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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., Accepted in Nature Astronomy

We present K-band interferometric observations of the PDS 70 protoplanets along with their host star using VLTI/GRAVITY. We obtained K-band spectra and 100 $\mu$as precision astrometry of both PDS 70 b and c in two epochs, as well as spatially resolving the hot inner disk around the star. Rejecting unstable orbits, we found a nonzero eccentricity for PDS 70 b of $0.17 \pm 0.06$, a near-circular orbit for PDS 70 c, and an orbital configuration that is consistent with the planets migrating into a 2:1 mean motion resonance. Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS 70 b of 10 $M_\textrm{Jup}$, while the mass of PDS 70 c was unconstrained. The GRAVITY K-band spectra rules out pure blackbody models for the photospheres of both planets. Instead, the models with the most support from the data are planetary atmospheres that are dusty, but the nature of the dust is unclear. Any circumplanetary dust around these planets is not well constrained by the planets' 1-5 $\mu$m spectral energy distributions (SEDs) and requires longer wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made the first observations of a circumplanetary environment with sub-au spatial resolution, placing an upper limit of 0.3~au on the size of a bright disk around PDS 70 b.

Context. Comets are planetesimals left over from the formation of planets in the solar system. With a growing number of observed molecular abundances in many comets, and an improved understanding of chemical evolution in protoplanetary disk midplanes, comparisons can be made between models and observations that could potentially constrain the formation histories of comets. Aims. Our aim is to carry out the first statistical comparison between cometary volatile ice abundances and modelled evolving abundances in a protoplanetary disk midplane. Methods. A χ2 method was used to determine maximum likelihood surfaces for 14 different comets that formed at a given time (up to 8 Myr) and place (out to beyond the CO iceline) in the pre-solar nebula midplane. This was done using observed volatile abundances for the 14 comets and the evolution of volatile abundances from chemical modelling of disk midplanes. Two assumptions for the chemical modelling starting conditions (cloud inheritance or chemical reset), as well as two different sets of cometary molecules (parent species, with or without sulphur species) were investigated. Results. Considering all parent species (ten molecules) in the reset scenario, χ2 likelihood surfaces show a characteristic trail in the parameter space with high likelihood of formation around 30 AU at early times and 12 AU at later times for ten comets. This trail roughly traces the vicinity of the CO iceline in time. Conclusions. A statistical comparison between observed and modelled chemical abundances in comets and comet-forming regions could be a powerful tool for constraining cometary formation histories. The formation histories for all comets were constrained to the vicinity of the CO iceline, assuming that the chemistry was partially reset early in the pre-solar nebula. This is found, both when considering carbon-, oxygen-, and sulphur-bearing molecules (ten in total), and when only considering carbon- and oxygen-bearing molecules (seven in total). Since these 14 comets did not previously fall into the same taxonomical categories together, this chemical constraint may be proposed as an alternative taxonomy for comets. Based on the most likely time for each of these comets to have formed during the disk chemical evolution, a formation time classification for the 14 comets is suggested.

Context. Snowlines in protoplanetary disks play an important role in planet formation and composition. Since the CO snowline is difficult to observe directly with CO emission, its location has been inferred in several disks from spatially resolved ALMA observations of DCO⁺ and N₂H⁺. Aims. N₂H⁺ is considered to be a good tracer of the CO snowline based on astrochemical considerations predicting an anti-correlation between N₂H⁺ and gas-phase CO. In this work, the robustness of N₂H⁺ as a tracer of the CO snowline is investigated. Methods. A simple chemical network was used in combination with the radiative transfer code LIME to model the N₂H⁺ distribution and corresponding emission in the disk around TW Hya. The assumed CO and N₂ abundances, corresponding binding energies, cosmic ray ionization rate, and degree of large-grain settling were varied to determine the effects on the N₂H⁺ emission and its relation to the CO snowline. Results. For the adopted physical structure of the TW Hya disk and molecular binding energies for pure ices, the balance between freeze-out and thermal desorption predicts a CO snowline at 19 AU, corresponding to a CO midplane freeze-out temperature of 20 K. The N₂H⁺ column density, however, peaks 5–30 AU outside the snowline for all conditions tested. In addition to the expected N₂H⁺ layer just below the CO snow surface, models with an N₂/CO ratio 0.2 predict an N₂H⁺ layer higher up in the disk due to a slightly lower photodissociation rate for N₂ as compared to CO. The influence of this N₂H⁺ surface layer on the position of the emission peak depends on the total CO and N₂ abundances and the disk physical structure, but the emission peak generally does not trace the column density peak. A model with a total (gas plus ice) CO abundance of 3 × 10−6 with respect to H₂ fits the position of the emission peak previously observed for the TW Hya disk. Conclusions. The relationship between N₂H⁺ and the CO snowline is more complicated than generally assumed: for the investigated parameters, the N₂H⁺ column density peaks at least 5 AU outside the CO snowline. Moreover, the N₂H⁺ emission can peak much further out, as far as ∼50 AU beyond the snowline. Hence, chemical modeling, as performed here, is necessary to derive a CO snowline location from N₂H⁺ observations.

We investigate the delivery of regular and deuterated forms of water from prestellar cores to circumstellar disks. We adopt a semi-analytical, axisymmetric, two-dimensional collapsing core model with post-processing gas-ice astrochemical simulations, in which a layered ice structure is considered. The physical and chemical evolutions are followed until the end of the main accretion phase. In our models, when mass averaged over the whole disk, a forming disk has a similar H₂O abundance and HDO/H₂O abundance ratio (within a factor of 2) as the precollapse values of these quantities, regardless of time. Consistent with previous studies, our models suggest that interstellar water ice is delivered to forming disks without significant alteration. On the other hand, the local vertically averaged H₂O ice abundance and HDO/H₂O ice ratio can differ more, by up to a factor of several, depending on time and distance from a central star. Key parameters for the local variations are the fluence of stellar UV photons en route into the disk and the ice layered structure, the latter of which is mostly established in the prestellar stages. We also find that even if interstellar water ice is destroyed by stellar UV and (partly) reformed prior to disk entry, the HDO/H₂O ratio in reformed water ice is similar to the original value. This finding indicates that some caution is needed in discussions on the prestellar inheritance of H₂O based on comparisons between the observationally derived HDO/H₂O ratio in clouds/cores and that in disks/comets. Alternatively, we propose that the ratio of D₂O/HDO to HDO/H₂O better probes the prestellar inheritance of H₂O. It is also found that in forming disks icy organics are more enriched in deuterium than water ice. The differential deuterium fractionation in water and organics is inherited from prestellar stages.

Molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material (O2/H2O ∼ 4 per cent) and is thought to have a primordial nature, i.e. coming from the interstellar cloud from which our Solar system was formed. However, interstellar O2 gas is notoriously difficult to detect and has only been observed in one potential precursor of a solar-like system. Here, the chemical and physical origin of O2 in comets is investigated using sophisticated astrochemical models. Three origins are considered: (i) in dark clouds; (ii) during forming protostellar discs; and (iii) during luminosity outbursts in discs. The dark cloud models show that reproduction of the observed abundance of O2 and related species in comet 67P/C-G requires a low H/O ratio facilitated by a high total density (≥105 cm−3), and a moderate cosmic ray ionization rate (≤10−16 s−1) while a temperature of 20 K, slightly higher than the typical temperatures found in dark clouds, also enhances the production of O2. Disc models show that O2 can only be formed in the gas phase in intermediate disc layers, and cannot explain the strong correlation between O2 and H2O in comet 67P/C-G together with the weak correlation between other volatiles and H2O. However, primordial O2 ice can survive transport into the comet-forming regions of discs. Taken together, these models favour a dark cloud (or ‘primordial’) origin for O2 in comets, albeit for dark clouds which are warmer and denser than those usually considered as Solar system progenitors.

Context. Nitrogen chemistry in protoplanetary disks and the freeze-out on dust particles is key for understanding the formation of nitrogen-bearing species in early solar system analogs. In dense cores, 10% to 20% of the nitrogen reservoir is locked up in ices such as NH3, NH4+ and OCN−. So far, ammonia has not been detected beyond the snowline in protoplanetary disks. Aims. We aim to find gas-phase ammonia in a protoplanetary disk and characterize its abundance with respect to water vapor. Methods. Using HIFI on the Herschel Space Observatory, we detected for the first time the ground-state rotational emission of ortho-NH3 in a protoplanetary disk around TW Hya. We used detailed models of the disk’s physical structure and the chemistry of ammonia and water to infer the amounts of gas-phase molecules of these species. We explored two radial distributions (extended across the disk and confined to

Aims. Our aim is to determine the critical parameters in water chemistry and the contribution of water to the oxygen budget by observing and modelling water gas and ice for a sample of eleven low-mass protostars, for which both forms of water have been observed. Methods. A simplified chemistry network, which is benchmarked against more sophisticated chemical networks, is developed that includes the necessary ingredients to determine the water vapour and ice abundance profiles in the cold, outer envelope in which the temperature increases towards the protostar. Comparing the results from this chemical network to observations of water emission lines and previously published water ice column densities, allows us to probe the influence of various agents (e.g., far-ultraviolet (FUV) field, initial abundances, timescales, and kinematics). Results. The observed water ice abundances with respect to hydrogen nuclei in our sample are 30–80 ppm, and therefore contain only 10–30% of the volatile oxygen budget of 320 ppm. The keys to reproduce this result are a low initial water ice abundance after the pre-collapse phase together with the fact that atomic oxygen cannot freeze-out and form water ice in regions with Tdust ≳ 15 K. This requires short prestellar core lifetimes ≲0.1 Myr. The water vapour profile is shaped through the interplay of FUV photodesorption, photodissociation, and freeze-out. The water vapour line profiles are an invaluable tracer for the FUV photon flux and envelope kinematics. Conclusions. The finding that only a fraction of the oxygen budget is locked in water ice can be explained either by a short pre-collapse time of ≲0.1 Myr at densities of nH ~ 10⁴ cm-³, or by some other process that resets the initial water ice abundance for the post-collapse phase. A key for the understanding of the water ice abundance is the binding energy of atomic oxygen on ice.

Context. The envelopes of asymptotic giant branch (AGB) stars are irradiated externally by ultraviolet photons; hence, the chemistry is sensitive to the photodissociation of N2 and CO, which are major reservoirs of nitrogen and carbon, respectively. The photodissociation of N2 has recently been quantified by laboratory and theoretical studies. Improvements have also been made for CO photodissociation. Aims. For the first time, we use accurate N2 and CO photodissociation rates and shielding functions in a model of the circumstellar envelope of the carbon-rich AGB star, IRC +10216. Methods. We use a state-of-the-art chemical model of an AGB envelope, the latest CO and N2 photodissociation data, and a new method for implementing molecular shielding functions in full spherical geometry with isotropic incident radiation. We compare computed column densities and radial distributions of molecules with observations. Results. The transition of N2→ N (also, CO → C → C+) is shifted towards the outer envelope relative to previous models. This leads to different column densities and radial distributions of N-bearing species, especially those species whose formation/destruction processes largely depend on the availability of atomic or molecular nitrogen, for example, CnN (n = 1, 3, 5), CnN− (n = 1, 3, 5), HCnN (n = 1, 3, 5, 7, 9), H2CN and CH2CN. Conclusions. The chemistry of many species is directly or indirectly affected by the photodissociation of N2 and CO, especially in the outer shell of AGB stars where photodissociation is important. Thus, it is important to include N2 and CO shielding in astrochemical models of AGB envelopes and other irradiated environments. In general, while differences remain between our model of IRC +10216 and the observed molecular column densities, better agreement is found between the calculated and observed radii of peak abundance.

We present the first detection of the v(3) ro-vibrational band of gas-phase SO2 in absorption in the mid-infrared spectral region around 7.3 mum of a sample of deeply embedded massive protostars. Comparison with model spectra shows that the derived excitation temperatures correlate with previous C2H2 and HCN studies, indicating that the same warm gas component is probed. The SO2 column densities are similar along all lines of sight suggesting that the SO2 formation has saturated, but not destroyed, and the absolute abundances of SO2 are high (similar to 10(7)). Both the high temperature and the high abundance of the detected SO2 are not easily explained by standard hot core chemistry models. Likewise, indicators of shock induced chemistry are lacking.

Observations with the Short Wavelength Spectrometer on board the Infrared Space Observatory have led to the first detection of the methyl radical CH(3) in the interstellar medium. The nu(2) Q-branch at 16.5 µm and the R(0) line at 16.0 µm have been unambiguously detected toward the Galactic center Sagittarius A*. The analysis of the measured bands gives a column density ofparl0;8.0+/-2.4parr0;x1014 cm(-2) and an excitation temperature of 17+/-2 K. Gaseous CO at a similarly low excitation temperature and C(2)H(2) are detected for the same line of sight. Using constraints on the H(2) column density obtained from C(18)O and visual extinction, the inferred CH(3) abundance isparl0;1.3+2.2-0.7parr0;x10-8. The chemically related CH(4) molecule is not detected, but the pure rotational lines of CH are seen with the Long Wavelength Spectrometer. The absolute abundances and the CH(3)/CH(4) and CH(3)/CH ratios are inconsistent with published pure gas-phase models of dense clouds. The data require a mix of diffuse and translucent clouds with different densities and extinctions, and/or the development of translucent models in which gas-grain chemistry, freeze-out, and reactions of H with polycyclic aromatic hydrocarbons and solid aliphatic material are included.

We present observations of the stretching mode of (CO2)-C-13 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS). Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to (CO2)-C-13 mixed with polar molecules such as H2O, and CH3OH. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282 cm(-1) (4.382 mu m), which we attribute to a polar, CH3OH-containing CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic C-12/C-13 ratio from the solid state as a function of Galacto-centric radius. The (CO2)-C-12/(CO2)-C-13 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase ((CO)-O-18, H2CO) studies. For the few individual objects for which gas phase values are available, the C-12/C-13 ratios derived from CO2 tend to be higher compared to CO studies (albeit with similar to 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.

Recent observations with the Submillimeter Wave Astronomy Satellite indicate abundances of gaseous H2O and O2 in dense molecular clouds which are significantly lower than found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H2O and O2 are computed for various density distributions, radiation field strengths and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H2O and O2 compared to the homogeneous case by more than an order of magnitude at the same A_V. O2 is particularly sensitive to the penetrating ultraviolet radiation, more so than H2O. The S140 and rho Oph clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with mean density of 2x10^3 cm-3 and enhancement I_UV=140 compared with the average interstellar radiation field, in agreement with observations of [CI] and 13CO of this cloud. Additional radiative transfer computations suggest that this diffuse H2O component is warm, ~60-90 K, and can account for the bulk of the 1_10-1_01 line emission observed by SWAS. The rho Oph model yields consistent O2 abundances but too much H2O, even for [C]/[O]=0.94, if I_UV, To be published in ApJL

Spectra of interstellar CO2 ice absorption features at a resolving power of lambda/Delta lambda approximate to 1500-2000 are presented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO2 near 4.27 mu m in all sources; the bending mode near 15.2 mu m is also detected in 12 of them. The selected sources include massive protostars (Elias 29 [in rho Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NCC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16); they thus probe a diverse range of environments. Column densities of interstellar CO2 ice relative to H2O ice fall in the range 10%-23%: this ratio displays remarkably little variation for such a physically diverse sample. Comparison of the observed profiles with laboratory data for CO2-bearing ice mixtures indicates that CO2 generally exists in at least two phases, one polar (H2O dominant) and one nonpolar (CO2 dominant). The observed CO2 profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for CO2 and implications for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing, rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO2-bearing ices.

ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.41 mu m. Comparison to interstellar ice analogs shows that the 7.24 mu m band can be well matched by the CH deformation mode of formic acid(HCOOH), while the 7.41 mu m band can be fitted both by the formate ion (HCOO-) and acetaldehyde (CH3HCO) The laboratory spectra reveal additional strong features of these molecules which should make a more definite identification straightforward. While an assignment of the 7.24 mu m band to the -CH3 deformation mode of aliphatic species maybe considered, the absence of a corresponding strong CH stretching mode argues against this possibility;These results and the earlier tentative detection of HCOOH towards NGC7538:IRS9 suggest that formic acid is a general component of the ices in the vicinity of embedded high-mass young stellar objects.

Data of the Infrared Space Observatory ISO have strongly influenced the current view of interstellar ice chemistry. ISO and ground-based results have confirmed that the most abundant ice species in warm regions close to massive protostars are H2O, CO2, and CH3OH. Ice segregation in those environments reflects the extensive thermal processing of grains over the Lifetime of protostars. We present here a systematic set of laboratory infrared spectra of ice mixtures dominated by H2O, CO2 and CH3OH which have been exposed to thermal and UV irradiation processing. It is shown that the infrared bands of CO2 and of CH3OH are particularly sensitive to the ice composition, temperature and applied UV irradiation. The laboratory data suggest partial crystallization of interstellar ices in the protostellar environment. We present a detailed laboratory study of the CO2 bending mode at 15.2 mu m. The observed multipeak structure of the CO2 bending mode is a result of thermal processing and can not be produced by UV irradiation in the laboratory. Laboratory results show that annealed CO2 ice has a lower stability against UV irradiation than cold amorphous CO2 ice. Annealed ice mixtures containing H2O, CO2 and CH3OH show that the multipeak structure of the CO2 bending mode is not destroyed by UV fluxes of less than or similar to 10(18) photons.cm(-2). Detailed analysis of H2O, CO2 and CH3OH bands show that their profiles can be effectively used to trace the Line of sight conditions and the origin and evolution of the ice composition in dense clouds. The datafiles discussed in this paper can be retrieved from the Leiden observatory database (www.strw.leidenuniv.nl/similar to lab/isodb).

We present the detection of re-vibrational absorption lines of the deformation mode of gaseous CH4 toward the massive protostars W 33A, and NGC 7538 : IRS9, using the SWS spectrometer an board of the Infrared Space Observatory. The observed lines indicate that the CH4 gas is warm (T similar to N 90 K), and has a low abundance (similar to 4 10(-7)). The CH4 ice in these Lines of sight is embedded in a polar matrix (Boogert et al. 1996), and the gas/solid state abundance ratio is low (similar to 0.5). We discuss models for the formation of interstellar CH4. The observations impose strong limitations on time dependent gas phase models, e.g. a low initial CO/C ratio would be required, the CH4 must have been formed and subsequently condensed on the grains within a narrow time window, and an additional mechanism would be necessary to form polar ice mantles. More likely, interstellar CH4 is formed through grain surface reactions at a high CO/C ratio, which explains the low observed CH4 abundance (similar to 10(-6)) the presence of CH4 in a polar ice, the low gas/solid ratio, and the absence of a strong cold CH4 gas component. The observed warm CH4 gas probably has sublimated from the grains in the 'hot core' region surrounding the protostar.

ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.40 mu m The position and width of these features indicates that they are due to the XH bending mode (X= C, N, or O) of molecules in the solid state. Comparison to interstellar ice analogs shows that the 7.24 mu m band can be well matched by solid formic acid (HCOOH), while the 7.40 mu m band can be fitted both by the formate ion (HCOO-) and acetaldehyde (CH3HCO). The laboratory spectra reveal additional strong features of these molecules which should make a more definite identification straightforward.

Apolar ices have been observed in several regions in dense clouds and are likely dominated by molecules such as CO, CO(2) and the infrared inactive molecules O(2) and N(2). Interstellar solid CO has been well characterized by ground-based high resolution measurements. Recent ISO results showed the ubiquitous presence of abundant CO(2) ice and the presence of CO(2)-rich ice mantles towards several molecular clouds. CO and CO(2) have sharp bands in the infrared and their band shape depends strongly on the ice composition. The profiles of the strong CO and CO(2) bands can therefore provide important information on the composition, temperature and thermal history of interstellar and precometary ices. We address, in this paper, the infrared spectra of 70 apolar ice mixtures of pure, binary, and multicomponent type. We studied their spectral properties at 10 K, during warm up and UV photolysis, and derived the optical constants. We discuss the importance of particle shape calculations for strong transitions such as CO and CO(2). In the laboratory context, we investigate the formation of CO(2) in the interstellar medium by UV photolysis of interstellar ices. Together with astronomical spectra taken by the ISO satellite these laboratory data will be extremely valuable for the determination of the grain mantle composition in dense clouds.

A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the first time, we have accurately and homogeneously determined the photospheric parameters, mass accretion rate, and disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an interstellar medium (ISM) gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rates and the disk mass measured by CO isotopologues emission lines, possibly owing to the small number of measured disk gas masses. This suggests that the mm-sized dust mass better traces the total disk mass and that masses derived from CO may be underestimated, at least in some cases., C.F.M. gratefully acknowledges an ESA Research Fellowship. G.R. is supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. A.N. would like to acknowledge funding from Science Foundation Ireland (Grant 13/ERC/I2907). Leiden is supported by the European Union A-ERC grant 291141 CHEMPLAN, by the Netherlands Research School for Astronomy (NOVA), and by grant 614.001.352 from the Netherlands Organization for Scientific Research (NWO). JPW and MA were supported by NSF and NASA grants AST-1208911 and NNX15AC92G., This is the final version of the article. It first appeared from EDP Sciences via http://dx.doi.org/10.1051/0004-6361/201628549

We present the first high-resolution sub-mm survey of both dust and gas for a large population of protoplanetary disks. Characterizing fundamental properties of protoplanetary disks on a statistical level is critical to understanding how disks evolve into the diverse exoplanet population. We use ALMA to survey 89 protoplanetary disks around stars with $M_{\ast}>0.1~M_{\odot}$ in the young (1--3~Myr), nearby (150--200~pc) Lupus complex. Our observations cover the 890~$\mu$m continuum and the $^{13}$CO and C$^{18}$O 3--2 lines. We use the sub-mm continuum to constrain $M_{\rm dust}$ to a few Martian masses (0.2--0.4~$M_{\oplus}$) and the CO isotopologue lines to constrain $M_{\rm gas}$ to roughly a Jupiter mass (assuming ISM-like $\rm {[CO]/[H_2]}$ abundance). Of 89 sources, we detect 62 in continuum, 36 in $^{13}$CO, and 11 in C$^{18}$O at $>3\sigma$ significance. Stacking individually undetected sources limits their average dust mass to $\lesssim6$ Lunar masses (0.03~$M_{\oplus}$), indicating rapid evolution once disk clearing begins. We find a positive correlation between $M_{\rm dust}$ and $M_{\ast}$, and present the first evidence for a positive correlation between $M_{\rm gas}$ and $M_{\ast}$, which may explain the dependence of giant planet frequency on host star mass. The mean dust mass in Lupus is 3$\times$ higher than in Upper Sco, while the dust mass distributions in Lupus and Taurus are statistically indistinguishable. Most detected disks have $M_{\rm gas}\lesssim1~M_{\rm Jup}$ and gas-to-dust ratios $

The distributions and abundances of molecules in protoplanetary disks are powerful tracers of the physical and chemical disk structures. The abundance ratios of HCN and its isomer HNC are known to be sensitive to gas temperature. Their line ratios might therefore offer a unique opportunity to probe the properties of the emitting gas. Using the 2D thermochemical code DALI, we ran a set of disk models accounting for different stellar and disk properties, with an updated chemical network for the nitrogen chemistry. These modeling results were then compared with observations, including new ALMA observations of HNC $J=3-2$ for the TW Hya disk and HNC $J=1-0$ for 29 disks in Lupus. Similar to CN, HCN and HNC have brighter line emission in models with larger disk flaring angles and higher UV fluxes. HNC and HCN are predicted to be abundant in the warm surface layer and outer midplane region, which results in ring-shaped emission patterns. However, the precise emitting regions and emission morphology depend on the probed transition, as well as on other parameters such as C and O abundances. The modeled HNC-to-HCN line intensity ratio increases from $

An era has started in which gas and dust can be observed independently in protoplanetary disks, thanks to the recent surveys with ALMA. The first near-complete high-resolution disk survey in both dust and gas in a single star-forming region has been carried out in Lupus, finding surprisingly low gas/dust ratios. The goal of this work is to fully exploit CO isotopologues observations in Lupus, comparing them with physical-chemical model results, in order to obtain gas masses for a large number of disks. We have employed physical-chemical models to analyze continuum and CO isotopologues observations of Lupus disks, including isotope-selective processes and freeze-out. Employing also the ALMA 13CO-only detections, disk gas masses have been calculated for a total of 34 sources, expanding the sample of 10 disks studied by Ansdell et al. (2016), where also C18O was detected. We confirm that overall gas-masses are very low, often smaller than 1 $M_{\rm J}$, if volatile carbon is not depleted. Accordingly, global gas/dust ratios predominantly between 1 and 10. Low CO-based gas masses and gas/dust ratios may indicate rapid loss of gas, or alternatively chemical evolution, e.g. via sequestering of carbon from CO to more complex molecules, or carbon locked up in larger bodies. Current ALMA observations cannot distinguish between these two hypotheses. We have simulated both scenarios, but chemical model results do not allow us to rule out one of the two. Assuming that all Lupus disks have evolved mainly due to viscous processes over the past few Myr, the observed correlation between the current mass accretion rate and dust mass found by Manara et al. (2016) implies a constant gas-to-dust ratio, which is close to 100 based on the observed $M_{\rm disk}/\dot{M}_{\rm acc}$ ratio. This in turn points to a scenario in which carbon depletion is responsible for the low CO isotopologue line luminosities.

Context. The formation of planets strongly depends on the total amount as well as on the spatial distribution of solids in protoplanetary disks. Thanks to the improvements in resolution and sensitivity provided by ALMA, measurements of the surface density of mm-sized grains are now possible on large samples of disks. Such measurements provide statistical constraints that can be used to inform our understanding of the initial conditions of planet formation. Aims. We aim to analyze spatially resolved observations of 36 protoplanetary disks in the Lupus star forming complex from our ALMA survey at 890 μm, aiming to determine physical properties such as the dust surface density, the disk mass and size, and to provide a constraint on the temperature profile. Methods. We fit the observations directly in the uv-plane using a two-layer disk model that computes the 890 μm emission by solving the energy balance at each disk radius. Results. For 22 out of 36 protoplanetary disks we derive robust estimates of their physical properties. The sample covers stellar masses between ∼0.1 and ∼ 2 M o , and we find no trend in the relationship between the average disk temperatures and the stellar parameters. We find, instead, a correlation between the integrated sub-mm flux (a proxy for the disk mass) and the exponential cut-off radii (a proxy of the disk size) of the Lupus disks. Comparing these results with observations at similar angular resolution of Taurus-Auriga and Ophiuchus disks found in literature and scaling them to the same distance, we observe that the Lupus disks are generally fainter and larger at a high level of statistical significance. Considering the 1-2 Myr age difference between these regions, it is possible to tentatively explain the offset in the disk mass-size relation with viscous spreading, however with the current measurements other mechanisms cannot be ruled out., M.T. and L.T. acknowledge support by the DFG cluster of excellence Origin and Structure of the Universe (www.universe-cluster. de). This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG. M.T. and L.T. thank Eva Wirström, the Gothenburg Centre for Advanced Studies in Science and Technology and the participants of the workshop Origins of Habitable Planets hosted at Chalmers (Gothenburg) in June 2016 where considerable part of this work was fruitfully discussed. M.T. is grateful to Cathie Clarke for insightful discussions. The fits have been carried out on the computing facilities of the Computational Center for Particle and Astrophysics (C2PAP) as part of the approved project “Dust evolution in protoplanetary disks”. M.T. acknowledges usage of the Max Planck Gesellschaft Hydra computing cluster for code testing, for which is grateful to Paola Caselli. Figures have been generated using the Python-based matplotlib package (Hunter 2007). Staircase plots of PDFs have been generated with a user-modified version of the Python-based triangle package (Foreman-Mackey et al. 2014). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This work was partly supported by the Italian Ministero dell’ Istruzione, Università e Ricerca through the grant Progetti Premiali 2012 – iALMA (CUP C52I13000140001). A.N. acknowledges funding from Science Foundation Ireland (Grant 13/ERC/I2907). C.F.M. acknowledges an ESA Research Fellowship. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00220.S, ADS/JAO.ALMA#2013.1.00226.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ