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Interpreting Spectral Energy Distributions from Young Stellar Objects. II. Fitting Observed SEDs Using a Large Grid of Precomputed Models
We present a method to analyze the spectral energy distributions (SEDs)of young stellar objects (YSOs). Our approach is to fit data withprecomputed two-dimensional (2D) radiation transfer models spanning alarge region of parameter space. This allows us to determine not only asingle set of physical parameter values but the entire range of valuesconsistent with the multiwavelength observations of a given source. Inthis way we hope to avoid any overinterpretation when modeling a set ofdata. We have constructed spectral energy distributions from optical tosubmillimeter wavelengths, including new Spitzer IRAC and MIPSphotometry, for 30 young and spatially resolved sources in theTaurus-Auriga star-forming region. We demonstrate fitting model SEDs tothese sources and find that we correctly identify the evolutionary stageand physical parameters found from previous independent studies, such asdisk mass, disk accretion rate, and stellar temperature. We also explorehow fluxes at various wavelengths help to constrain physical parametersand show examples of degeneracies that can occur when fitting SEDs. AWeb-based version of this tool is available to the community.
| High-Resolution Submillimeter Constraints on Circumstellar Disk Structure
We present a high spatial resolution submillimeter continuum survey of24 circumstellar disks in the Taurus-Auriga and Ophiuchus-Scorpius starformation regions using the SMA. In the context of a simple model, weuse broadband spectral energy distributions and submillimetervisibilities to derive constraints on some basic parameters thatdescribe the structure of these disks. For the typical disk in thesample we infer a radial surface density distributionΣr~r-p with a median p~0.5, althoughconsideration of the systematic effects of some of our assumptionssuggest that steeper distributions with p~1 are more reasonable. Thedistribution of the outer radii of these disks shows a distinct peak atRd~200 AU, with only a few cases where the disk emission iscompletely unresolved. Based on these disk structure measurements, themass accretion rates, and the typical spectral and spatial distributionsof submillimeter emission, we show that the observations are in goodagreement with similarity solutions for steady accretion disks that havea viscosity parameter α~0.01. We provide new estimates of thespectral dependence of the disk opacityκν~νβ with a mean β~1.0,corrected for optically thick emission. This typical value of β isconsistent with model predictions for the collisional growth of solidsto millimeter-size scales in the outer disk. Although direct constraintson planet formation in these disks are not currently available, theextrapolated density distributions inferred here are substantiallyshallower than those calculated based on the solar system or extrasolarplanets and typically used in planet formation models. It is possiblethat we are substantially underestimating disk densities due to anincomplete submillimeter opacity prescription.
| 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.
| Evidence of Stellar-driven Outflows from the Classical T Tauri Star RY Tau
RY Tau is a rapidly rotating classical T Tauri star observed close toedge-on. The combination of new HST/STIS observations obtained in 2001with HST/GHRS archive data from 1993 has allowed us to get, for thefirst time, information on the thermal structure and velocity of thewind. The repeated observations of the Si III] and C III] lines show alack of changes with time on the blue side of the profile (dominated bythe wind contribution). Very high temperature plasma(logTe=4.8) is detected at densities of9.5<=logne(cm-3)<=10.2 associated with thewind. The emitting volumes are ~(0.35 Rsolar)3,suggesting a stellar origin. The wind kinematics derived from theprofiles (Si III], C III], and [O II]) does not satisfy the theoreticalpredictions of MHD centrifugally driven disk winds. The profiles'asymmetry, large velocity dispersions, and small variability as well asthe small emitting volumes are best explained if the wind is produced bythe contributions of several outflows from atmospheric open-fieldstructures like those observed in the Sun.
| Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
Context: .T Tauri stars exhibit variability on all timescales, whoseorigin is still debated. Aims: .We investigate the long termvariability of CTTs over up to 20 years, characterize it from a set ofstatistical parameters and discuss its origin. Methods: .Wepresent a unique, homogeneous database of photometric measurements forClassical T Tauri stars extending up to 20 years. The database containsmore than 21 000 UBVR observations of 72 CTTs. All the data werecollected within the framework of the ROTOR-program at Mount MaidanakObservatory (Uzbekistan) and together they constitute the longesthomogeneous, accurate record of TTS variability ever assembled. Wecharacterize the long term photometric variations of 49 CTTs withsufficient data to allow a robust statistical analysis and propose anempirical classification scheme. Results: .Several patterns oflong term photometric variability are identified. The most commonpattern, exhibited by a group of 15 stars which includes T Tau itself,consists of low level variability (Δ V≤0.4 mag) with nosignificant changes occurring from season to season over many years. Arelated subgroup of 22 stars exhibits a similar stable long termvariability pattern, though with larger amplitudes (up to ΔV≃1.6 mag). Besides these representative groups, we identify threesmaller groups of 3-5 stars each which have distinctive photometricproperties. Conclusions: .The long term variability of most CTTsis fairly stable and merely reflects shorter term variability due tocold and hot surface spots. Only a small fraction of CTTs undergosignificant brightness changes on the long term (months, years), whichprobably arise from slowly varying circumstellar extinction.
| A kinematic study of the Taurus-Auriga T association
Aims.This is the first paper in a series dedicated to investigating thekinematic properties of nearby associations of young stellar objects.Here we study the Taurus-Auriga association, with the primary objectiveof deriving kinematic parallaxes for individual members of this low-massstar-forming region. Methods: .We took advantage of a recentlypublished catalog of proper motions for pre-main sequence stars, whichwe supplemented with radial velocities from various sources found in theCDS databases. We searched for stars of the Taurus-Auriga region thatshare the same space velocity, using a modified convergent point methodthat we tested with extensive Monte Carlo simulations. Results:.Among the sample of 217 Taurus-Auriga stars with known proper motions,we identify 94 pre-main sequence stars that are probable members of thesame moving group and several additional candidates whose pre-mainsequence evolutionary status needs to be confirmed. We derive individualparallaxes for the 67 moving group members with known radial velocitiesand give tentative parallaxes for other members based on the averagespatial velocity of the group. The Hertzsprung-Russell diagram for themoving group members and a discussion of their masses and ages arepresented in a companion paper.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars
T Tauri stars are variable stars that are in an early phase ofevolution, in which accretion and contraction to the main sequence arestill taking place. Their photometric variability is complex; it takesplace on a variety of timescales, due to a variety of physicalprocesses. Periodic variability occurs due to rotation and the presenceof cool or hot spots on the star. It may also occur due to periodicobscuration of the star by inhomogeneities in the still presentaccretion disk. But the periodicity may be masked by other forms ofvariability, or by time variation in the cool or hot spots, or theobscuring inhomogeneities. For other types of variable stars,self-correlation has proven to be a useful adjunct to Fourier analysisfor studying semiregular variability; it determines the cycle-to-cyclebehavior of the star, averaged over all the data. We have therefore usedit to investigate the photometric variability of about 30 T Tauri starsusing existing data. It has provided useful information about periodsand their coherence, about the amplitude of the periodic variation, orits upper limit, and about the ``profile'' of the amplitude-timescalebehavior. In most cases, it has confirmed periods previously determinedby Fourier analysis, but in some cases it has suggested that thepreviously determined period is spurious.
| FU Orionis: The MIDI VLTI Perspective
We present the first mid-infrared interferometric measurements of FUOrionis. We clearly resolve structures that are best explained with anoptically thick accretion disk. A simple accretion disk model fits theobserved SED and visibilities reasonably well and does not require thepresence of any additional structure such as a dusty envelope. Theinclination and also the position angle of the disk can be constrainedfrom the multibaseline interferometric observations. Our disk model isin general agreement with most published near-infrared interferometricmeasurements. From the shape and strength of the 8-13 μm spectrum,the dust composition of the accretion disk is derived for the firsttime. We conclude that most dust particles are amorphous and alreadymuch larger than those typically observed in the ISM. Although the highaccretion rate of the system provides both high temperatures out tolarge radii and an effective transport mechanism to distributecrystalline grains, we do not see any evidence for crystalline silicatesin either the total spectrum or the correlated flux spectra from theinner disk regions. Possible reasons for this nondetection arementioned. All results are discussed in context with other high spatialresolution observations of FU Ori and other FU Ori objects. We alsoaddress the question whether FU Ori is in a younger evolutionary stagethan a classical T Tauri star.Based on observations made with ESO Telescopes at the ParanalObservatories under program ID 074.C-0209 and 274.C-5032.
| 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.
| 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.
| A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus
We present mid-infrared spectra of T Tauri stars in the Taurusstar-forming region obtained with the Spitzer Infrared Spectrograph(IRS). For the first time, the 5-36 μm spectra of a large sample of TTauri stars belonging to the same star-forming region is studied,revealing details of the mid-infrared excess due to dust incircumstellar disks. We analyze common features and differences in themid-IR spectra based on disk structure, dust grain properties, and thepresence of companions. Our analysis encompasses spectral energydistributions from the optical to the far-infrared, a morphologicalsequence based on the IRS spectra, and spectral indices in IRS wavebands representative of continuum emission. By comparing the observedspectra to a grid of accretion disk models, we infer some basic diskproperties for our sample of T Tauri stars and find additional evidencefor dust settling.
| 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.
| Subaru/COMICS Study on Silicate Dust Processing around Young Low-Mass Stars
We have obtained 8-13 μm spectra of 30 young (1-10 Myr) low-masspre-main-sequence stars using COMICS on the 8.2 m Subaru Telescope toexamine dust evolution in protoplanetary disks. Most spectra showsilicate emission features of various strengths and shapes, indicativeof dust processing during the different stages of protoplanetary diskevolution. We have analyzed the observed silicate emission featuresusing a simple model previously applied to the more massive and luminousHerbig Ae/Be systems. We determined the feature strength and shape andderived the composition and typical size of the silicate dust grains. Weconfirm the previously reported dependency of the silicate featurestrength and shape on the grain size of the amorphous silicate dust. Weexamine the relation between the derived dust properties and stellar andcircumstellar disk parameters, such as systemic age, luminosity ofHα (LHα), disk mass, and opacity power-law index β atradio wavelengths. A possible relation is found between silicate featurestrength (grain size indicator) and the LHα, which may be anindicator of accretion activity. It implies that the turbulence inducedby accretion activity may be important for grain size evolution in thedisk. No clear correlation between the crystallinity and thestellar/disk parameters is found. We find that on average 5%-20% in massof the silicate dust grains is in crystalline form, irrespective ofsystemic age. This latter finding supports the idea that crystallinesilicate is formed at an early evolutionary phase, probably at theprotostellar phase, and is remaining during the later stages.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| On the interstellar extinction law toward young stars
We have determined the atomic hydrogen column density N HI toward all ofthe young stars from the Taurus-Auriga-Perseus star-forming complex forwhich the corresponding spectra are available in the Hubble SpaceTelescope archive (nine stars) by analyzing the Lyα line profile.We show that the stars studied, except DR Tau, lie not far from the edgeof the gaseous cloud of the star-forming region closest to us or, moreprecisely, inside the outer H I shell of the cloud. This shell with acolumn density of N HI ≃ 6 × 1020 cm-2 surrounds themolecular gas of the cloud composed of a diffuse component (theso-called diffuse screen) in which dense, compact TMC-1 cores areembedded. The properties of the dust grains toward the stars that lie atthe front edge of the cloud most likely differ only slightly from thoseof the interstellar dust outside star-forming regions. This casts doubton the validity of the hypothesis that the extinction curve toward youngstars has an anomalously low amplitude of the 2175 Åbump—such an extinction curve is observed for the field stars HD29647 and HD 283809 toward which the line of sight passes through theTMC-1 core.
| Light scattering by moving dust grains in the immediate vicinity of young stars
We consider the problem of the distortion of the photospheric spectrumfor a young star as its light is scattered in the inner accretion diskin the dust grain evaporation region. In T Tauri stars, this region isat a distance of the order of several stellar radii and is involved inthe large-scale motions of matter with velocities of 100 km s-1 orhigher. The light scattering in such a medium causes the frequency ofthe scattered radiation to be shifted due to the Doppler effect. Weanalyze the influence of this effect on the absorption line profiles inthe spectra of T Tauri stars using classical results of the theory ofradiative transfer. We consider two models of a scattering medium: (i) ahomogeneous cylindrical surface and (ii) a cylindrical surface with anazimuth-dependent height (such conditions take place during theaccretion of matter onto a star with an oblique magnetic dipole). Weshow that in the first case, the scattering of the photosphericradiation causes the absorption lines to broaden. If the motion of thecircumstellar matter in the dust evaporation region is characterized bytwo velocity components, then the line profile of the scatteredradiation is asymmetric, with the pattern of the asymmetry depending onthe direction of the radial velocity. In the second case, the scatteredradiation can cause periodic shifts of the absorption line centroid,which can be perceived by an observer as periodic radial-velocityvariations in the star. We suggest that precisely this effect isresponsible for the low-amplitude radial-velocity variations withperiods close to the stellar rotation periods that have recently beenfound in some of the T Tauri stars.
| Large dust particles in disks around T Tauri stars
We present 7-mm continuum observations of 14 low-mass pre-main-sequencestars in the Taurus-Auriga star-forming region obtained with the VeryLarge Array with ~1.''5 resolution and ~0.3 mJy rms sensitivity. For 10objects, the circumstellar emission has been spatially resolved. Thelarge outer disk radii derived suggest that the emission at thiswavelength is mostly optically thin. The millimetre spectral energydistributions are characterised by spectral indicesαmm= 2.3 to 3.2. After accounting for contributionsfrom free-free emission and corrections for optical depth, we determinedust opacity indices β in the range 0.5 to 1.6, which suggest thatmillimetre-sized dust aggregates are present in the circumstellar disks.Four of the sources with β>1 may be consistent withsubmicron-sized dust as found in the interstellar medium. Our findingsindicate that dust grain growth to millimetre-sized particles iscompleted within less than 1 Myr for the majority of circumstellardisks.
| Evidence for J- and H-Band Excess in Classical T Tauri Stars and the Implications for Disk Structure and Estimated Ages
We argue that classical T Tauri stars (CTTSs) possess significantnonphotospheric excess in the J and H bands (1.25 and 1.66 μm,respectively). We first show that normalizing the spectral energydistributions (SEDs) of CTTSs to the J band leads to a poor fit of theoptical fluxes (which are systematically overestimated), whilenormalizing the SEDs to the IC band (0.8 μm) produces abetter fit to the optical bands and in many cases reveals the presenceof a considerable excess at J and H. Near-infrared spectroscopic veilingmeasurements from the literature support this result. We find that J-and H-band excesses correlate well with the K-band (2.2 μm) excessand that the J-K and H-K colors of the excess emission are consistentwith that of a blackbody at the dust sublimation temperature (~1500-2000K). We propose that this near-IR excess originates at a hot inner rim,analogous to those suggested to explain the ``near-IR bump'' in the SEDsof Herbig Ae/Be stars. To test our hypothesis, we use the modelpresented by Dullemond and coworkers to fit the photometry data between0.5 and 24 μm of 10 CTTSs associated with the Chamaeleon II molecularcloud. We find that simple models that include luminosities calculatedfrom IC-band magnitudes and an inner rim may account for thereported J- and H-band excesses. The models that best fit the data arethose in which the inner radius of the disk is larger than expected fora rim in thermal equilibrium with the photospheric radiation fieldalone. In particular, we find that large inner rims are necessary toaccount for the mid-infrared fluxes (3.6-8.0 μm) obtained by theSpitzer Space Telescope (Spitzer). The large radius could be explainedif, as proposed by D'Alessio and colleagues, the UV radiation from theaccretion shock significantly affects the sizes of the inner holes indisks around CTTSs. Finally, we argue that deriving the stellarluminosities of CTTSs by making bolometric corrections to the J-bandfluxes, which is the ``standard'' procedure for obtaining CTTSluminosities, systematically overestimates these luminosities. Theoverestimated luminosities translate into underestimated ages when thestars are placed in the H-R diagram. Thus, the results presented hereinhave important implications for the dissipation timescale of inneraccretion disks.
| Molecular gas in irradiated protoplanetary discs
Gas discs around young stars typically disappear after a few Myr and ithas been suggested that the discs are internally photoevaporated, butthe stellar ultraviolet and X-ray radiation appears insufficient forthis mechanism to work. Archival data are used to search for anyrelation between X-ray luminosity and gas in the disc (traced by13CO molecules) for isolated young stars in Taurus. Asignificant correlation is found in that more irradiation increases theamount of 13CO, rather than dispersing the disc. Since highdust masses and accretion rates do not boost the X-ray luminosities,feeding of the radiation by the disc can be ruled out. It is moreplausible that energetic photons are releasing molecules that wouldotherwise be frozen onto dust grains. In this case, estimates of discmass using a canonical CO:H2 ratio are unrealistic. However,the mass released in CO molecules could contribute to building a belt oficy comets similar to the Sun's Kuiper Belt.
| Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective
We present a sensitive, multiwavelength submillimeter continuum surveyof 153 young stellar objects in the Taurus-Auriga star formation region.The submillimeter detection rate is 61% to a completeness limit of ~10mJy (3 σ) at 850 μm. The inferred circumstellar disk masses arelognormally distributed with a mean mass of ~5×10-3Msolar and a large dispersion (0.5 dex). Roughly one-third ofthe submillimeter sources have disk masses larger than the minimalnebula from which the solar system formed. The median disk-to-star massratio is 0.5%. The empirical behavior of the submillimeter continuum isbest described as Fν~ν2.0+/-0.5 between 350μm and 1.3 mm, which we argue is due to the combined effects of thefraction of optically thick emission and a flatter frequency behavior ofthe opacity compared to the interstellar medium. The latter effect couldbe due to a substantial population of large dust grains, whichpresumably would have grown through collisional agglomeration. In thissample, the only stellar property that is correlated with the outer diskis the presence of a companion. We find evidence for significantdecreases in submillimeter flux densities, disk masses, andsubmillimeter continuum slopes along the canonical infrared spectralenergy distribution evolution sequence for young stellar objects. Thefraction of objects detected in the submillimeter is essentiallyidentical to the fraction with excess near-infrared emission, suggestingthat dust in the inner and outer disks is removed nearly simultaneously.
| 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.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| A New Classification Scheme for T Tauri Light Curves
Based on many years of observational data from a photometric database onyoung stars, we propose a new classification scheme for the light curvesof classical T Tauri stars. Our analysis of master light-curve shapesfor 28 classical T Tauri stars is used to distinguish up to fivelight-curve types. The proposed scheme suggests a qualitativeinterpretation in terms of interaction of the central star with itscircumstellar accretion disk.
| Constraints on the ionizing flux emitted by T Tauri stars
We present the results of an analysis of ultraviolet observations of TTauri stars (TTs). By analysing emission measures taken from theliterature, we derive rates of ionizing photons from the chromospheresof five classical TTs in the range ~1041-1044photon s-1, although these values are subject to largeuncertainties. We propose that the HeII/CIV line ratio can be used as areddening-independent indicator of the hardness of the ultravioletspectrum emitted by TTs. By studying this line ratio in a much largersample of objects, we find evidence for an ionizing flux which does notdecrease, and may even increase, as TTs evolve. This implies that asignificant fraction of the ionizing flux from TTs is not powered by theaccretion of disc material on to the central object, and we discuss thesignificance of this result and its implications for models of discevolution. The presence of a significant ionizing flux in the laterstages of circumstellar disc evolution provides an important newconstraint on disc photoevaporation models.
| Observations and Modeling of the Inner Disk Region of T Tauri Stars
We present observations of four T Tauri stars using long baselineinfrared interferometry from the Palomar Testbed Interferometer. Thetarget sources, T Tau N, SU Aur, RY Tau, and DR Tau, are all known to besurrounded by dusty circumstellar disks. The observations directly tracethe inner regions (<1 AU) of the disk and can be used to constrainthe physical properties of this material. For three of the sourcesobserved, the infrared emission is clearly resolved. We first usegeometric models to characterize the emission region size, which rangesfrom 0.04 to 0.3 AU in radius. We then use Monte Carlo radiationtransfer models of accretion disks to jointly model the spectral energydistribution and the interferometric observations with disk modelsincluding accretion and scattering. With these models, we are able toreproduce the data set with extended emission arising from structureslarger than 10 mas contributing less than 6% of the K-band emission,consistent with little or no envelope remaining for these class IIsources [dlog(λFλ)/dlogλ~-2-0 in theinfrared]. The radiation transfer models have inner radii for the dustsimilar to the geometric models; however, for RY Tau, emission from gaswithin the inner dust radius contributes significantly to the model fluxand visibility at infrared wavelengths. The main conclusion of ourmodeling is that emission from inner gas disks (between the magnetictruncation radius and the dust destruction radius) can be a significantcomponent in the inner disk flux for sources with large inner dustradii.
| Einigen uberarbeitete und neue Vergleissternkarten.
Not Available
| On the alignment of Classical T Tauri stars with the magnetic field in the Taurus-Auriga molecular cloud
In this paper we readdress the issue of the alignment of Classical TTauri stars (CTTS) with the magnetic field in the Taurus-Aurigamolecular cloud. Previous studies have claimed that the jet axis ofactive young stellar objects (YSO), projected in the plane of the sky,is aligned preferentially along the projected direction of the localmagnetic field. We re-examine this issue in view of the numerous highangular resolution images of circumstellar disks and micro-jets nowavailable. The images show that T Tauri stars as a group are orientedrandomly with respect to the local magnetic field, contrary to previousclaims. This indicates that the magnetic field may play a lesser role inthe final stages of collapse of an individual prestellar core thanpreviously envisioned. The current database also suggests that asubsample of CTTS with resolved disks but without observations of brightand extended outflows have a tendency to align perpendicularly to themagnetic field. We discuss the possibility that this may trace a lessfavorable topology, e.g., quadrupolar, for the magnetic field in theinner disk, resulting in a weaker collimated outflow.
| The Mass Accretion Rates of Intermediate-Mass T Tauri Stars
We present Hubble Space Telescope ultraviolet spectra and supportingground-based data for a sample of nine intermediate-mass T Tauri stars(IMTTSs; 1.5-4 Msolar). The targets belong to threestar-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZOri, P2441, and V1044 Ori in the Ori OB1c association surrounding theOrion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring aroundλ Ori. The supporting ground-based observations include nearlysimultaneous UBV(R I)C photometry, 6 Å resolutionspectra covering the range 3900-7000 Å, optical echelleobservations in the range 5800-8600 Å, and K-band near-infraredspectra. We use these data to determine improved spectral types andreddening corrections and to obtain physical parameters of the targets.We find that an extinction law with a weak 2175 Å feature but highvalues of AUV/AV is required to explain thesimultaneous optical-UV data; the reddening laws for two B-type starslocated behind the Taurus clouds, HD 29647 and HD 283809, meet theseproperties. We argue that reddening laws with these characteristics maywell be representative of cold, dense molecular clouds. Spectral energydistributions and emission-line profiles of the IMTTSs are consistentwith expectations from magnetospheric accretion models. We compare oursimultaneous optical-UV data with predictions from accretion shockmodels to get accretion luminosities and mass accretion rates (M) forthe targets. We find that the average mass accretion rate for IMTTSs is~3×10-8 Msolar yr-1, a factor of~5 higher than that for their low-mass counterparts. The new data extendthe correlation between M and stellar mass to the intermediate-massrange. Since the IMTTSs are evolutionary descendants of the Herbig Ae/Bestars, our results put limits to the mass accretion rates of theirdisks. We present luminosities of the UV lines of highly ionized metalsand show that they are well above the saturation limit for magneticallyactive cool stars but correlate strongly with accretion luminosity,indicating that they are powered by accretion, in agreement withprevious claims but using a sample in which reddening and accretionluminosities have been determined self-consistently. Finally, we findthat the relation between accretion luminosity and Brγ luminosityfound for low-mass T Tauri stars extends to the intermediate-massregime.
| Dense gas discs around T Tauri stars
The circumstellar discs of six T Tauri stars, forming an age sequencefrom approximately 1 to 10 Myr, have been observed in the molecular lineHCO+J= 4 - 3. The dense gas traced by this line liestypically at less than half the radius characteristic of more easilyexcited CO lines, and the most compact disc solutions have outer radiiof only around 100 au. Within the uncertainties set by the discchemistry, the lowest-mass solutions are found to possess at least 2 to15 MJupiter of gas, in all but one of the six systems. Thedense gas present within ~100 au could provide fuel for the zone at tensof au where giants planets may form, and this mass reservoir issufficient to form such planets. Only the oldest star, LkCa 15 at 8-12Myr, has a large central cavity lacking in dense material that mayreflect gas removal at late times.
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
| 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.
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