High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum

Aki Takigawa1,2, Rhonda M. Stroud3, Larry R. Nittler4, Conel M. O’D Alexander4, and Akira Miyake2
Astrophysical Journal Letters 862, L13 Link to Article [DOI: 10.3847/2041-8213/aad1f5]
1The Hakubi Center for Advanced Research, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
2Division of Earth and Planetary Sciences, Kyoto University Kitashirakawa-Oiwakecho, Sakyo, Kyoto 606-8502, Japan
3Naval Research Laboratory, Code 6360, Washington, DC 20375, USA
4Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA

Corundum (α-Al2O3) and amorphous or metastable Al2O3 are common components of circumstellar dust observed around O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic compositions, this 1.4 μm diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral {011} faces of corundum and a rough, rounded face with cavities. High Mg contents (Mg/Al > 0.004) are due to the decay of radioactive 26Al. No spinel (MgAl2O4) inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas–grain and grain–grain collisions in the interstellar medium (ISM) and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of 26Al. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain–grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.

Icy Grains from the Nucleus of Comet C/2013 US10 (Catalina)

Silvia Protopapa1,2,6, Michael S. P. Kelley1,6, Bin Yang3,6, James M. Bauer1, Ludmilla Kolokolova1, Charles E. Woodward4,6, Jacqueline V. Keane5, and Jessica M. Sunshine1
Astrophysical Journal Letters 862, L16 Link to Article [DOI: 10.3847/2041-8213/aad33b]
1Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
2Southwest Research Institute, Boulder, CO 80302, USA
3European Southern Observatory, Santiago, Chile
4Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55455, USA
5Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
6Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration.

We present Infrared Telescope Facility/SpeX and NEOWISE observations of the dynamically new comet C/2013 US10 (Catalina), hereafter US10, from 5.8 au inbound, to near perihelion at 1.3 au, and back to 5.0 au outbound. We detect water ice in the coma of US10, assess and monitor the physical properties of the ice as insolation varies with heliocentric distance, and investigate the relationship between water ice and CO2. This set of measurements is unique in orbital coverage and can be used to infer both the physical evolution of the ice, and, potentially, the nucleus composition. We report (1) nearly identical near-infrared spectroscopic measurements of the coma at −5.8 au, −5.0 au, +3.9 au (where <0 au indicates pre-perihelion epochs), all presenting evidence of water-ice grains, (2) a dust-dominated coma at 1.3 and 2.3 au and, (3) an increasing CO2/Afρ ratio from −4.9 to 1.8 au. We propose that sublimation of the hyper-volatile CO2 is responsible for dragging water-ice grains into the coma throughout the orbit. Once in the coma, the observability of the water-ice grains is controlled by the ice grain sublimation lifetime, which seems to require some small dust contaminant (i.e., non-pure ice grains). At $| {R}_{{\rm{h}}}| \geqslant 3.9\,\mathrm{au}$, the ice grains are long-lived and may be unchanged since leaving the comet nucleus. We find that the nucleus of comet US10 is made of, among other components, ~1 μm water-ice grains containing up to 1% refractory materials.

Modeling Martian Atmospheric Losses over Time: Implications for Exoplanetary Climate Evolution and Habitability

Chuanfei Dong1,2 et al. (>10)
Astrophysical Journal Letters 859, L14 Link to Article [DOI: 10.3847/2041-8213/aac489]
1Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

In this Letter, we make use of sophisticated 3D numerical simulations to assess the extent of atmospheric ion and photochemical losses from Mars over time. We demonstrate that the atmospheric ion escape rates were significantly higher (by more than two orders of magnitude) in the past at ~4 Ga compared to the present-day value owing to the stronger solar wind and higher ultraviolet fluxes from the young Sun. We found that the photochemical loss of atomic hot oxygen dominates over the total ion loss at the current epoch, while the atmospheric ion loss is likely much more important at ancient times. We briefly discuss the ensuing implications of high atmospheric ion escape rates in the context of ancient Mars, and exoplanets with similar atmospheric compositions around young solar-type stars and M-dwarfs.

First Detection of the Simplest Organic Acid in a Protoplanetary Disk*

Cécile Favre1 et al. (>10)
Astrophysical Journal Letters 862, L2 Link to Article [DOI: 10.3847/2041-8213/aad046]
1INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125, Florence, Italy

The formation of asteroids, comets, and planets occurs in the interior of protoplanetary disks during the early phase of star formation. Consequently, the chemical composition of the disk might shape the properties of the emerging planetary system. In this context, it is crucial to understand whether and what organic molecules are synthesized in the disk. In this Letter, we report the first detection of formic acid (HCOOH) toward the TW Hydrae protoplanetary disk. The observations of the trans-HCOOH 6(1,6)–5(1,5) transition were carried out at 129 GHz with Atacama Large Millimeter/Submillimeter Array (ALMA). We measured a disk-averaged gas-phase t-HCOOH column density of ~(2–4) × 1012 cm−2, namely as large as that of methanol. HCOOH is the first organic molecule containing two oxygen atoms detected in a protoplanetary disk, a proof that organic chemistry is very active, albeit difficult to observe, in these objects. Specifically, this simplest acid stands as the basis for synthesis of more complex carboxylic acids used by life on Earth.

Measurements of Diffusion of Volatiles in Amorphous Solid Water: Application to Interstellar Medium Environments

Jiao He1, SM Emtiaz, and Gianfranco Vidali
The Astrophysical Journal 863, 156 Link to Article [https://doi.org/10.3847/1538-4357/aad227]
Physics Department, Syracuse University, Syracuse, NY 13244, USA
1Current address: Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands.

The diffusion of atoms and molecules in ices covering dust grains in dense clouds in interstellar space is an important but poorly characterized step in the formation of complex molecules in space. Here we report the measurement of diffusion of simple molecules in amorphous solid water (ASW), an analog of interstellar ices, which are amorphous and made mostly of water molecules. The new approach that we used relies on measuring, in situ, the change in band strength and position of mid-infrared features of OH dangling bonds as molecules move through pores and channels of ASW. We obtained the Arrhenius pre-exponents and activation energies for diffusion of CO, O2, N2, CH4, and Ar in ASW. The diffusion energy barrier of H2 and D2 were also measured, but only upper limits were obtained. These values constitute the first comprehensive set of diffusion parameters of simple molecules on the pore surface of ASW and can be used in simulations of the chemical evolution of Interstellar Medium environments, thus replacing unsupported estimates. We also present a set of argon temperature programmed desorption experiments to determine the desorption energy distribution of argon on non-porous ASW.

Nucleosynthesis Constraints on the Explosion Mechanism for Type Ia Supernovae

Kanji Mori1,2, Michael A. Famiano3,2, Toshitaka Kajino4,2,1, Toshio Suzuki5,2, Peter M. Garnavich6, Grant J. Mathews5,2, Roland Diehl7,2, Shing-Chi Leung8, and Ken’ichi Nomoto8
The Astrophysical Journal 863, 176 Link to Article [https://doi.org/10.3847/1538-4357/aad233]
1Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2National Astronomical Observatory of Japan 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
3Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA
4School of Physics and Nuclear Energy Engineering, and Internationsl Research Center for Big-Bang Cosmology and Element Genesis, Beihang University, Beijing 100083, People’s Republic of China
5Department of Physics, College of Humanities and Sciences, Nihon University 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156-8550, Japan
6Departmant of Physics, Center for Astrophysics, University of Notre Dame, Notre Dame, IN 46556, USA
7Max Planck Institut für extraterrestrische Physik, D-85748 Garching, Germany
8Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, Kashiwa, Chiba 277-8583, Japan

Observations of type Ia supernovae (SNe Ia) include information about the characteristic nucleosynthesis associated with these thermonuclear explosions. We consider observational constraints from iron-group elemental and isotopic ratios, to compare with various models obtained with the most realistic recent treatment of electron captures (ECs). The nucleosynthesis is sensitive to the highest white-dwarf central densities. Hence, nucleosynthesis yields can distinguish high-density Chandrasekhar-mass models from lower-density burning models such as white-dwarf mergers. We discuss new results of post-processing nucleosynthesis for two spherical models (deflagration and/or delayed-detonation models) based upon new EC rates. We also consider cylindrical and 3D explosion models (including deflagration, delayed-detonation, or a violent merger model). Although there are uncertainties in the observational constraints, we identify some trends in the observations and the models. We make a new comparison of the models with elemental and isotopic ratios from five observed supernovae and three supernova remnants. We find that the models and data tend to fall into two groups. In one group, low-density cores such as in a 3D merger model are slightly more consistent with the nucleosynthesis data, while the other group is slightly better identified with higher-density cores such as in single-degenerate 1D–3D deflagration models. Hence, we postulate that both types of environments appear to contribute nearly equally to observed SN Ia. We also note that observational constraints on the yields of 54Cr and 54Fe, if available, might be used as a means to clarify the degree of geometrical symmetry of SN Ia explosions.

Abundances of Ordinary Chondrites in Thermally Evolving Planetesimals

Shigeru Wakita1,2, Yasuhiro Hasegawa3, and Takaya Nozawa4
The Astrophysical Journal 863, 100 Link to Article [https://doi.org/10.3847/1538-4357/aad0a2]
1Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
2Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
4Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan

Chondrites are some of the most primitive objects in the solar system, and they maintain a record of the degree of thermal metamorphism experienced in their parent bodies. This thermal history can be classified by the petrologic type. We investigate the thermal evolution of planetesimals to account for the current abundances (known as the fall statistics) of petrologic types 3–6 of ordinary chondrites. We carry out a number of numerical calculations in which formation times and sizes of planetesimals are taken as parameters. We find that planetesimals that form within 2.0 Myr after the formation of Ca-Al-rich inclusions (CAIs) can contain all petrologic types of ordinary chondrites. Our results also indicate that plausible scenarios of planetesimal formation, which are consistent with the fall statistics, are that planetesimals with radii larger than 60 km start to form around 2.0 Myr after CAIs and/or that ones with radii less than 50 km should be formed within 1.5 Myr after CAIs. Thus, thermal modeling of planetesimals is important for revealing the occurrence and amount of metamorphosed chondrites and for providing invaluable insights into planetesimal formation.