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HD 95881: a gas rich to gas poor transition disk?
Context. Based on the far infrared excess the Herbig class of stars isdivided into a group with flaring circumstellar disks (group I) and agroup with flat circumstellar disks (group II). Dust sedimentation isgenerally proposed as an evolution mechanism to transform flaring disksinto flat disks. Theory predicts that during this process the diskspreserve their gas content, however observations of group II Herbig Aestars demonstrate a lack of gas. Aims: We map the spatialdistribution of the gas and dust around the group II Herbig Ae star HD95881. Methods: We analyze optical photometry, Q-band imaging,infrared spectroscopy, and K and N-band interferometric spectroscopy. Weuse a Monte Carlo radiative transfer code to create a model for thedensity and temperature structure which quite accurately reproduces allthe observables. Results: We derive a consistent picture in whichthe disk consists of a thick puffed up inner rim and an outer regionwhich has a flaring gas surface and is relatively void of“visible” dust grains. Conclusions: HD 95881 is in atransition phase from a gas rich flaring disk to a gas poorself-shadowed disk.Based on observations collected at the European Southern Observatory,Chile. Under program IDs: 060.A9234A, 076.C-0159A, 077.C-0367A,078.C-0281.
| Infrared Interferometry of Young Stellar Objects
The circumstellar disks around young stellar objects play a key role inthe formation process of stars and provide the stage for planetformation. Since recently, infrared interferometry provides the spatialresolution required to directly study the distribution of the gas anddust in the innermost AU around the forming star. We present recentinvestigations in which we employed the VLTI and its near- andmid-infrared interferometric instruments AMBER and MIDI to constrain thegeometry and physical conditions of the disks around Herbig Ae/Be starsand to study the accretion and outflow processes taking place close tothe central star.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. IV. Crystalline silicates
Aims: Dust grains in the planet-forming regions around young stars areexpected to be heavily processed due to coagulation, fragmentation, andcrystallization. This paper focuses on the crystalline silicate dustgrains in protoplanetary disks for a statistically significant number ofTTauri stars (96). Methods: As part of the cores to disks (c2d)legacy program, we obtained more than a hundred Spitzer/IRS spectra ofTTauri stars, over a spectral range of 5-35 μm where many silicateamorphous and crystalline solid-state features are present. At thesewavelengths, observations probe the upper layers of accretion disks upto distances of a dozen AU from the central object. Results: Morethan 3/4 of our objects show at least one crystalline silicate emissionfeature that can be essentially attributed to Mg-rich silicates. TheFe-rich crystalline silicates are largely absent in the c2d IRS spectra.The strength and detection frequency of the crystalline features seen atλ > 20 μm correlate with each other, while they are largelyuncorrelated with the observational properties of the amorphous silicate10 μm feature. This supports the idea that the IRS spectraessentially probe two independent disk regions: a warm zone (≤1 AU)emitting at λ ~ 10 μm and a much colder region emitting atλ > 20 μm (≤10 AU). We identify a crystallinity paradox,as the long-wavelength (λ > 20 μm) crystalline silicatefeatures are detected 3.5 times more frequently (~55% vs. ~15%) than thecrystalline features arising from much warmer disk regions (λ ~10μm). This suggests that the disk has an inhomogeneous dustcomposition within ~10 AU. The analysis of the shape and strength ofboth the amorphous 10 μm feature and the crystalline feature around23 μm provides evidence for the prevalence of μm-sized (amorphousand crystalline) grains in upper layers of disks. Conclusions:The abundant crystalline silicates found far from their presumedformation regions suggest efficient outward radial transport mechanismsin the disks around TTauri stars. The presence of μm-sized grains indisk atmospheres, despite the short timescales for settling to themidplane, suggests efficient (turbulent) vertical diffusion, probablyaccompanied by grain-grain fragmentation to balance the expectedefficient growth. In this scenario, the depletion of submicron-sizedgrains in the upper layers of the disks points toward removal mechanismssuch as stellar winds or radiation pressure.Tables [see full textsee full textsee full textsee full text], [see fulltextsee full textsee full textsee full text] and Figs. [see full textseefull textsee full textsee full text]-[see full textsee full textsee fulltextsee full text] are only available in electronic form athttp://www.aanda.org
| Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Context: Recently, evidence for the presence of weak magnetic fields inHerbig Ae/Be stars has been found in several studies. Aims: Weseek to expand the sample of intermediate-mass pre-main sequence starswith circular polarization data to measure their magnetic fields, and todetermine whether magnetic field properties in these stars arecorrelated with mass-accretion rate, disk inclination, companions,silicates, PAHs, or show a correlation with age and X-ray emission asexpected for the decay of a remnant dynamo. Methods:Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debrisdisk stars have been obtained at the European Southern Observatory withFORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM600B in the wavelength range 3250-6215 Å we were able to cover allhydrogen Balmer lines from Hβ to the Balmer jump. In allobservations a slit width of 0.4 arcsec was used to obtain a spectralresolving power of R ≈ 2000. Results: Among the 21 HerbigAe/Be stars studied, new detections of a magnetic field were achieved insix stars. For three Herbig Ae/Be stars, we confirm previous magneticfield detections. The largest longitudinal magnetic field, =-454±42 G, was detected in the Herbig Ae/Be star HD 101412 usinghydrogen lines. No field detection at a significance level of 3σwas achieved in stars with debris disks. Our study does not indicate anycorrelation of the strength of the longitudinal magnetic field with diskorientation, disk geometry, or the presence of a companion. We also donot see any simple dependence on the mass-accretion rate. However, it islikely that the range of observed field values qualitatively supportsthe expectations from magnetospheric accretion models giving support fordipole-like field geometries. Both the magnetic field strength and theX-ray emission show hints of a decline with age in the range of ~2-14Myr probed by our sample, supporting a dynamo mechanism that decays withage. However, our study of rotation does not show any obvious trend ofthe strength of the longitudinal magnetic field with rotation period.Furthermore, the stars seem to obey the universal power-law relationbetween magnetic flux and X-ray luminosity established for the Sun andmain-sequence active dwarf stars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes 077.C-0521(A) and 081.C-0410(A)).
| The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry
Context: Accretion and outflow processes are of fundamental importancefor our understanding of the formation of stars and planetary systems.To trace these processes, diagnostic spectral lines such as theBrγ 2.166 μm line are widely used, although due to a lack ofspatial resolution, the origin of the line emission is still unclear.Aims: Employing the AU-scale spatial resolution which can beachieved with infrared long-baseline interferometry, we aim todistinguish between theoretical models which associate the Brγline emission with mass infall (magnetospheric accretion, gaseous innerdisks) or mass outflow processes (stellar winds, X-winds, or diskwinds). Methods: Using the VLTI/AMBER instrument, we spatiallyand spectrally (λ/Δλ = 1500) resolved the inner(⪉5 AU) environment of five Herbig Ae/Be stars (HD 163296, HD104237, HD 98922, MWC 297, V921 Sco) in the Brγ emission line aswell as in the adjacent continuum. From the measuredwavelength-dependent visibilities, we derive the characteristic size ofthe continuum and Brγ line-emitting region. Additional informationis provided by the closure phase, which we could measure both in thecontinuum wavelength regime (for four objects) as well as in thespectrally resolved Brγ emission line (for one object). Thespectro-interferometric data is supplemented by archival and newVLT/ISAAC spectroscopy. Results: For all objects (except MWC297), we measure an increase of visibility within the Brγ emissionline, indicating that the Brγ-emitting region in these objects ismore compact than the dust sublimation radius. For HD 98922, ourquantitative analysis reveals that the line-emitting region is compactenough to be consistent with the magnetospheric accretion scenario. ForHD 163296, HD 104237, MWC 297, and V921 Sco we identify an extendedstellar wind or a disk wind as the most likely line-emitting mechanism.Since the stars in our sample cover a wide range of stellar parameters,we also search for general trends and find that the size of theBrγ-emitting region does not seem to depend on the basic stellarparameters (such as the stellar luminosity), but correlates withspectroscopic properties, in particular with the Hα line profileshape. Conclusions: By performing the first high-resolutionspectro-interferometric survey on Herbig Ae/Be stars, we find evidencefor at least two distinct Brγ line-formation mechanisms. Mostsignificant, stars with a P-Cygni Hα line profile and a highmass-accretion rate seem to show particularly compact Brγ-emittingregions (RBrγ/Rcont < 0.2), while starswith a double-peaked or single-peaked Hα-line profile show asignificantly more extended Brγ-emitting region (0.6 ⪉RBrγ/Rcont ⪉ 1.4), possibly tracing astellar wind or a disk wind.Based on observationsmade with ESO telescopes at the La Silla Paranal Observatory under opentime programme IDs 077.C-0694, 078.C-0360, and 078.C-0680.
| Searching for molecular hydrogen mid-infrared emission in the circumstellar environments of Herbig Be stars
Context: Molecular hydrogen (H2) is the most abundant molecule in thecircumstellar (CS) environments of young stars, and is a key element ingiant planet formation. The measurement of the H2 content provides themost direct probe of the total amount of CS gas, especially in the innerwarm planet-forming regions of the disks. Aims: Most Herbig Bestars (HBes) are distant from the Sun and their nature and evolution arestill debated. We therefore conducted mid-infrared observations of H2 asa tracer of warm gas around HBes known to have gas-rich CS environments.Methods: We report a search for the H2 S(1) emission line at17.0348 μm in the CS environments of 5 HBes with the high resolutionspectroscopic mode of VISIR (ESO VLT Imager and Spectrometer for themid-InfraRed). Results: No source shows evidence for H2 emissionat 17.0348 μm. Stringent 3σ upper limits on the integrated linefluxes are derived. Depending on the adopted temperature, limits oncolumn densities and masses of warm gas are also estimated. Thesenon-detections constrain the amount of warm (>150 K) gas in theimmediate CS environments of our target stars to be less than ˜1-10~ M_Jup.
| Spatially extended polycyclic aromatic hydrocarbons in circumstellar disks around T Tauri and Herbig Ae stars
Aims.Our aim is to determine the presence and location of the emissionfrom polycyclic aromatic hydrocarbons (PAHs) towards low andintermediate mass young stars with disks using large aperturetelescopes. Methods: VLT-VISIR N-band spectra and VLT-ISAAC andVLT-NACO L-band spectra of 29 sources are presented, spectrallyresolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features.Spatial-extent profiles of the features and the continuum emission havebeen derived and used to associate the PAH emission with the disks. Theresults are discussed in the context of recent PAH-emission disk models. Results: The 3.3, 8.6, and 11.2 μm PAH features are detected towarda small fraction of the T Tauri stars, with typical upper limits between1 × 10-15 and 5 × 10-17 Wm-2. All 11.2 μm detections from a previous Spitzer surveyare confirmed with (tentative) 3.3 μm detections, and both the 8.6and the 11.2 μm features are detected in all PAH sources. For 6detections, the spatial extent of the PAH features is confined to scalestypically smaller than 0.12-0.34'', consistent with the radii of 12-60AU disks at their distances (typically 150 pc). For 3 additionalsources, WL 16, HD 100546, and TY CrA, one or more of the PAH featuresare more extended than the hot dust continuum of the disk, whereas forOph IRS 48, the size of the resolved PAH emission is confirmed assmaller than for the large grains. For HD 100546, the 3.3 μm emissionis confined to a small radial extent of 12±3 AU, most likelyassociated with the outer rim of the gap in this disk. Gaps with radiiout to 10-30 AU may also affect the observed PAH extent for othersources. For both Herbig Ae and T Tauri stars, the small measuredextents of the 8.6 and 11.2 μm features are consistent with larger(≥100 carbon atoms) PAHs.Based on observations obtained at the European Southern Observatory,Paranal, Chile, within the observing programs 164.I-0605 (ISAAC May2002), 074.C-0413 (NACO, March/April 2005), 075.C-0420 (ISAAC August2005), 077.C-0668 (VISIR/ISAAC April/May 2006). Appendix A is onlyavailable in electronic form at http://www.aanda.org
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H2 Gas-Phase Lines
We present a survey of mid-infrared gas-phase lines toward a sample of76 circumstellar disks around low-mass pre-main-sequence stars from theSpitzer ``Cores to Disks'' legacy program. We report the firstdetections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20%and ~9% of our sources, respectively. The observed [Ne II] line fluxesand upper limits are consistent with [Ne II] excitation in an X-rayirradiated disk around stars with X-ray luminositiesLX=1029-1031 erg s-1. [Fe I]is detected at ~10-5 to 10-4 Lsolar,but no [S I] or [Fe II] is detected down to ~10-6Lsolar. The [Fe I] detections indicate the presence ofgas-rich disks with masses of >~0.1 MJ. No H20-0 S(0) and S(1) disk emission is detected, except for S(1) toward onesource. These data give upper limits on the warm (T~100-200 K) gas massof a few Jovian masses, consistent with recent T Tauri disk models thatinclude gas heating by stellar radiation. Compact disk emission of hot(T>~500 K) gas is observed through the H2 0-0 S(2) and/orS(3) lines toward ~8% of our sources. The line fluxes are, however,higher by more than an order of magnitude than those predicted by recentdisk models, even when X-ray and excess UV radiation are included. The[Ne II]/H2 0-0 S(2) ratios for these sources are similarlylower than predicted, consistent with the presence of an additional hotmolecular gas component not included in current disk models. Obliqueshocks of stellar winds interacting with the disk can explain manyaspects of the hot gas emission but are inconsistent with thenondetection of [S I] and [Fe II] lines.
| A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate- and high-mass stars isfundamentally a mystery. Clues towards solving this basic astrophysicalproblem can likely be found at the pre-main-sequence (PMS) evolutionarystage. With this work, we perform the largest and most sensitive searchfor magnetic fields in PMS Herbig Ae/Be (HAeBe) stars. We seek todetermine whether strong, ordered magnetic fields, similar to those ofmain-sequence Ap/Bp stars, can be detected in these objects, and if so,to determine the intensities, geometrical characteristics, andstatistical incidence of such fields. 68 observations of 50 HAeBe starshave been obtained in circularly polarized light using the FORS1spectropolarimeter at the ESO VLT. An analysis of both Balmer andmetallic lines reveals the possible presence of weak longitudinalmagnetic fields in photospheric lines of two HAeBe stars, HD 101412 andBF Ori. Results for two additional stars, CPD-53 295 and HD 36112, aresuggestive of the presence of magnetic fields, but no firm conclusionscan be drawn based on the available data. The intensity of thelongitudinal fields detected in HD 101412 and BF Ori suggest that theycorrespond to globally ordered magnetic fields with surface intensitiesof order 1 kG. On the other hand, no magnetic field is detected in 4other HAeBe stars in our sample in which magnetic fields had previouslybeen confirmed. Monte Carlo simulations of the longitudinal fieldmeasurements of the undetected stars allow us to place an upper limit ofabout 300 G on the general presence of aligned magnetic dipole magneticfields, and of about 500 G on perpendicular dipole fields. Taking intoaccount the results of our survey and other published results, we findthat the observed bulk incidence of magnetic HAeBe stars in our sampleis 8-12 per cent, in good agreement with that of magnetic main-sequencestars of similar masses. We also find that the rms longitudinal fieldintensity of magnetically detected HAeBe stars is about 200 G, similarto that of Ap stars and consistent with magnetic flux conservationduring stellar evolution. These results are all in agreement with thehypothesis that the magnetic fields of main-sequence Ap/Bp stars arefossils, which already exist within the stars at the PMS stage. Finally,we explore the ability of our new magnetic data to constrainmagnetospheric accretion in Herbig Ae/Be stars, showing that ourmagnetic data are not consistent with the general occurrence in HAeBestars of magnetospheric accretion as described by the theories ofKönigl and Shu et al..Based on observations from the ESO telescopes at the La Silla ParanalObservatory under programme ID 072.C-0447, DDT-272.C-5063, 074.C-0442.E-mail: wade-g@rmc.ca
| 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.
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| 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.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features
Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) featurestowards young low-mass (T Tauri) stars and compare them with surveys ofintermediate mass (Herbig Ae/Be) stars. The presence and strength of thePAH features are interpreted with disk radiative transfer modelsexploring the PAH feature dependence on the incident UV radiation, PAHabundance and disk parameters. Methods: .Spitzer Space Telescope5-35 μm spectra of 54 pre-main sequence stars with disks wereobtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars withunknown spectral type. Results: .Compact PAH emission is detectedtowards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2μm PAH feature is detected in all of these sources, as is the 6.2μm PAH feature for the 4 sources for which short wavelength data areavailable. However, the 7.7 and 8.6 μm features appear strongly inonly 1 of these 4 sources. Based on the 11.2 μm feature, PAH emissionis observed towards at least 3 T Tauri stars, with 14 tentativedetections, resulting in a lower limit to the PAH detection rate of 8%.The lowest mass source with PAH emission in our sample is TCha with a spectral type G8. All 4 sources in our sample withevidence for dust holes in their inner disk show PAH emission,increasing the feature/continuum ratio. Typical 11.2 μm lineintensities are an order of magnitude lower than those observed for themore massive Herbig Ae/Be stars. Measured line fluxes indicate PAHabundances that are factors of 10-100 lower than standard interstellarvalues. Conversely, PAH features from disks exposed to stars withT_eff≤ 4200 K without enhanced UV are predicted to be below thecurrent detection limit, even for high PAH abundances. Disk modelingshows that the 6.2 and 11.2 μm features are the best PAH tracers forT Tauri stars, whereas the 7.7 and 8.6 μm bands have low feature overcontinuum ratios due to the strongly rising silicate emission.
| 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.
| 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.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| A new approach to IRAS observations of Be stars
The IRAS associations for 193 Be stars are identified in this paper.From the infrared colors, the IRAS low-resolution spectra (LRS) and thespectral types, some physical properties and environment of the samplesare discussed. It can be concluded that not only free-free emission orfree-bound emission from the circumstellar ionized gas can beresponsible for the large IR excesses of Be stars as suggestedpreviously, but also, for some Be stars, thermal radiation from thecircumstellar dust and/or nebula around the star can produce large IRexcess as well. It is also found that the far IR excess of Be starsincreases with wavelengths.
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Grain growth in the inner regions of Herbig Ae/Be star disks
We present new mid-infrared spectroscopy of the emission from warmcircumstellar dust grains in Herbig Ae/Be stars. Our surveysignificantly extends the sample that was studied by Bouwman et al.(2001). We find a correlation between the strength of the silicatefeature and its shape. We interpret this as evidence for the removal ofsmall (0.1 mu m) grains from the disk surface while large (1-2 mu m)grains persist. If the evolution of the grain size distribution isdominated by gravitational settling, large grains are expected todisappear first, on a timescale which is much shorter than the typicalage of our programme stars. Our observations thus suggest a continuousreplenishment of micron sized grains at the disk surface. If the grainreplenishment is due to the dredge-up of dust from the disk interior,the mineralogy we observe is representative of the bulk composition ofdust in these stars.Based on observations obtained at the European Southern Observatory(ESO), La Silla, and on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
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