Abstract
In star formation, water is an essential molecule. It affects the oxygen based chemistry, the physical conditions and energy balance in the protostellar envelope, and it is associated with the emergence of life as we know it. Ground based observations of water are hampered by the high amount of water vapor in Earth’s atmosphere. Many of the lines that are observable from the ground are masing in star forming regions, making it hard to deduce abundances. The few lines that are observable, and shown not be masing are isotopologues, like HDO and D2O, making the estimates of the main isotopologue form of water difficult. A solution to this has been to send up space based observatories, which observe outside of the atmosphere free from disturbances. These observations are, because of the small telescope sizes, of low resolution, making it hard to separate the different source components of the water emission. An alternative is to target H18/2 O that can be observed toward protostars from the ground at high resolution, and is not masing in protostars under the conditions found in typical star forming regions. This isotopologue has the potential of revealing the spatial distribution of water in protostars.
In this thesis observations of the H18/2 O, and HDO isotopologues are presented toward four sources in a small survey conducted with several interferometric telescopes. H18/2 O is compared to the other molecules detected, i.e. CH3OCH3 (dimethyl ether), C2H5CN (ethyl cyanide) and SO2 (sulfur dioxide). The amount of warm water is deduced and its origin is observationally constrained. With both isotopologues observed, the HDO/H2O ratio is deduced. This ratio is then compared to other sources, e.g., comets and the Earth’s ocean, to gain understanding of the origin of the water in our own solar system.
The emission line fluxes are modeled with radiative transfer tools and compared to
other results of water abundances in the same source. The observed water emission, both H18(2 O and HDO is compact for all observed sources and traces the emission on R 150 AU scales or less. In one source the water is seen in absorption, with a inverse P-Cygni spectral profile - an indication of infalling motions. The similar line characteristics of water and the other detected molecules in
the frequency band, together with the absorption toward one source, shows that these water lines are not masing. Under the assumption of local thermal equilibrium (LTE) and a excitation temperature derived from two observed HDO lines toward one source, abundances are derived, and a corresponding HDO/H2O ratio. This method gives a partially model independent estimate of the amount of water deuterium fractionation in protostars. From non-LTE radiative transfer modeling of spherically symmetric models we show that while the HDO line is becoming marginally optically thick, the H18/2 O line is optically thin. The modeling results are useful to assess the validity of LTE and optical thickness. However, for deriving fractional abundances, the LTE approximation and modeling gives results that agree within the uncertainties. The other molecules detected in the observations of the H18/2 O show similar line widths to water. The abundances of dimethyl ether (CH3OCH3) and sulfur dioxide (SO2) in relation to water (H2O) are roughly equal between the individual sources. The ethyl cyanide (C2H5CN) on the other hand, is not. This indicates that similar chemical processes are at work for dimethyl ether, sulfur dioxide and water.
In this thesis observations of the H18/2 O, and HDO isotopologues are presented toward four sources in a small survey conducted with several interferometric telescopes. H18/2 O is compared to the other molecules detected, i.e. CH3OCH3 (dimethyl ether), C2H5CN (ethyl cyanide) and SO2 (sulfur dioxide). The amount of warm water is deduced and its origin is observationally constrained. With both isotopologues observed, the HDO/H2O ratio is deduced. This ratio is then compared to other sources, e.g., comets and the Earth’s ocean, to gain understanding of the origin of the water in our own solar system.
The emission line fluxes are modeled with radiative transfer tools and compared to
other results of water abundances in the same source. The observed water emission, both H18(2 O and HDO is compact for all observed sources and traces the emission on R 150 AU scales or less. In one source the water is seen in absorption, with a inverse P-Cygni spectral profile - an indication of infalling motions. The similar line characteristics of water and the other detected molecules in
the frequency band, together with the absorption toward one source, shows that these water lines are not masing. Under the assumption of local thermal equilibrium (LTE) and a excitation temperature derived from two observed HDO lines toward one source, abundances are derived, and a corresponding HDO/H2O ratio. This method gives a partially model independent estimate of the amount of water deuterium fractionation in protostars. From non-LTE radiative transfer modeling of spherically symmetric models we show that while the HDO line is becoming marginally optically thick, the H18/2 O line is optically thin. The modeling results are useful to assess the validity of LTE and optical thickness. However, for deriving fractional abundances, the LTE approximation and modeling gives results that agree within the uncertainties. The other molecules detected in the observations of the H18/2 O show similar line widths to water. The abundances of dimethyl ether (CH3OCH3) and sulfur dioxide (SO2) in relation to water (H2O) are roughly equal between the individual sources. The ethyl cyanide (C2H5CN) on the other hand, is not. This indicates that similar chemical processes are at work for dimethyl ether, sulfur dioxide and water.
| Original language | English |
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| Publisher | Natural History Museum of Denmark, Faculty of Science, University of Copenhagen |
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| Number of pages | 96 |
| Publication status | Published - 2013 |