Detection of OD towards the low-mass protostar IRAS 16293−2422 (original) (raw)
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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.
Astronomy & Astrophysics, 2015
OH 231.8+4.2, a bipolar outflow around a Mira-type variable star, displays a unique molecular richness amongst circumstellar envelopes (CSEs) around O-rich AGB and post-AGB stars. We report line observations of the HCO + and H 13 CO + molecular ions and the first detection of SO + , N 2 H + , and (tentatively) H 3 O + in this source. SO + and H 3 O + have not been detected before in CSEs around evolved stars. These data have been obtained as part of a full mm-wave and far-IR spectral line survey carried out with the IRAM 30 m radio telescope and with Herschel/HIFI. Except for H 3 O + , all the molecular ions detected in this work display emission lines with broad profiles (FWHM∼ 50-90 km s −1), which indicates that these ions are abundant in the fast bipolar outflow of OH 231.8. The narrow profile (FWHM∼14 km s −1) and high critical densities (>10 6 cm −3) of the H 3 O + transitions observed are consistent with this ion arising from denser, inner (and presumably warmer) layers of the fossil remnant of the slow AGB CSE at the core of the nebula. From rotational diagram analysis, we deduce excitation temperatures of T ex ∼10-20 K for all ions except for H 3 O + , which is most consistent with T ex ≈100 K. Although uncertain, the higher excitation temperature suspected for H 3 O + is similar to that recently found for H 2 O and a few other molecules, which selectively trace a previously unidentified, warm nebular component. The column densities of the molecular ions reported here are in the range N tot ≈[1-8]×10 13 cm −2 , leading to beam-averaged fractional abundances relative to H 2 of X(HCO +)≈10 −8 , X(H 13 CO +)≈2×10 −9 , X(SO +)≈4×10 −9 , X(N 2 H +)≈2×10 −9 , and X(H 3 O +)≈7×10 −9 cm −2. We have performed chemical kinetics models to investigate the formation of these ions in OH 231.8 as the result of standard gas phase reactions initiated by cosmic-ray and UV-photon ionization. The model predicts that HCO + , SO + , and H 3 O + can form with abundances comparable to the observed average values in the external layers of the slow central core (at ∼[3-8]×10 16 cm); H 3 O + would also form quite abundantly in regions closer to the center (X(H 3 O +)∼10 −9 at ∼10 16 cm). For N 2 H + , the model abundance is lower than the observed value by more than two orders of magnitude. The model fails to reproduce the abundance enrichment of HCO + , SO + , and N 2 H + in the lobes, which is directly inferred from the broad emission profiles of these ions. Also, in disagreement with the narrow H 3 O + spectra, the model predicts that this ion should form in relatively large, detectable amounts (≈10 −9) in the external layers of the slow central core and in the high-velocity lobes. Some of the model-data discrepancies are reduced, but not suppressed, by lowering the water content and enhancing the elemental nitrogen abundance in the envelope. The remarkable chemistry of OH 231.8 probably reflects the molecular regeneration process within its envelope after the passage of fast shocks that accelerated and dissociated molecules in the AGB wind ∼800 yr ago.
Astronomy and Astrophysics, 2010
Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin. Methods. The characteristic hyperfine pattern of the H 2 O + ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H 2 O + absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation. Results. In DR21, the velocity distribution of H 2 O + matches that of the [C ii] line at 158 µm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H ii-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H 2 O + column density of 7.2 × 10 12 cm −2 in NGC 6334, 2.3 × 10 13 cm −2 in DR21, and 1.1 × 10 15 cm −2 in Sgr B2.
First detection of ND in the solar-mass protostar IRAS16293-2422
Astronomy and Astrophysics, 2010
Context. In the past decade, much progress has been made in characterising the processes leading to the enhanced deuterium fractionation observed in the ISM and in particular in the cold, dense parts of star forming regions such as protostellar envelopes. Very high molecular D/H ratios have been found for saturated molecules and ions. However, little is known about the deuterium fractionation in radicals, even though simple radicals often represent an intermediate stage in the formation of more complex, saturated molecules. The imidogen radical NH is such an intermediate species for the ammonia synthesis in the gas phase. Many of these light molecules however have their fundamental transitions in the submillimetre domain and their detection is hampered by the opacity of the atmosphere at these wavelengths. Herschel/HIFI represents a unique opportunity to study the deuteration and formation mechanisms of species not observable from the ground. Aims. We searched here for the deuterated radical ND in order to determine the deuterium fractionation of imidogen and constrain the deuteration mechanism of this species. Methods. We observed the solar-mass Class 0 protostar IRAS16293-2422 with the heterodyne instrument HIFI in Bands 1a (480 -560 GHz), 3b (858 -961 GHz), and 4a (949 -1061 GHz) as part of the Herschel key programme CHESS (Chemical HErschel Surveys of Star forming regions). Results. The deuterated form of the imidogen radical ND was detected and securely identified with 2 hyperfine component groups of its fundamental transition (N=0 -1) at 522.1 and 546.2 GHz, in absorption against the continuum background emitted from the nascent protostar. The 3 groups of hyperfine components of its hydrogenated counterpart NH were also detected in absorption. The absorption arises from the cold envelope, where many deuterated species have been shown to be abundant. The estimated column densities are ∼ 2 × 10 14 cm −2 for NH and ∼ 1.3 × 10 14 cm −2 for ND. We derive a very high deuterium fractionation with an [ND]/[NH] ratio of between 30 and 100%. Conclusions. The deuterium fractionation of imidogen is of the same order of magnitude as that in other molecules, which suggests that an efficient deuterium fractionation mechanism is at play. We discuss two possible formation pathways for ND, by means of either the reaction of N + with HD, or deuteron/proton exchange with NH.
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.
First hyperfine resolved far-infrared OH spectrum from a star-forming region
Astronomy & Astrophysics, 2011
OH is an important molecule in the H2O chemistry and the cooling budget of star-forming regions. The goal of the Herschel key program `Water in Star-forming regions with Herschel' (WISH) is to study H2O and related species during protostellar evolution. Our aim in this letter is to assess the origin of the OH emission from star-forming regions and constrain the properties of the emitting gas. High-resolution observations of the OH 2Pi1/2 J = 3/2-1/2 triplet at 1837.8 GHz (163.1 micron) towards the high-mass star-forming region W3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel reveal the first hyperfine velocity-resolved OH far-infrared spectrum of a star-forming region. The line profile of the OH emission shows two components: a narrow component (FWHM approx. 4-5 km/s) with partially resolved hyperfine structure resides on top of a broad (FWHM approx. 30 km/s) component. The narrow emission agrees well with results from radiative transfer calculations of a spherical envelope model for W3 IRS 5 with a constant OH abundance of approx. 8e-9. Comparison with H2O yields OH/H2O abundance ratios of around 1e-3 for T > 100 K and around unity for T < 100K, consistent with the current picture of the dense cloud chemistry with freeze-out and photodesorption. The broad component is attributed to outflow emission. An abundance ratio of OH/H2O > 0.028 in the outflow is derived from comparison with results of water line modeling. This ratio can be explained by a fast J-type shock or a slower UV-irradiated C-type shock.
Herschel observations of the hydroxyl radical (OH) in young stellar objects
Astronomy & Astrophysics, 2010
Water in Star-forming regions with Herschel (WISH) is a Herschel Key Program investigating the water chemistry in young stellar objects (YSOs) during protostellar evolution. Hydroxyl (OH) is one of the reactants in the chemical network most closely linked to the formation and destruction of H2O. High-temperature chemistry connects OH and H2O through the OH + H2 <-> H2O + H reactions. Formation of H2O from OH is efficient in the high-temperature regime found in shocks and the innermost part of protostellar envelopes. Moreover, in the presence of UV photons, OH can be produced from the photo-dissociation of H2O. High-resolution spectroscopy of the OH 163.12 micron triplet towards HH 46 and NGC 1333 IRAS 2A was carried out with the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel. The low- and intermediate-mass YSOs HH 46, TMR 1, IRAS 15398-3359, DK Cha, NGC 7129 FIRS 2, and NGC 1333 IRAS 2A were observed with the Photodetector Array Camera and Spectrometer (PACS) in four transitions of OH and two [OI] lines. The OH transitions at 79, 84, 119, and 163 micron and [OI] emission at 63 and 145 micron were detected with PACS towards the class I low-mass YSOs as well as the intermediate-mass and class I Herbig Ae sources. No OH emission was detected from the class 0 YSO NGC 1333 IRAS 2A, though the 119 micron was detected in absorption. With HIFI, the 163.12 micron was not detected from HH 46 and only tentatively detected from NGC 1333 IRAS 2A. The combination of the PACS and HIFI results for HH 46 constrains the line width (FWHM > 11 km/s) and indicates that the OH emission likely originates from shocked gas. This scenario is supported by trends of the OH flux increasing with the [OI] flux and the bolometric luminosity. Similar OH line ratios for most sources suggest that OH has comparable excitation temperatures despite the different physical properties of the sources.
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.
Detection of doubly-deuterated methanol in the solar-type protostar IRAS?16293-2422
Astronomy and Astrophysics, 2002
We report the first detection of doublydeuterated methanol (CHD 2 OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH 2 DOH and CH 3 OD), towards the solar-type protostar IRAS16293−2422. From the present multifrequency observations, we derive the following abundance ratios:
Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B
Astronomy & Astrophysics, 2013
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/H2O abundance ratios. Methods: Using the Herschel/HIFI instrument as well as ground-based telescopes, we observed several HDO lines covering a large excitation range (Eup/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 H2 are estimated with a 3σ uncertainty at 7.5-3.0+3.5 × 10-9 and 1.2-0.4+0.4 × 10-11, respectively, whereas in NGC 1333 IRAS 4B they are 1-0.9+1.8 × 10-8 and 1.2-0.4+0.6 × 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) × 1013 cm-2, leading to an abundance of about (0.7-1.9) × 10-9. An HDO/H2O 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/H2O 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 H2O. 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 (Lbol, Lsmm, Lsmm/Lbol, Tbol, Lbol0.6/Menv). A tentative correlation is observed, however, between the ratio of the inner and outer abundances with the submillimeter luminosity.