Detection of interstellar ortho-D2H+ with SOFIA

TL;DR

This paper reports the first detection of ortho-D2H+ in space using SOFIA, providing insights into deuterium chemistry in cold protostellar environments and estimating the core's age through molecular analysis.

Contribution

It presents the first detection of ortho-D2H+ at 1.477 THz with SOFIA and combines this with previous data to model core chemistry and estimate its age.

Findings

Ortho-D2H+ detected in absorption against protostellar continuum.

Core's outer envelope is cold (<18 K) and chemically enriched in D2H+ and H2D+.

Estimated core age is approximately 500,000 years.

Abstract

We report on the detection of the ground-state rotational line of ortho-D2H+ at 1.477 THz (203 micron) using the German REceiver for Astronomy at Terahertz frequencies (GREAT) onboard the Stratospheric Observatory For Infrared Astronomy (SOFIA). The line is seen in absorption against far-infrared continuum from the protostellar binary IRAS 16293-2422 in Ophiuchus. The para-D2H+ line at 691.7 GHz was not detected with the APEX telescope toward this position. These D2H+ observations complement our previous detections of para-H2D+ and ortho-H2D+ using SOFIA and APEX. By modeling chemistry and radiative transfer in the dense core surrounding the protostars, we find that the ortho-D2H+ and para-H2D+ absorption features mainly originate in the cool (T<18 K) outer envelope of the core. In contrast, the ortho-H2D+ emission from the core is significantly absorbed by the ambient molecular cloud. Analyses of the combined D2H+ and H2D+ data result in an age estimate of ~500 000 yr for the core, with an uncertainty of ~200 000 yr. The core material has probably been pre-processed for another 500 000 years in conditions corresponding to those in the ambient molecular cloud. The inferred time scale is more than ten times the age of the embedded protobinary. The D2H+ and H2D+ ions have large and nearly equal total (ortho+para) fractional abundances of ~$10^{-9}$ in the outer envelope. This confirms the central role of H3+ in the deuterium chemistry in cool, dense gas, and adds support to the prediction of chemistry models that also D3+ should be abundant in these conditions.

Tables

Table 1. Table 1: Ground-state rotational transitions of ortho and para H 2 ​ D + subscript H 2 superscript D \rm H_{2}D^{+} and D 2 ​ H + subscript D 2 superscript H \rm D_{2}H^{+} . Columns 4 and 5 give the upper state energies and the Einstein coefficients for spontaneous emission.

Species	Transition	Frequency	$E_{\mathrm{upper}}$	$A_{\mathrm{ul}}$	Reference
	($J_{K_a{K}_c}^u-J_{K_a{K}_c}^l$)	(GHz)	(K)	(s-1)
para-$H_2{D}^{+}$	$1_{01}-0_{00}$	1370.1	65.8	$4.04\times {10}^{-3}$	a,c
ortho-$H_2{D}^{+}$	$1_{10}-1_{11}$	372.4	104.3	$1.22\times {10}^{-4}$	b,c
ortho-$D_2{H}^{+}$	$1_{11}-0_{00}$	1476.6	70.9	$3.30\times {10}^{-3}$	a,c
para-$D_2{H}^{+}$	$1_{10}-1_{01}$	691.7	83.4	$5.09\times {10}^{-4}$	b,c

Table 2. Table 2: Fractional abundances according to the models producing the spectra shown in Figure  10 . For the homogeneous ambient cloud model, abundances in the outer envelope R = 2500 − 6900 𝑅 2500 6900 R=2500-6900 AU and the ambient dark cloud are given separately. For the extended Crimier et al. model, the abundances are averages over the range R = 2500 − 10000 𝑅 2500 10000 R=2500-10000 AU. Columns 4, 5, and 7 give abundances derived from the observed lines using the two physical models and assuming constant fractional abundances in the cloud components where the observed signal is likely to arise from.

Species	Crimier + ambient dark cloud				Extended Crimier
	Chemistry model		Constant X		Chemistry model	Constant X
	Envelope	Ambient	Envelope	Ambient
(1)	(2)	(3)	(4)	(5)	(6)	(7)
ortho-$D_2{H}^{+}$	$1.6\times {10}^{-9}$	$1.1\times {10}^{-9}$	$\left(1.4\pm 0.4\right)\times {10}^{-9}$	$\left(1.0\pm 0.3\right)\times {10}^{-9}$	$1.5\times {10}^{-9}$	$\left(1.7\pm 0.3\right)\times {10}^{-9}$
para-$D_2{H}^{+}$	$1.8\times {10}^{-10}$	$1.9\times {10}^{-10}$			$2.1\times {10}^{-10}$
para-$H_2{D}^{+}$	$1.6\times {10}^{-9}$	$8.6\times {10}^{-10}$	$\left(1.9\pm 0.7\right)\times {10}^{-9}$	$\left(1.0\pm 0.4\right)\times {10}^{-9}$	$1.7\times {10}^{-9}$	$\left(1.6\pm 0.4\right)\times {10}^{-9}$
ortho-$H_2{D}^{+}$	$1.1\times {10}^{-10}$	$1.0\times {10}^{-9}$	$\left(1.0\pm 0.1\right)\times {10}^{-10}$	$\left(1.3\pm 0.4\right)\times {10}^{-9}$	$1.6\times {10}^{-10}$	$\left(7.5\pm 0.6\right)\times {10}^{-11}$

Equations

$$\mathrm{p}{\mathrm{H}}_2{\mathrm{D}}^{+}+\mathrm{o}{\mathrm{H}}_2\underset{{\mathrm{k}}_1^{-}}{\overset{{\mathrm{k}}_1^{+}}{\rightleftharpoons }}\mathrm{o}{\mathrm{H}}_2{\mathrm{D}}^{+}+\mathrm{p}{\mathrm{H}}_2.$$

$$K_1(T)\equiv \frac{k_1^{+}}{k_1^{-}}=\frac{Q_{\mathrm{o}{\mathrm{H}}_2{\mathrm{D}}^{+}}(T)}{Q_{\mathrm{p}{\mathrm{H}}_2{\mathrm{D}}^{+}}(T)}\frac{Q_{\mathrm{p}{\mathrm{H}}_2}(T)}{Q_{\mathrm{o}{\mathrm{H}}_2}(T)},$$

$${\mathrm{H}}_2{\mathrm{D}}^{+}+\mathrm{HD}$$

$${\mathrm{D}}_2{\mathrm{H}}^{+}+\mathrm{HD}$$

Full text

astrochemistry — ISM: molecules — ISM: individual objects (IRAS 16293-2422) — stars: formation

1 Introduction

The deuterated variants of the $\rm H_{3}^{+}$ ion are believed to be the most important agents of deuterium fractionation in cool ($T<20$ K) interstellar clouds (Millar et al., 1989). Deuteration driven by $\rm H_{3}^{+}$ should be particularly efficient in cold, dense cores. Firstly, substitution of H by D is favored in cold gas because it lowers the zero-point vibrational energy of a molecule. Secondly, $\rm H_{3}^{+}$ and its isotopologs are predicted to be enhanced greatly in conditions where common molecules like CO and N2 freeze onto dust grains (Dalgarno & Lepp 1984; Caselli et al. 2003; Roberts et al. 2003; Walmsley et al. 2004; Roberts et al. 2004). These chemistry models predict that gas with a large degree of CO depletion can have abundances of $\rm H_{2}D^{+}$ and $\rm D_{2}H^{+}$ similar to that of $\rm H_{3}^{+}$, and that $\rm D_{3}^{+}$ can become the most abundant molecular ion. The detection of multiply deuterated species in dense cores supports strongly these ideas (Millar, 2005). On the other hand, it has proven difficult to determine observationally the total abundances of deuterated isotopologs of $\rm H_{3}^{+}$.