title: Statistical Analysis of Simulated Molecular Clouds
creator: Molina Hernandez, Faviola Zuhé
subject: 500
subject: 500 Natural sciences and mathematics
subject: 520
subject: 520 Astronomy and allied sciences
subject: 530
subject: 530 Physics
description: Understanding molecular cloud formation is a major challenge in modern astrophysics. Although the improvements on computational power and novel astronomical instrumentation have allowed us to reach unprecedented accuracy, there are still many open questions. One key issue which helps us to understand the physics behind this problem is the correct comparison between numerical models and observations. Usually, the $^{12}$CO($J$=1-0) emission is considered to be a good tracer of the temperatures and structure of molecular clouds. However, it has been found that it may provide a biased picture of clouds, at best. In this thesis, we analyze a large set of numerical simulations with the aim of making direct comparison with observations. Using a 3D magneto-hydrodynamical simulation including time-dependent  chemistry, we find that most of the CO is located at number densities greater than 100 cm$^{-3}$ and kinetic temperatures ($T_{\rm K}$) below 40 K, regardless of the mean number density ($n_0$), metallicity ($Z$) and UV radiation field strength (UV). Radiative transfer calculations are performed to analyze the $^{12}$CO($J$=1-0) rotational transition line intensity that comes out of the cloud. We then calculate the excitation temperature ($T_{\rm ex}$) considering theoretical and observational approaches and find that the gas is mostly sub-thermally excited, indicating that $T_{\rm ex}$ represents a lower limit of $T_{\rm K}$. $T_{\rm ex}$ is used for estimating the CO column density ($N_{\rm CO}$). Considering the full position-position-velocity spectrum for inferring $T_{\rm ex}$, instead of the usual way of using the maximum of the intensity along the line of sight, improves the estimates of $N_{\rm CO}$ by $\sim$30\%. Besides, when a single Milky-Way like CO-to-H$_2$ conversion factor is assumed, the total inferred mass of H$_2$ is underestimated by a factor which typically increases from $\sim$0.1 to 1, as the product $n_0\times Z$ decreases, and/or UV becomes stronger. Moreover, we propose density variance--Mach number relations for supersonic, magnetized, turbulent gas, including an isothermal and non-isothermal equation of state. These analytical relations reproduce satisfactorily the measurements made on numerical simulations. We find that the magnetic field strength scales with density in a relationship of the kind $B\propto \rho^{\alpha}$ with $0\leq \alpha< 1/2$.  
date: 2013
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/15319/1/dissertation.pdf
identifier: DOI:10.11588/heidok.00015319
identifier: urn:nbn:de:bsz:16-heidok-153199
identifier:   Molina Hernandez, Faviola Zuhé  (2013) Statistical Analysis of Simulated Molecular Clouds.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/15319/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng