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Stabilization of the disk around βPictoris by extremely carbon-rich gas
The edge-on disk surrounding the nearby young star βPictoris is thearchetype of `debris disks', which are composed of dust and gas producedby collisions between-and evaporation of-planetesimals, analogues ofSolar System comets and asteroids. These disks may provide insight intothe formation and early evolution of terrestrial planets. Previous workon βPic concluded that the disk gas has roughly solar abundances ofelements, but this poses a problem because such gas should rapidly beblown away from the star, contrary to observations showing a stable gasdisk in keplerian rotation. Here we report the detection of singly anddoubly ionized carbon (CII, CIII) and neutral atomic oxygen (OI) gas inthe βPic disk. Carbon is extremely overabundant relative to everyother measured element. This appears to solve the problem of the stablegas disk, because the carbon overabundance should keep the gas disk inkeplerian rotation. The overabundance may indicate that the gas isproduced from material more carbon-rich than expected of Solar Systemanalogues.
| Near-infrared imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| Gravitationally unstable protoplanetary discs
The possibility that protoplanetary gaseous discs are dynamicallyunstable to axisymmetric and non-axisymmetric gravity perturbations(e.g. those produced by spontaneous disturbances) with characteristicscales larger than the vertical scale height is discussed analytically,using a local Wentzel-Kramers-Brillouin (WKB) approach. It is shown thatsuch discs might be clumpy, and these gravitationally bound clumps maylater collapse to become giant planets (`hot Jupiters'). The chief aimin this paper is to underscore a fact of vital importance forapplication in the planetary formation process: gravitationally unstablenon-axisymmetric (spiral) perturbations can effectively transport boththe angular momentum and the mass in a spatially inhomogeneous disc.
| Recent results on Pre-main sequence delta Scuti stars. .
Intermediate mass Pre-main sequence stars (1.5 Mȯ< M< 5Mȯ) cross the instability strip on their way tothe main sequence. They are therefore expected to be pulsating in asimilar way as the delta Scuti stars. In this contribution we presentthe status of the observational studies of pulsations in these starswith special emphasis on recent results from our group. The prospectsfor future investigations of these objects from the ground and fromspace are discussed.
| Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS)5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60μm excess. The spectra of five objects possess spectral features thatare well-modeled using micron-sized grains and silicates withcrystalline mass fractions 0%-80%, consistent with T Tauri and HerbigAeBe stars. With the exception of η Crv, these objects are youngwith ages <=50 Myr. Our fits require the presence of a cool blackbodycontinuum, Tgr=80-200 K, in addition to hot, amorphous, andcrystalline silicates, Tgr=290-600 K, suggesting thatmultiple parent body belts are present in some debris disks, analogousto the asteroid and Kuiper belts in our solar system. The spectra forthe majority of objects are featureless, suggesting that the emittinggrains probably have radii a>10 μm. We have modeled the excesscontinua using a continuous disk with a uniform surface densitydistribution, expected if Poynting-Robertson and stellar wind drag arethe dominant grain removal processes, and using a single-temperatureblackbody, expected if the dust is located in a narrow ring around thestar. The IRS spectra of many objects are better modeled with asingle-temperature blackbody, suggesting that the disks possess innerholes. The distribution of grain temperatures, based on our blackbodyfits, peaks at Tgr=110-120 K. Since the timescale for icesublimation of micron-sized grains with Tgr>110 K is afraction of a Myr, the lack of warmer material may be explained if thegrains are icy. If planets dynamically clear the central portions ofdebris disks, then the frequency of planets around other stars isprobably high. We estimate that the majority of debris disk systemspossess parent body masses, MPB<1 M⊕. Thelow inferred parent body masses suggest that planet formation is anefficient process.Based on observations with the NASA Spitzer Space Telescope, which isoperated by the California Institute of Technology for NASA.
| The SPITZER c2d Survey of Weak-Line T Tauri Stars. I. Initial Results
Using the Spitzer Space Telescope, we have observed 90 weak-line andclassical T Tauri stars in the vicinity of the Ophiuchus, Lupus,Chamaeleon, and Taurus star-forming regions as part of the Cores toDisks (c2d) Spitzer Legacy project. In addition to the Spitzer data, wehave obtained contemporaneous optical photometry to assist inconstructing spectral energy distributions. These objects werespecifically chosen as solar-type young stars with low levels ofHα emission, strong X-ray emission, and lithium absorption, i.e.,weak-line T Tauri stars, most of which were undetected in the mid- tofar-IR by the IRAS survey. Weak-line T Tauri stars are potentiallyextremely important objects in determining the timescale over which diskevolution may take place. Our objective is to determine whether theseyoung stars are diskless or have remnant disks that are below thedetection threshold of previous infrared missions. We find that only5/83 weak-line T Tauri stars have detectable excess emission between 3.6and 70 μm, which would indicate the presence of dust from the innerfew tenths of an AU out to the planet-forming regions a few tens of AUfrom the star. Of these sources, two have small excesses at 24 μmconsistent with optically thin disks; the others have optically thickdisks already detected by previous IR surveys. All of the sevenclassical T Tauri stars show excess emission at 24 and 70 μm althoughtheir properties vary at shorter wavelengths. Our initial results showthat disks are rare among young stars selected for their weak Hαemission.
| Discovery of an Optically Thick, Edge-on Disk around the Herbig Ae Star PDS 144N
We have discovered an optically thick, edge-on circumstellar disk arounda Herbig Ae star in the binary system PDS 144, providing the firstintermediate-mass analog of HK Tau and similar T Tauri stars. Thissystem consists of a V~13 mag primary and a fainter companion, with thespectra of both stars showing evidence for circumstellar disks andaccretion; both stars were classified as Herbig Ae by the Pico dos DiasSurvey. In Lick adaptive optics polarimetry, we resolved extendedpolarized light scattered from dust around the northern star. Follow-upKeck adaptive optics and mid-infrared observations show that this staris entirely hidden by an optically thick disk at all wavelengths from1.2 to 11.7 μm. The disk major axis subtends ~0.8" on the sky,corresponding to ~800 AU at a distance of 1000 pc. Bright ``wings''extend 0.3" above and below the disk ansae, due most likely toscattering from the edges of an outflow cavity in a circumstellarenvelope. We discuss the morphology of the disk and the spectral energydistributions of the two PDS 144 stars, present preliminary disk models,and identify a number of open questions regarding this fascinatingsystem.Some of the data presented here were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation.
| Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| The Nearest Young Moving Groups
The latest results in the research of forming planetary systems have ledseveral authors to compile a sample of candidates for searching forplanets in the vicinity of the Sun. Young stellar associations areindeed excellent laboratories for this study, but some of them are notclose enough to allow the detection of planets through adaptive opticstechniques. However, the existence of very close young moving groups cansolve this problem. Here we have compiled the members of the nearestyoung moving groups, as well as a list of new candidates from ourcatalog of late-type stars that are possible members of young stellarkinematic groups, studying their membership through spectroscopic andphotometric criteria.
| Braking the Gas in the β Pictoris Disk
The star β Pictoris hosts the best studied circumstellar disk todate. Nonetheless, a long-standing puzzle has been around since thedetection of metallic gas in the disk: radiation pressure from the starshould blow the gas away, yet the observed motion is consistent withKeplerian rotation. In this work we search for braking mechanisms thatcan resolve this discrepancy. We find that all species affected byradiation force are largely ionized and dynamically coupled into asingle fluid by Coulomb collisions, reducing the radiation force onspecies feeling the strongest acceleration. For a gas of solarcomposition, the effective radiation force still exceeds gravity, whilea gas of enhanced carbon abundance could be self-braking. We alsoexplore two other braking agents: collisions with dust grains andneutral gas. Grains surrounding β Pic are photoelectrically chargedto a positive electrostatic potential. If a significant fraction of thegrains are carbonaceous (10% in the midplane and larger at higheraltitudes), ions can be slowed down to satisfy the observed velocityconstraints. For neutral gas to brake the ion fluid, we find a minimumrequired mass ~0.03 M⊕, consistent with observed upperlimits on the hydrogen column density and substantially reduced relativeto previous estimates. Our results favor a scenario in which metallicgas is generated by grain evaporation in the disk, perhaps duringgrain-grain collisions. We exclude a primordial origin for the gas butcannot rule out its production by falling evaporating bodies near thestar.
| First Two-Micron Imaging Polarimetry of β Pictoris
High-resolution K-band imaging polarimetry of the β Pic dust diskhas been conducted with adaptive optics and a coronagraph using theSubaru 8.2 m telescope. Polarization of ~10% is detected out to r~120 AUwith a centrosymmetric vector pattern around the central star,confirming that the disk is seen as an infrared reflection nebula. Wehave modeled our near-infrared and previous optical polarization resultsin terms of dust scattering in the disk and have found that both thedegrees of polarization and the radial intensity profiles are wellreproduced. We argue that the observed characteristics of the disk dustare consistent with the presence of ice-filled fluffy aggregatesconsisting of submicron grains in the β Pic system. There is a gaparound 100 AU in both the intensity and polarization profiles, whichsuggests a paucity of planetesimals in this region. The radial intensityprofile also shows ripple-like structures, which are indicative of thepresence of multiple planetesimal belts, as in the case of the M-typeVega-like star AU Mic.
| The Warped Circumstellar Disk of HD 100546
We propose that the two-armed spiral features seen in visible HubbleSpace Telescope images of scattered light in HD 100546's circumstellardisk are caused by the illumination of a warped outer disk. A tilt of6°-15° from the symmetry plane can cause the observed surfacebrightness variations, providing the disk is very twisted (highlywarped) at radii greater than 200 AU where the spiral features are seen.Dust lanes are due in part to shadowing in the equatorial plane from theinner disk within a radius of 100 AU. HD 100546's outer disk, if viewededge-on, would appear similar to that of Beta Pictoris. A disk initiallymisaligned with a planetary system becomes warped due to precessioninduced by planetesimal bodies and planets. However, the twistedness ofHD 100546's disk cannot be explained by precession during the lifetimeof the system induced by a single Jovian-mass planet within the clearingat ~13 AU. One possible explanation for the corrugated disk is thatprecession was induced by massive bodies embedded in the disk at largerradius. This would require approximately a Jupiter mass of bodies welloutside the central clearing at 13 AU and within the location of thespiral features or at radii approximately between 50 and 200 AU.
| Testing Rotational Mixing Predictions with New Boron Abundances in Main-Sequence B-Type Stars
New boron abundances for seven main-sequence B-type stars are determinedfrom HST STIS spectroscopy around the B III 2066 Å line. Boronabundances provide a unique and critical test of stellar evolutionmodels that include rotational mixing, since boron is destroyed in thesurface layers of stars through shallow mixing long before otherelements are mixed from the stellar interior through deep mixing. Thestars in this study are all on or near the main sequence and are membersof young Galactic clusters. They show no evidence of mixing with gasfrom H-burning layers from their CNO abundances. Boron abundances rangefrom 12+log(B/H)<=1.0 to 2.2. The boron abundances are compared tothe published values of the stellar nitrogen abundances [all have12+log(N/H)<=7.8] and to their host cluster ages (4-16 Myr) toinvestigate the predictions from models of massive star evolution withrotational mixing effects. We find that the variations in boron andnitrogen are generally within the range of the predictions from thestellar evolution models with rotation (where predictions for modelswith rotation rates from 0 to 450 km s-1 and μ-barriersare examined), especially given their age and mass ranges. Three stars(of 34 B-type stars with detailed boron abundance determinations)deviate from the model predictions, showing either much larger borondepletions than can be explained by the rotating model predictions or aspectroscopic mass that is lower than expected, given the rotating modelpredictions for its age and abundances. The results from these threestars suggest that rotational mixing could be more efficient than thatcurrently modeled at the highest rotation rates.
| Dust in Resonant Extrasolar Kuiper Belts: Grain Size and Wavelength Dependence of Disk Structure
This paper considers the distribution of dust that originates in thebreakup of planetesimals that are trapped in resonance with a planet. Itis shown that there are three distinct grain populations with differentspatial distributions: (I) large grains have the same clumpy resonantdistribution as the planetesimals; (II) moderate-sized grains are nolonger in resonance and have an axisymmetric distribution; and (III)small grains are blown out of the system by radiation pressure and sohave a density distribution that falls off as τ~1/r. Population IIIcan be further divided into two subclasses: (IIIa) grains produced frompopulation I that exhibit trailing spiral structure that emanates fromthe resonant clumps and (IIIb) grains produced from population II thathave an axisymmetric distribution. Since observations in differentwavebands are sensitive to different dust sizes, multiwavelength imagingof debris disks can be used to test models that explain thesubmillimeter structure of debris disks as due to resonant trapping ofplanetesimals. For example, a collisional cascade without blowout grainswould appear clumpy in the submillimeter (which samples population I)and smooth at mid- to far-IR wavelengths (which sample population II).The wavelength of transition from clumpy to smooth structure isindicative of the mass of the perturbing planet. The size distributionof Vega's disk is modeled showing that the large quantities ofpopulation III grains detected recently by Spitzer must originate in thedestruction of the grains seen in the submillimeter images. Thus, athigh resolution and sensitivity the far- and mid-IR structure of Vega'sdisk is predicted to include spiral structure emanating from thesubmillimeter clumps.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| First Scattered Light Images of Debris Disks around HD 53143 and HD 139664
We present the first scattered light images of debris disks around a Kstar (HD 53143) and an F star (HD 139664) using the coronagraphic modeof the Advanced Camera for Surveys (ACS) on board the Hubble SpaceTelescope (HST). With ages of 0.3-1 Gyr, these are among the oldestoptically detected debris disks. HD 53143, viewed ~45° from edge-on,does not show radial variation in disk structure and has a width >55AU. HD 139664 is seen close to edge-on and has a beltlike morphologywith a dust peak 83 AU from the star and a distinct outer boundary at109 AU. We discuss evidence for significant diversity in the radialarchitecture of debris disks that appears unconnected to stellarspectral type or age. HD 139664 and possibly the solar system belong ina category of narrow belts 20-30 AU wide. HD 53143 represents a class ofwide-disk architecture with a characteristic width >50 AU.
| Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris
We present F435W (B), F606W (broad V), and F814W (broad I) coronagraphicimages of the debris disk around β Pictoris obtained with theHubble Space Telescope's Advanced Camera for Surveys. These imagesprovide the most photometrically accurate and morphologically detailedviews of the disk between 30 and 300 AU from the star ever recorded inscattered light. We confirm that the previously reported warp in theinner disk is a distinct secondary disk inclined by ~5° from themain disk. The projected spine of the secondary disk coincides with theisophotal inflections, or ``butterfly asymmetry,'' previously seen atlarge distances from the star. We also confirm that the opposingextensions of the main disk have different position angles, but we findthat this ``wing-tilt asymmetry'' is centered on the star rather thanoffset from it, as previously reported. The main disk's northeastextension is linear from 80 to 250 AU, but the southwest extension isdistinctly bowed with an amplitude of ~1 AU over the same region. Bothextensions of the secondary disk appear linear, but not collinear, from80 to 150 AU. Within ~120 AU of the star, the main disk is ~50% thinnerthan previously reported. The surface brightness profiles along thespine of the main disk are fitted with four distinct radial power lawsbetween 40 and 250 AU, while those of the secondary disk between 80 and150 AU are fitted with single power laws. These discrepancies suggestthat the two disks have different grain compositions or sizedistributions. The F606W/F435W and F814W/F435W flux ratios of thecomposite disk are nonuniform and asymmetric about both projected axesof the disk. The disk's northwest region appears 20%-30% redder than itssoutheast region, which is inconsistent with the notion that forwardscattering from the nearer northwest side of the disk should diminishwith increasing wavelength. Within ~120 AU, themF435W-mF606W andmF435W-mF814W colors along the spine of the maindisk are ~10% and ~20% redder, respectively, than those of β Pic.These colors increasingly redden beyond ~120 AU, becoming 25% and 40%redder, respectively, than the star at 250 AU. These measurementsoverrule previous determinations that the disk is composed of neutrallyscattering grains. The change in color gradient at ~120 AU nearlycoincides with the prominent inflection in the surface brightnessprofile at ~115 AU and the expected water-ice sublimation boundary. Wecompare the observed red colors within ~120 AU with the simulated colorsof nonicy grains having a radial number density ~r-3 anddifferent compositions, porosities, and minimum grain sizes. Theobserved colors are consistent with those of compact or moderatelyporous grains of astronomical silicate and/or graphite with sizes>~0.15-0.20 μm, but the colors are inconsistent with the bluecolors expected from grains with porosities >~90%. The increasinglyred colors beyond the ice sublimation zone may indicate the condensationof icy mantles on the refractory grains, or they may reflect anincreasing minimum grain size caused by the cessation of cometaryactivity.Based on guaranteed observing time awarded by the National Aeronauticsand Space Administration (NASA) to the ACS Investigation Definition Team(HST program 9987).
| A Hybrid N-Body-Coagulation Code for Planet Formation
We describe a hybrid algorithm to calculate the formation of planetsfrom an initial ensemble of planetesimals. The algorithm uses acoagulation code to treat the growth of planetesimals into oligarchs andexplicit N-body calculations to follow the evolution of oligarchs intoplanets. To validate the N-body portion of the algorithm, we use abattery of tests in planetary dynamics. Several complete calculations ofterrestrial planet formation with the hybrid code yield good agreementwith previously published calculations. These results demonstrate thatthe hybrid code provides an accurate treatment of the evolution ofplanetesimals into planets.
| Dynamical Evolution of the TW Hydrae Association
Using Galactic dynamics we have determined the age of the low-masspost-T Tauri stars in the TW Hya Association (TWA). To do so we appliedthe method of Ortega and coworkers to five stars of the association withHipparcos-measured distances (TWA 1, TWA 4, TWA 9, TWA 11, and TWA 19).The method is based on the calculation of the past three-dimensionalorbits of the stars. Of these stars, only TWA 9 presents a quitedifferent orbit so that it does not appear to be a dynamical member ofthe TWA. The four remaining stars have their first maximum orbitalconfinement at the age of -8.3+/-0.8 Myr, which is considered thedynamical age of the TWA. This confinement fixes the probablethree-dimensional forming region of the TWA within a mean radius of 14.5pc. This region is related to the older subgroups of the Sco-Cen OBassociation, Lower Centaurus Crux and Upper Centaurus Lupus, both with amean age of about 18 Myr. This dynamical age of the TWA and that of theβ Pic Moving Group, 11 Myr, also discussed here, introduce a moreprecise temporal scale for studies of disk evolution and planetaryformation around some stars of these associations. Using the retracedorbit of the runaway star HIP 82868 we examine the possibility that theformation of the TWA was triggered by a supernova explosion. It is shownthat for the four considered TWA stars, the expansion in volume is afactor of 5 from their origin to the present state. This is mainly dueto the currently more distant star TWA 19.
| Evidence for Companion-induced Secular Changes in the Turbulent Disk of a Be Star in the Large Magellanic Cloud MACHO Database
The light curve of a blue variable in the MACHO LMC database (FTS ID78.5979.72) appeared nearly unvarying for about 4 yr (the quasi-flatsegment) but then rapidly changed to become periodic with noisy minimafor the remaining 4 yr (the periodic segment); there are no antecedentindications of a gradual approach to this change. Lomb periodogramanalyses indicate the presence of two distinct periods of ~61 and 8 daysin both the quasi-flat and the periodic segments. Minima of the periodicsegment cover at least 50% of the orbital period and contain spikes oflight with the 8 day period; maxima do not show this short period. Thesystem typically shows maxima to be redder than minima. The most recentOGLE-III light curve shows only a 30 day periodicity. The variable's Vand R magnitudes and color are those of a Be star, and recent sets ofnear-infrared spectra 4 days apart, secured during the time of theOGLE-III data, show Hα emission near and at a maximum, confirmingits Be star characteristics. The model that best fits the photometricbehavior consists of a thin ringlike circumstellar disk of low mass withfour obscuring sectors orbiting the central B star in unison at the 61day period. The central star peers through the three equispacedseparations between the four sectors producing the 8 day period. Thesesectors could be dusty vortices comprised of particles larger thantypical interstellar dust grains that dim but selectively scatter thecentral star's light, while the remainder of the disk contains hydrogenin emission, making maxima appear redder. A companion star of lower massin an inclined and highly eccentric orbit produces an impulsiveperturbation near its periastron to change the disk's orientation,changing eclipses from partial to complete within ~10 days. The mostrecent change to a 30 day period observed in the OGLE-III data may becaused by obscuring sectors that have coalesced into larger ones andspread out along the disk.
| 3D continuum radiative transfer. A new adaptive grid construction algorithm based on the Monte Carlo method
Aims. The continuum radiative transfer is a common problem inAstrophysics, that must be solved to describe the physics and interpretthe observations of objects embedded (for instance) in a dusty medium(e.g. evolved stars, young stellar objects, ldots). Extreme high angularresolution observations reveal in general complex geometries (e.g.mediums with clumps of matter, warps ldots). It is thus necessary tosolve the radiative transfer problem making no simplifying assumptionson the geometry, i.e. resolve the full spatial three dimensionalproblem. Methods: . We use the flexible Monte Carlo (MC) method tosolve the continuum radiative transfer problem. We focus on thecomputation of the temperature structure of a dusty medium and theconstruction of the grid that samples this structure. We propose to usea natural by-product of the MC method, the locations of the photonpackage absorptions in the medium, to build an adaptive grid to reachthe finest resolution in the regions of interest. Results: . Wepresent the details of the new algorithm used for the construction ofthe grid. We also propose modifications to a widely used MC propagationframework. The reliability of our method is numerically tested againstwell established numerical results in one and two dimensions. In orderto illustrate the capabilities of our three dimensional methods, weexamine the observational evidence for the presence of warps incircumstellar discs.
| On the AU Microscopii debris disk. Density profiles, grain properties, and dust dynamics
Context: . AU Mic is a young M-type star surroundedby an edge-on optically thin debris disk that shares many commonobservational properties with the disk around βPictoris. In particular, the scattered light surfacebrightness profile falls off as r-5 outside 120 AU forβ Pictoris and 35 AU for AUMic. In both cases, the disk color rises as the distanceincreases beyond these reference radii. Aims: . In this paper, wepresent the first comprehensive analysis of the AUMic disk properties since the system was resolved by Kalas etal. (2004, Science, 303, 1990). We explore whether the dynamical model,which successfully reproduces the β Pictoris brightness profile(e.g., Augereau et al. 2001, A&A, 370, 447), could apply toAU Mic. Methods: . We calculate the surfacedensity profile of the AU Mic disk by performing theinversion of the near-IR and visible scattered light brightness profilesmeasured by Liu (2004, Science, 305, 1442) and Krist et al. (2005, AJ,129, 1008), respectively. We discuss the grain properties by analysingthe blue color of the disk in the visible (Krist et al. 2005) and byfitting the disk spectral energy distribution. Finally, we evaluate theradiation and wind forces on the grains. The impact of the recurrentX-ray and UV-flares on the dust dynamics is also discussed. Results: . We show that irrespective of the mean scattering asymmetryfactor of the grains, most of the emission arises from an asymmetric,collisionally-dominated region that peaks close to the surfacebrightness break around 35 AU. The elementary scatterers at visiblewavelengths are found to be sub-micronic, but the inferred sizedistribution underestimates the number of large grains, resulting insub-millimeter emissions that are too low compared to the observations.From our inversion procedure, we find that the V- to H-band scatteringcross sections ratio increases outside 40 AU, in line with the observedcolor gradient of the disk. This behavior is expected if the grains havenot been produced locally, but placed in orbits of high eccentricity bya size-dependent pressure force, resulting in a paucity of large grainsbeyond the outer edge of the parent bodies' disk. Because of the lowluminosity of AU Mic, radiation pressure isinefficient to diffuse the smallest grains in the outer disk, even whenthe flares are taken into account. Conversely, we show that a standard,solar-like stellar wind generates a pressure force onto the dustparticles that behaves much like a radiation pressure force. With anassumed dot{M} ≃ 3×102 dot{M}ȯ,the wind pressure overcomes the radiation pressure, and this effect isenhanced by the stellar flares. This greatly contributes to populatingthe extended AU Mic debris disk and explains thesimilarity between the β Pictoris andAU Mic brightness profiles. In both cases, the colorgradient beyond 120 AU for β Pictoris and 35 AUfor AU Mic, is believed to be a direct consequence ofthe dust dynamics.
| Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
Context: .Only a handful of debris disks have been imaged up to now. Dueto the need for high dynamic range and high angular resolution, verylittle is known about the inner planetary region, where small amounts ofwarm dust are expected to be found. Aims: .We investigate theclose neighbourhood of Vega with the help of infrared stellarinterferometry and estimate the integrated K-band flux originating fromthe central 8 AU of the debris disk. Methods: .We performedprecise visibility measurements at both short (~30 m) and long (~150 m)baselines with the FLUOR beam-combiner installed at the CHARA Array (MtWilson, California) in order to separately resolve the emissions fromthe extended debris disk (short baselines) and from the stellarphotosphere (long baselines). Results: .After revising Vega'sK-band angular diameter (θ_UD = 3.202 ± 0.005 mas), weshow that a significant deficit in squared visibility (ΔV2 = 1.88 ± 0.34%) is detected at short baselines withrespect to the best-fit uniform disk stellar model. This deficit can beeither attributed to the presence of a low-mass stellar companion aroundVega, or as the signature of the thermal and scattered emissions fromthe debris disk. We show that the presence of a close companion ishighly unlikely, as well as other possible perturbations (stellarmorphology, calibration), and deduce that we have most probably detectedthe presence of dust in the close neighbourhood of Vega. The resultingflux ratio between the stellar photosphere and the debris disk amountsto 1.29 ± 0.19% within the FLUOR field-of-view (~7.8 AU).Finally, we complement our K-band study with archival photometric andinterferometric data in order to evaluate the main physical propertiesof the inner dust disk. The inferred properties suggest that the Vegasystem could be currently undergoing major dynamical perturbations.
| Extrasolar planets and brown dwarfs around A-F type stars. III. β Pictoris: looking for planets, finding pulsations
In the frame of the search for extrasolar planets and brown dwarfsaround early-type stars, we present the results obtained on βPictoris, which is surrounded by a circumstellar disk that is warped bythe presence of a planet. We used 97 spectra acquired with CORALIE and230 spectra acquired with HARPS to characterize the radial velocitybehavior of β Pictoris and to infer constraints on the presence ofa planet close to this star. With these data, we were able to excludethe presence of an inner giant planet (2 M_Jup at a distance to the starof 0.05 AU, 9 M_Jup at 1 AU). We also discuss the origin of the observedradial velocity variations in terms of δ Scuti type pulsations.
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| Dust extinction and absorption: the challenge of porous grains
In many models of dusty objects in space the grains are assumed to becomposite or fluffy. However, the computation of the optical propertiesof such particles is still a very difficult problem. We analyze how theincrease of grain porosity influences basic features of cosmic dust -interstellar extinction, dust temperature, infrared bands and millimeteropacity. It is found that an increase of porosity leads to an increaseof extinction cross sections at some wavelengths and a decrease atothers depending on the grain model. However, this behaviour issufficient to reproduce the extinction curve in the direction of thestar σ Sco using current solar abundances. In the case of the starζ Oph our model requires larger amounts of carbon and iron in thedust-phase than is available. Porous grains can reproduce the flatextinction across the 3 - 8 μm wavelength range measured for severallines of sight by ISO and Spitzer. Porous grains are generally coolerthan compact grains. At the same time, the temperature of very porousgrains becomes slightly larger in the case of the EMT-Mie calculationsin comparison with the results found from the layered-sphere model. Thelayered-sphere model predicts a broadening of infrared bands and a shiftof the peak position to larger wavelengths as porosity grows. In thecase of the EMT-Mie model variations of the feature profile are lesssignificant. It is also shown that the millimeter mass absorptioncoefficients grow as porosity increases with a faster growth occurringfor particles with Rayleigh/non-Rayleigh inclusions. As a result, forvery porous particles the coefficients given by two models can differ bya factor of about 3.
| Disks Around Stars and the Growth of Planetary Systems
Circumstellar disks play a vital evolutionary role, providing a way tomove gas inward and onto a young star. The outward transfer of angularmomentum allows the star to contract without breaking up, and theremnant disk of gas and particles is the reservoir for forming planets.High-resolution spectroscopy is uncovering planetary dynamics and motionwithin the remnant disk, and imaging at infrared to millimeterwavelengths resolves disk structure over billions of years of evolution.Most stars are born with a disk, and models of planet formation need toform such bodies from the disk material within the disk's10-million-year life-span.
| Stellar Coronagraphy: Study and Test of a Hybrid Interfero-Coronagraph
We present a device for high dynamic range imaging of faint structuresin the close angular vicinity of stars. An advantage of this device isthe capacity to explore the field of view both uniformly and as close(or near) to the parent star as allowed by the diffraction limit of thetelescope. The design is adapted from the AchromaticInterfero-Coronagraph (AIC), which suppresses the light scattered bydiffraction from an on-axis unresolved source, but yields two twinimages of the stellar environment, displayed symmetrically in the focalplane (Gay & Rabbia 1996; Baudoz et al. 2000a). Our HybridInterfero-Coronagraph (HIC) avoids the 180° ambiguity and istherefore well suited for the study of extended objects distributed allaround the star. Like the AIC, the on-axis extinction with HIC is basedon destructive interference after amplitude division of the incidentfield from a single telescope. An achromatic π-dephasing and aspatial filtering of the wave front are performed on one arm of theinterferometer. The interferometric process occurs between the wavefront from the star and a spatially filtered wave front. Because of thisspatial filtering, the destructive process no longer remains achromatic.However, the residual chromaticity is compatible with astrophysicalapplications on ground-based telescopes. Numerical simulations show thatHIC performance does not suffer from telescope central obscuration. Anappropriate size of the spatial filtering mask and a suitableapodization allow the nulling of an on-axis unresolved star at the levelof 10-6, with a spectral resolutionR=λ0/Δλ=15 and in the absence ofwave-front phase or intensity errors. Preliminary laboratory tests witha HIC prototype are reported, and as a first step, an extinction of thesource by a factor of 200 was obtained, with a resolution R=6 in thevisible, using a simple setup.
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