Nan Liu¹, Larry R. Nittler¹, Marco Pignatari^2,3, 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.

O. Muñoz¹, F. Moreno¹, F. Vargas-Martín², D. Guirado¹, J. Escobar-Cerezo¹, M. Min^3,4, 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.

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.

Alexey Potapov¹, Cornelia Jäger¹, Thomas Henning², Mindaugas Jonusas^3,4, and Lahouari Krim^3,4
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.