Evolution of Molecular Abundances in Prohoplanetary Gaseous Disks

We investigate the chemical and physical evolution of protoplanetary gaseous disks and obtain the following results：1.We investigate the evolution of molecular abundances in a protoplanetary gaseous disk in which matter is accreting toward the central star by solving numerically the reaction equations of molecules as an initial value problem.We find that molecular abundances both in the gas phase and in ice mantles of dust grains depend crucially on the temperature and thus vary significantly with the distance from the central star.The timescale of molecular evolution，which is slightly smaller that the lifetime of the disk，is dependent on the physical conditions such as ionization rate and grain size in the disk。We also find that our results naturally explain the coexistence of oxidized(e.g.CO I D22 I D 2)ice and reduced(e.g.CH I D 24 I D 2，NH I D 23 I D 2)ice in the observed comets.2.We next investigate the regions of protoplanetary disks in which magnetorotational instability can operate the transport of angular momentum。We calculate the spatial distribution of various charges particles and compare the ohmic dissipation time of magnetic field with the Keplerian orbit period.We find that for a variety of disk models there exist inner regions where the gas is decoupled from the magnetic field and is magnetorotationally stable due to ohmic dissipation。This must made the accretion onto the central star non-steady.In the later evolutionary stages the stable region shrinks as the grain size increases and/or the sedimentation of grains toward the midplain of the disk proceeds。

We investigate the chemical and physical evolution of protoplanetary gaseous disks and obtain the following results :1. We investigate the evolution of molecular abundances in a protoplanetary gaseous disk in which matter is accreting toward the central star by solving numerically the reaction equations of molecules as an initial value problem. We find that molecular abundances both in the gas phase and in ice mantles of dust grains depend crucially on the temperature and thus vary significantly with the distance from the central star. The timescale of molecular evolution, which is slightly smaller that the lifetime of the disk, is dependent on the physical conditions such as ionization rate and grain size in the disk. We also find that our results naturally explain the coexistence of oxidized (e.g. COィイD22ィエD2) ice and reduced (e.g. CHィイD24ィエD2, NHィイD23ィエD2) ice in the observed comets.2. We next investigate the regions of protoplanetary disks in which magnetorotational instability can operate the transport of angular momentum. We calculate the spatial distribution of various charges particles and compare the ohmic dissipation time of magnetic field with the Keplerian orbit period. We find that for a variety of disk models there exist inner regions where the gas is decoupled from the magnetic field and is magnetorotationally stable due to ohmic dissipation. This must made the accretion onto the central star non-steady. In the later evolutionary stages the stable region shrinks as the grain size increases and/or the sedimentation of grains toward the midplain of the disk proceeds.