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

Nan Liu1, Thomas Stephan2,3, Patrick Boehnke2,3, Larry R. Nittler1, Conel M. O’D. Alexander1, Jianhua Wang1, Andrew M. Davis2,3,4, Reto Trappitsch2,3,5, and Michael J. Pellin2,3,4,6
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.

Reducing Uncertainties in the Production of the Gamma-emitting Nuclei 26Al, 44Ti, and 60Fe in Core-collapse Supernovae by Using Effective Helium Burning Rates

Sam M. Austin1,2, Christopher West2,3,4, and Alexander Heger2,3,5,6
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.

Origin and Evolution of Short-period Comets

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 ${N}_{{\rm{p}}}(q)$ perihelion passages with perihelion distance $q\lt 2.5\,\mathrm{au}$. 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 ${N}_{{\rm{p}}}(2.5)\simeq 500$ 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 ${N}_{{\rm{p}}}$, but the existing data are not good enough to constrain ${N}_{{\rm{p}}}$ from orbital fits. ${N}_{{\rm{p}}}(2.5)\gt 1000$ 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 ${N}_{{\rm{p}}}(2.5)\lesssim 10$, possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.

Stellar Origin of 15N-rich Presolar SiC Grains of Type AB: Supernovae with Explosive Hydrogen Burning

Nan Liu1, Larry R. Nittler1, Marco Pignatari2,3, Conel M. O’D. Alexander1, and Jianhua Wang1
The Astrophysical Journal Letters 842 L1 Link to Article [https://doi.org/10.3847/2041-8213/aa74e5]
1Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
2E. A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, Hull HU6 7RX, UK
3NuGrid collaboration, http://www.nugridstars.org.

We report C, N, and Si isotopic data for 59 highly 13C-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15N-rich (14N/15N ≤ solar) AB grains, and their Mg–Al, S, and Ca–Ti isotope data when available. These 37 grains are enriched in 13C, 15N, and 26Al with the PNGs showing more extreme enhancements. The 15N-rich AB grains show systematically higher 26Al and 30Si excesses than the 14N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (14N/15N < solar) and 2 (14N/15N ≥ solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found 32S and/or 50Ti enhancements. Interestingly, one AB1 grain had the largest 32S and 50Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32S excess and thus the initial presence of radiogenic 32Si (t 1/2 = 153 years). More importantly, we found that the 15N and 26Al excesses of AB1 grains form a trend that extends to the region in the N–Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors.

Experimental Phase Functions of Millimeter-sized Cosmic Dust Grains

O. Muñoz1, F. Moreno1, F. Vargas-Martín2, D. Guirado1, J. Escobar-Cerezo1, M. Min3,4, and J. W. Hovenier4
Astrophysical Journal 846, 85 Link to Article [https://doi.org/10.3847/1538-4357/aa7ff2]
1Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
2Department of Electromagnetism and Electronics, University of Murcia, E-30100 Murcia, Spain
3SRON Netherlands Institute for Space Research, Sobornnelaan 2, 3584 CA Utrecht, The Netherlands
4Astronomical Institute “Anton Pannekoek,” University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

We present the experimental phase functions of three types of millimeter-sized dust grains consisting of enstatite, quartz, and volcanic material from Mount Etna, respectively. The three grains present similar sizes but different absorbing properties. The measurements are performed at 527 nm covering the scattering angle range from 3° to 170°. The measured phase functions show two well-defined regions: (i) soft forward peaks and (ii) a continuous increase with the scattering angle at side- and back-scattering regions. This behavior at side- and back-scattering regions is in agreement with the observed phase functions of the Fomalhaut and HR 4796A dust rings. Further computations and measurements (including polarization) for millimeter-sized grains are needed to draw some conclusions about the fluffy or compact structure of the dust grains.

Destruction of Refractory Carbon in Protoplanetary Disks

Dana E. Anderson1, Edwin A. Bergin2, Geoffrey A. Blake1, Fred J. Ciesla3, Ruud Visser4, and Jeong-Eun Lee5
Astrophysical Journal 845, 13 Link to Article [https://doi.org/10.3847/1538-4357/aa7da1]
1Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
2Department of Astronomy, University of Michigan, 1085 S. University, Ann Arbor, MI 48109-1107, USA
3Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Ave., Chicago, IL 60637, USA
4European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748, Garching, Germany
5School of Space Research, Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Korea

The Earth and other rocky bodies in the inner solar system contain significantly less carbon than the primordial materials that seeded their formation. These carbon-poor objects include the parent bodies of primitive meteorites, suggesting that at least one process responsible for solid-phase carbon depletion was active prior to the early stages of planet formation. Potential mechanisms include the erosion of carbonaceous materials by photons or atomic oxygen in the surface layers of the protoplanetary disk. Under photochemically generated favorable conditions, these reactions can deplete the near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by several orders of magnitude on short timescales relative to the lifetime of the disk out to radii of ~20–100+ au from the central star depending on the form of refractory carbon present. Due to the reliance of destruction mechanisms on a high influx of photons, the extent of refractory carbon depletion is quite sensitive to the disk’s internal radiation field. Dust transport within the disk is required to affect the composition of the midplane. In our current model of a passive, constant-αdisk, where α = 0.01, carbon grains can be turbulently lofted into the destructive surface layers and depleted out to radii of ~3–10 au for 0.1–1 μm grains. Smaller grains can be cleared out of the planet-forming region completely. Destruction may be more effective in an actively accreting disk or when considering individual grain trajectories in non-idealized disks.

The Formation of Formaldehyde on Interstellar Carbonaceous Grain Analogs by O/H Atom Addition

Alexey Potapov1, Cornelia Jäger1, Thomas Henning2, Mindaugas Jonusas3,4, and Lahouari Krim3,4
Astrophysical Journal 846, 131 Link to Article [https://doi.org/10.3847/1538-4357/aa85e8]
1Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany
2Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
3Department of Chemistry, Sorbonne Universités, UPMC Univ Paris 06, UMR 8233, MONARIS, Paris F-75005, France
4CNRS, UMR 8233, MONARIS, Paris F-75005, France

An understanding of possible scenarios for the formation of astrophysically relevant molecules, particularly complex organic molecules, will bring us one step closer to the understanding of our astrochemical heritage. In this context, formaldehyde is an important molecule as a precursor of methanol, which in turn is a starting point for the formation of more complex organic species. In the present experiments, for the first time, following the synthesis of CO, formaldehyde has been produced on the surface of interstellar grain analogs, hydrogenated fullerene-like carbon grains, by O and H atom bombardment. The formation of H2CO is an indication for a possible methanol formation route in such systems.