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Chemical Substructure of High Mass Star Forming Regions
High mass stars (stars with mass M > 8 Msolar) are one of the most fundamental building blocks in the Universe. Deeply embedded in the dense clouds at further distances than their low mass counterparts, the forming processes of these fast evolving objects are still unclear. In the earliest phases of the high mass star forming regions (HMSFRs), many complicated astrophysical processes, such as fragmentation, accretion, inflows and outflows are coexistent that dynamic studies are not enough to understand all the mysteries. Therefore, chemistry has developed into a powerful tool in probing the nature of them. With the aim of understanding the chemical and physical processes in the very beginning of high mass star formation, I selected a series of HMSFRs at different evolutionary stages, and studied their chemical-physical properties via high spatial resolution observations at (sub)mm wavelengths. The results can be summarised shortly as follows: 1. At a spatial resolution of < 1500 AU, fragmentation process is observed in the continuum maps of all the resolved sources. Above all, the fragments in 4 starless clumps are on average more massive (with M > 10 Msolar) than the Jeans mass of the large-scale clump, indicating that thermal motions is not the dominant support to against collapse, and high mass stars may form in a “scaled up” version similar to the low mass stars. 2. Observations at a spatial resolution of 1 000 AU resolve NGC 7538 S into at least 3 fragments, having comparable sizes and masses derived from continuum emissions. However, these fragments exhibit distinguishing spectral line emissions at 1.3 mm wavelength, revealing different evolutionary stages. Combing with a 1-D gas-grain model fitting, for the first time, this project suggests that these fragments may result from different warm-up paces after synchronised fragmentation, and that the warm-up processes from one stage to another is rapid. 3. Chemical variations at small scales may be caused by the evolutionary stage diversity of fragments, but may also come from chemical difference of molecular species. With the first complimentary data obtained from both interferometric and single-dish telescopes at 1.3 mm, I analysed the continuum and spectral line features at a spatial resolution of 1 200 AU in Orion-KL. From the central warmer condensations to the cooler outflow regions, gas temperatures and densities differ, leading to spatial distribution inhomogeneity and abundance diversity of nitrogen (N-) bearing, sulfur (S-) bearing and oxygen (O-) bearing molecules. 4. Even at a spatial resolution of 1 000 AU, NGC 7538 IRS1 remains unresolved. At 1.3 mm, this core has a unique spectrum at the continuum peak: a majority of the lines exhibit absorption feature, while at least 3 lines from CH3OH and HCOOCH3 exhibit strong, pure emission. I proposed several hypotheses, attempting to interpret this by source geometry and/or line excitation states. 5. Complex organic molecules (COMs) are ubiquitously detected in the hot molecular cores (HMCs). Thanks to the high sensitivity observations, I identified several low abundant COMs demonstrating complicated chemistry in my sources. 6. In addition to the fragmentation process, the high mass starless cores are not completely chemically quiescent. None line detection in 4 infrared dark clouds at 1.1 mm may indicate these regions are in the early, cold evolutionary stage. However, detection of several dense gas tracers (> 5rms) especially at 3 mm, implies the presence of active gas phase chemistry. From our study, chemistry is an efficient tool in diagnosing evolutionary events coexistent at the small scales of HMSFRs.
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520 Astronomie
520 Astronomy and allied sciences
530 Physik
530 Physics
540 Chemie
540 Chemistry and allied sciences