EPoS Contribution
EPoS Contribution
Understading the X factor with MHD and radiative transfer simulations

Rahul Shetty
Institute for Theoretical Astrophysics, Heidelberg University, Heidelberg, Germany
Carbon monoxide (CO) is a key tracer which is often employed to estimate the physical conditions in the ISM, including molecular clouds (MCs), such as mass, morphology, and kinematics. It is thus essential to understand how well CO traces MC gas. I will discuss how CO (J=1-0) intensities are related to the total gas, molecular, and CO column densities, as well as the ``X factor,'' the ratio of H2 column density to CO intensity. Using MHD simulations that include a time-dependent treatment of chemistry to follow the formation and destruction of molecules, we perform non-LTE radiative transfer calculations to model CO line emission. Even though in a given model the X factor along different lines-of-sight can take on a range of values, the highest density regions generally produce a value consistent with observed trends. Consequently, the cloud averaged X factor does not vary much between clouds with densities > 100 cm^-3 and metallicities ~ 0.3 - 1.0 Z_{sun}. Clouds with lower metallicities have higher X factors. In such clouds with low CO abundances, the X factor can vary appreciably - sometimes by up to 4 orders of magnitude. I will discuss our results in the context of common interpretations of the X factor. I will show that a constant X factor does not require any linewidth-size relationship, or that MCs must be virialized objects. Instead, these simulations point to a very simple reason for the constant X factor found in Galactic MCS: it is a natural consequence of the limited ranges in dense gas temperature, turbulent velocities, and column densities found in MCs.
S. Glover - ITA, U. Heidelberg, Germany
C. Dullemond - ITA, U. Heidelberg, Germany
R. Klessen - ITA, U. Heidelberg, Germany
E. Ostriker - U. Maryland, USA
A. Harris - U. Maryland, USA
Key publication