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Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| HD 97048's Circumstellar Environment as Revealed by a Hubble Space Telescope ACS Coronagraphic Study of Disk Candidate Stars
We present the results of a coronagraphic scattered-light imaging surveyof six young disk candidate stars using the Hubble Space Telescope Advanced Camera for Surveys. The observations made use of the 1.8"occulting spot through the F606W (broad V) filter. Circumstellarmaterial was imaged around HD 97048, a Herbig Ae/Be star located in theChamaeleon I dark cloud at a distance of 180 pc. The material is seenbetween ~2" (360 AU) and ~4" (720 AU) from the star in all directions. A V-band azimuthally averaged radial surface brightness profile peaks atr=2'' with a value of 19.6+/-0.2 mag arcsec-2 andsmoothly decreases with projected distance from the star asI~r-3.3+/-0.5. An integrated flux of 16.8+/-0.1 mag ismeasured between 2" and 4", corresponding to a scattered-lightfractional luminosity lower limit ofLsca/L*>8.4×10-4. Filamentarystructure resembling spiral arms similar to that seen in Herbig Ae/Bedisks is observed. Such structure has been attributed to the influenceof orbiting planets or stellar encounters. Average surface brightnessupper limits are determined for the five nondetections: HD 34282, HD139450, HD 158643, HD 159492, and HD 195627. Possible reasons for thenondetections are disks that are too faint or disks hidden by theocculter.
| Observations of Herbig Ae Disks with Nulling Interferometry
We present the results of 10 μm nulling interferometric observationsof 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 mtelescopes. A portion of the observations was completed with theadaptive secondary at the MMT. We have conclusively spatially resolved 3of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter tworecently resolved using adaptive optics in combination with nullinginterferometry. For the resolved objects we find that the 10 μmemitting regions have a spatial extent of 15-30 AU in diameter. We alsohave some evidence for resolved emission surrounding an additional twostars (V892 Tau and R CrA). For those objects in our study with mid-IRSEDs in the classification of Meeus and coworkers, we find that thegroup I objects (those with constant to increasing mid-IR flux) are morelikely to be resolved, within our limited sample. This trend is evidentin correlations in the inferred disk sizes versus the submillimeter SEDslope and disk size versus fractional infrared luminosity of thesystems. We explore the spatial distribution and orientation of the warmdust in the resolved systems and constrain physical models that areconsistent with their observational signatures.The results presented here made use of the of MMT Observatory, a jointlyoperated facility of the University of Arizona and the SmithsonianInstitution. This paper also includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile.
| On the Nature of the Dust in the Debris Disk around HD 69830
We have used the infrared mineralogical model derived from the SpitzerIRS observations of the Deep Impact experiment to study the nature ofthe dust in the debris found around the K0 V star HD 69830. Using arobust approach to determine the bulk average mineralogical compositionof the dust, we show it to be substantially different from that foundfor comets 9P/Tempel 1 and C/Hale-Bopp 1995 O1 or for thecomet-dominated YSO HD 100546. Lacking in carbonaceous and ferrousmaterials but including small icy grains, the composition of the HD69830 dust most closely resembles that of a disrupted P- or D-typeasteroid. The amount of mass responsible for the observed emission isthe equivalent of a 30 km radius, 2500 kg m-3 sphere, whilethe radiative temperature of the dust implies that the bulk of theobserved material is at ~1.0 AU from the central source, coincident withthe 2:1 and 5:2 mean motion resonances of the outermost of threeNeptune-sized planets detected by Lovis and coworkers. In our solarsystem, P- and D-type asteroids are both large and numerous in the outermain belt and near Jupiter (e.g., the Hildas and Trojans) and haveundergone major disruptive events to produce debris disk-like structures(cf. the Karin and Veritas families 5-8 Myr ago). The short-lived natureof the small and icy dust implies that the disruption occurred withinthe last year, or that replenishment due to ongoing collisionalfragmentation is occurring.
| Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
Context: .Low-mass stars form with disks in which the coagulation ofgrains may eventually lead to the formation of planets. It is not knownwhen and where grain growth occurs, as models that explain theobservations are often degenerate. A way to break this degeneracy is toresolve the sources under study. Aims: .Our aim is to findevidence for the existence of grains of millimetre sizes in disks aroundT Tauri stars, implying grain growth. Methods: .The AustraliaTelescope Compact Array (ATCA) was used to observe 15 southern T Tauristars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm.The five Lupus sources were also observed with the SubMillimeter Array(SMA) at 1.4 mm. Our new data are complemented with data from theliterature to determine the slopes of the spectral energy distributionsin the millimetre regime. Results: .Ten sources were detected atbetter than 3σ with the ATCA, with σ ≈ 1-2 mJy, and allsources that were observed with the SMA were detected at better than15σ, with σ ≈ 4 mJy. Six of the sources in our sample areresolved to physical radii of ~100 AU. Assuming that the emission fromsuch large disks is predominantly optically thin, the millimetre slopecan be related directly to the opacity index. For the other sources, theopacity indices are lower limits. Four out of six resolved sources haveopacity indices ⪉1, indicating grain growth to millimetre sizes andlarger. The masses of the disks range from <0.01 to 0.08 M_ȯ,which is comparable to the minimum mass solar nebula. A tentativecorrelation is found between the millimetre slope and the strength andshape of the 10-μm silicate feature, indicating that grain growthoccurs on similar (short) timescales in both the inner and outerdisk.
| Collisional dust avalanches in debris discs
We quantitatively investigate how collisional avalanches may develop indebris discs as the result of the initial breakup of a planetesimal orcomet-like object, triggering a collisional chain reaction due tooutward escaping small dust grains. We use a specifically developednumerical code that follows both the spatial distribution of the dustgrains and the evolution of their size-frequency distribution due tocollisions. We investigate how strongly avalanche propagation depends ondifferent parameters (e.g., amount of dust released in the initialbreakup, collisional properties of dust grains, and their distributionin the disc). Our simulations show that avalanches evolve on timescalesof ~1000 years, propagating outwards following a spiral-like pattern,and that their amplitude exponentially depends on the number density ofdust grains in the system. We estimate the probability of witnessing anavalanche event as a function of disc densities, for a gas-free casearound an A-type star, and find that features created by avalanchepropagation can lead to observable asymmetries for dusty systems with aβ Pictoris-like dust content or higher. Characteristic observablefeatures include: (i) a brightness asymmetry of the two sides for a discviewed edge-on, and (ii) a one-armed open spiral or a lumpy structure inthe case of face-on orientation. A possible system in whichavalanche-induced structures might have been observed is the edge-onseen debris disc around HD 32297, which displays a strong luminositydifference between its two sides.
| Dust filtration at gap edges: implications for the spectral energy distributions of discs with embedded planets
The spectral energy distributions (SEDs) of some T Tauri stars display adeficit of near-infrared flux that could be a consequence of an embeddedJupiter-mass planet partially clearing an inner hole in thecircumstellar disc. Here, we use two-dimensional numerical simulationsof the planet-disc interaction, in concert with simple models for thedust dynamics, to quantify how a planet influences the dust at differentradii within the disc. We show that pressure gradients at the outer edgeof the gap cleared by the planet act as a filter - letting particlessmaller than a critical size through to the inner disc while holdingback larger particles in the outer disc. The critical particle sizedepends on the disc properties, but is typically of the order of10μm. This filtration process will lead to discontinuous grainpopulations across the planet's orbital radius, with small grains in theinner disc and an outer population of larger grains. We show that thistype of dust population is qualitatively consistent with SED modellingof systems that have optically thin inner holes in their circumstellardiscs. This process can also produce a very large gas-to-dust ratio inthe inner disc, potentially explaining those systems with optically thininner cavities that still have relatively high accretion rates.
| Mid-infrared imaging of the circumstellar dust around three Herbig Ae stars: HD 135344, CQ Tau, and HD 163296
Aims.Planet formation has been known for many years to be tied to thespatial distribution of gas and dust in disks around young stars. Toconstrain planet formation models, imaging observations ofprotoplanetary disks are required. Methods: . Given this, we haveundertaken a mid-infrared imaging survey of Herbig Ae stars, which arepre-main sequence stars of intermediate mass still surrounded by a largeamount of circumstellar material. The observations were made at awavelength of 20.5 μm with the CAMIRAS camera mounted at theCassegrain focus of the Canada France Hawaii Telescope. Results: .We report the observations of three stars, HD 135344, CQ Tau, and HD163296. The circumstellar material around the three objects is spatiallyresolved. The extensions feature a disk-like shape. The images providedirect information on two key parameters of the disk: its inclinationand its outer radius. The outer radius is found to be quite differentfrom the one deduced from disk models, which is only constrained byfitting the Spectral Energy Distribution of the object. Other parametersof the disk, such as flaring and dust mass have been deduced fromfitting both the observed extension and the spectral energy distributionwith sophisticated disk models. Conclusions: .Our results showhow important imaging data are to tighten constraints on the disk modelparameters.
| The Burst Mode of Protostellar Accretion
We present new numerical simulations in the thin disk approximation thatcharacterize the burst mode of protostellar accretion. The burst modebegins upon the formation of a centrifugally balanced disk around anewly formed protostar. It comprises prolonged quiescent periods of lowaccretion rate (typically <~10-7 Msolaryr-1) that are punctuated by intense bursts of accretion(typically >~10-4 Msolar yr-1, withduration <~100 yr) during which most of the protostellar mass isaccumulated. The accretion bursts are associated with the formation ofdense protostellar/protoplanetary embryos, which are later driven ontothe protostar by the gravitational torques that develop in the disk.Gravitational instability in the disk, driven by continuing infall fromthe envelope, is shown to be an effective means of transporting angularmomentum outward and mass inward to the protostar. We show that the diskmass always remains significantly less than the central protostar's massthroughout this process. The burst phenomenon is robust enough to occurfor a variety of initial values of rotation rate and frozen-in(supercritical) magnetic field and a variety of density-temperaturerelations. Even in cases where the bursts are nearly entirelysuppressed, a moderate increase in cloud size or rotation rate can leadto vigorous burst activity. We conclude that most (if not all)protostars undergo a burst mode of evolution during their earlyaccretion history, as inferred empirically from observations of FUOrionis variables.
| Discovery of an 86 AU Radius Debris Ring around HD 181327
HST NICMOS PSF-subtracted coronagraphic observations of HD 181327 haverevealed the presence of a ringlike disk of circumstellar debris seen in1.1 μm light scattered by the disk grains, surrounded by a diffuseouter region of lower surface brightness. The annular disk appears to beinclined by 31.7d+/-1.6d from face-on, with the disk major-axis P.A. at107deg+/-2deg. The total 1.1 μm flux density ofthe light scattered by the disk (at 1.2"
| Stellar parameters and evidence of circumstellar activity for a sample of Herbig Ae/Be stars
Aims.We investigate evidence of accretion in a sample of 15 Herbig Ae/Bestars to determine whether these events originate in a remnant gaseousstructure from the primordial cloud (rich in hydrogen) or in ametal-rich body (like comets in our Solar System). During such analysiswe also determine precise stellar parameters for this sample ofstars. Methods: .The stars were observed using high resolutionspectroscopy (R = 48 000). A synthetic photospheric spectrum wasconstructed and then subtracted from the observed one to obtain thecircumstellar component. An iterative procedure was applied to find thestellar parameters that were used to build the synthetic photosphericspectrum. Results: .Evidence of circumstellar activity were foundin four stars: HD 100546, HD 142666, HD 144432 and HD 145718. Thepresence of redshifted absorption features only in the Balmer linesimplies that the accreting material is hydrogen-rich, excluding thepossibility that the accretion events might have been created bycomet-like bodies. We determined effective temperature, surface gravity,metallicity and the projected rotational velocity for the stars in oursample.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Spitzer Spectral Observations of the Deep Impact Ejecta
Spitzer Space Telescope imaging spectrometer observations of comet9P/Tempel 1 during the Deep Impact encounter returned detailed, highlystructured, 5- to 35-micrometer spectra of the ejecta. Emissionsignatures due to amorphous and crystalline silicates, amorphous carbon,carbonates, phyllosilicates, polycyclic aromatic hydrocarbons, water gasand ice, and sulfides were found. Good agreement is seen between theejecta spectra and the material emitted from comet C/1995 O1 (Hale-Bopp)and the circumstellar material around the young stellar object HD100546.The atomic abundance of the observed material is consistent with solarand C1 chondritic abundances, and the dust-to-gas ratio was determinedto be greater than or equal to 1.3. The presence of the observed mix ofmaterials requires efficient methods of annealing amorphous silicatesand mixing of high- and low-temperature phases over large distances inthe early protosolar nebula.
| Low-temperature single crystal reflection spectra of forsterite
The infrared reflectivities of crystalline forsterite(Mg2SiO4) were measured for the temperature range295-50 K for each crystal axis, between wavenumber 5000 and 100cm-1. The reflection spectra show clear dependence oftemperature; most of the bands become more intense, sharper and theirpeak positions shift to higher wavenumber with decreasing temperature.Reflection spectra were fitted with dispersion formula of dampedoscillator model of the dielectric constants and the oscillatorparameters in the model were derived. The absorption spectra offorsterite particle are calculated with the derived dielectric constantsto show that the forsterite features are good thermal indicator for coldtemperature range below 295 K.
| 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.
| Circumstellar Activity in Herbig Ae/Be Stars
We presented evidences of accretion of matter in a sample of HerbigAe/Be stars and we determined whether these events originated in aremnant gaseous structure from the primordial cloud (rich in Hydrogen)or in a metal rich body (like comets in our Solar System). During suchanalysis we also determined precise stellar parameters for this sampleof stars. The stars were observed using high resolution spectroscopy (R= 48,000). A synthetic photospheric spectrum was constructed and thensubtracted from the observed one in order to obtain the circumstellarcomponent. An iterative procedure was applied in order to find thestellar parameters that were used to build the synthetic photosphericspectrum. Evidences of accretion were found for three stars: HD100546,HD142666 and HD145718. A chemical analysis of the accretion episodessuggest that the material is Hydrogen rich, what excludes thepossibility that the accretion events were created by comet-like bodies.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| 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.
| Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
We present adaptive-optics high-angular resolution (~0.1 arcsec)spectroscopic observations in the 3 μm region of eight well-knownHerbig Ae/Be stars with circumstellar disks. We detected the aromaticemission feature at 3.3 μm for four out of six of our objects withflared disks (HD 169142, HD 97048, HD 100453, HD 100546), someadditional features at 3.4 and 3.46 μm, and strong diamond featuresat 3.43 and 3.53 μm in two of our flared objects (HD 100546 and HD97048 respectively). We also detected hydrogen recombination line at3.74 μm in practically all the objects. The emission in thepolycyclic aromatic hydrocarbon (PAHs) feature at 3.3 μm and in theadditional carbon dust features in the 3.4-3.5 μm region is, for thefirst time, spatially resolved in all the sources where the features aredetected. The full-width at half-maximum sizes that we derive aretypical of emission arising in a circumstellar disk. On the other hand,the continuum emission remains unresolved, with the exception of HD97048 where it is marginally resolved. We compared the observed spatialdistribution of the 3.3 μm PAH feature and the adjacent continuum tothe predictions of a disk model that includes transiently heated smallgrains and PAHs, in addition to large grains in thermal equilibrium(Habart et al. 2004a). The model predicts that, as observed, the 3.3μm PAH emission feature is significantly broader than that of theadjacent continuum and also that about 50% of its integrated intensitycomes from a radius R< 30 AU. We find that the predicted brightnessprofiles reproduce the observed ones very well. This proves beyond doubtthat the energetic 3.3 μm PAH emission feature takes its origin inthe inner disk regions.
| Resolving the disk rotation of HD 97048 and HD 100546 in the [O I] 6300 Å line: evidence for a giant planet orbiting HD 100546
Aims.We intend to spatially and spectrally resolve the [O i] emissionregion in two nearby Herbig stars.Methods.We present high-resolution(λ/Δλ = 80 000) VLT/UVES echelle spectra of the [Oi] 6300 Å line in the Herbig Ae/Be stars HD97048 and HD 100546. Apart from thespectral signature, also the spatial extent of the [O i] emission regionis investigated. For both stars, we have obtained spectra with the slitpositioned at different position angles on the sky.Results.The [O i]emission region of HD 100546 appears to be coinciding with the dustdisk, its major axis located at 150±11° east of north. The SEpart of the disk moves towards the observer, while the NW side isredshifted. The [O i] emission region rotates counterclockwise aroundthe central star. For HD 97048, the position angle of the emissionregion is 160±19° east of north, which is the firstdetermination of this angle in the literature. The southern parts of thedisk are blueshifted, the northern side moves away from us. Our datasupport the idea that a gap is present at 10 AU in the disk of HD100546. Such a gap is likely planet-induced. We estimate the mass andorbital radius of this hypothetical companion responsible for this gapto be 20~M_Jupiter and 6.5 AU respectively.Conclusions.Based on temporalchanges in the [O i] line profile, we conclude that inhomogeneities arepresent in the [O i] emission region of HD 100546. These "clumps" couldbe in resonance with the suggested companion, orbiting the central starin about 11 yr. If confirmed, these observations could point to theexistence of an object straddling the line between giant planet andbrown dwarf in a system as young as 10 million years.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Near-Infrared and the Inner Regions of Protoplanetary Disks
We examine the ``puffed-up inner disk'' model proposed by Dullemond,Dominik, & Natta for explaining the near-IR excess radiation fromHerbig Ae/Be stars. Detailed model computations show that the observednear-IR excess requires more hot dust than is contained in the puffed-updisk rim. The rim can produce the observed near-IR excess only if itsdust has perfectly gray opacity, but such dust is in conflict with theobserved 10 μm spectral feature. We find that a compact (~10 AU),tenuous (τV<~0.4), dusty halo around the disk innerregions contains enough dust to readily explain the observations.Furthermore, this model also resolves the puzzling relationship noted byMonnier & Millan-Gabet between luminosity and the interferometricinner radii of disks.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| Mid-Infrared Spectra of Polycyclic Aromatic Hydrocarbon Emission in Herbig Ae/Be stars
We present spectra of four Herbig Ae/Be stars obtained with the InfraredSpectrograph (IRS) on the Spitzer Space Telescope. All four of thesources show strong emission from polycyclic aromatic hydrocarbons(PAHs), with the 6.2 μm emission feature shifted to 6.3 μm and thestrongest CC skeletal-mode feature occurring at 7.9 μm instead of at7.7 μm, as is often seen. Remarkably, none of the four stars hassilicate emission. The strength of the 7.9 μm feature varies withrespect to the 11.3 μm feature among the sources, indicating that wehave observed PAHs with a range of ionization fractions. The ionizationfraction is higher for systems with hotter and brighter central stars.Two sources, HD 34282 and HD 169142, show emission features fromaliphatic hydrocarbons at 6.85 and 7.25 μm. The spectrum of HD 141569shows a previously undetected emission feature at 12.4 μm that may berelated to the 12.7 μm PAH feature. The spectrum of HD 135344, thecoolest star in our sample, shows an unusual profile in the 7-9 μmregion, with the peak emission to the red of 8.0 μm and no 8.6 μmPAH feature.
| Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 μm interferometers
We present simulations of the interferometric visibilities of Herbig Aestar disks. We investigate whether interferometric measurements in the10 μm atmospheric window are sensitive to the presence of anincreased scale height at the inner disk edge, predicted by recentmodels. Furthermore, we investigate whether such measurements candiscriminate between disks with a “flaring” geometry anddisks with a “flat” geometry. We show that both thesequestions can be addressed, using measurements at a small number ofappropriately chosen baselines. The classification of Herbig Ae stars intwo groups, based on the appearance of the spectral energy distribution(SED), has been attributed to a difference in disk geometry. Sourceswith a group I SED would have a flaring outer disk geometry, whereas thedisk of group II sources is proposed to be flat (or“self-shadowed”). We show that this hypothesis can be testedusing long-baseline interferometric measurements in the 10 μmatmospheric window.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| FUSE observations of molecular hydrogen on the line of sight towards HD 141569A
We present an analysis of the Far Ultraviolet Spectroscopic Explorer(FUSE) spectrum of HD 141569A, a transitional object known to possess acircumstellar disk. We observe two components of gas at widely differenttemperatures along the line of sight. We detect cold H2, which isthermalized up to J=2 at a kinetic temperature of ~51 K. Such lowtemperatures are typical of the diffuse interstellar medium. Since theline of sight to HD 141569A does not pass through its disk, it appearsthat we are observing the cold H2 in a low extinction envelopeassociated with the high Galactic latitude dark cloud complex L134N,which is in the same direction and at nearly the same distance as HD141569A. The column densities of the higher J-levels of H2 suggest thepresence of warm gas along the line of sight. The excitation conditionsdo not seem to be consistent with what is generally observed in diffuseinterstellar clouds. The observed radial velocity of the gas impliesthat the UV spectral lines we observe are likely interstellar in originrather than circumstellar, although our absorption line study does notdefinitely rule out the possibility that the warm gas is close to thestar. The discovery of such warm gas along the line of sight may provideevidence for turbulent phenomena in the dark cloud L134N.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
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