arXiv subjects

Maria N. Drozdovskaya

Publications and source records attributed to Maria N. Drozdovskaya.

At least 19 recordsLinked to original sources

arXiv2026

Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) VI. Discovery of a class I methanol maser transition and its association with acetaldehyde

CH3OH masers that are collisionally pumped, referred to as `class I', commonly trace shock interfaces in star-forming regions, but their incidence in low-mass protostars remains poorly quantified. We use data from the COMPASS ALMA Large Program to establish the occurrence, spatial origin, and excitation of a newly detected class I CH3OH maser at 302.9 GHz towards low-mass protostars and to test whether its pumping environments exhibit signatures of shock-driven complex chemistry. COMPASS provides unbiased line surveys of 11 sources between 279.0 and 311.7 GHz at typical angular resolutions of 0.3"-0.5". Rotation diagram analysis of departures from LTE in the 302.9 GHz CH3OH excitation robustly reveals masing in seven of 11 sources, with peak brightness temperatures from a few to ~10^3 K. The brightest detection is towards HOPS 108 and is likely associated with the powerful outflow from the nearby intermediate-mass protostar HOPS 370. Excluding this case, the maser is detected in six of ten low-mass protostars (60%). All six are Class 0 sources interacting with dense ambient material, whereas the non-detections comprise three Class I sources and one isolated Class 0 core, indicating a preference for early evolutionary stages and gas-rich environments for outflow-ambient medium interactions. The maser emission is spatially coincident with thermal CH3OH along outflow cavity walls and bow shocks, but is more compact and knot-like. Among the surveyed COMs, only CH3CHO closely traces the shocked morphology of the 302.9 GHz maser, and only around HOPS 108. These results show that class I CH3OH masers in low-mass protostellar systems are more prevalent than previously recognised and preferentially occur in younger, more embedded sources. More sensitive CH3CHO observations are needed to test whether a similar relationship with shock-driven chemistry also applies in the low-mass regime.

astro-ph.GA↗

arXiv2026

Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) II. Approach to data reduction and products for line-rich broadband (sub)millimeter spectra

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. 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. 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. 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.

astro-ph.SR↗

arXiv2026

JWST Reveals Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks

We present JWST/NIRSpec PRISM observations of the interstellar comet 3I/ATLAS obtained on 2025 August 6 (Epoch 1), 2025 December 22 (Epoch 2), and 2026 April 1 (Epoch 3), spanning eight months around perihelion at heliocentric distances of 3.3, 2.4, and 5.7 au, respectively. The spectra reveal a broad 3 $μ$m absorption band together with H$_2$O, CO$_2$, and CO gas emission. Unlike previously reported water-ice-rich Solar System comae, the strong 3 $μ$m absorption is accompanied by weak or absent 1.5 and 2.0 $μ$m water-ice bands. Spectral modeling indicates that the observations are best reproduced by submicron- to micron-sized water-ice-bearing aggregates containing refractory material. Compared with Epoch 1, the Epoch 3 spectrum favors the presence of a second population of larger, micron-sized, ice-rich aggregates and exhibits a subtle Fresnel-like structure near 3.1 $μ$m, consistent with crystalline water ice. The observations can be explained by either crystalline water ice at both epochs, with the spectral evolution arising primarily from changes in grain size and refractory mixing, or an evolution from an amorphous-like to crystalline state. The spectral properties of 3I bridge those of water-ice-bearing Solar System comae and several spectral classes of mid-sized trans-Neptunian objects, suggesting that the icy building blocks of planetesimals formed in different protoplanetary disks may span a continuum in the physical state of water ice, ranging from pure ice grains to water-ice-bearing aggregates with varying refractory content at submicron-to-micron scales, with 3I extending toward the refractory-rich end of this continuum.

astro-ph.EP↗

arXiv2026

High water D/H ratio of the interstellar object 3I/ATLAS is consistent with a low-metallicity origin

Recent JWST observations have revealed unusually high $^{12}$C/$^{13}$C ratios in carbon-bearing molecules of the interstellar object 3I/ATLAS, consistent with formation in a lower-metallicity environment than the present-day local interstellar medium (ISM). 3I/ATLAS also exhibits an exceptionally high water D/H ratio, exceeding those in Solar System comets and nearby low-mass star-forming regions. Here we investigate whether this high water D/H ratio can be reproduced in a low-metallicity formation scenario, using gas-ice astrochemical models. Assuming that the water observed in 3I/ATLAS was inherited from the parent molecular cloud and core, we perform a grid of astrochemical models covering the cloud to core stages, varying the gas density, ultraviolet radiation field ($χ$), cosmic-ray ionization rate ($ζ$), and metallicity, while solving thermal balance for the gas temperature. We find that lower metallicity enhances H$_3^+$ deuteration and, more importantly, its transfer to water ice. In contrast, water D/H ratio depends non-monotonically on $χ$ and $ζ$, because of competing chemical and thermal effects. In our models, the observed water D/H ratio is most readily reproduced at subsolar metallicities, $\lesssim0.5Z_\odot$, and relatively high cloud densities of $\sim$10$^4$ cm$^{-3}$ without strong constraints on either $χ$ or $ζ$, as long as $ζ<10^{-15}$ s$^{-1}$. The D/H ratio of methane normalized by that of water is not sensitive to the metallicity, being consistent with the similar values observed in 67P/Churyumov-Gerasimenko and 3I/ATLAS. These results suggest that water deuteration may provide a complementary probe of the metallicity and physical condition of the parent molecular cloud and dense core of interstellar objects.

astro-ph.EP↗

arXiv2026

An Enriched Methane D/H Ratio in the Interstellar Object 3I/ATLAS

Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star. We present near-infrared spectra of the coma of interstellar object 3I/ATLAS measured with the James Webb Space Telescope. Our results demonstrate an unexpectedly high D/H = $(3.33\pm0.31)\%$ for methane and represent an exceedingly rare detection of deuterated organic molecules in an interstellar object. This D/H ratio exceeds any other value for methane measured in the solar system, and is a factor of $14\pm2$ higher than that measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, the only other comet for which CH$_3$D has been detected. Both 3I/ATLAS and 67P/Churyumov-Gerasimenko show a higher degree of methane dueteration compared to water, consistent with trends seen in other solar system bodies and the nearby interstellar medium, where deuteration in organics exceeds that of water by up to an order of magnitude. The D/H ratio in methane itself is observationally unconstrained in extrasolar sources to date, but the enriched ratio in 3I/ATLAS is similar to those measured in methanol and formaldehyde toward primitive environments. The elevated D/H ratio for methane in 3I/ATLAS is consistent with the hypothesis that its ices formed in a cold, low metallicity interstellar or protostellar environment under a high cosmic ray irradiation rate (Cordiner et al. 2026).

astro-ph.EP↗

arXiv2026

JWST Edge-on Disk Ice (JEDIce): Vibrationally hot, rotationally cold H$_2$ in the outer disk of Oph 163131 non-thermally excited by UV and cosmic rays

Constraining ionization and excitation processes in protoplanetary disks is essential for understanding the chemical structure and evolution of disk material, shaping planet formation pathways. We present JWST/NIRSpec IFU observations of the edge-on disk Oph 163131, which reveal a unusual ro-vibrational H$_2$ spectrum dominated by the 1--0 O(2) line (2.627 $μ$m), with suppressed higher-$J$ emission despite excitation to $v=2$ and $3$. This vibrationally hot, rotationally cold H$_2$ emission is spatially extended, broadly following the molecular disk traced by CO($J{=}2$--1), with emission increasing above and below a thin midplane dark lane and extending radially beyond $\sim$200 au, where near-IR scattered-light emission is no longer dominant. We interpret the observed H$_2$ emission as arising from non-thermal excitation in cold, dense outer-disk gas, where collisions depopulate higher-$J$ rotational levels within each vibrational manifold prior to emission, producing the characteristic ``$v$-hot, $J$-cold" spectrum. We consider both ultraviolet irradiation and cosmic-ray excitation as contributors to the H$_2$ emission and find that their combined action, together with collisional de-excitation of high-$J$ level populations, broadly reproduces the observed line ratios and morphology. Within this framework, we infer a rather high effective cosmic-ray ionization rate of $\sim(1$-$10)\times10^{-15}$ s$^{-1}$ in the presence of a moderate UV field ($χ_{UV}=100-1000$, in Draine units). These results for disks, together with the recent findings by Bialy et al. 2025 for the lower-density starless core B68, highlight the potential of ro-vibrational H$_2$ emission as a novel probe of cosmic-ray ionization.

astro-ph.SR↗

arXiv2026

Coma Physics of an Interstellar Object: JWST Spatial-Spectral Mapping of 3I/ATLAS

We report a survey of molecular emission from cometary volatiles using the James Webb Space Telescope (JWST) toward interstellar object 3I/ATLAS carried out on UT 2025 December 22 and 23 at a heliocentric distance ($r_H$) of $2.37-2.41$ au. These measurements of CO, CO$_2$, H$_2$O, CH$_3$OH, and CH$_4$ sampled molecular chemistry in 3I/ATLAS as it receded from its encounter with our Sun and entered the vicinity of the H$_2$O ice line -- the region between $r_H$ = $2-3$ au where the temperature becomes too low for H$_2$O to vigorously sublime and CO and CO$_2$ begin to control the overall activity. CO was the most abundant molecule, followed by H$_2$O and CO$_2$, whose molecular abundances with respect to CO were $(40.5\pm3.1)\%$ and ($41.6\pm0.3)\%$, respectively. This work presents spatial-spectral maps of column density and rotational temperature as a function of distance from the nucleus for all detected species. The spatial distributions of both quantities were highly anisotropic for the apolar species in the coma of 3I/ATLAS, yet were more nearly symmetric for the polar molecules. These results demonstrate how volatiles were segregated in the nucleus ices of 3I/ATLAS and reveal heating and cooling mechanisms in its coma. Derived maps of the ortho-to-para ratio (OPR) for H$_2$O were flat with increasing distance from the nucleus and consistent with a coma-averaged value $\mathrm{OPR}=2.7\pm0.2$, slightly less than the expected equilibrium value of three.

astro-ph.EP↗

arXiv2026

JWST Edge-on Disk Ice (JEDIce): Program overview and ice survey results

The icy material within protoplanetary disks plays a central role in planet formation, yet remains poorly characterized by observations. We present 1.6-28$μ$m spectra of five disks obtained as part of the JWST Edge-on Disk Ice (JEDIce) program, representing the largest survey of disk ices to date. The major ice species H$_2$O, CO$_2$, and CO are detected towards all disks, and exhibit a wide range of absolute optical depths and optical depth ratios across the sample. This is suggestive of a range of ice abundances and compositions, but quantitative constraints will require radiative transfer modeling. All disks exhibit ice features across the entire spatial region where the IR continuum is detected; vertically elevated ice grains therefore seem to be ubiquitous in disks. The CO ice is consistently dominated by apolar CO:CO$_2$ mixtures, implying that the disk ice compositions are neither completely reset nor pristinely inherited from the protostellar stage. The presence of these mixtures also suggests that entrapment may be important in shaping the spatial distribution of CO within the disks. Small molecules commonly seen in protostellar ices (CH$_4$, CH$_3$OH, NH$_3$) are generally not detected in our sample, though tracers of ammonium salts (OCN$^-$ and the 6.85 $μ$m band) are common, potentially reflecting an evolution towards comet-like ice compositions. The spectra also contain a wealth of information about the micron-sized dust, atomic and molecular gas, and PAH content, which together with the ice constraints will provide a comprehensive picture of the chemical, physical, and dynamical state of these systems.

astro-ph.EP↗

arXiv2026

Isotopic Evidence for a Cold and Distant Origin of the Interstellar Object 3I/ATLAS

Interstellar objects provide the only directly observable samples of icy planetesimals formed around other stars, and can therefore provide insight into the diversity of physical and chemical conditions occurring during exoplanet formation. Here we report isotopic measurements of the interstellar comet 3I/ATLAS, which reveal an elemental composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium, at a level of D/H = (0.95 +- 0.06)%, which is more than an order of magnitude higher than in known comets, while its range of 12C/13C ratios (141-191 for CO2 and 123-172 for CO) exceeds typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at temperatures $\lesssim30$ K in a relatively metal-poor environment, early in the history of the Galaxy. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS accreted roughly 10-12 billion years ago, following an early period of intense star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system, and provides direct evidence for active ice chemistry and volatile-rich planetesimal formation in the young Milky Way.

astro-ph.EP↗

arXiv2025

The Flying Saucer edge-on disc's Near Infrared silhouette revealed by the JWST JEDIce program

Edge-on discs offer a unique opportunity to probe radial and vertical dust and gas distributions in the protoplanetary phase. This study aims to investigate the distribution of micron-sized dust particles in the Flying Saucer (BKLT J162813-243139) in Rho Ophiuchi, leveraging the unique observational conditions of a bright infrared background that enables the edge-on disc to be seen in both silhouette and scattered light at certain, specific wavelengths. As part of the JWST Edge-on Disc Ice program ('JEDIce'), we use NIRSpec IFU observations of the Flying Saucer, serendipitously observed against a PAH-emitting background, to constrain the dust distribution and grain sizes through radiative transfer modelling. Observation of the Flying Saucer in silhouette at 3.29 microns reveals that the midplane radial extent of small dust grains is ~235 au, larger than the large-grain disc extent previously determined to be 190 au from millimetre data. The scattered light observed in emission probes micron sized icy grains at large vertical distances above the midplane. The vertical extent of the disc silhouette is similar at visible, near-IR, and mid-IR wavelengths, corroborating the conclusion that dust settling is inefficient for grains as large as tens of microns, vertically and radially.

astro-ph.EP↗

arXiv2025

First Detection of CH3OD in Prestellar Cores

The isotopic ratios of deuterated methanol derived around protostars are commonly used to infer the physical conditions under which they formed in the earlier prestellar stage. However, there is a discrepancy in the ratio of the singly deuterated methanol isotopologues, CH2DOH/CH3OD, between low- and high-mass protostars, which puts into question whether prestellar isotopic ratios are generally preserved during the star- and planet-forming process. Resolving this puzzle is only made harder by the complete lack of data on this ratio in the prestellar stage. This work presents observations with the IRAM 30m telescope that securely detect CH3OD in the prestellar core L1448 in Perseus and tentatively in B213-C6 in Taurus. This work constrains the ratio of CH2DOH/CH3OD and the D/H ratios for both singly deuterated methanol isotopologues for the first time at the prestellar stage. Column densities calculated under the assumption of local thermal equilibrium lead to a CH2DOH/CH3OD ratio of 2.8-8.5 in L1448 and $\leq$ 5.7 in B213-C6. The values are marginally consistent with the statistically expected ratio of 3, but most assumptions put the values in an elevated range in line with values found around low-mass protostars. The D/H ratio in CH2DOH is between 3.6% and 6.8% in L1448 and in the range of 2.4-5.8% in B213-C6. The D/H ratio derived for CH3OD is lower, namely 1.4-4.4% in L1448 and $\leq$ 3.8% in B213-C6.

astro-ph.GA↗

arXiv2025

JWST detection of a carbon dioxide dominated gas coma surrounding interstellar object 3I/ATLAS

3I/ATLAS is the third confirmed interstellar object to visit our Solar System, and only the second to display a clear coma. Infrared spectroscopy with the James Webb Space Telescope (JWST) provides the opportunity to measure its coma composition and determine the primary activity drivers. We report the first results from our JWST NIRSpec campaign for 3I/ATLAS, at an inbound heliocentric distance of $r_H=3.32$ au. The spectral images (spanning 0.6-5.3 $μ$m) reveal a CO2 dominated coma, with enhanced outgassing in the sunward direction, and the presence of H2O, CO, OCS, water ice and dust. The coma CO2/H2O mixing ratio of $7.6\pm0.3$ is among the highest ever observed in a comet, and is 4.5-sigma above the trend as a function of heliocentric distance for long-period and Jupiter-family comets (excluding the outlier C/2016 R2). Our observations are compatible with an intrinsically CO2-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO2 ice line in its parent protoplanetary disk. A low coma H2O gas abundance may also be implied, for example, due to inhibited heat penetration into the nucleus, which could suppress the H2O sublimation rate relative to CO2 and CO.

astro-ph.EP↗

arXiv2025

Nascent chemical complexity in prestellar core IRAS 16293E: complex organics and deuterated methanol

Prestellar cores represent early sites of low-mass ($M$ $\leq$ few M$_\odot$) star and planet formation and provide insight into initial chemical conditions of complex organic molecules (COMs). Deuterated COMs trace the degree of molecular inheritance and/or reprocessing, as high deuteration in protostellar systems suggests COMs forming during the prestellar stage when deuteration is enhanced. Within the L1689N molecular cloud, the prestellar core IRAS 16293E sits $90^{"}$ eastward of the chemically-rich IRAS 16293-2422 A and B protostellar system. A unique view of star formation inside a common natal cloud, IRAS 16293A, B, and E all show some of the highest levels of deuteration in the ISM, with a number of D/H ratios $10^{5}$ times higher than Solar. We investigate for the first time the deuteration levels of the simplest COM, methanol (CH$_3$OH), in IRAS 16293E. Using the Arizona Radio Observatory (ARO) 12 m telescope, we target favorable transitions of CH$_2$DOH, CHD$_2$OH, $^{13}$CH$_3$OH, and several higher complexity COMs (including acetaldehyde, CH$_3$CHO, methyl formate, HCOOCH$_3$, and dimethyl ether, CH$_3$OCH$_3$) in the 3 mm band. Follow-up observations with the Yebes 40 m telescope provided additional transitions in the 7 mm (Q-band). We report the first detections of these COMs and deuterated methanol in prestellar core IRAS 16293E and use our observations to calculate excitation temperatures, column densities, and relative abundance ratios. Striking similarities are found between relative molecular ratios and D/H values when comparing IRAS 16293E to the A and B protostars, as well as to a heterogeneous sample of other prestellar cores, protostars, and the comet 67P/Churyumov-Gerasimenko. Our results support the idea that there is a limited amount of chemical reprocessing of COMs when prestellar cores collapse and heat-up during the protostellar phase.

astro-ph.GA↗

arXiv2025

Modelling methanol and hydride formation in the JWST Ice Age era

(Abridged) JWST observations have measured the ice composition toward two highly-extinguished field stars in the Chamaeleon I cloud. The observed extinction excess on the long-wavelength side of the H2O ice band at 3 micron has been attributed to a mixture of CH3OH with ammonia hydrates, which suggests that CH3OH ice could have formed in a water-rich environment with little CO depletion. Laboratory experiments and quantum chemical calculations suggest that CH3OH could form via the grain surface reactions CH3+OH and/or C+H2O in water-rich ices. However, no dedicated chemical modelling has been carried out thus far to test their efficiency and dependence on the astrochemical code employed. We model the ice chemistry in the Chamaeleon I cloud using a set of astrochemical codes (MAGICKAL, MONACO, Nautilus, UCLCHEM, and KMC simulations) to test the effects of the different code architectures and of the assumed ice chemistry. Our models show that the JWST ice observations are better reproduced for gas densities >1e5 cm-3 and collapse times >1e5 yr. CH3OH ice forms predominantly (>99%) via CO hydrogenation. The contribution of reactions CH3+OH and C+H2O, is negligible. The CO2 ice may form either via CO+OH or CO+O depending on the code. However, KMC simulations reveal that both mechanisms are efficient despite the low rate constant of the CO+O surface reaction. CH4 is largely underproduced for all codes except for UCLCHEM, for which a higher amount of atomic C is available during the initial translucent cloud phase. Large differences in the ice abundances are found at Tdust<12 K between diffusive and non-diffusive chemistry codes. This is due to the fact that non-diffusive chemistry takes over diffusive chemistry at such low Tdust. This could explain the rather constant ice chemical composition found in Chamaeleon I and other dense cores despite the different visual extinctions probed.

astro-ph.GA↗

arXiv2024

Measuring the $\mathrm{^{34}S}$ and $\mathrm{^{33}S}$ isotopic ratios of volatile sulfur during planet formation

Stable isotopic ratios constitute powerful tools for unraveling the thermal and irradiation history of volatiles. In particular, we can use our knowledge of the isotopic fractionation processes active during the various stages of star, disk and planet formation to infer the origins of different volatiles with measured isotopic patterns in our own solar system. Observations of planet-forming disks with the Atacama Large Millimeter/submillimeter Array (ALMA) now readily detect the heavier isotopologues of C, O and N, while the isotopologue abundances and isotopic fractionation mechanisms of sulfur species are less well understood. Using ALMA observations of the SO and SO2 isotopologues in the nearby, molecule-rich disk around the young star Oph-IRS 48 we present the first constraints on the combined 32S/34S and 32S/33S isotope ratios in a planet-forming disk. Given that these isotopologues likely originate in relatively warm gas (>50 K), like most other Oph-IRS 48 volatiles, SO is depleted in heavy sulfur while SO2 is enriched compared to solar system values. However, we cannot completely rule out a cooler gas reservoir, which would put the SO sulfur ratios more in line with comets and other solar system bodies. We also constrain the S18O/SO ratio and find the limit to be consistent with solar system values given a temperature of 60 K. Together these observations show that we should not assume solar isotopic values for disk sulfur reservoirs, but additional observations are needed to determine the chemical origin of the abundant SO in this disk, inform on what isotopic fractionation mechanism(s) are at play, and aid in unravelling the history of the sulfur budget during the different stages of planet formation.

astro-ph.EP↗

arXiv2023

Spatially resolving the volatile sulfur abundance in the HD 100546 protoplanetary disk

Volatile elements play a crucial role in the formation of planetary systems. Their abundance and distribution in protoplanetary disks provide vital insights into the connection between formation processes and the atmospheric composition of individual planets. Sulfur, being one of the most abundant elements in planet-forming environments, is of great significance, and now observable in exoplanets with JWST. However, planetary formation models currently lack vital knowledge regarding sulfur chemistry in protoplanetary disks. Developing a deeper understanding of the major volatile sulfur carriers in disks is essential to building models that can meaningfully predict planetary atmospheric composition, and reconstruct planetary formation pathways. In this work, we combine archival observations with new data from ALMA and APEX, covering a range of sulfur-bearing species/isotopologs. We interpret this data using the DALI thermo-chemical code, for which our model is highly refined and disk-specific. We find that volatile sulfur is heavily depleted from the cosmic value by a factor of 1000, with a disk-averaged abundance of S/H = 1e-8. We show that the gas-phase sulfur abundance varies radially by 3 orders of magnitude, with the highest abundances inside the inner dust ring and coincident with the outer dust ring at 150 to 230 au. Extracting chemical abundances from our models, we find OCS, H2CS, and CS to be the dominant molecular carriers in the gas phase. We also infer the presence of a substantial OCS ice reservoir. We relate our results to the potential atmospheric composition of planets in HD 100546, and the wider exoplanet population

astro-ph.EP↗

arXiv2023

Low NH$_{3}$/H$_{2}$O ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun

A lower-than-solar elemental nitrogen content has been demonstrated for several comets, including 1P/Halley and 67P/C-G with independent in situ measurements of volatile and refractory budgets. The recently discovered semi-refractory ammonium salts in 67P/C-G are thought to be the missing nitrogen reservoir in comets. The thermal desorption of ammonium salts from cometary dust particles leads to their decomposition into ammonia and a corresponding acid. The NH$_{3}$/H$_{2}$O ratio is expected to increase with decreasing heliocentric distance with evidence for this in near-infrared observations. NH$_{3}$ has been claimed to be more extended than expected for a nuclear source. Here, the aim is to constrain the NH$_{3}$/H$_{2}$O ratio in comet C/2020 F3 (NEOWISE) during its July 2020 passage. OH emission from comet C/2020 F3 (NEOWISE) was monitored for 2 months with NRT and observed from GBT on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH observations, the NH$_{3}$ hyperfine lines were targeted with GBT. The concurrent GBT and NRT observations allowed the OH quenching radius to be determined at $\left(5.96\pm0.10\right)\times10^{4}$ km on 24 July 2020, which is important for accurately deriving $Q(\text{OH})$. C/2020 F3 (NEOWISE) was a highly active comet with $Q(\text{H}_{2}\text{O}) \approx 2\times10^{30}$ molec s$^{-1}$ one day before perihelion. The $3σ$ upper limit for $Q_{\text{NH}_{3}}/Q_{\text{H}_{2}\text{O}}$ is $<0.29\%$ at $0.7$ au from the Sun. The obtained NH$_{3}$/H$_{2}$O ratio is a factor of a few lower than measurements for other comets at such heliocentric distances. The abundance of NH$_{3}$ may vary strongly with time depending on the amount of water-poor dust in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby, ammonium salts, if present, can rapidly thermally disintegrate and modify the NH$_{3}$/H$_{2}$O ratio.

astro-ph.EP↗

arXiv2023

Azimuthal C/O Variations in a Planet-Forming Disk

The elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant planet is a promising diagnostic of that planet's formation history in a protoplanetary disk. Alongside efforts in the exoplanet community to measure C/O in planetary atmospheres, observational and theoretical studies of disks are increasingly focused on understanding how the gas-phase C/O varies both with radial location and between disks. This is mostly tied to the icelines of major volatile carriers such as CO and H2O. Using ALMA observations of CS and SO, we have unearthed evidence for an entirely novel type of C/O variation in the protoplanetary disk around HD 100546: an azimuthal variation from a typical, oxygen-dominated ratio (C/O=0.5) to a carbon-dominated ratio (C/O>1.0). We show that the spatial distribution and peculiar line kinematics of both CS and SO molecules can be well-explained by azimuthal variations in the C/O ratio. We propose a shadowing mechanism that could lead to such a chemical dichotomy. Our results imply that tracing the formation history of giant exoplanets using their atmospheric C/O ratios will need to take into account time-dependent azimuthal C/O variations in a planet's accretion zone.

astro-ph.EP↗