GROUND STATE ROTATIONAL LINES OF DOUBLY DEUTERATED AMMONIA AS TRACERS OF THE PHYSICAL CONDITIONS AND CHEMISTRY OF COLD INTERSTELLAR MEDIUM

摘要

We report the first detection of the N_(K_AK_C) = 1_(11) → 0_(00) and 1_(10) → 0_(00) ground state rotational lines of o-ND_2H at 335.5 and 388.7 GHz, obtained in the Lynds 1689N, Barnard 1, and Lynds 1544 molecular clouds using the Caltech Submillimeter Observatory (CSO). The submillimeter ND_2H lines have moderate opacities and simple hyperfine patterns, which allow accurate determination of the excitation temperature, H_2 volume density, and molecular column density. Both transitions have high critical densities. The 389 GHz line, in particular, traces molecular material with densities above a few x 10~6 cm~(-3). The strong 389 GHz ND_2H emission in LDN 1689N implies a high fraction of dense gas in this source, ～30%, as compared to ～15% in B1 and LDN 1544. All these regions are sites of strong molecular depletion and heavy deuteration. Nonaccreting molecules, H_3~+ and its isotopologues, are difficult to study, but in the sources studied here it appears that ammonia and its isotopologues are not completely frozen out, even in the high density gas. In the well-studied case of LDN 1544, the volume probed by the ND_2H emission has densities of ～10~6-10~7 cm~(-3), within the range where the "complete freezeout" has been predicted to occur. The critical density of the 389 GHz ND_2H line is close to that of the 309 GHz ND_3 line. Observations of these two transitions thus provide an accurate measure of the [ND_3]/[ND_2H] fractionation ratio in the very dense gas. The [ND_3]/[ND_2H] ratio in LDN 1689N (～3%) appears lower than the values measured in B1 and LDN 1544 (～7%-10%), indicating that different chemical processes may be at work in these environments. The submillimeter lines of deuteroammonia are relatively strong and detectable from good sites, such as Mauna Kea or Chajnantor. Interferometric observations of these lines with the Submillimeter Array (SMA), and subsequently the Atacama Large Millimeter Array (ALMA), will provide new opportunities to study the physics and chemistry of cold, dense ISM, where most molecules are depleted onto dust grains.