Astronomy &Astrophysics Herschel/HIFI: first science highlights Special feature Letter to the Editor Ortho-to-para ratio of interstellar heavy water (original) (raw)
Related papers
Ortho-to-para ratio of interstellar heavy water
Astronomy and Astrophysics, 2010
Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star forming regions, and in particular the Class 0 protostar IRAS 16293-2422. Aims. The CHESS (Chemical HErschel Surveys of Star forming regions) Key Program aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the HIFI instrument provide a unique opportunity to observe the fundamental 1 1,1 -0 0,0 transition of the ortho-D 2 O molecule, inaccessible from the ground, and to determine the ortho-to-para D 2 O ratio. Methods. We have detected the fundamental transition of the ortho-D 2 O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 ± 0.11 (1σ). From the previous ground-based observations of the fundamental 1 1,0 -1 0,1 transition of para-D 2 O seen in absorption at 316.80 GHz we estimate a line opacity of 0.26 ± 0.05 (1σ). Results. We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho to para D 2 O ratio to be lower than 2.6 at a 3 σ level of uncertainty, to be compared with the thermal equilibrium value of 2:1.
A study of deuterated water in the low-mass protostar IRAS 16293-2422
Astronomy & Astrophysics, 2012
Context. Water is a primordial species in the emergence of life, and comets may have brought a large fraction to Earth to form the oceans. To understand the evolution of water from the first stages of star formation to the formation of planets and comets, the HDO/H 2 O ratio is a powerful diagnostic. Aims. Our aim is to determine precisely the abundance distribution of HDO towards the low-mass protostar IRAS16293-2422 and learn more about the water formation mechanisms by determining the HDO/H 2 O abundance ratio. Methods. A spectral survey of the source IRAS16293-2422 was carried out in the framework of the CHESS (Chemical HErschel Surveys of Star forming regions) Herschel Key program with the HIFI (Heterodyne Instrument for the Far-Infrared) instrument, allowing detection of numerous HDO lines. Other transitions have been observed previously with ground-based telescopes. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of HDO by assuming an abundance jump. To determine the H 2 O abundance throughout the envelope, a similar study was made of the H 18 2 O observed lines, as the H 2 O main isotope lines are contaminated by the outflows. Results. It is the first time that so many HDO and H 18 2 O transitions have been detected towards the same source with high spectral resolution. We derive an inner HDO abundance (T ≥ 100 K) of about 1.7 × 10 −7 and an outer HDO abundance (T < 100 K) of about 8 × 10 −11 . To reproduce the HDO absorption lines observed at 894 and 465 GHz, it is necessary to add an absorbing layer in front of the envelope. It may correspond to a water-rich layer created by the photodesorption of the ices at the edges of the molecular cloud. At a 3σ uncertainty, the HDO/H 2 O ratio is 1.4-5.8% in the hot corino, whereas it is 0.2-2.2% in the outer envelope. It is estimated at ∼4.8% in the added absorbing layer. Conclusions. Although it is clearly higher than the cosmic D/H abundance, the HDO/H 2 O ratio remains lower than the D/H ratio derived for other deuterated molecules observed in the same source. The similarity of the ratios derived in the hot corino and in the added absorbing layer suggests that water formed before the gravitational collapse of the protostar, contrary to formaldehyde and methanol, which formed later once the CO molecules had depleted on the grains.
2010
We report the detection of absorption lines by the reactive ions OH, H2Oand H3O along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH at 971 GHz, H2O at 1115 and 607 GHz, and H3O at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O lines remains lower than 1 in the same velocity range, and the H3Oline shows only weak absorption. For LSR velocities between 7 and 50 km s−1 we estimate total column densities of N(OH) ≥ 2.5 × 1014 cm−2, N(H2O) ∼6 × 1013 cm−2 and N(H3O) ∼4.0 × 1013 cm−2. These detections confirm the role of O and OH in initiating th...
Heavy water stratification in a low-mass protostar
Astronomy & Astrophysics, 2013
Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular deuterium fractionation has been found in the environments of low-mass star-forming regions and, in particular, the Class 0 protostar IRAS 16293-2422. Aims. The key program Chemical HErschel Surveys of Star forming regions (CHESS) aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI (Heterodyne Instrument for Far-Infrared) instrument provide a unique opportunity to observe the fundamental 1 1,1-0 0,0 transition of ortho-D 2 O at 607 GHz and the higher energy 2 1,2-1 0,1 transition of para-D 2 O at 898 GHz, both of which are inaccessible from the ground. Methods. The ortho-D 2 O transition at 607 GHz was previously detected. We present in this paper the first tentative detection for the para-D 2 O transition at 898 GHz. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of D 2 O with the same method that was used to reproduce the HDO and H 2 18 O line profiles in IRAS 16293-2422. Results. As for HDO, the absorption component seen on the D 2 O lines can only be reproduced by adding an external absorbing layer, possibly created by the photodesorption of the ices at the edges of the molecular cloud. The D 2 O column density is found to be about 2.5 × 10 12 cm −2 in this added layer, leading to a D 2 O/H 2 O ratio of about 0.5%. At a 3σ uncertainty, upper limits of 0.03% and 0.2% are obtained for this ratio in the hot corino and the colder envelope of IRAS 16293-2422, respectively. Conclusions. The deuterium fractionation derived in our study suggests that the ices present in IRAS 16293-2422 formed on warm dust grains (∼15-20 K) in dense (∼10 4-5 × 10 4 cm −3) translucent clouds. These results allow us to address the earliest phases of star formation and the conditions in which ices form.
Measuring interstellar gas-phase D/H ratios in the presence of H 2
Astronomy and Astrophysics, 2006
Aims. To clarify the circumstances under which it is acceptable to approximate the interstellar gas-phase D/H ratio by D I/H I. Methods. Observed column densities of H I, D I, H 2 and HD are compared for six lines of sight having appreciable fractions of H 2. Results. The overall fraction of deuterium in HD varies by a factor 3−4 but is found to be much smaller than the fraction of H in H 2 in all cases, implying that deuterium appears as D I and N(D I)/N(H I) exceeds the gas-phase D/H ratio in H 2-bearing gas. Conclusions. Deuterium in H 2-bearing gas contributes to the observed D I absorption and the D/H ratio should be inferred from N(D)/N(H) where N(D) = (N(D I) + N(HD)), N(H) = N(H I) + 2N(H 2): failure to do so biases the resulting D/H ratio upward, typically by 5%−15% in present data. Along sightlines with multiple kinematic components having different molecular fractions, fractionation can cause velocity differences between D I and H I profiles. Shifts between H 2 and HD velocity centroids may arise when the molecule-bearing gas has kinematic substructure reflecting regions of different ionization balance and HD/H 2 ratios.
Monthly Notices of the Royal Astronomical Society, 2014
Understanding water deuterium fractionation is important for constraining the mechanisms of water formation in interstellar clouds. Observations of HDO and H2(18)O transitions were carried out towards the high-mass star-forming region G34.26+0.15 with the HIFI instrument onboard the Herschel Space Observatory, as well as with ground-based single-dish telescopes. Ten HDO lines and three H2(18)O lines covering a broad range of upper energy levels (22-204 K) were detected. We used a non-LTE 1D analysis to determine the HDO/H2O ratio as a function of radius in the envelope. Models with different water abundance distributions were considered in order to reproduce the observed line profiles. The HDO/H2O ratio is found to be lower in the hot core (3.5x10^(-4) - 7.5x10^(-4)) than in the colder envelope (1.0x10^(-3) - 2.2x10^(-3)). This is the first time that a radial variation of the HDO/H2O ratio has been found to occur in a high-mass source. The chemical evolution of this source was modeled as a function of its radius and the observations are relatively well reproduced. The comparison between the chemical model and the observations leads to an age of 10^5 years after the infrared dark cloud stage.
The structure and stability of interstellar molecular absorption line profiles at radio frequencies
Astronomy and Astrophysics, 2000
We have taken new, broader-band and higherresolution profiles of Galactic 1667 MHz OH and 89.2 GHz HCO + absorption toward several compact, extragalactic mmwave continuum sources. The profiles are generally stablequite similar between epochs and between the two speciesbut with occasional time-variations and differences. Typical linewidths are 1.0 km s −1 (FWHM) in either OH or HCO + and there are no differences in mean velocity. Profiles are compound but do not show broad wings, multiplicity, assymmetry, or other phenomena strikingly indicative of formation under extraordinary circumstances, consistent with the low ambient thermal pressures reflected in the weak rotational excitation of CO and HCO +. However, we have also discovered the existence of a lowlying, broad component of HCO + absorption covering just those portions of the spectrum where τ HI ≥ 0.1 − 0.2 at λ21 cm. Toward B0355+508 at b = −1.6 o , HCO + absorption extends continuously over more than 40 km s −1. The broadlydistributed HCO + absorption can be understood in terms of the known molecular fraction of local gas, as long as HCO + is generally present at about its typical abundance n(HCO +)/n(H 2) = 2 × 10 −9. The fact that CO forms rapidly from HCO + in diffuse gas then suffices to account for the abundance of CO in diffuse/translucent material over the entire range 10 12 ≤ N (CO) ≤ 10 16 cm −2 , 10 19 ≤ H 2 ≤ 10 21 cm −2 , using otherwise standard cloud models. Using models of molecular formation and excitation and the H-H 2 , C +-CO transition in diffuse gas, and noting the absence of HCO + emission at levels of 0.02-0.05 K, we show very directly that the line profile variations are not the result of AU-sized inclusions of high hydrogen volume density, in the manner usually inferred. Instead, it is necessary to account for small-scale chemical and other inhomogeneities.
HDO abundance in the envelope of the solar-type protostar IRAS�16293?2422
Astronomy and Astrophysics, 2005
We present IRAM 30 m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293−2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293−2422. We interpret the data by means of the Ceccarelli, Hollenbach and Tielens (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance x HDO in = 1×10 −7 and an outer abundance x HDO out ≤ 1 × 10 −9 (3σ). This result is in favor of HDO enhancement due to ice evaporation from the grains in the inner envelope. The deuteration ratio HDO/H2O is found to be fin = 3% and fout ≤ 0.2% (3σ) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H2O ices consistently derive a H2O ice abundance of several 10 −5 to 10 −4 , some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293−2422.
Detection of OD towards the low-mass protostar IRAS 16293−2422
Astronomy & Astrophysics, 2012
Context. Although water is an essential and widespread molecule in star-forming regions, its chemical formation pathways are still not very well constrained. Observing the level of deuterium fractionation of OH, a radical involved in the water chemical network, is a promising way to infer its chemical origin. Aims. We aim at understanding the formation mechanisms of water by investigating the origin of its deuterium fractionation. This can be achieved by observing the abundance of OD towards the low-mass protostar IRAS16293−2422, where the HDO distribution is already known. Methods. Using the GREAT receiver on board SOFIA, we observed the ground-state OD transition at 1391.5 GHz towards the lowmass protostar IRAS16293−2422. We also present the detection of the HDO 1 11 -0 00 line using the APEX telescope. We compare the OD/HDO abundance ratio inferred from these observations with the predictions of chemical models. Results. The OD line is detected in absorption towards the source continuum. This is the first detection of OD outside the solar system. The SOFIA observation, coupled to the observation of the HDO 1 11 -0 00 line, provides an estimate of the abundance ratio OD/HDO ∼ 17-90 in the gas where the absorption takes place. This value is fairly high compared with model predictions. This may be reconciled if reprocessing in the gas by means of the dissociative recombination of H 2 DO + further fractionates OH with respect to water.
Interstellar OH + , H 2 O + and H 3 O + along the sight-line to G10.6–0.4
Astronomy and Astrophysics, 2010
We report the detection of absorption lines by the reactive ions OH + , H 2 O + and H 3 O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H 2 O + at 1115 and 607 GHz, and H 3 O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H 2 O + lines remains lower than 1 in the same velocity range, and the H 3 O + line shows only weak absorption. For Local Standard of Rest (LSR) velocities between 7 and 50 kms −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H 2 O + ) ∼ 6 × 10 13 cm −2 and N(H 3 O + ) ∼ 4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H 2 O + column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.