Probing the Early Stages of Low‐Mass Star Formation in LDN 1689N: Dust and Water in IRAS 16293−2422A, B, and E

Abstract

We present deep images of dust continuum emission at 450, 800, and 850 m of the dark cloud LDN 1689N, which harbors the low-mass young stellar objects (YSOs) IRAS 16293À2422 A and B (I16293A and I16293B) and the cold prestellar object I16293E. Toward the positions of I16293A and I16293E we also obtained spectra of CO-isotopomers and deep submillimeter observations of chemically related molecules with high critical densities (HCO + , H 13 CO + , DCO + , H 2 O, HDO, and H 2 D + ). Toward I16293A we report the detection of the HDO 1 01 -0 00 and H 2 O 1 10 -1 01 ground-state transitions as broad self-reversed emission profiles with narrow absorption and a tentative detection of H 2 D + 1 10 -1 11 . Toward I16293E we detect weak emission of subthermally excited HDO 1 01 -0 00 . Based on this set of submillimeter continuum and line data, we model the envelopes around I16293A and I16293E. The density and velocity structure of I16293A is fitted by an inside-out collapse model, yielding a sound speed of a ¼ 0:7 km s À1 , an age of t ¼ (0:6 2:5) Â 10 4 yr, and a mass of 6.1 M . The density in the envelope of I16293E is fitted by a radial power law with index À1:0 AE 0:2, a mass of 4.4 M , and a constant temperature of 16 K. These respective models are used to study the chemistry of the envelopes of these pre-and protostellar objects. We made a large, fully sampled CO J ¼ 2 1 map of LDN 1689N, which clearly shows the two outflows from I16293A and I16293B and the interaction of one of the flows with I16293E. An outflow from I16293E reported elsewhere is not confirmed. Instead, we find that the motions around I16293E identified from small maps are part of a larger scale fossil flow from I16293B. Modeling of the I16293A outflow shows that the broad HDO, water ground state, and CO J ¼ 6 5 and 7-6 emission lines originate in this flow, while the HDO and H 2 O line cores originate in the envelope. The narrow absorption feature in the ground-state water lines is due to cold gas in the outer envelope. The derived H 2 O abundance is 3 Â 10 À9 in the cold regions of the envelope of I16293A (T kin < 14 K), 2 Â 10 À7 in warmer regions of the envelope (>14 K), and 10 À8 in the outflow. The HDO abundance is constant at a few times 10 À10 throughout the envelopes of I16293A and I16293E. Because the derived H 2 O and HDO abundances in the two objects can be understood through shock chemistry in the outflow and ion-molecule chemistry in the envelopes, we argue that both objects are related in chemical evolution. The [HDO]/[H 2 O] abundance ratio in the warm inner envelope of I16293A of a few times 10 À4 is comparable to that measured in comets. This supports the idea that the [HDO]/[H 2 O] ratio is determined in the cold prestellar core phase and conserved throughout the formation process of low-mass stars and planets.

A study of deuterated water in the low-mass protostar IRAS 16293-2422

“…The HDO 1 0,1 −0 0,0 fundamental transition at 464.924 GHz was previously observed by Stark et al (2004) Parise et al (2005) on the IRAS 16293 B source. The JCMT beam at this frequency is about 11 .…”

“…It is constituted of two cores IRAS 16293A and IRAS 16293B separated by ∼5 , and the source IRAS 16293A itself could be a binary system (Wootten 1989). Several outflows have also been detected in this source Stark et al 2004;Chandler et al 2005;Yeh et al 2008). This Class 0 protostar is the first source where a hot corino has been discovered (Ceccarelli et al 2000b;Cazaux et al 2003;Bottinelli et al 2004).…”

“…More recently, Demyk et al (2010) have determined a methyl formate deuterium fractionation of ∼15%, and Bacmann et al (2010) have concluded that there is a ND/NH ratio between 30% and 100%. Singly deuterated water in IRAS 16293 has been studied with groundbased telescopes by Stark et al (2004) and Parise et al (2005). The former find a constant abundance of 3 × 10 −10 throughout the envelope with the JCMT observation of the HDO 1 0,1 −0 0,0 fundamental line at 465 GHz alone, whereas the latter obtain an inner abundance (where T ≥ 100 K) X in = 1 × 10 −7 and an outer abundance X out ≤ 1 × 10 −9 using four transitions observed with the IRAM 1 -30 m telescope, as well as a JCMT 2 observation at 465 GHz.…”

Deuterium chemistry of dense gas in the vicinity of low-mass and massive star-forming regions

“…Stark et al 1999Stark et al , 2004Pillai et al 2012), DCN (e.g. Wilson et al 1973;van Dishoeck et al 1995), HDO (Henkel et al 1987;van Dishoeck et al 1995), and DCO + (Penzias 1979;Butner & Loren 1988).…”

Interferometric Studies of Low-Mass Protostars

“…Determining exactly when this ratio is established and how it varies, is an important for understanding, e.g., whether water undergoes significant processing in warm regions of protostellar envelopes or disks. Multi-transition HDO and H 2 O single-dish observations from ground-and space-based observations have previously been used to estimate the HDO/H 2 O abundance ratios in low-mass protostellar envelopes (e.g., Stark et al 2004;Parise et al 2005;Liu et al 2011). Despite the advantages offered by the ranges of excitation conditions probed by the multi-transitions observations, these studies are still complicated by the relatively large beam sizes of the ground-based single-dish and space observatories and are therefore strongly dependent on exact line radiative transfer models utilized for the interpretation.…”