Context.
Complex organic molecules are detected in many sources in the warm inner regions of envelopes surrounding deeply embedded protostars. Exactly how these species form remains an open question.
...Aims.
This study aims to constrain the formation of complex organic molecules through comparisons of their abundances towards the Class 0 protostellar binary IRAS 16293–2422.
Methods.
We utilised observations from the ALMA Protostellar Interferometric Line Survey of IRAS 16293–2422. The species identification and the rotational temperature and column density estimation were derived by fitting the extracted spectra towards IRAS 16293–2422 A and IRAS 16293–2422 B with synthetic spectra. The majority of the work in this paper pertains to the analysis of IRAS 16293–2422 A for a comparison with the results from the other binary component, which have already been published.
Results.
We detect 15 different complex species, as well as 16 isotopologues towards the most luminous companion protostar IRAS 16293–2422 A. Tentative detections of an additional 11 isotopologues are reported. We also searched for and report on the first detections of methoxymethanol (CH
3
OCH
2
OH) and trans-ethyl methyl ether (t-C
2
H
5
OCH
3
) towards IRAS 16293–2422 B and the follow-up detection of deuterated isotopologues of acetaldehyde (CH
2
DCHO and CH
3
CDO). Twenty-four lines of doubly-deuterated methanol (CHD
2
OH) are also identified.
Conclusions.
The comparison between the two protostars of the binary system shows significant differences in abundance for some of the species, which are partially correlated to their spatial distribution. The spatial distribution is consistent with the sublimation temperature of the species; those with higher expected sublimation temperatures are located in the most compact region of the hot corino towards IRAS 16293–2422 A. This spatial differentiation is not resolved in IRAS 16293–2422 B and will require observations at a higher angular resolution. In parallel, the list of identified CHD
2
OH lines shows the need of accurate spectroscopic data including their line strength.
Context. One of the important questions of astrochemistry is how complex organic molecules, including potential prebiotic species, are formed in the envelopes around embedded protostars. The ...abundances of minor isotopologues of a molecule, in particular the D- and 13C-bearing variants, are sensitive to the densities, temperatures and timescales characteristic of the environment in which they form, and can therefore provide important constraints on the formation routes and conditions of individual species. Aims. The aim of this paper is to systematically survey the deuteration and the 13C content of a variety of oxygen-bearing complex organic molecules on solar system scales toward the “B component” of the protostellar binary IRAS16293–2422. Methods. We have used the data from an unbiased molecular line survey of the protostellar binary IRAS16293−2422 between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The data probe scales of 60 AU (diameter) where most of the organic molecules are expected to have sublimated off dust grains and be present in the gas phase. The deuterated and 13C isotopic species of ketene, acetaldehyde and formic acid, as well as deuterated ethanol, are detected unambiguously for the first time in the interstellar medium. These species are analysed together with the 13C isotopic species of ethanol, dimethyl ether and methyl formate along with mono-deuterated methanol, dimethyl ether and methyl formate. Results. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of ≈2% and the other, the more complex species, D/H ratios of 4–8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short timescale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 and 300 K. This likely reflects the binding energies of the individual species, in good agreement with what has previously been found for high-mass sources. For dimethyl ether, the 12C/13C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. Tentative identifications suggest that the same may apply for 13C isotopologues of methyl formate and ethanol. If confirmed, this may be a clue to their formation at the late prestellar or early protostellar phases with an enhancement of the available 13C relative to 12C related to small differences in binding energies for CO isotopologues or the impact of FUV irradiation by the central protostar. Conclusions. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.
Context. Much attention has been placed on the dust distribution in protostellar envelopes, but there are still many unanswered questions regarding the physico-chemical structure of the gas. Aims. ...Our aim is to start identifying the factors that determine the chemical structure of protostellar regions, by studying and comparing low-mass embedded systems in key molecular tracers. Methods. The cold and warm chemical structures of two embedded Class 0 systems, IRAS 16293−2422 and VLA 1623−2417 were characterized through interferometric observations. DCO+, N2H+, and N2D+ were used to trace the spatial distribution and physics of the cold regions of the envelope, while c-C3H2 and C2H from models of the chemistry are expected to trace the warm (UV-irradiated) regions. Results. The two sources show a number of striking similarities and differences. DCO+ consistently traces the cold material at the disk-envelope interface, where gas and dust temperatures are lowered due to disk shadowing. N2H+ and N2D+, also tracing cold gas, show low abundances toward VLA 1623−2417, but for IRAS 16293−2422, the distribution of N2D+ is consistent with the same chemical models that reproduce DCO+. The two systems show different spatial distributions c-C3H2 and C2H. For IRAS 16293−2422, c-C3H2 traces the outflow cavity wall, while C2H is found in the envelope material but not the outflow cavity wall. In contrast, toward VLA 1623−2417 both molecules trace the outflow cavity wall. Finally, hot core molecules are abundantly observed toward IRAS 16293−2422 but not toward VLA 1623−2417. Conclusions. We identify temperature as one of the key factors in determining the chemical structure of protostars as seen in gaseous molecules. More luminous protostars, such as IRAS 16293−2422, will have chemical complexity out to larger distances than colder protostars, such as VLA 1623−2417. Additionally, disks in the embedded phase have a crucial role in controlling both the gas and dust temperature of the envelope, and consequently the chemical structure.
Context. The Class 0 protostellar binary IRAS 16293–2422 is an interesting target for (sub)millimeter observations due to, both, the rich chemistry toward the two main components of the binary and ...its complex morphology. Its proximity to Earth allows the study of its physical and chemical structure on solar system scales using high angular resolution observations. Such data reveal a complex morphology that cannot be accounted for in traditional, spherical 1D models of the envelope. Aims. The purpose of this paper is to study the environment of the two components of the binary through 3D radiative transfer modeling and to compare with data from the Atacama Large Millimeter/submillimeter Array. Such comparisons can be used to constrain the protoplanetary disk structures, the luminosities of the two components of the binary and the chemistry of simple species. Methods. We present 13CO, C17O and C18O J = 3–2 observations from the ALMA Protostellar Interferometric Line Survey (PILS), together with a qualitative study of the dust and gas density distribution of IRAS 16293–2422. A 3D dust and gas model including disks and a dust filament between the two protostars is constructed which qualitatively reproduces the dust continuum and gas line emission. Results. Radiative transfer modeling in our sampled parameter space suggests that, while the disk around source A could not be constrained, the disk around source B has to be vertically extended. This puffed-up structure can be obtained with both a protoplanetary disk model with an unexpectedly high scale-height and with the density solution from an infalling, rotating collapse. Combined constraints on our 3D model, from observed dust continuum and CO isotopologue emission between the sources, corroborate that source A should be at least six times more luminous than source B. We also demonstrate that the volume of high-temperature regions where complex organic molecules arise is sensitive to whether or not the total luminosity is in a single radiation source or distributed into two sources, affecting the interpretation of earlier chemical modeling efforts of the IRAS 16293–2422 hot corino which used a single-source approximation. Conclusions. Radiative transfer modeling of source A and B, with the density solution of an infalling, rotating collapse or a protoplanetary disk model, can match the constraints for the disk-like emission around source A and B from the observed dust continuum and CO isotopologue gas emission. If a protoplanetary disk model is used around source B, it has to have an unusually high scale-height in order to reach the dust continuum peak emission value, while fulfilling the other observational constraints. Our 3D model requires source A to be much more luminous than source B; LA ~ 18 L⊙ and LB ~ 3 L⊙.
Formamide (NH2CHO) has previously been detected in several star-forming regions and is thought to be a precursor for different prebiotic molecules. Its formation mechanism is still debated, however. ...Observations of formamide, related species, and their isopotologues may provide useful clues to the chemical pathways leading to their formation. The Protostellar Interferometric Line Survey (PILS) represents an unbiased, high angular resolution and sensitivity spectral survey of the low-mass protostellar binary IRAS 16293–2422 with the Atacama Large Millimeter/submillimeter Array (ALMA). For the first time, we detect the three singly deuterated forms of NH2CHO (NH2CDO, cis- and trans-NHDCHO), as well as DNCO towards the component B of this binary source. The images reveal that the different isotopologues are all present in the same region. Based on observations of the 13C isotopologues of formamide and a standard 12C/13C ratio, the deuterium fractionation is found to be similar for the three different forms with a value of about 2%. The DNCO/HNCO ratio is also comparable to the D/H ratio of formamide (~1%). These results are in agreement with the hypothesis that NH2CHO and HNCO are chemically related through grain-surface formation.
Context. One of the open questions in astrochemistry is how complex organic and prebiotic molecules are formed. The unsurpassed sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) ...takes the quest for discovering molecules in the warm and dense gas surrounding young stars to the next level. Aims. Our aim is to start the process of compiling an inventory of oxygen-bearing complex organic molecules toward the solar-type Class 0 protostellar binary IRAS 16293-2422 from an unbiased spectral survey with ALMA, Protostellar Interferometric Line Survey (PILS). Here we focus on the new detections of ethylene oxide (c-C2H4O), acetone (CH3COCH3), and propanal (C2H5CHO). Methods. With ALMA, we surveyed the spectral range from 329 to 363 GHz at 0.5″ (60 AU diameter) resolution. Using a simple model for the molecular emission in local thermodynamical equilibrium, the excitation temperatures and column densities of each species were constrained. Results. We successfully detect propanal (44 lines), ethylene oxide (20 lines) and acetone (186 lines) toward one component of the protostellar binary, IRAS 16293B. The high resolution maps demonstrate that the emission for all investigated species originates from the compact central region close to the protostar. This, along with a derived common excitation temperature of Tex ≈ 125 K, is consistent with a coexistence of these molecules in the same gas. Conclusions. The observations mark the first detections of acetone, propanal and ethylene oxide toward a low-mass protostar. The relative abundance ratios of the two sets of isomers, a CH3COCH3/C2H5CHO ratio of 8 and a CH3CHO/c-C2H4O ratio of 12, are comparable to previous observations toward high-mass protostars. The majority of observed abundance ratios from these results as well as those measured toward high-mass protostars are up to an order of magnitude above the predictions from chemical models. This may reflect either missing reactions or uncertain rates in the chemical networks. The physical conditions, such as temperatures or densities, used in the models, may not be applicable to solar-type protostars either.
Studies of deuterated isotopologues of complex organic molecules can provide important constraints on their origin in star formation regions. In particular, the abundances of deuterated species are ...very sensitive to the physical conditions in the environment where they form. Because the temperatures in star formation regions are low, these isotopologues are enhanced to significant levels, which enables the detection of multiply deuterated species. However, for complex organic species, so far only the multiply deuterated variants of methanol and methyl cyanide have been reported. The aim of this paper is to initiate the characterisation of multiply deuterated variants of complex organic species with the first detection of doubly deuterated methyl formate, CHD2OCHO. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) of the protostellar binary IRAS 16293–2422 in the spectral range of 329.1 GHz to 362.9 GHz. Spectra towards each of the two protostars are extracted and analysed using a local thermal equilibrium model in order to derive the abundances of methyl formate and its deuterated variants. We report the first detection of doubly deuterated methyl formate CHD2OCHO in the ISM. The D-to-H ratio (D/H ratio) of CHD2OCHO is found to be 2–3 times higher than the D/H ratio of CH2DOCHO for both sources, similar to the results for formaldehyde from the same dataset. The observations are compared to a gas-grain chemical network coupled to a dynamical physical model, tracing the evolution of a molecular cloud until the end of the Class 0 protostellar stage. The overall D/H ratio enhancements found in the observations are of about the same magnitude as the predictions from the model for the early stages of Class 0 protostars. However, that the D/H ratio of CHD2OCHO is higher than that of CH2DOCHO is still not predicted by the model. This suggests that a mechanism enhances the D/H ratio of singly and doubly deuterated methyl formate that is not in the model, for instance, mechanisms for H–D substitutions. This new detection provides an important constraint on the formation routes of methyl formate and outlines a path forward in terms of using these ratios to determine the formation of organic molecules through observations of differently deuterated isotopologues towards embedded protostars.
ABSTRACT
To understand how phosphorus (P)-bearing molecules are formed in star-forming regions, we have analysed the Atacama Large Millimeter/Submillimeter Array (ALMA) observations of PN and PO ...towards the massive star-forming region AFGL 5142, combined with a new analysis of the data of the comet 67P/Churyumov–Gerasimenko taken with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard Rosetta. The ALMA maps show that the emission of PN and PO arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. PO is more abundant than PN in most of the spots, with the PO/PN ratio increasing as a function of the distance to the protostar. Our data favour a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by UV photons from the protostar allows efficient formation of the two species in the cavity walls. Our analysis of the ROSINA data has revealed that PO is the main carrier of P in the comet, with PO/PN > 10. Since comets may have delivered a significant amount of prebiotic material to the early Earth, this finding suggests that PO could contribute significantly to the phosphorus reservoir during the dawn of our planet. There is evidence that PO was already in the cometary ices prior to the birth of the Sun, so the chemical budget of the comet might be inherited from the natal environment of the Solar system, which is thought to be a stellar cluster including also massive stars.
Abstract
The evolutionary past of our Solar system can be pieced together by comparing analogous low-mass protostars with remnants of our Protosolar Nebula – comets. Sulphur-bearing molecules may be ...unique tracers of the joint evolution of the volatile and refractory components. ALMA Band 7 data from the large unbiased Protostellar Interferometric Line Survey are used to search for S-bearing molecules in the outer disc-like structure, ∼60 au from IRAS 16293–2422 B, and are compared with data on 67P/Churyumov–Gerasimenko (67P/C–G) stemming from the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument aboard Rosetta. Species such as SO2, SO, OCS, CS, H2CS, H2S, and CH3SH are detected via at least one of their isotopologues towards IRAS 16293–2422 B. The search reveals a first-time detection of OC33S towards this source and a tentative first-time detection of C36S towards a low-mass protostar. The data show that IRAS 16293–2422 B contains much more OCS than H2S in comparison to 67P/C–G; meanwhile, the SO/SO2 ratio is in close agreement between the two targets. IRAS 16293–2422 B has a CH3SH/H2CS ratio in range of that of our Solar system (differences by a factor of 0.7–5.3). It is suggested that the levels of UV radiation during the initial collapse of the systems may have varied and have potentially been higher for IRAS 16293–2422 B due to its binary nature; thereby, converting more H2S into OCS. It remains to be conclusively tested if this also promotes the formation of S-bearing complex organics. Elevated UV levels of IRAS 16293–2422 B and a warmer birth cloud of our Solar system may jointly explain the variations between the two low-mass systems.
Context. The inner regions of the envelopes surrounding young protostars are characterized by a complex chemistry, with prebiotic molecules present on the scales where protoplanetary disks eventually ...may form. The Atacama Large Millimeter/submillimeter Array (ALMA) provides an unprecedented view of these regions zooming in on solar system scales of nearby protostars and mapping the emission from rare species. Aims. The goal is to introduce a systematic survey, the Protostellar Interferometric Line Survey (PILS), of the chemical complexity of one of the nearby astrochemical templates, the Class 0 protostellar binary IRAS 16293-2422, using ALMA in order to understand the origin of the complex molecules formed in its vicinity. In addition to presenting the overall survey, the analysis in this paper focuses on new results for the prebiotic molecule glycolaldehyde, its isomers, and rarer isotopologues and other related molecules. Methods. An unbiased spectral survey of IRAS 16293-2422 covering the full frequency range from 329 to 363 GHz (0.8 mm) has been obtained with ALMA, in addition to a few targeted observations at 3.0 and 1.3 mm. The data consist of full maps of the protostellar binary system with an angular resolution of 0.5" (60 AU diameter), a spectral resolution of 0.2 km s super(-1), and a sensitivity of 4-5 mJy beam super(-1) km s super(-1), which is approximately two orders of magnitude better than any previous studies. Results. More than 10000 features are detected toward one component in the protostellar binary, corresponding to an average line density of approximately one line per 3 km s super(-1). Glycolaldehyde; its isomers, methyl formate and acetic acid; and its reduced alcohol, ethylene glycol, are clearly detected and their emission well-modeled with an excitation temperature of 300 K. For ethylene glycol both lowest state conformers, aGg' and gGg', are detected, the latter for the first time in the interstellar medium (ISM). The abundance of glycolaldehyde is comparable to or slightly larger than that of ethylene glycol. In comparison to the Galactic Center these two species are over-abundant relative to methanol, possibly an indication of formation of the species at low temperatures in CO-rich ices during the infall of the material toward the central protostar. Both super(13) C and the deuterated isotopologues of glycolaldehyde are detected, also for the first time ever in the ISM. For the deuterated species, a D/H ratio of approximate5% is found with no differences between the deuteration in the different functional groups of glycolaldehyde, in contrast to previous estimates for methanol and recent suggestions of significant equilibration between water and -OH functional groups at high temperatures. Measurements of the super(13) C-species lead to a super(12) C: super(13) C ratio of approximate30, lower than the typical ISM value. This low ratio may reflect an enhancement of super(13) CO in the ice due to either ion-molecule reactions in the gas before freeze-out or to differences in the temperatures where super(12) CO and super(13) CO ices sublimate. Conclusions. The results reinforce the importance of low-temperature grain surface chemistry for the formation of prebiotic molecules seen here in the gas after sublimation of the entire ice mantle. Systematic surveys of the molecules thought to be chemically related, as well as the accurate measurements of their isotopic composition, hold strong promise for understanding the origin of prebiotic molecules in the earliest stages of young stars.
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