High Deuterium Fractionation Research Articles

Laboratory Microwave Spectroscopy of HDCS and Its Astronomical Detection toward TMC-1

The rotational spectrum of the deuterated species of thioformaldehyde, HDCS, has been measured in the laboratory by microwave spectroscopy. Fifty-two a-type R-branch transitions are observed, and the rotational constants and centrifugal distortion constants are determined accurately. On the basis of the laboratory results, two unidentified lines detected during the line survey toward a cold dark cloud TMC-1 are assigned to HDCS. The [HDCS]/[H2CS] ratio is determined to be 0.020 ± 0.005, indicating relatively high deuterium fractionation in H2CS. The ratio is discussed in terms of formation processes of H2CS in dark clouds.

Molecular Abundances and Low-Mass Star Formation. II. Organic and Deuterated Species toward IRAS 16293-2422

view Abstract Citations (305) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Abundances and Low-Mass Star Formation. II. Organic and Deuterated Species toward IRAS 16293-2422 van Dishoeck, Ewine F. ; Blake, Geoffrey A. ; Jansen, David J. ; Groesbeck, T. D. Abstract Millimeter and submillimeter line surveys of the protobinary source IRAS 16293-2422 are presented in the 230 and 345 GHz windows. In total, 265 lines belonging to 44 molecules and their isotopomers (24 chemically different species) have been detected. Here the data for organic and deuterated molecules are considered; the results for Si- and S-bearing species have been discussed in an earlier paper (Blake et al. 1994). The observations have been analyzed through a combination of rotation diagrams and full statistical equilibrium/ radiative transfer calculations. At least three physically and chemically different components can be distinguished within the 20" (3000 AU) beam. The first component is the warm (Tkin ≳ 80 K) and dense [n(H2) ≍ (0.5-1) × 107 cm-3] gas sampled by Si- and S-containing molecules such as SiO and SO2. This gas is found to be rich in organic molecules such as CH3OH, CH3CN, and H2CO, as well. It is only 3"-10" in size (500-1500 AU) and most likely represents the interaction of the bipolar outflow(s) with the circumbinary envelope. The second component is the circumbinary envelope itself, which has Tkin 40 K and n(H2) 106 - 107 cm-3 and is 10"-15" in size (2000 AU). It contains common molecules like CS, HCO+, and H2CO. The third component is the colder, lower density outer part of the envelope, which gradually fades into the ambient surrounding cloud core [Tkin ≍ 10-20 K; n(H2) (3 × 104)-(2 × 105) cm-3]. Radicals such as CN, C2H, and C3H2 appear to reside principally in this region. The ambient cloud material is also probed through self-absorbed features in the line profiles of molecules such as HCN, HCO+, and H2CO. Beam-averaged abundances over a 20" scale are presented and are compared with those observed in cold dark clouds such as L134N and TMC-1 and with high-mass star-forming cores such as Orion-KL. Remarkably high deuterium fractionation ratios are found, which are more characteristic of hot core regions than of cold, quiescent clouds. As a whole, the chemical and physical regimes that can be distinguished in the low-mass young stellar object IRAS 16293-2422 are qualitatively similar to those found in high-mass star-forming regions, confirming the earlier conclusion that the chemical composition depends more on the age of the object than its mass. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175915 Bibcode: 1995ApJ...447..760V Keywords: ISM: MOLECULES; STARS: CIRCUMSTELLAR MATTER; STARS: INDIVIDUAL ALPHANUMERIC: IRAS 16293-2422; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (10) Related Materials (1) Part 1: 1994ApJ...428..680B

Models of the gas-grain interaction - deuterium chemistry

We have extended chemical models of a dense quiescent cloud, including accretion, and a hot molecular core to include deuterium chemistry, both in the gas phase and on the surface of dust grains. For the dense cloud model we find that as accretion begins to dominate the chemistry, and atoms and molecules leave the gas phase, the destruction rate of many radicals is reduced. As a result their abundances increase sharply before decreasing as they too accrete on to the dust. This is particularly the case for deuterated species. We suggest that if observations of a cloud, such as TMC-1, reveal a high abundance of OD, for example, this would indicate that accretion was dominating the chemistry in that region. Further analysis may lead to some conclusions concerning the present discussion about the exact mode of dissociative recombination. In the model of a hot molecular core we are able to explain the enhanced abundances of NH2D and HDO, and the high deuterium fractionation observed by including grain surfaces reactions involving deuterium atoms.