arXiv subjects

John D. Ilee

Publications and source records attributed to John D. Ilee.

83 records · Page 5Linked to original sources

arXiv2019

The First Detection of 13C17O in a Protoplanetary Disk: a Robust Tracer of Disk Gas Mass

Measurements of the gas mass are necessary to determine the planet formation potential of protoplanetary disks. Observations of rare CO isotopologues are typically used to determine disk gas masses; however, if the line emission is optically thick this will result in an underestimated disk mass. With ALMA we have detected the rarest stable CO isotopologue, 13C17O, in a protoplanetary disk for the first time. We compare our observations with the existing detections of 12CO, 13CO, C18O and C17O in the HD163296 disk. Radiative transfer modelling using a previously benchmarked model, and assuming interstellar isotopic abundances, significantly underestimates the integrated intensity of the 13C17O J=3-2 line. Reconciliation between the observations and the model requires a global increase in CO gas mass by a factor of 3.5. This is a factor of 2-6 larger than previous gas mass estimates using C18O. We find that C18O emission is optically thick within the CO snow line, while the 13C17O emission is optically thin and is thus a robust tracer of the bulk disk CO gas mass.

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arXiv2019

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 HC15N (J=4-3) in the HD97048 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 H13CO+ 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 HD97048 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 < 10K, R > 200au). 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 HD97048 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 HD100546 protoplanetary disk.

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arXiv2019

Planet-forming material in a protoplanetary disc: the interplay between chemical evolution and pebble drift

The composition of gas and solids in protoplanetary discs sets the composition of planets that form out of them. Recent chemical models have shown that the composition of gas and dust in discs evolves on Myr time-scales, with volatile species disappearing from the gas phase. However, discs evolve due to gas accretion and radial drift of dust on time-scales similar to these chemical time-scales. Here we present the first model coupling the chemical evolution in the disc mid-planes with the evolution of discs due to accretion and radial drift of dust. Our models show that transport will always overcome the depletion of CO$_2$ from the gas phase, and can also overcome the depletion of CO and CH$_4$ unless both transport is slow (viscous $α\lesssim 10^{-3}$) and the ionization rate is high ($ζ\approx 10^{-17}$). Including radial drift further enhances the abundances of volatile species because they are carried in on the surface of grains before evaporating left at their ice lines. Due to large differences in the abundances within 10 au for models with and without efficient radial drift, we argue that composition can be used to constrain models of planet formation via pebble accretion.

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arXiv2018

Observing substructure in circumstellar discs around massive young stellar objects

Simulations of massive star formation predict the formation of discs with significant substructure, such as spiral arms and clumps due to fragmentation. Here we present a semi-analytic framework for producing synthetic observations of discs with substructure, in order to determine their observability in interferometric observations. Unlike post-processing of hydrodynamical models, the speed inherent to our approach permits us to explore a large parameter space of star and disc parameters, and thus constrain properties for real observations. We compute synthetic dust continuum and molecular line observations probing different disc masses, distances, inclinations, thermal structures, dust distributions, and number and orientation of spirals and fragments. With appropriate spatial and kinematic filtering applied, our models predict that ALMA observations of massive YSOs at <5 kpc distances should detect spirals in both gas and dust in strongly self-gravitating discs (i.e. discs with up to two spiral arms and strong kinematic perturbations). Detecting spirals will be possible in discs of arbitrary inclination, either by directly spatially resolving them for more face-on discs (inclinations up to ~50 degrees), or through a kinematic signature otherwise. Clumps resulting from disc fragmentation should be detectable in the continuum, if the clump is sufficiently hotter than the surrounding disc material.

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arXiv2018

The fate of formamide in a fragmenting protoplanetary disc

Recent high-sensitivity observations carried out with ALMA have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH$_{\rm 3}$CN) and methanol (CH$_{\rm 3}$OH) in relatively evolved protoplanetary discs. The behaviour and abundance of COMs in earlier phases of disc evolution remains unclear. Here we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disc with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH$_{\rm 2}$CHO), a pre-biotic species, in both the disc and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disc where the temperatures are $<$100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas-phase in three fragments as a result of the high temperatures ($\geq$200\,K), but remains in the solid-phase in one colder ($\leq$150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core which is formed would inherit their formamide content directly from the protosolar nebula.

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arXiv2018

High resolution millimetre imaging of the CI Tau protoplanetary disc - a massive ensemble of protoplanets from 0.1 - 100 au

We present high resolution millimeter continuum imaging of the disc surrounding the young star CI Tau, a system hosting the first hot Jupiter candidate in a protoplanetary disc system. The system has extended mm emission on which are superposed three prominent annular gaps at radii ~ 13, 39 and 100 au. We argue that these gaps are most likely to be generated by massive planets so that, including the hot Jupiter, the system contains four gas giant planets at an age of only 2 Myr. Two of the new planets are similarly located to those inferred in the famous HL Tau protoplanetary disc; in CI Tau, additional observational data enables a more complete analysis of the system properties than was possible for HL Tau. Our dust and gas dynamical modeling satisfies every available observational constraint and points to the most massive ensemble of exo-planets ever detected at this age, with its four planets spanning a factor 1000 in orbital radius.Our results show that the association between hot Jupiters and gas giants on wider orbits, observed in older stars, is apparently in place at an early evolutionary stage.

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arXiv2018

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 disc surrounding Elias 2-27 have been suggested as evidence of the disc being gravitationally unstable. However, previous work has shown that a massive, stable disc undergoing an encounter with a massive companion are also consistent with the observations. We compare the spiral morphology of smoothed particle hydrodynamic simulations modelling both cases. The gravitationally unstable disc produces symmetric, tightly wound spiral arms with constant pitch angle, as predicted by the literature. The companion disc'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 disc 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 disc.

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arXiv2017

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 disk 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 inward 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 Juno's measurement of Jupiter's C/O ratio should determine whether Jupiter accreted its metals from carbon rich gas or oxygen rich solids.

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arXiv2017

On the origin of the spiral morphology in the Elias 2-27 circumstellar disc

The young star Elias 2-27 has recently been observed to posses a massive circumstellar disc with two prominent large-scale spiral arms. In this Letter we perform three-dimensional Smoothed Particle Hydrodynamics simulations, radiative transfer modelling, 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 disc. We find that a gravitationally unstable disc and a disc 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 (less than approximately 10 - 13 MJup) and located at large radial distances (between approximately 300 - 700 au). We therefore suggest that Elias 2-27 may be one of the first detections of a disc undergoing gravitational instabilities, or a disc that has recently undergone fragmentation to produce a massive companion.

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arXiv2016

Grand challenges in protoplanetary disc modelling

The Protoplanetary Discussions conference --- held in Edinburgh, UK, from 7th --11th March 2016 --- included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multiphysics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on \textit{community driven input}, to encourage further progress in our understanding of circumstellar and protoplanetary discs.

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arXiv2014

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

Herbig 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 VLT/X-Shooter spectra of six Herbig Ae/Be stars, drawn from a sample of 91 targets, and high resolution VLT/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 Br γ 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 co-rotation 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.

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