Physics
Scientific paper
Mar 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt..........b&link_type=abstract
PhD Thesis, Rheinische Friedrich-Wilhelms-Universitaet Bonn, 2002
Physics
2
Scientific paper
The formation processes of high-mass stars are not well understood, and the basic question can be phrased as: Do massive stars form similarly to low-mass stars, but with enhanced accretion rates, or are different processes taking place, e.g., the coalescence and merging of intermediate-mass protostars in the very center of star-forming clusters? This thesis investigates the earliest known stages of massive star formation and studies many characteristic parameters to set constraints on the associated physical processes. The analysis and results of my work strongly support the accretion hypothesis and may be summarized as follows: Massive stars can form via accretion processes that are qualitatively similar and quantitatively enhanced compared to the low-mass case. Coalescence of protostars may occur in some sources, but our data indicate that merging of protostars is not the dominating process for high-mass star formation. Because the inner few AU of a star-forming cluster, where the accretion and/or coalescence processes take place, are difficult to resolve with current observational techniques at typical source distances of several kpc, indirect evidence has to be found to answer the initial question. A sample of 69 high-mass protostellar candidates was selected and studied first statistically from the cm and mm regime to near-infrared wavelengths. The analysis revealed that the chosen sample selection criteria very effectively selected a large number of luminous and massive sources at very early evolutionary stages prior to forming a significant ultracompact HII region. Most likely, these sources produce a large fraction of their luminosity by accretion. A detailed analysis of the intensity and density distributions of the sample from 1.2 mm dust continuum emission maps with 11'' spatial resolution shows that the single-dish radial profiles are not well fitted by single power-law distributions, but that they steepen towards the outside and flatten towards the center. While we interpret the steepening to the outside as a signature of the finite sizes of the star-forming cores, the inner flattening indicates fragmentation of the massive cores into a number of sub-sources. The latter is also observed in high-resolution interferometric data of some of the regions. Additionally, the inner power-law density distributions do not show strong deviations from density distributions of low-mass star-forming cores, which indicates that the initial conditions of low- and high-mass star formation are not very different. As massive molecular outflows on large scales provide insights into the star formation process at the center of the regions, we mapped the outflows of a sub-sample in the CO(2-1) line. The data, observed at higher spatial resolution (11'') than previous studies, reveal that massive bipolar and collimated outflows are as ubiquitous phenomena in high-mass star-forming regions as is also true for their low-mass counterparts. Such collimated outflows are most likely produced by star--disk interactions, and hence, massive stars should have disks as well. The observations presented in this thesis reveal accretion rate estimates in the high-mass regime around 10^-4 Msun/yr, rising as high as 10-3 Msun/yr. Such accretion rates should be sufficiently high to overcome the radiative pressure of the central protostar, and accretion can continue to form massive stars. Additionally, we find that the accretion rate of the most massive object in a core scales roughly linearly with the core mass. Furthermore, a high-resolution interferometric case study of one of our sample sources shows that its outflow, which was already known from single-dish observations, actually splits up into at least three bipolar outflows. One of this outflows is the most collimated massive outflow ever observed in molecular gas. These results give strong support to the hypothesis that massive stars form in analogous fashion to low-mass stars; they merely differ in having higher accretion rates, core masses, luminosities and outflow masses. A study of CH3OH and H2O masers confirms that both are good signposts of massive star formation in very early evolutionary stages. Furthermore, the data suggest that CH3OH and H2O masers need a similar environment (dense and warm molecular gas), but that, due to the different excitation processes (radiative pumping for CH3OH and collisional pumping for H2O), no spatial correlation exists. Kinematic maser features are shown to be not conclusive in many cases. Furthermore, high-energy processes were investigated by X-ray observations with the new satellite telescope CHANDRA, and a number of point sources in the hard X-ray band are detected. A comparison with infrared and mm data shows that the X-ray emission does not stem from the most deeply embedded and likely most massive central object, but from embedded nearby sources, some of them very likely intermediate-mass Herbig Ae/Be stars or precursors of them. The data are not sensitive enough to conclude whether the central object is simply not an X-ray source, or whether its emission from a central object is absorbed by the high gas column densities in the surrounding envelope. These X-ray observations mark the beginning of a new field of research, namely the understanding of the production mechanisms of high-energy photons emitted in the X-ray regime at early evolutionary stages of intermediate- and high-mass star formation.
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