Sachiko Amari^a, Ernst Zinner^a, Roberto Gallino^b

^aMcDonnell Center for the Space Sciences and the Physics Department, Washington University, St. Louis, MO 63130, USA
^bDipartimento di Fisica, Università di Torino, I-10125 Torino, Italy

We studied presolar graphite grains from four density fractions, KE3 (1.65 – 1.72 g/cm³), KFA1 (2.05 – 2.10 g/cm³), KFB1 (2.10 – 2.15 g/cm³), and KFC1 (2.15 – 2.20 g/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 0.24 ppm and 0.44 ppm, respectively, whereas those of SiC grains from supernovae and AGB stars are 0.065 ppm and 5.7 ppm, 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. Silicon carbide is mechanically more resistant than graphite and we assume that residence times of SiC are longer than those of graphite. Silicon carbide grains from AGB stars are much more abundant than graphite grains from AGB stars (5.7 ppm vs. 0.44 ppm). We speculate that one of the reasons that SiC grains from AGB stars are much more abundant than graphite grains from AGB stars is that major sources for graphite grains are 3 M_⊙ stars whereas for SiC lower-mass (1.5 – 2M_⊙) stars; lower-mass stars are more abundant. The abundances of supernova graphite grains and supernova SiC grains (0.24 ppm vs. 0.065 ppm) reflect grain formation and destruction in expanding supernova ejecta.

Reference
Amari S, Zinner E and Gallino R (in press) Isotopic study of presolar graphite from the murchison meteorite. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.01.006]
Copyright Elsevier

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Ertel S et al.¹ (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France

Context. The dust observed in debris disks is produced through collisions of larger bodies left over from the planet/planetesimal formation process. Spatially resolving these disks permits to constrain their architecture and thus that of the underlying planetary/planetesimal system.
Aims. Our Herschel open time key program DUNES aims at detecting and characterizing debris disks around nearby, sun-like stars. In addition to the statistical analysis of the data, the detailed study of single objects through spatially resolving the disk and detailed modeling of the data is a main goal of the project.
Methods. We obtained the first observations spatially resolving the debris disk around the sun-like star HIP 17439 (HD 23484) using the instruments PACS and SPIRE on board the Herschel Space Observatory. Simultaneous multi-wavelength modeling of these data together with ancillary data from the literature is presented.
Results. A standard single component disk model fails to reproduce the major axis radial profiles at 70  μm, 100  μm, and 160  μm simultaneously. Moreover, the best-fit parameters derived from such a model suggest a very broad disk extending from few au up to few hundreds of au from the star with a nearly constant surface density which seems physically unlikely. However, the constraints from both the data and our limited theoretical investigation are not strong enough to completely rule out this model. An alternative, more plausible, and better fitting model of the system consists of two rings of dust at approx. 30 au and 90 au, respectively, while the constraints on the parameters of this model are weak due to its complexity and intrinsic degeneracies.
Conclusions. The disk is probably composed of at least two components with different spatial locations (but not necessarily detached), while a single, broad disk is possible, but less likely. The two spatially well-separated rings of dust in our best-fit model suggest the presence of at least one high mass planet or several low-mass planets clearing the region between the two rings from planetesimals and dust.

Reference
Ertel S et al. (in press) Potential multi-component structure of the debris disk around HIP 17439 revealed by Herschel/DUNES. Astronomy & Astrophysics 561:A114.
[doi:10.1051/0004-6361/201219945]
Reproduced with permission © ESO

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