Evolutionary stages of massive stage formation

Massive stars play a vital role in the evolutionary of the Galaxy; they produce powerful outflows, strong stellar winds and large amounts of ultraviolet radiation, while being responsible for the chemical enrichment of heavy materials within the interstellar medium. Due to the importance and effects of these celestial objects, not only on their local environments but also on their host galaxy, it is necessary to understand the earliest stages of their formation. However, our knowledge of how massive stars form is still fairly limited due to their rarity and rapid evolution, along with observational limitations. This Thesis presents three studies of massive star forming regions and utilises various molecular line transitions to investigate the initial environments and evolutionary stages of massive star formation.

The first study presents the results from the ammonia mapping of 34 regions in the southern Galactic plane. The first two inversion transitions of the ammonia molecule have been used to map the dense gas within these star formation regions. Physical properties of these regions have been derived from the observed ammonia spectra and supplemented with the results from the ATLASGAL compact source catalogue. These data have been used to construct a rudimentary evolutionary sequence for massive star formation based on the morphology of the associated high-density gas; it is shown that there are no observed changes in the morphology of the dust emission within the regions of interest, however, changes are observed in the morphology of the molecular line emission, which is sensitive to high densities. Maser catalogues from the MMB and HOPS surveys are also employed, and are shown to be an effective and useful tool in the study of star formation; these provide an insight into regions of possible triggeredstar formation and as a tool to identify the locations of young massive stars.

In the second study, the association rate between the class II 6.7 GHz methanol maser and interstellar dust clumps is investigated, and found to be 99%. The physical parameters of maser associated clumps are determined and these clumps are found to be generally more compact, dense and luminous. Only clumps with volume densities of n > 10^4.1 cm^-3 are shown to be hosting masers, and using the bolometric luminosities of embedded objects, a minimum required mass for maser emission is derived to be 6 M_sun. The free-fall times and volume densities of maser associated clumps are used to calculate a statistical lifetime for the 6.7 GHz methanol maser, and this is found to be ~ 3.3 x 10^4 yrs, which is in excellent agreement with theoretical predictions.

The third study considers further maser species that are associated with star formation, which include the 12.2 GHz methanol maser, the 22.2 GHz water maser, and the four base transitions of the hydroxyl maser (1612, 1665, 1667 and 1720 MHz). The clumps associated with these species are found to be similar in nature to clumps coincident with the 6.7 GHz methanol maser. The timescales of these maser species are derived, in the same fashion as the 6.7 GHz methanol maser, and it is found that they all occupy the same phase of evolution, as shown by the L_bol/M_fwhm ratios (~ 10^0.6 and 10^2.5 L_sun/M_sun^-1). Statistical lifetimes are also calculated for each species and these are found to be ~ 2.0, 1.6, 0.5 x 10^4 yrs for the 12.2 GHz methanol, water and hydroxyl masers, respectively. The methanol maser lifetimes are found to be consistent with previous works, although the derived water and hydroxyl lifetimes are considerably shorter.

Overall, the work presented herein supports previous studies in constructing evolutionary sequences based on interstellar molecular emission. The spectral line emissions considered throughout are useful in showing that the physical properties of dense dust clumps across the Galactic plane are ingrained at their formation, and that maser emission of any type is present at the same period during star formation, casting doubt onthe previously accepted "straw man" model that proposes that masers can be used as a framework for the evolutionary sequence of massive star formation.