C or He/C supernova regions (Fujiya et al 2013;Pignatari et al 2013;Xu et al 2015). This process was proposed to explain the large 32 S excesses observed in presolar SiC C grains (Hoppe et al 2015;Pignatari et al 2013).…”

Nonadiabatic reaction dynamics to silicon monosulfide (SiS): A key molecular building block to sulfur-rich interstellar grains

“…In conjunction with astrochemical modeling, our study suggests that silicon monosulfide can form through this newly found reaction path toward star forming regions. This bimolecular reaction represents an important pathway to the simplest silicon-sulfur–bearing molecule—the silicon monosulfide diatomic species—as the starting point for a vigorous chemistry leading eventually to sulfide grains in deep space ( 32 , 34 ). Although there is, to date, no detailed description on the molecular level of how sulfide dust grains might form, silicon monosulfide is likely involved ( 31 ).…”

Sulphur isotope mass-independent fractionation observed in comet 67P/Churyumov–Gerasimenko by Rosetta/ROSINA

“…Since then, only during the last five years extensive studies of single grains could be done due to improvements of the analysis techniques (Hoppe et al 2010(Hoppe et al , 2015Xu et al 2015). Here we concentrate on sulphur isotopic ratios in those grains.…”

“…Recently, two studies examined SiC grains from Murchison meteorite for their isotopic composition: C, Si, N, S, Mg-Al, and Ca-Ti isotopic composition by Xu et al (2015), and Si, C, and S isotopic composition by Hoppe et al (2015). For their analysis, Xu et al (2015) used SiC grains of the separate KJE [described by Amari et al (1995)] which have a typical diameter between 0.5 and 0.8 μm.…”

“…The mean value over the entire image would be δ 34 S = (−703 ± 14) and δ 34 S = (−714 ± 24) . Hoppe et al (2015) studied 14 pre-solar SiC mainstream grains for their C, Si and S isotopic composition. They prepared a 48 g sample of Murchison meteorite following a similar procedure which was used for the separate KJE by Amari et al (1995) and developed by Besmehn & Hoppe (2003).…”