EPSC Abstracts, Vol. 4, EPSC2009-422, 2009 European Planetary Science Congress, © Author(s) 2009

Evolution of the due to the Galactic and passing and the implications for observations

A. Higuchi (1), E. Kokubo (2) and T. Ito (2)

(1) Tokyo Institute of Technology, Tokyo, Japan ([email protected] / Fax: +81-3-5734-3538), (2) National Astronomical Observatory of Japan, Mitaka, Japan

Abstract orbital elements). When the velocity change is large, some escape from the Solar We have investigated the formation of the Oort system to the interstellar space. Our previous study cloud due to two external forces: (1) Galactic tidal shows that the disk disappears before force, and (2) perturbation from the passing stars. attaining the isotropic distribution ([3], [4]). The We clarify the role of the two external forces in effect of the stellar encounters on the isotropic Oort cloud formation and evolution. We discuss Oort cloud, focusing on the production of long- the distribution of inclination of long-period period , has been examined by many comets from the evolving Oort cloud and its authors (e.g., [5], [6]). However, the results effects on the planetary region as a consequence. changes when we start with the initial condition as a flat planetesimal disk, not the isotropic Oort Oort Cloud Formation cloud. The Oort cloud is the reservoir of long-period comets surrounding the (e.g., [1]). Initial Condition and Calculation Method Observation of long-period comets predicts that the structure is spherical. Initial condition of the planetesimal disk Comets in the Oort cloud are believed to be We use the flat planetesimal disk on the ecliptic planetesimals, which are remnants of planet plane (i.e., the Galactic inclination is ~63 deg). formation. After planet scattering, their perihelia The semimajor axes of the planetesimals are large are lifted and inclinations are randomized by the enough to be affected by external forces (e.g., external forces such as the galactic tide and 10,000 AU) and the perihelion distances are inside perturbations from passing stars and/or giant the planetary region (e.g., 5 AU). molecular clouds. Since the planet scattering cannot randomize the inclination distribution of The Galactic tide the planetesimals, the spherical structure of the Oort cloud is attained by the external forces. The To calculate the orbital evolution of the tide from the Galactic disk, which is the strongest planetesimals due to the Galactic tide, we use the external force, causes the libration and/or analytical formulae derived in [2]. We neglect the circulation of the orbital elements of the radial component of the Galactic (e.g., planetesimals, but cannot randomize the [7]). distribution of the inclination (e.g., [2]). The Passing stars distribution of the inclination produced by the Galactic tide alone has two peaks at i~27 deg and The sets we use here are two types, first, we 153 deg ([2]).Thus, the random-walk nature of the use the realistic star set, i.e., stars with velocities encounters with passing stars must play an and masses derived from the observation of the important role in forming the spherical Oort cloud. solar neighbourhood. Second, we use the star set Passing stars give the velocity changes to the converted into the set with the identical velocity planetesimals and the planetesimals gain or lose and mass which reproduce the result of the first their energies and/or angular momentum (i.e., star set ([3], [4]). The effect of the stellar

EPSC Abstracts, Vol. 4, EPSC2009-422, 2009 European Planetary Science Congress, © Author(s) 2009 encounters is calculated using the same method References as [5]. [1] Dones, L. et al. (2004) in Comets II, ed. M. C.

Festou, H. U. Keller, & H. A. Weaver (Tuscon: Results Univ. Arizona Press), 153-174. Figure.1 shows the inclination distribution of [2] Higuchi, A. et al. (2007) The Astronomical planetesimals at 5 Gyr, evolved by the Galactic Journal, 134, 1693-1706. tide, passing stars, and both of them. [3] Higuchi, A. & Kokubo, E. (2009) in preparation [4] Higuchi, A. & Kokubo, E. (2008) presentation in EPSC2008 [5] Heisler, J. et al. (1987) Icarus, 70, 269-288. [6] Dybczński, P. A. (2002) Astronomy & Astrophysics, 396, 283-292.

Fig. 1 Inclination distribution of planetesimals at 5 Gyr for the initial semimajor axes 20,000 AU. Each color indicates the external force(s) considered in the calculations: [green: the Galactic tide], [red: passing stars], and [blue: the Galactic tide & passing stars]. The black solid curve shows the isotropic distribution.

As seen in Figure 1, the Galactic tide produces the strange distribution with two peaks ([2]), and passing stars produces the nearly isotropic distribution via the flat distribution ([3], [4]). On the other hand, the blue histogram in Fig. 1 shows almost isotropic distribution. Each of the Galactic tide and passing stars cannot attain the isotropic distribution, but it is attained when both of them are considered. One interesting aspect is that the two peaks related to the Galactic tidal effect are seen until just before reaching the isotropic distribution. This indicates that the inclination distribution of observed long period comets has not to be isotropic. The comparison of the results with the observation and the implication for the injection of Oort cloud comets into the planetary region will be discussed at the conference.