Born-again Asymptotic Giant Branch Research Articles

Isotopic Signatures of Supernova Nucleosynthesis in Presolar Silicon Carbide Grains of Type AB with Supersolar 14N/15N Ratios

Abstract We report high-resolution C, N, Al, Si, and S isotope data of 38 presolar SiC grains of type AB. Seventeen of these grains are of subtype AB1 (14N/15N < 440 = solar) and 20 of subtype AB2 (14N/15N ≥ 440), previously proposed to be mainly from supernovae (AB1) and J-type carbon stars (AB2), respectively. Our data are compatible with previously obtained isotope data of AB grains, except that 26Al/27Al ratios of AB1 grains span a narrower range. The data are compared with predictions from supernova models that consider H ingestion into the He shell during the pre-supernova phase. In these models a mixture of explosive H and He burning occurs at the bottom of the He shell during passage of the supernova shock, forming the so-called O/nova zone. Mixing matter from the O/nova zone with matter from the overlying He/C zone and the stellar envelope shows that the isotopic compositions and trends of both AB1 and AB2 grains can be matched within the model uncertainties. This demonstrates that supernovae should be considered as potential sources of AB2 grains, in addition to J-type carbon stars and born-again asymptotic giant branch stars, as previously proposed.

J-type Carbon Stars: A Dominant Source of 14N-rich Presolar SiC Grains of Type AB

Abstract We report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB (14N/15N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.

Origin of nanodiamonds from Allende constrained by statistical analysis of C isotopes from small clusters of acid residue by NanoSIMS

Meteoritic nanodiamonds carry noble gases with anomalies in their stable isotopes that have drawn attention to their potentially presolar origin. Measurements of 12C/13C isotope ratios of presolar nanodiamonds are essential to understanding their origins, but bulk studies do not show notable deviations from the solar system 12C/13C ratio.We implemented a technique using secondary ion mass spectrometry with maximized spatial resolution to measure carbon isotopes in the smallest clusters of nanodiamonds possible. We measured C and Si from clusters containing as few as 1000 nanodiamonds, the smallest clusters of nanodiamonds measured to date by traditional mass spectrometry. This allowed us to investigate many possible complex compositions of the nanodiamonds, both through direct methods and statistical analysis of the distributions of observed isotopic ratios.Analysis of the breadth of distributions of carbon isotopic ratios for a number of ∼1000-nanodiamond aggregates indicates that the 12C/13C ratio may be drawn from multiple Gaussian distributions about different isotopic ratios, which implies the presence of presolar material. The mean isotopic ratio is consistent with the solar system value, so presolar components are required to be either low in concentration, or to have a mean ratio close to that of the solar system. Supernovae are likely candidates for the source of such a presolar component, although asymptotic giant branch stars are not excluded.A few aggregates show deviations from the mean 12C/13C ratio large enough to be borderline detections of enrichments in 13C. These could be caused by the presence of a small population of nanodiamonds formed from sources that produce extremely 13C-rich material, such as J-stars, novae, born-again asymptotic giant branch stars, or supernovae. Of these possible sources, only supernovae would account for the anomalous noble gases carried in the nanodiamonds.

Titanium isotopic compositions of rare presolar SiC grain types from the Murchison meteorite

We report the Ti isotopic compositions of 8 mainstream, 22 Y, 9 Z, and 26 AB presolar SiC grains from two SiC-rich residues of the Murchison CM2 meteorite together with Si, C and some Mg-Al isotopic data for the same grains. Mainstream, Y and Z grains are believed to originate in asymptotic giant branch (AGB) stars of varying metallicities, but the stellar sources of AB grains are poorly understood. We find that the 46,47,49Ti/48Ti ratios are correlated with 29Si/28Si for all of the grain types, indicating that these ratios are mainly dominated by Galactic chemical evolution (GCE). The mainstream, Y and Z grains all show enrichments in 50Ti from neutron capture nucleosynthesis. However, AGB models predict smaller excesses in 50Ti (and 49Ti) than are observed in these grains. For Z grains and especially for Y grains, the enhancement of 50Ti is greater than the enhancement in 30Si, indicating that the 13C neutron source produced a greater total fluence of neutrons than the 22Ne source in the low metallicity parent AGB stars. The Z grains plot below the mainstream correlation lines at more 48Ti- and 28Si-rich compositions in plots of 46,47,49Ti/48Ti vs. 29Si/28Si. On the other hand, the Y grains plot close to the mainstream correlation line. This could imply that the Ti isotopes evolved non-linearly at metallicities below ∼1/3 solar. The AB grains in this study have Ti isotopic compositions that fall along correlation lines defined by the mainstream grains, suggesting origins in close to solar metallicity stars. However, these grains fall below the mainstream correlation lines in plots of 46,49,50Ti/48Ti vs. 29Si/28Si and do not show enhancements in 50Ti, indicating that their parent stars did not undergo significant s-process nucleosynthesis. These data support origins of AB grains in J-type C stars rather than born-again AGB stars that undergo s-process nucleosynthesis. AB grains that do not have 50Ti excesses may provide the best measure of Si and Ti isotope GCE since their parent stars were less affected by s-process nucleosynthesis than the mainstream grains.

R CORONAE BOREALIS STARS ARE VIABLE FACTORIES OF PRE-SOLAR GRAINS

We present a new theoretical estimate for the birthrate of R Coronae Borealis (RCB) stars that is in agreement with recent observational data. We find the current Galactic birthrate of RCB stars to be $\approx$ 25% of the Galactic rate of Type Ia supernovae, assuming that RCB stars are formed through the merger of carbon-oxygen and helium-rich white dwarfs. Our new RCB birthrate ($1.8 \times 10^{-3}$ yr$^{-1}$) is a factor of 10 lower than previous theoretical estimates. This results in roughly 180--540 RCB stars in the Galaxy, depending on the RCB lifetime. From the theoretical and observational estimates, we calculate the total dust production from RCB stars and compare this rate to dust production from novae and born-again asymptotic giant branch (AGB) stars. We find that the amount of dust produced by RCB stars is comparable to the amounts produced by novae or born-again post-AGB stars, indicating that these merger objects are a viable source of carbonaceous pre-solar grains in the Galaxy. There are graphite grains with carbon and oxygen isotopic ratios consistent with the observed composition of RCB stars, adding weight to the suggestion that these rare objects are a source of stardust grains.

Presolar graphite from the Murchison meteorite: An isotopic study

We studied presolar graphite grains from four density fractions, KE3 (1.65–1.72g/cm3), KFA1 (2.05–2.10g/cm3), KFB1 (2.10–2.15g/cm3), and KFC1 (2.15–2.20g/cm3), extracted from the Murchison (CM2) meteorite, with the ion microprobe. One of the most interesting features of presolar graphite is that isotopic features depend on density. There are grains with 15N and 18O excesses, Si isotopic anomalies, high 26Al/27Al ratios (∼0.1), and Ca and Ti isotopic anomalies, including the initial presence of short-lived 41Ca and 44Ti. These isotopic features are qualitatively explained by nucleosynthesis in core collapse supernovae. We estimate that 76%, 50%, 7% and 1% of the KE3, KFA1, KFB1 and KFC1 grains, respectively, are supernova grains. We performed 3- and 4-zone supernova mixing calculations to reproduce the C, O (18O/16O) and Al isotopic ratios of the KE3 grains, using 15M⊙ model calculations by Rauscher et al. (2002). Isotopic ratios of grains with high 12C/13C ratios (>200) can be reproduced, whereas those of grains with ratios ⩽200 are hard to explain if we assume that graphite grains form in C-rich conditions.We compared the distributions of the 12C/13C ratios of KFB1 and KFC1 grains and their s-process 86Kr/82Kr ratios inferred from bulk noble gas analysis to model calculations of asymptotic giant branch (AGB) stars with a range of mass and metallicity. We conclude that KFB1 grains with 12C/13C⩾100 formed in the outflow of low-mass (1.5, 2 and 3M⊙) low-metallicity (Z=3×10−3 for 1.5, 2 and 3M⊙, Z=6×10−3 for 3M⊙ only) AGB stars and that KFC1 grains with 12C/13C⩾60 formed in those stars as well as in 5M⊙ stars of solar and/or half-solar metallicities. Grains with 12C/13C<20 in all the fractions seem to have multiple origins. Some of them formed in the ejecta of core-collapse supernovae. J stars and born-again AGB stars are also possible stellar sources.We calculated the abundances of graphite grains from supernovae and AGB stars in the Murchison meteorite to be 0.24ppm and 0.44ppm, respectively, whereas those of SiC grains from supernovae and AGB stars are 0.063ppm and 5.6ppm, respectively. In contrast to graphite, AGB stars are a dominant source of SiC grains.Since different mineral types have different residence times in the interstellar medium, their abundances in meteorites may not reflect original yields in stellar sources. Even if graphite grains are more easily destroyed than SiC grains, graphite grains from supernovae are more abundant than SiC grains from supernovae (0.24ppm vs. 0.063ppm), indicating that supernovae are a prolific producer of graphite grains. Graphite grains from AGB stars are less abundant than SiC grains from AGB stars (0.44ppm vs. 5.6ppm). This difference may reflect the difference in their parent stars: graphite grains formed in low-metallicity stars, while SiC grains formed in close-to-solar metallicity stars.

EVIDENCE FOR RADIOGENIC SULFUR-32 IN TYPE AB PRESOLAR SILICON CARBIDE GRAINS?

We report C, Si, and S isotope measurements on 34 presolar silicon carbide grains of Type AB, characterized by 12C/13C < 10. Nitrogen, Mg-Al-, and Ca-Ti-isotopic compositions were measured on a subset of these grains. Three grains show large 32S excesses, a signature that has been previously observed for grains from supernovae (SNe). Enrichments in 32S may be due to contributions from the Si/S zone and the result of S molecule chemistry in still unmixed SN ejecta or due to incorporation of radioactive 32Si from C-rich explosive He shell ejecta. However, a SN origin remains unlikely for the three AB grains considered here, because of missing evidence for 44Ti, relatively low 26Al/27Al ratios (a few times 10–3), and radiogenic 32S along with low 12C/13C ratios. Instead, we show that born-again asymptotic giant branch (AGB) stars that have undergone a very-late thermal pulse (VLTP), known to have low 12C/13C ratios and enhanced abundances of the light s-process elements, can produce 32Si, which makes such stars attractive sources for AB grains with 32S excesses. This lends support to the proposal that at least some AB grains originate from born-again AGB stars, although uncertainties in the born-again AGB star models and possible variations of initial S-isotopic compositions in the parent stars of AB grains make it difficult to draw a definitive conclusion.

MAGNESIUM ISOTOPE EVIDENCE FOR SINGLE STAGE FORMATION OF CB CHONDRULES BY COLLIDING PLANETESIMALS

We report C, Si, and S isotope measurements on 34 presolar silicon carbide grains of Type AB, characterized by 12C/13C < 10. Nitrogen, Mg-Al-, and Ca-Ti-isotopic compositions were measured on a subset of these grains. Three grains show large 32S excesses, a signature that has been previously observed for grains from supernovae (SNe). Enrichments in 32S may be due to contributions from the Si/S zone and the result of S molecule chemistry in still unmixed SN ejecta or due to incorporation of radioactive 32Si from C-rich explosive He shell ejecta. However, a SN origin remains unlikely for the three AB grains considered here, because of missing evidence for 44Ti, relatively low 26Al/27Al ratios (a few times 10-3), and radiogenic 32S along with low 12C/13C ratios. Instead, we show that born-again asymptotic giant branch (AGB) stars that have undergone a very-late thermal pulse (VLTP), known to have low 12C/13C ratios and enhanced abundances of the light s-process elements, can produce 32Si, which makes such stars attractive sources for AB grains with 32S excesses. This lends support to the proposal that at least some AB grains originate from born-again AGB stars, although uncertainties in the born-again AGB star models and possible variations of initial S-isotopic compositions in the parent stars of AB grains make it difficult to draw a definitive conclusion.

Multi-element isotopic analyses of presolar graphite grains from Orgueil

We report C, N, O, Si, Al–Mg, K, Ca, and Ti isotopic analyses of presolar graphite grains from the Orgueil CI chondrite. NanoSIMS isotopic measurements were made on 345 grains from seven density fractions, with grain sizes >1μm: low-density grains from OR1b, OR1c, and OR1d; and high-density grains from OR1f, OR1g, OR1h, and OR1i. In all fractions, except OR1b and OR1h, we found presolar graphite as demonstrated by the large range of 12C/13C ratios (4–2480) measured in individual grains. Some isotopic properties are dependent on density: low-density grains contain 18O, 15N, and 28Si excesses, while the majority of high-density grains contain normal N and O, and are generally enriched in 29Si and 30Si. The 15N, 18O, and 28Si excesses and very high derived isotopic ratios for the extinct radionuclides 26Al, 41Ca, and 44Ti in low-density grains indicate an origin from supernovae. In order to explain the isotopic ratios measured in these grains, we present mixing scenarios between different layers of supernovae and discuss the limitations of various theoretical models. Silicon-30 and 12C excesses in high-density grains and lower values for short-lived radionuclides (26Al and 41Ca) indicate an origin in asymptotic giant branch stars with low metallicities. Some supernova grains, with 44Ca excesses, are also present amongst the high-density grains. Grains with low 12C/13C ratios (without evidence for 44Ti) and large excesses in 42,43Ca and 46,47,49,50Ti probably originate from post-asymptotic giant branch stars, that have suffered a very late thermal pulse, and can achieve low 12C/13C ratios and large neutron capture signatures in Ca and Ti isotopes.We conclude that most low-density graphite grains originate from supernovae while high-density graphite grains have multiple stellar sources: low-metallicity and born-again asymptotic giant branch stars, Type II supernovae, and possibly, J-type stars.

Ne ISOTOPES IN INDIVIDUAL PRESOLAR GRAPHITE GRAINS FROM THE MURCHISON METEORITE TOGETHER WITH He, C, O, Mg-Al ISOTOPIC ANALYSES AS TRACERS OF THEIR ORIGINS

Ne isotopes measured in individual presolar graphite grains, solid samples of extinct stars preserved in primitive meteorites, provide information on the type of stellar sources of the grains and on nucleosynthetic mixing and ion-trapping processes which were operating. We present Ne and He isotope analyses of single presolar graphite grains from the KFB1 density fraction extracted from the carbonaceous chondrite Murchison. In addition, we measured isotopes of C, O, and Mg-Al with the NanoSIMS ion microprobe to better constrain the origin of the grains. Eleven out of 51 presolar graphite grains contain nucleosynthetic {sup 22}Ne above our detection limit. This fraction of {sup 22}Ne-rich grains is similar to the one reported by Nichols et al. although we have a lower {sup 22}Ne detection limit. We detected rare He-shell {sup 20}Ne in one {sup 22}Ne-rich grain and obtained the {sup 20}Ne/{sup 22}Ne ratio (0.03 {+-} 0.02) of the He-shell of an Asymptotic Giant Branch (AGB) star with 1.5-2 M {sub sun} and subsolar metallicity. We also detected {sup 4}He in this grain, while in the other grains, which originally acquired He, He-loss seems to be significant. We found unequivocal evidence for radiogenic {sup 22}Ne (Ne-R) in another graphite grain, which likelymore » condensed in a core-collapse supernova and which incorporated live radioactive {sup 22}Na (t {sub 1/2} = 2.6 yr). For the other grains, a clear assignment to a stellar source is more difficult to make. Putative stellar sources are supernovae, AGB stars, born-again AGB stars, J-type carbon stars, and CO novae.« less

New Stellar Sources for High‐Density, Presolar Graphite Grains

WepresentC,N,O,Si,Al-Mg,K,Ca,andTiisotopicanalyses ofsevenhigh-density(ORG1f, � � 2:02 2:04 gcm � 3 ) graphite grains from Orgueil with 12 C/ 13 C ratios smaller than 20. The presence of 44Ti in three of these grains indicates an origin in Type II supernovae (SNe). The 13 C excesses in these SNe grains, however, remain enigmatic. The remaining grains have extremely large Ca and Ti isotopic anomalies, some of which are much larger than those predicted for envelopes of asymptotic giant branch (AGB) stars. These anomalies in conjunction with low 12 C/ 13 C ratios can only be explained by pure nucleosynthetic He-shell components of AGB stars. Born-again AGB stars that experience a late He flash are able to explain the low 12 C/ 13 C ratios of some of the grains along with the presence of extreme enrichments in the Ca and Ti isotopes. This study indicates that high-density graphite grains havemultiple stellar sources: SNe and born-again AGB stars, in addition to the previously established low-metallicity AGB stars. Subject headingg dust, extinction — meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — supernovae: general

Remarkable Changes in the Near-Infrared Spectrum of the Nova-like Variable V4332 Sagittarii

We report on recent near-IR observations of V4332 Sgr—the nova-like variable that erupted in 1994. Its rapid, post-outburst evolution to a cool M-type giant/supergiant, soon after its outburst, showed that it was an unusual object differing from other eruptive variables, such as classical/symbiotic novae or born-again asymptotic giant branch stars. The present study of V4332 Sgr was motivated by the keen interest in the recent eruption of V838 Mon—an object with a spectacular light echo that, along with V4332 Sgr, is believed to belong to a new class of objects (we propose that they be called quasi novae). Our observations show new developments in the evolution of V4332 Sgr. The most striking feature is the detection of several molecular bands of AlO—a rarely seen molecule in astronomical spectra—in the JHK spectra. Many of these bands are being detected for the first time. The only other detection of some of these AlO bands is in V838 Mon, thereby showing further spectral similarities between the two objects. JHK photometry shows the development of a new dust shell around V4332 Sgr with a temperature of ~900 K and a lower limit on the derived mass of Mdust = 3.7 × 10-12 M☉. This dust shell does not appear to be associated with ejecta of the 1994 outburst but is due to a second mass-loss episode, which is not expected in a classical nova outburst. The cold molecular environment, suggested by the AlO emission, is also not expected in novae ejecta. We model the AlO bands and also discuss the possible formation mechanism of the AlO. These results show the need to monitor V4332 Sgr regularly for unexpected developments. The results can also be significant in predicting possible changes in the future evolution of V838 Mon.

The Evolutionary Timescale of Sakurai’s Object: A Test of Convection Theory?

Sakurai's Object (V4334 Sagittarii) is a born-again asymptotic giant branch star following a very late thermal pulse. So far, no stellar evolution models have been able to explain the extremely fast evolution of this star, which has taken it from the pre-white dwarf stage to its current appearance as a giant within only a few years. A very high stellar mass can be ruled out as the cause of the fast evolution. Instead, the evolution timescale is reproduced in stellar models by making the assumption that the efficiency for element mixing in the He-flash convection zone during the very late thermal pulse is smaller than predicted by the mixing-length theory (MLT). As a result, the main energy generation from fast proton capture occurs closer to the surface, and the expansion to the giant state is accelerated to a few years. Assuming a mass of V4334 Sgr of 0.604 M☉—which is consistent with a distance of 4 kpc—a reduction of the MLT mixing efficiency by a factor of ~100 is required to match its evolutionary timescale. This value decreases if V4334 Sgr has a smaller mass and, accordingly, a smaller distance. However, the effect does not disappear for the smallest possible masses. These findings may present a semiempirical constraint on the element mixing in convective zones of the stellar interior.