Other
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........16s&link_type=abstract
Thesis (PhD). UNIVERSITY OF COLORADO AT BOULDER, Source DAI-B 61/04, p. 1992, Oct 2000, 164 pages.
Other
Scientific paper
In this thesis I present case studies of CO absorption for a diverse set of sightlines. The rotational- vibrational transitions at 2.3 & 4.6 μm are used to assess the CO abundance and excitation conditions on each line of sight. The depletion of CO onto dust grains (in the form of ices) is evaluated by directly comparing gas-phase absorption with the solid-state CO band at 4.67 μm. The objects and sightlines sampled include: W40 IRS 1a. CO and C2 absorption are compared toward the embedded OB star W40 IRS 1a. The excitation of both the CO and C2 indicate that the material is not dense but translucent, which is surprising since the visual extinction on the line of sight is AV ~ 9 magnitudes. No CO ice was detected, which confirms this conclusion. HD 29647. HD 29647 is a B star which lies behind the Taurus Molecular Clouds (AV = 3.7). The 4.6 μm gas-phase CO lines presented here could not constrain the conditions on the line of sight better than existing mm-wave CO measures because of saturation problems. As expected for low extinction sightlines, CO ices were not detected toward HD 29647. Ophiuchus: Elias 25. New observations of solid-state CO toward the embedded B star Elias 25 presented in this thesis imply that CO ices can form in the ρ Oph clouds (and presumably elsewhere) at much lower AV than previously thought. Comparisons to other ice data for ρ Oph suggests that ice growth and formation thresholds may indeed depend on the type of sightline observed. The data for the OB star sightlines may represent general ice growth trends for ρ Oph, unperturbed by star-formation, but further sensitive observations toward OB stars (especially of the CO ice band) are required to test this hypothesis. Mon R2 IRS 2. Gas-phase CO absorption at 4.6 μm toward the high-mass young stellar object (YSO) Mon R2 IRS 2 indicates T kin = 50-65K, nH >~ 5 × 1014cm-3, and no high velocity outflow (in the line of sight). The ratio of gas-phase CO to solid-state CO is 0.8 +/- 0.5, which is low compared to many other high-mass YSOs. Taurus: Elias 18. The gas-phase CO lines at 4.6 μm toward Elias 18 are very weak. In contrast, the CO solid-state feature is very strong, yielding a CO gas to ice ratio of ~1/30. It is argued that Elias 18 is in fact a low-mass YSO (Class I/II) with a circumstellar disk viewed very nearly edge-on. This high depletion result suggests that CO in the outer portion of protostellar disks is in the form of ices. In addition to the case studies, the overall abundance of CO in the interstellar medium is discussed for diffuse, translucent, and dense clouds. The abundance of CO in the circumstellar environs of YSOs (both disks and envelopes) is also addressed. The ratio of CO gas to ice is postulated to be an indicator of high-mass YSO evolutionary state and current data are shown which loosely confirm this idea. The depletion of CO onto grains in circumstellar YSO disks is also discussed in light of the results presented for Elias 18.
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