Judit Ferrer Asensio

Context. Symmetric molecules that contain several identical nuclei give rise to distinct isomers that are characterized by the total nuclear spin and produce separate ortho and para species that cannot be exchanged through radiative or inelastic transitions and whose relative populations can preserve information about formation conditions. In the interstellar medium, cyclopropenylidene (c-C ₃ H ₂ ) and its doubly deuterated isotopologue provide a prime laboratory for studying spin isomer ratios, which have been measured for the main species but are yet unavailable for c-C ₃ D ₂ . Aims. We present new detections of both ortho and para transitions of c-C ₃ D ₂ towards three sources in the Taurus molecular cloud from the GEMS Large program, and revisit previous detections towards four other cores from the literature in order to investigate the para-to-ortho ratio for doubly deuterated c-C ₃ D ₂ for the first time. Methods. We computed the ortho and para c-C ₃ D ₂ column densities towards all the sources using the non-local thermodynamic equilibrium (non-LTE) software RADEX thanks to the recent availability of dedicated collision rate coefficients. Results. The total derived column densities of c-C ₃ D ₂ for the new sources present a range of (2.5–5.3) × 10 ¹¹ cm ⁻² . The para-to-ortho ratio derived for B213-C16-1 and TMC1-C, 1.5 _−0.6 ^+0.7 and 1.3 _−0.7 ^+1.3 , respectively, are higher than the statistical ratio of 0.5, while for TMC1-NH3 the ratio is statistical within errors, 1.0 _−0.7 ^+1.4 . The para-to-ortho ratio for the revisited sources from the literature, L1544, TMC-1C, TMC-2, and L1517B are 0.4 _−0.1 ^+0.1 , 0.5 _−0.1 ^+0.1 , 0.6 _−0.1 ^+0.2 , and 1.4 _−0.5 ^+0.7 , respectively. In this case, the first three cores show para-to-ortho statistical values, and the latter a non-statistical one. Thus, from all the sources studied, four have statistical c-C ₃ D ₂ para-to-ortho ratios and three sources show higher non-statistical ratios. Conclusions. This is the first observational determination of the para-to-ortho ratio in a doubly deuterated molecule derived through a detailed radiative transfer tool. The observed larger-than-statistical c-C ₃ D ₂ para-to-ortho ratio values for some of the sources may result from p-D ₂ H ⁺ or D ₃ ⁺ driven reactions. The actual ratio is not directly related to the ortho-to-para ratios of H ₂ and c-C ₃ H ₂ . These findings provide an important reference point for future chemical models to investigate the formation pathways of c-C ₃ D ₂ .

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 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 . 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.

Context . In cores deuterium fractionation becomes highly efficient due to low temperatures and CO freeze-out. Cyclopropenylidene (c-C ₃ H ₂ ), a small cyclic molecule formed early in chemical evolution, and its deuterated forms serve as valuable tracers of gas-phase deuteration in these environments. Aims . In order to statistically explore the c-C ₃ H ₂ deuteration ratios towards starless and prestellar cores, we present observations of c-C ₃ H ₂ and its deuterated isotopologues on a sample of cores in the Perseus molecular cloud. Methods . Transitions of c-C ₃ H ₂ , c-C ₃ HD, and c-C ₃ D ₂ were observed with the Yebes 40m, the Arizona Radio Observatory (ARO) 12m and the Institut de Radioastronomie Millimétrique (IRAM) 30 metre telescopes towards a total of 16 starless and prestellar cores in the Perseus molecular cloud. The lines were fitted with Gaussian profiles and their column densities were computed using the non-local thermodynamic equilibrium (non-LTE) software RADEX. Results . The main isotopologue c-C ₃ H ₂ is detected in 93% (14/15) of the targeted cores (for one of the 16 cores, none of its transitions were covered), its singly deuterated form c-C ₃ HD is detected in 94% (15/16) of the targeted cores, and its doubly deuterated form c-C ₃ D ₂ is detected towards 56% (9/16) of the cores detected. A range of column densities towards the different cores was derived: for c-C ₃ H ₂ , (0.5–8.1)×10 ¹³ cm ⁻² ; for c-C ₃ HD, (0.2–2.1)×10 ¹² cm ⁻² ; and for c-C ₃ D ₂ , (0.6–1.6)×10 ¹¹ cm ⁻² . The ortho-to-para ratio of c-C ₃ H ₂ was obtained for all except one core with a median value of 3.5 ± 0.4. The D/H and D ₂ /D ratios were obtained for the cores with detections, yielding a statistically corrected D/H range of 0.5–9.2% with a median value of 1.5 ± 0.2% and a statistically corrected D ₂ /D range 9.0–55.2% with a median value of 25.9 ± 4.3%. Conclusions . No apparent trend is seen with the ortho-to-para ratio of c-C ₃ H ₂ and the evolutionary stage of the core, as traced by volume density n H ₂ . The median c-C ₃ H ₂ D/H ratio in Perseus’ starless cores appears lower than the value for the Taurus molecular cloud and the Chamaeleon molecular cloud. The D ₂ /D ratio is equivalent between the Perseus and Taurus molecular clouds within the uncertainties. There is a correlation with the D/H ratio and the n H ₂ of the cores in Perseus, strengthening the idea of D/H being a tracer of core evolution. The D ₂ /D ratio does not correlate with n H ₂ , but positively correlates with T _kin , suggesting that its formation is favoured by a slightly endothermic reaction.

The increased sensitivity and spectral resolution of observed spectra toward the prestellar core L1544 are challenging the current physical and chemical models. With the aim of further constraining the structure of L1544 as well as assessing the completeness of chemical networks, we turn to radiative transfer modeling of observed molecular lines toward this source. We obtained high-sensitivity and high spectral resolution observations of HCO⁺ (J = 1–0), CS (J = 2–1), C³⁴S (J = 2–1), H₂CO (J K a , K c = 2_1,2–1_1,1), c-C₃H₂ (J K a , K c = 2_1,2–1_0,1), SO (N, J = 2, 3–1, 2), and ³⁴SO (N, J = 2, 3–1, 2) with the IRAM 30 m telescope toward the dust peak of L1544. A non–local thermodynamic equilibrium radiative transfer code is coupled to the Markov Chain Monte Carlo method to model the observations. We find that with just one transition for each isotope, the modeling cannot find a global minimum that fits the observations. The derived fractional abundance profiles are compared to those computed with chemical models. The observed transitions trace gas components with distinct dynamics at different distances along the radius of the core. Moreover, the results evidence a poor reproduction of sulfur chemistry by chemical modeling and stress the need to include a more consistent S-depletion process to accurately reproduce the S-chemistry toward dense cores.

Phosphorus is a key element that plays an essential role in biological processes important for living organisms on Earth. The origin and connection of phosphorus-bearing molecules to early solar system objects and star-forming molecular clouds is therefore of great interest, yet there are limited observations throughout different stages of low-mass (M < a few solar masses) star formation. Observations from the Yebes 40 m and IRAM 30 m telescopes detect for the first time in the 7 mm, 3 mm, and 2 mm bands multiple transitions of PN and PO, as well as a single transition of PO⁺, toward a low-mass starless core. The presence of PN, PO, and PO⁺ is kinematically correlated with bright SiO(1–0) emission. Our results reveal not only that shocks are the main driver of releasing phosphorus from dust grains and into the gas phase but that the emission originates from gas not affiliated with the shock itself but quiescent gas that has been shocked in the recent past. From radiative transfer calculations, the PO/PN abundance ratio is found to be 3 . 1 − 0.6 + 0.4 , consistent with other high-mass and low-mass star-forming regions. This first detection of PO⁺ toward any low-mass star-forming region reveals a PO⁺/PO ratio of 0.011 5 − 0.0009 + 0.0008 , a factor of 10 lower than previously determined from observations of a Galactic center molecular cloud, suggesting its formation can occur under more standard Galactic cosmic-ray ionization rates. These results motivate the need for additional observations that can better disentangle the physical mechanisms and chemical drivers of this precursor of prebiotic chemistry.

Context. The abundances of deuterated molecules with respect to their main isotopologue counterparts have been determined to be orders of magnitude higher than expected from the cosmic abundance of deuterium relative to hydrogen. The increasing number of singly and multi-deuterated species detections helps us to constrain the interplay between gas-phase and solid-state chemistry and to understand better deuterium fractionation in the early stages of star formation. Acetaldehyde is one of the most abundant complex organic molecules (COMs) in star-forming regions and its singly deuterated isotopologues have already been observed towards protostars. Aims. A spectroscopic catalogue for astrophysical purposes is built for doubly deuterated acetaldehyde (CHD₂CHO) from measurements in the laboratory. With this accurate catalogue, we aim to search for and detect this species in the interstellar medium and retrieve its column density and abundance. Methods. Sub-millimetre wave transitions were measured for the non-rigid doubly deuterated acetaldehyde CHD₂CHO displaying hindered internal rotation of its asymmetrical CHD2 methyl group. An analysis of a dataset consisting of previously measured microwave transitions and of the newly measured ones was carried out with an effective Hamiltonian which accounts for the tunnelling of the asymmetrical methyl group. Results. A line position analysis was carried out, allowing us to reproduce 853 transition frequencies with a weighted root mean square standard deviation of 1.7, varying 40 spectroscopic constants. A spectroscopic catalogue for astrophysical purposes was built from the analysis results. Using this catalogue, we were able to detect, for the first time, CHD₂CHO towards the low-mass proto-stellar system IRAS 16293-2422 utilising data from the ALMA Proto-stellar Interferometric Line Survey. Conclusions. The first detection of the CHD₂CHO species allowed for the derivation of its column density with a value of 1.3×10¹⁵ cm⁻² and an uncertainty of 10–20%. The resulting D₂/D ratio of ~20% is found to be coincident with D₂/D ratios derived for other COMs towards IRAS 16293-2422, pointing to a common formation environment with enhanced deuterium fractionation.