Pengfei Tang and Liping Jin
Astrophysical Journal 871, 222 Link to Article [DOI: 10.3847/1538-4357/aafb6f ]
College of Physics, Jilin University Changchun, Jilin 130012, People’s Republic of China

We construct an analytical model of gravitationally unstable protoplanetary disks consisting of three regions: the inner region where the internal dissipation dominates the heating, the intermediate region where the central protostar irradiation dominates, and the outer region where background irradiation dominates. We use this analytical model and an evolutionary numerical model of protoplanetary disks to calculate the cooling time and find out the location of the isothermal region. We investigate the effects of the isothermal region on the disk instability model for giant planet formation. We find that the fragmentation region found in previous studies is contained in the isothermal region of a disk. In this case, the cooling time criterion is not applicable for fragmentation. Therefore, the constraint on the disk instability model caused by the cooling time criterion should be relieved. The viability of the disk instability model is improved. When the isothermal region is considered, the inner boundary of the fragmentation region is extended inward to ~20 au. We also show that if the contribution of the protostar irradiation to the disk surface temperature can be included in the cooling rate, the fragmentation region defined by the cooling time criterion can be extended inward to ~26 au. We find that a disk tends to be isothermal in the region where the cooling time criterion is satisfied. We also find that at the later stage of disk instability, the inner boundary of the fragmentation region is determined by the inner boundary of the gravitationally unstable region.

Thomas Pfeil and Hubert Klahr
Astrophysical Journal 871, 150 Link to Article [DOI: 10.3847/1538-4357/aaf962 ]
Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Hydrodynamic instabilities in disks around young stars depend on the thermodynamic stratification of the disk and on the local rate of thermal relaxation. Here, we map the spatial extent of unstable regions for the Vertical Shear Instability (VSI), the Convective Overstability (COS), and the amplification of vortices via the Subcritical Baroclinic Instability (SBI). We use steady-state accretion disk models, including stellar irradiation, accretion heating, and radiative transfer. We determine the local radial and vertical stratification and thermal relaxation rate in the disk, which depends on the stellar mass, disk mass, and mass accretion rate. We find that passive regions of disks—that is, the midplane temperature dominated by irradiation—are COS unstable about one pressure scale height above the midplane and VSI unstable at radii >10 au. Vortex amplification via SBI should operate in most parts of active and passive disks. For active parts of disks (midplane temperature determined by accretion power), COS can become active down to the midplane. The same is true for the VSI because of the vertically adiabatic stratification of an internally heated disk. If hydrodynamic instabilities or other nonideal MHD processes are able to create α-stresses (>10⁻⁵) and released accretion energy leads to internal heating of the disk, hydrodynamic instabilities are likely to operate in significant parts of the planet-forming zones in disks around young stars, driving gas accretion and flow structure formation. Thus, hydrodynamic instabilities are viable candidates to explain the rings and vortices observed with the Atacama Large Millimeter/submillimeter Array and Very Large Telescope.

^1,2Yunhua Wu,^3,4Weibiao Hsu
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005743]
¹Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
²University of Chinese Academy of Sciences, Beijing, China
³The State Key Laboratory of Planetary Science, Macau University of Science and Technology, Taipa, China
⁴CAS Center for Excellence in Comparative Planetology, China
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa_25.1±0.5) and pyroxenes (Low‐Ca pyroxene Fs_20.7±0.8Wo_4.2±1.0, and Ca‐rich pyroxene Fs_11.5±0.5Wo_37.6±1.2) with interstitial albitic plagioclase (Ab_80.7±1.7Or_5.0±0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4479±43 Myr and a lower intercept age of 465±47 Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., ²⁶Al) would act as a major heat source, suggesting melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, that is in coincidence with the disruption event of L chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting.

¹K. A. Shirley,¹T. D. Glotch
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005533]
¹Department of Geosciences, Stony Brook University, Stony Brook, New York, USA
Published by arrangement with John Wiley & Sons

Mid‐infrared spectroscopic analysis of the Moon and other airless bodies requires a full accounting of spectral variation due to the unique thermal environment in airless body regoliths and the substantial differences between spectra acquired under airless body conditions and those measured in an ambient environment on Earth. Because there exists a thermal gradient within the upper 100s of microns of lunar regolith, the data acquired by the Diviner Lunar Radiometer Experiment are not isothermal with wavelength. While this complication has been previously identified, its effect on other known variables that contribute to spectral variation, such as particle size and porosity, have yet to be well characterized in the laboratory. Here we examine the effect of particle size on mid‐infrared spectra of silicates common to the Moon measured within a simulated lunar environment chamber. Under simulated lunar conditions, decreasing particle size is shown to enhance the spectral contrast of the Reststrahlen bands and transparency features, as well as shift the location of the Christiansen feature to longer wavelengths. This study shows that these variations are detectable at Diviner spectral resolution, and emphasizes the need for simulated environment laboratory datasets, as well as hyperspectral mid‐infrared instruments on future missions to airless bodies.

Claudio Valletta and Ravit Helled
Astrophysical Journal 871, 127 Link to Article [DOI: 10.3847/1538-4357/aaf427 ]
Center for Theoretical Astrophysics and Cosmology Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

In the standard model for giant planet formation, the planetary growth begins with accretion of solids, followed by a buildup of a gaseous atmosphere as more solids are accreted and, finally, by rapid accretion of gas. The interaction of the solids with the gaseous envelope determines the subsequent planetary growth and the final internal structure. In this work we simulate the interaction of planetesimals with a growing giant planet (proto-Jupiter) and investigate how different treatments of the planetesimal–envelope interaction affect the heavy-element distribution and the inferred core mass. We consider various planetesimal sizes and compositions, as well as different ablation and radiation efficiencies and fragmentation models. We find that in most cases the core reaches a maximum mass of ~2 M _⊕. We show that the value of the core’s mass mainly depends on the assumed size and composition of the solids, while the heavy-element distribution is also affected by the fate of the accreted planetesimals (ablation/fragmentation). Fragmentation, which is found to be important for planetesimals >1 km, typically leads to enrichment of the inner part of the envelope, while ablation results in enrichment of the outer atmosphere. Finally, we present a semianalytical prescription for deriving the heavy-element distribution in giant protoplanets.

M. Nagasawa¹, K. K. Tanaka², H. Tanaka², H. Nomura³, T. Nakamoto³, and H. Miura⁴
Astrophysical Journal 871, 110 Link to Article [DOI: 10.3847/1538-4357/aaf795 ]
¹Department of Physics, School of Medicine, Kurume University, 67 Asahi-machi, Kurume-city, Fukuoka 830-0011, Japan
²Astronomical Institute, Tohoku University, 6-3 Aramaki, Aoba-ku Sendai, Miyagi 980-8578, Japan
³Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁴Graduate School of Natural Sciences, Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya 467-8501, Japan

We examined the excitations of planetesimals caused by the resonances of a giant planet in a protoplanetary gas disk. The highly excited planetesimals generate bow shocks, the mechanism of which results in chondrule formation, crystallization of silicate dust, and evaporation of icy planetesimals. The planetesimals beyond 2:1 resonance migrate owing to the gas drag and obtain the maximum eccentricity around 3:1 resonance, which is located at approximately half the planetary distance. The eccentricity depends on the parameters of the planetesimals and the Jovian planet, such as size and location, and the gas density of the disk. The maximum relative velocity of a 100 km sized planetesimal with respect to the gas disk reaches up to ~12 km s⁻¹ in the case of Jupiter owing to secular resonance, which occurs because of the disk’s gravity. We find that if a Jovian-mass planet is located within 10 au, the planetesimals larger than 100 km gain sufficient velocity to cause the melting of chondrule precursors and crystallization of the silicate. The maximum velocity is higher for large planetesimals and eccentric planets. Planetesimals are trapped temporarily in the resonances and continue to have high speed over 1 Myr after the formation of a Jovian planet. This duration fits into the timescale of chondrule formation suggested by the isotopic data. The evaporation of icy planetesimals occurs when a Jovian planet is located within 15 au. This mechanism can be a new indicator of planet formation in exosystems if some molecules ejected from icy planetesimals are detected.

¹S.J.Weidenschilling
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13270]
Planetary Science Institute, Tucson, Arizona, 85719 USA
Published by arrangement with John Wiley & Sons

Thermal models of asteroids generally assume that they accreted either instantaneously or over an extended interval with a prescribed growth rate. It is conventionally assumed that the onset of accretion of chondrite parent bodies was delayed until a substantial fraction of the initial 26Al had decayed. However, this interval is not consistent with the early melting, and differentiation of parent bodies of iron meteorites. Formation time scales are tested by dynamical simulations of accretion from small primary planetesimals. Gravitational accretion yields rapid runaway growth of large planetary embryos until most smaller bodies are depleted. In a given simulation, all asteroid‐sized bodies have comparable growth times, regardless of size. For plausible parameters, growth times are shorter than the lifetime of 26Al, consistent with thermal models that assume instantaneous accretion. Rapid growth after planetesimal formation is consistent with differentiation of parent bodies of iron meteorites, but not with the assumed delay in formation of chondritic bodies. After the initial growth stage, there is an interval of slower evolution until the belt is stirred and the embryos are dynamically removed. During this interval, a fraction of asteroid‐sized bodies experience large accretional impacts, allowing bodies of the same final size to have very different histories of radius versus time. Accretion from small primary planetesimals leaves some fraction of material in bodies small enough to preserve CAIs while avoiding heating by 26Al. Unheated material can be a significant fraction of the mass that remains after large embryos are removed from the Main Belt.

¹Hannah H. Kaplan,²Ralph E. Milliken,³Conel M. O’D. Alexander,⁴Christopher D. K. Herd
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13264]
¹Department of Space Studies, Southwest Research Institute, Boulder, Colorado, 80302 USA
²Department of Earth, Environmental, and Planetary Sciences, Brown University, , Providence, Rhode Island, 02912 USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, , Washington, DC, 20015 USA
⁴Department of Earth and Atmospheric Sciences, University of Alberta, , Edmonton, AB T6G 2E3 Canada
Published by arrangement with John Wiley & Sons

Insoluble organic matter (IOM) is the major organic component of chondritic meteorites and may be akin to organic materials from comets and interplanetary dust particles (IDPs). Reflectance spectra of IOM in the range 0.35–25 μm are presented as a tool for interpreting organic chemistry from remote measurements of asteroids, comets, IDPs, and other planetary bodies. Absorptions in the IOM spectra were strongly related to elemental H/C (atom) ratio. The aliphatic 3.4 μm absorption in IOM spectra increased linearly in strength with increasing H/C for H/C > 0.4, but was absent at lower H/C values. When meteorite spectra from the Reflectance Experiment Laboratory (RELAB) spectral catalog (n = 85) were reanalyzed at 3.4 μm, this detection limit (H/C > 0.4) persisted. Aromatic absorption features seen in IOM spectra were not observed in the meteorite spectra due to overlapping absorptions. However, the 3.4 μm aliphatic absorption strength for the bulk meteorites was correlated with both H/C of the meteorite’s IOM and bulk C (wt%). Gaussian modeling of the 3 μm region provided an additional estimate of bulk C for the meteorites, along with bulk H (wt%), which is related to phyllosilicate abundance. These relationships lay the foundation for determining organic and phyllosilicate abundances from reflectance spectra. Both the full IOM spectra and the spectral parameters discussed here will aid in the interpretation of data from asteroid missions (e.g., OSIRIS‐REx, Hayabusa2), and may be able to place unknown spectral samples within the context of the meteorite collection.

Kelly E. Miller, Christopher R. Glein, and J. Hunter Waite Jr.
Astrophysical Journal 871, 59 Link to Article [DOI: 10.3847/1538-4357/aaf561 ]
Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA

Since its discovery in the first half of the 20th century, scientists have puzzled over the origins of Titan’s atmosphere. Current models suggest that atmospheric N₂ on Titan may have originated from NH₃-bearing ice with N-isotopic ratios similar to those observed in NH₂ in cometary comae (¹⁴N/¹⁵N ~ 136). In contrast, N₂ ice appears to be too ¹⁵N poor to explain Titan’s atmosphere (¹⁴N/¹⁵N ~ 168). Additionally, data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko suggest that the Ar/N₂ ratio of outer solar system planetesimals may be too high for a comet-like N₂ source on Titan. The Rosetta mission also revealed an astonishing abundance of N-bearing complex organic material. While thermal fractionation of cometary sources during Titan accretion may explain the loss of N₂– and Ar-rich ices, more refractory materials such as complex organics would be retained. Later heating in the interior may lead to volatilization of accreted organics, consistent with Cassini–Huygens measurements of ⁴⁰Ar that suggest outgassing from the interior may have played a role in atmosphere formation. Here, we develop a three endmember mixing model for N isotopes and the ³⁶Ar/¹⁴N ratio of Titan’s atmosphere, and consider the implications for the source of atmospheric methane. Our model suggests that Titan’s interior is likely warm, and that N from accreted organics may contribute on the order of 50% of Titan’s present-day nitrogen atmosphere.

Chen-En Wei¹, Hideko Nomura¹, Jeong-Eun Lee², Wing-Huen Ip³, Catherine Walsh⁴, and T. J. Millar^5,6
Astrophysical Journal 870, 129 Link to Article [DOI: 10.3847/1538-4357/aaf390 ]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
²School of Space Research, Kyung Hee University, Seocheon-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do, 446-701, Republic of Korea
³Graduate Institute of Astronomy, National Central University, No. 300, Zhongda Road, Zhongli Dist., Taoyuan City 32001, Taiwan
⁴School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁵Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast, BT7 1NN, UK
⁶Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

The bulk composition of Earth is dramatically carbon-poor compared to that of the interstellar medium, and this phenomenon extends to the asteroid belt. To interpret this carbon deficit problem, the carbonaceous component in grains must have been converted into the gas phase in the inner regions of protoplanetary disks (PPDs) prior to planetary formation. We examine the effect of carbon grain destruction on the chemical structure of disks by calculating the molecular abundances and distributions using a comprehensive chemical reaction network. When carbon grains are destroyed and the elemental abundance of the gas becomes carbon-rich, the abundances of carbon-bearing molecules, such as HCN and carbon-chain molecules, increase dramatically near the midplane, while oxygen-bearing molecules, such as and , are depleted. We compare the results of these model calculations with the solid carbon-to-silicon fraction in the solar system. Although we find a carbon depletion gradient, there are some quantitative discrepancies: the model shows a higher value at the position of the asteroid belt and a lower value at the location of Earth. In addition, using the obtained molecular abundance distributions, coupled with line radiative transfer calculations, we make predictions for ALMA to potentially observe the effect of carbon grain destruction in nearby PPDs. The results indicate that HCN, , and c- may be good tracers.