arXiv subjects

S. Casassus

Publications and source records attributed to S. Casassus.

At least 37 records · Page 2Linked to original sources

arXiv2017

A 80 au cavity in the disk around HD 34282

Context: Large cavities in disks are important testing grounds for the mechanisms proposed to drive disk evolution and dispersion, such as dynamical clearing by planets and photo-evaporation. Aims: We aim to resolve the large cavity in the disk around HD 34282, such as has been predicted by previous studies modeling the spectral energy distribution Methods: Using ALMA band 7 observations we study HD 34282 with a spatial resolution of 0.10\arcsec x 0.17\arcsec at 345 GHz. Results: We resolve the disk around HD 34282 into a ring between 0.24\arcsec and 1.15\arcsec (78 and 374 au adopting a distance of 325 pc). The emission in this ring shows azimuthal asymmetry centered at a radial distance of 0.46\arcsec and a position angle of 135 degrees and an azimuthal FWHM of 51 degrees. We detect CO emission both inside the disk cavity and as far out as 2.7 times the radial extent of the dust emission. Conclusions: Both the large disk cavity and the azimuthal structure in the disk around HD 34282 can be explained by the presence of a 50 jupiter mass brown dwarf companion at a separation of ~ 0.1\arcsec.

astro-ph.SR↗

arXiv2016

The ALMA Early Science View of FUor/EXor objects. II. The Very Wide Outflow Driven by HBC 494

We present Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle-2 observations of the HBC 494 molecular outflow and envelope. HBC 494 is an FU Ori-like object embedded in the Orion A cloud and is associated with the reflection nebulae Re50 and Re50N. We use $^{12}$CO, $^{13}$CO and C$^{18}$O spectral line data to independently describe the outflow and envelope structures associated with HBC 494. The moment-1 map of the $^{12}$CO emission shows the widest outflow cavities in a Class I object known to date (opening angle $\sim$ 150$^{^{\circ}}$). The morphology of the wide outflow is likely to be due to the interaction between winds originating in the inner disc and the surrounding envelope. The low-velocity blue- and red-shifted $^{13}$CO and C$^{18}$O emission trace the rotation and infall motion of the circumstellar envelope. Using molecular line data and adopting standard methods for correcting optical depth effects, we estimate its kinematic properties, including an outflow mass on the order of 10$^{-1}$ M$_{\odot}$. Considering the large estimated outflow mass for HBC 494, our results support recent theoretical work suggesting that wind-driven processes might dominate the evolution of protoplanetary discs via energetic outflows.

astro-ph.SR↗

arXiv2016

Atomic Gas in Debris Discs

We have conducted a search for optical circumstellar absorption lines in the spectra of 16 debris disc host stars. None of the stars in our sample showed signs of emission line activity in either H$_α$, Ca II or Na I, confirming their more evolved nature. Four stars were found to exhibit narrow absorption features near the cores of the photospheric Ca II and Na I D lines (when Na I D data were available). We analyse the characteristics of these spectral features to determine whether they are of circumstellar or interstellar origins. The strongest evidence for circumstellar gas is seen in the spectrum of HD110058, which is known to host a debris disc observed close to edge-on. This is consistent with a recent ALMA detection of molecular gas in this debris disc, which shows many similarities to the $β$ Pictoris system.

astro-ph.EP↗

arXiv2016

A cavity and further radial substructures in the disk around HD~97048

Context: Gaps, cavities and rings in circumstellar disks are signposts of disk evolution and planet-disk interactions. We follow the recent suggestion that Herbig Ae/Be disks with a flared disk harbour a cavity, and investigate the disk around HD~97048. Aims: We aim to resolve the 34$\pm$ 4 au central cavity predicted by Maaskant et al. (2013) and to investigate the structure of the disk. Methods: We image the disk around HD~97048 using ALMA at 0.85~mm and 2.94~mm, and ATCA (multiple frequencies) observations. Our observations also include the 12CO J=1-0, 12CO J=3-2 and HCO+ J=4-3 emission lines. Results: A central cavity in the disk around HD~97048 is resolved with a 40-46 au radius. Additional radial structure present in the surface brightness profile can be accounted for either by an opacity gap at ~90 au or by an extra emitting ring at ~150 au. The continuum emission tracing the dust in the disk is detected out to 355 au. The 12CO J=3-2 disk is detected 2.4 times farther out. The 12CO emission can be traced down to $\approx$ 10 au scales. Non-Keplerian kinematics are detected inside the cavity via the HCO+ J=4-3 velocity map. The mm spectral index measured from ATCA observations suggests that grain growth has occurred in the HD~97048 disk. Finally, we resolve a highly inclined disk out to 150 au around the nearby 0.5~$M_{\odot}$ binary ISO-ChaI 126. Conclusions: The data presented here reveal a cavity in the disk of HD 97048, and prominent radial structure in the surface brightness. The cavity size varies for different continuum frequencies and gas tracers. The gas inside the cavity follows non-Keplerian kinematics seen in HCO+ emission. The variable cavity size along with the kinematical signature suggests the presence of a substellar companion or massive planet inside the cavity.

astro-ph.SR↗

arXiv2016

Exocometary gas in the HD 181327 debris ring

An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now we only knew of cases around A stars. Here we present the first detection of 12CO (2-1) disc emission around an F star, HD 181327, obtained with ALMA observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 +- 0.4 AU and a radial width of 23.2 +- 1.0 AU. At this frequency the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low level continuum emission beyond the main ring out to ~200 AU. We model the CO emission in the non-LTE regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2x10^-6 Mearth and 2.9x10^-6 Mearth, depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO2 cometary composition that is consistent with Solar system comets. Due to the low gas densities it is unlikely that the gas is shaping the dust distribution.

astro-ph.EP↗

arXiv2016

Azimuthal asymmetries in the debris disk around HD61005

Debris disks offer valuable insights into the latest stages of circumstellar disk evolution, and can possibly help us to trace the outcomes of planetary formation processes. In the age range 10 to 100\,Myr, most of the gas is expected to have been removed from the system, giant planets (if any) must have already been formed, and the formation of terrestrial planets may be on-going. Pluto-sized planetesimals, and their debris released in a collisional cascade, are under their mutual gravitational influence, which may result into non-axisymmetric structures in the debris disk. High angular resolution observations are required to investigate these effects and constrain the dynamical evolution of debris disks. Furthermore, multi-wavelength observations can provide information about the dust dynamics by probing different grain sizes. Here we present new VLT/SPHERE and ALMA observations of the debris disk around the 40\,Myr-old solar-type star HD\,61005. We resolve the disk at unprecedented resolution both in the near-infrared (in scattered and polarized light) and at millimeter wavelengths. Thanks to the new observations, we propose a solution for both the radial and azimuthal distribution of the dust grains in the debris disk. We find that the disk has a moderate eccentricity ($e \sim 0.1$) and that the dust density is two times larger at the pericenter compared to the apocenter. With no giant planets detected in our observations, we investigate alternative explanations besides planet-disk interactions to interpret the inferred disk morphology. We postulate that the morphology of the disk could be the consequence of a massive collision between $\sim$\,1000\,km-sized bodies at $\sim$\,61\,au. If this interpretation holds, it would put stringent constraints on the formation of massive planetesimals at large distances from the star.

astro-ph.SR↗

arXiv2016

Discovery of concentric broken rings at sub-arcsec separations in the HD 141569A gas-rich, debris disk with VLT/SPHERE

Transition disks correspond to a short stage between the young protoplanetary phase and older debris phase. Along this evolutionary sequence, the gas component disappears leaving room for a dust-dominated environment where already-formed planets signpost their gravitational perturbations. We endeavor to study the very inner region of the well-known and complex debris, but still gas-rich disk, around HD 141569A using the exquisite high-contrast capability of SPHERE at the VLT. Recent near-infrared (IR) images suggest a relatively depleted cavity within ~200 au, while former mid-IR data indicate the presence of dust at separations shorter than ~100 au. We obtained multi-wavelength images in the near-IR in J, H2, H3 and Ks bands with the IRDIS camera and a 0.95-1.35 micrometers spectral data cube with the IFS. Data were acquired in pupil-tracking mode, thus allowing for angular differential imaging. We discovered several new structures inside 1", of which the most prominent is a bright ring with sharp edges (semi-major axis: 0.4") featuring a strong north-south brightness asymmetry. Other faint structures are also detected from 0.4" to 1" in the form of concentric ringlets and at least one spiral arm. Finally, the VISIR data at 8.6 micrometers suggests the presence of an additional dust population closer in. Besides, we do not detect companions more massive than 1-3 mass of Jupiter. The performance of SPHERE allows us to resolve the extended dust component, which was previously detected at thermal and visible wavelengths, into very complex patterns with strong asymmetries ; the nature of these asymmetries remains to be understood. Scenarios involving shepherding by planets or dust-gas interactions will have to be tested against these observations.

astro-ph.EP↗

arXiv2016

Dust masses of disks around 8 Brown Dwarfs and Very Low-Mass Stars in Upper Sco OB1 and Ophiuchus

We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the $^{12}$CO J=3-2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M$_\oplus$, suggesting that the stellar mass / disk mass correlation can be extrapolated for brown dwarfs with masses as low as 0.05 M$_\odot$. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with warmest inner disk as traced by its H - [4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature originally derived for stellar mass objects to the brown dwarf regime to $\langle T_{dust} \rangle \approx 22 (L_{*} /L_{\odot})^{0.16} K$, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L$_\odot$, corresponding to masses between 0.1 and 0.2 M$_\odot$ depending on age.

astro-ph.SR↗

arXiv2015

Compact dust concentration in the MWC 758 protoplanetary disk

The formation of planetesimals requires that primordial dust grains grow from micron- to km-sized bodies. Dust traps caused by gas pressure maxima have been proposed as regions where grains can concentrate and grow fast enough to form planetesimals, before radially migrating onto the star. We report new VLA Ka & Ku observations of the protoplanetary disk around the Herbig Ae/Be star MWC 758. The Ka image shows a compact emission region in the outer disk indicating a strong concentration of big dust grains. Tracing smaller grains, archival ALMA data in band 7 continuum shows extended disk emission with an intensity maximum to the north-west of the central star, which matches the VLA clump position. The compactness of the Ka emission is expected in the context of dust trapping, as big grains are trapped more easily than smaller grains in gas pressure maxima. We develop a non-axisymmetric parametric model inspired by a steady state vortex solution with parameters adequately selected to reproduce the observations, including the spectral energy distribution. Finally, we compare the radio continuum with SPHERE scattered light data. The ALMA continuum spatially coincides with a spiral-like feature seen in scattered light, while the VLA clump is offset from the scattered light maximum. Moreover, the ALMA map shows a decrement that matches a region devoid of scattered polarised emission. Continuum observations at a different wavelength are necessary to conclude if the VLA-ALMA difference is an opacity or a real dust segregation.

astro-ph.EP↗

arXiv2015

A SCUBA-2 850 micron Survey of Protoplanetary Discs in the IC 348 Cluster

We present 850 micron observations of the 2-3 Myr cluster IC 348 in the Perseus molecular cloud using the SCUBA-2 camera on the James Clerk Maxwell Telescope. Our SCUBA-2 map has a diameter of 30 arcmin and contains ~370 cluster members, including ~200 objects with IR excesses. We detect a total of 13 discs. Assuming standard dust properties and a gas to dust mass ratio of 100, we derive disc masses ranging from 1.5 to 16 M_JUP . We also detect 8 Class 0/I protostars. We find that the most massive discs (M_Disc > 3 M_JUP ; 850 micron flux > 10 mJy) in IC 348 tend to be transition objects according to the characteristic "dip" in their infrared Spectral Energy Distributions (SEDs). This trend is also seen in other regions. We speculate that this could be an initial conditions effect (e.g., more massive discs tend to form giant planets that result in transition disc SEDs) and/or a disc evolution effect (the formation of one or more massive planets results in both a transition disc SED and a reduction of the accretion rate, increasing the lifetime of the outer disc). A stacking analysis of the discs that remain undetected in our SCUBA-2 observations suggests that their median 850 micron flux should be ~1 mJy, corresponding to a disc mass ~0.3 M_JUP (gas plus dust) or ~1 M_Earth of dust. While the available data are not deep enough to allow a meaningful comparison of the disc luminosity functions between IC 348 and other young stellar clusters, our results imply that disc masses exceeding the Minimum Mass Solar Nebula are very rare (~1%) at the age of IC 348, especially around very low-mass stars.

astro-ph.EP↗

arXiv2015

The inner environment of Z~CMa: High-Contrast Imaging Polarimetry with NaCo

Context. Z\,CMa is a binary composed of an embedded Herbig Be and an FU Ori class star separated by $\sim100$ au. Observational evidence indicate a complex environment in which each star has a circumstellar disk and drives a jet, and the whole system is embedded in a large dusty envelope. Aims. We aim to probe the circumbinary environment of Z\,CMa in the inner 400 au in scattered light. Methods. We use high contrast imaging polarimetry with VLT/NaCo at $H$ and $K_s$ bands. Results. The central binary is resolved in both bands. The polarized images show three bright and complex structures: a common dust envelope, a sharp extended feature previously reported in direct light, and an intriguing bright clump located $0\farcs3$ south of the binary, which appears spatially connected to the sharp extended feature. Conclusions.We detect orbital motion when compared to previous observations, and report a new outburst driven by the Herbig star. Our observations reveal the complex inner environment of Z\,CMa with unprecedented detail and contrast.

astro-ph.SR↗

arXiv2015

Gas inside the 97 au cavity around the transition disk Sz\,91

We present ALMA (Cycle 0) band-6 and band-3 observations of the transition disk Sz\,91. The disk inclination and position angle are determined to be $i=49.5\degr\pm3.5\degr$ and $\mathrm{PA}=18.2\degr\pm3.5\degr$ and the dusty and gaseous disk are detected up to $\sim220$ au and $\sim400$ au from the star, respectively. Most importantly, our continuum observations indicate that the cavity size in the mm-sized dust distribution must be $\sim97$ au in radius, the largest cavity observed around a T Tauri star. Our data clearly confirms the presence of \co(2-1) well inside the dust cavity. Based on these observational constrains we developed a disk model that simultaneously accounts for the \co and continuum observations (i.e., gaseous and dusty disk). According to our model, most of the millimeter emission comes from a ring located between 97 and 140 au. We also find that the dust cavity is divided into an innermost region largely depleted of dust particles ranging from the dust sublimation radius up to 85 au, and a second, moderately dust-depleted region, extending from 85 to 97 au. The extremely large size of the dust cavity, the presence of gas and small dust particles within the cavity and the accretion rate of Sz\,91 are consistent with the formation of multiple (giant) planets.

astro-ph.SR↗

arXiv2014

CO gas inside the protoplanetary disk cavity in HD 142527: disk structure from ALMA

Inner cavities and annular gaps in circumstellar disks are possible signposts of giant planet formation. The young star HD 142527 hosts a massive protoplanetary disk with a large cavity that extends up to 140 au from the central star, as seen in continuum images at infrared and millimeter wavelengths. Estimates of the survival of gas inside disk cavities are needed to discriminate between clearing scenarios. We present a spatially and spectrally resolved carbon monoxide isotopologue observations of the gas-rich disk HD 142527, in the J=2-1 line of 12CO, 13CO and C18O, obtained with the Atacama Large Millimeter Array (ALMA). We detect emission coming from inside the dust-depleted cavity in all three isotopologues. Based on our analysis of the gas in the dust cavity, the 12CO emission is optically thick, while 13CO and C18O emission are both optically thin. The total mass of residual gas inside the cavity is about 1.5-2 Jupiter masses. We model the gas with an axisymmetric disk model. Our best fit model shows that the cavity radius is much smaller in CO than it is in millimeter continuum and scattered light observations, with a gas cavity that does not extend beyond 105 au (at 3-sigma). The gap wall at its outer edge is diffuse and smooth in the gas distribution, while in dust continuum it is manifestly sharper. The inclination angle, as estimated from the high velocity channel maps, is 28+/-0.5 degrees, higher than in previous estimates, assuming a fix central star mass of 2.2 Solar masses.

astro-ph.EP↗

arXiv2014

Planck intermediate results. XVIII The millimetre and sub-millimetre emission from planetary nebulae

Late stages of stellar evolution are characterized by copious mass-loss events whose signature is the formation of circumstellar envelopes (CSE). Planck multi-frequency measurements have provided relevant information on a sample of Galactic planetary nebulae (PNe) in the important and relatively unexplored observational band between 30 and 857GHz. Planck enables the assembly of comprehensive PNe spectral energy distributions (SEDs) from radio {\bf to} far-infrared frequencies. Modelling of the derived SEDs provides us with information on physical properties of CSEs and the mass content of both main components: ionised gas, traced by the free-free emission at cm--mm waves; and thermal dust, traced by the millimetre and far-IR emission. In particular, the amount of ionised gas and dust has been derived here. Such quantities have also been estimated for the very young PN CRL618, where the strong variability observed in its radio and millimetre emission has previously prevented the construction of its SED. A morphological study of the Helix Nebula has also been performed. Planck maps reveal, for the first time, the spatial distribution of the dust inside the envelope, allowing us to identify different components, the most interesting of which is a very extended component (up to 1pc) that may be related to a region where the slow expanding envelope is interacting with the surrounding interstellar medium.

astro-ph.GA↗

arXiv2014

Spatially resolved HCN J=4--3 and CS J=7--6 emission from the disk around HD 142527

The disk around HD 142527 attracts a lot of attention, amongst others because of its resolved (sub) mm dust continuum that is concentrated into a horseshoe-shape towards the north of the star. In this manuscript we present spatially resolved ALMA detections of the HCN J=4-3 and CS J=7-6 emission lines. These lines give us a view deeper into the disk compared to the (optically thicker) CO isotopes. This is the first detection of CS J=7-6 coming from a protoplanetary disk. Both emission lines are azimuthally asymmetric and are suppressed under the horseshoe-shaped continuum emission peak. A possible mechanism to explain the decrease under the horseshoe-shaped continuum is the increased opacity coming from the higher dust concentration at the continuum peak. Lower {\gr dust and/or gas} temperatures and an optically thick radio-continuum reduce line emission by freeze-out and shielding of emission from the far side of the disk.

astro-ph.SR↗

arXiv2014

A CO survey in planet-forming disks: characterizing the gas content in the epoch of planet formation

We carried out a 12CO(3-2) survey of 52 southern stars with a wide range of IR excesses (LIR/L*) using the single dish telescopes APEX and ASTE. The main aims were (1) to characterize the evolution of molecular gas in circumstellar disks using LIR/L* values as a proxy of disk dust evolution, and (2) to identify new gas-rich disk systems suitable for detailed study with ALMA. About 60% of the sample (31 systems) have LIR/L* > 0.01 typical of T-Tauri or Herbig AeBe stars, and the rest (21 systems) have LIR/L* < 0.01 typical of debris disks. We detect CO(3-2) emission from 20 systems, and 18 (90%) of these have LIR/L* > 0.01. However, the spectra of only four of the newly detected systems appear free of contamination from background or foreground emission from molecular clouds. These include the early-type stars HD 104237 (A4/5V, 116 pc) and HD 98922 (A2 III, 507 pc, as determined in this work), where our observations reveal the presence of CO-rich circumstellar disks for the first time. Of the other detected sources, many could harbor gaseous circumstellar disks, but our data are inconclusive. For these two newly discovered gas-rich disks, we present radiative transfer models that simultaneously reproduce their spectral energy distributions and the 12CO(3-2) line profiles. For both of these systems, the data are fit well by geometrically flat disks, placing them in the small class of non-flaring disks with significant molecular gas reservoirs.

astro-ph.SR↗

arXiv2014

Planck intermediate results. XV. A study of anomalous microwave emission in Galactic clouds

Anomalous microwave emission (AME) is believed to be due to electric dipole radiation from small spinning dust grains. The aim of this paper is a statistical study of the basic properties of AME regions and the environment in which they emit. We used WMAP and Planck maps, combined with ancillary radio and IR data, to construct a sample of 98 candidate AME sources, assembling SEDs for each source using aperture photometry on 1deg-smoothed maps from 0.408 GHz up to 3000 GHz. Each spectrum is fitted with a simple model of free-free, synchrotron (where necessary), cosmic microwave background (CMB), thermal dust, and spinning dust components. We find that 42 of the 98 sources have significant (>5sigma) excess emission at frequencies between 20 and 60 GHz. An analysis of the potential contribution of optically thick free-free emission from ultra-compact HII regions, using IR colour criteria, reduces the significant AME sample to 27 regions. The spectrum of the AME is consistent with model spectra of spinning dust. The AME regions tend to be more spatially extended than regions with little or no AME. The AME intensity is strongly correlated with the submillimetre/IR flux densities and comparable to previous AME detections in the literature. AME emissivity, defined as the ratio of AME to dust optical depth, varies by an order of magnitude for the AME regions. The AME regions tend to be associated with cooler dust in the range 14-20 K and an average emissivity index of +1.8, while the non-AME regions are typically warmer, at 20-27 K. In agreement with previous studies, the AME emissivity appears to decrease with increasing column density. The emerging picture is that the bulk of the AME is coming from the polycyclic aromatic hydrocarbons and small dust grains from the colder neutral interstellar medium phase (Abridged).

astro-ph.GA↗

arXiv2014

Molecular Gas Clumps from the Destruction of Icy Bodies in the $β$ Pictoris Debris Disk

Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets and dwarf planets. But is gas also released in such events? Observations at submm wavelengths of the archetypal debris disk around $β$ Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85AU from the star, in a plane closely aligned with the orbit of the inner planet, $β$ Pic b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet, or from the remnants of a collision of Mars-mass planets.

astro-ph.SR↗