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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.
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| 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.
| 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.
| 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.
| 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.
| Analysis of the dust evolution in the circumstellar disks of T Tauri stars
Aims.We present a compositional analysis of 8{-}13 μ m spectra of 32young stellar objects (YSOs). Our sample consists of 5 intermediate-massstars and 27 low-mass stars. Although some previous studies give reasonsfor the similarity between the dust in circumstellar disks of T Tauristars and Herbig Ae/Be stars, a quantitative comparison has been lackingso far. Therefore, we include a discussion of the results of the 10μm spectroscopic survey of van Boekel et al. (2005, A&A, 437,189), who focus on Herbig Ae/Be stars, the higher mass counterparts of TTauri stars, and draw comparisons to this and other studies. Methods: .While the spectra of our 32 objects and first scientificresults have already been published elsewhere we perform a more detailedanalysis of the 10 μ m silicate feature. In our analysis weassume that this emission feature can be represented by a linearsuperposition of the wavelength-dependent opacity κ_abs(λ)describing the optical properties of silicate grains with differentchemical composition, structure, and grain size. Determining an adequatefitting equation is another goal of this study. Using a restrictednumber of fitting parameters, we investigate which silicate species arenecessary for the compositional fitting. Particles, with radii of 0.1μm- and 1.5 μm and consisting of amorphous olivine and pyroxene,forsterite, enstatite, and quartz are considered. Only compact,homogeneous dust grains are used in the presented fitting procedures. Inthis context we show that acceptable fitting results can also beachieved if emission properties of porous silicate grains are consideredinstead. Results: .Our analysis shows - in terms of the propertiesof the circumstellar dust-like crystallinity - T Tauri systems are acontinuation of HAeBe systems at their lower mass end. However, a weakcorrelation between grain growth and stellar luminosity could be found,in contrast to HAeBe systems.
| 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.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| First AU-scale observations of V1647 Orionis with VLTI/MIDI
The young eruptive star V1647 Ori was observed with MIDI, themid-infrared interferometric instrument at the Very Large TelescopeInterferometer (VLTI), on March 2, 2005. We present the first spectrallyresolved interferometric visibility points for this object. Our resultsshow that (1) the mid-infrared emitting region is extended, having asize of ≈7 AU at 10 μm; (2) no signatures of a close companion canbe seen; (3) the 8{-}13 μm spectrum exhibits no obvious spectralfeatures. Comparison with similar observations of Herbig Ae starssuggests that V1647 Ori probably possesses a disk of moderate flaring. Asimple disk model with T r-0.53, Σr-1.5, M_d=0.05 {M}ȯ is able to fit both thespectral energy distribution and the observed visibility valuessimultaneously.
| 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).
| 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.
| 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.
| Crystalline Silicate Emission in the Protostellar Binary Serpens SVS 20
We present spatially resolved mid-infrared spectroscopy of the ClassI/flat-spectrum protostellar binary system SVS 20 in the Serpens cloudcore. The spectra were obtained with the mid-infrared instrument T-ReCSon Gemini South. SVS 20-South, the more luminous of the two sources,exhibits a mid-infrared emission spectrum peaking near 11.3 μm, whileSVS 20-North exhibits a shallow amorphous silicate absorption spectrumwith a peak optical depth of τ~0.3. After removal of theline-of-sight extinction by the molecular common envelope, the``protostar-only'' spectra are found to be dominated by strong amorphousolivine emission peaking near 10 μm. We also find evidence foremission from crystalline forsterite and enstatite associated with bothSVS 20-S and SVS 20-N. The presence of crystalline silicate in such ayoung binary system indicates that the grain processing found in moreevolved Herbig Ae/Be and T Tauri pre-main-sequence stars likely beginsat a relatively young evolutionary stage, while mass accretion is stillongoing.
| 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.
| Spectroscopic behaviour of the unusual Ae star HD 190073
The results of high-resolution spectroscopy of the peculiar Ae star HD190073 obtained within the framework of a cooperative observingprogramme in 1994-2002 are presented. The temporal behaviour of theHα, Hβ, Hγ, Hδ, He i λ 5876 Å, Na iD and other circumstellar line profiles are investigated in detail.Special attention has been paid to the analysis of the deepmulticomponent blueshifted Ca ii H and K absorption lines. It has beenfound that the fine structure of their profiles is variable ontimescales from months to decades. The analysis of the circumstellarspectrum of HD 190073, rich in shell-like and emission lines with narrowabsorption cores, allows us to conclude that all absorption lines andcores are likely to be of photospheric origin. The emission lines arevariable in time and demonstrate signs of a stellar wind as well as adense equatorial circumstellar disk. As a preliminary hypothesis, wepropose that a global magnetic field of a specific topology can beresponsible for the formation of stable latitudinal stratification ofthe outflowing gas resulting in appearance of the complex structure ofthe Ca ii H and K line profiles. We emphasise that a measurement of thestellar magnetic field and an investigation of its detailedconfiguration would be an important step in understanding the nature ofHD 190073.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| [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.
| 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.
| Recent astrophysical results from the VLTI.
Not Available
| The Disk Atmospheres of Three Herbig Ae/Be Stars
We present infrared (IR) spectrophotometry (R~=180) of three HerbigAe/Be stars surrounded by possible protoplanetary disks: HD 150193, HD100546, and HD 179218. We construct a mid-IR spectral energydistributions (SEDs) for each object by using 7.6-13.2 μm HIFOGSspectra, 2.4-45 μm spectrophotometry from the Infrared SpaceObservatory Short-Wavelength Spectrometer, the 12, 25, 60, and 100 μmphotometric points from IRAS, and for HD 179218, photometric bolometricdata points from the Mount Lemmon Observing Facility. The SEDs aremodeled by using an expanded version of the Chiang & Goldreichtwo-layer, radiative and hydrostatic equilibrium, passive disk. Thisexpanded version includes the emission from Mg-pure crystalline olivine(forsterite) grains in the disk surface layer. Each of the three objectsstudied vary in the amount of crystals evident from theirspectrophotometry. HD 150193 contains no crystals, while HD 100546 andHD 179218, respectively, show evidence of having crystalline silicatesin the surface layers of their disks. We find that the inner region ofHD 100546 has a 37% higher crystalline-to-amorphous silicate ratio inits inner disk region (<=5 AU) than in the outer disk region, whilethe inner disk region of HD 179218 has a 84% highercrystalline-to-amorphous silicate ratio in its inner disk region (<=5AU) than in the outer region. All three objects are best fitted using agrain-size distribution power law that falls as a-3.5. HD150193 is best fitted by a small disk (~5 AU in radius), while HD 100546and HD 179218 are best fitted by larger disks (~150 AU in radius).Furthermore, HD 100546's disk flares larger than those of HD 150193 (25%more at 5 AU) and HD 179218 (80% more at 5 AU). We discuss theimplications of our results and compare them with other modelingefforts.
| 8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is 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/431/773
| Mid-IR observations of circumstellar disks. I. Pre-main sequence objects
We present new N-band photometry and spectroscopy for a sample of eightpre-main sequence stars including T Tauri, Herbig Ae/Be stars and FU Oriobjects using the ESO TIMMI2 camera at the La Silla observatory (Chile).For some objects this is their first N-band spectroscopic observationever. The FU Ori stars V 346 Nor, V 883 Ori and Z CMa show a broadabsorption band which we attribute to silicates, while for BBW 76 wefind silicate emission. A comparison with ISO-SWS spectra of V 346 Norand Z CMa taken in 1996/1997 reveals no differences in spectral shape.All T Tauri and Herbig Ae/Be stars possess N-band emission features. Wemodel the emission spectra with a mixture of silicates consisting ofdifferent grain sizes and composition. The Herbig Ae star HD 34282 showsstrong features of PAHs but none of silicate, while the emissionspectrum of the Herbig Ae star HD 72106 resembles those of solar-systemcomets and known Herbig sources of evolved dust. We demonstrate that HD72106 is host to highly processed silicates and find evidence forenstatite, which is not common in young objects. Evolved dust is alsoseen in the T Tauri stars HD 98800 and MP Mus. We further detected MPMus at 1200 μm with the bolometer array SIMBA at the SEST in LaSilla. The findings of our analysis are given in the context of previousdust studies of young stellar objects.Based on observations collected at the European Southern Observatory, LaSilla, Chile (69.C-0073, 70.C-0468, 71.C-0001, 73.C-0372).
| PAHs in circumstellar disks around Herbig Ae/Be stars
We investigate the presence and properties of PAHs on the surface ofcircumstellar disks around Herbig Ae/Be stars by comparing thepredictions of disk models with observations. We present results of aradiation transfer code for disks heated by the central star, inhydrostatic equilibrium in the vertical direction (flared disks). Thedust is a mixture of large grains in thermal equilibrium, transientlyheated small grains and PAHs. Special attention is given to theinfluence of the stellar, disk and PAH properties on the strength of thePAH emission lines and their spatial distribution. The models predict aninfrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 μmclearly visible above the continuum, and with some of them very strong.The PAH emission, spatially extended, comes mostly from the outer diskregion (R100 AU) while the continuum emission at similarwavelengths, mostly due to warm large grains, is confined to theinnermost disk regions (R few AU). We compare the model results toinfrared observations from ISO and ground-based telescopes of somethirty Herbig Ae/Be stars. Most of the observed PAH features in objectswith spectral type later than B9 are well described by our disk modelsand we discuss in some detail the PAH characteristics one can derivefrom the existing data. Objects with strong radiation field (generallyearlier than about B9) have the 3.3 μm feature (often the only oneobserved) much weaker than predicted, and we discuss possibleexplanations (dissipation of the disk, photoevaporation or modificationof the PAH properties).Appendix is only available in electronic form athttp://www.edpsciences.org
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks
Observations of interstellar gas-phase and solid-state species in the2.4 200 m range obtained with the spectrometers on board the InfraredSpace Observatory (ISO) are reviewed. Lines and bands caused by ices,polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms andmolecules (in particular H2, CO, H2O, OH, andCO2) are summarized and their diagnostic capabilitiesillustrated. The results are discussed in the context of the physicaland chemical evolution of star-forming regions, includingphoton-dominated regions, shocks, protostellar envelopes, and disksaround young stars.
| The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features
We present spectra of the 3.3 μm and 11.2 μm polycyclic aromatichydrocarbon (PAH) features of a large number of stellar sources,planetary nebulae, reflection nebulae, H II regions, and galaxies,obtained with Infrared Space Observatory Short Wavelength Spectrometer.Clear variations are present in the profiles of these features. Most ofthe sources show a symmetric 3.3 μm feature peaking at ~3.290 μm,while only very few show an asymmetric 3.3 μm feature peaking at aslightly longer wavelength. The profiles of the 11.2 μm feature aredistinctly asymmetric. The majority of the sources has a 11.2 μmfeature peaking between 11.20 and 11.24 μm, with a very steep bluerise and a low tail-to-top ratio. A few sources show a 11.2 μmfeature with a peak position of ~11.25 μm, a less steep blue rise,and a high tail-to-top ratio. The sources are classified independentlyon the basis of the 3.3 and 11.2 μm feature profiles and peakpositions. Correlations between these classes and those based on the 6-9μm features (Peeters et al.) are found. In particular, sources withthe most common profiles in the 6-9 μm region also show the mostcommon 3.3 and 11.2 μm feature profiles. However, the uncommonprofiles do not correlate with each other. Also, these classificationsdepend on the type of object. In general, H II regions, nonisolatedHerbig AeBe stars and young stellar objects show the same profiles forall 3-12 μm features. Many planetary nebulae and post-asymptoticgiant branch stars show uncommon feature profiles. The three galaxies inour sample show the same profiles as the H II regions for all but the11.2 μm feature, being similar to that of evolved stars. The observedpronounced contrast in the spectral variations for the CH modes (3.3 and11.2 μm bands) versus the CC modes (6.2, 7.7, and 8.6 μm bands) isstriking: the peak wavelengths of the features attributed to CC modesvary by ~15-80 cm-1, while for the CH modes the variationsare ~4-6.5 cm-1. We summarize existing laboratory data andtheoretical calculations of the modes emitting in the 3-12 μm regionof PAH molecules and complexes. In contrast to the 6.2 and 7.7 μmcomponents, which are attributed to PAH cations, the 3.3 μm featureappears to originate in neutral and/or negatively charged PAHs. Weattribute the variations in peak position and profile of these IRemission features to the composition of the PAH family. The variationsin FWHM of the 3.3 μm feature remains an enigma, while those of the11.2 μm can be explained by anharmonicity and molecular structure.The possible origin of the observed contrast in profile variationsbetween the CH modes and the CC modes is highlighted.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI
We present the first long baseline mid-infrared interferometricobservations of the circumstellar disks surrounding Herbig Ae/Be stars.The observations were obtained using the mid-infrared interferometricinstrument MIDI at the European Southern Observatory (ESO) Very LargeTelescope Interferometer VLTI on Cerro Paranal. The 102 m baseline givenby the telescopes UT1 and UT3 was employed, which provides a maximumfull spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10μm. The interferometric signal was spectrally dispersed at aresolution of 30, giving spectrally resolved visibility information from8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars andresolved all objects. The warm dust disk of HD 100546 could even beresolved in single-telescope imaging. Characteristic dimensions of theemitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10μm sizes of our sample stars correlate with the slope of the 10-25μm infrared spectrum in the sense that the reddest objects are thelargest ones. Such a correlation would be consistent with a differentgeometry in terms of flaring or flat (self-shadowed) disks for sourceswith strong or moderate mid-infrared excess, respectively. We comparethe observed spectrally resolved visibilities with predictions based onexisting models of passive centrally irradiated hydrostatic disks madeto fit the SEDs of the observed stars. We find broad qualitativeagreement of the spectral shape of visibilities corresponding to thesemodels with our observations. Quantitatively, there are discrepanciesthat show the need for a next step in modelling of circumstellar disks,satisfying both the spatial constraints such as are now available fromthe MIDI observations and the flux constraints from the SEDs in aconsistent way.Based on observations made with the Very Large Telescope Interferometerat Paranal Observatory.
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