arXiv subjects

Mark Wyatt

Publications and source records attributed to Mark Wyatt.

At least 37 records · Page 2Linked to original sources

arXiv2020

The effect of stellar multiplicity on protoplanetary discs. A NIR survey of the Lupus star forming region

We present results from a near-infrared (NIR) adaptive optics (AO) survey of pre-main-sequence stars in the Lupus Molecular Cloud with VLT/NACO to identify (sub)stellar companions down to $\sim$20 au separation and investigate the effects of multiplicity on circumstellar disc properties. We observe for the first time in the NIR with AO a total of 47 targets and complement our observations with archival data for another 58 objects previously observed with the same instrument. All 105 targets have millimetre ALMA data available, which provide constraints on disc masses and sizes. We identify a total of 13 multiple systems, including 11 doubles and 2 triples. In agreement with previous studies, we find that the most massive (M$_{\rm dust}$ $>$ 50 M$_{\oplus}$) and largest ($R_{\rm dust}>$ 70 au) discs are only seen around stars lacking visual companions (with separations of 20-4800 au) and that primaries tend to host more massive discs than secondaries. However, as recently shown in a very similar study of $>$200 PMS stars in the Ophiuchus Molecular Cloud, the distribution of disc masses and sizes are similar for single and multiple systems for M$_{\rm dust} < 50$ M$_{\oplus}$ and radii $R_{\rm dust}<$ 70 au. Such discs correspond to $\sim $80-90\% of the sample. This result can be seen in the combined sample of Lupus and Ophiuchus objects, which now includes more than 300 targets with ALMA imaging and NIR AO data, and implies that stellar companions with separations $>$20 au mostly affect discs in the upper 10$\%$ of the disc mass and size distributions.

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arXiv2020

Survey of planetesimal belts with ALMA: gas detected around the Sun-like star HD 129590

Gas detection around main sequence stars is becoming more common with around 20 systems showing the presence of CO. However, more detections are needed, especially around later spectral type stars to better understand the origin of this gas and refine our models. To do so, we carried out a survey of 10 stars with predicted high likelihoods of secondary CO detection using ALMA in band 6. We looked for continuum emission of mm-dust as well as gas emission (CO and CN transitions). The continuum emission was detected in 9/10 systems for which we derived the discs' dust masses and geometrical properties, providing the first mm-wave detection of the disc around HD 106906, the first mm-wave radius for HD 114082, 117214, HD 15745, HD 191089 and the first radius at all for HD 121191. A crucial finding of our paper is that we detect CO for the first time around the young 10-16 Myr old G1V star HD 129590, similar to our early Sun. The gas seems colocated with its planetesimal belt and its total mass is likely between $2-10 \times 10^{-5}$ M$_\oplus$. This first gas detection around a G-type main-sequence star raises questions as to whether gas may have been released in the Solar System as well in its youth, which could potentially have affected planet formation. We also detected CO gas around HD 121191 at a higher S/N than previously and find that the CO lies much closer-in than the planetesimals in the system, which could be evidence for the previously suspected CO viscous spreading owing to shielding preventing its photodissociation. Finally, we make estimates for the CO content in planetesimals and the HCN/CO outgassing rate (from CN upper limits), which we find are below the level seen in Solar System comets in some systems.

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arXiv2019

Debris Disk Composition: A Diagnostic Tool for Planet Formation and Migration

Debris disks are exoplanetary systems containing planets, minor bodies (such as asteroids and comets) and debris dust. Unseen planets are presumed to perturb the minor bodies into crossing orbits, generating small dust grains that are detected via remote sensing. Debris disks have been discovered around main sequence stars of a variety of ages (from 10 Myr to several Gyr) and stellar spectral types (from early A-type to M-type stars). As a result, they serve as excellent laboratories for understanding whether the architecture and the evolution of our Solar System is common or rare. This white paper addresses two outstanding questions in debris disk science: (1) Are debris disk minor bodies similar to asteroids and comets in our Solar System? (2) Do planets separate circumstellar material into distinct reservoirs and/or mix material during planet migration? We anticipate that SOFIA/HIRMES, JWST, and WFIRST/CGI will greatly improve our understanding of debris disk composition, enabling the astronomical community to answer these questions. However, we note that despite their observational power, these facilities will not provide large numbers of detections or detailed characterization of cold ices and silicates in the Trans Neptunian zone. Origins Space Telescope is needed to revolutionize our understanding of the bulk composition and mixing in exoplanetary systems.

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arXiv2019

The Mass of Stirring Bodies in the AU Mic Debris Disk Inferred from Resolved Vertical Structure

The vertical distribution of dust in debris disks is sensitive to the number and size of large planetesimals dynamically stirring the disk, and is therefore well-suited for constraining the prevalence of otherwise unobservable Uranus and Neptune analogs. Information regarding stirring bodies has previously been inferred from infrared and optical observations of debris disk vertical structure, but theoretical works predict that the small particles traced by short-wavelength observations will be `puffed up' by radiation pressure, yielding only upper limits. The large grains that dominate the disk emission at millimeter wavelengths are much less sensitive to the effects of stellar radiation or stellar winds, and therefore trace the underlying mass distribution more directly. Here we present ALMA 1.3 mm dust continuum observations of the debris disk around the nearby M star AU Mic. The 3 au spatial resolution of the observations, combined with the favorable edge-on geometry of the system, allows us to measure the vertical thickness of the disk. We report a scale height-to-radius aspect ratio of $h = 0.031_{-0.004}^{+0.005}$ between radii of $\sim 23$ au and $\sim 41$ au. Comparing this aspect ratio to a theoretical model of size-dependent velocity distributions in the collisional cascade, we find that the perturbing bodies embedded in the local disk must be larger than about 400 km, and the largest perturbing body must be smaller than roughly 1.8 M$_\odot$. These measurements rule out the presence of a gas giant or Neptune analog near the $\sim 40$ au outer edge of the debris ring, but are suggestive of large planetesimals or an Earth-sized planet stirring the dust distribution.

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arXiv2019

Exocometary Science

Evidence for exocomets, icy bodies in extrasolar planetary systems, has rapidly increased over the past decade. Volatiles are detected through the gas that exocomets release as they collide and grind down within their natal belts, or as they sublimate once scattered inwards to the regions closest to their host star. Most detections are in young, 10 to a few 100 Myr-old systems that are undergoing the final stages of terrestrial planet formation. This opens the exciting possibility to study exocomets at the epoch of volatile delivery to the inner regions of planetary systems. Detection of molecular and atomic gas in exocometary belts allows us to estimate molecular ice abundances and overall elemental abundances, enabling comparison with the Solar Nebula and Solar System comets. At the same time, observing star-grazing exocomets transiting in front of their star (for planetary systems viewed edge-on) and exozodiacal dust in the systems' innermost regions gives unique dynamical insights into the inward scattering process producing delivery to inner rocky planets. The rapid advances of this budding subfield of exoplanetary science will continue in the short term with the upcoming JWST, WFIRST and PLATO missions. In the longer term, the priority should be to explore the full composition of exocomets, including species crucial for delivery and later prebiotic synthesis. Doing so around an increasingly large population of exoplanetary systems is equally important, to enable comparative studies of young exocomets at the epoch of volatile delivery. We identify the proposed LUVOIR and Origins flagship missions as the most promising for a large-scale exploration of exocometary gas, a crucial component of the chemical heritage of young exo-Earths.

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arXiv2019

A planetesimal orbiting within the debris disc around a white dwarf star

Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4min periodic variation in the strength and shape of the CaII emission line profiles originating from the debris disc around the white dwarf SDSSJ122859.93+104032.9. We interpret this short-period signal as the signature of a solid body held together by its internal strength.

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arXiv2019

Probing Unseen Planet Populations with Resolved Debris Disk Structures

Thousands of exoplanets have been found with many widely different from the ones in our own system. Despite the success, systems with planets in wide orbits analogous to those of Jupiter and Saturn, in the critical first several hundred million years of evolution, are virtually unexplored. Where are the low-mass planets that are hidden from our exoplanet detection techniques? Is our Solar System's planetary architecture unique? High-fidelity debris disk images offer an effective method to answer these questions. We can use them to study the formation and evolution of low-mass planets from youth to the age of the Solar System, providing snapshots of the complex processes and valuable insights into the formation and migration history of giant planets at wide orbits. This white paper focuses on resolving debris structures in thermal emission that is applicable to a large unbiased sample. We summarize the properties of the known debris disks and assess the feasibility of resolving them within our current and future infrared and millimeter facilities by adopting uniform criteria. JWST and the 9-m Origins Space Telescope are the most promising missions in the coming decades to resolve almost half of the known disks at high fidelity. Resolved debris structures at multiple wavelengths and at all stages of evolution would reveal the properties of unseen planet populations, enabling a unique demographic study of overall planet formation and evolution.

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arXiv2018

Science with an ngVLA: Debris disk structure and composition

Debris disks, comprised of planetsimal belts and the dust and gas produced by their mutual collisions, are the longest-lived phase of circumstellar disks. Typically much fainter in emission than protoplanetary disks, debris disks can be found in associations in which some members still host protoplanetary disks rich in gas as well as around main sequence stars of all ages. There are even classes of debris disks around AGB stars and white dwarfs. Typically, these disks have been studied through dust emission though an increasing number of young disks are now found to host some gas which may be remnants of the protoplanetary disks or second-generation gas. The ngVLA will have a particular niche in the study of the dust population of these disks, since the size distribution of the dust can be derived from the spectral index of the spectral energy distribution from the far-infrared to the centimetre. With the longest lever arms provided by ngVLA, the size distribution can be characterized, testing models of collisional evolution. The ngVLA could also be capable of measuring the quantities of HI and OH emission associated with the disks. HI and OH are by-products of the dissociation of water, which is expected to be a primary component of outgassed material from cometary collisions. A probe of the water content of extrasolar systems has important implications for the delivery of water to terrestrial, potentially Earth-like planets.

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arXiv2018

The KIC 8462852 Light Curve From 2015.75 to 2018.18 Shows a Variable Secular Decline

The star KIC 8462852 (Boyajian's Star) displays both fast dips of up to 20% on time scales of days, plus long-term secular fading by up to 19% on time scales from a year to a century. We report on CCD photometry of KIC 8462852 from 2015.75 to 2018.18, with 19,176 images making for 1,866 nightly magnitudes in BVRI. Our light curves show a continuing secular decline (by 0.023 +- 0.003 mags in the B-band) with three superposed dips with duration 120-180 days. This demonstrates that there is a continuum of dip durations from a day to a century, so that the secular fading is seen to be by the same physical mechanism as the short-duration Kepler dips. The BVRI light curves all have the same shape, with the slopes and amplitudes for VRI being systematically smaller than in the B-band by factors of 0.77 +- 0.05, 0.50 +- 0.05, and 0.31 +- 0.05. We rule out any hypothesis involving occultation of the primary star by any star, planet, solid body, or optically thick cloud. But these ratios are the same as that expected for ordinary extinction by dust clouds. This chromatic extinction implies dust particle sizes going down to ~0.1 micron, suggesting that this dust will be rapidly blown away by stellar radiation pressure, so the dust clouds must have formed within months. The modern infrared observations were taken at a time when there was at least 12.4% +- 1.3% dust coverage (as part of the secular dimming), and this is consistent with dimming originating in circumstellar dust.

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arXiv2018

Unlocking the secrets of the midplane gas and dust distribution in the young hybrid disc HD 141569

HD141569 is a pre-main sequence star with a disc uniquely placed between protoplanetary and debris discs, similar to the older `hybrid' type discs. This work aims to place the mass and spatial structure of the disc midplane in the context of the debris, hybrid and protoplanetary discs. We observed HD~141569 with ALMA in 1.3~mm continuum and $^{13}$CO (2-1). This is the first detection and image of the optically thin gas emission from the midplane of this disc. In continuum emission, we detect a combination of an unresolved central peak and a ring of millimetre emission at 220$\pm$10~au, slightly interior to one of the rings discovered in scattered light. The minimum dust mass of the ring is 0.13$\pm$0.02~M$_{\oplus}$ while the unresolved millimetre peak at the stellar location is predominantly thermal emission due to a minimum of 1.2$\pm$0.2~M$_{\oplus}$ of dust. $^{13}$CO is distributed asymmetrically around the stellar position with a peak at 1.1" distance and a P.A. of -33$^\circ$. The gas is detected as far as 220$\pm$10~au, a radial separation the same as that of the mm ring. Assuming optically thin emission and standard ISM abundances, we use our $^{13}$CO data to derive the gas mass in the disc of (6.0$\pm$0.9) $\times 10^{-4}~$M$_\odot$. Comparison to published $^{12}$CO data shows that $^{12}$CO is optically thick, explaining why estimates based on $^{12}$CO underestimated the gas mass.

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arXiv2017

Extinction and the Dimming of KIC 8462852

To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from October 2015 through December 2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 +\- 9.7 milli-mag/yr in the Swift wavebands, with amounts of 21.0 +\- 4.5 mmag in the groundbased B measurements, 14.0 +\- 4.5 mmag in V, and 13.0 +\- 4.5 in R, and a rate of 5.0 +\- 1.2 mmag/yr averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at >= 3 sigma by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous SED models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with T_{eff} = 7000 - 7100 K and A_V ~ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 microns, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (i.e., R_V >= 5, but not flat across all the bands) compared with the extinction law for the general ISM (R_V = 3.1), suggesting that the dimming arises from circumstellar material.

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arXiv2017

Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals

This paper uses observations of dusty debris discs, including a growing number of gas detections in these systems, to test our understanding of the origin and evolution of this gaseous component. It is assumed that all debris discs with icy planetesimals create second generation CO, C and O gas at some level, and the aim of this paper is to predict that level and assess its observability. We present a new semi-analytical equivalent of the numerical model of Kral et al. (2016) allowing application to large numbers of systems.That model assumes CO is produced from volatile-rich solid bodies at a rate that can be predicted from the debris disc's fractional luminosity. CO photodissociates rapidly into C and O that then evolve by viscous spreading. This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas. The radial location of the debris and stellar luminosity explain some non-detections, e.g. close-in debris (like HD 172555) is too warm to retain CO, while high stellar luminosities (like $η$ Tel) result in short CO lifetimes. We list the most promising targets for gas detections, predicting $>15$ CO detections and $>30$ CI detections with ALMA, and tens of CII and OI detections with future far-IR missions. We find that CO, CI, CII and OI gas should be modelled in non-LTE for most stars, and that CO, CI and OI lines will be optically thick for the most gas-rich systems. Finally, we find that radiation pressure, which can blow out CI around early-type stars, can be suppressed by self-shielding.

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arXiv2017

Circumstellar discs: What will be next?

This prospective chapter gives our view on the evolution of the study of circumstellar discs within the next 20 years from both observational and theoretical sides. We first present the expected improvements in our knowledge of protoplanetary discs as for their masses, sizes, chemistry, the presence of planets as well as the evolutionary processes shaping these discs. We then explore the older debris disc stage and explain what will be learnt concerning their birth, the intrinsic links between these discs and planets, the hot dust and the gas detected around main sequence stars as well as discs around white dwarfs.

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arXiv2016

The SHARDDS survey: first resolved image of the HD114082 debris disk in Lower Centaurus Crux with SPHERE

We present the first resolved image of the debris disk around the 16+/-8 Myr old star, HD 114082. The observation was made in the H-band using the SPHERE instrument. The star is at a distance of 92+/-6 pc in the Lower Centaurus Crux association. Using a Markov Chain Monte Carlo analysis, we determined that the debris is likely in the form of a dust ring with an inner edge of 27.7+2.8/-3.5 au, position angle -74+0.5/-1.5 deg, and an inclination with respect to the line of sight of 6.7+3.8/-0.4 deg. The disk imaged in scattered light has a surface density declining with radius like ~r^(-4), steeper than expected for grain blowout by radiation pressure. We find only marginal evidence (2 sigma) of eccentricity, and rule out planets more massive than 1.0 Mjup orbiting within 1 au of the ring's inner edge, since such a planet would have disrupted the disk. The disk has roughly the same fractional disk luminosity (Ldisk/L*=3.3x10^(-3)) as HR4796A and Beta Pictoris, however it was not detected by previous instrument facilities most likely because of its small angular size (radius~0.4"), low albedo (~0.2) and low scattering efficiency far from the star due to high scattering anisotropy. With the arrival of extreme adaptive optics systems like SPHERE and GPI, the morphology of smaller, fainter and more distant debris disks are being revealed, providing clues to planet-disk interactions in young protoplanetary systems.

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arXiv2016

A self-consistent model for the evolution of the gas produced in the debris disc of $β$ Pictoris

This paper presents a self-consistent model for the evolution of gas produced in the debris disc of $β$ Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous $α$ model. We present new gas observations of $β$ Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity $α$ > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in $β$ Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.

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arXiv2015

Next Generation Very Large Array Memo No. 6, Science Working Group 1: The Cradle of Life

This paper discusses compelling science cases for a future long-baseline interferometer operating at millimeter and centimeter wavelengths, like the proposed Next Generation Vary Large Array (ngVLA). We report on the activities of the Cradle of Life science working group, which focused on the formation of low- and high-mass stars, the formation of planets and evolution of protoplanetary disks, the physical and compositional study of Solar System bodies, and the possible detection of radio signals from extraterrestrial civilizations. We propose 19 scientific projects based on the current specification of the ngVLA. Five of them are highlighted as possible Key Science Projects: (1) Resolving the density structure and dynamics of the youngest HII regions and high-mass protostellar jets, (2) Unveiling binary/multiple protostars at higher resolution, (3) Mapping planet formation regions in nearby disks on scales down to 1 AU, (4) Studying the formation of complex molecules, and (5) Deep atmospheric mapping of giant planets in the Solar System. For each of these projects, we discuss the scientific importance and feasibility. The results presented here should be considered as the beginning of a more in-depth analysis of the science enabled by such a facility, and are by no means complete or exhaustive.

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arXiv2015

Double-ringed debris discs could be the work of eccentric planets: explaining the strange morphology of HD 107146

We investigate the general interaction between an eccentric planet and a coplanar debris disc of the same mass, using analytical theory and n-body simulations. Such an interaction could result from a planet-planet scattering or merging event. We show that when the planet mass is comparable to that of the disc, the former is often circularised with little change to its semimajor axis. The secular effect of such a planet can cause debris to apsidally anti-align with the planet's orbit (the opposite of what may be naively expected), leading to the counter-intuitive result that a low-mass planet may clear a larger region of debris than a higher-mass body would. The interaction generally results in a double-ringed debris disc, which is comparable to those observed in HD 107146 and HD 92945. As an example we apply our results to HD 107146, and show that the disc's morphology and surface brightness profile can be well-reproduced if the disc is interacting with an eccentric planet of comparable mass (~10-100 Earth masses). This hypothetical planet had a pre-interaction semimajor axis of 30 or 40 au (similar to its present-day value) and an eccentricity of 0.4 or 0.5 (which would since have reduced to ~0.1). Thus the planet (if it exists) presently resides near the inner edge of the disc, rather than between the two debris peaks as may otherwise be expected. Finally we show that disc self-gravity can be important in this mass regime and, whilst it would not affect these results significantly, it should be considered when probing the interaction between a debris disc and a planet.

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arXiv2015

A compact concentration of large grains in the HD142527 protoplanetary dust trap

A pathway to the formation of planetesimals, and eventually giant planets, may occur in concentrations of dust grains trapped in pressure maxima. Dramatic crescent-shaped dust concentrations have been seen in recent radio images at sub-mm wavelengths. These disk asymmetries could represent the initial phases of planet formation in the dust trap scenario, provided that grain sizes are spatially segregated. A testable prediction of azimuthal dust trapping is that progressively larger grains should be more sharply confined and furthermore the trapped grains should follow a distribution that is markedly different from the gas. However, gas tracers such as CO and the infrared emission from small grains are both very optically thick where the submm continuum originates, so observations have been unable to test the trapping predictions or to identify compact concentrations of larger grains required for planet formation by core-accretion. Here we report multifrequency observations of HD142527, from 34GHz to 700GHz, that reveal a compact concentration of ~cm-sized grains, with a few Earth masses, embedded in a large-scale crescent of ~mm-sized particles. The emission peaks at wavelengths shorter than ~1mm are optically thick and trace the temperature structure resulting from shadows cast by the inner regions. Given this temperature structure, we infer that the largest dust grains are concentrated in the 34 GHz clump. We conclude that dust trapping is efficient for approximately cm-sized grains and leads to enhanced concentrations, while the smaller grains largely reflect the gas distribution.

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