Yao-Lun Yang

Massive stars play a critical role in the evolution of galaxies, but their formation remains poorly understood. One challenge is accurate measurement of the physical properties of massive protostars, such as current stellar mass, envelope mass, outflow cavity properties, and system orientation. Spectral energy distribution (SED) fitting is widely used to test models against observations. The far-infrared SED traces cold dust in envelopes, while the near- and mid-infrared (MIR) probes emission from outflow cavities and/or the inner envelope. However, SED fitting has degeneracies limiting its ability to yield accurate measurements of protostellar properties. Here, we develop image profile (IMPRO) fitting as a method to improve the characterization of protostars. We use brightness distributions from multiwavelength MIR images of massive protostars taken by SOFIA/FORCAST as part of the SOFIA Massive Star Formation (SOMA) survey to constrain protostellar properties by comparing them to a grid of radiative transfer models. We develop a fitting pipeline to extract information along the outflow axis, which is then combined with the SED fitting to yield improved constraints on protostellar properties. We apply the IMPRO fitting method to the nearby massive protostar Cepheus A, finding that its properties become more tightly constrained compared to SED fitting, especially in the inclination of the source. However, for the more distant G35.20–0.74N, we find that the spatial resolution of SOFIA/FORCAST limits the utility of this combined fitting pipeline. However, higher-resolution MIR observations, e.g., with JWST, are expected to greatly expand the applicability of this fitting technique to protostars across the Galaxy.

Context . There have been no robust identifications of fully deuterated methanol, CD ₃ OD, in the interstellar medium (ISM) to date. Aims . Our goal is to continue the search for CD ₃ OD in star-forming regions to determine the deuteration pattern of methanol and set constraints on the deuteration process. Methods . We used a sensitive imaging spectral survey of the protostellar system IRAS4A in the NGC1333 molecular cloud performed as part of the large program carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) called Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). The column densities of methanol and several of its less abundant isotopologs, including deuterated ones, were derived under the assumption of local thermodynamic equilibrium (LTE). Results . Along with the detection of several methanol isotopologs toward the binary component IRAS4A2, we report the first interstellar detection of CD ₃ OD. The deuteration pattern of methanol in IRAS4A2 is similar to the pattern measured in other low-mass protostars, with an increase in the apparent D/H ratio from the singly to multiply deuterated isotopologs. Chemical models of prestellar and protostellar cores focused on deuteration do not reproduce this pattern quantitatively. This discrepancy is even more exacerbated by the detection of CD ₃ OD. Our LTE modeling of the spectra yields low column density ratios of methanol to its ¹³ C and ¹⁸ O isotopologs while the ¹³ C to ¹⁸ O and ¹⁸ O to ¹⁷ O ratios are consistent with the isotopic ratios expected in the local ISM. We suspect that the column density of methanol is underestimated. One possible explanation is that the emission is clumpy rather than monolithic. Conclusions . The first detection of a fully deuterated COM highlights the power of deuterium fractionation as a diagnostic tool for investigating the formation and inheritance of organic molecules during the process of star formation.

Context . Studying the chemical composition of the warm gas surrounding embedded solar-type protostars is one of the areas for which the Atacama Large Millimeter/submillimeter Array (ALMA) is particularly well suited. Recently, the ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) carried out an observing campaign of a sample of 11 protostellar regions to investigate the chemical impact of their environments and evolutionary stages. Aims . The aim of this paper is to provide the observational and procedural information necessary for the community to understand the analysis presented in forthcoming scientific papers by the COMPASS collaboration, and to enable archival research using the data products provided by our team and the ALMA Science Archive. Methods . This paper describes the technical setup of the observations, and it outlines the data-processing pipeline and primary data products from this initiative. The Common Astronomy Software Applications (CASA) was used for initial calibration following the standard ALMA Pipeline, and then IMAGER was used for self-calibration and imaging. We describe post-processing done with Python to achieve favorable continuum subtraction, correct for the primary beam response, and extract spectra. Results . We discuss our strategy for mitigating data challenges, including those related to data volume and continuum subtraction for spectral line analysis. The incorporation of IMAGER in our workflow was advantageous in terms of the computing efficiency which allowed us to benchmark several imaging parameters and techniques; we also demonstrate that the imaging outcomes using IMAGER or CASA are scientifically comparable. Numerous data products are generated for the program, including image cubes for the 36 spectral windows targeting each source, extracted spectra, and continuum images.

Context . Methyl cyanide (CH ₃ CN) is one of the most abundant nitrogen-bearing complex organic molecules observed in the gas phase around both low- and high-mass protostars. However, very few observations exist with a broad enough frequency coverage and sufficiently high sensitivity to systematically characterize its minor isotopologues toward such regions. Aims . We add to this number by analyzing data toward BHR71-IRS1 taken with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) Large Program over a ∼33 GHz frequency range. Methods . We studied as many isotopologues of methyl cyanide as possible toward BHR71-IRS1. We found the column densities and excitation temperatures via spectral fitting and assuming local thermodynamic equilibrium. We then compared the derived column density ratios with those toward IRAS 16293-2422 A and B, V883 Ori, G31.41+0.31, and Sgr B2(N2b). Results . We detect CH ₃ CN, CH ₃ CN v ₈ =1, ¹³ CH ₃ CN, CH ₃ ¹³ CN, CH ₃ C ¹⁵ N, CH ₂ DCN, and CHD ₂ CN. The column density ratio of ¹³ CH ₃ CN/CH ₃ ¹³ CN is around unity, in agreement with previous observations. The CH ₂ DCN/CH ₃ CN ratios are similar for all the low-mass objects but are ∼1 order of magnitude higher than those toward Sgr B2(N2b). This could be due to a warmer pre-stellar phase or a different formation timescale of Sgr B2(N2b). Conclusions . The D/H ratios found from methyl cyanide isotopologues agree well with those of methanol toward BHR71-IRS1, likely pointing to the formation and deuteration of methyl cyanide in a similar star-formation phase as methanol (i.e., the pre-stellar phase). We find an increase in the methyl cyanide D/H ratios with the number of deuterium atoms, which suggests that the deuterated isotopologues may form later in the pre-stellar phase. Future COMPASS studies will increase the sample size of low-mass systems with methyl cyanide analysis and potentially examine some of the trends observed here statistically.

Context . Methanol (CH ₃ OH) masers that are collisionally pumped, referred to as ‘class I’, commonly trace shock interfaces in star-forming regions, but their incidence in embedded low-mass protostars remains poorly quantified. Aims . We aim to (i) establish the occurrence, spatial origin, and excitation of a newly detected class I CH ₃ OH maser at 302.913 GHz (12 _0,12 –11 _1,11 A, E _u = 180.9 K) towards low-mass protostars and (ii) test whether the environments that pump this maser exhibit signatures of shock-driven complex chemistry. Methods . We utilised data from the COMPASS ALMA Large Program to search for maser activity in the 302.9 GHz line in a sample of 11 low-mass protostellar sources. The COMPASS programme provides unbiased surveys of these sources between 279.0 and 311.7 GHz covering several transitions of CH ₃ OH as well as other complex organic molecules (COMs) at typical angular resolutions of 0.″3 − −0.″5. Measuring departures from local thermodynamic equilibrium (LTE) in the 302.9 GHz CH ₃ OH excitation via rotation diagram analysis robustly revealed masing. Results . The 302.9 GHz CH ₃ OH line is detected as a non-thermal excess (maser) in seven out of 11 COMPASS sources, with peak brightness temperatures ranging from a few to a thousand kelvin. One of these detections is towards HOPS 108, which hosts the brightest maser, and it is likely associated with the powerful outflow driven by the nearby intermediate-mass protostar HOPS 370. Excluding this case, the maser is detected in six of the ten low-mass protostars (60%). All six low-mass protostars in which maser emission is detected are Class 0 sources interacting with dense ambient material, whereas non-detections correspond to three Class I objects and one isolated Class 0 core. This indicates a preference for early evolutionary stages and environments rich in gas for outflow–ambient medium interactions. The maser emission is spatially coincident with thermal CH ₃ OH emission along the outflow cavity walls and bow shocks, but it has more compact knot-like morphologies. Lastly, among the surveyed COMs, only acetaldehyde (CH ₃ CHO) closely traces the shocked morphology of the 302.9 GHz CH ₃ OH maser. However, this CH ₃ CHO feature is observed only towards the brightest maser in the region around HOPS 108, which is influenced by the intermediate-mass protostellar outflow. Conclusions . Our study reveals that class I CH ₃ OH masers in low-mass protostellar systems are more prevalent than previously recognised and preferentially occur in younger, more embedded systems. Although this has only been observed in an intermediate-mass protostellar outflow, CH ₃ CHO is enhanced in a manner consistent with shock-driven chemistry along the outflow cavities. Testing whether a similar relationship applies in the low-mass regime will require more sensitive CH ₃ CHO observations towards regions where the class I CH ₃ OH masers are detected.

Context . Methanol is a complex organic molecule detected in both ice and gas toward star-forming regions. Due to its high abundance, its rarer isotopologues, including the deuterated ones, are also commonly detected. Measurements of deuteration can provide key constraints on the formation and evolution of molecules during the formation of stars and planets. It is not clear how the methanol deuteration varies between star-forming regions. Thanks to several recent spectroscopic studies, it is now possible to precisely derive the column densities of five deuterated isotopologues of methanol. Aims . Our aim is to determine the column density and deuterium fractionation of methanol in the low-mass protostar BHR71-IRS1. Methods . We analyzed data from a large spectral survey of BHR71-IRS1 carried out as part of the ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). We used the CASSIS software to identify the lines of the methanol isotopologues and constrain their column densities under the assumption of local thermodynamic equilibrium. Results . We detected CH ₃ OH, ¹³ CH ₃ OH, CH ₃ ¹⁸ OH, CH ₃ OD, CH ₂ DOH, CHD ₂ OH, and CD ₃ OH at the continuum peak position. In addition, CH ₃ ¹⁷ OH is tentatively detected, while CD ₃ OD is not detected, although a few faint lines (≲3 σ ) could be present toward a position redshifted from the continuum peak. The column densities of the main isotopologue obtained from ¹³ CH ₃ OH and CH ₃ ¹⁸ OH are consistent, assuming nominal ¹² C/ ¹³ C and ¹⁶ O/ ¹⁸ O ratios of 68 and 557, respectively. The CH ₂ DOH/CH ₃ OD column density ratio is equal to 7 ± 2. The statistically corrected D/H ratio is higher for CH ₂ DOH (1.9 ± 0.6%) than for CH ₃ OD (0.8 ± 0.2%). The level of deuteration after statistical correction increases with the number of D atoms in the methyl group, reaching 23 ± 5% for CD ₃ OH/CH ₃ OH. Conclusions . These trends are similar to other low-mass protostars such as IRAS 16293–2422 and HOPS 373SW, but seem to differ from the eruptive young star V883-Ori. Similar studies with comparable datasets will be carried out for the other COMPASS sources to determine if factors such as environmental conditions and evolutionary states impact the deuteration of methanol.

Context . Complex organic molecules (COMs) are detected in the interstellar medium, both in cold molecular clouds and in the warm inner regions of protostars. Whether these COMs are eventually directly inherited by protostars and planets or whether their chemistry is reset by in situ reactions remains unclear. Recent (sub)millimeter wavelength observations have spatially resolved the emission of gas-phase COMs around low-mass protostars, not only in compact hot corinos but also in the extended structures beyond them. Understanding the physical properties and desorption mechanisms of these molecules is essential for tracing their chemical evolution during the protostellar evolutionary sequence. Methanol (CH ₃ OH), the most abundant COM in star-forming regions, serves as an ideal tracer for investigating these processes. Aims . As part of the ALMA Cycle 9 Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS), we investigate the CH ₃ OH emission that extends beyond the hot corino of the low-mass protostar BHR71-IRS1. Methods . We identified spatially extended emission of CH ₃ OH around BHR71-IRS1 and subsequently explored plausible desorption scenarios. We assessed characteristic emission structures by applying a principal component analysis (PCA) to the identified emission lines. The physical properties of their line-emitting gas were derived under the local thermodynamic equilibrium (LTE) assumption through both rotational diagram and spectral modeling analyses. Results . In total, 26 spatially extended CH ₃ OH lines have been detected around BHR71-IRS1. These emission lines are distributed across three distinct structures: (1) X-shaped cavity walls, (2) compact northern and southern shocked knots, and (3) additional distant eastern and southeastern components. The cavity walls exhibit excitation temperatures of ∼25 K and column densities of ∼10 ¹⁴ cm ⁻² , while the knots show relatively higher excitation temperatures (70–120 K) and column densities (10 ¹⁴ –10 ¹⁶ cm ⁻² ). The eastern and southeastern components appear unrelated to BHR71-IRS1, and their physical properties are not well constrained under LTE conditions, suggesting that a non-LTE analysis may be required. Conclusions . Plausible desorption mechanisms are evaluated by comparing the physical parameters with model predictions that simulated the effects of far-ultraviolet (FUV) photons and C-type shocks. The low excitation temperatures and narrow line widths (∼0.7 km s ⁻¹ ) in the cavity walls favor a reactive desorption scenario driven by FUV irradiation from IRS1. In contrast, the knots with higher excitation temperatures and column densities support the dust sputtering scenario induced by C-type shocks.

Context . It remains a fundamental research question in astrochemistry to characterize the inventory of complex organic molecules formed during the early stages of star formation, which may affect the eventual chemical composition of protoplanetary disks and potentially planets. So far, only studies of individual sources or selected samples of molecular species have been conducted and the relative importance of environmental and evolutionary effects on the chemical composition remains unclear. Aims . The Atacama Large Millimeter/submillimeter Array (ALMA) provides the angular resolution and sensitivity to zoom in on the hot ( T > 100 K) gas surrounding embedded solar-type protostars and chart their molecular inventories with high accuracy. The ALMA Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) aims to provide comprehensive inventories of the molecular content of a sample of protostellar sources to understand the chemical impact of their environments and evolutionary stages. Methods . COMPASS is an unbiased spectral survey of 11 line-rich Class 0 and I protostellar sources in the spectral range from 279.0 to 311.7 GHz at 0.15–0.5″ angular resolution with ALMA, corresponding to the inner ∼100 au (radius) around the targeted sources. Through this survey, unique chemical fingerprints of each source can be established. Results . This paper provides an overview of the COMPASS program and the adopted strategy in terms of observational setups and targeted sources. We also present a qualitative comparison of the spatial distributions of selected molecules and the relative strengths of the line fluxes of selected complex organics. Conclusions . The qualitative comparisons suggest that chemical differences between sources may be present, for example, between the abundances of groups of oxygen- and nitrogen-bearing species, but also that such variations occur at levels smaller than an order of magnitude. Quantitative comparisons of abundances at these levels require the large bandwidth of the survey, as well as a careful analysis of the excitation and strengths of the several thousands of lines detected toward each source in the survey, as will be explored in forthcoming papers.

Molecular winds may play a key role in governing angular momentum transport and accretion during the early evolution of protostars. We present the morphology and kinematic properties of the H ₂ emission in five young, envelope-dominated protostars across a broad bolometric luminosity range, from 0.2 to 10 ⁴ L _⊙ , observed with the NIRSpec/integral field unit and MIRI/Medium Resolution Spectrograph on board JWST as part of the Investigating Protostellar Accretion (IPA) program. A rich set of pure rotational lines of H ₂ , up to v = 0–0 S (18), and a few rovibrational lines are detected in the winds, revealing bipolar structures. The H ₂ lines show a stratified/onion-like structure morphologically and kinematically, where the lines with higher E _u / k show a higher degree of collimation and higher velocities. Additionally, the wind velocity scales with the L _bol of the host protostellar system. In four out of five protostars, H ₂ emission fills the outflow cavity without showing pronounced limb brightening. We also report a tentative detection of H ₂ wind rotation in IRAS 16253, which suggests a launch radius of ∼4 au and a magnetic lever arm parameter of ∼5–10. Taken together, these properties of the H ₂ winds can be explained by the magnetohydrodynamic disk wind models. We detect a collimated, high-velocity H ₂ jet toward HOPS 370, which is more evolved than the extremely young source HH 211 but is accreting at a high accretion rate. This suggests that the presence of collimated molecular jets in protostars is more closely connected to accretion rate than system age.