arXiv subjects

Quentin Kral

Publications and source records attributed to Quentin Kral.

At least 55 records · Page 3Linked to original sources

arXiv2018

Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs

Despite being $>10$Myr, there are $\sim$10 debris discs with as much CO gas as in protoplanetary discs. Such discs have been assumed to be "hybrid", i.e., with secondary dust but primordial gas. Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C$^0$) that shields CO from photodissociating; i.e., these could be "shielded secondary discs". New ALMA observations are presented of HD131835 that detect $\sim 3 \times 10^{-3}$ M$_\oplus$ of C$^0$, the majority 40-200au from the star, in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO, C and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow (low $α$ parameter) and/or the CO production rate is high. Shielding by C$^0$ may also cause the CO content to reach levels at which CO self-shields, and the gas disc may become massive enough to affect the dust evolution. Application to the HD 131835 observations shows these can be explained if $α\sim 10^{-3}$; an inner cavity in C$^0$ and CO may also mean the system has yet to reach steady state. Application to other debris discs with high CO content finds general agreement for $α=10^{-3}$ to $0.1$. The shielded secondary nature of these gas discs can be tested by searching for C$^0$, as well as CN, N$_2$ and CH$^{+}$, which are also expected to be shielded by C$^0$.

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arXiv2018

Deep ALMA Search for CO Gas in the HD 95086 Debris Disc

One of the defining properties of debris discs compared to protoplanetary discs used to be their lack of gas, yet small amounts of gas have been found around an increasing number of debris discs in recent years. These debris discs found to have gas tend to be both young and bright. In this paper we conduct a deep search for CO gas in the system HD 95086 - a 17 Myr old, known planet host that also has a debris disc with a high fractional luminosity of $1.5\times10^{-3}$. Using the Atacama Large Millimeter/submillimeter Array (ALMA) we search for CO emission lines in bands 3, 6 and 7. By implementing a spectro-spatial filtering technique, we find tentative evidence for CO $J$=2-1 emission in the disc located at a velocity, 8.5$\pm$0.2 km s$^{-1}$, consistent with the radial velocity of the star. The tentative detection suggests that the gas on the East side of the disc is moving towards us. In the same region where continuum emission is detected, we find an integrated line flux of 9.5$\pm$3.6 mJy km s$^{-1}$, corresponding to a CO mass of (1.4-13)$\times10^{-6}$ M$_\oplus$. Our analysis confirms that the level of gas present in the disc is inconsistent with the presence of primordial gas in the system and is consistent with second generation production through the collisional cascade.

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arXiv2018

Evolution of protoplanetary disks from their taxonomy in scattered light: spirals, rings, cavities, and shadows

The variety of observed protoplanetary disks in polarimetric light motivates a taxonomical study to constrain their evolution and establish the current framework of this type of observations. We classified 58 disks with available polarimetric observations into six major categories (Ring, Spiral, Giant, Rim, Faint, and Small disks) based on their appearance in scattered light. We re-calculated the stellar and disk properties from the newly available GAIA DR2 and related these properties with the disk categories. More than a half of our sample shows disk sub-structures. For the remaining sources, the absence of detected features is due to their faintness, to their small size, or to the disk geometry. Faint disks are typically found around young stars and typically host no cavity. There is a possible dichotomy in the near-IR excess of sources with spiral-disks (high) and ring-disks (low). Like spirals, shadows are associated with a high near-IR excess. If we account for the pre-main sequence evolutionary timescale of stars with different mass, spiral arms are likely associated to old disks. We also found a loose, shallow declining trend for the disk dust mass with time. Protoplanetary disks may form sub-structures like rings very early in their evolution but their detectability in scattered light is limited to relatively old sources (more than 5 Myr) where the recurrently detected disk cavities allow to illuminate the outer disk. The shallow decrease of disk mass with time might be due to a selection effect, where disks observed thus far in scattered light are typically massive, bright transition disks with longer lifetime than most disks. Our study points toward spirals and shadows being generated by planets of fraction-to-few Jupiter masses that leave their (observed) imprint on both the inner disk near the star and the outer disk cavity.

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arXiv2018

Using warm dust to constrain unseen planets

Cold outer debris belts orbit a significant fraction of stars, many of which are planet-hosts. Radiative forces from the star lead to dust particles leaving the outer belts and spiralling inwards under Poynting-Robertson drag. We present an empirical model fitted to N-body simulations that allows the fate of these dust particles when they encounter a planet to be rapidly calculated. High mass planets eject most particles, whilst dust passes low mass planets relatively unperturbed. Close-in, high mass planets (hot Jupiters) are best at accreting dust. The model predicts the accretion rate of dust onto planets interior to debris belts, with mass accretions rates of up to hundreds of kilograms per second predicted for hot Jupiters interior to outer debris belts, when collisional evolution is also taken into account. The model can be used to infer the presence and likely masses of as yet undetected planets in systems with outer belts. The non-detection of warm dust with the Large Binocular Telescope Interferometer (LBTI) around Vega could be explained by the presence of a single Saturn mass planet, or a chain of lower mass planets. Similarly, the detection of warm dust in such systems implies the absence of planets above a quantifiable level, which can be lower than similar limits from direct imaging. The level of dust detected with LBTI around beta Leo can be used to rule out the presence of planets more massive than a few Saturn masses outside of ~5au.

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arXiv2018

Cometary impactors on the TRAPPIST-1 planets can destroy all planetary atmospheres and rebuild secondary atmospheres on planets f, g, h

The TRAPPIST-1 system is unique in that it has a chain of seven terrestrial Earth-like planets located close to or in its habitable zone. In this paper, we study the effect of potential cometary impacts on the TRAPPIST-1 planets and how they would affect the primordial atmospheres of these planets. We consider both atmospheric mass loss and volatile delivery with a view to assessing whether any sort of life has a chance to develop. We ran N-body simulations to investigate the orbital evolution of potential impacting comets, to determine which planets are more likely to be impacted and the distributions of impact velocities. We consider three scenarios that could potentially throw comets into the inner region (i.e within 0.1au where the seven planets are located) from an (as yet undetected) outer belt similar to the Kuiper belt or an Oort cloud: Planet scattering, the Kozai-Lidov mechanism and Galactic tides. For the different scenarios, we quantify, for each planet, how much atmospheric mass is lost and what mass of volatiles can be delivered over the age of the system depending on the mass scattered out of the outer belt. We find that the resulting high velocity impacts can easily destroy the primordial atmospheres of all seven planets, even if the mass scattered from the outer belt is as low as that of the Kuiper belt. However, we find that the atmospheres of the outermost planets f, g and h can also easily be replenished with cometary volatiles (e.g. $\sim$ an Earth ocean mass of water could be delivered). These scenarios would thus imply that the atmospheres of these outermost planets could be more massive than those of the innermost planets, and have volatiles-enriched composition.

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arXiv2018

Scattering of exocomets by a planet chain: exozodi levels and the delivery of cometary material to inner planets

Exocomets scattered by planets have been invoked to explain observations in multiple contexts, including the frequently found near- and mid-infrared excess around nearby stars arising from exozodiacal dust. Here we investigate how the process of inward scattering of comets originating in an outer belt, is affected by the architecture of a planetary system, to determine whether this could lead to observable exozodi levels or deliver volatiles to inner planets. Using N-body simulations, we model systems with different planet mass and orbital spacing distributions in the 1-50 AU region. We find that tightly packed ($Δa_\mathrm{p}<20 R_\mathrm{Hill,m}$) low mass planets are the most efficient at delivering material to exozodi regions (5-7% of scattered exocomets end up within 0.5 AU at some point), although the exozodi levels do not vary by more than a factor of ~7 for the architectures studied here. We suggest that emission from scattered dusty material in between the planets could provide a potential test for this delivery mechanism. We show that the surface density of scattered material can vary by two orders of magnitude (being highest for systems of low mass planets with medium spacing), whilst the exozodi delivery rate stays roughly constant, and that future instruments such as JWST could detect it. In fact for $η$ Corvi, the current Herschel upper limit rules out the scattering scenario by a chain of $\lesssim$30 M$_\oplus$ planets. Finally, we show that exocomets could be efficient at delivering cometary material to inner planets (0.1-1% of scattered comets are accreted per inner planet). Overall, the best systems at delivering comets to inner planets are the ones that have low mass outer planets and medium spacing ($\sim20 R_\mathrm{Hill,m}$).

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arXiv2018

Exoplanet recycling in massive white-dwarf debris discs

Several tens of white dwarfs are known to host circumstellar discs of dusty debris, thought to arise from the tidal disruption of rocky bodies originating in the star's remnant planetary system. This paper investigates the evolution of such discs if they are very massive, as may be the case if their progenitor was a terrestrial planet, moon, or dwarf planet. Assuming the discs are physically thin and flat, like Saturn's rings, their evolution is governed by Poynting-Robertson drag or viscous spreading, where the disc's effective viscosity is due to self-gravity wakes. For discs with masses >10^26 g, located in the outer parts of the tidal disruption zone, viscous spreading dominates the evolution, and mass is transported both in- and outwards. When outwards-spreading material flows beyond the Roche limit, it coagulates into new (minor) planets in a process analogous to the ongoing formation of moonlets at the outer edge of Saturn's rings. The newly formed bodies migrate outwards by exchanging angular momentum with the disc and coalesce into larger objects through mutual collisions. Eventually, the disc's Roche-limit overflow recycles tens of percent of the original disc mass; most ends up in a single large body near 2:1 mean-motion resonance with the disc's outer edge. Hence, the recycling of a tidally disrupted super-Earth, for example, could yield an Earth-mass planet on a ~10-h orbit, located in the habitable zone for 2-to-10-Gyr-old white dwarfs. The recycling process also creates a population of smaller bodies just outside the Roche limit, which may explain the minor planets recently postulated to orbit WD 1145+017.

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arXiv2018

Exozodiacal clouds: Hot and warm dust around main sequence stars

A warm/hot dust component (at temperature $>$ 300K) has been detected around $\sim$ 20% of stars. This component is called "exozodiacal dust" as it presents similarities with the zodiacal dust detected in our Solar System, even though its physical properties and spatial distribution can be significantly different. Understanding the origin and evolution of this dust is of crucial importance, not only because its presence could hamper future detections of Earth-like planets in their habitable zones, but also because it can provide invaluable information about the inner regions of planetary systems. In this review, we present a detailed overview of the observational techniques used in the detection and characterisation of exozodiacal dust clouds ("exozodis") and the results they have yielded so far, in particular regarding the incidence rate of exozodis as a function of crucial parameters such as stellar type and age, or the presence of an outer cold debris disc. We also present the important constraints that have been obtained, on dust size distribution and spatial location, by using state-of-the-art radiation transfer models on some of these systems. Finally, we investigate the crucial issue of how to explain the presence of exozodiacal dust around so many stars (regardless of their ages) despite the fact that such dust so close to its host star should disappear rapidly due to the coupled effect of collisions and stellar radiation pressure. Several potential mechanisms have been proposed to solve this paradox and are reviewed in detail in this paper. The review finishes by presenting the future of this growing field.

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arXiv2017

Transient events in bright debris discs: Collisional avalanches revisited

A collisional avalanche is set off by the breakup of a large planetesimal, releasing small unbound grains that enter a debris disc located further away from the star, triggering there a collisional chain reaction that can potentially create detectable transient structures. We explore this mechanism, using for the first time a code coupling dynamical and collisional evolutions, and investigate if avalanches could explain the short-term luminosity variations observed in some extremely bright discs. We consider two set-ups: a cold disc case, with a dust release at 10au and an outer disc extending from 50 to 120au, and a warm disc case with the release at 1au and a 5-12au outer disc. We find that avalanches could leave detectable structures on resolved images, for both cold and warm disc cases, in discs with optical depth $τ$ of a few $10^{-3}$, provided that large dust masses ($\gtrsim$10$^{20}$-5$\times$10$^{22}$g) are initially released. The integrated photometric excess due to an avalanche is limited, less than 10% for these released dust masses, peaking in the mid-IR and becoming insignificant beyond $\sim$40-50$μ$m. Contrary to earlier studies, we do not obtain stronger avalanches when increasing $τ$ to higher values. Likewise, we do not observe a significant luminosity deficit, as compared to the pre-avalanche level, after the passage of the avalanche. These two results concur to make avalanches an unlikely explanation for the sharp luminosity drops observed in some extremely bright debris discs. The ideal configuration for observing an avalanche would be a two-belt structure, with an inner belt of fractional luminosity >10$^{-4}$ where breakups of massive planetesimals occur, and a more massive outer belt, with $τ$ of a few $10^{-3}$, into which the avalanche chain reaction develops and propagates.

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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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arXiv2017

Near-infrared scattered light properties of the HR4796A dust ring: a measured scattering phase function from 13.6deg to 166.6deg

HR4796A is surrounded by a debris disc, observed in scattered light as an inclined ring. Past observations raised several questions. First, a strong brightness asymmetry detected in polarized reflected light recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date. We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR4796A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise. We determine empirically the scattering phase function of the dust in the H band from 13.6deg to 166.6deg. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30deg. We analyse the reflectance spectra of the disc from the 0.95 to 1.6 microns, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects. We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 microns. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50deg points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust.

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arXiv2016

Review of gas and dust in debris discs

This proceeding summarises a talk given on the state-of-the-art of debris disc modelling. We first review the basics of debris disc physics, which is followed by a short overview of the state-of-the-art in terms of modelling dust and gas in debris disc systems.

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arXiv2016

Planet Formation Imager (PFI): science vision and key requirements

The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to about 100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.

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arXiv2016

The unseen planets of double belt debris disk systems

The gap between two component debris disks is often taken to be carved by intervening planets scattering away the remnant planetesimals. We employ N-body simulations to determine how the time needed to clear the gap depends on the location of the gap and the mass of the planets. We invert this relation, and provide an equation for the minimum planet mass, and another for the expected number of such planets, that must be present to produce an observed gap for a star of a given age. We show how this can be combined with upper limits on the planetary system from direct imaging non-detections (such as with GPI or SPHERE) to produce approximate knowledge of the planetary system.

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arXiv2016

The magnetorotational instability in debris-disc gas

Debris discs are commonly swathed in gas which can be observed in UV, in fine structure lines in FIR, and in resolved maps of CO emission. Carbon and oxygen are overabundant in such gas, but it is severely depleted in hydrogen. As a consequence, its ionisation fraction is remarkably high, suggesting magnetohydrodynamic (MHD) processes may be important. In particular, the gas may be subject to the magnetorotational instability (MRI), and indeed recent modelling of $β$ Pictoris requires an anomalous viscosity to explain the gas's observed radial structure. In this paper we explore the possibility that the MRI is active in debris-disc gas and responsible for the observed mass transport. We find that non-ideal MHD and dust-gas interactions play a subdominant role, and that linear instability is viable at certain radii. However, owing to low gas densities, the outer parts of the disc could be stabilised by a weak ambient magnetic field, though it is difficult to constrain such a field. Even if the MRI is stabilised by too strong a field, a magnetocentrifugal wind may be launched in its place and this could lead to equivalent (non-turbulent) transport. Numerical simulations of the vertically stratified MRI in conditions appropriate to the debris disc gas should be able to determine the nature of the characteristic behaviour at different radii, and decide on the importance of the MRI (and MHD more generally) on the evolution of these discs.

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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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arXiv2016

Effects of disc asymmetries on astrometric measurements - Can they mimic planets?

Astrometry covers a parameter space that cannot be reached by RV or transit methods to detect terrestrial planets on wide orbits. In addition, high accuracy astrometric measurements are necessary to measure the inclination of the planet's orbits. Here we investigate the principles of an artefact of the astrometric approach. Namely, the displacement of the photo-centre due to inhomogeneities in a dust disc around the parent star. Indeed, theory and observations show that circumstellar discs can present strong asymmetries. We model the pseudo-astrometric signal caused by these inhomogeneities, asking whether a dust clump in a disc can mimic the astrometric signal of an Earth-like planet. We show that these inhomogeneities cannot be neglected when using astrometry to find terrestrial planets. We provide the parameter space for which these inhomogeneities can affect the astrometric signals but still not be detected by mid-IR observations. We find that a small cross section of dust corresponding to a cometary mass object is enough to mimic the astrometric signal of an Earth-like planet. Astrometric observations of protoplanetary discs to search for planets can also be affected by the presence of inhomogeneities. Some further tests are given to confirm whether an observation is a real planet astrometric signal or an impostor. Eventually, we also study the case where the cross section of dust is high enough to provide a detectable IR-excess and to have a measurable photometric displacement by actual instruments such as Gaia, IRAC or GRAVITY. We suggest a new method, which consists in using astrometry to quantify asymmetries (clumpiness) in inner debris discs that cannot be otherwise resolved.

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