Chemulator: Fast, accurate thermochemistry for dynamical models through emulation

Context. Chemical modelling serves two purposes in dynamical models: accounting for the effect of microphysics on the dynamics and providing observable signatures. Ideally, the former must be done as part of the hydrodynamic simulation but this comes with a prohibitive computational cost that leads to many simplifications being used in practice. / Aims. We aim to produce a statistical emulator that replicates a full chemical model capable of solving the temperature and abundances of a gas through time. This emulator should suffer only a minor loss of accuracy when compared to a full chemical solver and would have a fraction of the computational cost allowing it to be included in a dynamical model. / Methods. The gas-grain chemical code UCLCHEM was updated to include heating and cooling processes, and a large dataset of model outputs from possible starting conditions was produced. A neural network was then trained to map directly from inputs to outputs. / Results. Chemulator replicates the outputs of UCLCHEM with an overall mean squared error (MSE) of 1.7 × 10−4 for a single time step of 1000 yr, and it is shown to be stable over 1000 iterations with an MSE of 3 × 10−3 on the log-scaled temperature after one timzze step and 6 × 10−3 after 1000 time steps. Chemulator was found to be approximately 50 000 times faster than the time-dependent model it emulates but can introduce a significant error to some models

Are Elias 2-27's Spiral Arms Driven by Self-gravity, or by a Companion? A Comparative Spiral Morphology Study

The spiral waves detected in the protostellar disk surrounding Elias 2-27 have been suggested as evidence of the disk being gravitationally unstable. However, previous work has shown that a massive, stable disk undergoing an encounter with a massive companion are also consistent with the observations. We compare the spiral morphology of smoothed particle hydrodynamic simulations modeling both cases. The gravitationally unstable disk produces symmetric, tightly wound spiral arms with constant pitch angle, as predicted by the literature. The companion disk's arms are asymmetric, with pitch angles that increase with radius. However, these arms are not well-fitted by standard analytic expressions, due to the high disk mass and relatively low companion mass. We note that differences (or indeed similarities) in morphology between pairs of spirals is a crucial discriminant between scenarios for Elias 2-27, and hence future studies must fit spiral arms individually. If Elias 2-27 continues to show symmetric tightly wound spiral arms in future observations, then we posit that it is the first observed example of a gravitationally unstable protostellar disk

On the Origin of the Spiral Morphology in the Elias 2-27 Circumstellar Disk

The young star Elias 2-27 has recently been observed to posses a massive circumstellar disk with two prominent large-scale spiral arms. In this Letter, we perform three-dimensional Smoothed Particle Hydrodynamics simulations, radiative transfer modeling, synthetic ALMA imaging, and an unsharped masking technique to explore three possibilities for the origin of the observed structures - an undetected companion either internal or external to the spirals, and a self-gravitating disk. We find that a gravitationally unstable disk and a disk with an external companion can produce morphology that is consistent with the observations. In addition, for the latter, we find that the companion could be a relatively massive planetary-mass companion (≲10-13 M Jup ) and located at large radial distances (between ≈300-700 au). We therefore suggest that Elias 2-27 may be one of the first detections of a disk undergoing gravitational instabilities, or a disk that has recently undergone fragmentation to produce a massive companion.We acknowledge support from the DISCSIM project, grant agreement 341137 under ERC-2013-ADG. F.M. acknowledges support from The Leverhulme Trust. This Letter uses the following ALMA data: ADS/JAO.ALMA# 2013.1.00498.S. This work used the Darwin DiRAC HPC cluster at the University of Cambridge and was undertaken on the Cambridge COSMOS SMP system, part of the STFC DiRAC HPC Facility supported by BIS NeI capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1

First detections of H¹³CO+ and HC¹⁵N in the disk around HD 97048: Evidence for a cold gas reservoir in the outer disk

Observations of different molecular lines in protoplanetary disks provide valuable information on the gas kinematics, as well as constraints on the radial density and temperature structure of the gas. With ALMA we have detected H¹³CO+ (J = 4–3) and HC¹⁵ N (J = 4–3) in the HD 97048 protoplanetary disk for the first time. We compare these new detections to the ringed continuum mm-dust emission and the spatially resolved CO (J = 3–2) and HCO+ (J = 4–3) emission. The radial distributions of the H¹²CO+ and HC¹⁵N emission show hints of ringed sub-structure whereas, the optically thick tracers, CO and HCO+, do not. We calculate the HCO+/H¹³CO+ intensity ratio across the disk and find that it is radially constant (within our uncertainties). We use a physio-chemical parametric disk structure of the HD 97048 disk with an analytical prescription for the HCO+ abundance distribution to generate synthetic observations of the HCO+ and H¹³CO+ disk emission assuming LTE. The best by-eye fit models require radial variations in the HCO+/H¹³CO+ abundance ratio and an overall enhancement in H¹³CO+ relative to HCO+. This highlights the need to consider isotope selective chemistry and in particular low temperature carbon isotope exchange reactions. This also points to the presence of a reservoir of cold molecular gas in the outer disk (T ~ 10 K, R ~ 200 au). Chemical models are required to confirm that isotope-selective chemistry alone can explain the observations presented here. With these data, we cannot rule out that the known dust substructure in the HD 97048 disk is responsible for the observed trends in molecular line emission, and higher spatial resolution observations are required to fully explore the potential of optically thin tracers to probe planet-carved dust gaps. We also report non-detections of H¹³CO+ and HC¹⁵N in the HD 100546 protoplanetary disk

The extremely truncated circumstellar disc of V410 X-ray 1: A precursor to TRAPPIST-1?

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

Chemical enrichment of giant planets and discs due to pebble drift

Chemical compositions of giant planets provide a means to constrain how and where they form. Traditionally, super-stellar elemental abundances in giant planets were thought to be possible due to accretion of metal-rich solids. Such enrichments are accompanied by oxygen-rich compositions (i.e. C/O below the disc’s value, assumed to be solar, C/O = 0.54). Without solid accretion, the planets are expected to have sub-solar metallicity, but high C/O ratios. This arises because the solids are dominated by oxygen-rich species, e.g. H$_2$O and CO$_2$, which freeze out in the disc earlier than CO, leaving the gas metal poor but carbon rich. Here we demonstrate that super-solar metallicities can be achieved by gas accretion alone when growth and radial drift of pebbles are considered in protoplanetary discs. Through this mechanism, planets may simultaneously acquire super-solar metallicities and super-solar C/O ratios. This happens because the pebbles transport volatile species inwards as they migrate through the disc, enriching the gas at snow lines where the volatiles sublimate. Furthermore, the planet’s composition can be used to constrain where it formed. Since high C/H and C/O ratios cannot be created by accreting solids, it may be possible to distinguish between formation via pebble accretion and planetesimal accretion by the level of solid enrichment. Finally, we expect that Jupiter’s C/O ratio should be near or above solar if its enhanced carbon abundance came through accreting metal-rich gas. Thus, $\textit{Juno}$’s measurement of Jupiter’s C/O ratio should determine whether Jupiter accreted its metals from carbon-rich gas or oxygen-rich solids.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG

High-resolution observations of molecular emission lines toward the CI Tau proto-planetary disc: Planet-carved gaps or shadowing?

Recent observations have revealed that most proto-planetary discs show a pattern of bright rings and dark gaps. However, most of the high-resolution observations have focused only on the continuum emission. In this Paper we present high-resolution ALMA band 7 (0.89mm) observations of the disc around the star CI Tau in the $^{12}$CO & $^{13}$CO $J=3$-2 and CS $J=7$-6 emission lines. Our recent work demonstrated that the disc around CI Tau contains three gaps and rings in continuum emission, and we look for their counterparts in the gas emission. While we find no counterpart of the third gap and ring in $^{13}$CO, the disc has a gap in emission at the location of the second continuum ring (rather than gap). We demonstrate that this is mostly an artefact of the continuum subtraction, although a residual gap still remains after accounting for this effect. Through radiative transfer modelling we propose this is due to the inner disc shadowing the outer parts of the disc and making them colder. This raises a note of caution in mapping high-resolution gas emission lines observations to the gas surface density - while possible, this needs to be done carefully. In contrast to $^{13}$CO, CS emission shows instead a ring morphology, most likely due to chemical effects. Finally, we note that $^{12}$CO is heavily absorbed by the foreground preventing any morphological study using this line

Investigating the inner discs of Herbig Ae/Be stars with CO bandhead and Brγ emission

articleHerbig Ae/Be stars lie in the mass range between low- and high-mass young stars, and therefore offer a unique opportunity to observe any changes in the formation processes that may occur across this boundary. This paper presents medium-resolution Very Large Telescope (VLT)/X-shooter spectra of six Herbig Ae/Be stars, drawn from a sample of 91 targets, and high-resolution VLT/Cryogenic Infrared Echelle Spectrograph (CRIRES) spectra of five Herbig Ae/Be stars, chosen based on the presence of CO first overtone bandhead emission in their spectra. The X-shooter survey reveals a low detection rate of CO first overtone emission (7 per cent), consisting of objects mainly of spectral type B. A positive correlation is found between the strength of the CO v = 2–0 and Brγ emission lines, despite their intrinsic linewidths suggesting a separate kinematic origin. The high-resolution CRIRES spectra are modelled, and are well fitted under the assumption that the emission originates from small scale Keplerian discs, interior to the dust sublimation radius, but outside the corotation radius of the central stars. In addition, our findings are in very good agreement for the one object where spatially resolved near-infrared interferometric studies have also been performed. These results suggest that the Herbig Ae/Be stars in question are in the process of gaining mass via disc accretion, and that modelling of high spectral resolution spectra is able to provide a reliable probe into the process of stellar accretion in young stars of intermediate to high masses.European Union FP7-2011Science and Technology Facilities Counci

An inherited complex organic molecule reservoir in a warm planet-hosting disk

Quantifying the composition of the material in protoplanetary disks is essential to determining the potential for exoplanetary systems to produce and support habitable environments. When considering potential habitability, complex organic molecules are relevant, key among which is methanol (CH3OH). Methanol primarily forms at low temperatures via the hydrogenation of CO ice on the surface of icy dust grains and is a necessary basis for the formation of more complex species such as amino acids and proteins. We report the detection of CH3OH in a disk around a young, luminous A-type star, HD 100546. This disk is warm and therefore does not host an abundant reservoir of CO ice. We argue that the CH3OH cannot form in situ, and hence that this disk has probably inherited complex-organic-molecule-rich ice from an earlier cold dark cloud phase. This is strong evidence that at least some interstellar organic material survives the disk-formation process and can then be incorporated into forming planets, moons and comets. Therefore, crucial pre-biotic chemical evolution already takes place in dark star-forming clouds

Gravitational instabilities in a protosolar-like disc III: molecular line detection and sensitivities

At the time of formation, protoplanetary discs likely contain a comparable mass to their host protostars. As a result, gravitational instabilities (GIs) are expected to play a pivotal role in the early phases of disc evolution. However, as these young objects are heavily embedded, confirmation of GIs has remained elusive. Therefore, we use the radiative transfer code LIME to produce line images of a 0.17 M selfgravitating protosolar-like disc. We note the limitations of using LIME and explore methods to improve upon the default gridding. We synthesise noiseless observations to determine the sensitivities required to detect the spiral flux, and find that the line flux distribution does not necessarily correlate to the abundance density distribution; hence performing radiative transfer calculations is imperative. Moreover, the spiral features are seen in absorption, due to the GI-heated midplane and high extinction, which could be indicative of GI activity. If a small beamsize and appropriate molecular line are used then spatially resolving spirals in a protosolar-like disc should be possible with ALMA for an on-source time of 30 hr. Spectrally resolving non-axisymmetric structure takes only a tenth as long for a reasonable noise level, but attributing this structure to GI-induced activity would be tentative. Finally, we find that identifying finger-like features in PV diagrams of nearly edge-on discs, which are a direct indicator of spirals, is feasible with an on-source time of 19 hr, and hence likely offers the most promising means of confirming GI-driven spiral structure in young, embedded protoplanetary discs