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Speckle Suppression Through Dual Imaging Polarimetry, and a Ground-based Image of the HR 4796A Circumstellar Disk
We demonstrate the versatility of a dual imaging polarimeter working intandem with a Lyot coronagraph and adaptive optics to suppress thehighly static speckle noise pattern—the greatest hindrance toground-based direct imaging of planets and disks around nearby stars.Using a double difference technique with the polarimetric data, wequantify the level of speckle suppression, and hence improvedsensitivity, by placing an ensemble of artificial faint companions intoreal data, with given total brightness and polarization. For highlypolarized sources within 0farcs5, we show that we achieve 3 to4 mag greater sensitivity through polarimetric speckle suppressionthan simply using a coronagraph coupled to a high-order adaptive opticssystem. Using such a polarimeter with a classical Lyot coronagraph atthe 3.63 m Advanced Electro-Optical System telescope, we have obtained a6.5σ detection in the H band of the 76 AU diametercircumstellar debris disk around the star HR 4796A. Our data representthe first definitive ground-based near-IR polarimetric image of the HR4796A debris disk and clearly show the two outer ansae of the disk,evident in Hubble Space Telescope NICMOS/STIS imaging. Comparing ourpeak linearly polarized flux with the total intensity in the lobes asobserved by NICMOS, we derive a lower limit to the fractional linearpolarization of >29% caused by dust grains in the disk. In addition,we fit simple morphological models of optically thin disks to our dataallowing us to constrain the dust disk scale height (2.5+5.0–1.3 AU) and scattering asymmetry parameter (g =langcos θrang = 0.20+.07 –.10). Thesevalues are consistent with several lines of evidence suggesting that theHR 4796A disk is dominated by a micron-sized dust population, and areindeed typical of disks in transition between those surrounding theHerbig Ae stars to those associated with Vega-like stars.
| Infrared Spectrograph Characterization of a Debris Disk Around an M-Type Star in NGC 2547
We present 5 to 15 μm Spitzer Infrared Spectrograph (IRS)low-resolution spectral data of a candidate debris disk around an M4.5star identified as a likely member of the ~40 Myr old clusterNGC 2547. The IRS spectrum shows a silicate emission feature,indicating the presence of warm, small, (sub)micron-sized dust grains inthe disk. Of the 15 previously known candidate debris disks aroundM-type stars, the one we discuss in this paper is the first to have anobserved mid-infrared spectrum and is also the first to have measuredsilicate emission. We combined the IRS data with ancillary data(optical, JHKs , and Spitzer InfraRed Array Camera and 24μm data) to build the spectral energy distribution (SED) of thesource. Monte Carlo radiation transfer modeling of the SED characterizedthe dust disk as being very flat (h 100 = 2 AU) and extendinginward within at least 0.13 AU of the central star. Our analysisshows that the disk is collisionally dominated and is likely a debrisdisk.
| Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations
We have completed a high-resolution (R ≈ 60,000) opticalspectroscopic survey of 185 nearby M dwarfs identified using ROSAT datato select active, young objects with fractional X-ray luminositiescomparable to or greater than Pleiades members. Our targets are drawnfrom the NStars 20 pc census and the Moving-M sample with distancesdetermined from parallaxes or spectrophotometric relations. We limitedour sample to 25 pc from the Sun, prior to correcting forpre-main-sequence overluminosity or binarity. Nearly half of theresulting M dwarfs are not present in the Gliese catalog and have nopreviously published spectral types. We identified 30 spectroscopicbinaries (SBs) from the sample, which have strong X-ray emission due totidal spin-up rather than youth. This is equivalent to a 16% SBfraction, with at most a handful of undiscovered SBs. We estimate upperlimits on the age of the remaining M dwarfs using spectroscopic youthindicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements,a single CaH gravity index may not be sufficient, as highermetallicities mimic lower gravity. This is demonstrated in a subsampleof metal-rich radial velocity (RV) standards, which appear to have lowsurface gravity as measured by the CaH index, yet show no other evidenceof youth. We also use additional youth diagnostics such as lithiumabsorption and strong Hα emission to set more stringent agelimits. Eleven M dwarfs with no Hα emission or absorption arelikely old (>400 Myr) and were caught during an X-ray flare. Weestimate that our final sample of the 144 youngest and nearest low-massobjects in the field is less than 300 Myr old, with 30% of them beingyounger than 150 Myr and four very young (lap10 Myr), representing agenerally untapped and well-characterized resource of M dwarfs forintensive planet and disk searches.Based on observations collected at the W. M. Keck Observatory and theCanada-France-Hawaii Telescope. The Keck Observatory is operated as ascientific partnership between the California Institute of Technology,the University of California, and NASA, and was made possible by thegenerous financial support of the W. M. Keck Foundation. The CFHT isoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii.
| New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope
We present 24 μm and 70 μm Multiband Imaging Photometer forSpitzer (MIPS) observations of 70 A through M-type dwarfs with estimatedages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed timeprogram, including a re-analysis of some previously published sourcephotometry. Our sample is selected from stars with common youthindicators such as lithium abundance, X-ray activity, chromosphericactivity, and rapid rotation. We compare our MIPS observations toempirically derived Ks -[24] colors as a function of thestellar effective temperature to identify 24 μm and 70 μmexcesses. We place constraints or upper limits on dust temperatures andfractional infrared luminosities with a simple blackbody dust model. Weconfirm the previously published 70 μm excesses for HD 92945, HD112429, and AU Mic, and provide updated flux density measurements forthese sources. We present the discovery of 70 μm excesses for fivestars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599is also a known Spitzer IRS (InfraRed Spectrograph) excess source, andwe confirm the excess at 24 μm. We also present the detection of 24μm excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find thatlarge 70 μm excesses are less common around stars with effectivetemperatures of less than 5000 K (3.7+7.6–1.1%) than around stars with effective temperaturesbetween 5000 K and 6000 K (21.4+9.5 –5.7%),despite the cooler stars having a younger median age in our sample (12Myr vs. 340 Myr). We find that the previously reported excess for TWA13A at 70 μm is due to a nearby background galaxy, and the previouslyreported excess for HD 177724 is due to saturation of the near-infraredphotometry used to predict the mid-infrared stellar flux contribution.In the Appendix, we present an updated analysis of dust grain removaltimescales due to grain-grain collisions and radiation pressure,Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dustdynamical interaction. We find that drag forces can be important fordisk dynamics relative to grain-grain collisions for L IR/L* < 10–4, and that stellar wind drag ismore important than P-R drag for K and M dwarfs, and possibly for young(<1 Gyr) G dwarfs as well.
| Detecting circumstellar disks around gravitational microlenses
Aims. We investigate the chance of detecting proto-planetary or debrisdisks in stars that induce microlensing event (lenses), and consider themodification of the light curve shapes due to occultation and extinctionby the disks, as well as the gravitational deflection caused by theadditional mass. Methods: The magnification of gravitationalmicrolensing events is calculated using the ray shooting method. Theoccultation is taken into account by neglecting or weighting the imageson the lens plane according to a transmission map of the correspondingdisk for a point source point lens (PSPL) model. The estimated frequencyof events is obtained by considering the possible inclinations andoptical depths of the disk. Results: We conclude that gravitationalmicrolensing can be used, in principle, as a tool for detecting debrisdisks beyond 1 kpc, but estimate that each year around 1 debris disk isexpected for lens stars of F, G, or K spectral type and about 10 debrisdisks might have shown signatures in existing datasets.
| Modeling Porous Dust Grains with Ballistic Aggregates. II. Light Scattering Properties
We study the light scattering properties of random ballistic aggregatesconstructed in Shen et al. Using the discrete-dipole approximation, wecompute the scattering phase function and linear polarization for randomaggregates with various sizes and porosities, and with two differentcompositions: 100% silicate and 50% silicate +50% graphite. Weinvestigate the dependence of light scattering properties on wavelength,cluster size, and porosity using these aggregate models. We find thatwhile the shape of the phase function depends mainly on the sizeparameter of the aggregates, the linear polarization depends on both thesize parameter and the porosity of the aggregates, with increasingdegree of polarization as the porosity increases. Contrary to previousstudies, we argue that the monomer size has negligible effects on thelight scattering properties of ballistic aggregates, as long as theconstituent monomer is smaller than the incident wavelength up to 2πa0/λ ~ 1.6 where a 0 is the monomer radius.Previous claims for such monomer size effects are in fact the combinedeffects of size parameter and porosity. Finally, we present aggregatemodels that can reproduce the phase function and polarization ofscattered light from the AU Mic debris disk and from cometary dust,including the negative polarization observed for comets at scatteringangles 160° lsim θ < 180°. These aggregates havemoderate porosities, {\cal P}\approx 0.6, and are of sub-μm size forthe debris disk case, or μm size for the comet case.
| A Formation Scenario of Young Stellar Groups in the Region of the Scorpio Centaurus OB Association
The main objective of this work is to investigate the role played byLower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL), bothsubcomponents of the Scorpio Centaurus OB association (Sco-Cen), in theformation of the groups β Pictoris, TW Hydrae, and the ηChamaeleontis cluster. The dynamical evolution of all the stellar groupsinvolved and of the bubbles and shells blown by LCC and UCL arecalculated, and followed from the past to the present. This leads to aformation scenario in which (1) the groups β Pictoris, TW Hydraewere formed in the wake of the shells created by LCC and UCL, (2) theyoung cluster η Chamaeleontis was born as a consequence of thecollision of the shells of LCC and UCL, and (3) the formation of UpperScorpius (US), the other main subcomponent of the Sco-Cen association,may have been started by the same process that created ηChamaeleontis.
| Fomalhaut's Debris Disk and Planet: Constraining the Mass of Fomalhaut b from disk Morphology
Following the optical imaging of exoplanet candidate Fomalhaut b (Fomb), we present a numerical model of how Fomalhaut's debris disk isgravitationally shaped by a single interior planet. The model is simple,adaptable to other debris disks, and can be extended to accommodatemultiple planets. If Fom b is the dominant perturber of the belt, thento produce the observed disk morphology it must have a mass Mpl < 3M J, an orbital semimajor axis apl > 101.5 AU, and an orbital eccentricity e pl= 0.11-0.13. These conclusions are independent of Fom b's photometry. Tonot disrupt the disk, a greater mass for Fom b demands a smaller orbitfarther removed from the disk; thus, future astrometric measurement ofFom b's orbit, combined with our model of planet-disk interaction, canbe used to determine the mass more precisely. The inner edge of thedebris disk at a ≈ 133 AU lies at the periphery of Fom b's chaoticzone, and the mean disk eccentricity of e ≈ 0.11 is secularly forcedby the planet, supporting predictions made prior to the discovery of Fomb. However, previous mass constraints based on disk morphology rely onseveral oversimplifications. We explain why our constraint is morereliable. It is based on a global model of the disk that is notrestricted to the planet's chaotic zone boundary. Moreover, we screendisk parent bodies for dynamical stability over the system age of ~ 100Myr, and model them separately from their dust grain progeny; thelatter's orbits are strongly affected by radiation pressure and theirlifetimes are limited to ~ 0.1 Myr by destructive grain-graincollisions. The single planet model predicts that planet and disk orbitsbe apsidally aligned. Fomalhaut b's nominal space velocity does not bearthis out, but the astrometric uncertainties may be large. If the apsidalmisalignment proves real, our calculated upper mass limit of 3MJ still holds. If the orbits are aligned, our model predictsM pl = 0.5M J, a pl = 115 AU, and epl = 0.12. Parent bodies are evacuated from mean-motionresonances with Fom b; these empty resonances are akin to the Kirkwoodgaps opened by Jupiter. The belt contains at least 3M ⊕of solids that are grinding down to dust, their velocity dispersionsstirred so strongly by Fom b that collisions are destructive. Such alarge mass in solids is consistent with Fom b having formed in situ.
| Nearby Young Stars Selected by Proper Motion. I. Four New Members of the β Pictoris Moving Group From The Tycho-2 Catalog
We describe a procedure to identify stars from nearby moving groups andassociations out of catalogs of stars with large proper motions. We showthat from the mean motion vector of a known or suspected moving group,one can identify additional members of the group based on proper motiondata and photometry in the optical and infrared, with minimalcontamination from background field stars. We demonstrate this techniqueby conducting a search for low-mass members of the β Pictorismoving group in the Tycho-2 catalog. All known members of the movinggroup are easily recovered, and a list of 51 possible candidates isgenerated. Moving group membership is evaluated for 33 candidates basedon X-ray flux from ROSAT, Hα line emission, and radial velocitymeasurement from high-resolution infrared spectra obtained at InfraredTelescope Facility. We confirm three of the candidates to be new membersof the group: TYC 1186-706-1, TYC 7443-1102-1, and TYC 2211-1309-1 whichare late-K and early-M dwarfs 45-60 pc from the Sun. We also identify acommon proper motion companion to the known β Pictoris Moving Groupmember TYC 7443-1102-1, at a 26farcs3 separation; the new companion isassociated with the X-ray source 1RXS J195602.8 – 320720. We arguethat the present technique could be applied to other large proper motioncatalogs to identify most of the elusive, low-mass members of knownnearby moving groups and associations.Based on data obtained in part with the 2.4 m Hiltner telescope of theMDM observatory. Based on data obtained in part with the CTIO 1.5 mtelescope, operated by SMARTS, the Small and Medium Aperture TelescopeSystem consortium, under contract with the Associated Universities forResearch in Astronomy (AURA).
| Dust in the inner regions of debris disks around a stars
We present infrared interferometric observations of the inner regions oftwo A-star debris disks, β Leo and ζ Lep, using the FLUORinstrument at the CHARA interferometer on both short (30 m) and long(> 200 m) baselines. For the target stars, the short-baselinevisibilities are lower than expected for the stellar photosphere alone,while those of a check star, δ Leo, are not. We interpret thisvisibility offset of a few percent as a near-infrared (NIR) excessarising from dust grains which, due to the instrumental field of view,must be located within several AU of the central star. For β Leo,the NIR excess-producing grains are spatially distinct from the dustwhich produces the previously known mid-infrared (MIR) excess. Forζ Lep, the NIR excess may be spatially associated with the MIRexcess-producing material. We present simple geometric models which areconsistent with the NIR and MIR excesses and show that for both objects,the NIR-producing material is most consistent with a thin ring of dustnear the sublimation radius, with typical grain sizes smaller than thenominal radiation pressure blowout radius. Finally, we discuss possibleorigins of the NIR-emitting dust in the context of debris disk evolutionmodels.
| VLT/NACO coronagraphic observations of fine structures in the disk of β Pictoris
Aims. We present ground-based observations of the disk around the A-typestar β Pictoris to obtain scattered light images at the highestangular resolution (60 mas, equivalent to about 1 AU at the distance ofthe star) and the highest contrast in the very close environment of thestar. The purpose of this program is to perform a close inspection ofthe inner disk morphology. Methods: Images were collected with NACO,the AO-assisted near-IR instrument on the VLT (ESO) which includes twotypes of coronagraphs: classical Lyot masks and phase masks. In thisprogram we took advantage of both types of coronagraphs in two spectralbands, H-band for the Lyot mask and Ks-band for the phase mask. The Lyotmask blocks a large central region around the star (radius<0.35´´) but allows deep integrations and hence goodsignal-to-noise ratio at large distances, while the phase mask allowsimaging at very close separation (down to ~0.15´´ in theory)but conversely is more sensitive to residual aberrations. In addition,we simulated an extended object to understand the limitations indeconvolution of coronagraphic images. Results: The reducedcoronagraphic images allow us to carefully measure the structures of thedebris disk and reveal a number of asymmetries of which some were notreported before (position, elevation and thickness of the warp). Ouranalysis also demonstrates the advantage of the phase mask coronagraphto explore the very close environment of stars. In this program, thecircumstellar material is visible as close as 0.7´´ (13.5AU) owing to the phase mask while the Lyot mask generates artifactswhich hamper the detection of the dust at separations closer than1.2´´ (23.2 AU). The point source detection limit iscompared to recently published observations of a planet candidate.Finally, the simulations show that deconvolution of coronagraphic datamay indeed produce artificial patterns within the image of a disk.
| Variations on Debris Disks: Icy Planet Formation at 30-150 AU for 1-3 Msolar Main-Sequence Stars
We describe calculations for the formation of icy planets and debrisdisks at 30-150 AU around 1-3 Msolar stars. Debris diskformation coincides with the formation of planetary systems. Asprotoplanets grow, they stir leftover planetesimals to large velocities.A cascade of collisions then grinds the leftovers to dust, forming anobservable debris disk. Stellar lifetimes and the collisional cascadelimit the growth of protoplanets. The maximum radius of icy planets,rmax~1750 km, is remarkably independent of initial disk mass,stellar mass, and stellar age. These objects contain <~3%-4% of theinitial mass in solid material. Collisional cascades produce debrisdisks with maximum luminosity ~2×10-3 times the stellarluminosity. The peak 24 μm excess varies from ~1% times the stellarphotospheric flux for 1 Msolar stars to ~50 times thestellar photospheric flux for 3 Msolar stars. The peak 70-850μm excesses are ~30-100 times the stellar photospheric flux. For allstars, the 24-160 μm excesses rise at stellar ages of 5-20 Myr, peakat 10-50 Myr, and then decline. The decline is roughly a power law,f~t-n with n~0.6-1.0. This predicted evolution agrees withpublished observations of A-type and solar-type stars. The observedfar-IR color evolution of A-type stars also matches model predictions.
| Lithium Depletion of Nearby Young Stellar Associations
We estimate cluster ages from lithium depletion in fivepre-main-sequence groups found within 100 pc of the Sun: the TW Hydraeassociation, η Chamaeleontis cluster, β Pictoris moving group,Tucanae-Horologium association, and AB Doradus moving group. Wedetermine surface gravities, effective temperatures, and lithiumabundances for over 900 spectra through least-squares fitting tomodel-atmosphere spectra. For each group, we compare the dependence oflithium abundance on temperature with isochrones from pre-main-sequenceevolutionary tracks to obtain model-dependent ages. We find that theη Cha cluster and the TW Hydrae association are the youngest, withages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the βPic moving group at 21+/-9 Myr, the Tucanae-Horologium association at27+/-11 Myr, and the AB Dor moving group at an age of at least 45 Myr(whereby we can only set a lower limit, since the models-unlike realstars-do not show much lithium depletion beyond this age). Here theordering is robust, but the precise ages depend on our choice of bothatmospheric and evolutionary models. As a result, while our ages areconsistent with estimates based on Hertzsprung-Russell isochrone fittingand dynamical expansion, they are not yet more precise. Our observationsdo show that with improved models, much stronger constraints should befeasible, as the intrinsic uncertainties, as measured from the scatterbetween measurements from different spectra of the same star, are verylow: around 10 K in effective temperature, 0.05 dex in surface gravity,and 0.03 dex in lithium abundance.
| Coronal properties of the EQ Pegasi binary system
Context: The activity indicators of M dwarfs are distinctly differentfor early and late types. The coronae of early M dwarfs display highX-ray luminosities and temperatures, a pronounced inverse FIP effect,and frequent flaring to the extent that no quiescent level can bedefined in many cases. For late M dwarfs, fewer but more violent flareshave been observed, and the quiescent X-ray luminosity is much lower. Aims: To probe the relationship between coronal properties with spectraltype of active M dwarfs, we analyze the M3.5 and M4.5 components of theEQ Peg binary system in comparison with other active M dwarfs ofspectral types M0.5 to M5.5. Methods: We investigate the timingbehavior of both components of the EQ Peg system, reconstruct theirdifferential emission measure, and investigate the coronal abundanceratios based on emission-measure independent line ratios from theirChandra HETGS spectra. Finally we test for density variations indifferent states of activity. Results: The X-ray luminosity of EQ Peg A(M3.5) is by a factor of 6-10 brighter than that of EQ Peg B (M4.5).Like most other active M dwarfs, the EQ Peg system shows an inverse FIPeffect. The abundances of both components are consistent within theerrors; however, there seems to be a tendency toward the inverse FIPeffect being less pronounced in the less active EQ Peg B when comparingthe quiescent state of the two stars. This trend is supported by ourcomparison with other M dwarfs. Conclusions: As the X-ray luminositydecreases with later spectral type, so do coronal temperatures and flarerate. The amplitude of the observed abundance anomalies, i.e. theinverse FIP effect, declines; however, clear deviations from solarabundances remain.
| New M Dwarf Debris Disk Candidates in NGC 2547
With only six known examples, M dwarf debris disks are rare, even thoughM dwarfs constitute the majority of stars in the Galaxy. After finding anew M dwarf debris disk in a shallow mid-infrared observation of NGC2547, we present a considerably deeper Spitzer MIPS image of the region,with a maximum exposure time of 15 minutes pixel-1. Amongsources selected from a previously published membership list, weidentify nine new M dwarfs with excess emission at 24 μm tracing warmmaterial close to the snow line of these stars, at orbital radii of lessthan 1 AU. We argue that these are likely debris disks, suggesting thatplanet formation is under way in these systems. Interestingly, theestimated excess fraction of M stars appears to be higher than that of Gand K stars in our sample.
| Common Proper Motion Companions to Nearby Stars: Ages and Evolution
A set of 41 nearby stars (closer than 25 pc) is investigated which havevery wide binary and common proper motion (CPM) companions at projectedseparations between 1000 and 200,000 AU. These companions are identifiedby astrometric positions and proper motions from the NOMAD catalog.Based mainly on measures of chromospheric and X-ray activity, ageestimation is obtained for most of 85 identified companions.Color-absolute magnitude diagrams are constructed to test whether CPMcompanions are physically related to the primary nearby stars and havethe same age. Our carefully selected sample includes three remote whitedwarf companions to main-sequence stars and two systems (55 Cnc and GJ777A) of multiple planets and distant stellar companions. Ten new CPMcompanions, including three of extreme separations, are found. Multiplehierarchical systems are abundant; more than 25% of CPM components arespectroscopic or astrometric binaries or multiples themselves. Two newastrometric binaries are discovered among nearby CPM companions, GJ 264and HIP 59000, and preliminary orbital solutions are presented. TheHyades kinematic group (or stream) is presented broadly in the sample,but we find few possible thick-disk objects and no halo stars. Itfollows from our investigation that moderately young (age<~1 Gyr)thin-disk dwarfs are the dominating species in the near CPM systems, ingeneral agreement with the premises of the dynamical survival paradigm.
| Probing dust grain evolution in IM Lupi's circumstellar disc. Multi-wavelength observations and modelling of the dust disc
Aims. We present a panchromatic study, involving a multiple techniqueapproach, of the circumstellar disc surrounding the T Tauri star IM Lupi(Sz 82). Methods: We have undertaken a comprehensive observationalstudy of IM Lupi using photometry, spectroscopy, millimetreinterferometry and multi-wavelength imaging. For the first time, thedisc is resolved from optical and near-infrared wavelengths in scatteredlight, to the millimetre regime in thermal emission. Our data-set, inconjunction with existing photometric data, provides an extensivecoverage of the spectral energy distribution, including a detailedspectrum of the silicate emission bands. We have performed asimultaneous modelling of the various observations, using the radiativetransfer code MCFOST, and analysed a grid of models over a largefraction of the parameter space via Bayesian inference. Results: Wehave constructed a model that can reproduce all of the observations ofthe disc. Our analysis illustrates the importance of combining a widerange of observations in order to fully constrain the disc model, witheach observation providing a strong constraint only on some aspects ofthe disc structure and dust content. Quantitative evidence of dustevolution in the disc is obtained: grain growth up to millimetre-sizedparticles, vertical stratification of dust grains with micrometricgrains close to the disc surface and larger grains which have settledtowards the disc midplane, and possibly the formation of fluffyaggregates and/or ice mantles around grains.
| Evolution of Debris Disks
Circumstellar dust exists around several hundred main sequence stars.For the youngest stars, that dust could be a remnant of theprotoplanetary disk. Mostly it is inferred to be continuouslyreplenished through collisions between planetesimals in belts analogousto the Solar System's asteroid and Kuiper belts, or in collisionsbetween growing protoplanets. The evolution of a star's debris disk isindicative of the evolution of its planetesimal belts and may beinfluenced by planet formation processes, which can continue throughoutthe first gigayear as the planetary system settles to a stableconfiguration and planets form at large radii. Evidence for thatevolution comes from infrared photometry of large numbers of debrisdisks, providing snapshots of the dust present at different evolutionaryphases, as well as from images of debris disk structure. This reviewdescribes the theoretical framework within which debris disk evolutiontakes place and shows how that framework has been constrained byobservations.
| O VI Observations of the Onset of Convection Zones in Main-Sequence A Stars
If magnetic activity in outer stellar atmospheres is due to an interplaybetween rotation and subsurface convection, as is generally presumed,then one would not expect to observe indicators of activity in starswith Teff>~8300 K. Any X-ray or ultraviolet line emissionfrom hotter stars must be due either to a different mechanism or to anunresolved, active, binary companion. Due to their poor spatialresolution, X-ray instruments have been especially susceptible to sourceconfusion. At wavelengths longward of Lyα, the near-ultravioletspectra of stars hotter than this putative dividing line are dominatedby the photospheric continuum. We have used the Far-UltravioletSpectroscopic Explorer (FUSE) to obtain spectra of the subcoronal O VIemission lines, which lie at a wavelength where the photosphericcontinuum of the middle and early A stars is relatively weak. Weobserved 14 stars spanning a range in Teff from 7720 to10,000 K. Eleven of the 14 stars showed O VI emission lines, including 6of the 8 targets with Teff>8300 K. At face value, thissuggests that activity does not fall off with increasing temperature.However, the emission lines are narrower than expected from theprojected rotational velocities of these rapidly rotating stars,suggesting that the emission could come from unresolved late-typecompanions. Furthermore, the strength of the O VI emission is consistentwith that expected from an unseen active K or M dwarf binary companion,and the high LX/L(O VI) ratios observed indicate that thismust be the case. Our results are therefore consistent with earlierstudies that have shown a rapid drop-off in activity at theradiative/convective boundary expected at Teff~8300 K, inagreement with conventional stellar structure models.Based on observations made with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer (FUSE), operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
| Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs
We report on radio observations of the M8.5 dwarf LSR J1835+3259 and theL3.5 dwarf 2MASS J00361617+1821104, which provide the strongest evidenceto date that the electron cyclotron maser instability is the dominantmechanism producing radio emission in the magnetospheres of ultracooldwarfs. As has previously been reported for the M9 dwarf TVLM 513-46546,periodic pulses of 100% circularly polarized, coherent radio emissionare detected from both dwarfs with periods of 2.84+/-0.01 and3.08+/-0.05 hr, respectively, for LSR J1835+3259 and 2MASSJ00361617+1821104. Importantly, periodic unpolarized radio emission isalso detected from 2MASS J00361617+1821104, and brightness temperaturelimitations rule out gyrosynchrotron radiation as a source of this radioemission. The unpolarized emission from this and other ultracool dwarfsis also attributed to electron cyclotron maser emission, which hasbecome depolarized on traversing the ultracool dwarf magnetosphere,possibly due to propagations effects such as scattering. Based onavailable vsini data in the literature and rotation periods derived fromthe periodic radio data for the three confirmed sources of electroncyclotron maser emission, TVLM 513-46546, LSR J1835+3259, and 2MASSJ00361617+1821104, we determine that the rotation axes of all threedwarfs are close to perpendicular to our line of sight. This suggests apossible geometrical selection effect due to the inherent directivity ofelectron cyclotron maser emission, that may account for the previouslyreported relationship between radio activity and vsini observed forultracool dwarfs. We also determine the radius of the dwarf LSRJ1835+3259 to be >=0.117+/-0.012 Rsolar. The implied sizeof the radius, together with the bolometric luminosity of the dwarf,suggests that either LSR J1835 is a young- or intermediate-age browndwarf, or that current theoretical models underestimate the radii ofultracool dwarfs.
| An extremely wide and very low-mass pair with common proper motion. Is it representative of a nearby halo stream?
Aims. We describe the discovery of an extremely wide pair of low-massstars with a common large proper motion and discuss their possiblemembership in a Galactic halo stream crossing the Solar neighbourhood. Methods: In a high proper motion survey of the southern sky we usedmulti-epoch positions and photometry from the SuperCOSMOS Sky Surveys.New nearby ultracool dwarf and subdwarf candidates were selected amongthe faint and red high proper motion objects, and subsequently confirmedby low-resolution classification spectroscopy. The resultingspectroscopic distance estimates, approximate radial velocitymeasurements and improved proper motions involving additional epochsfrom the Two Micron All Sky Survey and from the DEep Near-InfraredSurvey were used to compute Galactic space velocities. Results: Thelate-type (M 7) dwarf SSSPM J2003-4433 and the ultracool subdwarf SSSPMJ1930-4311 (sdM 7) sharing the same very large proper motion of about860 mas/yr were found in the same sky region with an angular separationof about 6°. From the comparison with other high proper motioncatalogues we have estimated the probability of a chance alignment ofthe two new large proper motions to be less than 0.3%. From theindividually estimated spectroscopic distances of about38+10-7 pc and 72+21-16 pc,respectively for the M 7 dwarf and the sdM 7 subdwarf, and in view ofthe accurate agreement in their large proper motions we assume a commondistance of about 50 pc and a projected physical separation of about 5pc. The mean heliocentric space velocity of the pair (U,V,W)=(-232,-170, +74) km s-1, based on the correctness of thepreliminary radial velocity measurement for only one of the componentsand on the assumption of a common distance and velocity vector, istypical of the Galactic halo population. Conclusions: The largeseparation and the different metallicities of dwarfs and subdwarfs makea common formation scenario as a wide binary (later disrupted)improbable, although there remains some uncertainty in the spectroscopicclassification scheme of ultracool dwarfs/subdwarfs so that a dissolvedbinary origin cannot be fully ruled out yet. It seems more likely thatthis wide pair is part of an old halo stream. Higher-resolutionspectroscopic observations are needed to measure accurate radialvelocities of both components. Further, we suggest to check the M 7dwarf for an unresolved binary status, which would explain its shorterspectroscopic distance estimate, and to place both objects on atrigonometric parallax program.Based onobservations with the ESO 3.6 m/EFOSC2 at the European SouthernObservatory, La Silla (ESO program 70.C-0568).
| The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing
We present the first multicolor view of the scattered light disk of theHerbig Ae star HD 163296, based on coronagraphic observations from theHubble Space Telescope Advanced Camera for Surveys (HST ACS). Radialprofile fits of the surface brightness along the disk's semimajor axisindicate that the disk is not continuously flared, and extends to ~540AU. The disk's color (V-I)=1.1 at a radial distance of 3.5" is redderthan the observed stellar color (V-I)=0.15. This red disk color might beindicative of either an evolution in the grain size distribution (i.e.,grain growth) and/or composition, both of which would be consistent withthe observed nonflared geometry of the outer disk. We also identify asingle ansa morphological structure in our F435W ACS data, which isabsent from earlier epoch F606W and F814W ACS data, but corresponds toone of the two ansae observed in archival HST Space Telescope ImagingSpectrograph (STIS) coronagraphic data. Following transformation tosimilar bandpasses, we find that the scattered light disk of HD 163296is 1 mag arcsec-2 fainter at 3.5" in the STIS data than inthe ACS data. Moreover, variations are seen in (1) the visibility of theansa(e) structures, (2) the relative surface brightness of the ansa(e)structures, and (3) the (known) intrinsic polarization of the system.These results indicate that the scattered light from the HD 163296 diskis variable. We speculate that the inner disk wall, which Sitko et al.suggests has a variable scale height as diagnosed by near-IR SEDvariability, induces variable self-shadowing of the outer disk. Wefurther speculate that the observed surface brightness variability ofthe ansa(e) structures may indicate that the inner disk wall isazimuthally asymmetric.
| Spitzer MIPS Observations of Stars in the β Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observationsat 24 and 70 μm for 30 stars, and at 160 μm for a subset of 12stars, in the nearby (~30 pc), young (~12 Myr) β Pictoris movinggroup (BPMG). In several cases, the new MIPS measurements resolve sourceconfusion and background contamination issues in the IRAS data for thissample. We find that 7 members have 24 μm excesses, implying a debrisdisk fraction of 23%, and that at least 11 have 70 μm excesses (diskfraction of >=37%). Five disks are detected at 160 μm (out of abiased sample of 12 stars observed), with a range of 160/70 flux ratios.The disk fraction at 24 and 70 μm, and the size of the excessesmeasured at each wavelength, are both consistent with an ``inside-out''infrared excess decrease with time, wherein the shorter wavelengthexcesses disappear before longer wavelength excesses, and consistentwith the overall decrease of infrared excess frequency with stellar age,as seen in Spitzer studies of other young stellar groups. Assuming thatthe infrared excesses are entirely due to circumstellar disks, wecharacterize the disk properties using simple models and fractionalinfrared luminosities. Optically thick disks, seen in the younger TW Hyaand η Cha associations, are entirely absent in the BPMG. Additionalflux density measurements at 24 and 70 μm are reported for nineTucana-Horologium association member stars. Since this is <20% of theassociation membership, limited analysis on the complete disk fractionof this association is possible.
| Observational Possibility of the ``Snow Line'' on the Surface of Circumstellar Disks with the Scattered Light
We discuss how we obtain the spatial distribution of ice on the surfaceof the circumstellar disk around young stars. Ice in the disks plays avery important role in various issues, for instance, on the diskstructure, on planet formation, on an isotopic anomaly in meteorites,and on the origin of the oceans on Earth. Therefore, spatially resolvedobservations of the condensation/sublimation front of ice, the so-calledsnow line is strongly required. Here, we propose a new method forobtaining a spatially resolved snow line on circumstellar disks byobserving a 3μ H2O ice feature in the scattered light.Based on radiative transfer considerations, we show that the feature isclearly imprinted in the spectrum of the scattered light from bothoptically thick and thin circumstellar disks. We also show that thescattered light and the H2O ice feature from protoplanetarydisks are detectable and spatially resolvable with current instrumentsthrough anH2O narrowband filter around 3μm. Finally, wepresent a diagnostics of disk dust properties on K-H2O andK-L' two-color diagram.
| Further Constraints on the Presence of a Debris Disk in the Multiplanet System Gliese 876
Using both the Very Large Array (VLA) at 7 mm wavelength, and theAustralia Telescope Compact Array (ATCA) at 3 mm, we have searched formicrowave emission from cool dust in the extrasolar planetary systemGliese 876 (Gl 876). Having detected no emission above our 3σdetection threshold of 135 μJy, we rule out any dust disk with eithera mass greater than 0.0006 M ⊕ or less than ~250 AUacross. This result improves on previous detection aperture thresholdsby an order of magnitude, and it has some implications for the dynamicalmodeling of the system. It also is consistent with the Greaves et al.hypothesis that relates the presence of a debris disk to close-inplanets. Due to the dust-planetesimal relationship, our null result mayalso provide a constraint on the population or composition of the dustand small bodies around this nearby M dwarf.
| Spitzer/irac Search for Companions to Nearby, Young M Dwarfs
We present the results of a survey of nearby, young M stars for widelow-mass companions with the Infrared Array Camera (IRAC) on the SpitzerSpace Telescope. We observed 40 young M dwarfs within 20 pc of the Sun,selected through X-ray emission criteria. A total of ten candidatecompanions were found with IRAC colors consistent with T dwarfs.Extensive ground-based NIR follow-up observations rejected all thesecandidates. Two additional candidates were discovered via common propermotion measurements, one of which was rejected as a background objectand the other is a bona fide companion to GJ 2060, a member of the ABDoradus moving group.
| X-Atlas: An Online Archive of Chandra's Stellar High-Energy Transmission Grating Observations
The high-resolution X-ray spectroscopy made possible by the 1999deployment of the Chandra X-ray Observatory has revolutionized ourunderstanding of stellar X-ray emission. Many puzzles remain, however,particularly regarding the mechanisms of X-ray emission from OB stars.Although numerous individual stars have been observed in highresolution, realizing the full scientific potential of theseobservations will necessitate studying the high-resolution Chandra dataset as a whole. To facilitate the rapid comparison and characterizationof stellar spectra, we have compiled a uniformly processed database ofall stars observed with the Chandra High-Energy Transmission Grating(HETG). This database, known as X-Atlas, is accessible through a Webinterface with searching, data retrieval, and interactive plottingcapabilities. For each target, X-Atlas also features predictions of thelow-resolution ACIS spectra convolved from the HETG data for comparisonwith stellar sources in archival ACIS images. Preliminary analyses ofthe hardness ratios, quantiles, and spectral fits derived from thepredicted ACIS spectra reveal systematic differences between the high-and low-mass stars in the atlas and offer evidence for at least twodistinct classes of high-mass stars. A high degree of X-ray variabilityis also seen in both high- and low-mass stars, including Capella, longthought to exhibit minimal variability. X-Atlas contains over 130observations of approximately 25 high-mass stars and 40 low-mass starsand will be updated as additional stellar HETG observations becomepublic. The atlas has recently expanded to nonstellar point sources, andLow-Energy Transmission Grating (LETG) observations are currently beingadded as well.
| The Complete Census of 70 μm-bright Debris Disks within ``the Formation and Evolution of Planetary Systems'' Spitzer Legacy Survey of Sun-like Stars
We report detection of cool dust surrounding solar-type stars fromobservations performed as part of the Spitzer Legacy Science ProgramFEPS. From a sample of 328 stars having ages ~0.003-3 Gyr we haveselected sources with 70 μm flux densities indicating excess in theirSEDs above expected photospheric emission. Six strong excess sources arelikely primordial circumstellar disks, remnants of the star formationprocess. Another 25 sources having >=3 σ excesses areassociated with dusty debris disks, generated by collisions withinplanetesimal belts that are possibly stirred by existing planets. Sixadditional sources with >=2 σ excesses require confirmation asdebris disks. In our analysis, most (>80%) 70 μm excess sourceshave >=3 σ excesses at 33 μm as well, while only a minority(<40%) have >=3 σ excesses at 24 μm. The rising SEDstoward (and perhaps beyond) 70 μm imply dust temperatures <45-85 Kfor debris in equilibrium with the stellar radiation field. From fittedsingle-temperature blackbody models we infer bulk dust properties suchas characteristic temperature, location, fractional luminosity, andmass. For >1/3 of the debris sources we find that multipletemperature components are suggested, implying a dust distributionextending over many tens of AU. Because the disks are dominated bycollisional processes, the parent body (planetesimal) belts may beextended as well. Preliminary assessment of the statistics of colddebris around Sun-like stars shows that ~10% of FEPS targets with massesbetween 0.6 and 1.8 Msolar and ages between 30 Myr and 3 Gyrexhibit excess 70 μm emission. We find that fractional excessamplitudes appear higher for younger stars and that there may be a trendin 70 μm excess frequency with stellar mass.
| Outer edges of debris discs. How sharp is sharp?
Context: Rings or annulus-like features have been observed in mostimaged debris discs. Outside the main ring, while some systems (e.g.,β Pictoris and AU Mic) exhibit smooth surface brightness profiles(SB) that fall off roughly as ~r-3.5, others (e.g. HR 4796Aand HD 139664) display large drops in luminosity at the ring's outeredge and steeper radial luminosity profiles. Aims: We seek tounderstand this diversity of outer edge profiles under the“natural” collisional evolution of the system, withoutinvoking external agents such as planets or gas. Methods: We use amulti-annulus statistical code to follow the evolution of a collisionalpopulation, ranging in size from dust grains to planetesimals andinitially confined within a belt (the “birth ring”). Thecrucial effect of radiation pressure on the dynamics and spatialdistribution of the smallest grains is taken into account. We explorethe dependence of the resulting disc surface brightness profile onvarious parameters. Results: The disc typically evolves toward a“standard” steady state, where the radial surface brightnessprofile smoothly decreases with radius as r-3.5 outside thebirth ring. This confirms and extends the semi-analytical study ofStrubbe & Chiang (2006, ApJ, 648, 652) and provides a firm basis forinterpreting observed discs. Deviations from this typical profile, inthe form of a sharp outer edge and a steeper fall-off, occur for two“extreme” cases: 1) when the birth ring is so massive thatit becomes radially optically thick for the smallest grains. However,the required disc mass is probably too high here to be realistic; 2)when the dynamical excitation of the dust-producing planetesimals is solow (< e> and < i> ≤ 0.01) that the smallest grains,which otherwise dominate the optical depth of the system, arepreferentially depleted. This low-excitation case, although possibly notgeneric, cannot be ruled out by observations for most systems, . Conclusions: Our “standard” profile provides a satisfactoryexplanation for a large group of debris discs that show smooth outeredges and SB ∝ r-3.5. Systems with sharper outer edges,barring other confining agents, could still be explained by“natural” collisional evolution if their dynamicalexcitation is very low. We show that such a dynamically-cold caseprovides a satisfactory fit to the specific HR4796A ring.
| The SCUBA Legacy Catalogues: Submillimeter-Continuum Objects Detected by SCUBA
We present the SCUBA Legacy Catalogues, two comprehensive sets ofcontinuum maps (and catalogs) using data at 850 and 450 μm of thevarious astronomical objects obtained with the Submillimetre Common UserBolometer Array (SCUBA). The Fundamental Map Data Set contains data onlywhere superior atmospheric opacity calibration data were available. TheExtended Map Data Set contains data regardless of the quality of theopacity calibration. Each data set contains1.2deg×1.2deg maps at locations where dataexisted in the JCMT archive, imaged using the matrix inversion method.The Fundamental Data Set is composed of 1423 maps at 850 μm and 1357maps at 450 μm. The Extended Data Set is composed of 1547 maps at 850μm. Neither data set includes high sensitivity, single-chop SCUBAmaps of ``cosmological fields'' nor solar system objects. Each data setwas used to determine a respective object catalogue, consisting ofobjects identified within the respective 850 μm maps using anautomated identification algorithm. The Fundamental and Extended MapObject Catalogues contain 5061 and 6118 objects, respectively. Objectsare named based on their respective J2000.0 position of peak 850 μmintensity. The catalogues provide for each object the respective maximum850 μm intensity, estimates of total 850 μm flux and size, andtentative identifications from the SIMBAD Database. Where possible, thecatalogues also provide for each object its maximum 450 μm intensityand total 450 μm flux and flux ratios.
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