CH3OH Lines Research Articles

CH3OH and Its Deuterated Species in the Disk/Envelope System of the Low-mass Protostellar Source B335

Deuterium fractionation in the closest vicinity of a protostar is important in understanding its potential heritage to a planetary system. Here, we have detected the spectral line emission of CH3OH and its three deuterated species, CH2DOH, CHD2OH, and CH3OD, toward the low-mass protostellar source B335 at a resolution of 0.″03 (5 au) with Atacama Large Millimeter/submillimeter Array. They have a ring distribution within the radius of 24 au with the intensity depression at the continuum peak. We derive the column densities and abundance ratios of the above species at six positions in the disk/envelope system as well as the continuum peak. The D/H ratio of CH3OH is ∼[0.03–0.13], which is derived by correcting the statistical weight of 3 for CH2DOH. The [CHD2OH]/[CH2DOH] ratio is derived to be higher ([0.14–0.29]). On the other hand, the [CH2DOH]/[CH3OD] ratio ([4.9–15]) is higher than the statistical ratio of 3 and is comparable to those reported for other low-mass sources. We study the physical structure on a few astronomical unit scales in B335 by analyzing the CH3OH (183,15 − 182,16, A) and HCOOH (120,12 − 110,11) line emission. Velocity structures of these lines are reasonably explained as the infalling-rotating motion. The protostellar mass and the upper limit to the centrifugal barrier are thus derived to be 0.03–0.07 M ⊙ and <7 au, respectively, showing that B335 harbors a young protostar with a tiny disk structure. Such youth of the protostar may be related to the relatively high [CH2DOH]/[CH3OH] ratio.

ALMA view of the L1448-mm protostellar system on disk scales: CH3OH and H13CN as new disk wind tracers

Protostellar disks are known to accrete; however, the exact mechanism that extracts the angular momentum and drives accretion in the low-ionization “dead” region of the disk is under debate. In recent years, magnetohydrodynamic (MHD) disk winds have become a popular solution. Even so, observations of these winds require both high spatial resolution (~10 s au) and high sensitivity, which has resulted in only a handful of MHD disk wind candidates to date. In this work we present high angular resolution (~30 au) ALMA observations of the emblematic L1448-mm protostellar system and find suggestive evidence for an MHD disk wind. The disk seen in dust continuum (~0.9 mm) has a radius of ~23 au. Rotating infall signatures in H13CO+ indicate a central mass of 0.4 ± 0.1 M⊙ and a centrifugal radius similar to the dust disk radius. Above the disk, we identify rotation signatures in the outflow traced by H13CN, CH3OH, and SO lines and find a kinematical structure consistent with theoretical predictions for MHD disk winds. This is the first detection of an MHD disk wind candidate in H13CN and CH3OH. The wind launching region estimated from cold MHD wind theory extends out to the disk edge. The magnetic lever arm parameter would be λϕ ≃ 1.7, in line with recent non-ideal MHD disk models. The estimated mass-loss rate is approximately four times the protostellar accretion rate (Ṁacc ≃ 2 × 10−6M⊙ yr−1) and suggests that the rotating wind could carry enough angular momentum to drive disk accretion.

Torsion-Rotational Transitions in Methanol as a Probe of Fundamental Physical Constants—Electron and Proton Masses

We report on the using of torsion-rotational transitions in the CH3OH and 13CH3OH molecules to evaluate possible variations of the electron-to-proton mass ratio μ = me/mp from spectral observations of emission lines detected in the microwave range towards the dense molecular cloud Orion-KL. An estimate of the upper limit on the relative changes in μ is obtained by two independent ways with 13CH3OH lines and with the combination of 13CH3OH and CH3OH lines. The calculated upper limit Δμ/μ < 1.1 × 10–8(1σ) is in line with the most stringent constraints on the variability of fundamental physical constants established by other astrophysical methods.

Broadband VLA Spectral-line Survey of a Sample of Ionized Jet Candidates

The study of the interaction between ionized jets, molecular outflows, and their environments is critical to understanding high-mass star formation, especially because jets and outflows are thought to be key in the transfer of angular momentum outward from accretion disks. We report a low spectral resolution Karl G. Jansky Very Large Array (VLA) survey for OH, NH3, CH3OH, and hydrogen radio recombination lines, toward a sample of 58 high-mass star-forming regions that contain numerous ionized jet candidates. The observations are from a survey designed to detect radio continuum; the novel aspect of this work is to search for spectral lines in broadband VLA data (we provide the script developed in this work to facilitate exploration of other data sets). We report detection of 25 GHz CH3OH transitions toward 10 sources; 5 of them also show NH3 emission. We found that most of the sources detected in CH3OH and NH3 have been classified as ionized jets or jet candidates and that the emission lines are coincident with, or very near (≲0.1 pc), these sources; hence, these molecular lines could be used as probes of the environment near the launching site of jets/outflows. No radio recombination lines were detected, but we found that the rms noise of stacked spectra decreases following the radiometer equation. Therefore, detecting radio recombination lines in a sample of brighter free–free continuum sources should be possible. This work demonstrates the potential of broadband VLA continuum observations as low resolution spectral-line scans.

First observations of warm and cold methanol in Class 0/I proto-brown dwarfs

ABSTRACT We present results from the first molecular line survey to search for the fundamental complex organic molecule, methanol (CH3OH), in 14 Class 0/I proto-brown dwarfs (proto-BDs). IRAM 30-m observations over the frequency range of 92–116 and 213–280 GHz have revealed emission in 14 CH3OH transition lines, at upper state energy level, Eupper ∼7–49 K, and critical densities, ncrit of 105–109 cm−3. The most commonly detected lines are at Eupper &amp;lt; 20 K, while 11 proto-BDs also show emission in the higher excitation lines at Eupper ∼21–49 K and ncrit ∼ 105 to 108 cm−3. In comparison with the brown dwarf formation models, the high excitation lines likely probe the warm (∼25–50 K) corino region at ∼10–50 au in the proto-BDs, while the low-excitation lines trace the cold (&amp;lt;20 K) gas at ∼50–150 au. The column density for the cold component is an order of magnitude higher than the warm component. The CH3OH ortho-to-para ratios range between ∼0.3 and 2.3. The volume-averaged CH3OH column densities show a rise with decreasing bolometric luminosity among the proto-BDs, with the median column density higher by a factor of ∼3 compared to low-mass protostars. Emission in high-excitation (Eupper &amp;gt; 25 K) CH3OH lines together with the model predictions suggest that a warm corino is present in ∼78 per cent of the proto-BDs in our sample. The remaining shows evidence of only the cold component, possibly due to the absence of a strong, high-velocity jet that can stir up the warm gas around it.

FAUST. VII. Detection of a Hot Corino in the Prototypical Warm Carbon-chain Chemistry Source IRAS 15398–3359

We have observed the low-mass protostellar source IRAS 15398−3359 at a resolution of 0.″2–0.″3, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST, to examine the presence of a hot corino in the vicinity of the protostar. We detect nine CH3OH lines including the high-excitation lines with upper-state energies up to 500 K. The CH3OH rotational temperature and the column density are derived to be K and cm−2, respectively. The beam filling factor is derived to be , indicating that the emitting region of CH3OH is much smaller than the synthesized beam size and is not resolved. The emitting region of three high-excitation lines, 183,15–182,16, A (E u = 447 K), 193,16–192,17, A (E u = 491 K), and 203,17–202,18, A (E u = 537 K), is located within the 50 au area around the protostar and seems to have a slight extension toward the northwest. Toward the continuum peak, we also detect one emission line from CH2DOH and two features of multiple CH3OCHO lines. These results, in combination with previous reports, indicate that IRAS 15398−3359 is a source with hybrid properties showing both hot corino chemistry rich in complex organic molecules on small scales (∼10 au) and warm carbon-chain chemistry rich in carbon-chain species on large scales (∼100–1000 au). A possible implication of the small emitting region is further discussed in relation to the origin of the hot corino activity.

The Detection of Hot Molecular Cores in the Small Magellanic Cloud

We report the first detection of hot molecular cores in the Small Magellanic Cloud (SMC), a nearby dwarf galaxy with 0.2 solar metallicity. We observed two high-mass young stellar objects in the SMC with the Atacama Large Millimeter/submillimeter Array and detected emission lines of CO, HCO+, H13CO+, SiO, H2CO, CH3OH, SO, and SO2. Compact hot-core regions are traced by SO2, whose spatial extent is about 0.1 pc, and the gas temperature is higher than 100 K based on the rotation diagram analysis. In contrast, CH3OH, a classical hot-core tracer, is dominated by extended (∼0.2–0.3 pc) components in both sources, and the gas temperature is estimated to be K for one source. Protostellar outflows are also detected from both sources as high-velocity components of CO. The metallicity-scaled abundances of SO2 in hot cores are comparable among the SMC, Large Magellanic Cloud (LMC), and Galactic sources, suggesting that the chemical reactions leading to SO2 formation would be regulated by elemental abundances. On the other hand, CH3OH shows a large abundance variation within SMC and LMC hot cores. The diversity in the initial condition of star formation (e.g., degree of shielding, local radiation field strength) may lead to the large abundance variation of organic molecules in hot cores. This work, in conjunction with previous hot-core studies in the LMC and outer/inner Galaxy, suggests that the formation of a hot core would be a common phenomenon during high-mass star formation across the metallicity range of 0.2–1 Z⊙. High-excitation SO2 lines will be a useful hot-core tracer in the low-metallicity environments of the SMC and LMC.

Submillimeter observations of molecular gas interacting with the supernova remnant W28

Context. Supernovae (SNe) inject large amounts of energy and chemically enriched materials into their surrounding interstellar medium and, in some instances, into molecular clouds (MCs). The interaction of a supernova remnant (SNR) with a MC plays a crucial role in the evolution of the cloud’s physical and chemical properties. Despite their importance, only a handful of studies have been made addressing the molecular richness in molecular clouds impacted by SNRs. (Sub)millimter wavelength observations of MCs affected by SNRs can be used to build a census of their molecular richness, which in turn can motivate various chemical and physical models aimed at explaining the chemical evolution of the clouds. Aims. We carried out multi-molecule and multi-transition observations toward the molecular region F abutting the SNR W28, containing 1720 MHz OH masers, well-established tracers of SNR-MC interactions. We used the detected lines to constrain the physical conditions of this region. Methods. We used the APEX Telescope to observe molecular lines in the frequency range 213–374 GHz. We used non-local thermodynamic equilibrium (non-LTE) RADEX modeling to interpret the observational data. Results. We detected emission from multiple molecular species in the region, namely CH3OH, H2CO, SO, SiO, CN, CCH, NO, CS, HCO+, HCN, HNC, N2H+, CO, and from the isotopologues of some of them. We report the first detection of thermally excited (nonmaser) CH3OH emission toward a SNR. Employing non-LTE RADEX modeling of multiple H2CO and CH3OH lines, we constrained the kinetic temperature and spatial density in the molecular gas. The gas kinetic temperatures range from 60 to 100 K while the spatial density of the gas ranges from 9 × 105 to 5 × 106 cm−3. We obtained an ortho-para ratio ~2 for H2CO, which indicates that formaldehyde is most likely formed on dust grain surfaces and not in the gas phase. Conclusions. Our results show that molecules as complex as H2CO and CH3OH can be detected in SNR-MC interactions. This could motivate chemical modeling to explore their formation pathways.

OMC-2 FIR 4 under the microscope: Shocks, filaments, and a highly collimated jet at 100 au scales

Context. Star-forming molecular clouds are characterised by the ubiquity of intertwined filaments. The filaments have been observed in both high- and low-mass star-forming regions, and they are thought to split into collections of sonic fibres. The locations where filaments converge are termed hubs, and these are associated with the young stellar clusters. However, the observations of filamentary structures within hubs at distances of 75–300 pc require a high angular resolution &lt;2″ (~ 150–600 au) that limits the number of such studies conducted so far. Aims. The integral shaped filament (ISF) of the Orion A molecular cloud is noted for harbouring several hubs within which no filamentary structures have been observed so far. The goal of our study is to investigate the nature of the filamentary structures within one of these hubs, which is the chemically rich hub OMC-2 FIR 4, and to analyse their emission with high density and shock tracers. Methods. We observed the OMC-2 FIR 4 proto-cluster using Band 6 of the Atacama Large (sub-)Millimetre Array (ALMA) in Cycle 4 with an angular resolution of ~0.26″ (100 au). We analysed the spatial distribution of dust, the shock tracer SiO, and dense gas tracers (i.e., CH3OH, CS, and H13CN). We also studied the gas kinematics using SiO and CH3OH maps. Results. Our observations for the first time reveal interwoven filamentary structures within OMC-2 FIR 4 that are probed by several tracers. Each filamentary structure is characterised by a distinct velocity as seen from the emission peak of CH3OH lines. They also show transonic and supersonic motions. SiO is associated with filaments and also with multiple bow-shock features. The bow-shock features have sizes between ~500 and 2700 au and are likely produced by the outflow from HOPS-370. Their dynamical ages are &lt;800 yr. In addition, for the first time, we reveal a highly collimated SiO jet (~1°) with a projected length of ~5200 au from the embedded protostar VLA 15. Conclusions. Our study unveiled the previously unresolved filamentary structures as well as the shocks within OMC-2 FIR 4. The kinematics of the filamentary structures might be altered by external and/or internal mechanisms such as the wind from H II regions, the precessing jet from the protostellar source HOPS-370, or the jet from VLA 15. While the complexity of the region, coupled with the limited number of molecular lines in our dataset, makes any clear association with these mechanisms challenging, our study shows that multi-scale observations of these regions are crucial for understanding the accretion processes and flow of material that shape star formation.

Confirmation of the outflow in L1451-mm: SiO line and CH3OH maser detections

Context. The observational counterparts of theoretically predicted first hydrostatic cores (FHSC) have been searched for in the interstellar medium for nearly two decades now. Distinguishing them from other types of more evolved but still embedded objects remains a challenge because these objects have a short lifetime, are small, and embedded in a dense cocoon. One possible lead to finding them is the characterization of the outflows that are launched by these objects, which are assumed to have a low velocity and be small extent. Aims. We observed the L1451-mm FHSC candidate with the NOEMA interferometer (and complementary IRAM 30m data) in order to study the emission of several molecules. Methods. Molecular lines were reduced and analyzed with the GILDAS package network, the CASSIS software, and some python packages. A nonlocal thermodynamic equilibrium analysis of the CH3OH detected lines was performed to retrieve the physical conditions of the emitting region around the central source, together with the CH3OH, SiO, CS, and H2CO column densities. Results. Of the targeted molecules, we detected lines of c-C3H2, CH3OH, CS, C34S, SO, DCN, DCO+, H2CO, HC3N, HDCO, and SiO. One of the methanol lines appears to be a maser line. The detection of this class I maser and the SiO line in L1451-mm support the presence of a low-velocity and compact outflow. The excitation conditions of the thermal lines of methanol are also compatible with shocks (H2 density of ~3 × 106 cm−3 and a temperature higher than 40 K). Conclusions. Although these low-velocity outflows are theoretically predicted by some models of FHSC, these models also predict the shock temperature to be below 20 K, that is, not evaporating methanol. In addition, the predicted velocities would not erode the grains and release silicon in the gas phase. We therefore conclude that these new observations favor the hypothesis that L1451-mm would be at a very early protostellar stage, launching an outflow nearly on the plane of the sky with a higher velocity than is observed.

Multiline observations of CH3OH, c-C3H2, and HNCO toward L1544

Context. Pre-stellar cores are the basic unit for the formation of stars and stellar systems. The anatomy of the physical and chemical structures of pre-stellar cores is critical for understanding the star formation process. Aims. L1544 is a prototypical pre-stellar core that shows significant chemical differentiation surrounding the dust peak. We aim to constrain the physical conditions at the different molecular emission peaks. This study allows us to compare the abundance profiles predicted from chemical models with the classical density structure of the Bonnor-Ebert (BE) sphere. Methods. We conducted multi-transition pointed observations of CH3OH, c-C3H2, and HNCO with the IRAM 30m telescope toward the dust peak and the respective molecular peaks of L1544. Using this data set, with nonlocal-thermodynamic-equilibrium radiative transfer calculations and a one-dimensional model, we revisit the physical structure of L1544 and benchmark the observations with the abundance profiles from current chemical models. Results. We find that the HNCO, c-C3H2, and CH3OH lines in L1544 trace progressively higher-density gas, from ~104 to several times 105 cm−3. Particularly, we find that to produce the observed intensities and ratios of the CH3OH lines, a local gas density enhancement above that of the BE sphere is required. This suggests that the physical structure of an early-stage core may not necessarily follow a smooth decrease in gas density profile locally, but can be intercepted by clumpy substructures that surround the gravitational center. Conclusions. Multiple transitions of molecular lines from different molecular species can provide a tomographic view of the density structure of pre-stellar cores. The local gas density enhancement deviating from the BE sphere may reflect the impact of accretion flows that appear asymmetric and are enhanced at the meeting point of large-scale cloud structures.

Tracking the Ice Mantle History in the Solar-type Protostars of NGC 1333 IRAS 4

To understand the origin of the diversity observed in exoplanetary systems, it is crucial to characterize the early stages of their formation, represented by solar-type protostars. Likely, the gaseous chemical content of these objects directly depends on the composition of the dust-grain mantles formed before the collapse. Directly retrieving the ice mantle composition is challenging, but it can be done indirectly by observing the major components, such as NH3 and CH3OH at centimeter wavelengths, once they are released into the gas phase during the warm protostellar stage. We observed several CH3OH and NH3 lines toward three Class 0 protostars in NGC 1333 (IRAS 4A1, IRAS 4A2, and IRAS 4B), at high angular resolution (1″; ∼300 au) with the VLA interferometer at 24–26 GHz. Using a non-LTE LVG analysis, we derived a similar NH3/CH3OH abundance ratio in the three protostars (≤0.5, 0.015–0.5, and 0.003–0.3 for IRAS 4A1, 4A2, and 4B, respectively). Hence, we infer they were born from precollapse material with similar physical conditions. Comparing the observed abundance ratios with astrochemical model predictions, we constrained the dust temperature at the time of the mantle formation to be ∼17 K, which coincides with the average temperature of the southern NGC 1333 diffuse cloud. We suggest that a brutal event started the collapse that eventually formed IRAS 4A1, 4A2, and 4B, which, therefore, did not experience the usual prestellar core phase. This event could be the clash of a bubble with NGC 1333 South, which has previously been evoked in the literature.

Chemical and Physical Characterization of the Isolated Protostellar Source CB68: FAUST IV

The chemical diversity of low-mass protostellar sources has so far been recognized, and environmental effects are invoked as its origin. In this context, observations of isolated protostellar sources without the influence of nearby objects are of particular importance. Here, we report the chemical and physical structures of the low-mass Class 0 protostellar source IRAS 16544−1604 in the Bok globule CB 68, based on 1.3 mm Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of ∼70 au that were conducted as part of the large program FAUST. Three interstellar saturated complex organic molecules (iCOMs), CH3OH, HCOOCH3, and CH3OCH3, are detected toward the protostar. The rotation temperature and the emitting region size for CH3OH are derived to be 131 ± 11 K and ∼10 au, respectively. The detection of iCOMs in close proximity to the protostar indicates that CB 68 harbors a hot corino. The kinematic structure of the C18O, CH3OH, and OCS lines is explained by an infalling–rotating envelope model, and the protostellar mass and the radius of the centrifugal barrier are estimated to be 0.08–0.30 M ⊙ and <30 au, respectively. The small radius of the centrifugal barrier seems to be related to the small emitting region of iCOMs. In addition, we detect emission lines of c-C3H2 and CCH associated with the protostar, revealing a warm carbon-chain chemistry on a 1000 au scale. We therefore find that the chemical structure of CB 68 is described by a hybrid chemistry. The molecular abundances are discussed in comparison with those in other hot corino sources and reported chemical models.

FAUST VI. VLA1623−2417 B: a new laboratory for astrochemistry around protostars on 50 au scale

ABSTRACT The ALMA (Atacama Large Millimeter Array) interferometer, with its unprecedented combination of high sensitivity and high angular resolution, allows for (sub-)mm wavelength mapping of protostellar systems at Solar system scales. Astrochemistry has benefitted from imaging interstellar complex organic molecules in these jet–disc systems. Here, we report the first detection of methanol (CH3OH) and methyl formate (HCOOCH3) emission towards the triple protostellar system VLA1623−2417 A1+A2+B, obtained in the context of the ALMA Large Programme FAUST (Fifty AU STudy of the chemistry in the disc/envelope system of solar-like protostars). Compact methanol emission is detected in lines from Eu = 45 K up to 61 K and 537 K towards components A1 and B, respectively. Large velocity gradient analysis of the CH3OH lines towards VLA1623−2417 B indicates a size of 0.11–0.34 arcsec (14–45 au), a column density $N_{\rm CH_3OH}$ = 1016–1017 cm−2, kinetic temperature ≥ 170 K, and volume density ≥ 108 cm−3. A local thermodynamic equilibrium approach is used for VLA1623−2417 A1, given the limited Eu range, and yields Trot ≤ 135 K. The methanol emission around both VLA1623−2417 A1 and B shows velocity gradients along the main axis of each disc. Although the axial geometry of the two discs is similar, the observed velocity gradients are reversed. The CH3OH spectra from B show two broad (4–5 km s−1) peaks, which are red- and blueshifted by ∼ 6–7 km s−1 from the systemic velocity. Assuming a chemically enriched ring within the accretion disc, close to the centrifugal barrier, its radius is calculated to be 33 au. The methanol spectra towards A1 are somewhat narrower (∼ 4 km s−1), implying a radius of 12–24 au.

Importance of source structure on complex organics emission

Context. Complex organic molecules (COMs) are often observed toward embedded Class 0 and I protostars. However, not all Class 0 and I protostars exhibit COM emission. Aims. The aim is to study variations in methanol (CH3OH) emission and use this as an observational tracer of hot cores to test if the absence of CH3OH emission can be linked to source properties. Methods. A sample of 148 low-mass and high-mass protostars is investigated using new and archival observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that contain lines of CH3OH and its isotopologues. Data for an additional 36 sources are added from the literature, giving a total of 184 different sources. The warm (T ≳ 100 K) gaseous CH3OH mass, MCH3OH, is determined for each source using primarily optically thin isotopologues and is compared to a simple toy model of a spherically symmetric infalling envelope that is passively heated by the central protostar. Results. A scatter of more than four orders of magnitude is found for MCH3OH among the low-mass protostars, with values ranging between 10−7 M⊙ and ≲10−11 M⊙. On average, Class I protostellar systems seem to have less warm MCH3OH(≲10−10 M⊙) than younger Class 0 sources (~10−7 M⊙). High-mass sources in our sample show more warm MCH3OH, up to ~10−7−10−3 M⊙. To take into account the effect of the source’s overall mass on MCH3OH, a normalized CH3OH mass is defined as MCH3OH/Mdust,0, where Mdust,0 is the cold plus warm dust mass in the disk and inner envelope within a fixed radius measured from the ALMA dust continuum. A correlation between MCH3OH/Mdust,0 and Lbol is found. Excluding upper limits, a simple power-law fit to the normalized warm CH3OH masses results in MCH3OH/Mdust,0 ∝ Lbol0.70±0.05 over an Lbol range of 10−1−106 L⊙. This is in good agreement with the toy model, which predicts that the normalized MCH3OH increases with Lbol0.70 due to the snow line moving outward. Sources for which the size of the disk is equivalent to or smaller than the estimated 100 K radius fall within the 3σ range of the best-fit power-law model, whereas sources with significantly larger disks show normalized warm CH3OH masses that are up to two orders of magnitude lower. Conclusions. The agreement between sources that are rich in CH3OH with the toy model of a spherically symmetric infalling envelope implies that the thermal structure of the envelopes in these sources is likely not strongly affected by a disk. However, based on the disagreement between the toy model and sources that show less warm CH3OH mass, we suggest that source structure such as a disk can result in colder gas and thus fewer COMs in the gas phase. Additionally, optically thick dust can hide the emission of COMs. Advanced modeling is necessary to quantify the effects of a disk and/or continuum optical depth on the presence of gaseous COMs in young protostellar systems.

Chemical survey of Class I protostars with the IRAM-30 m

Context. Class I protostars are a bridge between Class 0 protostars (≤105 yr old), and Class II (≥106 yr) protoplanetary disks. Recent studies show gaps and rings in the dust distribution of disks younger than 1 Myr, suggesting that planet formation may start already at the Class I stage. To understand what chemistry planets will inherit, it is crucial to characterize the chemistry of Class I sources and to investigate how chemical complexity evolves from Class 0 protostars to protoplanetary disks. Aims. There are two goals: (i) to perform a census of the molecular complexity in a sample of four Class I protostars, and (ii) to compare the data with the chemical compositions of earlier and later phases of the Sun-like star formation process. Methods. We performed IRAM-30 m observations at 1.3 mm towards four Class I objects (L1489-IRS, B5-IRS1, L1455-IRS1, and L1551-IRS5). The column densities of the detected species were derived assuming local thermodynamic equilibrium (LTE) or large velocity gradients (LVGs). Results. We detected 27 species: C-chains, N-bearing species, S-bearing species, Si-bearing species, deuterated molecules, and interstellar complex organic molecules (iCOMs; CH3OH, CH3CN, CH3CHO, and HCOOCH3). Among the members of the observed sample, L1551-IRS5 is the most chemically rich source. Different spectral profiles are observed: (i) narrow lines (~1 km s−1) towards all the sources, (ii) broader lines (~4 km s−1) towards L1551-IRS5, and (iii) line wings due to outflows (in B5-IRS1, L1455-IRS1, and L1551-IRS5). Narrow c-C3H2 emission originates from the envelope with temperatures of 5–25 K and sizes of ~2′′−10′′. The iCOMs in L1551-IRS5 reveal the occurrence of hot corino chemistry, with CH3OH and CH3CN lines originating from a compact (~0.′′15) and warm (T &gt; 50 K) region. Finally, OCS and H2S seem to probe the circumbinary disks in the L1455-IRS1 and L1551-IRS5 binary systems. The deuteration in terms of elemental D/H in the molecular envelopes is: ~10−70% (D2CO/H2CO), ~5−15% (HDCS/H2CS), and ~1−23% (CH2DOH/CH3OH). For the L1551-IRS5 hot corino we derive D/H ~2% (CH2DOH/CH3OH). Conclusions. Carbon chain chemistry in extended envelopes is revealed towards all the sources. In addition, B5-IRS1, L1455-IRS1, and L1551-IRS5 show a low-excitation methanol line that is narrow and centered at systemic velocity, suggesting an origin from an extended structure, plausibly UV-illuminated. The abundance ratios of CH3CN, CH3CHO, and HCOOCH3 with respect to CH3OH measured towards the L1551-IRS5 hot corino are comparable to that estimated at earlier stages (prestellar cores, Class 0 protostars), and to that found in comets. The deuteration in our sample is also consistent with the values estimated for sources at earlier stages. These findings support the inheritance scenario from prestellar cores to the Class I phase when planets start forming.

The evolution of temperature and density structures of OB cluster-forming molecular clumps

Context. OB star clusters originate from parsec-scale massive molecular clumps, while individual stars may form in ≲0.1 pc scale dense cores. The thermal properties of the clump gas are key factors governing the fragmentation process, and are closely affected by gas dynamics and feedback of forming stars. Aims. We aim to understand the evolution of temperature and density structures on the intermediate-scale (≲0.1–1 pc) extended gas of massive clumps. This gas mass reservoir is critical for the formation of OB clusters, due to their extended inflow activities and intense thermal feedback during and after formation. Methods. We performed ~0.1 pc resolution observations of multiple molecular line tracers (e.g., CH3CCH, H2CS, CH3CN, CH3OH) that cover a wide range of excitation conditions, toward a sample of eight massive clumps. The sample covers different stages of evolution, and includes infrared-weak clumps and sources that are already hosting an HII region, spanning a wide luminosity-to-mass ratio (L∕M) range from ~1 to ~100 (L⊙/M⊙). Based on various radiative transfer models, we constrain the gas temperature and density structures and establish an evolutionary picture, aided by a spatially dependent virial analysis and abundance ratios of multiple species. Results. We determine temperature profiles varying in the range 30–200 K over a continuous scale, from the center of the clumps out to 0.3–0.4 pc radii. The clumps’ radial gas density profiles, described by radial power laws with slopes between −0.6 and ~−1.5, are steeper for more evolved sources, as suggested by results based on dust continuum, representing the bulk of the gas (~104 cm−3), and on CH3OH lines probing the dense gas (≳106–108 cm−3) regime. The density contrast between the dense gas and the bulk gas increases with evolution, and may be indicative of spatially and temporally varying star formation efficiencies. The radial profiles of the virial parameter show a global variation toward a sub-virial state as the clump evolves. The linewidths probed by multiple tracers decline with increasing radius around the central core region and increase in the outer envelope, with a slope shallower than the case of the supersonic turbulence (σv ∝ r0.5) and the subsonic Kolmogorov scaling (σv ∝ r0.33). In the context of evolutionary indicators for massive clumps, we also find that the abundance ratios of [CCH]/[CH3OH] and [CH3CN]/[CH3OH] show correlations with clump L∕M.

ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions – VIII. A search for hot cores by using C2H5CN, CH3OCHO, and CH3OH lines

ABSTRACT Hot cores characterized by rich lines of complex organic molecules are considered as ideal sites for investigating the physical and chemical environments of massive star formation. We present a search for hot cores by using typical nitrogen- and oxygen-bearing complex organic molecules (C2H5CN, CH3OCHO, and CH3OH), based on ALMA Three-millimeter Observations of Massive Star-forming regions (ATOMS). The angular resolutions and line sensitivities of the ALMA observations are better than 2 arcsec and 10 mJy beam−1, respectively. A total of 60 hot cores are identified with 45 being newly detected, in which the complex organic molecules have high gas temperatures (&amp;gt; 100 K) and hot cores have small source sizes (&amp;lt; 0.1 pc). So far, this is the largest sample of hot cores observed with similar angular resolution and spectral coverage. The observations have also shown nitrogen and oxygen differentiation in both line emission and gas distribution in 29 hot cores. Column densities of CH3OH and CH3OCHO increase as rotation temperatures rise. The column density of CH3OCHO correlates tightly with that of CH3OH. The pathways for production of different species are discussed. Based on the spatial position difference between hot cores and ultracompact H ii (UC H ii) regions, we conclude that 24 hot cores are externally heated, while the other hot cores are internally heated. The observations presented here will potentially help establish a hot core template for studying massive star formation and astrochemistry.

Clump-scale chemistry in the NGC 2264-D cluster-forming region

Abstract We have conducted mapping observations toward the n3 and n5 positions in the NGC 2264-D cluster-forming region with the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 3. Observations with a 10000 au scale beam reveal the chemical composition at the clump scale. The spatial distributions of the observed low upper-state-energy lines of CH3OH are similar to those of CS and SO, and the HC3N emission seems to be predominantly associated with clumps containing young stellar objects. The turbulent gas induced by the star formation activities produces large-scale shock regions in NGC 2264-D, which are traced by the CH3OH, CS, and SO emissions. We derive the HC3N, CH3CN, and CH3CHO abundances with respect to CH3OH. Compared to the n5 field, the n3 field is farther (in projected apparent distance) from the neighboring NGC 2264-C, yet the chemical composition in the n3 field tends to be similar to that of the protostellar candidate CMM3 in NGC 2264-C. The HC3N$/$CH3OH ratios in the n3 field are higher than those in the n5 field. We find an anti-correlation between the HC3N$/$CH3OH ratio and their excitation temperatures. The low HC3N$/$CH3OH abundance ratio at the n5 field implies that the n5 field is an environment with more active star formation compared with the n3 field.

Exploring the 100 au Scale Structure of the Protobinary System NGC 2264 CMM3 with ALMA

Abstract We have observed the young protostellar system NGC 2264 CMM3 in the 1.3 mm and 2.0 mm bands at a resolution of about 0.″1 (70 au) with ALMA. The structures of two distinct components, CMM3A and CMM3B, are resolved in the continuum images of both bands. CMM3A has an elliptical structure extending along the direction almost perpendicular to the known outflow, while CMM3B reveals a round shape. We have fitted two 2D Gaussian components to the elliptical structure of CMM3A and CMM3B and have separated the disk and envelope components for each source. The spectral index α between 2.0 and 0.8 mm is derived to be 2.4–2.7 and 2.4–2.6 for CMM3A and CMM3B, respectively, indicating optically thick dust emission and/or grain growth. A velocity gradient in the disk/envelope direction is detected for CMM3A in the CH3CN, CH3OH, and 13CH3OH lines detected in the 1.3 mm band, which can be interpreted as the rotation of the disk/envelope system. From this result, the protostellar mass of CMM3A is roughly evaluated to be 0.1–0.5 M ⊙ by assuming Keplerian rotation. The mass accretion rate is thus estimated to be 5 × 10−5 − 4 × 10−3 M ⊙ yr−1, which is higher than the typical mass accretion rate of low-mass protostars. The OCS emission line shows a velocity gradient in both outflow direction and disk/envelope direction. A hint of outflow rotation is found, and the specific angular momentum of the outflow is estimated to be comparable to that of the disk. These results provide us with novel information on the initial stage of a binary/multiple system.