Bonanza: An extremely large dust grain from a supernova

Abstract

We report the morphology, microstructure, and isotopic composition of the largest SiC stardust grain known to have condensed from a supernova. The 25-μm diameter grain, termed Bonanza, was found in an acid-resistant residue of the Murchison meteorite. Grains of such large size have neither been observed around supernovae nor predicted to form in stellar environments. The large size of Bonanza has allowed the measurement of the isotopic composition of more elements in it than any other previous presolar grain, including: Li, B, C, N, Mg, Al, Si, S, Ca, Ti, Fe, and Ni. Bonanza exhibits large isotopic anomalies in the elements C, N, Mg, Si, Ca, Ti, Fe, and Ni typical of an astrophysical origin in ejecta of a Type II core-collapse supernova and comparable to those previously observed for other presolar SiC grains of type X. Additionally, we extracted multiple focused ion beam lift-out sections from different regions of the grain. Our transmission electron microscopy demonstrates that the crystalline order varies at the micrometer scale, and includes rare, higher order polytype domains (e.g., 15 R). Analyses with STEM−EDS show Bonanza contains a heterogeneous distribution of subgrains with sizes ranging from <10 nm to >100 nm of Ti(N, C); Fe, Ni-rich grains with variable Fe:Ni; and (Al, Mg)N. Bonanza also has the highest ever inferred initial ²⁶Al/²⁷Al ratio, consistent with its supernova origin. This unique grain affords us the largest expanse of data, both microstructurally and isotopically, to compare with detailed calculations of nucleosynthesis and dust condensation in supernovae.

Introduction

Primitive meteorites, interplanetary dust particles, Antarctic micrometeorites, and cometary material contain tiny grains of stardust that condensed in outflows of late-type stars and the ejecta of core-collapse supernovae (Clayton and Nittler, 2004, Lodders and Amari, 2005, Lugaro, 2005, Zinner, 2014). After a long interstellar history, these grains ended up in the molecular cloud from which our Solar System formed. The stellar origin of these so-called presolar stardust grains is inferred from their isotopic compositions, which are highly anomalous and completely different from those of the Solar System. Their detailed study in the laboratory provides information on stellar evolution and nucleosynthesis, the chemical evolution of the Galaxy, as well as physical and chemical conditions in stellar atmospheres and in the early Solar System.

Silicon carbide is the most studied type of presolar grain (Hynes and Gyngard, 2009, Gyngard et al., 2009b, Nittler and Ciesla, 2016). Different subtypes of presolar SiC have been defined based on their C, N, and Si isotopic ratios (Hoppe et al., 1994, Zinner, 2014). While most SiC grains have an origin in asymptotic giant branch (AGB) stars, type X and C grains, comprising only 1% and ∼0.1%, respectively, of all SiC grains, condensed in the ejecta of core-collapse (Type II) supernovae (SNe) (Nittler et al., 1996, Hoppe et al., 2012). These grains are characterized by excesses in ²⁸Si and ¹⁵N relative to solar, a wide range of ¹²C/¹³C ratios (from ∼15 to ∼10,000) and very high inferred ²⁶Al/²⁷Al ratios, ranging up to 0.8. A subset of grains that had their Ca-Ti isotopes measured show large ⁴⁴Ca excesses from the decay of short-lived (t_1/2 = 60 y) ⁴⁴Ti (Nittler et al., 1996). Both ²⁸Si and ⁴⁴Ti are produced only in SNe; the first by oxygen burning (e.g., Meyer and Zinner, 2006) and the second by an α-rich freezeout from nuclear statistical equilibrium (Timmes et al., 1996). Their presence in X grains provides direct evidence for their SN origin.

In spite of their relatively low abundance, a fairly large number of X grains has been located by ion imaging (e.g., Nittler et al., 1997 and Hoppe et al., 2000) and automatic isotopic measurements (e.g., Nittler and Alexander, 2003). However, because many of these grains were relatively small (<1 μm), not many isotopic analyses of different elements have been performed on them. Most isotopic measurements have been restricted to C and Si. Of the ∼700 X grains that have been analyzed to date (e.g., Hynes and Gyngard, 2009, Gyngard et al., 2009b), the approximate percentages of grains analyzed for the following elements are: C 67%, Si 93%, N 44%, Al-Mg 20%, S 4.0%, Ca (^40,42,44Ca) 21%, Ti (^46,48,49Ti) 16%, Fe (^54,56,57Fe) 7%, Ni (^58,60,61,62Ni) 6%, Ba (^{135,136,137,138}Ba) 1.2%. The fractions for other trace elements, such as Sr, Zr, Mo, and Ru, analyzed by resonance ionization mass spectrometry (RIMS) are even smaller. There is also no overlap for many of these analyses. For example, none of the X grains that had their Fe and Ni isotopic ratios measured (Marhas et al., 2008) were analyzed for their Ti isotopic ratios.

We report the discovery of an extremely large X grain, named Bonanza, with a diameter of 25 μm. This corresponds to a volume some 15,000 times larger than that of a typical ∼1 μm X grain. Such a large grain offers a unique opportunity to measure as many elements as possible for their isotopic compositions in the same grain and to explore the nucleosynthesis mixing and condensation conditions that occurred in a presolar Type II SN. Bonanza was found among grains from the size fraction LU of the Murchison L-series chemical and physical separation (Amari et al., 1994). Here we report transmission electron microscopy (TEM) and NanoSIMS isotopic data of this unique grain. Preliminary results have been presented by Zinner et al., 2010, Zinner et al., 2011 and Stroud et al., 2014, Stroud et al., 2015. Some Raman spectroscopic data have been also reported for this grain (Wopenka et al., 2010) but are not discussed in detail here.