Prestellar Phase Research Articles

FAUST XVII: Super deuteration in the planet-forming system IRS 63 where the streamer strikes the disk

Recent observations suggest that planet formation starts early, in protostellar disks of $ yrs, which are characterized by strong interactions with the environment, such as through accretion streamers and molecular outflows. To investigate the impact of such phenomena on the physical and chemical properties of a disk, it is key to understand what chemistry planets inherit from their natal environment. In the context of the ALMA large program Fifty AU Study of the chemistry in the disk/envelope system of solar-like protostars (FAUST), we present observations on scales from sim 1500 au to sim 60 au of H$_2$CO, HDCO, and D$_2$CO toward the young planet-forming disk IRS 63. The H$_2$CO probes the gas in the disk as well as in a large scale streamer ($ 1500$ au) impacting onto the southeast disk side. We detected for the first time deuterated formaldehyde, HDCO and D$_2$CO, in a planet-forming disk and HDCO in the streamer that is feeding it. These detections allowed us to estimate the deuterium fractionation of H$_2$CO in the disk: HDCO H$_2$CO 0.1-0.3$ and D$_2$CO H$_2$CO 0.1$. Interestingly, while HDCO follows the H$_2$CO distribution in the disk and in the streamer, the distribution of D$_2$CO is highly asymmetric, with a peak of the emission (and D H ratio) in the southeast disk side, where the streamer crashes onto the disk. In addition, D$_2$CO was detected in two spots along the blue- and redshifted outflow. This suggests that (i) in the disk, HDCO formation is dominated by gas-phase reactions in a manner similar to H$_2$CO, while (ii) D$_2$CO is mainly formed on the grain mantles during the prestellar phase and/or in the disk itself and is at present released in the gas phase in the shocks driven by the streamer and the outflow. These findings testify to the key role of streamers in the buildup of the disk concerning both the final mass available for planet formation and its chemical composition.

Carbon Isotope Fractionation of Complex Organic Molecules in Star-forming Cores

Recent high-resolution and sensitivity Atacama Large Millimeter/submillimeter Array observations have unveiled the carbon isotope ratios (12C/13C) of complex organic molecules (COMs) in a low-mass protostellar source. To understand the 12C/13C ratios of COMs, we investigated the carbon isotope fractionation of COMs from prestellar cores to protostellar cores with a gas-grain chemical network model. We confirmed that the 12C/13C ratios of small molecules are bimodal in the prestellar phase: CO and species formed from CO (e.g., CH3OH) are slightly enriched in 13C compared to the local interstellar medium (by ∼10%), while those from C and C+ are depleted in 13C owing to isotope exchange reactions. COMs are mainly formed on the grain surface and in the hot gas (> 100 K) in the protostellar phase. The 12C/13C ratios of COMs depend on which molecules the COMs are formed from. In our base model, some COMs in the hot gas are depleted in 13C compared to the observations. Thus, we additionally incorporate reactions between gaseous atomic C and H2O ice or CO ice on the grain surface to form H2CO ice or C2O ice, as suggested by recent laboratory studies. The direct C-atom addition reactions open pathways to form 13C-enriched COMs from atomic C and CO ice. We find that these direct C-atom addition reactions mitigate 13C-depletion of COMs, and the model with the direct C-atom addition reactions better reproduces the observations than our base model. We also discuss the impact of the cosmic-ray ionization rate on the 12C/13C ratio of COMs.

Hunting for complex cyanides in protostellar ices with the JWST

Nitrogen-bearing complex organic molecules have been commonly detected in the gas phase but not yet in interstellar ices. This has led to the long-standing question of whether these molecules form in the gas phase or in ices. The James Webb Space Telescope (JWST) offers the sensitivity, spectral resolution, and wavelength coverage needed to detect them in ices and investigate whether their abundance ratios are similar in gas and ice. We report the first tentative detection of CH3CN, C2H5CN and the simple molecule, N2O, based on the CN-stretch band in interstellar ices toward three (HOPS 153, HOPS 370, and IRAS 20126+4104) out of the five protostellar systems observed as part of the Investigating Protostellar Accretion (IPA) GO program with JWST-NIRSpec. We also provide upper limits for the two other sources with smaller luminosities in the sample. We detect OCN− in the ices of all sources with typical CH3CN/OCN− ratios of around 1. Ice and gas column density ratios of the nitrogen-bearing species with respect to each other are better matched than those with respect to methanol, which are a factor of ~5 larger in the ices than the gas. We attribute the elevated ice column densities with respect to methanol to the difference in snowline locations of nitrogen-bearing molecules and of methanol, biasing the gas-phase observations toward fewer nitrogen-bearing molecules. Moreover, we find tentative evidence of the enhancement of OCN−, CH3CN, and C2H5CN in warmer ices; although, the formation of these molecules likely starts along with methanol in the cold prestellar phase. Future surveys combining NIRSpec and MIRI, and additional laboratory spectroscopic measurements of C2H5CN ice, are necessary for robust detection and conclusions on the formation history of complex cyanides.

Grain growth and its chemical impact in the first hydrostatic core phase

Context. The first hydrostatic core (FHSC) phase is a brief stage in the protostellar evolution that is difficult to detect. Its chemical composition determine that of later evolutionary stages. Numerical simulations are the tool of choice to study these objects. Aims. Our goal is to characterize the chemical evolution of gas and dust during the formation of the FHSC. Moreover, we are interested in analyzing, for the first time with 3D magnetohydrodynamic (MHD) simulations, the role of grain growth in its chemistry. Methods. We postprocessed 2 × 105 tracer particles from a RAMSES non-ideal MHD simulation using the codes NAUTILUS and SHARK to follow the chemistry and grain growth throughout the simulation. Results. Gas-phase abundances of most of the C, O, N, and S reservoirs in the hot corino at the end of the simulation match the ice-phase abundances from the prestellar phase. Interstellar complex organic molecules such as methyl formate, acetaldehyde, and formamide are formed during the warm-up process. Grain size in the hot corino (nH > 1011 cm−3) increases forty-fold during the last 30 kyr, with negligible effects on its chemical composition. At moderate densities (1010 < nH < 1011 cm−3) and cool temperatures 15 < T < 50 K, increasing grain sizes delay molecular depletion. At low densities (nH ~ 107 cm−3), grains do not grow significantly. To assess the need to perform chemo-MHD calculations, we compared our results with a two-step model that reproduces well the abundances of C and O reservoirs, but not the N and S reservoirs. Conclusions. The chemical composition of the FHSC is heavily determined by that of the parent prestellar core. Chemo-MHD computations are needed for an accurate prediction of the abundances of the main N and S elemental reservoirs. The impact of grain growth in moderately dense areas delaying depletion permits the use of abundance ratios as grain growth proxies.

Complex Organic Molecules in a Very Young Hot Corino, HOPS 373SW

We present the spectra of complex organic molecules (COMs) detected in HOPS 373SW with the Atacama Large Millimeter/submillimeter Array (ALMA). HOPS 373SW, which is a component of a protostellar binary with a separation of 1500au, has been discovered as a variable protostar by the JCMT transient monitoring survey with a modest (∼30%) brightness increase at submillimeter wavelengths. Our ALMA target-of-opportunity observation at ∼345 GHz for HOPS 373SW revealed extremely young chemical characteristics with strong deuteration of methanol. The dust continuum opacity is very high toward the source center, obscuring line emission from within 0.″03. The other binary component, HOPS 373NE, was detected only in C17O in our observation, implying a cold and quiescent environment. We compare the COM abundances relative to CH3OH in HOPS 373SW with those of V883 Ori, which is an eruptive disk object, as well as other hot corinos, to demonstrate the chemical evolution from envelope to disk. High abundances of singly, doubly, and triply deuterated methanol (CH2DOH, CHD2OH, and CD3OH) and a low CH3CN abundance in HOPS 373SW compared to other hot corinos suggest a very early evolutionary stage of HOPS 373SW in the hot corino phase. Since the COMs detected in HOPS 373SW would have been sublimated very recently from grain surfaces, HOPS 373SW is a promising place to study the surface chemistry of COMs in the cold prestellar phase before sublimation.

ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): A Forming Quadruple System with Continuum “Ribbons” and Intricate Outflows

One of the most poorly understood aspects of low-mass star formation is how multiple-star systems are formed. Here we present the results of Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations toward a forming quadruple protostellar system, G206.93-16.61E2, in the Orion B molecular cloud. ALMA 1.3 mm continuum emission reveals four compact objects, of which two are Class I young stellar objects and the other two are likely in prestellar phase. The 1.3 mm continuum emission also shows three asymmetric ribbon-like structures that are connected to the four objects, with lengths ranging from ∼500 to ∼2200 au. By comparing our data with magnetohydrodynamic simulations, we suggest that these ribbons trace accretion flows and also function as gas bridges connecting the member protostars. Additionally, ALMA CO J = 2−1 line emission reveals a complicated molecular outflow associated with G206.93-16.61E2, with arc-like structures suggestive of an outflow cavity viewed pole-on.

The First Interferometric Measurements of NH2D/NH3 Ratio in Hot Corinos

The chemical evolution of nitrogen during star and planet formation is still not fully understood. Ammonia (NH3) is a key specie in the understanding of the molecular evolution in star-forming clouds and nitrogen isotope fractionation. In this paper, we present high-spatial-resolution observations of multiple emission lines of NH3 toward the protobinary system NGC1333 IRAS4A with the Karl G. Jansky Very Large Array. We spatially resolved the binary (hereafter, 4A1 and 4A2) and detected compact emission of NH3 transitions with high excitation energies (≳100 K) from the vicinity of the protostars, indicating the NH3 ice has sublimated at the inner hot region. The NH3 column density is estimated to be ∼1017–1018 cm−2. We also detected two NH2D transitions, allowing us to constrain the deuterium fractionation of ammonia. The NH2D/NH3 ratios are as high as ∼0.3–1 in both 4A1 and 4A2. From comparisons with the astrochemical models in the literature, the high NH2D/NH3 ratios suggest that the formation of NH3 ices mainly started in the prestellar phase after the formation of bulk water ice finished, and that the primary nitrogen reservoir in the star-forming cloud could be atomic nitrogen (or N atoms) rather than nitrogen-bearing species such as N2 and NH3. The implications on the physical properties of IRAS4A’s cores are discussed as well.

Methanol deuteration in high-mass protostars

Context. The deuteration of molecules forming in the ices such as methanol (CH3OH) is sensitive to the physical conditions during their formation in dense cold clouds and can be probed through observations of deuterated methanol in hot cores. Aims. The aim is to determine the D/H ratio of methanol for a large sample of 99 high-mass protostars and to link this to the physical conditions during the formation of methanol in the prestellar phases. Methods. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) containing transitions of CH3OH, CH2DOH, CHD2OH, 13CH3OH, and CH318OH are investigated. The column densities of CH2DOH, CHD2OH, and CH3OH are determined for all sources, where the column density of CH3OH is derived from optically thin 13C and 18O isotopologues. Consequently, the D/H ratio of methanol is derived taking statistical effects into account. Results. Singly deuterated methanol (CH2DOH) is detected at the 3σ level toward 25 of the 99 sources in our sample of the high-mass protostars. Including upper limits, the (D/H)CH3OH ratio inferred from NCH2DOH/NCH3OH was derived for 38 of the 99 sources and varies between ~10−3-10−2. Including other high-mass hot cores from the literature, the mean methanol D/H ratio is 1.1 ± 0.7 × 10−3. This is more than one order of magnitude lower than what is seen for low-mass protostellar systems (2.2 ± 1.2 × 10−2). Doubly deuterated methanol (CHD2OH) is detected at the 3σ level toward 11 of the 99 sources. Including upper limits for 15 sources, the (D/H)CH2DOH ratios derived from NCHD2OH/NCH2DOH are more than two orders of magnitude higher than (D/H)CH3OH with an average of 2.0 ± 0.8 × 10−1 which is similar to what is found for low-mass sources. Comparison with literature GRAINOBLE models suggests that the high-mass prestellar phases are either warm (>20 K) or live shorter than the free-fall timescale. In contrast, for low-mass protostars, both a low temperature of <15 K and a prestellar phase timescale longer than the free-fall timescale are necessary. Conclusions. The (D/H)CH3OH ratio drops by more than an order of magnitude between low-mass and high-mass protostars due to either a higher temperature during the prestellar phases or shorter prestellar phases. However, successive deuteration toward CHD2OH seems equally effective between low-mass and high-mass systems.

SOLIS

Context.Deuteration is a precious tool for investigating the origin and formation routes of interstellar complex organic molecules in the different stages of the star formation process. Methyl cyanide (CH3CN) is one of the most abundant interstellar complex organic molecules (iCOMs); it is of particular interest because it is among the very few iCOMs detected not only around protostars but also in protoplanetary disks. However, its formation pathways are not well known and only a few measurements of its deuterated isotopologue (CH2DCN) have been made to date.Aims.We studied the line emission from CH3CN and its deuterated isotopologue CH2DCN towards the prototypical Class I object SVS13-A, where the deuteration of a large number of species has already been reported. Our goal is to measure the CH3CN deuteration in a Class I protostar, for the first time, in order to constrain the CH3CN formation pathways and the chemical evolution from the early prestellar core and Class 0 to the evolved Class I stages.Methods.We imaged CH2DCN towards SVS13-A using the IRAM NOEMA interferometer at 3mm in the context of the Large Program SOLIS (with a spatial resolution of 1″.8 × 1″.2). The NOEMA images were complemented by the CH3CN and CH2DCN spectra collected by the IRAM-30m Large Program ASAI, which provided an unbiased spectral survey at 3 mm, 2 mm, and 1.3 mm. The observed line emission was analysed using local thermodynamic equilibrium (LTE) and non-LTE large velocity gradient (LVG) approaches.Results.The NOEMA/SOLIS images of CH2DCN show that this species emits in an unresolved area centred towards the SVS13-A continuum emission peak, suggesting that methyl cyanide and its isotopologues are associated with the hot corino of SVS13-A, previously imaged via other iCOMs. In addition, we detected 41 and 11 ASAI transitions of CH3CN and CH2DCN, respectively, which cover upper level energies (Eup) from 13 to 442 K and from 18 K to 200 K. The non-LTE LVG analysis of the CH3CN lines points to a kinetic temperature of (140 ± 20) K, a gas density nH2≥ 107 cm−3, and an emitting size of ~0″.3, in agreement with the hypothesis that CH3CN lines are emitted in the SVS13-A hot corino. The derived [CH2DCN]/[CH3CN] ratio is ~9%. This value is consistent with those measured towards prestellar cores and a factor 2–3 higher than those measured in Class 0 protostars.Conclusions.Contrarily to what expected for other molecular species, the CH3CN deuteration does not show a decrease in SVS13-A with respect to measurements in younger prestellar cores and Class 0 protostars. Finally, we discuss why our new results suggest that CH3CN was likely synthesised via gas-phase reactions and frozen onto the dust grain mantles during the cold prestellar phase.

H2CS deuteration maps towards the pre-stellar core L1544

Context. Deuteration is a crucial tool for understanding the complexity of interstellar chemical processes, especially when they involve the interplay of gas-phase and grain-surface chemistry. In the case of multiple deuteration, comparing observation with the results of chemical modelling is particularly effective to study how molecules are inherited in the different stages within the process of star and planet formation. Aims. We aim to study the D/H ratio in H2CS across the prototypical pre-stellar core L1544. This study allows us to test current gas–dust chemical models involving sulfur in dense cores. Methods. Here, we present single-dish observations of H2CS, HDCS and D2CS with the IRAM 30 m telescope. We analysed their column densities and distributions and compared these observations with gas–grain chemical models. The deuteration maps of H2CS in L1544 were compared with the deuteration maps of methanol, H2CO, N2H+, and HCO+ towards the same source. Furthermore, we compared the single and double deuteration of H2CS towards the dust peak of L1544 with H2CO and c-C3H2. The difference between the deuteration of these molecules in L1544 is discussed and compared with the prediction of chemical models. Results. The maximum deuterium fractionation for the first deuteration of H2CS is N(HDCS)/N(H2CS) ~ 30% and is located towards the north-east at a distance of about 10000 AU from the dust peak. While for c-C3H2 the first and second deuteration have a similar efficiency, for H2CS and H2CO the second deuteration is more efficient, leading to D2CX/HDCX ~ 100% (with X = O or S). Conclusions. Our results imply that the large deuteration of H2CO and H2CS observed in protostellar cores as well as in comets is likely inherited from the pre-stellar phase. However, comparison with state-of-the-art chemical models suggests that the reaction network for the formation of the doubly deuterated H2CS and H2CO it is not complete yet.

Deuterium Fractionation as a Multiphase Component Tracer in the Galactic Center

Abstract The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disk. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm (>100 K) and turbulent phase with little material in the prestellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO+, and N2H+ toward the CMZ molecular cloud G+0.693–0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent narrow component, with a line width of ∼9 km s−1, in addition to the warm, less dense, and turbulent broad component with a line width of ∼20 km s−1. The very low D/H ratio ≤6 × 10−5 for HCO+ and N2H+, close to the cosmic value (∼2.5 × 10−5), and the high D/H ratios, >4 × 10−4 for HCN and HNC, derived for the broad component confirm the presence of high-temperature deuteration routes for nitriles. For the narrow component we have derived D/H ratios >10−4 and excitation temperatures of 7 K for all molecules, suggesting kinetic temperatures ≤30 K and H2 densities ≥5 × 104 cm−3, at least one order of magnitude larger than that for the broad component. The method presented in this Letter allows us to identify clouds on the verge of star formation, i.e., under prestellar conditions, toward the CMZ. This method can also be used for the identification of such clouds in external galaxies.

Protostellar Interferometric Line Survey of the Cygnus X region (PILS-Cygnus)

Context. Complex organic molecules (COMs) are commonly detected in and near star-forming regions. However, the dominant process in the release of these COMs from the icy grains – where they predominately form – to the gas phase is still an open question. Aims. We investigate the origin of COM emission in a high-mass protostellar source, CygX-N30 MM1, through high-angular-resolution interferometric observations over a continuous broad frequency range. Methods. We used 32 GHz Submillimeter Array observations with continuous frequency coverage from 329 to 361 GHz at an angular resolution of ~1″ to do a line survey and obtain a chemical inventory of the source. The line emission in the frequency range was used to determine column densities and excitation temperatures for the COMs. We also mapped out the intensity distribution of the different species. Results. We identified approximately 400 lines that can be attributed to 29 different molecular species and their isotopologues. We find that the molecular peak emission is along a linear gradient, and coincides with the axis of red- and blueshifted H2CO and CS emission. Chemical differentiation is detected along this gradient, with the O-bearing molecular species peaking towards one component of the system and the N- and S-bearing species peaking towards the other. The chemical gradient is offset from but parallel to the axis through the two continuum sources. The inferred column densities and excitation temperatures are compared to other sources where COMs are abundant. Only one deuterated molecule is detected, HDO, while an upper limit for CH2DOH is derived, leading to a D/H ratio of <0.1%. Conclusions. We conclude that the origin of the observed COM emission is probably a combination of the young stellar sources along with accretion of infalling material onto a disc-like structure surrounding a young protostar and located close to one of the continuum sources. This disc and protostar are associated with the O-bearing molecular species, while the S- and N-bearing species on the other hand are associated with the other continuum core, which is probably a protostar that is slightly more evolved than the other component of the system. The low D/H ratio likely reflects a pre-stellar phase where the COMs formed on the ices at warm temperatures (~30 K), where the deuterium fractionation would have been inefficient. The observations and results presented here demonstrate the importance of good frequency coverage and high angular resolution when disentangling the origin of COM emission.

Constraining Spatial Densities of Early Ice Formation in Small Dense Molecular Cores from Extinction Maps

Abstract Tracing dust in small dense molecular cores is a powerful tool to study the conditions required for ices to form during the prestellar phase. To study these environments, five molecular cores were observed: three with ongoing low-mass star formation (B59, B335, and L483), and two starless collapsing cores (L63 and L694-2). Deep images were taken in the infrared JHK bands with the United Kingdom Infrared Telescope WFCAM (Wide Field Camera) instrument and IRAC channels 1 and 2 on the Spitzer Space Telescope. These five photometric bands were used to calculate extinction along the line of sight toward background stars. After smoothing the data, we produced high spatial resolution extinction maps (∼13″–29″). The maps were then projected into the third dimension using the AVIATOR algorithm implementing the inverse Abel transform. The volume densities of the total hydrogen were measured along lines of sight where ices (H2O, CO, and CH3OH) have previously been detected. We find that lines of sight with pure CH3OH or a mixture of CH3OH with CO have maximum volume densities above 1.0 × 105 cm−3. These densities are only reached within a small fraction of each of the cores (∼0.3%–2.1%). CH3OH presence may indicate the onset of complex organic molecule formation within dense cores, and thus we can constrain the region where this onset can begin. The maximum volume densities toward star-forming cores in our sample (∼(1.2–1.7) × 106 cm−3) are higher than those toward starless cores (∼(3.5–9.5) × 105 cm−3).

ALMA observations of doubly deuterated water: inheritance of water from the prestellar environment

Context. Establishing the origin of the water D/H ratio in the Solar System is central to our understanding of the chemical trail of water during the star and planet formation process. Recent modeling suggests that comparisons of the D2O/HDO and HDO/H2O ratios are a powerful way to trace the chemical evolution of water and, in particular, determine whether the D/H ratio is inherited from the molecular cloud or established locally. Aims. We seek to determine the D2O column density and derive the D2O/HDO ratios in the warm region toward the low-mass Class 0 sources B335 and L483. The results are compared with astrochemical models and previous observations to determine their implications for the chemical evolution of water. Methods. We present ALMA observations of the D2O 11,0–10,1 transition at 316.8 GHz toward B335 and L483 at ≲0.′′5 (≲100 au) resolution, probing the inner warm envelope gas. The column densities of D2O, HDO, and H218O are determined by synthetic spectrum modeling and direct Gaussian fitting, under the assumption of a single excitation temperature and similar spatial extent for the three water isotopologs. Results. D2O is detected toward both sources in the inner warm envelope. The derived D2O/HDO ratio is (1.0 ± 0.2) × 10−2 for L483 and (1.4 ± 0.1) × 10−2 for B335. These values indicate that the D2O/HDO ratio is higher than the HDO/H2O ratios by a factor of ≳2 toward both sources. Conclusions. The high D2O/HDO ratios are a strong indication of chemical inheritance of water from the prestellar phase down to the inner warm envelope. This implies that the local cloud conditions in the prestellar phase, such as temperatures and timescales, determine the water chemistry at later stages and could provide a source of chemical differentiation in young systems. In addition, the observed D2O/H2O ratios support an observed dichotomy in the deuterium fractionation of water toward isolated and clustered protostars, namely, a higher D/H ratio toward isolated sources.

Modeling chemistry during star formation: water deuteration in dynamic star-forming regions

Context. Recent observations of the HDO/H2O ratio toward protostars in isolated and clustered environments show an apparent dichotomy, where isolated sources show higher D/H ratios than clustered counterparts. Establishing which physical and chemical processes create this differentiation can provide new insights into the chemical evolution of water during star formation and the chemical diversity during the star formation process and in young planetary systems. Aims. We seek to determine to what degree the local cloud environment influences the D/H ratio of water in the hot corinos toward low-mass protostars and establish which physical and chemical conditions can reproduce the observed HDO/H2O and D2O/HDO ratios in hot corinos. Methods. The evolution of water during star formation is modeled using 3D physicochemical models of a dynamic star-forming environment. The physical evolution during the protostellar collapse is described by tracer particles from a 3D MHD simulation of a molecular cloud region. Each particle trajectory is post-processed using RADMC-3D to calculate the temperature and radiation field. The chemical evolution is simulated using a three-phase grain-surface chemistry model and the results are compared with interferometric observations of H2O, HDO, and D2O in hot corinos toward low-mass protostars. Results. The physicochemical model reproduces the observed HDO/H2O and D2O/HDO ratios in hot corinos, but shows no correlation with cloud environment when similar initial conditions are tested. The observed dichotomy in water D/H ratios requires variation in the initial conditions, for example the duration and temperature of the prestellar phase. Reproducing the observed D/H ratios in hot corinos requires a prestellar phase duration t ~ 1−3 Myr and temperatures in the range T ~ 10−20 K prior to collapse. Furthermore, high cosmic-ray ionization rates (ξH2 ~ 10−15 s−1) appear to be incompatible with the observed D/H ratios toward low-mass protostars. Conclusions. This work demonstrates that the observed differentiation between clustered and isolated protostars stems from differences in the molecular cloud or prestellar core conditions and does not arise during the protostellar collapse itself. The observed D/H ratios for water in hot corinos are consistent with chemical inheritance of water, and no resetting during the protostellar collapse, providing a direct link between the prestellar chemistry and the hot corino.

Observations of the Onset of Complex Organic Molecule Formation in Interstellar Ices

Abstract Isolated dense molecular cores are investigated to study the onset of complex organic molecule formation in interstellar ice. Sampling three cores with ongoing formation of low-mass stars (B59, B335, and L483) and one starless core (L694-2), we sample lines of sight to nine background stars and five young stellar objects (YSOs; A K ∼ 0.5–4.7). Spectra of these stars from 2 to 5 μm with NASA’s Infrared Telescope Facility (IRTF) simultaneously display signatures from the cores of H2O (3.0 μm), CH3OH (C–H stretching mode, 3.53 μm), and CO (4.67 μm) ices. The CO ice is traced by nine stars, in which five show a long-wavelength wing due to a mixture of CO with polar ice (CO r ), presumably CH3OH. Two of these sightlines also show independent detections of CH3OH. For these we find the ratio of the CH3OH:CO r is 0.55 ± 0.06 and 0.73 ± 0.07 from L483 and L694-2, respectively. The detections of both CO and CH3OH for the first time through lines of sight toward background stars observationally constrains the conversion of CO into CH3OH ice. Along the lines of sight, most of the CO exists in the gas phase and ≤15% of the CO is frozen out. However, CH3OH ice is abundant with respect to CO (∼50%) and exists mainly as a CH3OH-rich CO ice layer. Only a small fraction of the lines of sight contains CH3OH ice, presumably those with the highest density. The high conversion of CO to CH3OH can explain the abundances of CH3OH ice found in later stage Class 1 low-mass YSO envelopes (CH3OH:CO r ∼ 0.5–0.6). For high-mass YSOs and one Class 0 YSO, this ratio varies significantly, implying local variations can affect ice formation. The large CH3OH ice abundance indicates that the formation of complex organic molecules is likely during the prestellar phase in cold environments without higher energy particle interactions (e.g., cosmic rays).

The census of dense cores in the Serpens region from the Herschel Gould Belt Survey

ABSTRACT The Herschel Gould Belt survey mapped the nearby (d < 500 pc) star-forming regions to understand better how the prestellar phase influences the star formation process. Here, we report a complete census of dense cores in a ∼15 deg2 area of the Serpens star-forming region located between d ∼ 420 and 484 pc. The PACS and SPIRE cameras imaged this cloud from 70 to 500 μm. With the multiwavelength source extraction algorithm getsources, we extract 833 sources, of which 709 are starless cores and 124 are candidate protostellar cores. We obtain temperatures and masses for all the sample, classifying the starless cores in 604 prestellar cores and 105 unbound cores. Our census of sources is $80{{\ \rm per\ cent}}$ complete for M > 0.8 M⊙ overall. We produce the core mass function (CMF) and compare it with the initial mass function (IMF). The prestellar CMF is consistent with lognormal trend up to ∼2 M⊙, after which it follows a power law with slope of −2.05 ± 0.34. The tail of its CMF is steeper but still compatible with the IMF for the region we studied in this work. We also extract the filaments network of the Serpens region, finding that $81{{\ \rm per\ cent}}$ of prestellar cores lie on filamentary structures. The spatial association between cores and filamentary structure supports the paradigm, suggested by other Herschel observations, that prestellar cores mostly form on filaments. Serpens is confirmed to be a young, low-mass and active star-forming region.

Chemical evolution during the formation of a protoplanetary disk

Context.The chemical composition of protoplanetary disks is expected to impact the composition of the forming planets. Characterizing the diversity of chemical composition in disks and the physicochemical factors that lead to this diversity is consequently of high interest.Aims.The aim of this study is to investigate the chemical evolution from the prestellar phase to the formation of the disk, and to determine the impact that the chemical composition of the cold and dense core has on the final composition of the disk.Methods.We performed 3D nonideal magneto-hydrodynamic (MHD) simulations of a dense core collapse using the adaptive-mesh-refinement RAMSES code. For each particle ending in the young rotationally supported disk, we ran chemical simulations with the three-phase gas-grain chemistry code Nautilus. Two different sets of initial abundances, which are characteristic of cold cores, were considered. The final distributions of the abundances of common species were compared to each other, as well as with the initial abundances of the cold core.Results.We find that the spatial distributions of molecules reflect their sensitivity to the temperature distribution. The main carriers of the chemical elements in the disk are usually the same as the ones in the cold core, except for the S-bearing species, where HS is replaced by H2S3, and the P-bearing species, where atomic P leads to the formation of PO, PN, HCP, and CP. However, the abundances of less abundant species change over time. This is especially the case for “large” complex organic molecules (COMs) such as CH3CHO, CH3NH2, CH3OCH3, and HCOOCH3which see their abundances significantly increase during the collapse. These COMs often present similar abundances in the disk despite significantly different abundances in the cold core. In contrast, the abundances of many radicals decrease with time. A significant number of species still show the same abundances in the cold core and the disk, which indicates efficient formation of these molecules in the cold core. This includes H2O, H2CO, HNCO, and “small” COMs such as CH3OH, CH3CN, and NH2CHO. We computed the MHD resistivities within the disk for the full gas–grain chemical evolution and find results in qualitative agreement with the literature assuming simpler chemical networks.Conclusions.In conclusion, the chemical content of prestellar cores is expected to affect the chemical composition of disks. The impact is more or less important depending on the type of species. Users of stand-alone chemical models of disks should pay special attention to the initial abundances they choose.

First detection of NHD and ND2 in the interstellar medium

Context. Deuterium fractionation processes in the interstellar medium (ISM) have been shown to be highly efficient in the family of nitrogen hydrides. To date, observations have been limited to ammonia (NH2D, NHD2, ND3) and imidogen radical (ND) isotopologues. Aims. We want to explore the high-frequency windows offered by the Herschel Space Observatory to search for deuterated forms of the amidogen radical NH2 and to compare the observations against the predictions of our comprehensive gas-grain chemical model. Methods. Making use of the new molecular spectroscopy data recently obtained at high frequencies for NHD and ND2, we searched for both isotopologues in the spectral survey toward the Class 0 protostar IRAS 16293-2422, a source in which NH3, NH, and their deuterated variants have previously been detected. We used the observations carried out with HIFI (Heterodyne Instrument for the Far-Infrared) in the framework of the key program “Chemical Herschel surveys of star forming regions” (CHESS). Results. We report the first detection of interstellar NHD and ND2. Both species are observed in absorption against the continuum of the protostar. From the analysis of their hyperfine structure, accurate excitation temperature and column density values are determined. The latter were combined with the column density of the parent species NH2 to derive the deuterium fractionation in amidogen. We find a high deuteration level of amidogen radical in IRAS 16293-2422, with a deuterium enhancement about one order of magnitude higher than that predicted by earlier astrochemical models. Such a high enhancement can only be reproduced by a gas-grain chemical model if the pre-stellar phase preceding the formation of the protostellar system has a long duration: on the order of one million years. Conclusions. The amidogen D/H ratio measured in the low-mass protostar IRAS 16293-2422 is comparable to that derived for the related species imidogen and much higher than that observed for ammonia. Additional observations of these species will provide more insights into the mechanism of ammonia formation and deuteration in the ISM. Finally, we indicate the current possibilities to further explore these species at submillimeter wavelengths.

Chemical Variation among Protostellar Cores: Dependence on Prestellar Core Conditions

Abstract Hot corino chemistry and warm carbon chain chemistry (WCCC) are driven by gas–grain interactions in star-forming cores: radical–radical recombination reactions to form complex organic molecules (COMs) in the ice mantle, sublimation of CH4 and COMs, and their subsequent gas-phase reactions. These chemical features are expected to depend on the composition of the ice mantle, which is set in the prestellar phase. We calculated the gas–grain chemical reaction network considering a layered ice mantle structure in star-forming cores to investigate how the hot corino chemistry and WCCC depend on the physical condition of the static phase before the onset of gravitational collapse. We found that WCCC becomes more active if the temperature is lower, or the visual extinction is lower in the static phase, or the static phase is longer. The dependence of hot corino chemistry on the static-phase condition is more complex. While CH3OH is less abundant in the models with a warmer static phase, some COMs are formed efficiently in those warm models because there are various formation paths of COMs. If the visual extinction is lower, photolysis makes COMs less abundant in the static phase. Once the collapse starts and visual extinction increases, however, COMs can be formed efficiently. The duration of the static phase does not largely affect COM abundances. The chemical diversity between prototypical hot corinos and hybrid sources, in which both COMs and carbon chains are reasonably abundant, can be explained by the variation of prestellar conditions. Deficiency of gaseous COMs in prototypical WCCC sources is, however, hard to reproduce within our models.