STELLAR ORIGINS OF EXTREMELY 13C- AND 15N-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?

Extreme excesses of 13C (12C/13C < 10) and 15N (14N/15N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized 13C- and 15N-enriched presolar SiC grains (12C/13C < 16 and 14N/15N < ∼100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in 13C and 15N, but with quite diverse Si isotopic signatures. Four grains with 29,30Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with 30Si excesses and 29Si depletions show lower-than-solar 34S/32S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that 15N-enriched AB grains (∼50 < 14N/15N < ∼100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower 26Al/27Al, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments.


INTRODUCTION
Primitive meteorites contain several types of dust grains that condensed in stellar winds or in the ejecta accompanying stellar explosions, became part of the protosolar molecular cloud, and survived destruction in the early Solar System before incorporation into meteorites. These presolar grains are identified by their anomalous isotopic compositions, which cannot be explained by any known mass fractionation process in the Solar System and instead suggest different stellar origins (Zinner 2014). The mineral silicon carbide (SiC) is the best studied presolar grain phase because (1) it can be quite easily separated from bulk meteorites by acid dissolution ; and (2) SiC is predominantly formed in C-rich gas according to grain condensation calculations in stellar conditions (Lodders & Fegley 1995;Ebel & Grossman 2001), so its formation in the generally O-rich (O/C > 1) Solar System is rare; (3) Compared to other presolar mineral phases such as oxides (typically submicron in size), many SiC grains are relatively larger so that isotopic analysis of multiple elements can be made in each single grain.
Based on the measured isotopic compositions of a large number of elements, more than 90% of presolar SiC (mainstream grains) have been inferred to originate in winds from low-mass asymptotic giant branch (AGB) stars with close-tosolar metallicity Nittler 2003;Davis 2011;Zinner 2014). Mainstream grains have 12 C/ 13 C between 10 and 100 with a wide range of 14 N/ 15 N ratios; their 9 δ 29 Si/ 28 Si and δ 30 Si/ 28 Si vary from −100‰ to 200‰ (see Zinner 2014 for details). The remaining 10% of presolar grains have been divided into isotopic sub-groups, including X and C grains from core collapse supernovae (CCSNe), Y and Z grains from low metallicity AGB stars, and AB grains, which possibly have multiple stellar sources (e.g., Alexander 1993;Nittler et al. 1996;Hoppe et al. 1997;Amari et al. 1999, Amari et al. 2001bPignatari et al. 2015, hereafter P15). Relative to mainstream grains, Y and Z grains are more enriched in 30 Si and additionally, Y grains have 12 C/ 13 C ratios higher than 100. Although X and C grains both came from CCSNe with similar C and N isotopic compositions, X grains are enriched in 28 Si (negative δ 29,30 Si/ 28 Si), while C grains show excesses in both 29 Si and 30 Si (positive δ 29,30 Si/ 28 Si). Finally, AB grains are mainly characterized by 12 C/ 13 C ratios lower than 10, and their Si isotopic compositions are overlapped with those of mainstream grains (see Zinner 2014 for details).
A small fraction of presolar SiC grains are characterized by extremely low 12 C/ 13 C and 14 N/ 15 N ratios and excesses in 30 Si relative to 29 Si (Amari et al. 2001a). Such isotopic signatures agree well with the characteristic proton-capture nucleosynthetic signatures in novae. Novae are powered by thermonuclear explosions in the H-rich envelopes of white dwarf (WD) stars. These H-rich envelopes are produced by transfer of material from a low-mass stellar companion that is still on the main sequence or on the giant branch. There are two types of classical novae, depending on the nature of the WD: CO WDs are the remnants of evolved AGB stars less massive than ∼6-8 M e , and ONe WDs are remnants of more massive AGB stars (8-10 M e , José et al. 2004;Herwig 2005;Jones et al. 2013;Karakas & Lattanzio 2014). Nucleosynthetic models predict ubiquitous production of 13 C and 15 N by explosive H burning in both CO (M > 0.8 M e ) and ONe novae (José & Hernanz 2007).
These rare presolar SiC grains are therefore called putative nova grains. A number of problems, however, are faced by nova models in explaining the grain data: (1) putative nova grains of different mineral phases including SiC, oxides, and silicates, all have much less anomalous isotopic compositions compared to nova ejecta that therefore needs to be highly diluted with isotopically normal (i.e., ∼solar) material (>90%) to match the grain data Amari et al. 2001a;Nittler & Hoppe 2005;Gyngard et al. 2010;Leitner et al. 2012;Nguyen & Messenger 2014); (2) although thermodynamic equilibrium calculations predict that SiC grains can condense in the innermost ejected shell of ONe novae (José et al. 2004), mixing with a large amount of solar-like composition material (C < O) would greatly lower the C/O ratio in nova ejecta, and consequently lower the possibility for SiC to condense during nova outbursts; (3) CO novae are more abundant (70%-80%), and also more efficient in producing dust than ONe novae (e.g., Gehrz et al. 1986;Mason et al. 1996), but most of the putative nova grains seem to be from ONe novae based on their Si isotope data (Amari et al. 2001a). These difficulties therefore make it quite problematic to link 13 C-and 15 N-enriched grains to nova origins. On the other hand, it is important to point out that both C-and O-rich dust has been simultaneously observed in some novae (e.g., Gehrz et al. 1998). Thus, the strong radiation from the underlying WD may cause the process of condensation to proceed kinetically rather than at equilibrium. Consequently, the strong role of the CO molecule in determining formation of C-rich dust in reduced stellar environments (C/O > 1) could be dramatically decreased (Shore & Gehrz 2004).
Interestingly, Nittler & Hoppe (2005) reported one 13 C-and 15 N-enriched SiC grain (M11-334-2) with excesses in 28 Si, 44 Ca, and 49 Ti that point toward a CCSN origin. Consequently, this result suggests that the proton-capture nucleosynthesis associated with classical novae may also occur in some CCSNe, perhaps due to ingestion of H from the stellar envelope into underlying layers (e.g., P15). To better resolve the stellar origin(s) of putative nova grains, isotope data of a number of elements in single grains are needed to constrain the nucleosynthetic processes in their parent stars. Nova models predict that 26 Al/ 27 Al ratios of nova ejecta are generally lower than the initial 26 Al/ 27 Al ratios typically measured in X grains from CCSNe and that Ti lies beyond the standard nucleosynthetic endpoint for classical nova outbursts, i.e., Ti isotopic compositions of nova ejecta should be close to those of mainstream grains in most cases (José et al. 2004). In contrast, 49 Ti and 50 Ti excesses, relative to 48 Ti, were found in a majority of X grains (Nittler et al. 1996;Lin et al. 2010). Thus, the initial 26 Al/ 27 Al and Ti isotope ratios can be used as diagnostic tools to distinguish CCSNe from novae. Limited by their extremely low abundances (<1%) and small sizes (<1 μm), most of the fewer than 10 putative nova grains in the literature were only analyzed for the isotopic compositions of their C, N, and Si (Amari et al. 2001a). Magnesium-Al isotope ratios were only reported for four grains, and Ti isotope ratios for two grains, both of which had Ti isotope anomalies contradicting with a nova origin (Amari et al. 2001a;Nittler & Hoppe 2005).
In this paper, we report C, N, and Si isotope data for 11 new putative nova grains and three 15 N-enriched AB grains from Murchison and their correlated Mg-Al, S, and Ca-Ti isotope data when available. These new 13 C-and 15 N-enriched grains double the number of putative nova grains found in meteorites. More importantly, they provide valuable information on nucleosynthesis in their parent stars through isotope data of heavier elements (e.g., S, Ti). While seven of the 11 new putative nova grains are enriched in 30 Si relative to 29 Si, as seen in previous nova grains (Amari et al. 2001a), four grains show enrichments in both 29 Si and 30 Si, relative to 28 Si, by up to 15 times their solar abundances, which are quite similar to the Si isotopic compositions of C grains that are believed to have originated in CCSNe Pignatari et al. 2013b;Xu et al. 2015). By definition, these grains cannot be classified as either putative nova grains or C grains. Historically, presolar SiC grains with unusual C, N, and Si isotope ratios that could not be assigned to any existing groups have often been named type U (unusual or ungrouped) grains. The definition of U grains, however, is quite ambiguous, and not all grains classified as U-type in the recent literature may be genetically related. As will be discussed in Section 4.1, 13 C-and 15 Nenriched grains with excesses in 29,30 Si essentially originated from similar regions in CCSNe as type C grains. We therefore propose to rename type C grains ( 12 C/ 13 C > 10) "C1 grains" and name the 13 C-and 15 N-enriched grains ( 12 C/ 13 C < 10) with 29,30 Si excesses "C2 grains." By comparing with recent nucleosynthetic calculations by P15, the C2 grains from this study confirm the possibility raised by Nittler & Hoppe (2005) that some, if not all, of 13 C-and 15 N-enriched presolar SiC grains are from CCSNe.

EXPERIMENTAL METHODS
The SiC grains in this study were extracted from the Murchison meteorite by means of the CsF isolation method described by Nittler & Alexander (2003). The grains were separated in size by sedimentation. Thousands of grains with typical diameter ∼1 μm were dispersed on a high purity Au foil from a water suspension and further pressed into the Au with a flat sapphire disk.
We searched for rare SiC grains by manually acquiring simultaneous ion images of 12 C 2 -, 12 C 13 C − , 12 C 14 N − , 12 C 15 N − , 28 Si − , 29 Si − , and 30 Si − in multi-collection mode using a focused Cs + ion beam (1-2 pA, 100-150 nm) with a Cameca NanoSIMS 50L ion microprobe at the Carnegie Institution for Science. The use of C 2 rather than C ions allows (1) better alignment of the C and N isotopic secondary beams in the mass spectrometer because of similar energy distributions (differing energy distributions between atomic and molecular ions, De Gregorio et al. 2013); and (2) better alignment of the C and Si isotopic secondary beams because of smaller differences in mass. Data were acquired in imaging mode and isotope ratios for individual grains determined with the L'image processing software written by L. R. Nittler. Aggregates of synthetic SiC and Si 3 N 4 grains were used as isotopic standards and measured in between every 10 to 20 grain measurements (∼5-10 minute integration time for each grain analysis) in order to monitor and correct for instrumental drift in the isotopic mass fractionations. After NanoSIMS analysis, we obtained high-resolution images (∼5 nm/pixel) and energy-dispersive X-ray spectra of candidate SiC grains with a JEOL 6500F field-emission SEM. The SEM images showed that some SiC grains that appear to be single grains in NanoSIMS ion images are in fact clumps of smaller SiC grains (<1 μm). Thus, there exist large uncertainties in estimating the total number of single SiC grains analyzed only based on the NanoSIMS ion images, making it difficult to accurately ascertain SiC sub-type abundances in the Murchison acid residue.
Out of ∼700 ion images collected on more than 1000 grains, we identified 10 SiC grains that are extremely enriched in 13 C and 15 N ( 12 C/ 13 C < 16 and 14 N/ 15 N < 50) and three 15 Nenriched AB grains. One of the 13 grains was only 200 nm across and sputtered away during the analysis of C, N, and Si isotopes. We measured S isotopes in four of the 12 remaining grains. These measurements were made by simultaneously obtaining ion images of 12 C 2 -, 12 C 13 C − , 12 C 14 N − , 12 C 15 N − , 32 S − , 33 S − , and 34 S − in three of the four grains, and ion images of 12 C 2 -, 28 Si − , 29 Si − , 30 Si − , 32 S − , 33 S − , and 34 S − in grain GAB in multicollection mode by rastering a Cs + beam over the grain areas. Grains G270_2 and Ag2_6 were completely sputtered away during S isotope analysis. Sulfur contamination in the synthetic SiC standard is high, so it was used as a standard for the S analysis. Also, because the S isotope ratios of mainstream grains are quite normal according to previous studies (e.g., Hoppe et al. 2015), we also measured mainstream grains adjacent to the four grains to monitor the instrumental isotope mass fractionation. The S concentrations of the mainstream grains ( 32 S/ 28 Si ion ratios in the range of 0.007-0.07) are 10-100 times lower than that of the SiC standard ( 32 S/ 28 Si ion ratios of ∼0.7). Overall, the δ 33 S/ 32 S and δ 34 S/ 32 S values of all the mainstream grains are solar within uncertainties (∼20‰), when normalized to the mean ratios of the synthetic SiC standard.
Magnesium isotopes were measured along with Al by simultaneously collecting 24 Mg + , 25 Mg + , 26 Mg + , 27 Al + , 28 Si + , 30 Si + , and 48 Ti + in the 10 remaining grains in multi-collection mode by rastering a primary O − beam (4 pA, ∼300 nm). Subsequently, the 10 grains were also measured for 28 Si + , 40 Ca + , 44 Ca + , 47 Ti + , 48 Ti + , 49 Ti + , 50 Ti + , and 50 Cr + in combined-analysis mode (Set 1: 28 Si + ; Set 2: the rest of the isotopes). Seven of the 10 grains had detectable Ti counts that were correlated with Si in the ion images. Burma spinel was used as a standard for Mg isotope analysis and also used to determine the Mg and Al relative sensitivity factors, giving a relationship between secondary ion yields and the atomic ratios of Al + / Mg + = 1.16×Al/Mg. The TiC standard was found to contain significant Ca contamination and was thus used as standard for both Ca and Ti isotope analysis. All the isotope data are reported in Table 1 with 1σ uncertainties. Note that an additional 13 C-and 15 N-enriched SiC grain from Murchison, G240-1, previously reported in an abstract by Nittler et al. (2006), is also discussed here and included in Table 1. This grain was found by automated C and Si isotopic measurements of 1550 Murchison SiC grains with the Carnegie ims-6f ion probe (Nittler & Alexander 2003) and re-measured for C, N, Si, and Mg-Al by NanoSIMS, using similar methods to those described above.

Putative Nova and 15 N-enriched AB Grains
Presolar SiC grains that are highly enriched in 13 C ( 12 C/ 13 C < 10) are mainly assigned to one of two sub-types: AB and putative nova grains. AB grains have a wide range of 14 N/ 15 N ratios from ∼50 to ∼10 4 (solar 14 N/ 15 N ratio: 441±5, Marty et al. 2011), and Si isotopic ratios that are similar to those of mainstream SiC grains. The Si isotopes of mainstream SiC grains (δ 29 Si/ 28 Si = (1.37±0.01)×δ 30 Si/ 28 Si+(−19.9±0.6), Zinner et al. 2007) are roughly consistent with the predictions of Galactic chemical evolution (GCE, Lugaro et al. 1999;Zinner et al. 2006). On the other hand, putative nova grains are more enriched in 15 N and have distinctive Si and Mg-Al isotopes (e.g., Figure 2 of Zinner 2014). In Figure 1, three new 15 N-enriched AB grains and seven new putative nova grains from this study are plotted along with literature data for C, N, Si, and Al isotope ratios. Two grains (M11-334-2 and M11-151-4) reported by Nittler & Hoppe (2005) with Ti and/or Si isotopic compositions that are incompatible (e.g., 28 Si, 44 Ca excesses in M11-334-2; 47 Ti excess in M11-151-4) with nova nucleosynthesis (e.g., 30 Si excess and Ti isotope ratios close to mainstream grain data) are labeled in the plot. In addition, these two grains have the highest 26 Al/ 27 Al ratios among all 13 C-and 15 N-enriched presolar SiC grains. Because of these distinctive isotopic signatures, we do not consider them as putative nova grains hereafter. Figure 1 shows that the isotopic compositions of the seven new putative nova grains and three AB grains from this study are similar to previous ones for C, N, and Al. Interestingly, putative nova grains show higher initial 26 Al/ 27 Al ratios than AB grains, but lower than X grains (X grains: 0.1< 26 Al/ 27 Al < 1, e.g., Zinner 2014). Grain 577 is classified as an AB grain in the presolar grain database (Hynes & Gyngard 2009), while Huss et al. (1997) argued that G577 was from a low metallicity AGB star that had experienced very extensive deep mixing. In fact, although the 14 N/ 15 N ratio of G577 is in the range of AB grains, its 26 Al/ 27 Al and Si isotope ratios are more like those of the putative nova grains and it should probably be thus classified as such. Nitrogen-15 enriched AB grains and putative nova grains bear some similarity in that most grains of each type have solar Ti isotopic compositions and no excess in 44 Ca (ascribed to decay of 44 Ti, with a half-life of 60 year, in grains from CCSNe), and that a few grains of each type show negative δ 34 S/ 32 S values (Fujiya et al. 2013). Putative nova grains, however, can be distinguished from 15 N-enriched AB grains by (1) relatively lower 14 N/ 15 N ratios (<70); (2) higher 26 Al/ 27 Al ratios (>0.01); and (3) excesses in 30 Si relative to 29 Si.

Al Contamination in Presolar SiC Grains
In previous studies, surface contamination has been found in presolar SiC grains for both major (e.g., C, N, Nittler & Alexander 2003) and trace elements (e.g., Sr, Ba, Liu et al. 2015). The most common practices to remove surface contamination include: (1) acid-cleaning grains prior to isotopic and elemental analysis (Liu et al. 2014(Liu et al. , 2015; and (2) surface-cleaning of grains by removing a few atomic layers using the Cs + ion source during NanoSIMS analysis. Groopman et al. (2015), however, found that 26 Mg/ 24 Mg and 27 Al/ 24 Mg ratios are well correlated in depth profiles of many presolar SiC grains, and the obtained linear fits often have negative 26 Mg/ 24 Mg intercepts that are unphysical, indicating that there exists constant Al contamination (up to 60%) throughout NanoSIMS measurements (see Groopman et al. 2015 for calculations in detail). Thus, the "true" initial 26 Al/ 27 Al ratios should be 1.5-2 times higher on average than the values calculated using the standard approach (e.g., Equation (2) of Nittler et al. 1997). We also examined the depth profiles of Mg-Al isotope data for the 10 grains reported in Table 1 and found similar "internal isochrones." These 13 C-and 15 N-enriched SiC grains, however, have extremely high 27 Al/ 24 Mg ratios (>500 in most of the cases), so the intercepts of the linear fits have large uncertainties, and the amounts of Al contamination therefore cannot be accurately determined for these grains using the method of Groopman et al. (2015).
The grain GAB has the lowest 27 Al/ 24 Mg of the measured grains and its depth profile is shown in Figure 2 for illustration. During the first five cycles, because of surface Al contamination and variation in the Al + / Mg + ion ratio, the corresponding data points (gray circles) deviate from the linear fit (solid black, with 95% confidence bands shown as gray area) to the data points that reached a steady state Al + / 24 Mg + ion ratio (red circles). Despite large uncertainties, Figure 2(b) shows that the intercept is slightly negative, indicating that there might be 20 % 15 60 -+ constant Al contamination, considering the 95% uncertainties in the linear fit. Because of the large uncertainties in the estimated amounts of Al contamination, the effect of Al contamination is not included in the initial 26 Al/ 27 Al ratios reported in Table 1 and these thus represent lower limits. The relative difference in 26 Al/ 27 Al ratios between putative nova and AB grains, however, cannot be explained by Al contamination, because 26 Al/ 27 Al ratios of all putative nova grains are higher than those of 15 N-enriched AB grains despite different instruments, samples, and analysis conditions used in these studies. Carbon-13 is produced in stars by proton capture on preexisting 12 C followed by β-decay, 12 C(p,γ) 13 N(β + ν) 13 C, which occurs during H burning in the CNO cycle at temperatures ranging from 1.5×10 7 to 3.5×10 8 K. Although the 13 C Notes.All isotope data are reported with 1σ uncertainties; a The 26 Al/ 27 Al ratio is calculated based on the Equation and δ( 34 S/ 32 S). The NanoSIMS sensitivity factor ratio of S to Si is taken to be three . Because S isotopes were simultaneously measured with C and N isotopes in grain G270_2, Ag2, and Ag2_6, the sensitivity factor ratio of Si to C 2 was needed to calculate 32 Si/ 28 Si (Λ(Si/C 2 ) ∼unity according to the ratio determined in both synthetic and presolar SiC).
production is enhanced with increasing temperature, 13 C is also more efficiently consumed via 13 N(p,γ) 14 O(β + ν) 14 N at sufficiently high T during the nova thermal nuclear runaway (TNR). As a consequence, the 12 C/ 13 C production ratio does not depend strongly on temperature. On the other hand, 15 N can only be significantly produced at high temperatures ( 14 N/ 15 N < 0.1). Thus, hot H burning during the CNO cycle (>10 8 K) is needed in order to produce significant amounts of 13 C and 15 N (José et al. 2004). Amari et al. (2001a) discussed difficulties in explaining both low 12 C/ 13 C and 14 N/ 15 N ratios of putative nova grains by nucleosynthesis in AGB stars, CCSNe, and Wolf-Rayet stars, which led to the conclusion that  (Hoppe et al. 1994(Hoppe et al. , 1996Huss et al. 1997;Amari et al. 2001b), and eight putative nova grains Amari et al. 2001a;Nittler & Hoppe 2005)  novae, ONe novae in particular, are the most likely stellar source of highly 13 C-and 15 N-enriched presolar SiC grains.
The only 13 C-and 15 N-enriched stellar site identified by both astrophysical observations (e.g., Sneden & Lambert 1975) and nucleosynthetic simulations (e.g., José et al. 2004;Denissenkov et al. 2014) is classical novae. José et al. (2004) pointed out that the main effect of classical nova nucleosynthesis on the Si isotopes is an increase of δ 30 Si/ 28 Si with δ 29 Si/ 28 Si below or approaching zero. Putative nova, C1, and C2 grains are compared to ONe nova model predictions of José et al. (2004) with updated nuclear inputs for Si isotope ratios in Figure 3. The nova model predictions are shown as mixing lines between weighted average isotopic compositions of pure ONe nova ejecta and the solar isotopic composition. This figure shows that all the putative nova grains lie between the 1.15 and 1.35 M e ONe nova model predictions, although the pure nova ejecta need to be diluted with a large amount of isotopically normal material to match the grain data. In contrast, type C2 grains show enrichments in both 29 Si and 30 Si relative to 28 Si that are incompatible with the nova nucleosynthetic calculations. For instance, even though individual shells of nova ejecta are more variable with respect to their bulk compositions, the lowest ratio of δ 30 Si/ 28 Si to δ 29 Si/ 28 Si reached in individual shells of the 1.35 M e ONe nova model (∼6) is still a factor of three to six higher than those of C2 grains ( Figure 5 of Amari et al. 2001a), which cannot be explained by mixing with materials of solar isotopic composition (i.e., isotopic dilution cannot cause the ratio of δ 30 Si/ 28 Si to δ 29 Si/ 28 Si to vary). Thus, ONe nova models are incompatible with the Si isotopic signatures of C2 grains.
Other types of stars that can produce low 12 C/ 13 C ratios include born-again AGB stars and J-type stars. Born-again AGB stars (Fujimoto 1977) are post-AGB stars that undergo a very late thermal pulse (VLTP, e.g., Sakuraiʼs object). At this stage, the remnant star has only a thin residual H shell left that could be ingested into the convective He-rich shell during the VLTP, producing extremely low 12 C/ 13 C ratios. However, models of such stars predict large excesses in 14 N ( Figure 10 of Herwig et al. 2011) and are thus unlikely as sources of the 13 Cand 15 N-rich SiC grains. J-type giants are C-rich stars that are mainly identified by their low 12 C/ 13 C and the absence of selement enrichments (Abia & Isern 2000); their origin is poorly understood. Hedrosa et al. (2013) reported N isotope ratios for six J-type giants, one of which, WX cyg, may have a 14 N/ 15 N ratio as low as six and could show the same 15 N-excess seen in the putative nova grains. Thus, C-rich J-type stars might have contributed 13 C-and 15 N-enriched SiC dust grains to the Solar System. However, according to the 29,30 Si excesses in C2 grains (see also Section 4.1.2 for details), s-process element enrichments are expected in these grains, while such enrichments are not observed in J-type stars (Abia & Isern 2000). Thus, at present they are less likely to be the stellar progenitors of C2 grains. Similar to type C2 grains, another rare group of presolar SiC grains, type C grains (hereafter type C1 grains), also have large excesses in 29 Si and 30 Si, relative to 28 Si. However, C1 grains are enriched in 12 C and 15 N, relative to solar materials, similar to the isotopic signatures of most X grains (∼1%-2% of presolar SiC grains, see Zinner 2014 for references). Figure 3 shows that the Si isotopic compositions of C1 and C2 grains fall on similar trends. For reference, the slopes of the linear fits for C1 and C2 grains in Figure 3 are 0.89±0.05 and 1.12±0.57, respectively, which are consistent with each other within 95% confidence bands. In contrast to the 28 Si-enriched X grains that came from CCSNe, the rarer type C1 and C2 grains (<0.1%) are characterized by large excesses in 29 Si and 30 Si. These are most likely explained by neutron capture triggered by activation of the 22 Ne(α,n) 25 Mg neutron source during CCSN explosions (Meyer et al. 2000;Pignatari et al. 2013b). The associated neutron-capture nucleosynthesis is called a neutron burst process, i.e., n process (Blake & Schramm 1976;Thielemann et al. 1979;Meyer et al. 2000). Figure 4 shows the 12 M e , Z e CCSN model of Woosley & Heger (2007, W&H07), which predicts a neutron burst zone in the outer C/O and inner He/C zones (shaded area in the left panel of Figure 4). Model predictions for the Si isotope ratios of these shells are compared with those of C1 and C2 grains (right panel of Figure 4); all the grain data are well matched by mixing neutron-burst products with H envelope material (with an assumed initial solar Si isotopic composition). The good agreement is consistent with the 29 Si and 30 Si excesses of both C1 and C2 grains being the result of neutron-burst nucleosynthesis in CCSNe. Pignatari et al. (2013b) proposed that the large negative δ 33 S/ 32 S and δ 34 S/ 32 S values found in some C1 grains can be explained by the presence of 32 S from the in situ decay of short-lived 32 Si (t 1/2 = 153 a) produced by the n process (predicted 32 Si/ 28 Si on the order of 10 −3 , Figure 5 of Pignatari et al. 2013b), which could be incorporated into SiC grains as 32 Si and then decay to 32 S after grain condensation. This 12 M e CCSN model, as discussed in Xu et al. (2015), predicts high 32 Si/ 28 Si ratios in the neutron burst zone (Figure 4), in agreement with the 15 M e model by Pignatari et al. (2013a). The Si and S isotopic compositions of C1 grains therefore can be explained by the 12 M e CCSN model predictions for the C-rich neutron burst zone, if mixing with H envelope material is considered.
In addition to the good agreement between type C2 grain data and neutron burst predictions for Si isotopes, additional neutron-burst isotopic signatures were observed in the grain GAB and also strongly support the CCSN origin ( Figure 5). To qualitatively compare model predictions with the grain data, we mixed materials from individual neutron burst shells of different CCSNe with different amounts of H envelope material (with solar Si and Ti isotopic compositions to dilute the nprocess products) to match the measured δ 29 Si/ 28 Si value of GAB (230±6‰) and calculated predictions for the other neutron-rich Si and Ti isotopes. Figure 5 clearly shows that all of the baseline CCSN models (e.g., 12 and 25 M e CCSN model by W&H07) predict elevated δ 30 Si/ 28 Si, 32 Si/ 28 Si, and δ 50 Ti/ 48 Ti values in the neutron burst zone, and the 25 M e CCSN model predictions agree best with the grain data. Note that the 25 M e CCSN model of W&H07 predicts that the neutron burst zone is located within the C/O zone, where the C/O ratio is less than unity. So the main prerequisite for SiC formation (C > O) therefore cannot be satisfied. Overall, the Si, S, and Ti isotopic compositions of grain GAB are well reproduced by pure signatures of the neutron burst zone without the need of further mixing from deeper zones. In addition, no measurable excess in δ 44 Ca/ 40 Ca was found in grain GAB, indicating that its parental material was mainly from outer zones of the supernova without significant incorporation of 44 Ti from deeper layers, in good agreement with the constraints from other n-process isotopic signatures.
CCSN models, however, predict extremely high 12 C/ 13 C ratios for the C/O and He/C zones (10 7 -10 9 for 12 C/ 13 C,  is abundantly produced as a result of the triple-alpha reaction, while the lack of H in these zones precludes abundant production of 13 C. Xu et al. (2015) therefore highlighted difficulties in explaining 13 C and 15 N excesses in SiC X grains and low density graphite grains that were discussed in previous studies (e.g., Travaglio et al. 1999;Yoshida 2007). Nittler & Hoppe (2005) pointed out that the CCSN isotopic signatures (excesses in 28 Si, 44 Ca, and 49 Ti) observed in the 13 C-and 15 Nenriched grain M11-334-2 demonstrates that high-temperature H burning must occur in the He-shell zones of at least some CCSNe and they proposed that some kind of extra mixing process(es) brought extra H into the He shell of a presupernova star, which could eventually enhance 13 C and 15 N production in the He shell via proton-capture reactions at high temperatures. For the first time, CCSN models of P15 show that an H ingestion event in the pre-supernova phase can affect the usual He-burning signature, and the impact of the following explosive H burning in the He shell during the supernova shock is considered in the models. In the next section, we compare our new grain measurements on C, N, and Al isotopic abundances with theoretical predictions by P15, and we analyze their main nucleosynthetic features.

13 C and 15 N Excesses of Type C2 Grains: Proton Capture Signatures in CCSNe
In addition to extra mixing processes occurring along the boundary of two convective zones caused by e.g., rotation (e.g., Meynet et al. 2006) and gravity waves (Arnett & Meakin 2011), more tumultuous mixing events can occur in stars, such as shell mergers (e.g., Woosley & Weaver 1995;Rauscher et al. 2002), and convective-reactive events following the ingestion of less processed material into hotter and deeper stellar zones (e.g., Herwig et al. 2014). For instance, Woosley & Weaver (1995) found that in one finely zoned 10 M e model of solar metallicity, the He shell is fully convectively linked with the H envelope, which could possibly bring a large amount of H into the He/C shell in CCSNe. Similarly, Pignatari et al. (2013a) found that H is ingested into the He shell in a 25 M e supernova model with solar metallicity.
Recently, P15 have qualitatively investigated the effect of H ingestion in the He shell material (e.g., Herwig et al. 2011Herwig et al. , 2014Stancliffe et al. 2011;Woodward et al. 2015) and the presence of residual H during explosive He burning in the 25 M e supernova model of Pignatari et al. (2013a). In particular, during pre-supernova evolution, H was ingested in the He shell and shell convection was then turned off, allowing survival of the ingested H in the He shell. P15 adopted two sets of supernova shock models (T peak = 0.7×10 9 K at the bottom of the He-rich zone in model 25d, and T peak = 2.3×10 9 K in model 25T, with the latter reproducing the stellar condition of the 15 M e CCSN model 15d in Pignatari et al. 2013a). The amount of H remaining from the pre-supernova evolution is varied from 1.2%(−H, the value predicted by the stellar model 25d), 0.12%(−H10), to 0.0024% (−H500) in the He shell. The predictions of these models for the He/C zone are compared to the 13 C-and 15 N-enriched presolar SiC grains from this study in Figure 6. Comparing the predictions of the −H500 (the least amount of H ingested) models with those of the other two models clearly indicates that explosive H burning greatly enhances the production of 13 C, 15 N, and 26 Al in the He/C zone.
Three of the four C2 grains have 12 C/ 13 C ratios lower than three, which can only be reached by the 25T-H and 25d-H models. Although Al contamination could lower the initial 26 Al/ 27 Al ratios estimated for the grains, the inferred amount of Al contamination found in most SiC grains is only up to 65%, i.e., a factor of two higher at most (Groopman et al. 2015), which generally agree with the amount of Al contamination found in the grain GAB, (Figure 2(b)). By mixing 13 C-and 15 N-enriched shells with H envelope material to match C isotope ratios of C2 grains, we found that the 25d-H predictions are more comparable ( 26 Al/ 27 Al ≈ 0.02) to the grain data ( 26 Al/ 27 Al ≈ 0.01-0.03) than the 25T-H model predictions ( 26 Al/ 27 Al ≈ 0.1-0.3). Finally, we point out that as indicated by their 28 Si and 44 Ti excesses, X grains are likely to have incorporated more material from deeper layers, e.g., from the C/Si zone (Pignatari et al. 2013a). So even if explosive H burning also occurred in the outer He shells of their parent CCSNe, the isotopic signatures of explosive H burning can be modified by incorporating materials with 12 C-rich explosive He-burning signatures in X grains.

Constraints on the Neutron Burst Environment
During Explosive H Burning Figure 5 shows differences in the predicted Si and Ti isotopes of the CCSN models for the neutron burst zone. In the 25d-H to 25d-H500 models, no resolvable neutron burst zone can be found and these are therefore not shown. A neutron burst zone exists in the 25T-H500 model, but there is a limited amount of H ingestion and the effect of proton-capture nucleosynthesis is negligible ( Figure 5). Differences are found between P15 and W&H07 model predictions in Figure 5. For instance, while the 25d-H500 model predictions for 32 Si/ 28 Si and δ 50 Ti/ 48 Ti lie between the 12 and 25 M e CCSN model predictions of W&H07, the corresponding predictions for δ 30 Si/ 28 Si are higher than both the W&H07 models. Together with the intrinsic differences between the two sets of models, the variations that we consider here could be caused by different nuclear cross sections adopted for the nucleosynthetic calculations of W&H07 and P15. For neutron capture on 30 Si, W&H07 adopted the experimental MACS (Maxwellian Averaged Cross Section) at 30 keV from Bao et al. (2000), 6.5±0.6 mb. On the other hand, P15 adopted the more recent 30 Si MACS values reported by Guber et al. (2003), which is more than a factor of three times lower (1.82±0.33 mb) than that used by W&H07 at 30 keV. Because of the lowered probability for neutron capture on 30 Si by adopting the Guber et al. (2003) MACS values, the P15 models predict higher 30 Si abundance relative to that of 28 Si. Thus, the difference in the δ 30 Si/ 28 Si predictions is mainly caused by different 30 Si MACS values adopted in the two sets of models.
When the ingested amount of H is increased from 0.0024% to 1.2% (25T-H model), the neutron burst nucleosynthesis is suppressed because the 22 Ne neutron source abundance ( 22 Ne(α,n) 25 Mg) for the n process is greatly reduced due to competing proton captures on 22 Ne via 22 Ne(p,γ) 23 Na (P15). Consequently, 49 Ti is more abundantly made than 50 Ti, and 32 Si/ 28 Si is about 20 orders of magnitude lower, because of the lowered neutron density and the consequent negligible production of 32 Si ( Figure 5). Nucleosynthetic calculations based on one-dimensional (1D) stellar models by P15 therefore cannot explain the n-process isotopic signatures of the C2 grains. Highly likely, the discrepancy between the grain data and theoretical predictions is caused by the fact that 1D stellar models cannot obviously predict the large heterogeneities of H ingestion into the He shell that have been found in multidimensional hydrodynamic simulations (e.g., pre Herwig et al. 2014). Thus, although 1D nucleosynthetic calculations can provide robust predictions for specific nuclide abundances resulting from the H ingestion event when the relevant reaction rates are experimentally well determined (e.g., reactions to produce 12 C, 13 C, 14 N, 15 N, 26 Al, 27 Al), different neutron-and proton-capture isotopic signatures may exist in different regions of the He shell, resulting from the multi-dimensional nature of H-ingestion events, i.e., varying amounts of H mixed into different regions of the He shell due to the heterogeneous H-ingestion process predicted by multi-dimensional models.
Overall, the P15 models suggest that H has to be effectively injected into the He shell (∼1.2%) in order to account for the Figure 5. Si-and Ti-isotopic compositions of grain GAB are compared to different CCSN model predictions for the neutron burst zone, which are normalized to δ 29 Si/ 28 Si at 230‰ by assuming different dilution factors for different shells. The isotope data are plotted with 1σ uncertainties. CCSN models include (1) 12 and 25 M e CCSN model predictions by Woosley & Heger (2007, W&H07); and (2) 25 M e CCSN model predictions with 1.2% (25T) and 0.0024% (25T-H500) of H ingested into the He shells by Pignatari et al. (2015, P15). low 12 C/ 13 C isotope ratios of C2 grains. Furthermore, the neutron burst isotopic signatures of C2 grains strongly support the heterogeneous distribution of ingested H and asymmetries predicted by multi-dimensional simulations for the H-ingestion event. That is to say, C2 grains probably incorporated materials from different regions of the He shell, so proton-capture isotopic signatures are the result of explosive H burning in regions with higher amounts of ingested H (∼1.2%), while nprocess isotopic signatures are records of nucleosynthesis during pre-supernova evolution in regions with negligible amounts of ingested H (<0.0024%). Therefore, although 1D stellar models provide important insights into different nucleosynthetic processes in CCSNe, complete multi-dimensional hydrodynamic simulations of the H ingestion event are needed to illustrate how parental materials of C2 grains are mixed prior to their condensation and to quantitatively compare nucleosynthetic predictions with C2 grain data. Currently, detailed multi-dimensional simulations for H ingestion are only available for the He shell in low mass stars. Therefore, similar calculations are also needed for massive stars with solar-like metallicity. To conclude, comparison of the C2 grain data with nucleosynthetic calculations based on 1D stellar models is critical to capture physical process(es) that are not previously considered, while the multi-element isotope data of C2 grains also presents a challenge for state-of-the-art 1D simulations of the hydrogen ingestion event, which provides a unique opportunity to constrain both neutron-and proton-capture environments prior to and during the supernova explosions for multi-dimensional hydrodynamic simulations.

Parent Stars of Putative Nova Grains:
Novae or Supernovae?
Hydrogen burning through the CNO cycle at high temperatures can yield both low 12 C/ 13 C and 14 N/ 15 N ratios (Section 4.1.1), and could occur in both novae and supernovae. During classical nova outbursts, the main nucleosynthetic path is driven by (p,γ), (p,α), and β + processes, with little contribution from n-and α-capture processes (Figure 2 of José 2016). In contrast, all of these processes could occur in the He/C zone during CCSN explosions, assuming ingestion of H depending on the peak temperature, density, and the amount of ingested H.
We determined Ca-Ti isotope ratios in four of the seven putative nova grains from this study, all of which have solar 44 Ca/ 40 Ca and Ti isotope ratios. Although these isotopic signatures are in good agreement with model predictions for classical nova nucleosynthesis, they do not rule out explosive H burning during supernova explosions, because the normal isotopic compositions could be simply caused by surface contamination, as in the case of Al (see Section 3.2). Even if we assume negligible amounts of surface contamination, the normal Ca-Ti isotope ratios are still consistent with CCSN nucleosynthesis because (1) 44 Ti is not produced in the He shell. So 44 Ca excesses from 44 Ti decay would not be expected in grains that condensed there, in agreement with the solar 44 Ca/ 40 Ca ratio of putative nova grains; (2) additionally, Ti isotope anomalies are predicted to be only a factor of two higher than those of Si in the outer He shell, and this corresponds to predicted Ti isotope anomalies lower than 200‰ in these four grains, in agreement with the grain data within uncertainties. Nucleosynthetic processes in different CCSN zones are quite diverse and the resulting isotopic signatures become complicated by including H ingestion and ad hoc mixing among different zones. As a result, protoncapture isotopic signatures of classical nova nucleosynthesis could be produced by explosive H burning in CCSNe, with contribution from nova-like isotopic signatures of the O zone in the 25T-H model of P15. Although Parikh et al. (2014) have recently pointed out enhancement in the 33 S/ 32 S ratio as a diagnostic isotopic signature of nova grains, in fact, the 25T-H model predictions for the 33 S/ 32 S ratio are also positive in both the O (nova-like) zone and the He/C zone of CCSNe. Thus, the Figure 6. Isotopic compositions of C, N, Si, and Al of three new 15 N-enriched AB grains, four C2 grains and seven new putative nova grains from this study are compared to H-ingestion CCSN model predictions for the He/C zone by P15. "Inner" refers to the innermost shell of the He/C zone and "Outer" refers to the outermost shell. The isotope data are plotted with 1σ uncertainties. The dashed lines represent terrestrial isotopic compositions. For 14 N/ 15 N, the solar value reported by Marty et al. (2011) is also shown. 33 S/ 32 S ratio is not specifically diagnostic of grains from novae, but more in general of H-burning at high temperatures in stars.
In the following sections, we first examine if classical nova models can explain the grain data. If not, the possibility of explaining these isotopic signatures by CCSNe will be discussed. We remind the readers that C-rich J-type stars also remain a possibility as the stellar site of putative nova grains. However, due to the lack of knowledge about the origin and detailed nature of these stars, nothing is known about isotopic signatures of elements other than C and N (astrophysical observations, e.g., Abia & Isern 2000) in them. In addition, recent observations of Nova Vul 1670 showed that the object is best explained as the remnant of a merger of two stars that were surrounded by an outflow in the form of O-rich molecular gas that was extremely enriched in 13 C and 15 N ( 12 C/ 13 C∼2, 14 N/ 15 N∼3, Kamiński et al. 2015). Although events exactly like Vul 1670 are unlikely to be the progenitors of putative nova SiC grains because of its oxidized environment, it remains an open question if there exist C-rich Vul 1670 like objects that are enriched in 13 C and 15 N that could be the progenitors of putative nova grains.

Putative Nova Grains versus Nova Nucleosynthesis
A comparison of putative nova grains and nova models was given by Amari et al. (2001a). The new putative nova grains in our study span the same range as previous grains for C, N, and Si isotopic compositions (Figure 1), so the reader is referred to Amari et al. (2001a) for discussion on these isotope ratios. In this section, we focus on comparison of the grain data with nova model predictions for 26 Al/ 27 Al and S isotope ratios. Figure 7(a) shows that mixing curves for ONe and CO nova model predictions follow different trajectories in the plot of 26 Al/ 27 Al versus 12 C/ 13 C, mainly because 26 Al is highly overproduced in ONe novae, which occur at higher outburst temperatures. Overall, four putative nova grains with 26 Al/ 27 Al > 0.06 can be roughly matched by ONe nova model predictions. The mixing ratios required to match the grain data indicate that if they are indeed ONe nova grains, they must have come from ONe novae with masses lower than 1.35 M e , because the extreme δ 30 Si/ 28 Si values predicted by the 1.35 M e ONe nova model are not observed (Figure 3). In addition, putative nova grains with 26 Al/ 27 Al ∼ 0.01 overlap with CO nova model predictions, in contrast to the previous argument that all of the putative nova grains originated from ONe novae solely based on Si isotope ratios (Amari et al. 2001a). The Si isotope anomalies of these CO nova grains are within ±100‰ with small excesses in 30 Si relative to 29 Si, while the main effect of CO nova nucleosynthesis is to decrease δ 29 Si/ 28 Si while leaving δ 30 Si/ 28 Si unchanged. Variations in the Si isotopes of mainstream presolar SiC grains (±200‰) are believed to result from the effect of GCE on their parent stars (Alexander & Nittler 1999;Lugaro et al. 1999;Zinner et al. 2006). Thus, Si isotope ratios of the companion main-sequence star in a binary system that forms a nova outburst later should fall along a line with a slope of 1.38 defined by the mainstream grain data in the plot of δ 29 Si/ 28 Si versus δ 30 Si/ 28 Si (Zinner et al. 2007). Consequently, CO nucleosynthesis could lower the δ 29 Si/ 28 Si value during nova outburst and therefore result in excess 30 Si relative to 29 Si as observed in putative CO nova grains. As a result, the possibility of grains from CO novae mitigates one of the difficulties in linking putative nova grains to novae (problem 3 discussed in Section 1), but thermodynamic calculations for the innermost shell of CO novae predict that SiC cannot be condensed in such an oxidized environment (José et al. 2004). Condensation calculations including kinetic effects and mixing among different shells of CO novae, however, are needed to fully investigate this issue.
More importantly, none of the nova models can explain the 34 S anomalies found in two putative nova grains (G270_2 and Ag2_6). As shown in Figure 1 of José (2016), the protoncapture path cannot reach the S mass region in novae with WD masses lower than 1.35 M e , so that S isotope abundances are barely affected by lower-mass nova nucleosynthesis. In 1.35 M e ONe novae, the proton-capture nucleosynthesis overproduces S isotopes, and 32,33 S isotopes are more abundantly made than 34 S, corresponding to negative δ 34 S/ 32 S values. The 1.35 M e ONe nova model, however, predicts huge excesses in δ 30 Si/ 28 Si, inconsistent with the grain data (Figure 7(b)). In addition, neutron-rich isotopes cannot be made in novae because nova nucleosynthesis is dominated by proton-capture and β + decay (Figure 2 of José 2016). Negative δ 34 S/ 32 S values of nova grains therefore cannot be explained by 32 S excess from 32 Si β − decay as argued for the C1 and C2 grains from CCSNe. Thus, the isotopic compositions of grains G270_2 and Ag2_6 seem to be inconsistent with predictions of classical nova nucleosynthesis. However, it is noteworthy to point out that the production of S isotopic abundances is still affected by nuclear uncertainties, e.g., the treatment of the 34 Cl ground and isomeric nuclear states (e.g., Coc et al. 2000). Furthermore, these nova nucleosynthetic models are all based on 1D hydrodynamic model calculations that cannot predict asymmetries during nova outbursts and do not account for the effect of rotation. Thus, the mismatch with the multi-element isotopic data by 1D nova model predictions could also indicate incorporation of products from regions with varying nucleosynthetic environments within the nova ejecta during a single nova outburst (similar to the discussion for CCSNe in Section 4.1.4). Thus, multi-dimensional hydrodynamic calculations of nova outbursts (e.g., Casanova et al. 2011) are needed to confirm the mismatch of nova model predictions with the isotopic compositions of G270_2 and Ag2_6.

Putative Nova Grains versus CCSN Nucleosynthesis
In Figure 6, predictions for explosive H burning events in CCSNe are overlapped with most of the putative nova grain data, if one considers variations in the peak temperature and peak density during explosions, the amount of ingested H, and the percentage of mixed H envelope material. This demonstrates the strong similarities between explosive H burning events in novae and CCSNe. Thus, many isotopic signatures can be diagnostic for both stellar sources. The explosive H ingestion scenario in CCSNe, however, at the moment has many advantages over classical novae in explaining the putative nova grain data: (1) Explosive H burning in CCSNe only affects a thin shell of the He/C zone, and mixing with a large amount of isotopically close-to normal material should be a natural consequence of grain condensation during explosions. In contrast, a mechanism for mixing such a large amount of normal material with pure nova ejecta has not been identified; (2) A C-rich environment is obtainable by mixing explosive H ingestion products with extended He-shell layers and more external H-rich material. The main prerequisite for SiC formation (C > O) can therefore be satisfied.
While S isotopic anomalies cannot be explained by models for low-mass novae (<1.35 M e ), the negative δ 34 S/ 32 S values of grains G270_2 and Ag2_6 could be matched by local mixing between the He/C zone and zones with 28 Si, 32 S excesses in CCSNe ( Figure 8). Traditionally, the Si/S zone of the classical W&H07 models was considered as the only zone with large excesses in 28 Si and 32 S because of alpha captures. Pignatari et al. (2013a) have recently shown that increased shock velocities and consequently higher peak temperatures can result in efficient α-capture at the bottom of the He/C zone, and a C/Si zone can be formed there. The 25T-H model predicts such a C/Si zone (blue circles in Figure 8), which lies below the 13 C-and 15 N-enriched shells. As shown in Figure 8, local mixing between the He/C and C/Si zones can explain both the 34 S depletion (or 32 S excess) and 30 Si excesses of G270_2 and Ag2_6. Thus, the state-of-the-art model calculations suggest that grains G270_2 and Ag2_6 might have originated from CCSNe. Negligible amounts of 32 Si are produced in the He/C zone ( Figure 5) and the C/Si zone of the 25T-H model, so the negative δ 34 S/ 32 S values cannot be explained by 32 S excesses from radioactive 32 Si decay in this scenario.
Finally, the stellar origin of 15 N-enriched AB grains is even more ambiguous than that of the putative nova grains. J-type stars, CCSNe with H-ingestion, and CO novae are all possible stellar sites, while the CO novae are less favored because of the oxidized stellar environment, which make it less likely for SiC to condense. It should also be noted that there are consistent differences between the 15 N-enriched AB grains and putative nova grains (higher 14 N/ 15 N, lower 26 Al/ 27 Al, no or smaller excesses in 30 Si relative to 29 Si in AB grains, see Section 3.1 for details). This seems to indicate that 15 N-enriched AB grains are carriers of weaker proton-capture processes compared to putative nova grains.

Additional Evidence for Presolar
Dust Grains from Novae?
Novae can produce significant amounts of 22 Na with a halflife of 2.6 year, which could be incorporated into condensing SiC grains where it would decay in situ to 22 Ne (Starrfield et al. 1997). Heck et al. (2007) measured isotopic compositions of Ne in single presolar SiC grains and identified one out of the 110 grains analyzed as an AB grain with excess in 22 Ne, which was proposed as an indication of CO nova origin. In fact, CCSN models also predict significant production of 22 Na. For instance, the CCSN models of W&H07 predict that the 22 Ne/ 20 Ne ratio can reach ∼10 in the He/C zone, which could explain the grain data ( 22 Ne/ 20 Ne < 0.36) by mixing with isotopically normal material (solar 22 Ne/ 20 Ne = 0.1). Additionally, P15 models predict large amounts of 22 Na produced in the He shell that could be incorporated into C-rich dust grains and in situ decay to 22 Ne. Thus, both CCSNe and novae can produce higher-than-solar 22 Ne/ 20 Ne isotopes, which are therefore not diagnostic of a nova origin. To conclude, none of the isotopic signatures that have been found in putative nova SiC grains so far unequivocally link the grains to a nova origin.
A few highly 17 O-enriched presolar silicates and oxides have been found in primitive meteorites in previous studies (Gyngard et al. 2011;Leitner et al. 2012;Nguyen & Messenger 2014), which might have originated from novae. These grains generally show enrichments in 25 Mg and 26 Mg (the latter ascribed to 26 Al decay), in agreement with both supernova and nova model predictions. In addition, large excess in 30 Si relative to 29 Si was found in one of the two silicate grains Figure 7. Plots of 26 Al/ 27 Al vs. 12 C/ 13 C, and δ 34 S/ 32 S vs. δ 30 Si/ 28 Si. Putative nova grains in the literature and from this study are compared to the bulk composition of pure CO and ONe nova ejecta. The model predictions are shown as mixing lines between pure nova ejecta and the solar material. The isotope data are plotted with 1σ uncertainties. For each nova model, the number in parenthesis refers to the percentage of H-rich material accreted onto the WD. studied by Nguyen & Messenger (2014), which is similar to the Si isotopic signatures of putative nova SiC grains. The highest 17 O/ 16 O ratio predicted by the H-ingestion models of P15 is only up to 2×10 −3 (25T-H model), which is about one order of magnitude lower than the highest ratio observed in the 17 Oenriched silicate and oxide grains. Thus, the O isotopic compositions of the putative nova silicates and oxides are in better agreement with CO nova nucleosynthetic model predictions of José (2016) than are the putative nova SiC grains.

CONCLUSIONS
Multi-element isotope data clearly show that the rare population of presolar SiC grains with very large enrichments in both 13 C and 15 N, previously argued to mainly originate in classical novae, can be divided into two groups, referred to here as C2 grains and putative nova grains. Based on detailed comparisons of new and literature isotope data on these grains with state-of-the-art nucleosynthetic calculations, we can come to the following conclusions: (1) Type C2 grains are characterized by large 13 C and 15 N enrichments, and 29 Si and 30 Si excesses, which incorporated materials from regions of CCSNe similar to the 29 Si-and 30 Sienriched C1 grains, but with higher 12 C/ 13 C ratios (>10). The n-process isotopic signatures of C2 grains (excesses in 29,30 Si,50 Ti) are well explained by neutron-burst nucleosynthesis in CCSNe, pointing toward a CCSN origin. The 13 C and 15 N enrichments of C2 grains therefore strongly suggest the occurrence of explosive H burning in the He shell during CCSN explosions. By ingesting ∼1.2% H into the He shell, the P15 models predict that the neutron source for the n process, 22 Ne, is consumed by proton capture during explosive H burning, so that the n process cannot occur. In contrast to the 1D model predictions, type C2 grains show both proton-and neutron-capture isotopic signatures with the n-process signatures unaffected by the proton capture process, thus strongly supporting the large heterogeneity predicted for H ingestion into the He shell by multi-dimensional simulations. Therefore, while comparison with 1D stellar models helps to capture missing physical processes in stellar models, complete multidimensional model calculations for hydrogen ingestion events in massive stars at solar and close-to-solar metallicity are needed to match the multi-element isotope data. Presolar grain data allows derivation of stringent constraints on both the neutron-and proton-capture environments in the He shell of CCSNe in detail. We point out that because of the uncertain physical mechanism(s) responsible for mixing H into the He shell in pre-supernovae, there still remain a number of open questions about H-ingestion events, e.g., what fraction of CCSNe experience explosive H burning? How varying is the effect of explosive H burning on nucleosynthetic products of the He/C zone in different CCSNe?
(2) Putative nova grains are characterized by 13 C and 15 N enrichments and large excesses in 30 Si relative to 29 Si. In fact, nucleosynthetic model predictions of explosive H burning events in novae and CCSNe are quite similar for a number of elements, so the stellar origin of putative nova grains is quite ambiguous. Comparison with nova models shows that if putative nova grains are from novae, (1) they came from both CO and ONe novae; and (2) the mass of their parent stars should be lower than 1.35 M e . The current nova models, however, cannot explain the negative δ 34 S/ 32 S ratios found in two putative nova grains from this study. Instead, local mixing in He-shell material of CCSNe can reproduce these isotopic signatures. It is therefore more likely that these two putative nova grains were actually sourced from CCSNe. Putative nova oxides and silicates, however, show extremely high 17 O/ 16 O ratios, while H-ingestion model predictions of P15 are an order of magnitude lower and therefore cannot explain the grain data. So far, 17 O-enriched oxides and silicates seem to be the most likely nova grain candidates.
(3) Nitrogen-15 enriched AB grains show systematic differences compared to putative nova grains, such as higher 14 N/ 15 N ratios (>70), and lower 26 Al/ 27 Al ratios. Also, they are overlapped with mainstream grains for Si isotope ratios, and lack the 30 Si excesses found in putative nova grains. J-type stars, CO novae, and CCSNe are all potential stellar progenitors Figure 8. Si-and S-isotopic compositions of putative nova and AB grains are compared to model predictions for the He/C zones of different models (solid and dashed lines) and for the C/Si zone of the 25T model (blue circles) by P15. The dotted dashed lines are mixing lines between the He/C and Si/C zones, and the shaded areas therefore represent isotopic compositions that can be produced by local mixing in the 25T model. of 15 N-enriched AB grains. The distinctive differences in N, Si, and Al isotope ratios between 15 N-enriched AB and nova grains indicate either that they came from different types of stars, or that they came from the same stellar source, but incorporated stellar materials processed in different nucleosynthetic conditions.