The chemical composition of disks around pre-main sequence stars is an active topic of research, not only because disks provide the material from which future planetary systems are made, but also because molecules are excellent probes of the physical processes in disks. Molecules other than CO are only just starting to be revealed in disks (e.g., Dutrey et al. 1997, A&A 317, L55; Kastner et al. 1997, Science 277, 67; Qi 2001, PhD thesis Caltech). Most of these studies focus on the lower rotational transitions of the molecules. In late 1998, our group started a JCMT program using the new dual polarization receiver B3 to search for high frequency transitions. In disks, the densities are sufficiently high (10⁶-10⁸ cm^-3) that the higher-J transitions are at least as strong as the lower-J lines. Moreover, the beam is smaller at high frequencies, reducing the beam dilution of the emission.

During our first run, we succeeded in detecting several lines at the level of T_A^*~0.05-0.2 K in about 2 hours of telescope time each, testifying to the excellent performance of receiver B3. Previous studies on other telescopes often required almost an entire night of integration per line. We subsequently pursued the project looking for other molecules, the results of which will be published in Thi et al. (2003, submitted).

Of particular interest is the first detection of the DCO⁺ ion in a circumstellar disk (see Figure). The DCO⁺ J=5-4 line at 360.169 GHz was detected, together with the HCO⁺ and H¹³CO⁺ J=4-3 lines at 356.734 and 346.998 GHz, in the disk around the isolated T Tauri star TW Hya. This is one of the nearest T Tauri stars at a distance of only 56 pc and has an age of 7-15 Myr, somewhat older than most classical T Tauri stars. The disk has a (gas + dust) mass of ~0.02 M_Sun and a size of ~200 AU, corresponding to a 3.6'' radius at 56 pc. Because the disk is seen nearly face-on, the molecular lines do not show the characteristic profiles of a rotating disk but are single peaked.

JCMT detection of the DCO⁺ J=5-4 line, together with the HCO⁺ J=4-3 and H¹³CO⁺ J=4-3 lines, in the disk around the pre-main sequence star TW Hya. The H¹³CO⁺ and DCO⁺ spectra have been shifted by -0.15 and -0.3 K, respectively, for clarity. The near-infrared scattered light HST images by Weinberger et al. (2002, ApJ 566, 409) are shown on the left.

The three lines allow an accurate value of the DCO⁺/HCO⁺ ratio of 0.035+-0.015 to be determined. This value for the TW Hya disk is close to that found in cold pre-stellar cores but somewhat higher than that measured in the envelope around the low-mass protostar IRAS 16293-2422 which has started to heat its surroundings. It is also close to the DCN/HCN ratio obtained for pristine cometary material in the jet of comet Hale-Bopp (Blake et al. 1999, Nature 398, 213).

Deuterated molecules are excellent probes of the temperature history of interstellar and circumstellar gas. It is well known that the abundances of deuterated molecules in cold regions are enhanced by orders of magnitude over the elemental [D]/[H] abundance ratio of 1.5 x 10^-5 through a process called fractionation: at temperatures below ~50 K, the H₃⁺ + HD -> H₂D⁺ + H₂ reaction is driven strongly in the forward direction. H₂D⁺ can subsequently transfer a deuteron to the abundant CO molecule, leading to enhanced DCO⁺. Moreover, the DCO⁺/HCO⁺ ratio is increased if the main destroyer of H₃⁺, i.e. CO, is depleted onto grains. Thus, the DCO⁺/HCO⁺ ratio traces both low temperatures and the level of depletion. The observed DCO⁺/HCO⁺ of 0.035 is consistent with theoretical models of flaring disks of Aikawa et al. (2002, A&A 386, 622) which consider gas-phase fractionation processes within a realistic 2-D temperature distribution and which include the effects of freeze-out onto grains. Most of the emission is found to arise from an intermediate layer of the disk, where temperatures are 20-30 K and densities at least 10⁶ cm^-3.

The similarity of the deuterium fractionation ratios in cold clouds, disks and pristine cometary material suggests that the gas spends most of its lifetime at low temperatures and is incorporated in disks before the envelope is heated, i.e., before the class I stage. Alternatively, the ratio may be reset in disks by low-temperature gas-phase chemistry. Searches for other deuterated molecules which are likely incorporated as ices (e.g., CH₃OH) can distinguish these scenarios. An exciting future prospect is to probe the D/H ratio down to planet-forming regions with ALMA.

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