Physics
Scientific paper
Mar 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........14b&link_type=abstract
Ph.D. Thesis, Groningen University, (1999)
Physics
9
Ism: Dust, Extinction, Ism: Molecules, Ism: Abundances, Infrared: Ism: Lines And Bands, Stars:Formation, Galaxy: Abundances
Scientific paper
One may expect that the chemical and physical evolution of dust, gas, and ice in protostellar envelopes follows the evolution of protostars. During the cloud collapse, simple molecules are efficiently formed on grain surfaces, creating ice mantles. The shocks created by the protostellar outflow may sputter the ice mantles into the gas phase, while also species with high reaction barriers may be formed in the gas. Heating close to the protostar evaporates the ice mantles. Evaporated ices can drive a rich and varied chemistry in the hot cores, leading to the formation of more complex species. Furthermore, the UV field near massive stars, or bombardment by cosmic rays, may initiate reactions within ice mantles, leading to refractory dust material. Finally, the icy grains that have survived in the circumstellar disk, may be the building blocks of comets, which may have delivered the volatile reservoir to the terrestrial planets. Many of the fundamental questions in our current picture of interstellar chemistry, and the interaction with the protostar are yet unanswered. What is the composition of the gas and ice mantles? How important are the various processes for molecule formation, and destruction? Are these processes the same for high and low mass objects? How does the protostellar disk evolve to planets and comets? In this thesis, observations of the Short Wavelength Spectrometer on board of the Infrared Space Observatory are analyzed, addressing many of the key questions. The detection of solid and gaseous CH4 are reported. The profile of the ice band shows that CH4 is embedded in a mantle of polar molecules. The low abundance, and low gas/solid ratio indicate that CH4 is formed on grain surfaces at low C/CO ratios. The detected warm CH4 gas originates from out-gassing in the hot core. The importance of grain surface chemistry is sustained by the dominance of simple molecules in molecular clouds and star forming regions. For the first time, solid 12CO2 and 13CO2 band profiles are studied toward a large sample of high and low mass protostars in various stages of their evolution. A great variety of absorption band profiles is seen, which appears to be the result of thermal processing. In particular, the massive hot core sources show a sequence of increasing heating time. However, compact H II regions show less sign of thermal processing, and thus there may be a fundamental difference in the way these sources have evolved. The carbon isotope ratio is derived from solid 12CO2 and 13CO2 column densities. A value of 69+/-15 derived in the local ISM. A tentative gradient with galacto-centric radius is observed, in agreement with gas phase studies (CO, H2CO). The CO2 isotope ratio tends to be higher compared to CO, which would have important consequences for the origin of interstellar CO2. A complete 1.6-190 um spectrum is presented for the low mass protostar Elias 29 in the Rho Ophiuchi molecular cloud. It contains a wealth of information, which is put in a general picture of the structure of this object. Hot CO and H2O gas is detected, which we derive to be present close to the central heat source, on a scale of a circumstellar disk. Given the large abundance of hot gas, Elias 29 resembles the more evolved massive hot core sources. However, in contrast, the detected ice features (CO, H2O, "6.8 um", CO2) show no sign of processing, and may be present in a disk or in the foreground cloud. Given the high extinction of Elias 29, the SED is surprisingly flat, and resembles that of the Herbig Ae star AB Aur, which is known to have a circumstellar disk. Broad H I emission lines, perhaps originating from accretion shocks, and extended CO J=6-5 line emission, revealing a dense outflow, are also reported. Finally, laboratory experiments of CH4, 12CO2, and 13CO2 in a large variety of ices and at a range of temperatures are presented in a way that facilitates comparison with observed interstellar features. Also, it is shown that the CO and CO2 optical constants available in the literature are inconsistent. Using Mie scattering calculations in the Rayleigh limit, it is shown that this translates in a large uncertainty in calculated cross sections for various grain shapes.
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