Methanol: a diagnostic tool for star formation

Abstract

A powerful diagnostic tool of interstellar clouds is found in the analysis of their molecular spectra, which can be seen as fingerprints of a star forming region. Since the main gas-phase molecular component, H₂, is not directly observable under "normal'' conditions, one has to rely on other molecules to trace the physical state of an interstellar cloud. Often, traditionally symmetric rotors such as NH₃ are used to probe a cloud's kinetic temperature, while linear molecules, e.g. CS, are used to determine its density. However, different spatial distributions of the tracers ("chemistry") complicate the picture, as they often trace physically different and spatially non-coexisting gas components. It is thus desirable to trace all relevant physical parameters with a single molecule. Promising candidates exist among slightly asymmetric rotors, which have properties qualifying them as tracers for physical conditions. Since they are almost symmetric, they share a strong sensitivity to kinetic temperature with symmetric molecules, but they also allow determinations of spatial density. <br /> Methanol, CH₃OH, a slightly asymmetric rotor, is a proven tracer of high-density environments; moreover it is particularly well suited for high mass star forming regions, as it is ubiquitously found in different regimes of star formation, from quiescent, cold (T ~ 10 K), dark clouds to "hot core" sources near high-mass (proto)stellar objects. However, up to now an extremely poor knowledge of the CH₃OH collisional rates and of their propensity rules has prevented realistic systematic studies exploiting methanol's full potential as an interstellar tracer. Recently, this situation has changed with the calculation of collisional rate coefficients Pottage et al. (2002). <br /> Part I of my thesis is a pilot study dedicated to the analysis of the excitation of CH₃OH in the interstellar medium and to its tracing properties in the centimeter, millimeter and submillimeter spectrum; the new CH₃OH-He collisional rates have been integrated in a "standard" Large Velocity Gradient program aimed at modelling methanol excitation. An innovative way of analysing CH₃OH spectra, based on a multicomponent-simultaneous fitting of all the lines observed towards a given position, is also discussed. <br /> In Part II the analysis and the technique discussed in Part I are applied to high-mass star forming regions. A sample of 13 sources in the early stages of star formation is analysed by single dish observations of centimeter and millimeter CH₃OH lines. All the sources, in the earliest stages of star formation prior to forming an ultracompact H<font size="-2">II</font> region, have a homogenous distance to the solar system and show strong CH₃OH emission at least in the less excited lines we observed. CH₃OH observations at high spatial resolution are discussed in chapter 4; the High Mass Protostellar Object IRAS 05358+3543 has been mapped in the 5_k → 4_k v_t=0 band at 241.7GHz with the Plateau de Bure Interferometer, reaching a resolution of 2.6" × 1.36". Our observations reveal indeed a hot, dense phase around the main millimeter dust condensations, still unresolved with our resolution, with detection of torsionally excited lines.