Presolar grains from novae: their isotopic ratios and radioactivities
Introduction
During the last 10 years, presolar grains have been extensively studied and this new field of astronomy has contributed to our understanding of processes occurring in stars, mixing of stellar ejecta, and grain formation in circumstellar envelopes (e.g., Zinner, 1997, Zinner, 1998). Presolar grains are stardust extracted from meteorites. They formed in circumstellar envelopes or in stellar ejecta, and remained intact throughout the journey to the earth.
Abundances of various kinds of presolar grains are typically on the order of a few ppm of meteorites or even less. Thus, a series of chemical treatments is required in order to extract grains with such low abundances (Amari et al., 1994). Presolar grains identified to date are diamond (Lewis et al., 1987), silicon carbide (Bernatowicz et al., 1987, Tang and Anders, 1988), graphite (Amari et al., 1990), oxide (Nittler et al., 1997), silicon nitride (Nittler et al., 1995), and refractory carbides inside of presolar grains (Bernatowicz et al., 1991, Bernatowicz et al., 1996).
Secondary Ion Mass Spectrometry (SIMS) has been the method of choice to study isotopic ratios of individual presolar grains. Silicon carbide grains have been most widely studied, since they are relatively abundant in various types of meteorites (Huss and Lewis, 1995). Isotopic ratios of presolar grains enable us to infer stellar sources of the grains. Mainstream SiC grains (about 90% of total SiC) (Hoppe and Ott, 1997) and the minor Y (Amari et al., 2001b) and Z (Hoppe et al., 1997) SiC grains, as well as oxide grains (Nittler et al., 1997), are considered to have formed in asymptotic giant branch stars which have a range of metallicities. Silicon carbide of type X (1% of total SiC), low-density graphite, and Si3N4 most likely formed in supernova ejecta (e.g., Amari and Zinner, 1997). Other minor SiC populations called A+B grains are believed to have formed in J-stars or Sakurai’s object (Amari et al., 2001c).
In contrast to those mentioned above, grains of a putative nova origin are extremely rare, even though novae have been linked to an anomalous noble gas component, Ne-E(L), found in meteorites (Clayton, 1975, Clayton and Hoyle, 1976). In this paper, the isotopic signature of nova candidate grains will be discussed. More detailed discussions can be found in Amari et al. (2001a)
Section snippets
Isotopic ratios
Seven grains have been found whose isotopic ratios are suggestive of a nova origin (Table 1). They are characterized by low 12C/13C (4–9, solar=89) and 14N/15N (5–20, solar=272), high 30Si/28Si (up to 2×solar) with close-to-normal 29Si/28Si (Fig. 1, Fig. 2, Fig. 3). 14N/15N ratios of two graphite grains, KFB1a-161 and KFC1a-551, are solar. However, many graphite grains have solar N isotopic ratios in spite of anomalous 12C/13C ratios. It has been argued that part of the N in many graphite
Problems and puzzles
Although the present nova models can qualitatively account for the isotopic ratios of the grains, there are problems to be addressed. First, the isotopic ratios of the grains are less extreme than the predicted values in the models. It is illustrated in Fig. 1, Fig. 2, Fig. 3, where the nova grain data and the predicted ratios of CO and ONe novae are plotted (Kovetz and Prialnik, 1997, Starrfield et al., 1997, Starrfield et al., 1998, José et al., 1997, José and Hernanz, 1998, José et al., 1999
Summary
The isotopic ratios of seven presolar grains are consistent with an origin in ONe novae. Their isotopic characteristics qualitatively agree with the predicted ratios of nova models. However, quantitative agreement is achieved only if newly synthesized material during nova explosion is mixed with more than 95% of isotopically close-to-solar material.
Acknowledgements
I thank Peter Hoppe and Michael Smith for critical reading of the manuscript. I am grateful to the Max Planck Gesellschaft for the financial support to attend workshop ‘Astronomy with Radioactivities III’. This work was supported by NASA grant NAG5-8336.
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