Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B

Abstract

Context. The measure of the water deuterium fractionation is a relevant tool for understanding mechanisms of water formation and evolution from the prestellar phase to the formation of planets and comets. Aims. The aim of this paper is to study deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B, to compare their HDO abundance distributions with other star-forming regions, and to constrain their HDO/H 2 O abundance ratios. Methods. Using the Herschel/HIFI instrument as well as ground-based telescopes, we observed several HDO lines covering a large excitation range (E up /k = 22-168 K) towards these protostars and an outflow position. Non-local thermal equilibrium radiative transfer codes were then used to determine the HDO abundance profiles in these sources. Results. The HDO fundamental line profiles show a very broad component, tracing the molecular outflows, in addition to a narrower emission component and a narrow absorbing component. In the protostellar envelope of NGC 1333 IRAS 4A, the HDO inner (T ≥ 100 K) and outer (T < 100 K) abundances with respect to H 2 are estimated with a 3σ uncertainty at 7.5 +3.5 −3.0 × 10 −9 and 1.2 +0.4 −0.4 × 10 −11 , respectively, whereas in NGC 1333 IRAS 4B they are 1 +1.8 −0.9 × 10 −8 and 1.2 +0.6 −0.4 × 10 −10 , respectively. Similarly to the low-mass protostar IRAS 16293-2422, an absorbing outer layer with an enhanced abundance of deuterated water is required to reproduce the absorbing components seen in the fundamental lines at 465 and 894 GHz in both sources. This water-rich layer is probably extended enough to encompass the two sources, as well as parts of the outflows. In the outflows emanating from NGC 1333 IRAS 4A, the HDO column density is estimated at about (2-4) × 10 13 cm −2 , leading to an abundance of about (0.7-1.9) × 10 −9 . An HDO/H 2 O ratio between 7 × 10 −4 and 9 × 10 −2 is also derived in the outflows. In the warm inner regions of these two sources, we estimate the HDO/H 2 O ratios at about 1 × 10 −4 -4 × 10 −3 . This ratio seems higher (a few %) in the cold envelope of IRAS 4A, whose possible origin is discussed in relation to formation processes of HDO and H 2 O. Conclusions. In low-mass protostars, the HDO outer abundances range in a small interval, between ∼10 −11 and a few 10 −10 . No clear trends are found between the HDO abundance and various source parameters (L bol , L smm , L smm /L bol , T bol , L 0.6 bol /M env ). A tentative correlation is observed, however, between the ratio of the inner and outer abundances with the submillimeter luminosity.

The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars

“…The transitions could originate in slightly different regions depending on the source structure. Coutens et al (2013) used Herschel and several groundbased single dish telescopes to observe lines of HDO toward IRAS 4A and IRAS 4B. From 1D radiative transfer models and using the H 18 2 O column density by Persson et al (2012), they estimated HDO/H 2 O ratios in the inner warm regions of IRAS 4A (4.0−30.0 × 10 −4 ) and IRAS 4B (1.0−37.0 ± 10 −4 ).…”

“…Using SMA data of the HDO 3 1,2 −2 2,1 transition at 225 GHz, the above mentioned HDO/H 2 O limit of <6.4 × 10 −4 was found toward NGC 1333-IRAS 4B (Jørgensen & van Dishoeck 2010b). Coutens et al (2013) obtained values of 4−30 × 10 −4 for IRAS 4A and 1−37 × 10 −4 for IRAS 4B using new Herschel data combined with the H 18 2 O column densities found by Persson et al (2012) based on interferometric data. Taquet et al (2013) derived the warm HDO/H 2 O ratios for IRAS 2A and IRAS 4A using the determinations of the H 18 2 O column density from Persson et al (2012) and lower spatial-and spectral-resolution interferometric observations targeting the HDO 4 2,2 −4 2,3 transition (143.7 GHz, E u = 316.2 K) together with single dish (Liu et al 2011) observations of HDO.…”

“…4.2, full radiative transfer models of spherically symmetric envelopes have been run. These same type of models have been used to interpret the water and HDO emission lines observed by Herschel and single-dish ground based telescopes (Coutens et al 2012(Coutens et al , 2013. In these models, the molecular excitation is computed at each position in the envelope and subsequent line radiative transfer is performed.…”

Detection of a dense clump in a filament interacting with W51e2

“…After the formation of water and deuterated water in the ice mantles in the star formation scenario, the environment undergoes gravitational collapse and the local density increases, leading to the formation of a protostar. The surrounding medium then heats up and releases the icy mantles into the gas phase, leading to a significant HDO abundance in the gas phase (Coutens et al 2012(Coutens et al , 2013b. Based on the non-LTE modeling of the five observed NH 3 transitions, we characterize the region of the filament that gives rise to the 70 km s −1 feature as having T kin = 30 K and n(H 2 ) = 5 × 10 5 cm −3 .…”

Water in low-mass star-forming regions withHerschel

“…Previous studies used step-or drop-abundance profiles for water vapour (Herpin et al 2012;Coutens et al 2012Coutens et al , 2013. These profiles are characterised by distinct regions of constant abundance.…”