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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.
| Mid-Infrared Observations of T Tauri Stars: Probing the Star-Disk Connection in Rotational Evolution
We present mid-IR N-band (λeff=10.2 μm) photometryof a carefully selected sample of T Tauri stars thought to be singlefrom the Taurus-Auriga molecular cloud. Infrared excesses in these starsare generally attributed to circumstellar dust disks. Combiningobservations at 2.16 (Ks band) and 10.2 μm (N band) weprobe a region in the circumstellar dust disk from a few stellar radiithrough the terrestrial planet zone (0.02-1.0 AU). By analyzing thedistribution of the (Ks-N) color index with respect topreviously measured photometric rotation periods we investigate whatrole circumstellar disks play in the rotational evolution of the centralstar. The resulting positive correlation between these two variables isconsistent with the notion that a star-disk interaction facilitates theregulation of angular momentum during the T Tauri stage. We alsodemonstrate how including nonsingle stars in such an analysis weakensany correlation in the relation between (Ks-N) color andperiod. To further understand disk properties we also present spectralenergy distributions for a few objects with new ground-based M-band(λeff=4.8 μm) and Q-band (λeff=20μm) data and compare them to a geometrically thin, optically thickdisk model.
| 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.
| θ1 Orionis E as a Spectroscopic Binary
θ1 Ori E (θ1E), the fifth brightestmember of the Orion Nebula Trapezium, has been discovered to be adouble-line spectroscopic binary. This paper describes the 1998-2006Keck HIRES observations, the resulting spectroscopic orbit, and what canbe inferred from the limited photometric information now available. Theperiod is 9.89 days, and the components are essentially identicalmid-G-type giants; their location in the H-R diagram and theirpossession of strong Li I λ6707 lines indicate that they arepre-main-sequence members of the Orion Nebula Cluster (ONC), with massesof about 3-4 Msolar. θ1E is among thebrightest X-ray sources in the ONC. It is variable in X-rays and is alsoa variable nonthermal radio source, but it is uncertain whether theobject is also optically variable.
| 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.
| A Simultaneous Optical and X-Ray Variability Study of the Orion Nebula Cluster. I. Incidence of Time-correlated X-Ray/Optical Variations
We present a database of BVRI time-series photometry of the Orion NebulaCluster obtained with two ground-based telescopes at differentlongitudes to provide simultaneous coverage with the 13 day Chandraobservation of the cluster. The resulting database of simultaneousoptical and X-ray light curves for some 800 pre-main-sequence (PMS)stars represents, by a factor of hundreds, the largest synoptic,multi-wavelength-regime, time-series study of young stars to date. Thisdatabase will permit detailed analyses of the relationship betweenoptical and X-ray variability among a statistically significant ensembleof PMS stars, with the goal of elucidating the origins of PMS X-rayproduction. In this first paper, we present the optical observations,describe the combined X-ray/optical database, and perform an analysis oftime-correlated variability in the optical and X-ray light curves. Weidentify 40 stars (representing 5% of our study sample) with possibletime-correlated optical and X-ray variability. Examples of both positiveand negative time correlations are found, possibly representing X-rayflares and persistent coronal features associated with both cool and hotsurface spots (i.e., magnetically active regions and accretion shocks).We also find two possible examples of ``white-light'' flares coincidentwith X-ray flares; these may correspond to the impulsive heating phasein solar-analog flares. However, although interesting, these representunusual cases. More generally, we find very little evidence to suggest adirect causal link between the sources of optical and X-ray variabilityin PMS stars. The conclusion that accretion is a primary driver of X-rayproduction in PMS stars is not supported by our findings.This study is part of the Chandra Orion Ultradeep Project (COUP).
| 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.
| 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.
| Probing T Tauri Accretion and Outflow with 1 Micron Spectroscopy
In a high-dispersion 1 μm survey of 39 classical T Tauri stars(CTTSs) veiling is detected in 80% of the stars, and He I λ10830and Pγ line emission in 97%. On average, the 1 μm veilingexceeds the level expected from previously identified sources of excessemission, suggesting the presence of an additional contributor toaccretion luminosity in the star-disk interface region. Strengths ofboth lines correlate with veiling, and at Pγ there is a systematicprogression in profile morphology with veiling. He I λ10830 hasan unprecedented sensitivity to inner winds, showing blueshiftedabsorption below the continuum in 71% of the CTTSs, compared to 0% atPγ. This line is also sensitive to magnetospheric accretion flows,with redshifted absorption below the continuum found in 47% of theCTTSs, compared to 24% at Pγ. The blueshifted absorption at He Iλ10830 shows considerable diversity in its breadth andpenetration depth into the continuum, indicating that a range of innerwind conditions exist in accreting stars. We interpret the broadest anddeepest blue absorptions as formed from scattering of the 1 μmcontinuum by outflowing gas whose full acceleration region envelopes thestar, suggesting radial outflow from the star. In contrast, narrow blueabsorption with a range of radial velocities more likely arises viascattering of the 1 μm continuum by a wind emerging from the innerdisk. Both stellar and disk winds are accretion powered, since neitheris seen in nonaccreting WTTSs and among the CTTSs helium strengthcorrelates with veiling.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| XMM-Newton X-ray spectroscopy of classical T Tauri stars
We present results from a comparative study of XMM-Newton observationsof four classical T Tauri stars (CTTS), namely BPTau, CR Cha, SU Aur andTW Hya. In these objects coronal, i.e. magnetic,activity and as recently shown, magnetically funneled accretion are theprocesses likely to be responsible for the generation of X-ray emission.Variable X-ray emission with luminosities in the order of1030 erg/s is observed for all targets. We investigate lightcurves as well as medium and high-resolution X-ray spectra to determinethe plasma properties of the sample CTTS and to study the origin oftheir X-ray emission and its variability. The emission measuredistributions and observed temperatures differ significantly and thetargets are dominated either by plasma at high densities as produced byaccretion shocks or by predominantly hotter plasma of coronal origin.Likewise the variability of the X-ray luminosity is found to begenerated by both mechanisms. Cool plasma at high densities is found inall stars with detected O VII triplet emission, prevented only for SUAur due to strong absorption. A general trend is present in theabundance pattern, with neon being at solar value or enhanced whileoxygen, iron and most other metals are depleted, pointing to thepresence of the inverse FIP effect in active coronae and possibly grainformation in evolved disks. We find that both accretion shocks andcoronal activity contribute to the observed X-ray emission of thetargets. While coronal activity is the dominant source of X-ray activityin the majority of the CTTS, the fraction for each process differssignificantly between the individual objects.
| 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.
| 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).
| First Optical Images of Circumstellar Dust Surrounding the Debris Disk Candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealeda circumstellar dust disk around the A star HD 32297. Dust-scatteredlight is detected as far as 400 AU radius, and the linear morphology isconsistent with a disk ~10° away from an edge-on orientation. Herewe present the first optical images that show the dust-scattered lightmorphology from 560 to 1680 AU radius. The position angle of theputative disk midplane diverges by ~31°, and the color of dustscattering is most likely blue. We associate HD 32297 with a wall ofinterstellar gas and the enigmatic region south of the Taurus molecularcloud. We propose that the extreme asymmetries and blue disk colororiginate from a collision with a clump of interstellar material as HD32297 moves southward, and discuss evidence consistent with an age of 30Myr or younger.
| Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAMPlateau de Bure interferometer in the isotopologues of CO, and incontinuum at 3 and 1.3 mm. These observations allow us to trace thestructure of the circumstellar material of AB Aur inregions where optical and IR imaging is impossible because of theemission from the star. These images reveal that the environment of ABAur is widely different from the proto-planetary disks that surround TTauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 inseveral aspects. Instead of being centrally peaked, the continuumemission is dominated by a bright, asymmetric (spiral-like) feature atabout 140 AU from the central star. Little emission is associated withthe star itself. The molecular emission shows that AB Aur is surroundedby a very extended flattened structure ("disk"), which is rotatingaround the star. Bright molecular emission is also found towards thecontinuum "spiral". The large-scale molecular structure suggests the ABAur disk is inclined between 23 and 43 degrees, but the strong asymmetryof the continuum and molecular emission prevents an accuratedetermination of the inclination of the inner parts. Analysis of theemission in terms of a Keplerian disk provides a reasonable fit to thedata, but fails to give a consistent picture because the inclinationsdetermined from 12CO J~= 2~→ 1, 13CO J~=2~→ 1, 13CO J~= 1~→ 0 and C18O J~=1~→ 0 do not agree. The mass predicted for the central star in suchKeplerian models is in the range 0.9 to 1.2 Mȯ, muchsmaller than the expected 2.2 Mȯ from the spectral typeof AB Aur. Better and more consistent fits to the 13CO J~=2~→ 1, 13CO J~= 1~→ 0 data are obtained by relaxingthe Keplerian hypothesis. We find significant non-Keplerian motion, witha best fit exponent for the rotation velocity law of 0.41 ± 0.01,but no evidence for radial motion. The disk has an inner hole about 70AU in radius. The disk is warm and shows no evidence of depletion of CO.The dust properties suggest that the dust is less evolved than intypical T Tauri disks. Both the spiral-like feature and the departurefrom purely Keplerian motion indicates the AB Aur disk is not inquasi-equilibrium. Disk self-gravity is insufficient to create theperturbation. This behavior may be related either to an early phase ofstar formation in which the Keplerian regime is not yet fullyestablished and/or to a disturbance of yet unknown origin. An alternate,but unproven, possibility is that of a low mass companion located about40 AU from AB Aur.
| Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective
We present a sensitive, multiwavelength submillimeter continuum surveyof 153 young stellar objects in the Taurus-Auriga star formation region.The submillimeter detection rate is 61% to a completeness limit of ~10mJy (3 σ) at 850 μm. The inferred circumstellar disk masses arelognormally distributed with a mean mass of ~5×10-3Msolar and a large dispersion (0.5 dex). Roughly one-third ofthe submillimeter sources have disk masses larger than the minimalnebula from which the solar system formed. The median disk-to-star massratio is 0.5%. The empirical behavior of the submillimeter continuum isbest described as Fν~ν2.0+/-0.5 between 350μm and 1.3 mm, which we argue is due to the combined effects of thefraction of optically thick emission and a flatter frequency behavior ofthe opacity compared to the interstellar medium. The latter effect couldbe due to a substantial population of large dust grains, whichpresumably would have grown through collisional agglomeration. In thissample, the only stellar property that is correlated with the outer diskis the presence of a companion. We find evidence for significantdecreases in submillimeter flux densities, disk masses, andsubmillimeter continuum slopes along the canonical infrared spectralenergy distribution evolution sequence for young stellar objects. Thefraction of objects detected in the submillimeter is essentiallyidentical to the fraction with excess near-infrared emission, suggestingthat dust in the inner and outer disks is removed nearly simultaneously.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Spatially resolving the accretion shocks on the rapidly-rotating M0 T-Tauri star MN Lupi
We obtained high-resolution, high-quality VLT/UVES spectra toreconstruct the two-dimensional surface structure of therapidly-rotating classical T-Tauri star MN Lupi on two separate nights.Both surface maps show a structured warm (5000 K) band centered aroundthe pole at a latitude of ≈65°. Located within the band are twohot spots with temperatures of approximately or possibly even in excessof 5800 K, i.e. 2000 K above the effective photospheric temperature.Both maps appear with an adjacent equatorial band of temperature 3400 K,some 400-500 K below the effective photospheric temperature. While weinterpret the two hot spots and the warm high-latitude band to be theheating points from two accretion impacts at the time of ourobservations and their redistributed energy trailed due to the faststellar rotation, respectively, the cool equatorial band may not be coolafter all but due to obscuration of the stellar surface by the innermostregion of the disk. The fact that the hot spots appear at high stellarlatitude is in agreement with the magnetospheric accretion model thatproposes material funnelling onto the star along a predominantly dipolarmagnetic field at roughly 50° latitude. The evidence of ongoing diskaccretion, together with the very fast rotation of MN Lupi of just 3-4times below its break-up velocity, suggests that the accretion mechanismis the cause of its rapid surface rotation. We present a model ofmagnetic star-disk coupling for MN Lupi that predicts a polar surfacemagnetic field of ≈3 kG.
| IRAC Observations of Taurus Pre-Main-Sequence Stars
We present infrared photometry obtained with the IRAC camera on theSpitzer Space Telescope of a sample of 82 pre-main-sequence stars andbrown dwarfs in the Taurus star-forming region. We find a clearseparation in some IRAC color-color diagrams between objects with andwithout disks. A few ``transition'' objects are noted, which correspondto systems in which the inner disk has been evacuated of small dust.Separating pure disk systems from objects with remnant protostellarenvelopes is more difficult at IRAC wavelengths, especially for objectswith infall at low rates and large angular momenta. Our resultsgenerally confirm the IRAC color classification scheme used in previouspapers by Allen et al. and Megeath et al. to distinguish betweenprotostars, T Tauri stars with disks, and young stars without (inner)disks. The observed IRAC colors are in good agreement with recentimproved disk models, and in general accord with models for protostellarenvelopes derived from analyzing a larger wavelength region. We alsocomment on a few Taurus objects of special interest. Our results shouldbe useful for interpreting IRAC results in other, less well studiedstar-forming regions.
| 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.
| Unusual Color Variability of Eruptive Stars
We substantiate the conclusion that the unusual color variability foundpreviously in some eruptive stars is typical of a broad class ofnonstationary objects, manifests itself over a wide temperature range(from B0 to K 3), and can be regarded as a new type of stellarvariability.
| Colors of Classical T Tauri Stars in Taurus Derived from Spitzer Infrared Spectrograph Spectra: Indication of Dust Settling
We analyzed Spitzer Infrared Spectrograph data of a representativesample of classical T Tauri stars in the Taurus star-forming region bycomputing color indices in wave bands that characterize the continuumemission from dust in circumstellar disks. We compared these indices tothose derived from a grid of accretion disk models with varyinginclination angles, mass accretion rates, and amounts of dust settling.We conclude that most T Tauri stars in our sample have experienced somedegree of dust settling and grain growth in their disks, indicating diskevolution.
| 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 Search for Substellar Companions around 15 Weak-Lined T Tauri Stars with the Planetary Camera 2 of the Hubble Space Telescope
To search for thermal emission from substellar companions, we haveobtained Z-band images of 15 weak-lined T Tauri stars in theTaurus-Auriga and Ophiuchus star-forming regions using the PlanetaryCamera 2 on the Hubble Space Telescope. We found 18 faint nearby objectsat separations larger than 4" but none at smaller separations. For 11 ofthese we have enough color information to suggest that they are notsubstellar members of the star-forming regions. The remaining sevenfaint nearby objects are candidate substellar objects. Our detectionlimit for companions with separations larger than about 1" is apparentZ=19.5 mag, corresponding to substellar objects in the mass range3MJ-15MJ for the age range of our targets, 1.2-25Myr. Our detection limit degrades for smaller separations and is 4-6 magpoorer at a separation of 0.2", corresponding to a projected separationof 30 AU. Inside 0.2" our sensitivity is insufficient to detectsubstellar companions. To complement the imaging program, extensivespectroscopic observations were obtained with the Center forAstrophysics Digital Speedometers. Four of our targets proved to bespectroscopic binaries, one of them double-lined. We report orbitalsolutions for these four stars. Our radial velocities support themembership of all our targets to their respective star-forming regions,except for HD 283759, which must be rejected as a member.Some of the results presented here used observations made with the MMT,a joint facility of the Smithsonian Institution and the University ofArizona.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation (NSF).
| Time-series Paschen-β spectroscopy of SU Aurigae
We present time-series echelle spectra of the Paβ line of the TTauri star SU Aur, observed over three consecutive nights. The lineshows strong variability (~10 per cent) over the velocity range 100-420km s-1 in the red broad absorption component, and weakervariability (~2 per cent) over the velocity range -200-0kms-1in the blue wing. The variability in the velocity range-200-0kms-1 is correlated with that in200-400kms-1, and the variability in these velocity rangesanticorrelates with that in 0-100kms-1. The mean spectrumfrom the second night shows the suggestion of a blueshifted absorptioncomponent at about -150kms-1, similar to that found in theHα and Hβ lines. We find the position of the subpeak in thered absorption component changes steadily with time, and its motionmodulates at half the rotational period. We also find that themodulation of the line equivalent width is possibly associated with ahalf and a third of the rotational period, which is consistent with thesurface Doppler images of SU Aur. Radiative transfer models of arotationally modulated Paβ line, produced in the shock-heatedmagnetospheric accretion flow, are also presented. Models with amagnetic dipole offset reproduce the overall characteristics of theobserved line variability, including the line equivalent width and themotion of the subpeak in the red absorption trough.
| 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.
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