Methanol Masers as Signposts of Star Formation and VLBI Observations of Coronal Emission from Young Stars
Doctoral thesis, 2004
Using a consistent survey of the galactic plane at 6.7 GHz as a starting point, this thesis reports on the observational as well as theoretical results achieved by extensively studying of the phenomenon of class II methanol masers.
The general catalogue of 6.7 GHz methanol masers in the Milky Way, gathers all known sources discovered since 1991, including the 4 new sources detected during the Onsala Blind Survey of the galactic plane. Their spatial distribution in the Galaxy follows the distribution of OB-associations hosting massive star formation, suggesting that methanol masers arise where massive stars form. Statistical considerations are presented with the aim of estimating the luminosity function of the masers.
Dust continuum observations of methanol maser sites not uniquely associated to known infrared sources, were used to trace the Spectral Energy Distribution (SED) of these sites, yielding dust temperatures and enclosed masses. This study confirmed that many of 6.7 GHz methanol maser sources host large amounts of cold dust, making further evidence for classifying them as sites of (massive) star formation in an early stage. VLBI observations of strong 6.7 GHz methanol masers conducted for the first time including short baseline spacings showed that there is a considerably large, low brightness emission that was systematically resolved out at the usual spatial resolution. Various scenarios for the nature of this emission are presented as the basis for the investigation toward an explanation to this phenomenon.
Theoretical work on modelling a circumstellar disc traced by methanol masers gave estimates for the mass of the central object and the extent of the masing disc in NGC7538-IRS1 N. The model fits both the spectrum and the integrated brightness profile using two parameters, the line of sight velocity gradient and the ratio between inner and outer radius.
As a side project, results of 3.6 cm-continuum global VLBI observations of the young active stars YZ CMi, AD Leo and T Tauri Sb (first detection at this frequency and resolution) are presented, where size and polarisation of the coronal emission as well as accurate positions of all targets were measured. The emission mechanism for the corona could be non-thermal,and particularly for T Tauri Sb, an electron cyclotron maser is proposed to explain the flaring activity.