arXiv subjects

Simon Casassus

Publications and source records attributed to Simon Casassus.

At least 55 records · Page 3Linked to original sources

arXiv2019

From scattered-light to millimeter emission: A comprehensive view of the Gyr-old system of HD 202628 and its eccentric debris ring

We present here new observations of the eccentric debris ring surrounding the Gyr-old solar-type star HD 202628: at millimeter wavelengths with ALMA, at far-infrared wavelengths with \textit{Herschel}, and in scattered light with \textit{HST}. The ring inner edge is found to be consistent between ALMA and \textit{HST} data. As radiation pressure affects small grains seen in scattered-light, the ring appears broader at optical than at millimeter wavelengths. The best fit to the ring seen with ALMA has inner and outer edges at $143.1 \pm 1.7$ AU and $165.5 \pm 1.4$, respectively, and an inclination of $57.4^\circ \pm 0.4$ from face-on. The offset of the ring centre of symmetry from the star allows us to quantify its eccentricity to be $e=0.09_{-0.01}^{+0.02}$. This eccentric feature is also detected in low resolution \textit{Herschel}/PACS observations, under the form of a pericenter-glow. Combining the infrared and millimeter photometry, we retrieve a disk grain size distribution index of $\sim -3.4$, and therefore exclude in-situ formation of the inferred belt-shaping perturber, for which we provide new dynamical constraints. Finally, ALMA images show four point-like sources that exceed 100$\,μ$Jy, one of them being just interior to the ring. Although the presence of a background object cannot be excluded, we cannot exclude either that this source is circumplanetary material surrounding the belt-shaper, in which case degeneracies between its mass and orbital parameters could be lifted, allowing us to fully characterize such a distant planet in this mass and age regime for the very first time.

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arXiv2019

Upper limits on protolunar disc masses using ALMA observations of directly-imaged exoplanets

The Solar System gas giants are each surrounded by many moons, with at least 50 prograde satellites thought to have formed from circumplanetary material. Just like the Sun is not the only star surrounded by planets, extrasolar gas giants are likely surrounded by satellite systems. Here, we report on ALMA observations of four <40 Myr old stars with directly-imaged companions: PZ Tel, AB Pic, 51 Eri, and $κ$ And. Continuum emission at 1.3 mm is undetected for any of the systems. Since these are directly-imaged companions, there is knowledge of their temperatures, masses and locations. These allow for upper limits on the amount of circumplanetary dust to be derived from detailed radiative transfer models. These protolunar disc models consider two disc sizes: 0.4 and 0.04 times the exoplanet's Hill radius. The former is representative of hydrodynamic simulations of circumplanetary discs while the latter a case with significant radial drift of solids. The more compact case is also motivated by the semi-major axis of Callisto, enclosing Jupiter's Galilean satellites. All upper limits fall below the expected amount of dust required to explain regular satellite systems ($10^{-4}$ times the mass of their central planet). Upper limits are compared with viscous evolution and debris disc models. Our analysis suggests that the non detections can be interpreted as evidence of dust growth beyond metre sizes to form moonetesimals in timescales <10 Myr. This sample increases by 50% the number of ALMA non-detections of young companions available in the literature.

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arXiv2019

Dust unveils the formation of a mini-Neptune planet in a protoplanetary ring

Rings and radial gaps are ubiquitous in protoplanetary disks, yet their possible connection to planet formation is currently subject to intense debates. In principle, giant planet formation leads to wide gaps which separate the gas and dust mass reservoir in the outer disk, while lower mass planets lead to shallow gaps which are manifested mainly on the dust component. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the star HD169142, host to a prominent disk with deep wide gaps that sever the disk into inner and outer regions. The new ALMA high resolution images allow for the outer ring to be resolved as three narrow rings. The HD169142 disk thus hosts both the wide gaps trait of transition disks and a narrow ring system similar to those observed in the TW Hya and HL Tau systems. The mass reservoir beyond a deep gap can thus host ring systems. The observed rings are narrow in radial extent (width/radius of 1.5/57.3, 1.8/64.2 and 3.4/76.0, in au) and have asymmetric mutual separations: the first and middle ring are separated by 7 au while the middle and outermost ring are distanced by ~12 au. Using hydrodynamical modeling we found that a simple explanation, involving a single migrating low mass planet (10 M$_\oplus$), entirely accounts for such an apparently complex phenomenon. Inward migration of the planet naturally explains the ring's asymmetric mutual separation. The isolation of HD169142's outer rings thus allows a proof of concept to interpret the detailed architecture of the outer region of protoplanetary disks with low mass planet formation of mini-Neptune's size, i.e. as in the protosolar nebula.

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arXiv2019

Resolved observations of transition disks

Resolved observations are bringing new constraints on the origin of radial gaps in protoplanetary disks. The kinematics, sampled in detail in one case-study, are indicative of non-Keplerian flows, corresponding to warped structures and accretion which may both play a role in the development of cavities. Disk asymmetries seen in the radio continuum are being interpreted in the context of dust segregation via aerodynamic trapping. We summarise recent observational progress, and also describe prospects for improvements in the near term.

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arXiv2019

Cooling in the shade of warped transition disks

The mass of the gaseous reservoir in young circumstellar disks is a crucial initial condition for the formation of planetary systems, but estimates vary by orders of magnitude. In some disks with resolvable cavities, sharp inner disk warps cast two-sided shadows on the outer rings; can the cooling of the gas as it crosses the shadows bring constraints on its mass? The finite cooling timescale should result in dust temperature decrements shifted ahead of the optical/IR shadows in the direction of rotation. However, some systems show temperature drops, while others do not. The depth of the drops and the amplitude of the shift depend on the outer disk surface density Sigma through the extent of cooling during the shadow crossing time, and also on the efficiency of radiative diffusion. These phenomena may bear observational counterparts, which we describe with a simple one-dimensional model. An application to the HD142527 disk suggests an asymmetry in its shadows, and predicts a >~10deg shift for a massive gaseous disk, with peak Sigma > 8.3 g/cm2. Another application to the DoAr44 disk limits the peak surface density to Sigma < 13g/cm2

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arXiv2019

The Ophiuchus DIsk Survey Employing ALMA (ODISEA): Disk Dust Mass Distributions across Protostellar Evolutionary Classes

As protostars evolve from optically faint / infrared bright (Class I) sources to optically bright / infrared faint (Class II) the solid material in their surrounding disks accumulates into planetesimals and protoplanets. The nearby, young Ophiuchus star-forming region contains hundreds of protostars in a range of evolutionary states. Using the Atacama Large Millimeter Array to observe their millimeter continuum emission, we have measured masses of, or placed strong upper limits on, the dust content of 279 disks. The masses follow a log-normal distribution with a clear trend of decreasing mass from less to more evolved protostellar infrared class. The (logarithmic) mean Class I disk mass, M = 3.8 M_Earth, is about 5 times greater than the mean Class II disk mass, but the dispersion in each class is so high, sigma(logM) ~ 0.8-1, that there is a large overlap between the two distributions. The disk mass distribution of flat-spectrum protostars lies in between Classes I and II. In addition, three Class III sources with little to no infrared excess are detected with low disk masses, M ~ 0.3 M_Earth. Despite the clear trend of decreasing disk mass with protostellar evolutionary state in this region, a comparison with surveys of Class II disks in other regions shows that masses do not decrease monotonically with age. This suggests that the cloud-scale environment may determine the initial disk mass scale or that there is substantial dust regeneration after 1 Myr.

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arXiv2019

Dust traps in the protoplanetary disc MWC 758: two vortices produced by two giant planets?

Resolved ALMA and VLA observations indicate the existence of two dust traps in the protoplanetary disc MWC 758. By means of 2D gas+dust hydrodynamical simulations post-processed with 3D dust radiative transfer calculations, we show that the spirals in scattered light, the eccentric, asymmetric ring and the crescent-shaped structure in the (sub)millimetre can all be caused by two giant planets: a 1.5-Jupiter mass planet at 35 au (inside the spirals) and a 5-Jupiter mass planet at 140 au (outside the spirals). The outer planet forms a dust-trapping vortex at the inner edge of its gap (at ~85 au), and the continuum emission of this dust trap reproduces the ALMA and VLA observations well. The outer planet triggers several spiral arms which are similar to those observed in polarised scattered light. The inner planet also forms a vortex at the outer edge of its gap (at ~50 au), but it decays faster than the vortex induced by the outer planet, as a result of the disc's turbulent viscosity. The vortex decay can explain the eccentric inner ring seen with ALMA as well as the low signal and larger azimuthal spread of this dust trap in VLA observations. Finding the thermal and kinematic signatures of both giant planets could verify the proposed scenario.

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arXiv2019

Planet formation: The case for large efforts on the computational side

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade.

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arXiv2019

The ice composition in the disk around V883 Ori revealed by its stellar outburst

Complex organic molecules (COMs), which are the seeds of prebiotic material and precursors of amino acids and sugars, form in the icy mantles of circumstellar dust grains but cannot be detected remotely unless they are heated and released to the gas phase. Around solar-mass stars, water and COMs only sublimate in the inner few au of the disk, making them extremely difficult to spatially resolve and study. Sudden increases in the luminosity of the central star will quickly expand the sublimation front (so-called snow line) to larger radii, as seen previously in the FU Ori outburst of the young star V883 Ori. In this paper, we take advantage of the rapid increase in disk temperature of V883 Ori to detect and analyze five different COMs, methanol, acetone, acetonitrile, acetaldehyde, and methyl formate, in spatially-resolved submillimeter observations. The COMs abundances in V883 Ori is in reasonable agreement with cometary values. This result suggests that outbursting young stars can provide a special opportunity to study the ice composition of material directly related to planet formation.

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arXiv2019

Modelling the spinning dust emission from LDN 1780

We study the anomalous microwave emission (AME) in the Lynds Dark Nebula (LDN) 1780 on two angular scales. Using available ancillary data at an angular resolution of 1 degree, we construct an SED between 0.408 GHz to 2997 GHz. We show that there is a significant amount of AME at these angular scales and the excess is compatible with a physical spinning dust model. We find that LDN 1780 is one of the clearest examples of AME on 1 degree scales. We detected AME with a significance > 20$σ$. We also find at these angular scales that the location of the peak of the emission at frequencies between 23-70 GHz differs from the one on the 90-3000 GHz map. In order to investigate the origin of the AME in this cloud, we use data obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) that provides 2 arcmin resolution at 30 GHz. We study the connection between the radio and IR emissions using morphological correlations. The best correlation is found to be with MIPS 70$μ$m, which traces warm dust (T$\sim$50K). Finally, we study the difference in radio emissivity between two locations within the cloud. We measured a factor $\approx 6$ of difference in 30 GHz emissivity. We show that this variation can be explained, using the spinning dust model, by a variation on the dust grain size distribution across the cloud, particularly changing the carbon fraction and hence the amount of PAHs.

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arXiv2018

Cm-wavelength observations of MWC758: resolved dust trapping in a vortex

The large crescents imaged by ALMA in transition disks suggest that azimuthal dust trapping concentrates the larger grains, but centimetre-wavelengths continuum observations are required to map the distribution of the largest observable grains. A previous detection at ~1cm of an unresolved clump along the outer ring of MWC758 (Clump1), and buried inside more extended sub-mm continuum, motivates followup VLA observations. Deep multiconfiguration integrations reveal the morphology of Clump 1 and additional cm-wave components which we characterize via comparison with a deconvolution of recent 342GHz data (~1mm). Clump1, which concentrates ~1/3 of the whole disk flux density at ~1cm, is resolved as a narrow arc with a deprojected aspect ratio Chi>5.6, and with half the azimuthal width than at 342 GHz. The spectral trends in the morphology of Clump1 are quantitatively consistent with the Lyra-Lin prescriptions for dust trapping in an anticyclonic vortex, provided with porous grains (f~0.2+-0.2) in a very elongated (Chi~14+-3) and cold (T~23+-2K) vortex. The same prescriptions constrain the turbulence parameter alpha and the gas surface density Sigma_g through log10( alpha x Sigma_g /g/cm2)~-2.3+-0.4, thus requiring values for Sigma_g larger than a factor of a few compared to that reported in the literature from the CO isotopologues, if alpha <~ 1E-3. Such physical conditions imply an appreciably optically thick continuum even at cm-wavelengths (tau(33GHz)~0.2). A secondary and shallower peak at 342GHz is about twice fainter relative to Clump1 at 33GHz. Clump2 appears to be less efficient at trapping large grains.

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arXiv2018

Disks ARound TTauri Stars with Sphere (DARTTS-S) I: Sphere / IRDIS Polarimetric Imaging of 8 prominent TTauri Disks

We present the first part of our DARTTS-S (Disks ARound TTauri Stars with SPHERE) survey: Observations of 8 TTauri stars which were selected based on their strong (sub-)mm excesses using SPHERE / IRDIS polarimetric differential imaging (PDI) in the J and H bands. All observations successfully detect the disks, which appear vastly different in size, from $\approx$80 au in scattered light to $>$400 au, and display total polarized disk fluxes between 0.06% and 0.89% of the stellar flux. For five of these disks, we are able to determine the three-dimensional structure and the flaring of the disk surface, which appears to be relatively consistent across the different disks, with flaring exponents $α$ between $\approx$1.1 and $\approx$1.6. We also confirm literature results w.r.t. the inclination and position angle of several of our disk, and are able to determine which side is the near side of the disk in most cases. While there is a clear trend of disk mass with stellar ages ($\approx$1 Myr to $>$10 Myr), no correlations of disk structures with age were found. There are also no correlations with either stellar mass or sub-mm flux. We do not detect significant differences between the J and H bands. However, we note that while a high fraction (7/8) of the disks in our sample show ring-shaped sub-structures, none of them display spirals, in contrast to the disks around more massive Herbig Ae/Be stars, where spiral features are common.

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arXiv2018

The Ophiuchus DIsc Survey Employing ALMA (ODISEA) - I : project description and continuum images at 28 au resolution

We introduce the Ophiuchus DIsc Survey Employing ALMA (ODISEA), a project aiming to study the entire population of Spitzer-selected protoplanetary discs in the Ophiuchus Molecular Cloud (~300 objects) from both millimeter continuum and CO isotopologues data. Here we present 1.3 mm/230 GHz continuum images of 147 targets at 0.2" (28 au) resolution and a typical rms of 0.15 mJy. We detect a total of 133 discs, including the individual components of 11 binary systems and 1 triple system. Fifty-three of these discs are spatially resolved. We find clear substructures (inner cavities, rings, gaps, and/or spiral arms) in 8 of the sources and hints of such structures in another 4 discs. We construct the disc luminosity function for our targets and perform comparisons to other regions. A simple conversion between flux and dust mass (adopting standard assumptions) indicates that all discs detected at 1.3 mm are massive enough to form one or more rocky planets. In contrast, only ~50 discs (~1/3 of the sample) have enough mass in the form of dust to form the canonical 10 M_Earth core needed to trigger runaway gas accretion and the formation of gas giant planets, although the total mass of solids already incorporated into bodies larger than cm scales is mostly unconstrained. The distribution in continuum disc sizes in our sample is heavily weighted towards compact discs: most detected discs have radii < 15 au, while only 23 discs (~15% of the targets) have radii > 30 au.

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arXiv2018

Observability of planet-disc interactions in CO kinematics

Empirical evidence of planets in gas-rich circumstellar discs is required to constrain giant planet formation theories. Here we study the kinematic patterns which arise from planet-disc interactions and their observability in CO rotational emission lines. We perform three-dimensional hydrodynamical simulations of single giant planets, and predict the emergent intensity field with radiative transfer. Pressure gradients at planet-carved gaps, spiral wakes and vortices bear strong kinematic counterparts. The iso-velocity contours in the CO(2-1) line centroids $v_\circ$ reveal large-scale perturbations, corresponding to abrupt transitions from below sub-Keplerian to super-Keplerian rotation along with radial and vertical flows. The increase in line optical depth at the edge of the gap also modulates $v_\circ$, but this is a mild effect compared to the dynamical imprint of the planet-disc interaction. The large-scale deviations from the Keplerian rotation thus allow the planets to be indirectly detected via the first moment maps of molecular gas tracers, at ALMA angular resolutions. The strength of these deviations depends on the mass of the perturber. This initial study paves the way to eventually determine the mass of the planet by comparison with more detailed models.

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arXiv2018

The Eccentric Cavity, Triple Rings, Two-Armed Spirals, and Double Clumps of the MWC 758 Disk

Spatially resolved structures in protoplanetary disks hint at unseen planets. Previous imaging observations of the transitional disk around MWC 758 revealed an inner cavity, a ring-like outer disk, emission clumps, and spiral arms, all possibly generated by companions. We present ALMA dust continuum observations of MWC 758 at 0.87 millimeter (mm) wavelength with 43$\times$39 mas angular resolution (6.9$\times$6.2 AU) and 20 $μ$Jy beam$^{-1}$ rms. The central sub-mm emission cavity is revealed to be eccentric; once deprojected, its outer edge can be well-fitted by an ellipse with an eccentricity of 0.1 and one focus on the star. The broad ring-like outer disk is resolved into three narrow rings with two gaps in between. The outer two rings tentatively show the same eccentricity and orientation as the innermost ring bounding the inner cavity. The two previously known dust emission clumps are resolved in both the radial and azimuthal directions, with radial widths equal to $\sim$4$\times$ the local scale height. Only one of the two spiral arms previously imaged in near-infrared (NIR) scattered light is revealed in ALMA dust emission, at a slightly larger stellocentric distance owing to projection effects. We also submit evidence of disk truncation at $\sim$100 AU based on comparing NIR imaging observations with models. The spirals, the north clump, and the truncated disk edge are all broadly consistent with the presence of one companion exterior to the spirals at roughly 100 AU.

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arXiv2018

An inner warp in the DoAr 44 T Tauri transition disk

Optical/IR images of transition disks (TDs) have revealed deep intensity decrements in the rings of HAeBes HD142527 and HD100453, that can be interpreted as shadowing from sharply tilted inner disks, such that the outer disks are directly exposed to stellar light. Here we report similar dips in SPHERE+IRDIS differential polarized imaging (DPI) of TTauri DoAr44. With a fairly axially symmetric ring in the submm radio continuum, DoAr44 is likely also a warped system. We constrain the warp geometry by comparing radiative transfer predictions with the DPI data in H band (Q_ϕ(H)) and with a re-processing of archival 336GHz ALMA observations. The observed DPI shadows have coincident radio counterparts, but the intensity drops are much deeper in Q_ϕ(H) (~88%), compared to the shallow drops at 336GHz (~24%). Radiative transfer predictions with an inner disk tilt of ~30+-5deg approximately account for the observations. ALMA long-baseline observations should allow the observation of the warped gas kinematics inside the cavity of DoAr44.

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arXiv2018

Circumbinary, not transitional: On the spiral arms, cavity, shadows, fast radial flows, streamers and horseshoe in the HD142527 disc

We present 3D hydrodynamical models of the HD142527 protoplanetary disc, a bright and well studied disc that shows spirals and shadows in scattered light around a 100 au gas cavity, a large horseshoe dust structure in mm continuum emission, together with mysterious fast radial flows and streamers seen in gas kinematics. By considering several possible orbits consistent with the observed arc, we show that all of the main observational features can be explained by one mechanism - the interaction between the disc and the observed binary companion. We find that the spirals, shadows and horseshoe are only produced in the correct position angles by a companion on an inclined and eccentric orbit approaching periastron - the 'red' family from Lacour et al. (2016). Dust-gas simulations show radial and azimuthal concentration of dust around the cavity, consistent with the observed horseshoe. The success of this model in the HD142527 disc suggests other mm-bright transition discs showing cavities, spirals and dust asymmetries may also be explained by the interaction with central companions.

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arXiv2017

The ALMA early science view of FUor/EXor objects - V: continuum disc masses and sizes

Low-mass stars build a significant fraction of their total mass during short outbursts of enhanced accretion known as FUor and EXor outbursts. FUor objects are characterized by a sudden brightening of ~5 magnitudes at visible wavelengths within one year and remain bright for decades. EXor objects have lower amplitude outbursts on shorter timescales. Here we discuss a 1.3 mm ALMA mini-survey of eight outbursting sources (three FUor, four EXor, and the borderline object V1647 Ori) in the Orion Molecular Cloud. While previous papers in this series discuss the remarkable molecular outflows observed in the three FUor objects and V1647 Ori, here we focus on the continuum data and the differences and similarities between the FUor and EXor populations. We find that FUor discs are significantly more massive (~80-600 M_JUP) than the EXor objects (~0.5-40 M_JUP ). We also report that the EXor sources lack the prominent outflows seen in the FUor population. Even though our sample is small, the large differences in disc masses and outflow activity suggest that the two types of objects represent different evolutionary stages. The FUor sources seem to be rather compact (Rc < 20-40 au) and to have a smaller characteristic radius for a given disc mass when compared to T Tauri stars. V1118 Ori, the only known close binary systemin our sample, is shown to host a disc around each one of the stellar components. The disc around HBC 494 is asymmetric, hinting at a structure in the outer disc or the presence of a second disc.

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