16. 20
暴走的成長の様子 KOKUBO AND IDA
KOKUBO AND IDA
質量 [1023g]
最大の天体
軌道離心率
平均値
FIG. 4. Time evolution of the maximum mass (solid curve) and the mean
mass (dashed curve) of the system.
than this range are not statistically valid since each mass bin often
has only a few bodies. First, the distribution tends to relax to a
decreasing function of mass through dynamical friction among
(energy equipartition of) bodies (t = 50,000, 100,000 years).
時間 [年]
Second, the distributions tend to flatten (t = 200,000 years). This
is because as a runaway body grows, the system is mainly heated
by the runaway body (Ida and Makino 1993). In this case, the
eccentricity and inclination of planetesimals are scaled by the
FIG. 4. Time evolution of the maximum mass (solid curve) and the mean
微惑星の暴走的成長
mass (dashed curve) of the system.
軌道長半径 [AU] → are not statistically valid since each mass bin often
than this range
原始惑星が誕生する
FIG. 3. Snapshots of a planetesimal system on the a–e plane. The circles
represent planetesimals and their radii are proportional to the radii of planetesi-
has only a few bodies. First, the distribution tends to relax to a
17. 寡占的成長の様子
FORMATION OF PROTOPLANETS FROM PLANETESIMALS 23
各場所で微惑星が暴走的成長
→ 等サイズの原始惑星が並ぶ
＝
寡占的成長とよぶ
軌道離心率
各軌道での原始惑星
質量 [kg] 形成時間 [yr]
地球軌道 1×1024 7×105
木星軌道 3×1025 4×107
軌道長半径 [AU]
天王星軌道 8×1025 2×109
FIG. 7. Snapshots of a planetesimal system on the a–e plane. The cir- FIG. 8. The number of bodies in linear mass bins is plotted for t = 100,000,
20. ジャイアントインパクトの様子
1134 KOKUBO, KOMIN
軌道離心率
軌道長半径 [AU]
Fig. 2.—Snapshots of the system on the a-e (left) and a-i (right) planes at t ¼ 0, 1
are proportional to the physical sizes of the planets.
長い時間をかけて原始惑星同士の軌道が乱れる
planets is hnM i ’ 2:0 Æ 0:6, which means that the typical result-
ing system consists of two Earth-sized planets and a smaller
planet. In this model, we obtain hna i ’ 1:8 Æ 0:7. In other words,
one or two planets tend to form outside the initial distribution of
→ 互いに衝突・合体してより大きな天体に成長
protoplanets. In most runs, these planets are smaller scattered
planets. Thus we obtain a high efficiency of h fa i ¼ 0:79 Æ 0:15.
The accretion timescale is hTacc i ¼ ð1:05 Æ 0:58Þ ; 108 yr. These
results are consistent with Agnor et al. (1999), whose initial con-
24. 1226 MACHIDA ET AL.
巨大ガス惑星の形成の様子
1.— Time sequence for model M04. The density (color scale) and velocity distributions (arrows) on the cross section in the z ¼ 0 plane are plotted. The bottom
˜
¼ 3) are 4 times the spatial magnification of the top panels (l ¼ 1). Three levels of grids are shown in each top (l ¼ 1, 2, and 3) and bottom (l ¼ 3, 4, and 5) panel.
周囲の円盤ガスが原始惑星の重力圏内に捕獲される
l of the outermost grid is denoted in the top left corner of each panel. The elapsed time ˜p and the central density c on the midplane are denoted above each of the
ls. The velocity scale in units of the sound speed is denoted below each panel.
t ˜
