Context.
Complex organic molecules (COMs), especially the oxygen-bearing species, have been observed to be abundant in the gas phase toward low-mass and high-mass protostars. Deep line surveys have ...been carried out only for a limited number of well-known star-forming regions using the Atacama Large Millimeter/submillimeter Array (ALMA), which has unprecedented resolution and sensitivity, and statistical studies on oxygen-bearing COMs (O-COMs) in a large sample of high-mass protostars using ALMA are still lacking.
Aims.
We aim to determine the column density ratios of six O-COMs with respect to methanol (CH
3
OH) in a sample of 14 high-mass protostellar sources in order to investigate their origin through ice and/or gas-phase chemistry. The selected species are: acetalde-hyde (CH
3
CHO), ethanol (C
2
H
5
OH), dimethyl ether (DME; CH
3
OCH
3
), methyl formate (MF; CH
3
OCHO), glycolaldehyde (GA; CH
2
OHCHO), and ethylene glycol (EG; (CH
2
OH)
2
).
Methods.
We fit the spectra of 14 high-mass sources observed as part of the Complex Chemistry in hot Cores with ALMA (CoCCoA) survey and derived the column densities and excitation temperatures of the six selected O-COMs. We used the minor isotopologue of methanol CH
3
18
OH to infer the column density of the main isotopologue CH
3
OH, of which the lines are generally optically thick. We compared our O-COM ratios with those of five low-mass protostars studied with ALMA from the available literature as well as with the results from experiments and simulations.
Results.
Although the CoCCoA sources have different morphologies and brightness in their continuum and methanol emission, the O-COM ratios with respect to methanol have very similar values in the high-mass and low-mass samples. The DME and MF have the highest and most constant ratios within one order of magnitude, while the other four species have lower ratios and exhibit larger scatter by one to two orders of magnitude. The ratio between DME and MF is close to one, which agrees well with previous observational findings. Current simulations and experiments can reproduce most observational trends with a few exceptions. For example, they tend to overestimate the abundance of ethanol and GA with respect to methanol.
Conclusions.
The constant column density ratios of the selected O-COMs among the low- and high-mass sources suggest that these species are formed in similar environments during star formation, probably on icy dust grains in the pre-stellar stages. Where deviations are found, hypotheses exist to explain the differences between observations, simulations, and experiments, such as the involvement of gas-phase chemistry and different emitting areas of molecules.
Context. Observations of edge-on disks are an important tool for constraining general protoplanetary disk properties that cannot be determined in any other way. However, most radiative transfer ...models cannot simultaneously reproduce the spectral energy distributions (SEDs) and resolved scattered light and submillimeter observations of these systems because the geometry and dust properties are different at different wavelengths. Aims. We simultaneously constrain the geometry of the edge-on protoplanetary disk HH 48 NE and the characteristics of the host star. HH 48 NE is part of the JWST early-release science program Ice Age. This work serves as a stepping stone toward a better understanding of the physical structure of the disk and of the icy chemistry in this particular source. This type of modeling lays the groundwork for studying other edge-on sources that are to be observed with the JWST. Methods. We fit a parameterized dust model to HH 48 NE by coupling the radiative transfer code RADMC-3D and a Markov chain Monte Carlo framework. The dust structure was fit independently to a compiled SED, a scattered light image at 0.8 µm, and an ALMA dust continuum observation at 890 µm. Results. We find that 90% of the dust mass in HH 48 NE is settled to the disk midplane. This is less than in average disks. The atmospheric layers of the disk also exclusively contain large grains (0.3–10 µm). The exclusion of small grains in the upper atmosphere likely has important consequences for the chemistry because high-energy photons can penetrate very deeply. The addition of a relatively large cavity (~50 au in radius) is necessary to explain the strong mid-infrared emission and to fit the scattered light and continuum observations simultaneously.
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 com- paring 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. Key
Context. Herschel observations of water and highly excited CO (J > 9) have allowed the physical and chemical conditions in the more active parts of protostellar outflows to be quantified in detail ...for the first time. However, to date, the studied samples of Class 0/I protostars in nearby star-forming regions have been selected from bright, well-known sources and have not been large enough for statistically significant trends to be firmly established. Aims. We aim to explore the relationships between the outflow, envelope and physical properties of a flux-limited sample of embedded low-mass Class 0/I protostars. Methods. We present spectroscopic observations in H2O, CO and related species with Herschel HIFI and PACS, as well as ground-based follow-up with the JCMT and APEX in CO, HCO+ and isotopologues, of a sample of 49 nearby (d < 500 pc) candidate protostars selected from Spitzer and Herschel photometric surveys of the Gould Belt. This more than doubles the sample of sources observed by the WISH and DIGIT surveys. These data are used to study the outflow and envelope properties of these sources. We also compile their continuum spectral energy distributions (SEDs) from the near-IR to mm wavelengths in order to constrain their physical properties (e.g. Lbol, Tbol and Menv). Results. Water emission is dominated by shocks associated with the outflow, rather than the cooler, slower entrained outflowing gas probed by ground-based CO observations. These shocks become less energetic as sources evolve from Class 0 to Class I. Outflow force, measured from low-J CO, also decreases with source evolutionary stage, while the fraction of mass in the outflow relative to the total envelope (i.e. Mout/Menv) remains broadly constant between Class 0 and I. The median value of ~1% is consistent with a core to star formation efficiency on the order of 50% and an outflow duty cycle on the order of 5%. Entrainment efficiency, as probed by FCO/Ṁacc, is also invariant with source properties and evolutionary stage. The median value implies a velocity at the wind launching radius of 6.3 km s-1, which in turn suggests an entrainment efficiency of between 30 and 60% if the wind is launched at ~1 AU, or close to 100% if launched further out. LO i is strongly correlated with Lbol but not with Menv, in contrast to low-J CO, which is more closely correlated with the latter than the former. This suggests that O i traces the present-day accretion activity of the source while CO traces time-averaged accretion over the dynamical timescale of the outflow. H2O is more strongly correlated with Menv than Lbol, but the difference is smaller than low-J CO, consistent with water emission primarily tracing actively shocked material between the wind, traced by O i, and the entrained molecular outflow, traced by low-J CO. LO i does not vary from Class 0 to Class I, unlike CO and H2O. This is likely due to the ratio of atomic to molecular gas in the wind increasing as the source evolves, balancing out the decrease in mass accretion rate. Infall signatures are detected in HCO+ and H2O in a few sources, but still remain surprisingly illusive in single-dish observations.
Physicochemical models: source-tailored or generic? Kulterer, Beatrice M; Drozdovskaya, Maria N; Coutens, Audrey ...
Monthly notices of the Royal Astronomical Society,
10/2020, Letnik:
498, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Physicochemical models can be powerful tools to trace the chemical evolution of a protostellar system and allow to constrain its physical conditions at formation. The aim of this work is to ...assess whether source-tailored modelling is needed to explain the observed molecular abundances around young, low-mass protostars or if, and to what extent, generic models can improve our understanding of the chemistry in the earliest stages of star formation. The physical conditions and the abundances of simple, most abundant molecules based on three models are compared. After establishing the discrepancies between the calculated chemical output, the calculations are redone with the same chemical model for all three sets of physical input parameters. With the differences arising from the chemical models eliminated, the output is compared based on the influence of the physical model. Results suggest that the impact of the chemical model is small compared to the influence of the physical conditions, with considered time-scales having the most drastic effect. Source-tailored models may be simpler by design; however, likely do not sufficiently constrain the physical and chemical parameters within the global picture of star-forming regions. Generic models with more comprehensive physics may not provide the optimal match to observations of a particular protostellar system, but allow a source to be studied in perspective of other star-forming regions.
Context
. Observations of edge-on disks are an important tool for constraining general protoplanetary disk properties that cannot be determined in any other way. However, most radiative transfer ...models cannot simultaneously reproduce the spectral energy distributions (SEDs) and resolved scattered light and submillimeter observations of these systems because the geometry and dust properties are different at different wavelengths.
Aims
. We simultaneously constrain the geometry of the edge-on protoplanetary disk HH 48 NE and the characteristics of the host star. HH 48 NE is part of the JWST early-release science program Ice Age. This work serves as a stepping stone toward a better understanding of the physical structure of the disk and of the icy chemistry in this particular source. This type of modeling lays the groundwork for studying other edge-on sources that are to be observed with the JWST.
Methods
. We fit a parameterized dust model to HH 48 NE by coupling the radiative transfer code
RADMC-3D
and a Markov chain Monte Carlo framework. The dust structure was fit independently to a compiled SED, a scattered light image at 0.8 µm, and an ALMA dust continuum observation at 890 µm.
Results
. We find that 90% of the dust mass in HH 48 NE is settled to the disk midplane. This is less than in average disks. The atmospheric layers of the disk also exclusively contain large grains (0.3–10 µm). The exclusion of small grains in the upper atmosphere likely has important consequences for the chemistry because high-energy photons can penetrate very deeply. The addition of a relatively large cavity (~50 au in radius) is necessary to explain the strong mid-infrared emission and to fit the scattered light and continuum observations simultaneously.
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-1, and a sensitivity of 4−5 mJy beam-1 km s-1, which is approximately two orders of magnitude better than any previous studies. Results. More than 10 000 features are detected toward one component in the protostellar binary, corresponding to an average line density of approximately one line per 3 km s-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 13C 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 ≈5% 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 13C-species lead to a 12C:13C ratio of ≈30, lower than the typical ISM value. This low ratio may reflect an enhancement of 13CO in the ice due to either ion-molecule reactions in the gas before freeze-out or to differences in the temperatures where 12CO and 13CO 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.
The edge-on protoplanetary disk HH 48 NE Sturm, J. A.; McClure, M. K.; Bergner, J. B. ...
Astronomy and astrophysics (Berlin),
9/2023, Letnik:
677
Journal Article
Recenzirano
Context.
The abundance and distribution of ice in protoplanetary disks is critical for an understanding of the link between the composition of circumstellar matter and the composition of exoplanets. ...Edge-on protoplanetary disks are a useful tool for constraining this ice composition and its location in the disk because the spectral signatures of the ice can be observed in absorption against the continuum emission that arises from the warmer regions in the central disk.
Aims.
The aim of this work is to model ice absorption features in protoplanetary disks and to determine how well the abundance of the main ice species throughout the disk can be determined within the uncertainty of the physical parameter space. The edge-on proto-planetary disk around HH 48 NE, a target of the
James Webb
Space Telescope Early Release program Ice Age, is used as a reference system.
Methods.
We used the full anisotropic scattering capabilities of the radiative transfer code RADMC-3D to ray-trace the mid-infrared continuum. Using a constant parameterized ice abundance, we added ice opacities to the dust opacity in regions in which the disk was cold enough for the main carbon, oxygen, and nitrogen carriers to freeze out.
Results.
The global abundance relative to the dust content of the main ice carriers in HH 48 NE can be determined within a factor of 3 when the uncertainty of the physical parameters is taken into account. Ice features in protoplanetary disks can be saturated at an optical depth of ≲1 due to local saturation. Ices are observed at various heights in the disk model, but in this model, spatial information is lost for features at wavelengths >7 µm when observing with
James Webb
Space Telescope because the angular resolution decreases towards longer wavelengths. Spatially observed ice optical depths cannot be directly related to column densities, as would be the case for direct absorption against a bright continuum source, because of radiative transfer effects. Vertical snowlines will not be a clear transition because the height of the snow surface increases radially, but their location may be constrained from observations using radiative transfer modeling. Radial snowlines are not really accessible. Not only the ice abundance, but also the inclination, the settling, the grain size distribution, and the disk mass have a strong impact on the observed ice absorption features in disks. Relative changes in the ice abundance can only be inferred from observations if the source structure is well constrained.
ABSTRACT
Deuterated methanol is one of the most robust windows astrochemists have on the individual chemical reactions forming deuterium-bearing molecules and the physicochemical history of the ...regions where they reside. The first-time detection of mono- and di-deuterated methanol in a cometary coma is presented for comet 67P/Churyumov–Gerasimenko using Rosetta–ROSINA data. D-methanol (CH3OD and CH2DOH combined) and D2-methanol (CH2DOD and CHD2OH combined) have an abundance of 5.5 ± 0.46 and 0.00069 ± 0.00014 per cent relative to normal methanol. The data span a methanol deuteration fraction (D/H ratio) in the 0.71−6.6 per cent range, accounting for statistical corrections for the location of D in the molecule and including statistical error propagation in the ROSINA measurements. It is argued that cometary CH2DOH forms from CO hydrogenation to CH3OH and subsequent H–D substitution reactions in CH3–R. CHD2OH is likely produced from deuterated formaldehyde. Meanwhile, CH3OD and CH2DOD could form via H–D exchange reactions in OH–R in the presence of deuterated water ice. Methanol formation and deuteration is argued to occur at the same epoch as D2O formation from HDO, with formation of mono-deuterated water, hydrogen sulphide, and ammonia occurring prior to that. The cometary D-methanol/methanol ratio is demonstrated to agree most closely with that in prestellar cores and low-mass protostellar regions. The results suggest that cometary methanol stems from the innate cold (10–20 K) prestellar core that birthed our Solar system. Cometary volatiles individually reflect the evolutionary phases of star formation from cloud to core to protostar.
We investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. We adopt a semi-analytical, axisymmetric, two-dimensional collapsing core model with ...post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. The physical and chemical evolutions are followed until the end of the main accretion phase. In our models, when mass averaged over the whole disk, a forming disk has a similar H sub(2) O abundance and HDO/H sub(2) O abundance ratio (within a factor of 2) as the precollapse values of these quantities, regardless of time. Consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. On the other hand, the local vertically averaged H sub(2) O ice abundance and HDO/H sub(2) O ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. Key parameters for the local variations are the fluence of stellar UV photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. We also find that even if interstellar water ice is destroyed by stellar UV and (partly) reformed prior to disk entry, the HDO/H sub(2) O ratio in reformed water ice is similar to the original value. This finding indicates that some caution is needed in discussions on the prestellar inheritance of H sub(2) O based on comparisons between the observationally derived HDO/H sub(2) O ratio in clouds/cores and that in disks/comets. Alternatively, we propose that the ratio of D sub(2) O/HDO to HDO/H sub(2) O better probes the prestellar inheritance of H sub(2) O. It is also found that in forming disks icy organics are more enriched in deuterium than water ice. The differential deuterium fractionation in water and organics is inherited from prestellar stages.
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