Ground State Rotational Lines of Doubly Deuterated Ammonia
Ground State Rotational Lines of Doubly Deuterated Ammonia as Tracers of Physical Conditions and Chemistry of Cold ISM Darek Lis (Caltech) M. Gerin, E. Roueff, T. G. Phillips, C. Vastel Columbus, June 22, 2003
Deuterium Fractionation in Cold, Dense ISM • Initial detection of deuteration in heavy molecules in the ISM was for HCN (Jefferts, Penzias, & Wilson 1973) • Chemical fractionation under lowtemperature conditions (Solomon & Woolf 1973) arising from differences in molecular binding energies (e. g. HD energetically favored over H 2) • Over 20 deuterated molecules detected to date; typical abundances 10 -4– 10 -1 • Amazingly, doubly and triply deuterated species detected ND 3: Lis et al. (2002 a) CD 3 OH: Parise et al. (2004)
Textbook fractionation
Roberts et al. (2003) Phillips & Vastel (2003) D 2 H+: Vastel et al. (2004)
Deuterated Molecules as Tracers of Star Forming Regions • • • Multiply deuterated molecules often found in very young protostars (e. g. IRAS 16293, NGC 1333 IRAS 4 A) or in dense cores prior to star formation (e. g. L 1544, B 1, L 1689 N) Analysis of the line profiles provides information about the gas kinematics at the onset of dynamical collapse Molecular fractionation ratios provide information about the physical conditions (temperature, depletion)
Molecular Depletion • • • Condensation of most stable species onto dust grains leads to systematic molecular differentiation in starless cores (e. g. Tafalla et al. 2002, 2003) Strong drop in abundance of CO and CS toward the center naturally explained by depletion at densities above a few 104 cm-3 ; N-bearing species less affected by the process up to a few 105 cm-3 Recent models (e. g. Walmsley et al. 2004) show that even the N-bearing species should condense onto dust grains at densities in excess of ~106 cm-3 Under such “complete freeze-out” conditions, H 3+ and its deuterated isotopologues would become the best tracers of the molecular gas H 2 D+ difficult to observe; are N-bearing species completely frozen-out?
ND 2 H Energy Levels • …
Wide-band Spectra • Both ortho- and para-ND 2 H 111 – 000 transitions detected with the wideband CSO spectrometer • Ortho/para ratio consistent with the expected LTE value of 2: 1 • Good atmospheric transmission; easily accessible frequency
L 1689 N • Hyperfine splitting clearly detected in both the 111 – 000 and 110 – 000 transitions; simple hyperfine pattern • Direct measure of the optical depth and excitation temperature • Accurate estimate of the ND 2 H column density • Different critical densities (~106 vs. 5 107 cm-3); 389 GHz line only excited in very dense gas • Linewidth much smaller than most molecules; comparable to H 2 D+ (0. 45 vs. 0. 36 kms-1)
Barnard 1 • Source with a larger linewidth – blending of the hyperfine components • Emission weaker compared to L 1689 N • B 1 is a dark core known to harbor some low-luminosity embedded sources • L 1689 N is a shock compressed interaction region between a molecular outflow and an ambient cloud (Lis et al. 2002 b)
L 1689 N • Deuterated species enhanced in the gas impinged upon by the outflow • Shock velocity 8– 10 kms-1; grain mantles evaporated • High deuteration levels may be present in the mantle material; molecules released into gas phase by the shock • Gas-phase chemistry may drive up abundances of deuterated species in the dense post-shock gas, after it cools down Lis et al. (2002 b)
NGC 1333 Lis et al. (2004) • CO outflow directions outlined by white arrows • Bright DCO+ emission near IRAS 4 A, where ND 3 was first detected • Secondary DCO+ peak ~120" north, directly onaxis of the HH 7 -11 outflow • ND 3 emission also detected at this location (Roueff et al. 2005)
L 1689 N and B 1 Results • • Strong 389 GHz o-ND 2 H emission in L 1689 N implies a high fraction of dense gas (5 107 cm-3 critical density); ~25%, compared to ~15% in B 1 Both regions are sites of strong molecular depletion and heavy deuteration However, ammonia isotopologues in these sources are not completely frozen out, even in the high density gas Are these sources special? What happens in more quiescent clouds?
L 1544 • Strong ND 2 H emission detected in both transitions • Linewidths 0. 32/0. 26 kms-1 • For Tk=10 K, the turbulent linewidth of the 389 GHz ND 2 H line is only 0. 20 kms-1; compared to 0. 5 kms-1 for H 2 D+ • ND 2 H emission in L 1544 originates from more quiescent gas than H 2 D+ • ND 2 H emission not tracing the low-density envelope (density 106 – 107 cm-3 in 18”– 5” radius; Caselli et al. 2003, Doty et al. 2005)
Summary • • • Submillimeter, ground state rotational lines of ND 2 H detected in L 1689 N, B 1, and L 1544 Ammonia isotopologues not completely frozen out, even in the high-density gas (L 1544: density in the range where the “complete freeze-out” has been predicted to occur) The two submm ND 2 H transitions have different critical densities and with careful modeling will provide good constrains on the gas kinematics and physical conditions in cold, dense ISM Mapping observations critically needed Interferometric observations with SMA, and subsequently ALMA, will provide new opportunities to test the current chemical models
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