The Star Formation Newsletter

s of recently accepted papers Evolution of Molecular Abundances in Protoplanetary Disks with Accretion Flow Y. Aikawa, T. Umebayashi, T. Nakano, and S. M. Miyama Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA Computing Service Center, Yamagata University, Yamagata 990-8560, Japan Nobeyama Radio Observatory, National Astronomical Observatory, Nobeyama, Minamisaku, Nagano 384-1305, Japan National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan E-mail contact: [email protected] We investigate the evolution of molecular abundances in a protoplanetary disk in which matter is accreting toward the central star by solving numerically the reaction equations of molecules as an initial-value problem. We obtain the abundances of molecules, both in the gas phase and in ice mantles of grains, as functions of time and position in the disk. In the region of surface density less than 10 g cm−2 (distance from the star ∼> 10 AU for the mass accretion rate 10−8M yr−1), cosmic rays are barely attenuated even on the midplane of the disk, and produce chemically active ions such as H3 and He . We find that through reactions with these ions considerable amounts of CO and N2, which are initially the dominant species in the disk, are transformed into CO2, CH4, NH3, and HCN. In the regions where the temperature is low enough for these products to freeze onto grains, they accumulate in ice mantles. As the matter migrates toward inner warmer regions of the disk, some of the molecules in the ice mantles evaporate. It is found that most of the molecules desorbed in this way are transformed into less volatile molecules by the gas-phase reactions, which then freeze out. Molecular abundances both in the gas phase and in ice mantles crucially depend on the temperature and thus vary significantly with the distance from the central star. Although molecular evolution proceeds in protoplanetary disks, our model also shows that significant amount of interstellar ice, especially water ice, survives and is included in ice mantles in the outer region of the disks. We also find that the time scale of molecular evolution is dependent on the ionization rate and the grain size in the disk. If the ionization rate and the grain size are the same as those in molecular clouds, the time scale of the molecular evolution, in which CO and N2 are transformed into other molecules, is about 10yr, which is slightly smaller than the lifetime of the disk. The time scale for molecular evolution is larger (smaller) in the case of lower (higher) ionization rate or larger (smaller) grain size. We compare our results with the molecular composition of comets, which are considered to be the most primitive bodies in our solar system. The molecular abundances derived from our model naturally explain the coexistence of oxidized ice and reduced ice in the observed comets. Our model also suggests that comets formed in different regions of the disk have different molecular compositions. Finally we give some predictions for future millimeter-wave and sub-millimeter-wave observations of protoplanetary disks. Accepted by Ap. J. VLA Continuum Observations of Suspected Massive Hot Cores P. Carral, S. Kurtz, L. F. Rodŕıguez, J. Mart́ı, S. Lizano, and M. Osorio 1 Dept. de Astronomı́a, Universidad de Guanajuato, Apdo. Postal 144, Guanajuato, Guanajuato, 36000, México 2 Instituto de Astronomı́a, UNAM, Unidad Morelia, J. J. Tablada 1006, Morelia, Michoacán, 58090, México 3 Departamento de F́ısica Aplicada, Escuela Politécnica Superior, Universidad de Jaén, Calle Virgen de la Cabeza 2, E-23071, Jaén, Spain E-mail contact: [email protected]