Opacity broadening of 13 co linewidths and its effect on the variance-sonic mach number relation

• Main Author: Correia, C.
• Publication Date: 2014
• Other Authors: Burkhart, B., Lazarian, A., Ossenkopf, V., Stutzki, J., Kainulainen, J., Kowal, G., Medeiros, José Renan de
• Format: Article
• Language: eng
• Source: Repositório Institucional da UFRN
• Download full: https://repositorio.ufrn.br/jspui/handle/123456789/29025; http://dx.doi.org/10.1088/2041-8205/785/1/L1
• topic: ISM: structure; Magnetohydrodynamics (MHD); Methods: numerical Online-only material: color figures

We study how the estimation of the sonic Mach number (Ms ) from 13CO linewidths relates to the actual three-dimensional sonic Mach number. For this purpose we analyze MHD simulations that include post-processing to take radiative transfer effects into account. As expected, we find very good agreement between the linewidth estimated sonic Mach number and the actual sonic Mach number of the simulations for optically thin tracers. However, we find that opacity broadening causes Ms to be overestimated by a factor of ≈1.16-1.3 when calculated from optically thick 13CO lines. We also find that there is a dependence on the magnetic field: super-Alfvénic turbulence shows increased line broadening compared with sub-Alfvénic turbulence for all values of optical depth for supersonic turbulence. Our results have implications for the observationally derived sonic Mach number-density standard deviation (σρ/langρrang) relationship, $\sigma ^2_{\rho /\langle \rho \rangle }=b^2M_s^2$, and the related column density standard deviation (σ N/langNrang) sonic Mach number relationship. In particular, we find that the parameter b, as an indicator of solenoidal versus compressive driving, will be underestimated as a result of opacity broadening. We compare the σ N/langNrang-Ms relation derived from synthetic dust extinction maps and 13CO linewidths with recent observational studies and find that solenoidally driven MHD turbulence simulations have values of σ N/langNrangwhich are lower than real molecular clouds. This may be due to the influence of self-gravity which should be included in simulations of molecular cloud dynamics.