Nan Liu¹, Thomas Stephan^2,3, Patrick Boehnke^2,3, Larry R. Nittler¹, Conel M. O’D. Alexander¹, Jianhua Wang¹, Andrew M. Davis^2,3,4, Reto Trappitsch^2,3,5, and Michael J. Pellin^2,3,4,6
The Astrophysical Journal Letters 844 L12 Link to Article [https://doi.org/10.3847/2041-8213/aa7d4c]
¹Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
³Chicago Center for Cosmochemistry, Chicago, IL, USA
⁴The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
⁵Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁶Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA

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.

Sam M. Austin^1,2, Christopher West^2,3,4, and Alexander Heger^2,3,5,6
The Astrophysical Journal Letters 839 L9 Link to Article [https://doi.org/10.3847/2041-8213/aa68e7]
¹National Superconducting Cyclotron Laboratory, Michigan State University, 640 South Shaw Lane, East Lansing, MI 48824-1321, USA
²Joint Institute for Nuclear Astrophysics—Center for the Evolution of the Elements, Michigan State University, East Lansing, MI 48824-1321, USA
³Minnesota Institute for Astronomy, School of Physics and Astronomy, University of Minnesota, Twin Cities, Minneapolis, MN 55455-0149, USA
⁴Center for Academic Excellence, Metropolitan State University, St. Paul, MN, 55106, USA
⁵Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia
⁶Center for Nuclear Astrophysics, Department of Physics and Astronomy, Shanghai Jiao-Tong University, Shanghai 200240, P. R. China

We have used effective reaction rates (ERRs) for the helium burning reactions to predict the yield of the gamma-emitting nuclei ²⁶Al, ⁴⁴Ti, and ⁶⁰Fe in core-collapse supernovae (SNe). The variations in the predicted yields for values of the reaction rates allowed by the ERR are much smaller than obtained previously, and smaller than other uncertainties. A “filter” for SN nucleosynthesis yields based on pre-SN structure was used to estimate the effect of failed SNe on the initial mass function averaged yields; this substantially reduced the yields of all these isotopes, but the predicted yield ratio ⁶⁰Fe/²⁶Al was little affected. The robustness of this ratio is promising for comparison with data, but it is larger than observed in nature; possible causes for this discrepancy are discussed.

David Nesvorný¹, David Vokrouhlický², Luke Dones¹, Harold F. Levison¹, Nathan Kaib³, and Alessandro Morbidelli⁴
Astrophysical Journal 843, 120 Link to Article [https://doi.org/10.3847/1538-4357/aa7cf6]
¹Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
²Institute of Astronomy, Charles University, V Holešovičkách 2, CZ-18000 Prague 8, Czech Republic
³H.L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
⁴Département Cassiopée, University of Nice, CNRS, Observatoire de la Côte d’Azur, Nice, F-06304, France

Comets are icy objects that orbitally evolve from the trans-Neptunian region into the inner solar system, where they are heated by solar radiation and become active due to the sublimation of water ice. Here we perform simulations in which cometary reservoirs are formed in the early solar system and evolved over 4.5 Gyr. The gravitational effects of Planet 9 (P9) are included in some simulations. Different models are considered for comets to be active, including a simple assumption that comets remain active for perihelion passages with perihelion distance . The orbital distribution and number of active comets produced in our model is compared to observations. The orbital distribution of ecliptic comets (ECs) is well reproduced in models with and without P9. With P9, the inclination distribution of model ECs is wider than the observed one. We find that the known Halley-type comets (HTCs) have a nearly isotropic inclination distribution. The HTCs appear to be an extension of the population of returning Oort-cloud comets (OCCs) to shorter orbital periods. The inclination distribution of model HTCs becomes broader with increasing , but the existing data are not good enough to constrain from orbital fits. is required to obtain a steady-state population of large active HTCs that is consistent with observations. To fit the ratio of the returning-to-new OCCs, by contrast, our model implies that , possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.