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.
Context.
Complex organic molecules (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases ...toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on Solar- System scales in the Class 0/I stage is relevant.
Aims.
Our aim is to determine the abundance ratios of oxygen-bearing COMs in Class 0 protostellar systems on scales of ~100 AU radius. We aim to compare these abundances with one another, and to the abundances of other low-mass protostars such as IRAS 16293-2422B and HH 212. Additionally, using both cold and hot COM lines, the gas-phase abundances can be tracked from a cold to a hot component, and ultimately be compared with those in ices to be measured with the
James Webb
Space Telescope (JWST). The abundance of deuterated methanol allows us to probe the ambient temperature during the formation of this species.
Methods.
ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained for seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using local thermodynamic equilibrium models, the excitation temperature and column densities are determined for several O-bearing COMs including methanol (CH
3
OH), acetaldehyde (CH
3
CHO), methyl formate (CH
3
OCHO), and dimethyl ether (CH
3
OCH
3
). Abundance ratios are taken with respect to CH
3
OH.
Results.
Three out of the seven of the observed sources, B1-c, B1-bS (both Perseus), and Serpens S68N (Serpens), show COM emission. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs such as CH
3
OCHO, CH
3
OCH
3
, acetone (CH
3
COCH
3
), and ethylene glycol ((CH
2
OH)
2
) are remarkably similar for the three COM-rich sources; this similarity also extends to IRAS 16293-2422B and HH 212, even though collectively these sources originate from four different star-forming regions (i.e., Perseus, Serpens, Ophiuchus, and Orion). For other COMs like CH
3
CHO, ethanol (CH
3
CH
2
OH), and glycolaldehyde (CH
2
OHCHO), the abundances differ by up to an order of magnitude, indicating that local source conditions become important. B1-c hosts a cold (
T
ex
≈ 60 K), more extended component of COM emission with a column density of typically a few percent of the warm/hot (
T
ex
~ 200 K) central component. A D/H ratio of 1–3% is derived for B1-c, S68N, and B1-bS based on the CH
2
DOH/CH
3
OH ratio (taking into account statistical weighting) suggesting a temperature of ~15 K during the formation of methanol. This ratio is consistent with other low-mass protostars, but is lower than for high-mass star-forming regions.
Conclusions.
The abundance ratios of most O-bearing COMs are roughly fixed between different star-forming regions, and are presumably set at an earlier cold prestellar phase. For several COMs, local source properties become important. Future mid-infrared facilities such as JWST/MIRI will be essential for the direct observation of COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow ice and gas-phase abundances to be directly linked in order to constrain the routes that produce and maintain chemical complexity during the star formation process.
Context
. Cold cores are one of the first steps of star formation, characterized by densities of a few 10
4
–10
5
cm
−3
, low temperatures (15 K and below), and very low external UV radiation. In ...these dense environments, a rich chemistry takes place on the surfaces of dust grains. Understanding the physico-chemical processes at play in these environments is essential to tracing the origin of molecules that are predominantly formed via reactions on dust grain surfaces.
Aims
. We observed the cold core LDN 429-C (hereafter L429-C) with the NOEMA interferometer and the IRAM 30 m single dish telescope in order to obtain the gas-phase abundances of key species, including CO and CH
3
OH. Comparing the data for methanol to the methanol ice abundance previously observed with
Spitzer
allows us to put quantitative constraints on the efficiency of the non-thermal desorption of this species.
Methods
. With physical parameters determined from available
Herschel
data, we computed abundance maps of 11 detected molecules with a non-local thermal equilibrium (LTE) radiative transfer model. These observations allowed us to probe the molecular abundances as a function of density (ranging from a few 10
3
to a few 10
6
cm
−3
) and visual extinction (ranging from 7 to over 75), with the variation in temperature being restrained between 12 and 18 K. We then compared the observed abundances to the predictions of the Nautilus astrochemical model.
Results
. We find that all molecules have lower abundances at high densities and visual extinctions with respect to lower density regions, except for methanol, whose abundance remains around 4.5 × 10
−10
with respect to H
2
. The CO abundance spreads over a factor of 10 (from an abundance of 10
−4
with respect to H
2
at low density to 1.8 × 10
−5
at high density) while the CS, SO, and H
2
S abundances vary by several orders of magnitude. No conclusion can be drawn for CCS, HC
3
N, and CN because of the lack of detections at low densities. Comparing these observations with a grid of chemical models based on the local physical conditions, we were able to reproduce these observations, allowing only the parameter time to vary. Higher density regions require shorter times than lower density regions. This result can provide insights on the timescale of the dynamical evolution of this region. The increase in density up to a few 10
4
cm
−3
may have taken approximately 10
5
yr, while the increase to 10
6
cm
−3
occurs over a much shorter time span (10
4
yr). Comparing the observed gas-phase abundance of methanol with previous measurements of the methanol ice, we estimate a non-thermal desorption efficiency between 0.002 and 0.09%, increasing with density. The apparent increase in the desorption efficiency cannot be reproduced by our model unless the yield of cosmic-ray sputtering is altered due to the ice composition varying as a function of density.
Ices are the main carriers of volatiles in protoplanetary disks and are crucial to our understanding of the protoplanetary disk chemistry that ultimately sets the organic composition of planets. The ...Director’s Discretionary-Early Release Science (DD-ERS) program Ice Age on the
James Webb
Space Telescope (JWST) follows the ice evolution through all stages of star and planet formation. JWST’s exquisite sensitivity and angular resolution uniquely enable detailed and spatially resolved inventories of ices in protoplanetary disks. JWST/NIRSpec observations of the edge-on Class II protoplanetary disk HH 48 NE reveal spatially resolved absorption features of the major ice components H
2
O, CO
2
, and CO, and multiple weaker signatures from less abundant ices NH
3
, OCN
−
, and OCS. Isotopologue
13
CO
2
ice has been detected for the first time in a protoplanetary disk. Since multiple complex light paths contribute to the observed flux, the ice absorption features are filled in by ice-free scattered light. This implies that observed optical depths should be interpreted as lower limits to the total ice column in the disk and that abundance ratios cannot be determined directly from the spectrum. The
12
CO
2
/
13
CO
2
integrated absorption ratio of 14 implies that the
12
CO
2
feature is saturated, without the flux approaching zero, indicative of a very high CO
2
column density on the line of sight, and a corresponding abundance with respect to hydrogen that is higher than interstellar medium values by a factor of at least a few. Observations of rare isotopologues are crucial, as we show that the
13
CO
2
observation allowed us to determine the column density of CO
2
to be at least 1.6 × 10
18
cm
−2
, which is more than an order of magnitude higher than the lower limit directly inferred from the observed optical depth. Spatial variations in the depth of the strong ice features are smaller than a factor of two. Radial variations in ice abundance, for example snowlines, are significantly modified since all observed photons have passed through the full radial extent of the disk. CO ice is observed at perplexing heights in the disk, extending to the top of the CO-emitting gas layer. Although poorly understood radiative transfer effects could contribute to this, we argue that the most likely interpretation is that we observed some CO ice at high temperatures, trapped in less volatile ices such as H
2
O and CO
2
. Future radiative transfer models will be required to constrain the physical origin of the ice absorption and the implications of these observations for our current understanding of disk physics and chemistry.
Context.
The chemical inventory of planets is determined by the physical and chemical processes that govern the early phases of star formation. Nitrogen-bearing species are of interest as many ...provide crucial precursors in the formation of life-related matter.
Aims.
The aim is to investigate nitrogen-bearing complex organic molecules towards two deeply embedded Class 0 low-mass protostars (Perseus B1-c and Serpens S68N) at millimetre wavelengths with the Atacama Large Millimeter/submillimeter Array (ALMA). Next, the results of the detected nitrogen-bearing species are compared with those of oxygen-bearing species for the same and other sources. The similarities and differences are used as further input to investigate the underlying formation pathways.
Methods.
ALMA observations of B1-c and S68N in Band 6 (~1 mm) and Band 5 (~2 mm) are studied at ~0.5′′ resolution, complemented by Band 3 (~3 mm) data in a ~2.5′′ beam. The spectra are analysed for nitrogen-bearing species using the CASSIS spectral analysis tool, and the column densities and excitation temperatures are determined. A toy model is developed to investigate the effect of source structure on the molecular emission.
Results.
Formamide (NH
2
CHO), ethyl cyanide (C
2
H
5
CN), isocyanic acid (HNCO, HN
13
CO, DNCO), and methyl cyanide (CH
3
CN, CH
2
DCN, and CHD
2
CN) are identified towards the investigated sources. Their abundances relative to CH
3
OH and HNCO are similar for the two sources, with column densities that are typically an order of magnitude lower than those of oxygen-bearing species. The largest variations, of an order of magnitude, are seen for NH
2
CHO abundance ratios with respect to HNCO and CH
3
OH and do not correlate with the protostellar luminosity. In addition, within uncertainties, the nitrogen-bearing species have similar excitation temperatures to those of oxygen-bearing species (~100–300 K). The measured excitation temperatures are larger than the sublimation temperatures for the respective species.
Conclusions.
The similarity of most abundances with respect to HNCO for the investigated sources, including those of CH
2
DCN and CHD
2
CN, hints at a shared chemical history, especially the high D-to-H ratio in cold regions prior to star formation. However, some of the variations in abundances may reflect the sensitivity of the chemistry to local conditions such as temperature (e.g. NH
2
CHO), while others may arise from differences in the emitting areas of the molecules linked to their different binding energies in the ice. The excitation temperatures likely reflect the mass-weighted kinetic temperature of a gas that follows a power law structure. The two sources discussed in this work add to the small number of sources that have been subjected to such a detailed chemical analysis on Solar System scales. Future data from the
James Webb
Space Telescope will allow a direct comparison between the ice and gas abundances of both smaller and larger nitrogen-bearing species.
Context.
The number of identified complex organic molecules (COMs) in inter- and circumstellar gas-phase environments is steadily increasing. Recent laboratory studies show that many such species ...form on icy dust grains. At present only smaller molecular species have been directly identified in space in the solid state. Accurate spectroscopic laboratory data of frozen COMs, embedded in ice matrices containing ingredients related to their formation scheme, are still largely lacking.
Aim.
This work provides infrared reference spectra of acetaldehyde (CH
3
CHO), ethanol (CH
3
CH
2
OH), and dimethyl ether (CH
3
OCH
3
) recorded in a variety of ice environments and for astronomically relevant temperatures, as needed to guide or interpret astronomical observations, specifically for upcoming
James Webb
Space Telescope observations.
Methods.
Fourier transform transmission spectroscopy (500–4000 cm
−1
/20–2.5
μ
m, 1.0 cm
−1
resolution) was used to investigate solid acetaldehyde, ethanol and dimethyl ether, pure or mixed with water, CO, methanol, or CO:methanol. These species were deposited on a cryogenically cooled infrared transmissive window at 15 K. A heating ramp was applied, during which IR spectra were recorded until all ice constituents were thermally desorbed.
Results.
We present a large number of reference spectra that can be compared with astronomical data. Accurate band positions and band widths are provided for the studied ice mixtures and temperatures. Special efforts have been put into those bands of each molecule that are best suited for identification. For acetaldehyde the 7.427 and 5.803
μ
m bands are recommended, for ethanol the 11.36 and 7.240
μ
m bands are good candidates, and for dimethyl ether bands at 9.141 and 8.011
μ
m can be used. All spectra are publicly available in the Leiden Database for Ice.
Context. The Ophiuchus clouds, in particular L 1688, are an excellent region to study the embedded phases of star formation, due to the relatively large number of protostars. However, the standard ...method of finding and characterizing embedded young stellar objects (YSOs) through just their infrared spectral slope does not yield a reliable sample. This may affect the age determinations, often derived from the statistics on the total number of embedded YSOs and pre-main sequence stars within a cloud. Aims. Our aim is to characterize the structure of protostellar envelopes on an individual basis and to correctly identify the embedded YSO population of L 1688. Methods. Spectral maps of the HCO+ $J=$ 4-3 and C18O $J=$ 3-2 lines, using the HARP-B array on the James Clerk Maxwell Telescope and SCUBA 850 μm dust maps, are obtained of all sources in the L 1688 region with infrared spectral slopes consistent with, or close to, that of embedded YSOs. Selected 350 μm maps obtained with the Caltech Submillimeter Observatory are presented as well. The properties, extent and variation of dense gas, column density and dust up to 1' (~7500 AU) are probed at 15'' resolution. Using the spatial variation of the gas and dust, together with the intensity of the HCO+ $J=$ 4-3 line, we are able to accurately identify the truly embedded YSOs and determine their properties. Results. The protostellar envelopes range from 0.05 to 0.5 $M_{\odot}$ in mass. The concentration of HCO+ emission (~0.5 to 0.9) is generally higher than that of the dust concentration. Combined with absolute intensities, HCO+ proves to be a better tracer of protostellar envelopes than dust, which can contain disk and cloud contributions. Our total sample of 45 sources, including all previously classified Class I sources, several flat-spectrum sources and some known disks, was re-classified using the molecular emission. Of these, only 17 sources are definitely embedded YSOs. Four of these embedded YSOs have little (0.1-0.2 $M_{\odot}$) envelope material remaining and are likely at the interesting transitional stage from embedded YSO to T Tauri star. About half of the flat-spectrum sources are found to be embedded YSOs and about half are disks. Conclusions. The presented classification method is successful in separating embedded YSOs from edge-on disks and confused sources. The total embedded population of the Ophiuchus L 1688 cloud is found almost exclusively in Oph-A, Oph-B2 and the Ophiuchus ridge with only three embedded YSOs not related to these regions. The detailed characterization presented will be necessary to interpret deep interferometric ALMA and future Herschel observations.
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