¹Munan Gong (龚 慕南),²Xiaochen Zheng (郑 晓晨),^2,3,4Douglas N. C. Lin (林潮),¹Kedron Silsbee,⁵Clement Baruteau,¹Shude Mao (毛 淑德)
The Astrophysical Journal 883, 164 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab3e70]
¹Max-Planck Institute for Extraterrestrial Physics, Garching by Munich, D-85748, Germany
²Department of Astronomy and Center for Astrophysics, Tsinghua University, Beijing 10086, People’s Republic of China
³Institute for Advanced Studies, Tsinghua University, Beijing 10086, People’s Republic of China
⁴Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 95064, USA
⁵Institut de Recherche en Astrophysique et Planétologie (IRAP), 14 avenue Edouard Belin, F-31400 Toulouse, France

Chondrules are silicate spheroids found in meteorites, and they serve as important fossil records of the early solar system. In order to form chondrules, chondrule precursors must be heated to temperatures much higher than the typical conditions in the current asteroid belt. One proposed mechanism for chondrule heating is the passage through bow shocks of highly eccentric planetesimals in the protoplanetary disk in the early solar system. However, it is difficult for planetesimals to gain and maintain such high eccentricities. In this paper, we present a new scenario in which planetesimals in the asteroid belt region are excited to high eccentricities by the Jovian sweeping secular resonance in a depleting disk, leading to efficient formation of chondrules. We study the orbital evolution of planetesimals in the disk using semi-analytic models and numerical simulations. We investigate the dependence of eccentricity excitation on the planetesimal’s size, as well as the physical environment and the probability for chondrule formation. We find that 50–2000 km planetesimals can obtain eccentricities larger than 0.6 and cause effective chondrule heating. Most chondrules form in high-velocity shocks, in low-density gas, and in the inner disk. The fraction of chondrule precursors that become chondrules is about 4%–9% between 1.5 and 3 au. Our model implies that the disk depletion timescale is τ _dep ≈ 1 Myr, comparable to the age spread of chondrules, and that Jupiter formed before chondrules, no more than 0.7 Myr after the formation of calcium aluminum inclusions.

^1,2Fei Dai,^2,4Kento Masuda,²Joshua N. Winn,³Li Zeng
The Astrophysical Journal 883, 79 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab3a3b]
¹Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
³Department of Earth & Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
⁴NASA Sagan Fellow.

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods—some as short as 4 hr. These “ultra-short-period planets” or “hot Earths” are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from Gaia to improve the stellar and planetary parameters, and use Gaussian process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35% iron and 65% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55 Cnc e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8 M _⊕, despite the selection bias toward more massive planets, suggesting that 8 M _⊕ is the critical mass for runaway accretion.

¹Tetsuya Yokoyama,^1,2Yuichiro Nagai,^1,2Ryota Fukai,²Takafumi Hirata
The Astrophysical Journal 883, 62 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab39e7]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
²Geochemical Research Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

New high-precision Mo isotopic data were obtained for 10 iron meteorites and two carbonaceous, five ordinary, and two rumuruti chondrites. A clear isotopic dichotomy is observed in μ ⁱ Mo−μ ⁹⁴Mo diagrams between the CC meteorites (carbonaceous chondrites and IVB irons) and other noncarbonaceous (NC) meteorites. The Mo isotope variabilities within the CC meteorites can indicate either s-process matter distributed heterogeneously throughout various chondritic components in the different outer solar system materials or that generated by a local parent-body processing. In contrast, the presence of two end-member components for the Mo isotope composition, that is, NC-A and NC-B, was suggested in the NC reservoir. The NC-B component represents the remaining counterpart of the gaseous source reservoir for type B calcium-aluminum-rich inclusions, which was presumably formed via thermal processing that destroyed r-process-rich carriers. Two models were proposed to consider the observed Mo isotope variability among the NCs. In model 1, the NC-A reservoir was formed closer to the Sun than the NC-B reservoir by another thermal processing that destroyed s-process-depleted phases. The Mo isotopic composition of the NC region changed via outward motion of particles from the two reservoirs, resulting in a gradual change from NC-A- to NC-B-like components as a function of the heliocentric distance. In model 2, the Mo isotopic composition in individual NCs is controlled by the amount of metal and matrix-like material that is removed from and added to the NC-B reservoir. Such a fractionation process most likely occurred locally in time and/or space in the inner solar system.