Nan Liu¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA

We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ(¹³⁸Ba/¹³⁶Ba) values are found, down to –400‰, which can only be modeled with a flatter ¹³C profile within the ¹³C pocket than is normally used. The dependence of δ(¹³⁸Ba/¹³⁶Ba) predictions on the distribution of ¹³C within the pocket in asymptotic giant branch (AGB) models allows us to probe the ¹³C profile within the ¹³C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the ²²Ne (α, n) ²⁵Mg rate in the stellar temperature regime relevant to AGB stars, based on δ(¹³⁴Ba/¹³⁶Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ(¹³⁴Ba/¹³⁶Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i process), which is activated by the very late thermal pulse during the post-AGB phase and characterized by a neutron density much higher than the s process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, ¹³⁰Ba and ¹³²Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of ¹³⁵Ba, we conclude that there is no measurable decay of ¹³⁵Cs (t 1/2 = 2.3 Ma) to ¹³⁵Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before ¹³⁵Cs decayed.

Reference
Liu et al. (2014) Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-process Nucleosynthesis in Asymptotic Giant Branch Stars. The Astrophysical Journal 786:66.
[doi:10.1088/0004-637X/786/1/48]

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D. Bodewits¹, T. L. Farnham¹, M. F. A’Hearn¹, L. M. Feaga¹, A. McKay^2,3, D. G. Schleicher⁴ and J. M. Sunshine¹

¹Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
²Astronomy Department, New Mexico State University, Las Cruces, NM 88003, USA
³University of Texas Austin/McDonald Observatory, 1 University Station, Austin, TX 78712, USA
⁴Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA

We used the Ultraviolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km², estimated to be >20% of the surface. Based on our measurements, the comet lost 4 × 10¹¹ kg of ice and dust during this apparition, corresponding to at most a few meters of its surface. Even though this was likely not the comet’s first passage through the inner solar system, the activity of Garradd was complex and changed significantly during the time it was observed.

Reference
Bodewits D, Farnham TL, A’Hearn MF, Feaga LM, McKay A, Schleicher DG and Sunshine JM (2014) The Evolving Activity of the Dynamically Young Comet C/2009 P1 (Garradd). The Astrophysical Journal 786:48.
[doi:10.1088/0004-637X/786/1/48]

Link to Article