Shock heating of dusts and icy planetesimals due to bow shocks in protoplanetary disks
K. K. Tanaka1, M. Nagasawa2, H. Nomura 3, H. Miura4, T. Nakamoto3, H. Tanaka5 andT. Yamamoto1
1

Hokkaido University, Japan, 2Kurume University, Japan, 3Tokyo Institute of Technology,Japan, 4Nagoya City University, Japan, 5Tohoku University, Japan

In protoplanetary disks, planetesimals grow to planets through mutual collisions andaccretions. The gravitational interactions among planetesimals increase the eccentricities oftheir orbits. Especially, large planeteimals have high eccentric orbits and supersonicvelocities relative to the nebular gas. Small silicate particles in the nebular gas are heated andcould be melted by passages though the bow shocks of such planetesimals [1-3]. It is
proposed that the planetesimals perturbed by Jovian mean-motion resonances are thepromising source of shock waves that form chondrules in chondritic meteorites because theshocks give short-time-scale heating of dust grains appropriate for the formation ofchondrules [1]. In the previous studies on the planetesimal excitation, however, the velocities
obtained were at most 8 kms-1 in the asteroid belt, which is insufficient to account for theubiquitous existence of chondrules. Recently Nagasawa et al. [4] reexamined the effect ofJovian resonances, taking into account the secular resonance in the asteroid belt caused by thegravity of the gas disk. They found that the velocities of planetesimals relative to the gas disk
exceed 12 kms-1 and the heating region is restricted to a relatively narrowband (1.5-3.5 au).
These results suggest that chondrules were produced effectively in the asteroid region.
The planetesimal bow shocks also lead to heating of the planetesimal itself. Tanaka et al.
[5] presented that icy planetesimals with radii larger than 100 km suffer from significant
evaporation even outside the snow line. In [5], we focused on the oligarchic growth stage of
propoplanets, in which planetesimals have velocities of 1-5 kms-1 relative to the gas due tostirring by protoplanets. In this study, we investigate the evaporation of planetesimal in thestage of the formation of gas giant planets. In such a stage, the evaporation of planetesimalproceeds much more effectively because of the stronger shocks and the evaporation time ismuch shorter than the disk lifetime of 106 yr. The evaporated H2O vapor from the planetesimal
surface recondenses as it cools and form a large amount of icy fine grains. During theevaporation of icy planetesimals, the silicate dust is also expected to be released from theplanetesimal surface by the strong flow of the disk gas. This process provides a large amountof chondrule precursors. Our results may also relate to the origin of crystalline silicate
observed in various comets and protoplanetary disks. The planetesimal bow shocks also leadto chemical reactions among various evaporated molecules. We propose a possibility that wecan diagnose the shock heating and evaporation of icy planetesimals, using observations oflines of the molecules in the protoplanetary disks.
References[1] S. J. Weidenschilling, F. Marzari, & L. L. Hood, (1998), Sci. 279, 681.
[2] H. Miura, & T. Nakamoto, 2005, Icar., 175, 289[3] H. Miura, K. K. Tanaka, T. Yamamoto, T.Nakamoto, J. Yamada, & K. Tsukamoto, 2010, ApJ, 642[4] M. Nagasawa, K. K. Tanaka, H. Tanaka, T. Nakamoto, H. Miura, & T. Yamamoto, 2014, ApJL, 794:L7.
[5] K. K. Tanaka, T. Yamamoto, H. Tanaka, H. Miura, M. Nagasawa, & T. Nakamoto, 2013, ApJ, 764, 120.