CO Snowline Research Articles

Are These Planets or Brown Dwarfs? Broadly Solar Compositions from High-resolution Atmospheric Retrievals of ∼10–30 M Jup Companions

Using Keck Planet Imager and Characterizer high-resolution (R ∼ 35,000) spectroscopy from 2.29 to 2.49 μm, we present uniform atmospheric retrievals for eight young substellar companions with masses of ∼10–30 M Jup, orbital separations spanning ∼50–360 au, and T eff between ∼1500 and 2600 K. We find that all companions have solar C/O ratios and metallicities to within the 1σ–2σ level, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets with m < 10 M Jup, which show tentative atmospheric metal enrichment compared to their host stars. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions with T eff ∼ 1700–2000 K (κ And b and GSC 6214–210 b), our best-fit models prefer a nongray cloud model with >3σ significance. The cloudy models yield 2σ−3σ lower T eff for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1σ level. Finally, we constrain 12CO/13CO for three companions with the highest signal-to-noise ratio data (GQ Lup b, HIP 79098b, and DH Tau b) and report vsini and radial velocities for all companions.

Mind the trap

Aims. The ability of bulk ices (H2O, CO2) to trap volatiles has been well studied in any experimental sense, but largely ignored in protoplanetary disk and planet formation models as well as the interpretation of their observations. We demonstrate the influence of volatile trapping on C/O ratios in planet-forming environments. Methods. We created a simple model of CO, CO2, and H2O snowlines in protoplanetary disks and calculated the C/O ratio at different radii and temperatures. We included a trapping factor, which partially inhibits the release of volatiles (CO, CO2) at their snowline and releases them instead, together with the bulk ice species (H2O, CO2). Our aim has been to assess its influence of trapping solid-state and gas phase C/O ratios throughout planet-forming environments. Results. Volatile trapping significantly affects C/O ratios in protoplanetary disks. Variations in the ratio are reduced and become more homogeneous throughout the disk when compared to models that do not include volatile trapping. Trapping reduces the proportion of volatiles in the gas and, as such, reduces the available carbon- and oxygen-bearing molecules for gaseous accretion to planetary atmospheres. Volatile trapping is expected to also affect the elemental hydrogen and nitrogen budgets. Conclusions. Volatile trapping is an overlooked, but important effect to consider when assessing the C/O ratios in protoplanetary disks and exoplanet atmospheres. Due to volatile trapping, exoplanets with stellar C/O have the possibility to be formed within the CO and CO2 snowline.

Mapping the Vertical Gas Structure of the Planet-hosting PDS 70 Disk

PDS 70 hosts two massive, still-accreting planets and the inclined orientation of its protoplanetary disk presents a unique opportunity to directly probe the vertical gas structure of a planet-hosting disk. Here, we use high-spatial-resolution (≈0.″1; 10 au) observations in a set of CO isotopologue lines and HCO+ J = 4−3 to map the full 2D (r, z) disk structure from the disk atmosphere, as traced by 12CO, to closer to the midplane, as probed by less abundant isotopologues and HCO+. In the PDS 70 disk, 12CO traces a height of z/r ≈ 0.3, 13CO is found at z/r ≈ 0.1, and C18O originates at, or near, the midplane. The HCO+ surface arises from z/r ≈ 0.2 and is one of the few non-CO emission surfaces constrained with high-fidelity in disks to date. In the 12CO J = 3−2 line, we resolve a vertical dip and steep rise in height at the cavity wall, making PDS 70 the first transition disk where this effect is directly seen in line-emitting heights. In the outer disk, the CO emission heights of PDS 70 appear typical for its stellar mass and disk size and are not substantially altered by the two inner embedded planets. By combining CO isotopologue and HCO+ lines, we derive the 2D gas temperature structure and estimate a midplane CO snowline of ≈ 56–85 au. This implies that both PDS 70b and 70c are located interior to the CO snowline and are likely accreting gas with a high C/O ratio of ≈ 1.0, which provides context for future planetary atmospheric measurements from, e.g., JWST, and for properly modeling their formation histories.

Shaping the CO snowline in protoplanetary disks

Context. Characterizing the dust thermal structure in protoplanetary disks is a fundamental task because the dust surface temperature can affect both the planetary formation and the chemical evolution. Because the temperature depends on many parameters, including the grain size, it can be challenging to properly model the grain temperature structure. Many chemistry disk models usually employ a sophisticated single dust structure designed to reproduce the effect of a realistic population presumably composed of a large diversity of sizes. This generally represents a good approximation in most cases. Nonetheless, it dilutes the effects of the complex radiative interactions between the different grain populations on the resulting dust temperature, and thus, the chemistry. Aims. We seek to show that the radiative interactions between dust grains of different sizes can induce a nontrivial dust temperature structure that cannot be reproduced by a single dust population and that can significantly affect the chemical outcome. Methods. The disk thermal structures were computed using the Monte Carlo radiative transfer code RADMC-3D. The thermal structures were postprocessed using the gas-grain code NAUTILUS to calculate the evolution of the chemical abundance. Results. We find that simultaneously using at least two independent dust grain populations in disk models produces a complex temperature structure due to the starlight that is intercepted by the upper layers of the disk. In particular, we find that micron-sized dust grains are warmer than larger grains and can even show a radial temperature bump in some conditions. This dust temperature spread between the grain populations results in the segregation of the CO snowline and in an unexpected CO gas hole in the midplane. We compare the results with observed close to edge-on class I/II disks. Conclusions. Our study shows that the size dependence of the dust temperature significantly impacts the chemistry, and that two dust populations at least are required to account for this property of the thermal structure in protoplanetary disk models over a wide range of disk masses and dust properties.

Tracing snowlines and C/O ratio in a planet-hosting disk

The composition of a forming planet is set by the material it accretes from its parent protoplanetary disk. Therefore, it is crucial to map the chemical make-up of the gas in disks to understand the chemical environment of planet formation. This paper presents molecular line observations taken with the Atacama Large Millimeter/submillimeter Array of the planet-hosting disk around the young star HD 169142. We detect N2H+, CH3OH, [CI], DCN, CS, C34S, 13CS, H2CS, H2CO, HC3N, and c–C3H2 in this system for the first time. Combining these data with the recent detection of SO and previously published DCO+ data, we estimated the location of H2O and CO snowlines and investigate radial variations in the gas phase C/O ratio. We find that the HD 169142 disk has a relatively low N2H+ flux compared to the disks around Herbig stars HD 163296 and MWC 480, indicating less CO freeze-out, and place the CO snowline beyond the millimetre disk at ≈150 au. The detection of CH3OH from the inner disk is consistent with the H2O snowline being located at the edge of the central dust cavity at ≈20 au. The radially varying CS/SO ratio across the proposed H2O snowline location is consistent with this interpretation. Additionally, the detection of CH3OH in such a warm disk adds to the growing evidence supporting the inheritance of complex ices in disks from the earlier colder stages of star formation. Finally, we propose that the giant HD 169142 b located at 37 au is forming between the CO2 and H2O snowlines where the local elemental make-up of the gas is expected to have C/O ≈ 1.0.

Dust Enrichment and Grain Growth in a Smooth Disk around the DG Tau Protostar Revealed by ALMA Triple Bands Frequency Observations

Characterizing the physical properties of dust grains in a protoplanetary disk is critical to comprehending the planet formation process. Our study presents Atacama Large Millimeter/submillimeter Array (ALMA) high-resolution observations of the young protoplanetary disk around DG Tau at a 1.3 mm dust continuum. The observations, with a spatial resolution of ≈0.″04, or ≈5 au, revealed a geometrically thin and smooth disk without substantial substructures, suggesting that the disk retains the initial conditions of the planet formation. To further analyze the distributions of dust surface density, temperature, and grain size, we conducted a multiband analysis with several dust models, incorporating ALMA archival data of the 0.87 and 3.1 mm dust polarization. The results showed that the Toomre Q parameter is ≲2 at a 20 au radius, assuming a dust-to-gas mass ratio of 0.01. This implies that a higher dust-to-gas mass ratio is necessary to stabilize the disk. The grain sizes depend on the dust models, and for the DSHARP compact dust, they were found to be smaller than ∼400 μm in the inner region (r ≲ 20 au) while exceeding larger than 3 mm in the outer part. Radiative transfer calculations show that the dust scale height is lower than at least one-third of the gas scale height. These distributions of dust enrichment, grain sizes, and weak turbulence strength may have significant implications for the formation of planetesimals through mechanisms such as streaming instability. We also discuss the CO snowline effect and collisional fragmentation in dust coagulation for the origin of the dust size distribution.

Comet 2I/Borisov in Comparison with Comets of the Solar System

The brief review summarizes data on the chemical and mineral composition, as well as on the phys ical properties, of the first extrasolar comet 2I/Borisov, obtained from observations that were carried out from September 2019 to the end of March 2020. It is noted that the qualitative chemical composition of the volatile and mineral components comet 2I/Borisov is similar to the composition of comets in the Solar System, but there are differences that indicate the specific conditions for the formation of its nucleus in a circumstellar gas and dust disk. Different release rates of CO and H2O molecules in the vicinity of perihelion indicate the pos sible heterogeneity of the comet’s nucleus, which was formed from more homogeneous ice blocks, but differ ing in composition. These constituent blocks could have formed over a wide range of radial distances: from the snow line of H2O to the CO snow line. Their accumulation in the comet’s nucleus indicates large-scale mixing of protocometary bodies in the circumstellar disk. No spectra of finely crystalline magnesium silicates were found in cometary coma of 2I/Borisov, which can be interpreted as the absence of a significant amount of gas and dust transfer from the inner hot regions of the disk to the outside, into the zone of formation of protocometary bodies.

A Survey of CO, CO2, and H2O in Comets and Centaurs

CO and CO2 are the two dominant carbon-bearing molecules in comae and have major roles in driving activity. Their relative abundances also provide strong observational constraints to models of solar system formation and evolution but have never been studied together in a large sample of comets. We carefully compiled and analyzed published measurements of simultaneous CO and CO2 production rates for 25 comets. Approximately half of the comae have substantially more CO2 than CO, about one-third are CO-dominated, and about 1/10 produce a comparable amount of both. There may be a heliocentric dependence to this ratio with CO dominating comae beyond 3.5 au. Eight out of nine of the Jupiter Family Comets in our study produce more CO2 than CO. The six dynamically new comets produce more CO2 relative to CO than the eight Oort Cloud comets that have made multiple passes through the inner solar system. This may be explained by long-term cosmic-ray processing of a comet nucleus’s outer layers. We find (Q CO/Q )median = 3 ± 1% and (Q /Q )median = 12 ± 2%. The inorganic volatile carbon budget was estimated to be (Q CO+Q )/Q ∼ 18% for most comets. Between 0.7 and 4.6 au, CO2 outgassing appears to be more intimately tied to the water production in a way that the CO is not. The volatile carbon/oxygen ratio for 18 comets is C/Omedian ∼ 13%, which is consistent with a comet formation environment that is well within the CO snow line.

Broadband spectroscopy of astrophysical ice analogues

Context. Broadband optical constants of astrophysical ice analogues in the infrared (IR) and terahertz (THz) ranges are required for modeling the dust continuum emission and radiative transfer in dense and cold regions, where thick icy mantles are formed on the surface of dust grains. Such data are still missing from the literature, which can be attributed to the lack of appropriate spectroscopic systems and methods for laboratory studies. Aims. In this paper, the THz time-domain spectroscopy (TDS) and the Fourier-transform IR spectroscopy (FTIR) are combined to study optical constants of CO and CO2 ices in the broad THz-IR spectral range. Methods. The measured ices were grown at cryogenic temperatures by gas deposition on a cold silicon window. We developed a method to quantify the broadband THz-IR optical constants of ices, based on the direct reconstruction of the complex refractive index of ices in the THz range from the TDS data and the use of the Kramers-Kronig relation in the IR range for the reconstruction from the FTIR data. Uncertainties introduced by the Kramers-Kronig relations were eliminated by merging the THz and IR spectra. Finally, the reconstructed THz-IR response was analyzed using classical models of complex dielectric permittivity. Results. The complex refractive index of CO and CO2 ices deposited at the temperature of 28 K was obtained in the range of 0.312.0 THz and fitted using the analytical Lorentz model. Based on the measured dielectric constants, opacities of the astrophysical dust with CO and CO2 icy mantles were computed. Conclusions. The method developed in this work can be used for a model-independent reconstructions of optical constants of various astrophysical ice analogs in a broad THz-IR range. Such data can provide important benchmarks for interpreting broadband observations from existing and future ground-based facilities and space telescopes. The reported results will be useful in modeling sources that exhibit a drastic molecular freeze-out, such as the central regions of prestellar cores and mid-planes of protoplanetary disks, as well as CO and CO2 snow lines in disks.

Modeling snowline locations in protostars: The impact of the structure of protostellar cloud cores

Context. Snowlines during star and disk formation are responsible for a range of effects during the evolution of protostars, such as setting the chemical composition of the envelope and disk. This in turn influences the formation of planets by changing the elemental compositions of solids and affecting the collisional properties and outcomes of dust grains. Snowlines can also reveal echoes of past accretion bursts, providing insight into the formation process of stars. Aims. The objective is to identify which parameters (e.g., luminosity, gas density, and presence of disk) dictate the location of snowlines during the early, deeply embedded phase and to quantify how each parameter changes the observed snowline location. Methods. A numerical chemical network coupled with a grid of cylindrical-symmetric physical models was used to identify what physical parameters alter the CO and H2O snowline locations. The investigated parameters are the initial molecular abundances, binding energies of CO and H2O, heating source, cloud core density, outflow cavity opening angle, and disk geometry. Simulated molecular line emission maps were used to quantify the change in the snowline location with each parameter. Results. The snowline radius of molecules with low sublimation temperatures (≲30 K), such as CO, shift outward on the order of 103 AU with an order of magnitude increase in protostellar luminosity. An order of magnitude decrease in cloud core density also shifts the CO snowline position outward by a few 103 AU. The presence of disk(-like) structures cause inward shifts by a factor of a few, and mainly along the disk mid-plane. For molecules that sublimate at higher temperatures, such as H2O, increasing the protostellar luminosity or decreasing the cloud core density by an order of magnitude shifts the snowline position outward by a factor of a few. The presence of a disk concentrates molecules with high sublimation temperatures to compact regions (a few 10 AU) around the protostar by limiting the outward shift of snowline positions. Successful observational measurements of snowline locations are strongly dependent on spatial resolution, the presence or lack thereof of disk(-like) structures, and the inclination of the disk(-like) structure. Conclusions. The CO and H2O snowline locations do not occur at a single, well-defined temperature as is commonly assumed. Instead, the snowline position depends on luminosity, cloud core density, and whether a disk is present or not. Inclination and spatial resolution affect the observability and successful measurement of snowline locations. We note that N2H+ and HCO+ emission serve as good observational tracers of CO and H2O snowline locations. However, constraints on whether or not a disk is present, the observation of additional molecular tracers, and estimating envelope density will help in accurately determining the cause of the observed snowline position. Plots of the N2H+ and HCO+ peak emission radius versus luminosity are provided to compare the models with observations of deeply embedded protostars aiming to measure the CO and H2O snowline locations.

Discovery of Line Pressure Broadening and Direct Constraint on Gas Surface Density in a Protoplanetary Disk

The gas surface density profile of protoplanetary disks is one of the most fundamental physical properties to understanding planet formation. However, it is challenging to determine the surface density profile observationally, because the H2 emission cannot be observed in low-temperature regions. We analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) archival data of the 12CO J = 3 − 2 line toward the protoplanetary disk around TW Hya and discovered extremely broad line wings due to the pressure broadening. In conjunction with a previously reported optically thin CO isotopologue line, the pressure broadened line wings enabled us to directly determine the midplane gas density for the first time. The gas surface density at ∼5 au from the central star reaches ∼103 g cm−2, which suggests that the inner region of the disk has enough mass to form a Jupiter-mass planet. Additionally, the gas surface density drops at the inner cavity by ∼2 orders of magnitude compared to outside the cavity. We also found a low CO abundance of ∼10−6 with respect to H2, even inside the CO snow line, which suggests conversion of CO to less volatile species. Combining our results with previous studies, the gas surface density jumps at r ∼ 20 au, suggesting that the inner region (3 < r < 20 au) might be the magnetorotational instability dead zone. This study sheds light on the direct gas surface density constraint without assuming the CO/H2 ratio using ALMA.

Molecular tracers of planet formation in the atmospheres of hot Jupiters

ABSTRACT The atmospheric chemical composition of a hot Jupiter can lead to insights into where in its natal protoplanetary disc it formed and its subsequent migration pathway. We use a 1D chemical kinetics code to compute a suite of models across a range of elemental abundances to investigate the resultant abundances of key molecules in hot Jupiter atmospheres. Our parameter sweep spans metallicities between 0.1x and 10x solar values for the C/H, O/H, and N/H ratios, and equilibrium temperatures of 1000 and 2000 K. We link this parameter sweep to the formation and migration models from previous works to predict connections between the atmospheric molecular abundances and formation pathways, for the molecules H2O, CO, CH4, CO2, HCN, and NH3. We investigate atmospheric H2O abundances in eight hot Jupiters reported in the literature. All eight planets fall within our predicted ranges for various formation models; however, six of them are degenerate between multiple models and hence require additional molecular detections for constraining their formation histories. The other two planets, HD 189733 b and HD 209458 b, have water abundances that fall within ranges expected from planets that formed beyond the CO2 snowline. Finally, we investigate the detections of H2O, CO, CH4, CO2, HCN, and NH3 in the atmosphere of HD 209458 b and find that, within the framework of our model, the abundances of these molecules best match with a planet that formed between the CO2 and CO snowlines and then underwent disc-free migration to reach its current location.

The Volatile Carbon-to-oxygen Ratio as a Tracer for the Formation Locations of Interstellar Comets

Based on the occurrence rates implied by the discoveries of 1I/‘Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect ≥one interstellar object every year. We advocate for future measurements of the production rates of H2O, CO2, and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. By quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO2 relative to H2O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q(CO)/Q(H2O) < 0.2 and Q(CO)/Q(H2O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.

Chemical Feedback of Pebble Growth: Impacts on CO depletion and C/O ratios

Abstract Observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet formation. Previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. In this paper, we present a new 1D method for modeling pebble growth and chemistry simultaneously. Gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. Pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. We present the results of this model after 1 Myr of disk evolution around a 1M ⊙ star at various locations both inside and outside the CO snowline. We find that for a turbulent disk (α = 10−3), CO is depleted from the surface layers of the disk by roughly 1–2 orders of magnitude, consistent with observations of protoplanetary disks. This is achieved by a combination of ice sequestration and decreasing UV opacity, both driven by pebble growth. Further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase C/O ratios to values of approximately unity. However, our model is unable to produce C/O values of ∼1.5–2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain C/O ratios &gt;1.

The sulphur depletion problem in molecular clouds: The H2S case

Sulphur is one of the most abundant elements in the Universe and plays a crucial role in biological systems. However, sulphuretted molecules in the ISM are not as abundant as expected and there is no clear answer of where the missing Sulphur is yet. To shed light onto this open question, we focus our attention on the chemistry of H2S, thought to be an important reservoir of Sulphur and formed mainly by grain-phase reactions. To understand the formation of H2S, the growth of ices, and the chemical desorption process, we study the CO, CH3OH, N2H+, and H2S abundances towards Barnard 1b, a Sulphur-rich cloud hosting a first Larson core. We look for correlations between gas-phase abundances of H2S and CH3OH that better constrain the location of the CO snowline in dark cores. Finally, this provides additional data to benchmark models for a deeper insight on the chemical desorption process and its efficiency.

Planetesimal rings as the cause of the Solar System’s planetary architecture

Astronomical observations reveal that protoplanetary disks around young stars commonly have ring- and gap-like structures in their dust distributions. These features are associated with pressure bumps trapping dust particles at specific locations, which simulations show are ideal sites for planetesimal formation. Here we show that our Solar System may have formed from rings of planetesimals—created by pressure bumps—rather than a continuous disk. We model the gaseous disk phase assuming the existence of pressure bumps near the silicate sublimation line (at T ~ 1,400 K), water snowline (at T ~ 170 K) and CO snowline (at T ~ 30 K). Our simulations show that dust piles up at the bumps and forms up to three rings of planetesimals: a narrow ring near 1 au, a wide ring between ~3–4 au and ~10–20 au and a distant ring between ~20 au and ~45 au. We use a series of simulations to follow the evolution of the innermost ring and show how it can explain the orbital structure of the inner Solar System and provides a framework to explain the origins of isotopic signatures of Earth, Mars and different classes of meteorites. The central ring contains enough mass to explain the rapid growth of the giant planets’ cores. The outermost ring is consistent with dynamical models of Solar System evolution proposing that the early Solar System had a primordial planetesimal disk beyond the current orbit of Uranus. The planetary architecture of the Solar System and its isotopic dichotomy can be reproduced using a protoplanetary disk model structured with rings and gaps, as commonly seen in protoplanetary disks around other stars.

The 12CO/13CO isotopologue ratio of a young, isolated brown dwarf

Context. Linking atmospheric characteristics of planets to their formation pathways is a central theme in the study of extrasolar planets. Although the 12C/13C isotope ratio shows little variation in the Solar System, the atmosphere of a super-Jupiter was recently shown to be rich in 13CO, possibly as a result of dominant ice accretion beyond the CO snow line during its formation. Carbon isotope ratios are therefore suggested to be a potential tracer of formation pathways of planets. Aims. In this work, we aim to measure the 12CO/13CO isotopologue ratio of a young, isolated brown dwarf. While the general atmospheric characteristics of young, low-mass brown dwarfs are expected to be very similar to those of super-Jupiters, their formation pathways may be different, leading to distinct isotopologue ratios. In addition, such objects allow high-dispersion spectroscopy at high signal-to-noise ratios. Methods. We analysed archival K-band spectra of the L dwarf 2MASS J03552337+1133437 taken with NIRSPEC at the Keck telescope. A free retrieval analysis was applied to the data using the radiative transfer code petitRADTRANS coupled with the nested sampling tool PyMultiNest to determine the isotopologue ratio 12CO/13CO in its atmosphere. Results. The isotopologue 13CO is detected in the atmosphere through the cross-correlation method at a signal-to-noise of ~8.4. The detection significance is determined to be ~9.5σ using a Bayesian model comparison between two retrieval models (including or excluding 13CO). We retrieve an isotopologue 12CO/13CO ratio of 97−18+25 (90% uncertainty), marginally higher than the local interstellar standard. Its C/O ratio of ~0.56 is consistent with the solar value. Conclusions. Although only one super-Jupiter and one brown dwarf now have a measured 12CO/13CO ratio, it is intriguing that they are different, possibly hinting to distinct formation pathways. Regardless of spectroscopic similarities, isolated brown dwarfs may experience a top-down formation via gravitational collapse, which resembles star formation, while giant exoplanets favourably form through core accretion, which potentially alters isotopologue ratios in their atmospheres depending on the material they accrete from protoplanetary disks. This further emphasises atmospheric carbon isotopologue ratio as a tracer of the formation history of exoplanets. In the future, analyses such as those presented here should be conducted on a wide range of exoplanets using medium-to-high-resolution spectroscopy to further assess planet formation processes.

Molecules with ALMA at Planet-forming Scales (MAPS). XIII. HCO+ and Disk Ionization Structure

Abstract We observed HCO+ J = 1 − 0 and H13CO+ J = 1 − 0 emission toward the five protoplanetary disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480 as part of the MAPS project. HCO+ is detected and mapped at 0.″3 resolution in all five disks, while H13CO+ is detected (S/N &gt; 6σ) toward GM Aur and HD 163296 and tentatively detected (S/N &gt; 3σ) toward the other disks by a matched filter analysis. Inside a radius of R ∼ 100 au, the HCO+ column density is flat or shows a central dip. At outer radii (≳100 au), the HCO+ column density decreases outward, while the column density ratio of HCO+/CO is mostly in the range of ∼10−5–10−4. We derived the HCO+ abundance in the warm CO-rich layer, where HCO+ is expected to be the dominant molecular ion. At R ≳ 100 au, the HCO+ abundance is ∼3 × 10−11 − 3 × 10−10, which is consistent with a template disk model with X-ray ionization. At the smaller radii, the abundance decreases inward, which indicates that the ionization degree is lower in denser gas, especially inside the CO snow line, where the CO-rich layer is in the midplane. Comparison of template disk models with the column densities of HCO+, N2H+, and N2D+ indicates that the midplane ionization rate is ≳10−18 s−1 for the disks around IM Lup, AS 209, and HD 163296. We also find hints of an increased HCO+ abundance around the location of dust continuum gaps in AS 209, HD 163296, and MWC 480. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Molecules with ALMA at Planet-forming Scales (MAPS). VI. Distribution of the Small Organics HCN, C2H, and H2CO

Abstract Small organic molecules, such as C2H, HCN, and H2CO, are tracers of the C, N, and O budget in protoplanetary disks. We present high-angular-resolution (10–50 au) observations of C2H, HCN, and H2CO lines in five protoplanetary disks from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We derive column density and excitation temperature profiles for HCN and C2H, and find that the HCN emission arises in a temperate (20–30 K) layer in the disk, while C2H is present in relatively warmer (20–60 K) layers. In the case of HD 163296, we find a decrease in column density for HCN and C2H inside one of the dust gaps near ∼83 au, where a planet has been proposed to be located. We derive H2CO column density profiles assuming temperatures between 20 and 50 K, and find slightly higher column densities in the colder disks around T Tauri stars than around Herbig Ae stars. The H2CO column densities rise near the location of the CO snowline and/or millimeter dust edge, suggesting an efficient release of H2CO ices in the outer disk. Finally, we find that the inner 50 au of these disks are rich in organic species, with abundances relative to water that are similar to cometary values. Comets could therefore deliver water and key organics to future planets in these disks, similar to what might have happened here on Earth. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

If you like C/O variations, you should have put a ring on it

Context. The C/O ratio as traced with C2H emission in protoplanetary disks is fundamental for constraining the formation mechanisms of exoplanets and for our understanding of volatile depletion in disks, but current C2H observations show an apparent bimodal distribution that is not well understood, indicating that the C/O distribution is not described by a simple radial dependence. Aims. The transport of icy pebbles has been suggested to alter the local elemental abundances in protoplanetary disks through settling, drift, and trapping in pressure bumps, resulting in a depletion of volatiles in the surface layer and an increase in the elemental C/O. Methods. We combine all disks with spatially resolved ALMA C2H observations with high-resolution continuum images and constraints on the CO snow line to determine if the C2H emission is indeed related to the location of the icy pebbles. Results. We report a possible correlation between the presence of a significant CO-ice dust reservoir and high C2H emission, which is only found in disks with dust rings outside the CO snow line. In contrast, compact dust disks (without pressure bumps) and warm transition disks (with their dust ring inside the CO snow line) are not detected in C2H, suggesting that such disks may have never contained a significant CO ice reservoir. Conclusions. This correlation provides evidence for the regulation of the C/O profile by the complex interplay of CO snow line and pressure bump locations in the disk. These results demonstrate the importance of including dust transport in chemical disk models for a proper interpretation of exoplanet atmospheric compositions and a better understanding of volatile depletion in disks, in particular the use of CO isotopologs to determine gas surface densities.