MG: Simulating chemistry in star forming environments -- KS: Producing distant solar system bodies by mutual scattering of planetary embryos

 MG Abstract: Chemistry plays an important role in the interstellar medium (ISM), regulating the heating and cooling of the gas and determining abundances of molecular species that trace gas properties in observations. One of the most abundant and important molecules in the ISM is CO. CO is a main coolant for the molecular ISM, and the CO(J=1-0) line emission is a widely used observational tracer for molecular clouds.We propose a new simplified chemical network for hydrogen and carbon chemistry in the atomic and molecular ISM. We compare results from our chemical network in detail with results from a full  photodissociation region (PDR) code, as well as with observations of diffuse and translucent clouds. We apply our new chemistry network to a study of the X_CO conversion factor, which is used to convert theCO luminosity to the total H2 mass. Synthetic CO maps are obtained by post-processing the MHD simulations with chemistry and radiation transfer. We find that  CO is only an approximate tracer of H2. Nevertheless, X_CO=0.7-1.0 10^20 cm^-2 K^-1 km^-1 s consistent with observations, insensitive to the evolutionary ISM state or the far-ultraviolet (FUV) radiation field strength.Our numerical simulations successfully reproduce the observed variations of X_CO on parsec scales, as well as the dependence of X_CO on extinction and the CO excitation temperature                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      KS Abstract:It is likely that multiple bodies with masses between those of Mars and Earth formed in the outer planetesimal disk of the solar system, and that some were not incorporated into the giant planets. These would have been scattered to higher semi-major axis via gravitational interactions with the giant planets. For test particles, this scattering process continues until they are either ejected from the solar system, or experience sufficient torque from Galactic tides or passing stars that they no longer return close to the giant planets (thus forming the Oort cloud). These torques are only relevant when the semi-major axes of the bodies are thousands of AU. If there are massive bodies in the scattering disk, then their mutual torques can raise their perihelion distances while their semi-major axis is only a few hundred AU. We conduct N-body simulations of this process and find that in a substantial fraction of cases, one (or occasionally more) of the bodies survives to the present day in an orbit with perihelion distance between 40 and 70 AU, semi-major axis of up to a few hundred AU, and inclination less than 30 degrees. Any surviving embryos could likely be detected by current or planned optical surveys, or have a noticeable effect on solar system ephemerides. Regardless of whether any large bodies remain, their dynamical influence can explain many of the observed properties of the detached disk (bodies such as Sedna with perihelion distances that place them outside the region of influence of the giant planets, and aphelion distances too small for them to be influenced by Galactic tides or passing stars) 

Munan Gong (MG,Princeton University) - Kedron Silsbee (KS,Princeton University)
DoA, Rm 2907
Mon, 2017-10-23 12:00 to 13:00