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Register a new userHD far infrared emission as a measure of protoplanetary disk mass
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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 $6cdot10^{-3}$ M$_odot$ < M$_{rm disk} < 9cdot10^{-3}$ M$_odot$. Future observations require a 5$sigma$ sensitivity of $1.8cdot10^{-20}$ W m$^{-2}$ ($2.5cdot10^{-20}$ 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.
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We assess the ionising effect of low energy protostellar cosmic rays in protoplanetary disks around a young solar mass star for a wide range of disk parameters. We assume a source of low energy cosmic rays located close to the young star which travel diffusively through the protoplanetary disk. We use observationally inferred values from nearby star-forming regions for the total disk mass and the radial density profile. We investigate the influence of varying the disk mass within the observed scatter for a solar mass star. We find that for a large range of disk masses and density profiles that protoplanetary disks are optically thin to low energy ($sim$3 GeV) cosmic rays. At $Rsim10$au, for all of the disks that we consider ($M_mathrm{disk}=6.0times10^{-4} - 2.4times 10^{-2}M_odot$), the ionisation rate due to low energy stellar cosmic rays is larger than that expected from unmodulated galactic cosmic rays. This is in contrast to our previous results which assumed a much denser disk which may be appropriate for a more embedded source. At $Rsim70$au, the ionisation rate due to stellar cosmic rays dominates in $sim$50% of the disks. These are the less massive disks with less steep density profiles. At this radius there is at least an order of magnitude difference in the ionisation rate between the least and most massive disk that we consider. Our results indicate, for a wide range of disk masses, that low energy stellar cosmic rays provide an important source of ionisation at the disk midplane at large radii ($sim$70au).
We present a detailed multi-wavelength characterization of the multi-ring disk of HD 169142. We report new ALMA observations at 3 mm and analyze them together with archival 0.89 and 1.3 mm data. Our observations resolve three out of the four rings in the disk previously seen in high-resolution ALMA data. A simple parametric model is used to estimate the radial profile of the dust optical depth, temperature, density, and particle size distribution. We find that the multiple ring features of the disk are produced by annular accumulations of large particles, probably associated with gas pressure bumps. Our model indicates that the maximum dust grain size in the rings is $sim1$ cm, with slightly flatter power-law size distributions than the ISM-like size distribution ($psim3.5$) found in the gaps. In particular, the inner ring ($sim26$ au) is associated with a strong and narrow buildup of dust particles that could harbor the necessary conditions to trigger the streaming instability. According to our analysis, the snowlines of the most important volatiles do not coincide with the observed substructures. We explore different ring formation mechanisms and find that planet-disk interactions are the most likely scenario to explain the main features of HD 169142. Overall, our multi-wavelength analysis provides some of the first unambiguous evidence of the presence of radial dust traps in the rings of HD 169142. A similar analysis in a larger sample of disks could provide key insights on the impact that disk substructures have on the dust evolution and planet formation processes.
[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.
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 <30 AU and a broad, ring-like structure from ~50-230 AU with a peak at 100 AU just beyond the millimeter grain edge. We modeled DCO$^+$ emission in HD 169142 with a physical disk structure adapted from the literature, and employed a simple deuterium chemical network to investigate the formation of DCO$^+$ through the cold deuterium fractionation pathway via H$_2$D$^+$. Contributions from the warm deuterium fractionation pathway via CH$_2$D$^+$ are approximated using a constant abundance in the intermediate disk layers. Parameterized models show that alterations to the midplane gas temperature and CO abundance of the literature model are both needed to recover the observed DCO$^+$ radial intensity profile. The best-fit model contains a shadowed, cold midplane in the region z/r < 0.1 with an 8 K decrease in gas temperature and a factor of five CO depletion just beyond the millimeter grain edge, and a 2 K decrease in gas temperature for r > 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.
Near-IR observations of protoplanetary disks provide information about the properties of the inner disk. High resolution spectra of abundant molecules such as CO can be used to determine the disk structure in the warm inner parts. The $v2/v1$ ro-vibrational ratio of $v_{1-0}$ and $v_{2-1}$ transitions has been recently observed to follow distinct trends with the CO emitting radius, in a sample of TTauri and Herbig disks; these trends have been empirically interpreted as due to inner disk depletion from gas and dust. In this work we use existing thermo-chemical disk models to explore the interpretation of these observed trends in ro-vibrational CO emission. We use the radiation thermo-chemical code ProDiMo, exploring a set of previously published models with different disk properties and varying one parameter at a time: the inner radius, the dust-to-gas mass ratio, the gas mass. In addition, we use models where we change the surface density power law index, and employ a larger set of CO ro-vibrational levels, including also fluorescence from the first electronic state. We investigate these models for both TTauri and Herbig star disks. Finally, we include a set of DIANA models for individual TTauri and Herbig disks which were constructed to reproduce a large set of multi-wavelength observations.
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