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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.
| 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.
| On the interstellar extinction law toward young stars
We have determined the atomic hydrogen column density N HI toward all ofthe young stars from the Taurus-Auriga-Perseus star-forming complex forwhich the corresponding spectra are available in the Hubble SpaceTelescope archive (nine stars) by analyzing the Lyα line profile.We show that the stars studied, except DR Tau, lie not far from the edgeof the gaseous cloud of the star-forming region closest to us or, moreprecisely, inside the outer H I shell of the cloud. This shell with acolumn density of N HI ≃ 6 × 1020 cm-2 surrounds themolecular gas of the cloud composed of a diffuse component (theso-called diffuse screen) in which dense, compact TMC-1 cores areembedded. The properties of the dust grains toward the stars that lie atthe front edge of the cloud most likely differ only slightly from thoseof the interstellar dust outside star-forming regions. This casts doubton the validity of the hypothesis that the extinction curve toward youngstars has an anomalously low amplitude of the 2175 Åbump—such an extinction curve is observed for the field stars HD29647 and HD 283809 toward which the line of sight passes through theTMC-1 core.
| A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus
We present mid-infrared spectra of T Tauri stars in the Taurusstar-forming region obtained with the Spitzer Infrared Spectrograph(IRS). For the first time, the 5-36 μm spectra of a large sample of TTauri stars belonging to the same star-forming region is studied,revealing details of the mid-infrared excess due to dust incircumstellar disks. We analyze common features and differences in themid-IR spectra based on disk structure, dust grain properties, and thepresence of companions. Our analysis encompasses spectral energydistributions from the optical to the far-infrared, a morphologicalsequence based on the IRS spectra, and spectral indices in IRS wavebands representative of continuum emission. By comparing the observedspectra to a grid of accretion disk models, we infer some basic diskproperties for our sample of T Tauri stars and find additional evidencefor dust settling.
| Mid-Infrared Imaging of the Herbig Ae Star AB Aurigae: Extended Emission on Several Scales
We present high-sensitivity subarcsecond resolution images of the HerbigAe star AB Aurigae at 11.6 and 18.5 μm taken with Michelle on GeminiNorth. Bright extended dust emission close to the star is resolved atboth wavelengths, with a quadratically subtracted FWHM of 17+/-4 AU at11.6 μm and 22+/-5 AU at 18.5 μm. Additional fainter emission isdetected out to a radius of 280 AU at 11.6 μm and 350 AU at 18.5μm down to the sensitivity limit of the observations. The lattervalue is identical to the measured size of the millimeter-continuumdisk, but much smaller than the CO disk. Assuming moderately absorbingmaterial, we find that larger particles (~1 μm) dominate the mid-IRemission in the inner (<100 AU) regions of the disk, and smallerparticles (<0.3 μm) dominate in the outer regions of the disk. Amodel of a nearly face-on passive flared disk with an inner rim accountswell for our observations.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).
| 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.
| Why Do T Tauri Disks Accrete?
Observations of T Tauri stars and young brown dwarfs suggest that theaccretion rates of their disks scale strongly with the central stellarmass, approximately M˙~M2*. No dependence ofaccretion rate on stellar mass is predicted by the simplest version ofthe Gammie layered disk model, in which nonthermal ionization of upperdisk layers allows accretion to occur via the magnetorotationalinstability. We show that a minor modification of Gammie's model toinclude heating by irradiation from the central star yields a modestdependence of M˙ on the mass of the central star. A purely viscousdisk model could provide a strong dependence of accretion rate onstellar mass if the initial disk radius (before much viscous evolutionhas occurred) has a strong dependence on stellar mass. However, it isfar from clear that at least the most massive pre-main-sequence diskscan be totally magnetically activated by X-rays or cosmic rays. Wesuggest that a combination of effects are responsible for the observeddependence, with the lowest mass stars having the lowest mass disks,which can be thoroughly magnetically active, while the higher mass starshave higher mass disks that have layered accretion and relativelyinactive or ``dead'' central zones at some radii. In such dead zones, wesuggest that gravitational instabilities may play a role in allowingaccretion to proceed. In this connection, we emphasize the uncertaintyin disk masses derived from dust emission and argue that T Tauri diskmasses have been systematically underestimated by conventional analyses.Further study of accretion rates, especially in the lowest mass stars,would help to clarify the mechanisms of accretion in T Tauri stars.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| Bright Localized Near-Infrared Emission at 1-4 AU in the AB Aurigae Disk Revealed by IOTA Closure Phases
We report on the detection of localized off-center emission at 1-4 AU inthe circumstellar environment of the young stellar object AB Aurigae. Weused closure-phase measurements in the near-infrared that were made atthe long-baseline interferometer IOTA, the first obtained on a youngstellar object using this technique. When probing sub-AU scales, allclosure phases are close to zero degrees, as expected given thepreviously determined size of the AB Aurigae inner-dust disk. However, aclear closure-phase signal of -3.5d +/- 0.5d is detected on one trianglecontaining relatively short baselines, requiring a high degree ofnon-point symmetry from emission at larger (AU-sized) scales in thedisk. We have not identified any alternative explanation for theseclosure-phase results, and we demonstrate that a ``disk hot spot'' modelcan fit our data. We speculate that such detected asymmetricnear-infrared emission might arise as a result of localized viscousheating due to a gravitational instability in the AB Aurigae disk, or tothe presence of a close stellar companion or accreting substellarobject.
| Possible Molecular Spiral Arms in the Protoplanetary Disk of AB Aurigae
The circumstellar dust disk of the Herbig Ae star AB Aur has been foundto exhibit complex spiral-like structures in the near-IR image obtainedwith the Subaru Telescope. We present maps of the disk in both 12CO (3-2) and dust continuum at 345 GHz with the SubmillimeterArray at an angular resolution of 1.0"×0.7" (144×100 AU).The continuum emission traces a dust disk with a central depression anda maximum overall dimension of 450 AU (FWHM). This dust disk exhibitsseveral distinct peaks that appear to coincide with bright features inthe near-IR image, in particular the brightest inner spiral arm. The COemission traces a rotating gas disk of size 530×330 AU with adeprojected maximum velocity of 2.8 km s-1 at 450 AU. Incontrast with the dust disk, the gas disk exhibits an intensity peak atthe stellar position. Furthermore, the CO emission in several velocitychannels traces the innermost spiral arm seen in the near-IR. We comparethe observed spatial-kinematic structure of the CO emission to a simplemodel of a disk in Keplerian rotation and find that only the emissiontracing the main spiral arm clearly lies outside the confines of ourmodel. This emission has a net outward radial motion compared with theradial velocity predicted by the model at the location of the mainspiral arms. The disk of AB Aur is therefore quite different from theKeplerian disks seen around many Herbig Ae stars. The spiral-likestructures of the disk with non-Keplerian motions we revealed in12CO (3-2), together with the central depression of the dustdisk, could be explained to be driven by the possible existence of agiant planet forming in the disk.
| The Remarkable Far-Ultraviolet Spectrum of FK Comae Berenices: King of Spin
A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on theultrafast rotating yellow giant FK Comae Berenices (HD 117555; vsini~163km s-1) recorded emission profiles of C III λ977(T~8×104 K) and O VI λ1031(T~3×105 K) that are exceptionally broad andasymmetric, but nearly identical in shape, aside from a bluewardabsorption component in the latter (identified as interstellar O I,rather than, say, a C III outflow feature). The FWHMs exceed 500 kms-1, twice the broadest far-UV line shape of any normallate-type star observed to date, but similar to the Hα profiles ofFK Com, and following the trend of other fast spinning early G giantsthat often display ``superrotational'' broadening of their UV ``hot''lines. Although the red-asymmetric O VI λ1031 profile issuggestive of an outflow at ~3×105 K, the weaker memberof the doublet, λ1037, does not display the differentialabsorption pattern expected from a warm wind. Furthermore, at times thechromospheric Mg II λ2796 + λ2803 composite profile, froma collection of International Ultraviolet Explorer (IUE) echellegramsobtained two decades earlier, is nearly identical in shape tored-asymmetric O VI λ1031. A contemporaneous optical Doppler mapplaces the photospheric dark spots mainly in the polar regions of theapproaching hemisphere. The dominantly redward biased profiles of C IIIand O VI could be explained if the associated emission zones wereleading the starspots in phase and partially rooted in lower latitudes.
| On the Formation of Gas Giant Planets on Wide Orbits
A new suite of three-dimensional radiative, gravitational hydrodynamicalmodels is used to show that gas giant planets are unlikely to form bythe disk-instability mechanism at distances of ~100-200 AU from youngstars. A similar result seems to hold for the core accretion mechanism.These results appear to be consistent with the paucity of detections ofgas giant planets on wide orbits by infrared imaging surveys, and theyalso imply that if the object orbiting GQ Lupus is a gas giant planet,it most likely did not form at a separation of ~100 AU. Instead, a wideplanet around GQ Lup must have undergone a close encounter with a thirdbody that tossed the planet outward to its present distance from itsprotostar. If it exists, the third body may be detectable by NASA'sSpace Interferometry Mission.
| Circumstellar Disk around HD 143275 and Interstellar Absorption
This paper presents a collection of spectra of the star HD 143275(δ Sco), acquired during the last 11 years. During this period,stellar absorption lines turned into emission features possiblyoriginating in the circumstellar disk. We demonstrate that thecircumstellar shell does not produce any components of interstellarabsorption features (atomic lines, molecular features, and diffuseinterstellar bands), which remain of the same equivalent widths duringthe whole period covered by our observations.
| The Darkest Bright Star: Chandra X-Ray Observations of Vega
We present X-ray observations of Vega obtained with the Chandra HighResolution Camera and Advanced CCD Imaging Spectrometer. After a totalof 29 ks of observation with Chandra, X-rays from Vega remainundetected. We derive upper limits to the X-ray luminosity of Vega as afunction of temperature over the range of 105-107K and find a 99.7% upper limit as low as ~2×1025 ergss-1 at T=106.2 K. We also compare these new deeperobservations with the limit derived from a reanalysis of ROSAT PSPCdata. Our X-ray luminosity limit for Vega is still greater thanpredictions of post-Herbig Ae phase X-rays from the shear dynamo modelproposed by Tout & Pringle for a Vega age of 350 Myr. If the age ofVega is closer to 100 Myr, as suggested by some indicators, our X-raylimit is then similar to Tout-Pringle model predictions. Current X-rayobservations of Vega are therefore unable to discriminate betweendifferent scenarios explaining the X-ray activity of the convectivelystable Herbig Ae/Be stars. Further progress is more likely to beachieved through X-ray observations of younger main-sequence early-typeA stars, whose conjectured residual post-Herbig Ae phase X-ray activitywould be significantly higher.
| 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.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| The magnetic Bp star 36 Lyncis. I. Magnetic and photospheric properties
Aims.This paper reports the photospheric, magnetic and circumstellar gascharacteristics of the magnetic B8p star 36 Lyncis (HD 79158).Methods: .Using archival data and new polarised and unpolarisedhigh-resolution spectra, we redetermine the basic physical properties,the rotational period and the geometry of the magnetic field, and thephotospheric abundances of various elements. Results: .Based onmagnetic and spectroscopic measurements, we infer an improved rotationalperiod of 3.83475± 0.00002 d. We determine a current epoch of thelongitudinal magnetic field positive extremum (HJD 2 452 246.033), andprovide constraints on the geometry of the dipole magnetic field (i≥56°, 3210~G≤ B_d≤ 3930 G, β unconstrained). Weredetermine the effective temperature and surface gravity using theoptical and UV energy distributions, optical photometry and Balmer lineprofiles (T_eff=13 300± 300 K, log g=3.7-4.2), and based on theHipparcos parallax we redetermine the luminosity, mass, radius and truerotational speed (L=2.54± 0.16~L_ȯ, M=4.0±0.2~M_ȯ, R=3.4± 0.7~R_ȯ, v_eq=45-61.5 kms-1). We measure photospheric abundances for 21 elementsusing optical and UV spectra, and constrain the presence of verticalstratification of these elements. We perform preliminary Doppler Imagingof the surface distribution of Fe, finding that Fe is distributed in apatchy belt near the rotational equator. Most remarkably, we confirmstrong variations of the Hα line core which we interpret as due tooccultations of the star by magnetically-confined circumstellar gas.
| 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.
| Large dust grains in the inner region of circumstellar disks
Context. Simple geometrical ring models account well for near-infraredinterferometric observations of dusty disks surrounding pre-mainsequence stars of intermediate mass. Such models demonstrate that thedust distribution in these disks has an inner hole and puffed-up inneredge consistent with theoretical expectations. Aims. In thispaper, we reanalyze the available interferometric observations of sixintermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC758, V1295 Aql and AB Aur) in the framework of a more detailed physicalmodel of the inner region of the dusty disk. Our aim is to verifywhether the model will allow us to constrain the disk and dustproperties. Methods. Observed visibilities from the literature arecompared with theoretical visibilities from our model. With theassumption that silicates are the most refractory dust species, ourmodel computes self-consistently the shape and emission of the inneredge of the dusty disk (and hence its visibilities for giveninterferometer configurations). The only free parameters in our modelare the inner disk orientation and the size of the dust grains. Results.In all objects with the exception of AB Aur, ourself-consistent models reproduce both the interferometric results andthe near-infrared spectral energy distribution. In four cases, grainslarger than ~1.2 μm, and possibly much larger are either required byor consistent with the observations. The inclination of the inner diskis found to be always larger than ~30°, and in at least two objectsmuch larger.
| 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.
| Chandra X-ray observations of the stellar group near the Herbig Be star MWC 297. A revision of the X-ray properties of MWC 297
We present a Chandra ACIS-I X-ray observation of the region near theHerbig early-Be star MWC 297, where we detect a tight group of pointX-ray sources. These are probably physically associated to MWC 297,because of their obvious clustering with respect to the more scatteredfield-source population. These data are compared to earlier ASCA datawith much poorer spatial resolution, from which the detection of strongquiescent and flaring emission from MWC 297 itself was claimed. We arguethat this star, contributing only 5% to the total X-ray emission of thegroup, was probably not the dominant contributor to the observed ASCAemission, while the X-ray brightest star in the group is a much bettercandidate. This is also supported by the spectral analysis of theChandra data, with reference to the ASCA spectra. We conclude that noneof the X-ray data available for MWC 297 justify the earlier claim ofstrong magnetic activity in this star. The X-ray emission of MWC 297during the Chandra observation is even weaker than that found in otherHerbig stars with the same spectral type, even accounting for its largeline-of-sight absorption.
| 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.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Spectroscopic survey of the Galaxy with Gaia- II. The expected science yield from the Radial Velocity Spectrometer
The Gaia mission is designed as a Galaxy explorer, and will measuresimultaneously, in a survey mode, the five or six phase-space parametersof all stars brighter than 20th magnitude, as well as providing adescription of their astrophysical characteristics. These measurementsare obtained by combining an astrometric instrument with micro-arcsecondcapabilities, a photometric system giving the magnitudes and colours in15 bands and a medium-resolution spectrograph named the Radial VelocitySpectrometer (RVS). The latter instrument will produce spectra in the848- to 874-nm wavelength range, with a resolving power R= 11500, fromwhich radial velocities, rotational velocities, atmospheric parametersand abundances can be derived. A companion paper has presented thecharacteristics of the RVS and its performance. The present paperdetails the outstanding scientific impact of this important part of theGaia satellite on some key open questions in present-day astrophysics.The unbiased and simultaneous acquisition of multi-epoch radialvelocities and individual abundances of key elements in parallel withthe astrometric parameters is essential for the determination of thedynamical state and formation history of our Galaxy. Moreover, for starsbrighter than V~= 15, the resolving power of the RVS will giveinformation about most of the effects that influence the position of astar in the Hertzsprung-Russell diagram, placing unprecedentedconstraints on the age, internal structure and evolution of stars of alltypes. Finally, the RVS multi-epoch observations are ideally suited tothe identification, classification and characterization of the manytypes of double, multiple and variable stars.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| The Origin of Episodic Accretion Bursts in the Early Stages of Star Formation
We study numerically the evolution of rotating cloud cores, from thecollapse of a magnetically supercritical core to the formation of aprotostar and the development of a protostellar disk during the mainaccretion phase. We find that the disk quickly becomes unstable to thedevelopment of a spiral structure similar to that observed recently inAB Aurigae. A continuous infall of matter from the protostellar envelopemakes the protostellar disk unstable, leading to spiral arms and theformation of dense protostellar/protoplanetary clumps within them. Thegrowing strength of spiral arms and ensuing redistribution of mass andangular momentum creates a strong centrifugal disbalance in the disk andtriggers bursts of mass accretion during which the denseprotostellar/protoplanetary clumps fall onto the central protostar.These episodes of clump infall may manifest themselves as episodes ofvigorous accretion (>=10-4 Msolaryr-1), as is observed in FU Orionis variables. Between theseaccretion bursts, the protostar is characterized by a low accretion rate(<10-6 Msolar yr-1). During thephase of episodic accretion, the mass of the protostellar disk remainsless than the mass of the protostar.
| Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective
We present a sensitive, multiwavelength submillimeter continuum surveyof 153 young stellar objects in the Taurus-Auriga star formation region.The submillimeter detection rate is 61% to a completeness limit of ~10mJy (3 σ) at 850 μm. The inferred circumstellar disk masses arelognormally distributed with a mean mass of ~5×10-3Msolar and a large dispersion (0.5 dex). Roughly one-third ofthe submillimeter sources have disk masses larger than the minimalnebula from which the solar system formed. The median disk-to-star massratio is 0.5%. The empirical behavior of the submillimeter continuum isbest described as Fν~ν2.0+/-0.5 between 350μm and 1.3 mm, which we argue is due to the combined effects of thefraction of optically thick emission and a flatter frequency behavior ofthe opacity compared to the interstellar medium. The latter effect couldbe due to a substantial population of large dust grains, whichpresumably would have grown through collisional agglomeration. In thissample, the only stellar property that is correlated with the outer diskis the presence of a companion. We find evidence for significantdecreases in submillimeter flux densities, disk masses, andsubmillimeter continuum slopes along the canonical infrared spectralenergy distribution evolution sequence for young stellar objects. Thefraction of objects detected in the submillimeter is essentiallyidentical to the fraction with excess near-infrared emission, suggestingthat dust in the inner and outer disks is removed nearly simultaneously.
| 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.
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