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Large inner dust gaps in transition disks are frequently posited as evidence of giant planets sculpting gas and dust in the disk, or the opening of a gap by photoevaporative winds. Although the former hypothesis is strongly supported by the observations of planets and deep depletions in gas within the gap some disks, many T Tauri stars hosting transition disks accrete at rates typical for an undepleted disk, raising the question of how gap opening occurs in these objects. We thus present an analysis of the structure of the transition disk around the T Tauri star DM Tau, which is strongly accreting ($\sim 10^{-8.3}~\mathrm{M}_\odot~ \mathrm{yr}^{-1}$) and turbulent ($\alpha=0.078 \pm 0.02$). Using the DALI thermochemical code, we fit disk models to simultaneously reproduce the accretion rate, high level of turbulence, the gas traced by ALMA band 6 observations of $^{12}$CO, $^{13}$CO, and C$^{18}$O J=2--1 lines, and the observed dust emission from the mm continuum and spectral energy distribution. We find a shallow depletion in gas surface density of $\sim 10$ relative to the outer disk and a gas rich inner disk is consistent with the observations. The planet mass of $, Comment: Accepted for publication in the Astronomical Journal. 36 pages, 28 figures

This work presents ground-based spectrally-resolved water emission at R = 30000-100000 over infrared wavelengths covered by JWST (2.9-12.8 $\mu$m). Two new surveys with iSHELL and VISIR are combined with previous spectra from CRIRES and TEXES to cover parts of multiple ro-vibrational and rotational bands observable within telluric transmission bands, for a total of $\approx160$ spectra and 85 disks (30 of which are JWST targets in Cycle 1). The general expectation of a range of regions and excitation conditions traced by infrared water spectra is for the first time supported by the combined kinematics and excitation as spectrally resolved at multiple wavelengths. The main findings from this analysis are: 1) water lines are progressively narrower from the ro-vibrational bands at 2-9 $\mu$m to the rotational lines at 12 $\mu$m, and partly match a broad (BC) and narrow (NC) emission components, respectively, as extracted from ro-vibrational CO spectra; 2) rotation diagrams of resolved water lines from upper level energies of 4000-9500 K show vertical spread and curvatures indicative of optically thick emission ($\approx 10^{18}$ cm$^{-2}$) from a range of excitation temperatures ($\approx 800$-1100 K); 3) the new 5 $\mu$m spectra demonstrate that slab model fits to the rotational lines at $> 10$ $\mu$m strongly over-predict the ro-vibrational emission bands at $< 9$ $\mu$m, implying non-LTE vibrational excitation. We discuss these findings in the context of emission from a disk surface and a molecular inner disk wind, and provide a list of guidelines to support the analysis of spectrally-unresolved JWST spectra., Comment: Accepted for publication on AJ

A large fraction of observed protoplanetary disks in nearby Star-Forming Regions (SFRs) are fainter than expected in CO isotopologue emission. Disks not detected in 13CO line emission are also faint and often unresolved in the continuum emission at an angular resolution of around 0.2 arcseconds. Focusing on the Lupus SFR, the aim of this work is to investigate whether the population of CO-faint disks comprises radially extended and low mass disks - as commonly assumed so far - or if it is of intrinsically radially compact disks, an interpretation that we propose in this paper. The latter scenario was already proposed for individual sources or small samples of disks, while this work targets a large population of disks in a single SFR for which statistical arguments can be made. A new grid of physical-chemical models of compact disks has been run with DALI in order to cover a region of the parameter space that had not been explored before with this code. Such models have been compared with 12CO and 13CO ALMA observations of faint disks in Lupus. Disks that are not detected in 13CO emission and with faint or undetected 12CO emission are consistent with compact disk models. For radially compact disk, the emission of CO isotopologues is mostly optically thick and it scales with the surface area: i.e., it is fainter for smaller objects. The fraction of compact disks is potentially between roughly 50% and 60% of the entire Lupus sample. Deeper observations of 12CO and 13CO at a moderate angular resolution will allow us to distinguish whether faint disks are intrinsically compact, or if they are extended but faint, without the need of resolving them. If the fainter end of the disk population observed by ALMA disk surveys is consistent with such objects being very compact, this will either create a tension with viscous spreading or require MHD winds or external processes to truncate the disks., Comment: 10 pages, 3 main figures, 3 appendix figures. Accepted to A&A

Recent ALMA surveys of protoplanetary disks have shown that for most disks the extent of the gas emission is greater than the extent of the thermal emission of the millimeter-sized dust. Both line optical depth and the combined effect of radially dependent grain growth and radial drift may contribute to this observed effect. For a sample of 10 disks from the Lupus survey we investigate how well dust-based models without radial dust evolution reproduce the observed 12CO outer radius, and determine whether radial dust evolution is required to match the observed gas-dust size difference. We used the thermochemical code DALI to obtain 12CO synthetic emission maps and measure gas and dust outer radii (Rco, Rmm) using the same methods as applied to the observations, which were compared to observations on a source-by-source basis. For 5 disks we find that the observed gas-dust size difference is larger than the gas-dust size difference due to optical depth, indicating that we need both dust evolution and optical depth effects to explain the observed gas-dust size difference. For the other 5 disks the observed gas-dust size difference can be explained using only line optical depth effects. We also identify 6 disks not included in our initial sample but part of a survey of the same star-forming region that show significant 12CO emission beyond 4 x Rmm. These disks, for which no Rco is available, likely have gas-dust size differences greater than 4 and are difficult to explain without substantial dust evolution. Our results suggest that radial drift and grain growth are common features among both bright and fain disks. The effects of radial drift and grain growth can be observed in disks where the dust and gas radii are significantly different, while more detailed models and deeper observations are needed to see this effect in disks with smaller differences., Comment: 17 pages, 11 figures, accepted in A&A

The formation of planets occurs within protoplanetary disks surrounding young stars, resulting in perturbation of the gas and dust surface densities. Here we report the first evidence of spatially resolved gas surface density (Σ g ) perturbation toward the AS 209 protoplanetary disk from the optically thin C18O (J = 2-1) emission. The observations were carried out at 1.3 mm with Atacama Large Millimeter/submillimeter Array at a spatial resolution of about 0.″3 0.″2 (corresponding to ~38 25 au). The C18O emission shows a compact (≤60 au), centrally peaked emission and an outer ring peaking at 140 au, consistent with that observed in the continuum emission, and its azimuthally averaged radial intensity profile presents a deficit that is spatially coincident with the previously reported dust map. This deficit can only be reproduced with our physico-thermochemical disk model by lowering Σgas by nearly an order of magnitude in the dust gaps. Another salient result is that, contrary to C18O, the DCO+ (J = 3-2) emission peaks between the two dust gaps. We infer that the best scenario to explain our observations (C18O deficit and DCO+ enhancement) is a gas perturbation due to one or more forming planets, which is commensurate with previous continuum observations of the source along with hydrodynamical simulations. Our findings confirm that the previously observed dust gaps are very likely due to perturbation of the gas surface density that is induced by a planet of at least 0.2M J in formation. Finally, our observations also show the potential of using CO isotopologues to probe the presence of one or more Saturn-mass planets.

The combination of high-resolution and sensitivity offered by ALMA is revolutionizing our understanding of protoplanetary discs, as their bulk gas and dust distributions can be studied independently. In this paper we present resolved ALMA observations of the continuum emission (λ = 1.3 mm) and CO isotopologues (12CO, 13CO, C18O, J = 2 - 1) integrated intensity from the disc around the nearby (d = 162 pc), intermediate-mass (M_{\star }=1.67 M_{\odot }) pre-main-sequence star CQ Tau. The data show an inner depression in continuum and in both 13CO and C18O emission. We employ a thermo-chemical model of the disc reproducing both continuum and gas radial intensity profiles, together with the disc spectral energy distribution. The models show that a gas inner cavity with size between 15 and 25 au is needed to reproduce the data with a density depletion factor between ~10-1 and ~10-3. The radial profile of the distinct cavity in the dust continuum is described by a Gaussian ring centred at R_dust=53 au and with a width of σ =13 au. Three-dimensional gas and dust numerical simulations of a disc with an embedded planet at a separation from the central star of {~ }20 au and with a mass of {~ } 6 - 9 M_Jup reproduce qualitatively the gas and dust profiles of the CQ Tau disc. However, a one-planet model appears not to be able to reproduce the dust Gaussian density profile predicted using the thermo-chemical modeling.

Hydrogen deuteride (HD) rotational line emission can provide reliable protoplanetary disk gas mass measurements, but it is difficult to observe and detections have been limited to three T-Tauri disks. No new data have been available since the \emph{Herschel} Space Observatory mission ended in 2013. We set out to obtain new disk gas mass constraints by analysing upper limits on HD 1-0 emission in \emph{Herschel}/PACS archival data from the DIGIT key programme. With a focus on the Herbig Ae/Be disks, whose stars are more luminous than T Tauris, we determine upper limits for HD in data previosly analysed for its line detections. Their significance is studied with a grid of models run with the DALI physical-chemical code, customised to include deuterium chemistry. Nearly all the disks are constrained to $M_{\rm gas}\leq0.1\,$M$_{\odot}$, ruling out global gravitational instability. A strong constraint is obtained for the HD 163296 disk mass, $M_{\rm gas} \leq 0.067\,$M$_{\odot}$, implying $\Delta_{\rm g/d}\leq100$. This HD-based mass limit is towards the low end of CO-based mass estimates for the disk, highlighting the large uncertainty in using only CO and suggesting that gas-phase CO depletion in HD 163296 is at most a factor of a few. The $M_{\rm gas}$ limits for HD 163296 and HD 100546, both bright disks with massive candidate protoplanetary systems, suggest disk-to-planet mass conversion efficiencies of $M_{\rm p}/(M_{\rm gas} + M_{\rm p})\approx10$ to $40\,$% for present-day values. Near-future observations with SOFIA/HIRMES will be able to detect HD in the brightest Herbig~Ae/Be disks within $150\,$pc with $\approx10\,$h integration time., Comment: Accepted for publication in A&A

[abridged]CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks. Recent studies have found a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as $v2/v1$, and the emitting radius of CO. In disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii, and NIR excess, and identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars. Slab models and full disk thermo chemical models are calculated. Simulated observations from the models are directly compared to the data. Broad CO lines with low vibrational ratios are best explained by a warm (400-1300 K) inner disk surface with gas-to-dust ratios below 1000; no CO is detected within/at the inner dust rim, due to dissociation at high temperatures. In contrast, explaining the narrow lines with high vibrational ratios requires an inner cavity of a least 5 AU in both dust and gas, followed by a cool (100-300 K) molecular gas reservoir with gas-to-dust ratios greater than 10000 at the cavity wall. In all cases the CO gas must be close to thermalization with the dust. The high gas-to-dust ratios needed to explain high $v2/v1$ in narrow CO lines for a subset of group I disks can naturally be interpreted as due to the dust traps that have been proposed to explain millimeter dust cavities. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA., Comment: 17 pages, 16 figures, accepted by A&A

We are conducting a multi-wavelength analysis of high-resolution molecular spectra that probe the evolution of gas in the inner 10 au in protoplanetary disks. A sample of 15 disks has been combined to probe a range of inner disk structures including small and large dust cavities. Half of the sample has been observed in far-ultraviolet H2 emission with a new HST-COS program (GO-14703), that we have combined to near-infrared spectra of CO emission as observed with VLT-CRIRES and IRTF-iSHELL. This synergic dataset traces the evolution and depletion of CO and H2 in inner disk cavities and shows an evolving radial stratification of the molecular gas, where CO lines are narrower than H2 lines in disks with cavities. CO rotation diagrams also show significant evolution, suggesting a change in gas excitation as CO emission recedes to larger disk radii.

We are conducting a survey of high-resolution near-infrared CO rovibrational spectra in protoplanetary disks around young intermediate-mass stars. We use IRTF-iSHELL with the 0.″375 and 0.″75 slits, providing 4 km s−1 and 6 km s−1 resolution and covering 4.52–5.25 μm in one single exposure. This includes part of the rovibrational R branch and most of the P branch of the CO fundamental band (Δv = 1), one of the best tracers of warm/hot gas in disks. The high quality of the spectra and the large sample covered in this survey will support multiple investigations to study the structure, kinematics, composition, and evolution of the inner 10 au in disks for years to come. The survey currently includes ∼30 stars mostly within 200 pc, complementing extensive imaging campaigns that are observing their disks at optical, infrared, and millimeter wavelengths and revealing structures and planets.

Outflows driven by large-scale magnetic fields likely play an important role in the evolution and dispersal of protoplanetary disks, and in setting the conditions for planet formation. We extend our 2-D axisymmetric non-ideal MHD model of these outflows by incorporating radiative transfer and simplified thermochemistry, with the twin aims of exploring how heating influences wind launching, and illustrating how such models can be tested through observations of diagnostic spectral lines. Our model disks launch magnetocentrifugal outflows primarily through magnetic tension forces, so the mass-loss rate increases only moderately when thermochemical effects are switched on. For typical field strengths, thermochemical and irradiation heating are more important than magnetic dissipation. We furthermore find that the entrained vertical magnetic flux diffuses out of the disk on secular timescales as a result of non-ideal MHD. Through post-processing line radiative transfer, we demonstrate that spectral line intensities and moment-1 maps of atomic oxygen, the HCN molecule, and other species show potentially observable differences between a model with a magnetically driven outflow and one with a weaker, photoevaporative outflow. In particular, the line shapes and velocity asymmetries in the moment-1 maps could enable the identification of outflows emanating from the disk surface., Comment: 35 pages, 20 figures, accepted for publication in ApJ

High resolution ALMA observations of protoplanetary disks have revealed that many, if not all primordial disks consist of ring-like dust structures. The origin of these dust rings remains unclear, but a common explanation is the presence of planetary companions that have cleared gaps along their orbit and trapped the dust at the gap edge. A signature of this scenario is a decrease of gas density inside these gaps. In recent work, Isella et al. 2016 derived drops in gas density consistent with Saturn-mass planets inside the gaps in the HD163296 disk through spatially resolved CO isotopologue observations. However, as CO abundance and temperature depends on a large range of factors, the interpretation of CO emission is non-trivial. We use the physical-chemical code DALI to show that the gas temperature increases inside dust density gaps, implying that any gaps in the gas, if present, would have to be much deeper, consistent with planet masses higher than a Jupiter mass. Furthermore, we show that a model with increased grain growth at certain radii, as expected at a snowline, can reproduce the dust rings in HD163296 equally well without the need for companions. This scenario can explain both younger and older disks with observed gaps, as gaps have been seen in systems as young, Accepted by ApJL

Transition disks with large dust cavities around young stars are promising targets for studying planet formation. Previous studies have revealed the presence of gas cavities inside the dust cavities hinting at recently formed, giant planets. However, many of these studies are biased towards the brightest disks in the nearby star forming regions, and it is not possible to derive reliable statistics that can be compared with exoplanet populations. We present the analysis of 11 transition disks with large cavities (>20 AU radius) from a complete disk survey of the Lupus star forming region, using ALMA Band 7 observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum, 13CO and C18O 3-2 observations and the Spectral Energy Distribution of each source. Gas and dust surface density profiles are derived using the physical-chemical modeling code DALI. This is the first study of transition disks of large cavities within a complete disk survey within a star forming region. The dust cavity sizes range from 20-90 AU radius and in three cases, a gas cavity is resolved as well. The deep drops in gas density and large dust cavity sizes are consistent with clearing by giant planets. The fraction of transition disks with large cavities in Lupus is ~11%, which is inconsistent with exoplanet population studies of giant planets at wide orbits. Furthermore, we present a hypothesis of an evolutionary path for large massive disks evolving into transition disks with large cavities., 29 pages, 15 figures, Accepted by ApJ

Bright ring-like structure emission of the CN molecule has been observed in protoplanetary disks. We investigate whether such structures are due to the morphology of the disk itself or if they are instead an intrinsic feature of CN emission. With the intention of using CN as a diagnostic, we also address to which physical and chemical parameters CN is most sensitive. A set of disk models were run for different stellar spectra, masses, and physical structures via the 2D thermochemical code DALI. An updated chemical network that accounts for the most relevant CN reactions was adopted. Ring-shaped emission is found to be a common feature of all adopted models; the highest abundance is found in the upper outer regions of the disk, and the column density peaks at 30-100 AU for T Tauri stars with standard accretion rates. Higher mass disks generally show brighter CN. Higher UV fields, such as those appropriate for T Tauri stars with high accretion rates or for Herbig Ae stars or for higher disk flaring, generally result in brighter and larger rings. These trends are due to the main formation paths of CN, which all start with vibrationally excited H2* molecules, that are produced through far ultraviolet (FUV) pumping of H2. The model results compare well with observed disk-integrated CN fluxes and the observed location of the CN ring for the TW Hya disk. CN rings are produced naturally in protoplanetary disks and do not require a specific underlying disk structure such as a dust cavity or gap. The strong link between FUV flux and CN emission can provide critical information regarding the vertical structure of the disk and the distribution of dust grains which affects the UV penetration, and could help to break some degeneracies in the SED fitting. In contrast with C2H or c-C3H2, the CN flux is not very sensitive to carbon and oxygen depletion., Comment: New version of paper, correcting too high H2 excitation rates and consequently too high CN column densities. Qualitative conclusions of the paper remain unchanged. Quantitatively, the CN column densities are an order of magnitude lower whereas fluxes decrease by a factor of 3-4. Rings are larger by up to a factor of 2. 13 pages, 19 figures, accepted for publication in A&A

The oxygen-bearing molecular ions OH+, H2O+, and H3O+ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH+ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ~1000 km s^{-1}. In addition, OH+ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H2O+ are also detected at systemic velocities, but H3O+ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of OH/OH+~5-10 for the outflowing components and ~20 for the torus, and an OH+ abundance relative to H nuclei of ~>10^{-7}. We also find high OH+/H2O+ and OH+/H3O+ ratios, both are ~>4 in the torus and ~>10-20 in the outflowing gas components. Chemical models indicate that these high OH+ abundances relative to OH, H2O+, and H3O+ are characteristic of gas with a high ionization rate per unit density, ��/n_H~(1-5)x10^{-17} cm^3 s^{-1} and ~(1-2)x10^{-16} cm^3 s^{-1} for the above components, respectively, and an ionization rate of ��~(0.5-2)x10^{-12} s^{-1}. X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10-400 MeV) cosmic-rays are primarily responsible for the ionization with \dot{M}_{CR}~0.01 M_{sun} yr^{-1} and \dot{E}_{CR}~10^{44} erg s^{-1}, the latter corresponding to 1% of the AGN luminosity and similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream., 25 pages, 11 figures

This work aims to understand which midplane conditions are probed by the DCO$^+$ emission in the disk around the Herbig Ae star HD 169142. We explore the sensitivity of the DCO$^+$ formation pathways to the gas temperature and the CO abundance. The DCO$^+$ $J$=3-2 transition was observed with ALMA at a spatial resolution of 0.3". The HD 169142 DCO$^+$ radial intensity profile reveals a warm, inner component at radii 120 AU. The warm deuterium fractionation pathway is implemented as a constant DCO$^+$ abundance of 2.0$\times$10$^{-12}$ between 30-70 K. The DCO$^+$ emission probes a reservoir of cold material in the HD 169142 outer disk that is not revealed by the millimeter continuum, the SED, nor the emission from the 12CO, 13CO, or C18O $J$=2-1 lines., Accepted to Astronomy & Astrophysics

Aims: The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15 N ratio of 323 +/- 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15N: low-temperature exchange reactions or isotope-selective photodissociation of N2. Methods: Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results: The formation of CN and HCN is closely coupled to the vibrational excitation of H2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N2. The column density ratio of HCN over HC15N in the disk's inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios consistent with observations of disks and comets. The HCN/HC15N and CN/C15N column density ratios in the models are a factor of 2-3 higher than those inferred from the ALMA observations., Accepted by A&A; 17 pages, 17 figures; note added in proof

Protoplanetary discs around brown dwarfs and very low mass stars offer some of the best prospects for forming Earth-sized planets in their habitable zones. To this end, we study the nature of the disc around the very low mass star V410 X-ray 1, whose SED is indicative of an optically thick and very truncated dust disc, with our modelling suggesting an outer radius of only 0.6 au. We investigate two scenarios that could lead to such a truncation, and find that the observed SED is compatible with both. The first scenario involves the truncation of both the dust and gas in the disc, perhaps due to a previous dynamical interaction or the presence of an undetected companion. The second scenario involves the fact that a radial location of 0.6 au is close to the expected location of the H$_2$O snowline in the disc. As such, a combination of efficient dust growth, radial migration, and subsequent fragmentation within the snowline leads to an optically thick inner dust disc and larger, optically thin outer dust disc. We find that a firm measurement of the CO $J=2$--1 line flux would enable us to distinguish between these two scenarios, by enabling a measurement of the radial extent of gas in the disc. Many models we consider contain at least several Earth-masses of dust interior to 0.6 au, suggesting that V410 X-ray 1 could be a precursor to a system with tightly-packed inner planets, such as TRAPPIST-1., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

ALMA observations of protoplanetary disks confirm earlier indications that there is a clear difference between the dust and gas radial extents. The origin of this difference is still debated, with both radial drift of the dust and optical depth effects suggested in the literature. In this work, the feedback of realistic dust particle distributions onto the gas chemistry and molecular emissivity is investigated, with a particular focus on CO isotopologues. The radial dust grain size distribution is determined using dust evolution models that include growth, fragmentation and radial drift. A new version of the code DALI is used to take into account how dust surface area and density influence the disk thermal structure, molecular abundances and excitation. The difference of dust and gas radial sizes is largely due to differences in the optical depth of CO lines and millimeter continuum, without the need to invoke radial drift. The effect of radial drift is primarily visible in the sharp outer edge of the continuum intensity profile. The gas outer radius probed by $^{12}$CO emission can easily differ by a factor of $\sim 2$ between the models for a turbulent $\alpha$ ranging between typical values. Grain growth and settling concur in thermally decoupling the gas and dust components, due to the low collision rate with large grains. As a result, the gas can be much colder than the dust at intermediate heights, reducing the CO excitation and emission, especially for low turbulence values. Also, due to disk mid-plane shadowing, a second CO thermal desorption (rather than photodesorption) front can occur in the warmer outer mid-plane disk. The models are compared to ALMA observations of HD 163296 as a test case. In order to reproduce the observed CO snowline of the system, a binding energy for CO typical of ice mixtures needs to be used rather than the lower pure CO value., Comment: 26 pages, 25 figures, accepted for publication in A&A, abstract shortened to fit arXiv requirements

Protoplanetary disks around young stars are the sites of planet formation. While the dust mass can be estimated using standard methods, determining the gas mass - and thus the amount of material available to form giant planets - has proven to be very difficult. Hydrogen deuteride (HD) is a promising alternative to the commonly-used gas mass tracer, CO. We aim to examine the robustness of HD as tracer of the disk gas mass, specifically the effect of gas mass on the HD FIR emission and its sensitivity to the vertical structure. Deuterium chemistry reactions relevant for HD were implemented in the thermochemical code DALI and models were run for a range of disk masses and vertical structures. The HD J=1-0 line intensity depends directly on the gas mass through a sublinear power law relation with a slope of ~0.8. Assuming no prior knowledge about the vertical structure of a disk and using only the HD 1-0 flux, gas masses can be estimated to within a factor of 2 for low mass disks (M$_{\rm disk} < 10^{-3}$ M$_\odot$). For more massive disks, this uncertainty increases to more than an order of magnitude. Adding the HD 2-1 line or independent information about the vertical structure can reduce this uncertainty to a factor of ~3 for all disk masses. For TW Hya, using the radial and vertical structure from Kama et al. 2016b the observations constrain the gas mass to $6\cdot10^{-3}$ M$_\odot$ < M$_{\rm disk} < 9\cdot10^{-3}$ M$_\odot$. Future observations require a 5$��$ sensitivity of $1.8\cdot10^{-20}$ W m$^{-2}$ ($2.5\cdot10^{-20}$ W m$^{-2}$) and a spectral resolving power R > 300 (1000) to detect HD 1-0 (HD 2-1) for all disk masses above $10^{-5}$ M$_\odot$ with a line-to-continuum ratio > 0.01. These results show that HD can be used as an independent gas mass tracer with a relatively low uncertainty and should be considered as an important science goal for future FIR missions., 18 pages, 13 figures, accepted in A&A

The protoplanetary system HD 169142 is one of the few cases where a potential candidate protoplanet has been recently detected via direct imaging. To study the interaction between the protoplanet and the disk itself observations of the gas and dust surface density structure are needed. This paper reports new ALMA observations of the dust continuum at 1.3\,mm, $^{12}$CO, $^{13}$CO and C$^{18}$O $J=2-1$ emission from the system HD 169142 at angular resolution of $\sim 0".18 - 0".28$ ($\sim 20\,$au$ - 33\,$au). The dust continuum emission reveals a double-ring structure with an inner ring between $0".17-0".28$ ($\sim 20 - 35\,$au) and an outer ring between $0".48-0".64$ ($\sim 56 - 83\,$au). The size and position of the inner ring is in good agreement with previous polarimetric observations in the near-infrared and is consistent with dust trapping by a massive planet. No dust emission is detected inside the inner dust cavity ($R \lesssim 20\,$au) or within the dust gap ($\sim 35 - 56\,$au). In contrast, the channel maps of the $J=2-1$ line of the three CO isotopologues reveal the presence of gas inside the dust cavity and dust gap. The gaseous disk is also much larger than the compact dust emission extending to $\sim 1'.5$ ($\sim 180\,$au) in radius. This difference and the sharp drop of the continuum emission at large radii point to radial drift of large dust grains ($>$ \micron-size). Using the thermo-chemical disk code \textsc{dali}, the continuum and the CO isotopologues emission are modelled to quantitatively measure the gas and dust surface densities. The resulting gas surface density is reduced by a factor of $\sim 30-40$ inward of the dust gap. The gas and dust distribution hint at the presence of multiple planets shaping the disk structure via dynamical clearing (dust cavity and gap) and dust trapping (double ring dust distribution)., Accepted for publication in A&A

We performed very deep searches for 2 ground-state water transitions in 13 protoplanetary disks with the HIFI instrument on-board the Herschel Space Observatory, with integration times up to 12 hours per line. Two other water transitions that sample warmer gas were also searched for with shallower integrations. The detection rate is low, and the upper limits provided by the observations are generally much lower than predictions of thermo-chemical models with canonical inputs. One ground-state transition is newly detected in the stacked spectrum of AA Tau, DM Tau, LkCa 15, and MWC 480. We run a grid of models to show that the abundance of gas-phase oxygen needs to be reduced by a factor of at least ~100 to be consistent with the observational upper limits (and positive detections) if a dust-to-gas mass ratio of 0.01 were to be assumed. As a continuation of previous ideas, we propose that the underlying reason for the depletion of oxygen (hence the low detection rate) is the freeze-out of volatiles such as water and CO onto dust grains followed by grain growth and settling/migration, which permanently removes these gas-phase molecules from the emissive upper layers of the outer disk. Such depletion of volatiles is likely ubiquitous among different disks, though not necessarily to the same degree. The volatiles might be returned back to the gas phase in the inner disk (within about 15 AU), which is consistent with current constraints. Comparison with studies on disk dispersal due to photoevaporation indicates that the timescale for volatile depletion is shorter than that of photoevaporation., Comment: Accepted for publication in ApJ

[Abridged] The infrared ro-vibrational emission lines from organic molecules in the inner regions of protoplanetary disks are unique probes of the physical and chemical structure of planet forming regions and the processes that shape them. The non-LTE excitation effects of carbon dioxide (CO2) are studied in a full disk model to evaluate: (i) what the emitting regions of the different CO2 ro-vibrational bands are; (ii) how the CO2 abundance can be best traced using CO2 ro-vibrational lines using future JWST data and; (iii) what the excitation and abundances tell us about the inner disk physics and chemistry. CO2 is a major ice component and its abundance can potentially test models with migrating icy pebbles across the iceline. A full non-LTE CO2 excitation model has been built. The characteristics of the model are tested using non-LTE slab models. Subsequently the CO2 line formation has been modelled using a two-dimensional disk model representative of T-Tauri disks. The CO2 gas that emits in the 15 $\mu$m and 4.5 $\mu$m regions of the spectrum is not in LTE and arises in the upper layers of disks, pumped by infrared radiation. The v$_2$ 15 $\mu$m feature is dominated by optically thick emission for most of the models that fit the observations and increases linearly with source luminosity. Its narrowness compared with that of other molecules stems from a combination of the low rotational excitation temperature (~250 K) and the inherently narrower feature for CO2. The inferred CO2 abundances derived for observed disks are more than two orders of magnitude lower than those in interstellar ices (~10$^5$), similar to earlier LTE disk estimates. Line-to-continuum ratios are low, of order a few %, thus high signal-to-noise (S/N > 300) observations are needed for individual line detections. Prospects of accurate abundance retreival with JWST-MIRI and JWST-NIRSpec are discussed., Comment: 18 pages, 16 figures, 7 appendix figures. Accepted for publication in A&A

The extent of the gas in protoplanetary disks is observed to be universally larger than the extent of the dust. This is often attributed to radial drift and grain growth of the mm grains, but line optical depth produces a similar observational signature. We investigate in what parts of the disk structure parameter space dust evolution and line optical depth are the dominant drivers of the observed gas and dust size difference. Using the thermochemical model DALI with dust evolution included we ran a grid of models aimed at reproducing the observed gas and dust size dichotomy. The relation between R_dust and dust evolution is non-monotonic and depends on the disk structure. R_gas is directly related to the radius where the CO column density drops below 10^15 cm^-2 and CO becomes photodissociated. R_gas is not affected by dust evolution but scales with the total CO content of the disk. R_gas/R_dust > 4 is a clear sign for dust evolution and radial drift in disks, but these cases are rare in current observations. For disks with a smaller R_gas/R_dust, identifying dust evolution from R_gas/R_dust requires modelling the disk structure including the total CO content. To minimize the uncertainties due to observational factors requires FWHM_beam < 1x the characteristic radius and a peak SNR > 10 on the 12CO emission moment zero map. For the dust outer radius to enclose most of the disk mass, it should be defined using a high fraction (90-95%) of the total flux. For the gas, any radius enclosing > 60% of the 12CO flux will contain most of the disk mass. To distinguish radial drift and grain growth from line optical depth effects based on size ratios requires disks to be observed at high enough angular resolution and the disk structure should to be modelled to account for the total CO content of the disk., 18 pages, 27 figures, accepted in A&A

Planet formation and clearing of protoplanetary disks is one of the long standing problems in disk evolution theory. The best test of clearing scenarios is observing systems that are most likely to be actively forming planets: the transitional disks with large inner dust cavities. We present the first results of our ALMA (Atacama Large Millimeter/submillimeter Array) Cycle 0 program using Band 9, imaging the Herbig Ae star Oph IRS 48 in CO 6−5 and the submillimeter continuum in the extended configuration. The resulting ~0.2″ spatial resolution completely resolves the cavity of this disk in the gas and the dust. The gas cavity of IRS 48 is half as large as the dust cavity, ruling out grain growth and photoevaporation as the primary cause of the truncation. On the other hand, the continuum emission reveals an unexpected large azimuthal asymmetry and steep edges in the dust distribution along the ring, suggestive of dust trapping. We will discuss the implications of the combined gas and dust distribution for planet formation at a very early stage. This is one of the first transition disks with spatially resolved gas inside the cavity, demonstrating the superb capabilities of the Band 9 receivers.

We report on Herschel/PACS observations of absorption lines of OH+, H2O+ and H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with E_lower of at least 285 and \sim200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH+/H2O+ of 1-2.5 are estimated in the nuclei of both sources. The inferred OH+ column and abundance relative to H nuclei are (0.5-1)x10^{16} cm-2 and \sim2x10^{-8}, respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH+/H2O+\sim5-10. H3O+ is detected in both sources with N(H3O+)\sim(0.5-2)x10^{16} cm-2, and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ~500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H+ - O+ - OH+ - H2O+ - H3O+, and we also argue that the H+ production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (\gtrsim10^4 cm-3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is zeta>10^{-13} s-1, a lower limit that is severalx10^2 times the highest rate estimates for Galactic regions. In Arp 220, our lower limit for zeta is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an AGN is responsible for the molecular ion production., Comment: 24 pages, 13 figures. Accepted for publication in Astronomy & Astrophysics

[Context] Hydrides are simple compounds containing one or a few hydrogen atoms bonded to a heavier atom. They are fundamental precursor molecules in cosmic chemistry and many hydride ions have become observable in high quality for the first time thanks to the Herschel Space Observatory. Ionized hydrides such as CH+ and OH+ (and also HCO+), which affect the chemistry of molecules such as water, provide complementary information on irradiation by far-UV (FUV) or X-rays and gas temperature., [Aims] We explore hydrides of the most abundant heavier elements in an observational survey covering young stellar objects (YSOs) with different mass and evolutionary state. The focus is on hydrides associated with the dense protostellar envelope and outflows, contrary to previous work that focused on hydrides in diffuse foreground clouds., [Methods] Twelve YSOs were observed with HIFI on Herschel in six spectral settings providing fully velocity-resolved line profiles as part of the Water in star-forming regions with Herschel (WISH) program. The YSOs include objects of low (Class 0 and I), intermediate, and high mass, with luminosities ranging from 4 L⊙ to 2 × 105 L⊙., [Results] The targeted lines of CH+, OH+, H2O+, C+, and CH are detected mostly in blue-shifted absorption. H3O+ and SH+ are detected in emission and only toward some high-mass objects. The observed line parameters and correlations suggest two different origins related to gas entrained by the outflows and to the circumstellar envelope. The derived column densities correlate with bolometric luminosity and envelope mass for all molecules, best for CH, CH+, and HCO+. The column density ratios of CH+/OH+ are estimated from chemical slab models, assuming that the H2 density is given by the specific density model of each object at the beam radius. For the low-mass YSOs the observed ratio can be reproduced for an FUV flux of 2–400 times the interstellar radiation field (ISRF) at the location of the molecules. In two high-mass objects, the UV flux is 20–200 times the ISRF derived from absorption lines, and 300–600 ISRF using emission lines. Upper limits for the X-ray luminosity can be derived from H3O+ observations for some low-mass objects., [Conclusions] If the FUV flux required for low-mass objects originates at the central protostar, a substantial FUV luminosity, up to 1.5 L⊙, is required. There is no molecular evidence for X-ray induced chemistry in the low-mass objects on the observed scales of a few 1000 AU. For high-mass regions, the FUV flux required to produce the observed molecular ratios is smaller than the unattenuated flux expected from the central object(s) at the Herschel beam radius. This is consistent with an FUV flux reduced by circumstellar extinction or by bloating of the protostar., The work on star formation at ETH Zurich was partially funded by the Swiss National Science Foundation (grant No. 200020-113556). Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA), by a Royal Netherlands Academy of Arts and Sciences (KNAW) professor prize, and by the European Union A-ERC grant 291141 CHEMPLAN. Support for this work was also provided by NASA (Herschel OT funding) through an award issued by JPL/ Caltech.

Transitional disks around young stars are promising candidates to look for recently formed, embedded planets. Planet-disk interaction models predict that planets clear a gap in the gas while trapping dust at larger radii. Other physical mechanisms could be responsible for cavities as well. Previous observations have revealed that gas is still present inside these cavities, but the spatial distribution of this gas remains uncertain. We present high spatial resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of 13CO and C18O lines of four well-studied transitional disks. The observations are used to set constraints on the gas surface density, specifically cavity size and density drop inside the cavity. The physical-chemical model DALI is used to analyze the gas images of SR21, HD135344B, DoAr44 and IRS48. The main parameters of interest are the size, depth and shape of the gas cavity. CO isotope-selective photodissociation is included to properly constrain the surface density in the outer disk from C18O emission. The gas cavities are up to 3 times smaller than those of the dust in all four disks. Model fits indicate that the surface density inside the gas cavities decreases by a factor of 100-10000 compared with the surface density profile derived from the outer disk. A comparison with an analytical model of gap depths by planet-disk interaction shows that the disk viscosities are likely low, with a, Accepted by A&A; version after language-editing

Despite intensive studies of protoplanetary disks, there is still no reliable way to determine their total mass and their surface density distribution, quantities that are crucial for describing both the structure and the evolution of disks up to the formation of planets. The goal of this work is to use less abundant CO isotopologues, whose detection is routine for ALMA, to infer the gas mass of disks. Isotope-selective effects need to be taken into account in the analysis, because they can significantly modify CO isotopologues line intensities. CO isotope-selective photodissociation has been implemented in the physical-chemical code DALI and 800 disk models have been run for a range of disk and stellar parameters. Dust and gas temperature structures have been computed self-consistently, together with a chemical calculation of the main species. Both disk structure and stellar parameters have been investigated. Total fluxes have been ray-traced for different CO isotopologues and for various transitions for different inclinations. A combination of 13CO and C18O total intensities allows inference of the total disk mass, although with non-negligible uncertainties. These can be overcome by employing spatially resolved observations, i.e. the disk's radial extent and inclination. Comparison with parametric models shows differences at the factor of a few level, especially for extremely low and high disk masses. Finally, total line intensities for different CO isotopologue and for various low-J transitions are provided and are fitted to simple formulae. The effects of a lower gas-phase carbon abundance and different gas/dust ratios are investigated as well, and comparison with other tracers is made. Disk masses can be determined within a factor of a few by comparing CO isotopologue lines observations with the simulated line fluxes, modulo the uncertainties in the volatile elemental abundances., Comment: 20 pages, 15 figures, A&A accepted

The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. We combine our recent detections of C$^{0}$ in these disks with observations of other carbon reservoirs (CO, C$^{+}$, C$_{2}$H) and gas mass and warm gas tracers (HD, O$^{0}$), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of $\leq0.1\,M_{\odot}$. The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is at most moderately depleted compared to the interstellar medium, with [C]/[H]$_{\rm gas}=(0.1-1.5)\times 10^{-4}$. HD 100546 itself is a $\lambda\,$Bo\"{o}tis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H]$_{\rm gas}=(0.2-5.0)\times 10^{-6}$ and C/O${>}1$. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H]$_{\rm gas}$ in the outer disk of TW Hya and allows to make predictions for other systems., Comment: Accepted for publication in A&A

The gas temperature structure of protoplanetary disks is a key ingredient for interpreting various disk observations and for quantifying the subsequent evolution of these systems. The comparison of low- and mid-$J$ CO rotational lines is a powerful tool to assess the temperature gradient in the warm molecular layer of disks. Spectrally resolved high-$J$ ($J_{\rm u} > 14$) CO lines probe intermediate distances and heights from the star that are not sampled by (sub-)millimeter CO spectroscopy. This paper presents new {\it Herschel}/HIFI and archival PACS observations of $^{12}$CO, $^{13}$CO and \cii \ emission in 4 Herbig AeBe (HD 100546, HD 97048, IRS 48, HD 163296) and 3 T Tauri (AS 205, S CrA, TW Hya) disks. In the case of the T Tauri systems AS 205 and S CrA, the CO emission has a single-peaked profile, likely due to a slow wind. For all other systems, the {\it Herschel} CO spectra are consistent with pure disk emission and the spectrally-resolved lines (HIFI) and the CO rotational ladder (PACS) are analyzed simultaneously assuming power-law temperature and column density profiles, using the velocity profile to locate the emission in the disk. The temperature profile varies substantially from disk to disk. In particular, $T_{\rm gas}$ in the disk surface layers can differ by up to an order of magnitude among the 4 Herbig AeBe systems with HD 100546 being the hottest and HD 163296 the coldest disk of the sample. Clear evidence of a warm disk layer where $T_{\rm gas} > T_{\rm dust}$ is found in all the Herbig Ae disks. The observed CO fluxes and line profiles are compared to predictions of physical-chemical models. The primary parameters affecting the disk temperature structure are the flaring angle, the gas-to-dust mass ratio the scale height and the dust settling., Accepted for publication in A&A

Water is a key molecule for tracing physical and chemical processes in star-forming regions. The key program “Water in star-forming regions with Herschel ” is observing several water transitions towards low-mass protostars with HIFI. Results regarding the 557 GHz transition of water are reported here showing that the line is surprisingly broad, and consists of several different velocity components. The bulk of the emission comes from shocks, where the abundance is increased by several orders of magnitude to ~10 -4 . The abundance of water in the outer envelope is determined to ~10 -8 , whereas only an upper limit of 10 -5 is derived for the inner, warm envelope.

Molecules containing one or a few hydrogen atoms and a heavier atom (hydrides) have been predicted to trace FUV radiation. In some chemical models, FUV emission by the central object or protostar of a star forming region greatly enhances some of the hydride abundances. Two massive regions, W3 IRS5 and AFGL 2591, have been observed in hydride lines by HIFI onboard the {\it Herschel Space Observatory}. We use published results as well as new observations of CH$^+$ towards W3 IRS5. Molecular column densities are derived from ground state absorption lines, radiative transfer modeling or rotational diagrams. Models assuming no internal FUV are compared with two-dimensional models including FUV irradiation of outflow walls. We confirm that the effect of FUV is clearly noticeable and greatly improves the fit. The most sensitive molecules to FUV irradiation are CH$^+$ and OH$^+$, enhanced in abundance by many orders of magnitude. Modeling in addition also full line radiative transfer, Bruderer et al (2010b) achieve good agreement of a two-dimensional FUV model with observations of CH$^+$ in AFGL 2591. It is concluded that CH$^+$ and OH$^+$ are good FUV tracers in star-forming regions., EAS Publications Series, 5th Zermatt conference on Conditions and impact of star formation: New results with Herschel and beyond (invited), submitted

The Herschel Space Observatory enables observations in the far-infrared at high spectral and spatial resolution. A particular class of molecules will be directly observable: light diatomic hydrides and their ions (CH, OH, SH, NH, CH + ,O H + ,S H + ,N H + ). These simple constituents are important both for the chemical evolution of the region and as tracers of high-energy radiation. If outflows of a forming star erode cavities in the envelope, protostellar far-UV (FUV; 6

The paper presents new high angular resolution ALMA 1.3 mm dust continuum observations of the protoplanetary system AS 209 in the Ophiuchus star forming region. The dust continuum emission is characterized by a main central core and two prominent rings at $r = 75\,$au and $r = 130\,$au intervaled by two gaps at at $r = 62\,$au and $r = 103\,$au. The two gaps have different widths and depths, with the inner one being narrower and shallower. We determined the surface density of the millimeter dust grains using the 3D radiative transfer disk code \textsc{dali}. According to our fiducial model the inner gap is partially filled with millimeter grains while the outer gap is largely devoid of dust. The inferred surface density is compared to 3D hydrodynamical simulations (FARGO-3D) of planet-disk interaction. The outer dust gap is consistent with the presence of a giant planet ($M_{\rm planet} \sim 0.8\,M_{\rm Staturn}$); the planet is responsible for the gap opening and for the pile-up of dust at the outer edge of the planet orbit. The simulations also show that the same planet can give origin to the inner gap at $r = 62\,$au. The relative position of the two dust gaps is close to the 2:1 resonance and we have investigated the possibility of a second planet inside the inner gap. The resulting surface density (including location, width and depth of the two dust gaps) are in agreement with the observations. The properties of the inner gap pose a strong constraint to the mass of the inner planet ($M_{\rm planet} < 0.1\,M_{\rm J}$). In both scenarios (single or pair of planets), the hydrodynamical simulations suggest a very low disk viscosity ($��< 10^{-4}$). Given the young age of the system (0.5 - 1 Myr), this result implies that the formation of giant planets occurs on a timescale of $\lesssim$ 1\,Myr., Accepted by A&A

Context. Outflow cavities in envelopes of young stellar objects (YSOs) have been pr edicted to allow far-UV (FUV) photons to escape far from the central source, with significant observable e ffects, especially if the protostar is a forming high-mass star suspected of emitting a copious amount of FUV radiation. Indirect evidence of this picture has been provided by models and unresolved single-dish observations, but direct high-resolution data are necessary for con firmation. Previous chemical modeling has suggested that CS and HCN are good probes of the local FUV field, so make good target specie s. Aims. We directly probe the physical conditions of the material in the outflow walls to te st this prediction. Methods. Interferometric observations of the CS(7-6) and HCN(4-3) rotational lines in the high-mass star-forming region AFGL 2591 are carried out in the compact and extended configuration of the SubMillim eter Array (SMA). The velocity structure was analyzed, and integrated maps compared to K-band near-IR observations. A chemical model predicts abundances of CS and HCN for a gas under protostellar X-ray and FUV irradiation, and was used in conjunction with the data to distinguish between physical scenarios. Results. CS and HCN emission was found in spatial coincidence in extended sources displaced up to 7 ′′ from the position of the young star. Their line widths are small, excluding major shocks. Chemical model calculations predict an enhanced abundance of the two molecules in warm, dense, and FUV irradiated gas. Hot dust observed between the molecular emission and the outflow accounts for the necessary attenuation to prevent photodissociation of the molecule s. Conclusions. The SMA data suggest that the outflow walls are heated and chemically altere d by the FUV emission of the central high-mass object, providing the best direct evidence yet of large-sca le direct irradiation of outflow walls.

Observations of CO+ suggest column densities on the order 10^12 cm^-2 that can not be reproduced by many chemical models. CO+ is more likely to be destroyed than excited in collisions with hydrogen. An anomalous excitation mechanism may thus have to be considered when interpreting CO^+ observations. Chemical models are used to perform a parameter study of CO^+ abundances. Line fluxes are calculated for N(CO+)=10^12 cm^-2 and different gas densities and temperatures using a non-LTE escape probability method. The chemical formation and destruction rates are considered explicitly in the detailed balance equations of the radiative transfer. In addition, the rotational levels of CO+ are assumed to be excited upon chemical formation according to a formation temperature. It is found, that chemical models are generally able to produce high fractional CO+ abundances (x(CO+) =10^-10). In a far-ultraviolet (FUV) dominated environment, however, high abundances of CO+ are only produced in regions with a Habing field G0 > 100 and T(kin) > 600 K, posing a strong constraint on the gas temperature. For gas densities >10^6 cm^-3 and temperatures > 600 K, the combination of chemical and radiative transfer analysis shows little effect on intensities of CO+ lines with upper levels N_up 10^6 cm ^ -3. The Herschel Space Observatory is able to reveal such effects in the terahertz wavelength regime. Ideal objects to observe are protoplanetary disks with densities 10^6 cm^-3. It is finally suggested that the CO+ chemistry may be well understood and that the abundances observed so far can be explained with a high enough gas temperature and a proper geometry., 9 pages, 7 figures

Observations of the high-mass star forming region AFGL 2591 reveal a large abundance of CO+, a molecule known to be enhanced by far UV (FUV) and X-ray irradiation. In chemical models assuming a spherically symmetric envelope, the volume of gas irradiated by protostellar FUV radiation is very small due to the high extinction by dust. The abundance of CO+ is thus underpredicted by orders of magnitude. In a more realistic model, FUV photons can escape through an outflow region and irradiate gas at the border to the envelope. Thus, we introduce the first 2D axi-symmetric chemical model of the envelope of a high-mass star forming region to explain the CO+ observations as a prototypical FUV tracer. The model assumes an axi-symmetric power-law density structure with a cavity due to the outflow. The local FUV flux is calculated by a Monte Carlo radiative transfer code taking scattering on dust into account. A grid of precalculated chemical abundances, introduced in the first part of this series of papers, is used to quickly interpolate chemical abundances. This approach allows to calculate the temperature structure of the FUV heated outflow walls self-consistently with the chemistry. Synthetic maps of the line flux are calculated using a raytracer code. Single-dish and interferometric observations are simulated and the model results are compared to published and new JCMT and SMA observations. The two-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dish observations and also explains the non-detection by the SMA. We conclude that the observed CO+ line flux and its narrow width can be interpreted by emission from the warm and dense outflow walls irradiated by protostellar FUV radiation.

(Abridged*) Models of the young solar nebula assume a hot initial disk with most volatiles are in the gas phase. The question remains whether an actively accreting disk is warm enough to have gas-phase water up to 50 AU radius. No detailed studies have yet been performed on the extent of snowlines in an embedded accreting disk (Stage 0). Quantify the location of gas-phase volatiles in embedded actively accreting disk system. Two-dimensional physical and radiative transfer models have been used to calculate the temperature structure of embedded protostellar systems. Gas and ice abundances of H$_2$O, CO$_2$, and CO are calculated using the density-dependent thermal desorption formulation. The midplane water snowline increases from 3 to 55 AU for accretion rates through the disk onto the star between $10^{-9}$-$10^{-4} \ M_{\odot} \ {\rm yr^{-1}}$. CO$_2$ can remain in the solid phase within the disk for $\dot{M} \leq 10^{-5} \ M_{\odot} \ {\rm yr^{-1}}$ down to $\sim 20$ AU. Most of the CO is in the gas phase within an actively accreting disk independent of disk properties and accretion rate. The predicted optically thin water isotopolog emission is consistent with the detected H$_2^{18}$O emission toward the Stage 0 embedded young stellar objects, originating from both the disk and the warm inner envelope (hot core). An accreting embedded disk can only account for water emission arising from $R < 50$ AU, however, and the extent rapidly decreases for low accretion rates. Thus, the radial extent of the emission can be measured with ALMA observations and compared to this limit. Volatiles sublimate out to 50 AU in young disks and can reset the chemical content inherited from the envelope in periods of high accretion rates. A hot young solar nebula out to 30 AU can only have occurred during the deeply embedded Stage 0, not during the T-Tauri phase of our early solar system., 15 pages, 10 figures, accepted for publication in A&A

Context. Outflows are an important part of the star formation process as both the result of ongoing active accretion and one of the main sources of mechanical feedback on small scales. Water is the ideal tracer of these effects because it is present in high abundance for the conditions expected in various parts of the protostar, particularly the outflow. Aims. We constrain and quantify the physical conditions probed by water in the outflow-jet system for Class 0 and I sources. Methods. We present velocity-resolved Herschel HIFI spectra of multiple water-transitions observed towards 29 nearby Class 0/ Ip rotostars as part of the WISH guaranteed time key programme. The lines are decomposed into different Gaussian components, with each component related to one of three parts of the protostellar system; quiescent envelope, cavity shock and spot shocks in the jet and at the base of the outflow. We then use non-LTE radex models to constrain the excitation conditions present in the two outflow-related components. Results. Water emission at the source position is optically thick but effectively thin, with line ratios that do not vary with velocity, in contrast to CO. The physical conditions of the cavity and spot shocks are similar, with post-shock H2 densities of order 10 5 −10 8 cm −3 and H2O column densities of order 10 16 −10 18 cm −2 .H 2O emission originates in compact emitting regions: for the spot shocks these correspond to point sources with radii of order 10−200 AU, while for the cavity shocks these come from a thin layer along the outflow cavity wall with thickness of order 1−30 AU. Conclusions. Water emission at the source position traces two distinct kinematic components in the outflow; J shocks at the base of the outflow or in the jet, and C shocks in a thin layer in the cavity wall. The similarity of the physical conditions is in contrast to off-source determinations which show similar densities but lower column densities and larger filling factors. We propose that this is due to the differences in shock properties and geometry between these positions. Class I sources have similar excitation conditions to Class 0 sources, but generally smaller line-widths and emitting region sizes. We suggest that it is the velocity of the wind driving the outflow, rather than the decrease in envelope density or mass, that is the cause of the decrease in H2O intensity between Class 0 and I sources.

Due to instrumental limitations and a lack of disk detections, the structure between the envelope and the rotationally supported disk has been poorly studied. This is now possible with ALMA through observations of CO isotopologs and tracers of freezeout. Class 0 sources are ideal for such studies given their almost intact envelope and young disk. The structure of the disk-envelope interface of the prototypical Class 0 source, VLA1623A which has a confirmed Keplerian disk, is constrained from ALMA observations of DCO+ 3-2 and C18O 2-1. The physical structure of VLA1623 is obtained from the large-scale SED and continuum radiative transfer. An analytic model using a simple network coupled with radial density and temperature profiles is used as input for a 2D line radiative transfer calculation for comparison with the ALMA Cycle 0 12m array and Cycle 2 ACA observations of VLA1623. DCO+ emission shows a clumpy structure bordering VLA1623A's Keplerian disk, suggesting a cold ring-like structure at the disk-envelope interface. The radial position of the observed DCO+ peak is reproduced in our model only if the region's temperature is between 11-16K, lower than expected from models constrained by continuum and SED. Altering the density has little effect on the DCO+ position, but increased density is needed to reproduce the disk traced in C18O. The DCO+ emission around VLA1623A is the product of shadowing of the envelope by the disk. Disk-shadowing causes a drop in the gas temperature outside of the disk on >200AU scales, encouraging deuterated molecule production. This indicates that the physical structure of the disk-envelope interface differs from the rest of the envelope, highlighting the drastic impact that the disk has on the envelope and temperature structure. The results presented here show that DCO+ is an excellent cold temperature tracer., Accepted for publication in Astronomy & Astrophysics, 16 pages with appendices, 17 figures and 3 tables

We utilize magnetohydrodynamic (MHD) simulations to develop a numerical model for GMC-GMC collisions between nearly magnetically critical clouds. The goal is to determine if, and under what circumstances, cloud collisions can cause pre-existing magnetically subcritical clumps to become supercritical and undergo gravitational collapse. We first develop and implement new photodissociation region (PDR) based heating and cooling functions that span the atomic to molecular transition, creating a multiphase ISM and allowing modeling of non-equilibrium temperature structures. Then in 2D and with ideal MHD, we explore a wide parameter space of magnetic field strength, magnetic field geometry, collision velocity, and impact parameter, and compare isolated versus colliding clouds. We find factors of ~2-3 increase in mean clump density from typical collisions, with strong dependence on collision velocity and magnetic field strength, but ultimately limited by flux-freezing in 2D geometries. For geometries enabling flow along magnetic field lines, greater degrees of collapse are seen. We discuss observational diagnostics of cloud collisions, focussing on 13CO(J=2-1), 13CO(J=3-2), and 12CO(J=8-7) integrated intensity maps and spectra, which we synthesize from our simulation outputs. We find the ratio of J=8-7 to lower-J emission is a powerful diagnostic probe of GMC collisions., Comment: 22 pages, 23 figures. Accepted for publication in ApJ

Abridged: Recent simulations have explored different ways to form accretion disks around low-mass stars. We aim to present observables to differentiate a rotationally supported disk from an infalling rotating envelope toward deeply embedded young stellar objects and infer their masses and sizes. Two 3D magnetohydrodynamics (MHD) formation simulations and 2D semi-analytical model are studied. The dust temperature structure is determined through continuum radiative transfer RADMC3D modelling. A simple temperature dependent CO abundance structure is adopted and synthetic spectrally resolved submm rotational molecular lines up to $J_{\rm u} = 10$ are simulated. All models predict similar compact components in continuum if observed at the spatial resolutions of 0.5-1$"$ (70-140 AU) typical of the observations to date. A spatial resolution of $\sim$14 AU and high dynamic range ($> 1000$) are required to differentiate between RSD and pseudo-disk in the continuum. The peak-position velocity diagrams indicate that the pseudo-disk shows a flatter velocity profile with radius than an RSD. On larger-scales, the CO isotopolog single-dish line profiles are similar and are narrower than the observed line widths of low-$J$ lines, indicating significant turbulence in the large-scale envelopes. However a forming RSD can provide the observed line widths of high-$J$ lines. Thus, either RSDs are common or a higher level of turbulence ($b \sim 0.8 \ {\rm km \ s^{-1}}$ ) is required in the inner envelope compared with the outer part. Multiple spatially and spectrally resolved molecular line observations are needed. The continuum data give a better estimate on disk masses whereas the disk sizes can be estimated from the spatially resolved molecular lines observations. The general observable trends are similar between the 2D semi-analytical models and 3D MHD RSD simulations., 16 pages, 14 figures, accepted for publication, A&A

We present multi-epoch high-spectral resolution observations with VLT/CRIRES of the OH doublet $^2\Pi_{3/2}$ P4.5 (1+,1-) (2.934 $\mu$m) towards the protoplanetary disk around HD 100546. The OH doublet is detected at all epochs and is spectrally resolved while nearby H$_2$O lines remains undetected. The OH line velocity profile is different in the three datasets: in the first epoch (April 2012, PA=26$^{\circ}$) the OH lines are symmetric and line broadening is consistent with the gas being in Keplerian rotation around the star. No OH emission is detected within a radius of 8-11 au from the star: the line emitting region is similar in size and extent to that of the CO ro-vibrational lines. In the other two epochs (March 2013 and April 2014, PA=90$^{\circ}$ and 10$^{\circ}$, respectively) the OH lines appear asymmetric and fainter compared to April 2012. We investigate the origin of these line asymmetries which were taken by previous authors as evidence for tidal interaction between an (unseen) massive planet and the disk. We show that the observed asymmetries can be fully explained by a misalignment of the slit of order 0\farcs04-0\farcs20 with respect to the stellar position. The disk is spatially resolved and the slit misalignment is likely caused by the extended dust emission which is brighter than the stellar photosphere at near-infrared wavelengths which is the wavelength used for the pointing. This can cause the photo-center of HD 100546 to be mis-aligned with the stellar position at near-infrared wavelengths., Comment: Accepted for publication in ApJ