Physics
Scientific paper
Sep 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........2v&link_type=abstract
PhD Thesis, Sterrewacht Leiden, The Netherlands, 2000.
Physics
4
Scientific paper
This thesis studies the physical and chemical structure of a set of massive young stars which are surrounded by a thick envelope of dust and gas, the earliest known phase of massive star formation. The primary scientific questions addressed are: (i) What is the evolutionary order of the phenomena associated with massive star formation? (ii) What is the physical and chemical structure of the envelopes of massive young stars? How do they compare to those of low-mass stars? Do specific molecules trace different stages? (iii) What are the masses of any circumstellar disks, and on what time scales are they dispersed? To answer these questions, a sample of infrared and submillimeter sources has been selected on high luminosity, close distance, isolated location and high mid-infrared flux. We present observations of these sources with single-dish submillimeter antennas, millimeter interferometers and near-infrared spectroscopy, and also discuss ISO spectra. For the interpretation, we have developed models with a detailed physical structure, combined with chemical differentiation, which is strongly coupled to the temperature. Some of the conclusions are: The envelopes of massive young stars are well described by centrally heated spherical models, with masses of ~ 100-1000 Modot within radii of ~0.1 pc. For a power-law density structure n(r) = n0 (r / r0)-α, we find α = 1.0-1.5 for the younger sources, significantly lower than α ≅ 2 found for the envelopes of low-mass stars at a comparable stage of evolution. This difference may indicate that the support against gravitational collapse in high-mass cores is by nonthermal (e.g., turbulent) pressure, and in low-mass cores by thermal pressure. For the more evolved sources, α = 1.5-2.0 fits the data best. Unlike in low-mass star formation, the near-infrared emission decreases as the envelope warms up, indicates that the hot dust close to the star is destroyed and/or pushed out by stellar radiation or mass loss. The evolutionary order of phenomena associated with massive star formation appears to be: embedded infrared source to hot core to (ultra-) compact H II region. In the first two stages, CH3OH and H2O masers occur, which are replaced by OH masers in the third. Chemically, the low deuterium fractionation and total abundances of H2CO and CH3OH argue for a short duration of the cold (≅ 10 K) pre-stellar phase. The fractional ionization of the envelopes of massive young stars is consistent with cosmic-ray ionization at an average rate of ζCR = (2.6 +/- 1.8) × 10-17 s-1. Variations in ζCR are a factor ≅ 2 on a ~kpc scale, in good agreement with γ-ray data. Ionization by local X-rays and shielding against cosmic rays appear unimportant. The differences in ζCR for a given source derived using H3+ or H13CO+, and the correlation of the H3+ column density with distance suggest that significant H3+ absorption occurs in intervening translucent clouds. The abundances of several molecules are found to increase with increasing temperature. Four types of species can be distinguished: (i) `passive' molecules, which are formed in the gas phase, freeze out onto grains during the cold collapse phase and are released during warm-up without chemical modification (e.g. CO); (ii) molecules which form by reactions on or inside the ice layer, and subsequently evaporate (e.g. CH3OH); (iii) molecules formed by gas-phase reactions with evaporated species (e.g. CH3OCH3); (iv) molecules which are formed in the hot gas by high-temperature reactions (e.g. HCN). In 5 out of 5 sources, compact (radius < 300 AU) dust emission is detected with the OVRO interferometer, which resides in a dense shell or in a disk. The mass of this component is not well constrained because of optical depth effects, but it may be as high as 10 Modot, which is comparable to the stellar mass.
No associations
LandOfFree
The Embedded Phase of Massive Star Formation does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The Embedded Phase of Massive Star Formation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The Embedded Phase of Massive Star Formation will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-764515