Stellar origin of 15N-rich presolar SiC grains of type AB: supernovae with explosive hydrogen burning

We report C, N, and Si isotopic data for 59 highly 13C-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15N-rich (14N/15N<=solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in 13C, 15N and 26Al with the PNGs showing more extreme enhancements. The 15N-rich AB grains show systematically higher 26Al and 30Si excesses than the 14N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (14N/15N=solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG, and found 32S and/or 50Ti enhancements. Interestingly, one AB1 grain had the largest 32S and 50Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32S excess and thus the initial presence of radiogenic 32Si (t1/2=153 yr). More importantly, we found that the 15N and 26Al excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from SNe that experienced H ingestion into the He/C zones of their progenitors.

reported observational evidence for higher-than-solar 14 N/ 15 N ratios in six of seven J-type carbon stars, but the lower-than-solar value for the one exception was a lower limit. State-of-the-art born-again AGB stellar models by Herwig et al. (2011) predict 14 N/ 15 N ratios up to ~50,000 in the He intershell region with 12 C/ 13 C<10 (Jadhav et al. 2013). Thus, neither scenario seems able to explain the lower-than-solar 14 N/ 15 N ratios of ~50% of AB grains, though they cannot be completely ruled out due to poor statistics in the observational measurements and uncertainties in stellar models. Novae are well known stellar explosions in which high-temperature H burning can produce extremely low 14 N/ 15 N (0.2−~1000, Jose et al. 2004;Jose & Hernanz 2007) and 12 C/ 13 C (<2), and have been proposed as the parent stars of the so-called putative nova grains (PNGs, <0.1% of all SiC) that have extreme 13 C and 15 N enrichments (Amari et al. 2001b).
Despite the systematic difference between 15 N-rich AB grains and PNGs, Liu et al. (2016, hereafter L16) pointed out that the isotopic signatures of 15 N-rich AB grains could also reflect nucleosynthetic processes in novae but probably with lower masses relative to PNGs.
Intriguingly, an enhancement in two p-process isotopes, 92 Mo and 94 Mo, was found in one AB grain studied by Savina et al. (2003), raising the possibility of a SN origin, the most likely stellar site for p-process nucleosynthesis. SN models (Woosley & Heger 2007, hereafter WH07;Pignatari et al. 2013a), however, generally predict extremely high 12 C/ 13 C ratios (>10 5 ) in the SN C-rich He/C zone that must have contributed a majority of precursor material to the SiC (e.g., Table 2 of Xu et al. 2015) because thermodynamic equilibrium models require C/O>1 to form SiC. Note that the stable CO molecule could be dissociated in SNe by electrons arising from 56 Co decay, allowing carbonaceous grains to grow even in O>C conditions (e.g., Clayton 2013). However, quasi-equilibrium calculations (Ebel & Grossman 2001) show that although graphite could be stable in O-dominated SN zones, SiC is not stable under such O-rich conditions. Moreover, these zones also have extremely high 12 C/ 13 C ratios. More importantly, AB and PNG grains show similar Raman characteristics to mainstream SiC grains from lowmass AGB stars after correcting for the isotope-induced peak shifts and broadening (Liu et al. 2017a), suggesting similar condensation environments in their parent stars, i.e., equilibrium condensation with C/O>1. Nittler & Hoppe (2005) reported a presolar SiC grain with the Si and Ti isotopic signature of X grains from SNe, but C and N isotope ratios similar to PNGs, raising the possibility of explosive H-burning occurring in the C-rich He /C zone during a SN explosion.
Furthermore, L16 found a subgroup of 13 C-and 15 N-rich presolar SiC, C2 grains (<0.1% of presolar SiC), with neutron-capture isotopic signatures ( 29,30 Si, 50 Ti excesses), pointing to a SN origin. Pignatari et al. (2015, hereafter P15) investigated H ingestion into the He/C zone during the pre-SN stage of a massive star's evolution followed by the core-collapse SN explosion. They found that a H-ingestion event before the SN explosion, which is predicted by the stellar model calculations, can switch off the convective mixing process, leaving some H in the He/C zone during the explosion (see P15 and L16 for model details). The combination of the pre-SN Hingestion event followed by the SN explosive nucleosynthesis can lead to low 12 C/ 13 C and 14 N/ 15 N, and high 26 Al/ 27 Al in the H-ingested He/C zone. Thus, SNe should also be considered as possible sources of 15 N-rich AB grains and PNGs.

METHODS AND RESULTS
The SiC grains in this study were extracted from the Murchison CM2 chondrite using the CsF isolation method described by Nittler & Alexander (2003). Three presolar SiC grain mounts, labeled #1, #2, and #3, were prepared by dispersing grains separated in size by sedimentation (~1 µm) on high purity Au foils from a water suspension. The grains were subsequently pressed into the Au foils with a flat sapphire disk. The 13 C-and 15 N-rich presolar SiC grains analyzed in our previous study were all found on mount #3 (L16). The AB grains and PNGs in this study were nondestructively identified with a Raman microscope by their lower-shifted Raman TO peak (Liu et al. 2017a). Isotopic analyses of C, N, Si, Mg-Al, and Ca-Ti were performed with the NanoSIMS 50L ion microprobe at the Carnegie Institution. Analytical procedures are described in L16. The isotopic data are reported in Table 1 and all the plots with 1σ uncertainties. We relaxed the definition of AB grains ( 12 C/ 13 C<10) to 12 C/ 13 C<16 because, for instance, grain M2-A3-G1225 has a 12 C/ 13 C ratio of 13 but a 26 Al/ 27 Al ratio of 0.02, an order of magnitude higher than those of MS grains. Among the potential sources of AB grains discussed above, at present only novae and SNe appear likely to produce 15 N-rich N, indicating that it may be useful to redivide the group into: group 1 grains with 14 N/ 15 N<440 (solar value), and group 2 grains with 14 N/ 15 N ≥440. The 14 N-rich and 15 N-rich AB grains are therefore referred to hereafter as AB1 and AB2 grains, respectively.
Also, there exist distinctive differences between AB1 and AB2 grains: (1) Presolar SiC with 12 C/ 13 C lower than the equilibrium CNO cycle value of 3.5 are all 15 N-rich (Table 1); (2) For AB1 grains, the lower the 14 N/ 15 N ratio, the higher the 26 Al/ 27 Al ratio, whereas AB2 grains show no correlation in Figure 1a; (3) The linear fits to AB grain Si three-isotope data show that compared to AB2 grains, AB1 grains are more enriched in 30 Si relative to 29 Si ( Figure 1b). Our N and Al isotopic data for AB1 grains are well correlated and show less scatter than the literature data from A01a, likely indicating less contamination in our study. This is expected because we removed adjacent materials around the grains of interest with a focused ion beam instrument and performed isotope analyses in imaging mode to further exclude contamination during data reduction. Moreover, the size of the O − beam of the NanoSIMS 50L used here is a factor of several smaller than that of the ims-3f instrument used by A01a, which likely further reduced the amount of contamination sampled during our analyses.
Three out of eight AB1 grains and one PNG from this study had negative δ 33,34 S values outside of 1σ errors (Figure 2a), i.e., 32 S excesses, which could result from the decay of radiogenic 32 Si (t 1/2 =153 yr). Novae are predicted to have normal or positive δ 33 S compositions (Parikh et al. 2014) and thus cannot explain the 32 S excesses observed in these grains. Pignatari et al. (2013a) first identified 32 S excesses in type C SiC grains as most likely indicating the initial presence of radiogenic 32 Si that was produced by neutron capture in the so-called neutron-burst zone (Meyer et al. 2000) within the He/C zone during a SN explosion. Later, Fujiya et al. (2013) found such radiogenic 32 Si signatures in AB grains and pointed out that the 32 Si excesses could also be produced by the intermediate neutron-capture process (i-process, Cowan & Rose 1997) in born-again AGB stars with typical neutron densities of ~10 15 cm −3 . Although the AB grains of Fujiya et al. (2013) were not measured for N, we used the 26 Al/ 27 Al ratios of these grains to tentatively separate AB1 and AB2 grains based on a dividing value of 26 Al/ 27 Al of 0.003 ( Figure   1a), shown in Figure 2a. As a matter of fact, two of the three grains from Fujiya et al. (2013) with 32 S excesses are most likely AB1 grains. While the third grain was not measured for its Mg-Al isotopes, it is more enriched in 30 Si relative to 29 Si, increasing the likelihood of it also being an AB1 grain (Figure 1b). In contrast, none of the AB2 grains show any 32 S excesses, and the 32 S excesses therefore seem to be mainly associated with the 15 N-enriched AB1 grains, possibly challenging the born-again AGB scenario suggested by Fujiya et al. (2013). Finally, five out of 12 AB1 grains from our study (Figure 2b) show clear neutron-capture isotopic signatures, i.e., large excesses in 50 Ti (δ 50 Ti≥~200‰). In contrast, although large 32 S excesses are found in two out of four PNGs, none of the eight PNGs studied so far show any 50 Ti enhancement. All the AB grains and PNGs show quite close-to-solar δ 46,47 Ti values, indicating close-to-solar initial metallicities for their parent stars (Alexander & Nittler 1999).

DISCUSSIONS
Overviews of nova nucleosynthesis in the context of PNG isotopic signatures are given by José et al. (2004) and José & Hernanz (2007). To summarize, although PNGs show isotopic signatures similar to nova nucleosynthetic yields, several fundamental inconsistencies make it problematic to link AB1 grains and PNGs to classical novae: (1) In the context of nova nucleosynthesis, the Si isotope ratios of PNGs indicate that these grains are more likely to originate from ONe novae (1.15−1.35 M ¤ ), resulting in large excesses in 30 Si relative to 29 Si.  (1) is that if in the parent CO novae of AB1 grains, the companion low-mass stars had evolved to their AGB phases, the accreted H-rich envelope material onto the white dwarfs would be 14 Nand 26 Al-enriched ( 14 N/ 15 N>1000, 26 Al/ 27 Al=0.001−0.01, Palmerini et al. 2011) compared to the solar-composition envelope adopted in the models. This would lead to a better match with the AB1 grain data by CO nova models, but requires finely tuned timing since the AGB phase only lasts on the order of ~10 6 yr. (3) A strong piece of evidence against a nova origin is the neutroncapture isotopic signatures preserved in these grains. In detail, AB1 grains generally show 32 S and 50 Ti excesses, both of which point to neutron irradiation that does not occur in novae (José & Hernanz 2007). Although none of the PNGs analyzed so far show Ti isotopic anomalies indicating neutron capture, two out of four PNGs had large 32 S excesses. The disagreement between isotopic signatures of Si and S measured in PNGs and nova models is illustrated in Figure 3b. Thus, it is unlikely that AB1 grains and at least some of PNGs originated from novae.
Finally, stellar models for lower-than 1.00 M ¤ CO novae predict higher-than-solar 14 N/ 15 N and could potentially explain AB2 grains that do not show neutron-capture isotopic signatures  Pignatari et al. 2013aPignatari et al. ,b, 2015 generally agree that only material from the outermost layers is 14 N-rich but 26 Al-poor within a SN (Lin et al. 2010;Xu et al. 2015). Thus, the model predictions in Figure 4a are shown as mixing lines between explosive H-burning products from the He/C zone with weight-averaged isotopic compositions and material from the outermost zones (8.4 to 13.3 M ¤ ) in 25T and 25d models. Figure 4a shows that most of the PNGs and two AB1 grains with close-to-terrestrial 14 N/ 15 N ratios (if not caused by terrestrial N contamination) are well matched by the mixed ejecta.
On the other hand, both the WH07 and Pignatari models show that the 26 Al/ 27 Al ratios of the outermost layers are significantly increased in SN models ≥20 M ¤ relative to those <20 M ¤ (Figures 4b,c), resulting from the loss of the H envelope in the former with the H-burning zone exposed on the surface. Since the mismatch of the models with the C2 and AB1 grain data in  Figure 4b. In this case, the N-Al isotope correlation of the AB1 grains is well reproduced by the C-rich 25d and 25d-H5 ejecta, implying that AB1 grains could have come from low-mass core collapse SNe with H ingestion. Alternatively, AB1 grains could also be explained by SNe ≥20 M ¤ if they incorporated a significant amount of materials from the surrounding 14 N-rich H-envelope material with much lowered 26 Al/ 27 Al ratio lost during the pre-supernova phase. There is clear observational evidence for interactions between the SN ejecta and the ejected pre-supernova material (e.g., Smith et al. 2015).
More importantly, the N, Al, Si, and Ti isotopic compositions of C2 and AB1 grains strongly indicate that they share a common origin in SNe. Figure 4b shows that C2 and AB1 grains fall quite close to the same mixing line, with C2 grains lying closer to the explosive H-burning products end-member. Moreover, L16 found a δ 50 Ti excess of 800±200‰ in one C2 grain with a δ 30 Si value of ~400‰. In comparison, all the AB1 grains have Si isotope ratios within 100‰ of terrestrial values and δ 50 Ti excesses up to 200‰, which could be explained by diluting the C2 isotopic signature with roughly four times more material from the outermost H envelope that have normal Si and Ti isotopic compositions. In comparison, most of the PNGs can be better explained by more massive SNe compared to AB1 grains (Figure 4a). Moreover, Figure 4b implies that AB1 grains with closer-to-solar N isotope ratios probably can be matched by C-rich ejecta in SNe that achieved peak temperatures lower than 7×10 8 K (the value adopted in the 25d models) at the bottom of their He/C zones during explosions, implying higher-mass progenitor stars, lower explosive energies, and/or SN asymmetries.
Differences exist between the WH07 and P15 model predictions (Figures 4c,d), which result from differences in the final stellar structures and the adopted nuclear reaction rates. For instance, P15 used the more recent LUNA rates for 14 N(p,γ) 15 O that are lowered by up to 40% below 1.5×10 8 K (Imbriani et al. 2005) compared to the previously recommended values (Angulo et al. 1999), explaining the higher 14 N/ 15 N in the P15 models. In addition, both N and Al ratios in Interestingly, grain M1-A7-G897 had the largest 32 S and 50 Ti excesses (Figure 2), strongly supporting the concomitant production of both nuclei by neutron capture in the parent SNe. Our results, therefore, support the initial presence of 32 Si in AB1 grains. This also agrees with the fact that AB1 grains are more enriched in 30 Si relative to 29 Si than AB2 grains as a result of stronger neutron-capture nucleosynthesis. As with previous studies (Fujiya et al. 2013;Hoppe et al. 2012;Xu et al. 2015), the derived 32 Si/ 28 Si ratios of the three AB1 grains with 32 S excesses from our study are on the order of 10 −4 −10 −3 , which are consistent with the 15 M ¤ SN models of Pignatari et al. (2013a), but are generally lower than those of type C grains. Also, in the 12 M ¤ WH07 SN model, enough 32 Si is made in the C-rich He/C zone to explain the grain data, along with their Si and Ti isotope ratios (Figure 5 of L16). In contrast, PNGs show large 32 S excesses but no measurable 50 Ti excesses. If a large amount of Ti from the neutron-burst zone within the He/C zone were removed by condensing TiC, for example, before mixing into the envelope, the decoupled 32 S and 50 Ti correlation of PNGs could be explained. We, however, do not favor this explanation, because Liu et al. (2017b) recently found well correlated Si and Ti isotope ratios in X grains from SNe, suggesting insignificant amounts of fractionation between Si and Ti prior to mixing of ejecta from different zones. The correlated 32 S and 50 Ti excesses in grain M1-A7-G897 also seem not to support this scenario. On the other hand, the decoupled 32 S and 50 Ti excesses of PNGs could be explained if 32 S excesses were incorporated into the grains as 32 S made by alpha capture (Figure 8 of L16).

CONCLUSIONS
Multi-element isotopic data clearly show distinctive differences between 14 N-rich and 15 N-rich AB grains. We thus propose to divide AB grains into AB1 grains with 14 N/ 15 N<440 and AB2 grains with 14 N/ 15 N≥440. Detailed comparisons of the new isotope data on these grains with state-of-the-art nucleosynthetic calculations show fundamental problems in linking AB1 grains and PNGs to classical novae. In contrast, there is a satisfactory agreement between the AB1, PNG, and C2 grain data and models for SNe with H ingestion from the envelope into the He/C zone prior to a SN explosion. Interestingly, one AB1 grain with the largest S and Ti isotopic anomalies found in this study, for the first time, shows concomitant overproduction of 32 S and 50 Ti in the parent star, providing additional evidence to the initial presence of radiogenic 32 Si in presolar grains.   study are compared to literature data (Amari et al. 2001a,b;Nittler & Hoppe 2005;L16 AB1  25d-H  25d-H5  25d-H50  25T-H  25T-H5  25T-H50