arXiv subjects

Shota Notsu

Publications and source records attributed to Shota Notsu.

At least 19 recordsLinked to original sources

arXiv2026

Probing the Innermost Region of the V883 Ori Disk Using ALMA Band 1 Methanol Line Observations

The snowlines of major volatiles in protoplanetary disks play a pivotal role in dust evolutions and volatile delivery to nascent planetary systems. In this paper, we report the Atacama Large Millimeter/submillimeter Array Band 1 ($\approx7.5\,\mathrm{mm}$) observations of methanol (CH$_3$OH) emission lines in the disk around the FU-Ori type star V883 Ori, where accretion outburst heats the disk and the majority of ices has sublimated. We detect three CH$_3$OH emission lines at an angular resolution of $\approx0.\!\!^{\prime\prime}2$. The stacked CH$_3$OH image exhibits a centrally-peaked morphology in contrast to the previous (sub-)mm observations that show a central depression. By fitting radially-resolved line profiles, we derive the radial intensity profile of the CH$_3$OH emission where we find a steep increase at $\lesssim40\,\mathrm{au}$. The column density of CH$_3$OH reaches at least $\sim10^{19}\mathrm{-}10^{20}\,\mathrm{cm^{-2}}$ at $\sim10\,\mathrm{au}$. This provides direct evidence that a significant amount of warm gaseous methanol is present in the innermost region of the disk where its emission has been suppressed in previous (sub-)mm observations due to the optically thick dust emission. The steep increase in the intensity profile may indicate that the CH$_3$OH snowline in the midplane is located at $\sim30\mathrm{-}55\,\mathrm{au}$, or that the CH$_3$OH emission traces the temperature structure given that the emission is likely optically thick. Our results demonstrate the capability and significance of (sub-)cm observations in probing the innermost opaque region of disks, paving the way for the future observations with upcoming facilities.

astro-ph.EP↗

arXiv2026

GREX-PLUS Science Book v2

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA strategic L-class mission to be launched in the 2030s. Its primary science goals are two-fold: galaxy formation and evolution, and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a telescope with a 1 m primary mirror aperture cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2--8 $\mu$m wavelength band and a high-resolution spectrometer with a wavelength resolution of 30,000 in the 10--18 $\mu$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high-resolution spectrometer aims to identify the location of the water ``snowline'' in protoplanetary disks. Both instruments will provide unique datasets for a broad range of scientific topics, including galaxy mass assembly, the origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy of exoplanet atmospheres, planetary atmospheres in the Solar System, and so on. This document is the second version of a collection of scientific themes that can be achieved with GREX-PLUS. Each section in Chapters~2 and 3 is based on presentations at several GREX-PLUS Science Workshops.

astro-ph.IM↗

arXiv2026

Compact Hydrogen Sulfide Emission Indicates Sulfur-bearing Ice Sublimation in the Inner Disk of HD 163296

The sulfur chemistry in protoplanetary disks directly affects the composition and potential habitability of nascent planets, but its volatile inventory remains highly uncertain. Here, we present deep Atacama Large Millimeter/submillimeter Array (ALMA) observations of hydrogen sulfide (H$_2$S) along with SO and SO$_2$ in the disk around HD 163296 at an angular resolution of $\approx0.\!\!^{\prime\prime}3$ (or $\approx$30 au). We detect unresolved, compact emission of H$_2$S and SO (and tentatively SO$_2$) at the disk center with a broad line width of $\sim$40 km s$^{-1}$, suggesting that the emission is originating from the innermost regions. By fitting line profiles with a geometrically-thin Keplerian-rotating disk model, we constrain the emitting radii and gas temperatures of these molecules to be $\approx$3-5 au and $\gtrsim$90-120 K, respectively, consistent with sublimation of sulfur-bearing molecules along with water ice in the inner warm region. While the higher or comparable column density of H$_2$S with respect to SO and SO$_2$ indicates that H$_2$S is an important volatile sulfur reservoir in the disk, the limited constraints mean that we cannot rule out significantly depleted volatile sulfur as also commonly inferred in other planet-forming disks. Further observations are needed to better constrain disk sulfur inventory, unravel how sulfur compounds are reprocessed in disks, and shed light on the nature of less-volatile species, such as salts and sulfide minerals, which may occupy a significant portion of sulfur budget.

astro-ph.EP↗

arXiv2025

ALMA Band 7 Observations of Water Lines in the Protoplanetary Disk of V883 Ori

The FU Orionis star V883 Ori provides a unique opportunity to probe the water snowline in a protoplanetary disk. During an accretion burst, the enhanced stellar luminosity heats the disk, sublimating ices and bringing volatile species into the gas-phase. The water snowline, located at $\sim$80 au in the midplane, represents a key boundary for dust growth and volatile delivery to forming planets. We present Atacama Large Millimeter/submillimeter Array Band 7 observations of V883 Ori that detect two targeted water isotopologue transitions: para-H$_2$$^{18}$O $5_{1,5}$-$4_{2,2}$ at 322 GHz and HDO $3_{3,1}$-$4_{2,2}$ at 335 GHz. After correcting for Keplerian rotation, we detect HDO and H$_2$$^{18}$O at 23.6$\sigma$ and 9.3$\sigma$, respectively. Rotational-diagram analysis using a Markov Chain Monte Carlo approach yields $T_\mathrm{rot}=116.89\pm12.81$ K and $N=(4.90\pm1.69)\times10^{15}\,\mathrm{cm}^{-2}$ for H$_2$$^{18}$O, and $T_\mathrm{rot}=87.46\pm4.95$ K and $N=(4.47\pm0.62)\times10^{15}\,\mathrm{cm}^{-2}$ for HDO. These results imply water vapor abundances of $N_{\mathrm{H_2O}}/N_{\mathrm{H_2}}\sim3\times10^{-7}$-$5\times10^{-6}$ and an HDO/H$_2$O ratio of $(0.4$-$2.0)\times10^{-3}$ just inside the water snowline, broadly consistent with inheritance from protostellar envelopes. The HDO line in Band 7 is significantly weaker than predicted from Band 6 extrapolation, showing only $\sim$26% of the expected strength. This attenuation can be explained by a more compact, hotter emitting region with an effective radius of $\sim$53 au and/or frequency-dependent dust absorption that enlarges the apparent inner cavity at higher frequency. Our results highlight both the diagnostic power of water isotopologue lines and the need for higher angular resolution observations to resolve the water snowline and test these scenarios.

astro-ph.EP↗

arXiv2025

The chemical diversity of giant-planet nurseries as revealed by ALMA

With the giant exoplanet occurrence rate peaking around stars of 1.5-2 solar masses, there is strong motivation to characterize the disks that set their formation conditions. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to investigate both the availability of different molecules in disks and infer the radial distribution of elemental abundances, enabling us to make connections to exoplanet systems. Here we present a survey of six transition disks around young F-, A-, and B-type stars using ALMA. We find 13C18O, CS, SO, and H2CO in all six systems, as well as ten additional molecules in a subset of disks, including detections of H2S, 33SO, and CH3OCH3. Using these data, and literature data where available, we construct the first comprehensive picture of Herbig disk chemistry. We find clear correlations between molecular tracers of C/O>1 environments (e.g., CS, C2H) and disk mass, as traced by C18O line flux. In contrast, tracers of C/O<1 environments (e.g., SO, CH3OH) do not show significant correlations with disk mass. Interestingly, these molecules are relatively brighter in lower-mass disks, with their presence primarily linked to disks with central cavities and spirals. Finally, we show that the observed chemical diversity seen across Herbig disks leads to varying C/O regimes at the orbital radii of candidate proto-planets identified within these disks. When comparing these inferred disk C/O ratios with those measured for directly imaged exoplanets, we find a notable overlap and show that low C/O gas is common on 10's of au scales in Herbig disks.

astro-ph.EP↗

arXiv2025

Ice sublimation in the dynamic HD 100453 disk reveals a rich reservoir of inherited complex organics

Protoplanetary disks around luminous young A-type stars are prime observational laboratories to determine the abundances of complex organic molecules (COMs) present during planet formation. In contrast to their lower stellar mass counterparts, these warmer disks contain the sublimation fronts of complex molecules such as CH3OH on spatial scales accessible with the Atacama Large Millimeter/submillimeter Array (ALMA). We present ALMA observations of the Herbig Ae disk HD 100453 that uncover a rich reservoir of COMs sublimating from the dust cavity edge. In addition to CH3OH, we detect 13CH3OH for the first time in a Class II disk, revealing a factor of three enhancement of 13C in the disk large organics. A tentative detection of CH2DOH is also reported, resulting in a D/H of 1-2%, which is consistent with the expected deuterium enhancement from the low temperature CH3OH formation in molecular clouds and with the deuteration of CH3OH measured in comets. The detection of methyl-formate (CH3OCHO), at only a few percent level of CH3OH is an order of magnitude lower compared to claims towards other organic-rich Herbig Ae disks but is more in line with organic abundance patterns towards the earlier stages of star formation. Together these data provide multiple lines of evidence that disks, and therefore the planet and comet-forming materials, contain inherited interstellar ices and perhaps the strongest evidence to date that much of the interstellar organic ice composition survives the early stages of planet formation.

astro-ph.EP↗

arXiv2025

ALMA reveals thermal and non-thermal desorption of methanol ice in the HD 100546 protoplanetary disk

Methanol (CH$_3$OH) and formaldehyde (H$_2$CO) are chemically coupled organic molecules proposed to act as an intermediate step between simple molecules and more complex prebiotic compounds. Their abundance distributions across disks regulate the prebiotic potential of material at different disk radii. We present observations of multiple methanol and formaldehyde transitions toward the Herbig Ae disk HD 100546 obtained with ALMA, building upon the previous serendipitous detection of methanol in this source. We find that methanol has a higher rotational temperature ($T_\mathrm{rot}$) than formaldehyde towards both the centrally concentrated emission component in the inner disk ($0-110$ au) and a radially separate dust ring farther out in the disk ($180-260$ au). $T_\mathrm{rot}$ decreases for methanol and formaldehyde from the inner ($152^{+35}_{-27}$ K and $76^{+9}_{-8}$ K) to the outer disk ($52^{+8}_{-6}$ K and $31^{+2}_{-2}$ K), suggesting that we are tracing two different chemical environments. $T_\mathrm{rot}$ for both species in the inner disk is consistent with thermal desorption as the origin, while the outer disk reservoir is driven by non-thermal desorption. The CH$_3$OH/H$_2$CO column density ratio decreases from $14.6^{+5.2}_{-4.6}$ in the inner disk to $1.3^{+0.3}_{-0.2}$ in the outer disk, consistent with modelling predictions. The CH$_3$OH/H$_2$CO column density ratio for the inner disk is consistent with the median value in the range of column density ratios compiled from Solar System comets which would have formed at a similar distance. This supports the notion that interstellar ice is inherited and preserved by protoplanetary disks around solar-mass and intermediate-mass stars as we are seeing 'fresh' ice sublimation, as well as providing more evidence for the presence of prebiotic precursor molecules in planet-forming regions.

astro-ph.EP↗

arXiv2024

Characterising the molecular line emission in the asymmetric Oph-IRS 48 dust trap: Temperatures, timescales, and sub-thermal excitation

The ongoing physical and chemical processes in planet-forming disks set the stage for planet formation. The asymmetric disk around the young star Oph-IRS 48 has one of the most well-characterised chemical inventories, showing molecular emission from a wide variety of species at the dust trap. One of the explanations for the asymmetric structure is dust trapping by a perturbation-induced vortex. We aim to constrain the excitation properties of the molecular species SO$_2$, CH$_3$OH, and H$_2$CO. We further characterise the extent of the molecular emission, through the determination of important physical and chemical timescales at the location of the dust trap. We also investigate whether the potential vortex can influence the observable temperature structure of the gas. Through a pixel-by-pixel rotational diagram analysis, we create rotational temperature and column density maps for SO$_2$ and CH$_3$OH, while temperature maps for H$_2$CO are created using line ratios. We find temperatures of $T\sim$55 K and $T\sim$125 K for SO$_2$ and CH$_3$OH, respectively, while the line ratios point towards temperatures of T$\sim$150-300 K for H$_2$CO. The rotational diagram of CH$_3$OH is dominated by scatter and subsequent non-LTE RADEX calculations suggest that both CH$_3$OH and H$_2$CO must be sub-thermally excited. The temperatures suggest that SO$_2$ comes from a layer deep in the disk, while CH$_3$OH and H$_2$CO originate from a higher layer. While a potential radial gradient is seen in the temperature map of SO$_2$, we do not find any hints of a vortex influencing the temperature structure. The determined turbulent mixing timescale is not able to explain the emitting heights of the molecules, but the photodissociation timescales are able to explain the wider azimuthal extents of SO$_2$ and H$_2$CO compared to CH$_3$OH, where a secondary, gas-phase formation reservoir is required for H$_2$CO.

astro-ph.EP↗

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↗

arXiv2024

Detection of Dimethyl Ether in the Central Region of the MWC 480 Protoplanetary Disk

Characterizing the chemistry of complex organic molecules (COMs) at the epoch of planet formation provides insights into the chemical evolution of the interstellar medium (ISM) and the origin of organic materials in our Solar System. We report a detection of dimethyl ether (CH$_3$OCH$_3$) in the disk around the Herbig Ae star MWC 480 with the sensitive Atacama Large Millimeter/submillimeter Array observations. This is the first detection of CH$_3$OCH$_3$ in a non-transitional Class II disk. The spatially unresolved, compact (${\lesssim}$25 au in radius) nature, the broad line width ($\sim$30 km s$^{-1}$), and the high excitation temperature (${\sim}$200 K) indicate sublimation of COMs in the warm inner disk. Despite the detection of CH$_3$OCH$_3$, methanol (CH$_3$OH), the most abundant COM in the ISM, has not been detected, from which we constrain the column density ratio of CH$_3$OCH$_3$/CH$_3$OH ${\gtrsim}$7. This high ratio may indicate the reprocessing of COMs during the disk phase, as well as the effect of the physical structure in the inner disk. We also find that this ratio is higher than in COM-rich transition disks recently discovered. This may indicate that, in the full disk of MWC 480, COMs have experienced substantial chemical reprocessing in the innermost region, while the COM emission in the transition disks predominantly traces the inherited ice sublimating at the dust cavity edge located at larger radii (${\gtrsim}$20 au).

astro-ph.EP↗

arXiv2024

The Japanese Vision for the Black Hole Explorer Mission

The Black Hole Explorer (BHEX) is a next-generation space very long baseline interferometry (VLBI) mission concept that will extend the ground-based millimeter/submillimeter arrays into space. The mission, closely aligned with the science priorities of the Japanese VLBI community, involves an active engagement of this community in the development of the mission, resulting in the formation of the Black Hole Explorer Japan Consortium. Here we present the current Japanese vision for the mission, ranging from scientific objectives to instrumentation. The Consortium anticipates a wide range of scientific investigations, from diverse black hole physics and astrophysics studied through the primary VLBI mode, to the molecular universe explored via a potential single-dish observation mode in the previously unexplored 50-70\,GHz band that would make BHEX the highest-sensitivity explorer ever of molecular oxygen. A potential major contribution for the onboard instrument involves supplying essential elements for its high-sensitivity dual-band receiving system, which includes a broadband 300\,GHz SIS mixer and a space-certified multi-stage 4.5K cryocooler akin to those used in the Hitomi and XRISM satellites by the Japan Aerospace Exploration Agency. Additionally, the Consortium explores enhancing and supporting BHEX operations through the use of millimeter/submillimeter facilities developed by the National Astronomical Observatory of Japan, coupled with a network of laser communication stations operated by the National Institute of Information and Communication Technology.

astro-ph.IM↗

arXiv2024

An ALMA molecular inventory of warm Herbig Ae disks: I. Molecular rings, asymmetries and complexity in the HD 100546 disk

Observations of disks with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to map the chemical makeup of nearby protoplanetary disks with unprecedented spatial resolution and sensitivity. The typical outer Class II disk observed with ALMA is one with an elevated C/O ratio and a lack of oxygen-bearing complex organic molecules, but there are now some interesting exceptions: three transition disks around Herbig Ae stars all show oxygen-rich gas traced via the unique detections of the molecules SO and CH3OH. We present the first results of an ALMA line survey at 337 to 357 GHz of such disks and focus this paper on the first Herbig Ae disk to exhibit this chemical signature - HD 100546. In these data, we detect 19 different molecules including NO, SO and CH3OCHO (methyl formate). We also make the first tentative detections of H213CO and 34SO in protoplanetary disks. Multiple molecular species are detected in rings, which are, surprisingly, all peaking just beyond the underlying millimeter continuum ring at 200 au. This result demonstrates a clear connection between the large dust distribution and the chemistry in this flat outer disk. We discuss the physical and/or chemical origin of these sub-structures in relation to ongoing planet formation in the HD 100546 disk. We also investigate how similar and/or different the molecular make up of this disk is to other chemically well-characterised Herbig Ae disks. The line-rich data we present motivates the need for more ALMA line surveys to probe the observable chemistry in Herbig Ae systems which offer unique insight into the composition of disk ices, including complex organic molecules.

astro-ph.EP↗

arXiv2024

An ALMA molecular inventory of warm Herbig Ae disks: II. Abundant complex organics and volatile sulphur in the IRS 48 disk

The Atacama Large Millimeter/submillimeter Array (ALMA) can probe the molecular content of planet-forming disks with unprecedented sensitivity. These observations allow us to build up an inventory of the volatiles available for forming planets and comets. Herbig Ae transition disks are fruitful targets due to the thermal sublimation of complex organic molecule (COM) and likely H2O-rich ices in these disks. The IRS 48 disk shows a particularly rich chemistry that can be directly linked to its asymmetric dust trap. Here, we present ALMA observations of the IRS 48 disk where we detect 16 different molecules and make the first robust detections of H213CO, 34SO, 33SO and c-H2COCH2 (ethylene oxide) in a protoplanetary disk. All of the molecular emissions, aside from CO, are colocated with the dust trap and this includes newly detected simple molecules such as HCO+, HCN and CS. Interestingly, there are spatial offsets between different molecular families, including between the COMs and sulphur-bearing species, with the latter being more azimuthally extended and located radially further from the star. The abundances of the newly detected COMs relative to CH3OH are higher than the expected protostellar ratios, which implies some degree of chemical processing of the inherited ices during the disk lifetime. These data highlight IRS 48 as a unique astrochemical laboratory to unravel the full volatile reservoir at the epoch of planet and comet formation and the role of the disk in (re)setting chemical complexity.

astro-ph.EP↗

arXiv2023

Chemistry of Complex Organic Molecules in the V883 Ori Disk Revealed by ALMA Band 3 Observations

Complex organic molecules (COMs) in protoplanetary disks are key to understanding the origin of volatiles in comets in our solar system, yet the chemistry of COMs in protoplanetary disks remains poorly understood. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations of the disk around the young outbursting star V883 Ori, where the COMs sublimate from ices and are thus observable thanks to the warm condition of the disk. We have robustly identified ten oxygen-bearing COMs including $^{13}$C-isotopologues in the disk-integrated spectra. The radial distributions of the COM emission, revealed by the detailed analyses of the line profiles, show the inner emission cavity, similar to the previous observations in Band 6 and Band 7. We found that the COMs abundance ratios with respect to methanol are significantly higher than those in the warm protostellar envelopes of IRAS 16293-2422 and similar to the ratios in the solar system comet 67P/Churyumov-Gerasimenko, suggesting the efficient (re-)formation of COMs in protoplanetary disks. We also constrained the $^{12}$C/$^{13}$C and D/H ratios of COMs in protoplanetary disks for the first time. The $^{12}$C/$^{13}$C ratios of acetaldehyde, methyl formate, and dimethyl ether are consistently lower ($\sim$ 20-30) than the canonical ratio in the interstellar medium ($\sim$ 69), indicating the efficient $^{13}$C-fractionation of CO. The D/H ratios of methyl formate are slightly lower than the values in IRAS 16293-2422, possibly pointing to the destruction and reformation of COMs in disks. We also discuss the implications for nitrogen and sulfur chemistry in protoplanetary disks.

astro-ph.EP↗

arXiv2023

GREX-PLUS Science Book

GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a mission candidate for a JAXA's strategic L-class mission to be launched in the 2030s. Its primary sciences are two-fold: galaxy formation and evolution and planetary system formation and evolution. The GREX-PLUS spacecraft will carry a 1.2 m primary mirror aperture telescope cooled down to 50 K. The two science instruments will be onboard: a wide-field camera in the 2-8 $μ$m wavelength band and a high resolution spectrometer with a wavelength resolution of 30,000 in the 10-18 $μ$m band. The GREX-PLUS wide-field camera aims to detect the first generation of galaxies at redshift $z>15$. The GREX-PLUS high resolution spectrometer aims to identify the location of the water ``snow line'' in proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of supermassive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on.

astro-ph.CO↗

arXiv2022

The Molecular Composition of Shadowed Protosolar Disk Midplanes beyond the Water Snowline

The disk midplane temperature is potentially affected by the dust traps/rings. The dust depletion beyond the water snowline will cast a shadow. In this study, we adopt a detailed gas-grain chemical reaction network, and investigate the radial gas and ice abundance distributions of dominant carbon-, oxygen-, and nitrogen-bearing molecules in disks with shadow structures beyond the water snowline around a protosolar-like star. In shadowed disks, the dust grains at around $3-8$ au are predicted to have more than around $5-10$ times amounts of ices of organic molecules such as H$_{2}$CO, CH$_{3}$OH, and NH$_{2}$CHO, saturated hydrocarbon ices such as CH$_{4}$ and C$_{2}$H$_{6}$, in addition to H$_{2}$O, CO, CO$_{2}$, NH$_{3}$, N$_{2}$, and HCN ices, compared with those in non-shadowed disks. In the shadowed regions, we find that hydrogenation (especially of CO ice) is the dominant formation mechanism of complex organic molecules. The gas-phase N/O ratios show much larger spatial variations than the gas-phase C/O ratios, thus the N/O ratio is predicted to be a useful tracer of the shadowed region. N$_{2}$H$^{+}$ line emission is a potential tracer of the shadowed region. We conclude that a shadowed region allows the recondensation of key volatiles onto dust grains, provides a region of chemical enrichment of ices that is much closer to the star than within a non-shadowed disk, and may explain to some degree the trapping of O$_{2}$ ice in dust grains that formed comet 67P/Churyumov-Gerasimenko. We discuss that, if formed in a shadowed disk, Jupiter does not need to have migrated vast distances.

astro-ph.EP↗

arXiv2021

X-ray induced chemistry of water and related molecules in low-mass protostellar envelopes

Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances in the inner warm envelopes. Water destruction by X-rays has been proposed to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood. In this study, we aim to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen bearing species. We compute the chemical composition of two low-mass protostellar envelopes using a 1D gas-grain chemical reaction network, under various X-ray field strengths. According to our calculations, outside the water snowline, the water gas abundance increases with $L_{\mathrm{X}}$. Inside the water snowline, water maintains a high abundance of $\sim 10^{-4}$ for small $L_{\mathrm{X}}$, with water and CO being the dominant oxygen carriers. For large $L_{\mathrm{X}}$, the water gas abundances significantly decrease just inside the water snowline (down to $\sim10^{-8}-10^{-7}$) and in the innermost regions ($\sim10^{-6}$). For these cases, the O$_{2}$ and O gas abundances reach $\sim 10^{-4}$ within the water snowline, and they become the dominant oxygen carriers. The HCO$^{+}$ and CH$_{3}$OH abundances, which have been used as tracers of the water snowline, significantly increase/decrease within the water snowline, respectively, as the X-ray fluxes become larger. The abundances of some other dominant molecules, such as CO$_{2}$, OH, CH$_{4}$, HCN, and NH$_{3}$, are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower density envelope models. Future observations of water and related molecules (using e.g., ALMA and ngVLA) will access the regions around protostars where such X-ray induced chemistry is effective.

astro-ph.GA↗

arXiv2020

The composition of hot Jupiter atmospheres assembled within chemically evolved protoplanetary discs

The radial-dependent positions of snowlines of abundant oxygen- and carbon-bearing molecules in protoplanetary discs will result in systematic radial variations in the C/O ratios in the gas and ice. This variation is proposed as a tracer of the formation location of gas-giant planets. However, disc chemistry can affect the C/O ratios in the gas and ice, thus potentially erasing the chemical fingerprint of snowlines in gas-giant atmospheres. We calculate the molecular composition of hot Jupiter atmospheres using elemental abundances extracted from a chemical kinetics model of a disc midplane where we have varied the initial abundances and ionization rates. The models predict a wider diversity of possible atmospheres than those predicted using elemental ratios from snowlines only. As found in previous work, as the C/O ratio exceeds the solar value, the mixing ratio of CH$_{4}$ increases in the lower atmosphere, and those of C$_{2}$H$_{2}$ and HCN increase mainly in the upper atmosphere. The mixing ratio of H$_{2}$O correspondingly decreases. We find that hot Jupiters with C/O$>1$ can only form between the CO$_{2}$ and CH$_{4}$ snowlines. Moreover, they can only form in a disc which has fully inherited interstellar abundances, and where negligible chemistry has occurred. Hence, carbon-rich planets are likely rare, unless efficient transport of hydrocarbon-rich ices via pebble drift to within the CH$_{4}$ snowline is a common phenomenon. We predict combinations of C/O ratios and elemental abundances that can constrain gas-giant planet formation locations relative to snowline positions, and that can provide insight into the disc chemical history.

astro-ph.EP↗