Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| Collimation, Proper Motions, and Physical Conditions in the HH 30 Jet from Hubble Space Telescope Slitless Spectroscopy
We present STIS spectral images of the HH 30 stellar jet taken through awide slit over two epochs. The jet is unresolved spectrally, so theobservations produce emission-line images for each line in the spectrum.This rich data set shows how physical conditions in the jet vary withdistance and time, produces precise proper motions of knots within thejet, resolves the jet width close to the star, and gives a spectrum ofthe reflected light from the disk over a large wavelength range atseveral positions. We introduce a new method for analyzing a set of lineratios based on minimizing a quadratic form between models and data. Themethod generates images of the density, temperature, and ionizationfraction computed using all the possible line ratios appropriatelyweighted. In HH 30, the density declines with distance from the sourcein a manner consistent with an expanding flow and is larger by a factorof 2 along the axis of the jet than it is at the periphery. Ionizationin the jet ranges from ~5% to 40%, and high-ionization/excitation knotsform at about 100 AU from the star and propagate outward with the flow.These high-excitation knots are not accompanied by correspondingincreases in the density, so if formed by velocity variations the knotsmust have a strong internal magnetic pressure to smooth out densityincreases while lengthening recombination times.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| On the Flaring of Jet-sustaining Accretion Disks
Jet systems with two unequal components interact with their parentaccretion disks through the asymmetric removal of linear momentum fromthe star-disk system. We show that as a result of this interaction, thedisk's state of least energy is not made up of orbits that lie in aplane containing the star's equator, as in a disk without jets. Thedisk's profile has the shape of a sombrero, curved in the direction ofacceleration. For this novel state of minimum energy, we derive thetemperature profile of thin disks. The flaring geometry caused by thesombrero profile increases the disk temperature, especially in its outerregions. The jet-induced acceleration disturbs the vertical equilibriumof the disk, leading to mass loss in the form of a secondary windemanating from the upper face of the disk. The jets' time variabilitycauses the disk to radially expand or contract, depending on whether theinduced acceleration increases or decreases. It also excites verticalmotion and eccentric distortions in the disk and affects the sombreroprofile's curvature. These perturbations lead to the heating of the diskthrough its viscous stresses as it tries to settle into the varyingstate of minimum energy. The jet-disk interaction studied here will helpus to estimate the duration of the jet episode in star-disk systems andmay explain the origin of the recently observed one-sided molecularoutflow of the HH 30 disk-jet system.
| 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.
| Modeling T Tauri Winds from He I λ10830 Profiles
The high opacity of He I λ10830 makes it an exceptionallysensitive probe of the inner wind geometry of accreting T Tauri stars.In this line, blueshifted absorption below the continuum results fromsimple scattering of stellar photons, a situation that is readilymodeled without definite knowledge of the physical conditions andrecourse to multilevel radiative transfer. We present theoretical lineprofiles for scattering in two possible wind geometries, a disk wind anda wind emerging radially from the star, and compare them to observed He I λ10830 profiles from a survey of classical T Tauri stars. Thecomparison indicates that subcontinuum blueshifted absorption ischaracteristic of disk winds in ~30% of the stars and of stellar windsin ~40%. We further conclude that for many stars the emission profile ofhelium likely arises in stellar winds, increasing the fraction ofaccreting stars inferred to have accretion-powered stellar winds to~60%. Stars with the highest disk accretion rates are more likely tohave stellar wind than disk wind signatures and less likely to haveredshifted absorption from magnetospheric funnel flows. This suggeststhe possibility that when accretion rates are high, disks can extendcloser to the star, magnetospheric accretion zones can be reduced insize, and conditions can arise that favor radially outflowing stellarwinds.
| Periodic radial velocity variations in RU Lupi
Context: . RU Lup is a Classical T Tauri star withunusually strong emission lines, which has been interpreted asmanifestations of accretion. Recently, evidence has accumulated thatthis star might have a variable radial velocity. Aims: .Weintended to investigate in more detail the possible variability inradial velocity using a set of 68 high-resolution spectra taken at theVLT (UVES), the AAT (UCLES) and the CTIO (echelle). Methods:.Using standard cross-correlation techniques, we determined the radialvelocity of RU Lup. We analysed these results with Phase-dispersionminimization and the Lomb-Scargle periodogram and searched for possibleperiodicities in the obtained radial velocities. We also analysedchanges in the absorption line shapes and the photometric variability ofRU Lup. Results: .Our analysis indicated that RU Lup exhibitsvariations in radial velocity with a periodicity of 3.71 days and anamplitude of 2.17 km s-1. These variations can be explainedby the presence of large spots, or groups of spots, on the surface of RULup. We also considered a low-mass companion and stellar pulsations asalternative sources for these variations but found these to beunlikely.
| 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.
| A VLT/NACO survey for triple and quadruple systems among visual pre-main sequence binaries
Aims.This paper describes a systematic search for high-ordermultiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods: .We conducted an Adaptive Optics survey of a sample of 58 PMSwide binaries from various star-forming regions, which include 52 TTauri systems with mostly K- and M-type primaries, with the NIRinstrument NACO at the VLT. Results: .Of these 52 systems, 7 arefound to be triple (2 new) and 7 quadruple (1 new). The new closecompanions are most likely physically bound based on their probabilityof chance projection and, for some of them, on their position on acolor-color diagram. The corresponding degree of multiplicity among widebinaries (number of triples and quadruples divided by the number ofsystems) is 26.9 ± 7.2% in the projected separation range ~0.07arcsec -12'', with the largest contribution from the Taurus-Aurigacloud. We also found that this degree of multiplicity is twice in Tauruscompared to Ophiuchus and Chamaeleon for which the same number ofsources are present in our sample. Considering a restricted samplecomposed of systems at distance 140-190 pc, the degree of multiplicityis 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30binaries, 5 triples, 6 quadruples). The observed frequency agrees withresults from previous multiplicity surveys within the uncertainties,although a significant overabundance of quadruple systems compared totriple systems is apparent. Tentatively including the spectroscopicpairs in our restricted sample and comparing the multiplicity fractionsto those measured for solar-type main-sequence stars in the solarneighborhood leads to the conclusion that both the ratio of triples tobinaries and the ratio of quadruples to triples seems to be in excessamong young stars. Most of the current numerical simulations of multiplestar formation, and especially smoothed particles hydrodynamicssimulations, over-predict the fraction of high-order multiplicity whencompared to our results. The circumstellar properties around theindividual components of our high-order multiple systems tend to favormixed systems (i.e. systems including components of wTTS and cTTS type),which is in general agreement with previous studies of disks inbinaries, with the exception of Taurus, where we find a preponderance ofsimilar type of components among the multiples studied.
| Plateau de Bure interferometer observations of the disk and outflow of HH 30
Context: .HH 30 is a well-known Pre-Main-Sequence star in Taurus. HSTobservations have revealed a flared, edge-on disk driving ahighly-collimated optical jet, making this object a case study for thedisk-jet-outflow paradigm. Aims: .We searched for a molecularoutflow, and attempted to better constrain the star and diskparameters. Methods: .We obtained high angular resolution (sin1'') observations of the dust continuum at 2.7 and 1.3 mm, and of the12CO~J=2-1, 13CO~J=2-1 and J=1-0,C18O~J=1-0 emissions around HH 30. A standard disk model isused to fit the 13CO J=2-1 uv-plane visibilities and derivethe disk properties, and the stellar mass. An ad hoc outflow model isused to reproduce the main properties of the 12CO~J=2-1emission. Results: .The rotation vector of the disk points towardthe North-Eastern jet. The disk rotation is Keplerian: using a distanceof 140 pc, we deduce a mass of 0.45 Msun for the central star. The diskouter radius is 420 AU. A highly asymmetric outflow originates from theinner parts of the disk. Only its North-Eastern lobe was detected: itpresents to first order a conical morphology with a 30° half openingangle and a constant (12 km s-1) radial velocity field.Outflow rotation was searched for but not found. The upper limit of theoutflow rotation velocity is 1 km s-1 at 200 AU of the jetaxis. Conclusions: .HH 30 is a low mass TTauri of spectral typearound M1 and age 1 to 4 Myr, surrounded by a medium size Kepleriandisk, of mass around 4 × 10-3 Msun. It beautifullyillustrates the jet-disk-outflow interaction, being so far the only starto display a jet and outflow connected to a well defined Keplerian disk,but reveals a surprisingly asymmetric (one-sided) outflow despite arelatively symmetric jet. Furthermore, these observations do not enableto assign the origin of the molecular outflow to entrainment by theoptical jet or to a disk wind. In the latter hypothesis, the lack ofrotation would imply an origin in the inner 15 AU of the disk.
| 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.
| Adaptive Optics Spectroscopy of the [Fe II] Outflows from HL Tauri and RW Aurigae
We present new results of [Fe II] λ1.644 μm spectroscopytoward the jets from HL Tau and RW Aur carried out with the SubaruTelescope combined with the adaptive optics system. We observed theregions within 2"-3" from the stars with the subarcsecond resolutions of0.5" and 0.2" for HL Tau and RW Aur, respectively. In addition to thestrong high-velocity component (HVC) extended along each jet, wedetected a blueshifted low-velocity component (LVC) seen as a wing orshoulder of the HVC at each stellar position. The position velocitydiagrams of the two objects show a characteristic similar to those ofthe cold disk wind and X-wind models in that the [Fe II] line width isbroad close to the stellar position and narrower at the extended jet. Acloser comparison suggests, however, that the disk wind model tends tohave too large a line width at the HVC, while the X-wind model hasexcess redshifted emission at the stellar position. The narrow velocitywidth with symmetric line profiles of the observed HVC supports anX-wind-type model, while the LVC, located away from the star, favors thepresence of a disk wind. The [Fe II] emission shows a gap of 0.8" for HLTau and a marked drop of Y~-0.2" for RW Aur between the redshifted jetand the star, which indicate optically thick disks of ~160 and <40 AUin radius, respectively. Part of the Br12 emission of HL Tau originatesfrom the jet itself because its normalized line profile shows asignigicantly large deviation from the normalized continuum in spatialprofile.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| 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.
| Which jet launching mechanism(s) in T Tauri stars?
Aims.We examine whether ejection phenomena from accreting T Tauri starscan be described by only one type of self-collimated jet model.Methods: .We present analytical kinematic predictions valid soon afterthe Alfvén surface for all types of steady magneticallyself-confined jets. Results: .We show that extended disc winds,X-winds, and stellar winds occupy distinct regions in the poloidal speedvs. specific angular momentum plane. Comparisons with currentobservations of T Tauri jets yield quantitative constraints on the rangeof launching radii, magnetic lever arms, and specific energy input indisc and stellar winds. Implications on the origin of jet asymmetriesand disc magnetic fields are outlined. Conclusions: .We argue thatejection phenomena from accreting T Tauri stars most likely includethree dynamical components: (1) an outer self-collimated steady discwind carrying most of the mass-flux in the optical jet (when present);confining (2) a pressure-driven coronal stellar wind; and (3) a hotinner flow made of blobs sporadically ejected from the magnetopause. Ifthe stellar magnetic moment is parallel to the disc magnetic field, thenthe highly variable inner flow resembles a "Reconnection X-wind", thathas been proven to efficiently brake down an accreting and contractingyoung star. If the magnetic moment is anti-parallel, then largerversions of the solar coronal mass ejections are likely to occur. Therelative importance of these three components in the observed outflowsand the range of radii involved in the disc wind are expected to varywith time, from the stage of embedded source to the optically revealed TTauri star phase.
| Tidal stripping and disk kinematics in the RW Aurigae system
We present interferometric maps of the RW Aur system obtained with theIRAM Plateau de Bure Interferometer in CO(J=2-1), CO(J=1-0), and nearbycontinuum. The sub-arcsecond angular resolution (0.89''×0.58'')and high-sensitivity reached at 1.3 mm enable us to resolve threemolecular structures: (1) an optically thick disk around RW Aur A inrotation about the optical jet axis, (2) a disturbed asymmetric peakaround RW Aur B, (3) a 600 AU-long “arm” of materialtrailing from the RW Aur A disk. Comparison with Keplerian modelsindicates that the RW Aur A disk is the smallest detected so far arounda T Tauri star (radius 40-57 AU) and that the CO emitting layer at theouter edge is warmer than the dust ({Tout}≃ 60-100 K)and relatively thick ({Nwarm}≃0.1-10×1022cm-2). The morphology andkinematics of the detected features strongly suggest that we arewitnessing tidal stripping of the primary disk by the recent fly-by ofRW Aur B. We speculate that tidal dissipation might explain the warmergas temperatures in the RW Aur A disk compared with typical T Tauristars, and perhaps play a role in its elevated accretion rate. We alsofind that the rotation sense of the RW Aur A disk is opposite totransverse velocity shifts in the optical jet reported by Woitas et al.(2005, A&A, 432, 149). We argue that these transverse shifts arelikely to represent only upper limits to the true jet rotation speed.The limits remain consistent with current models of MHD launching fromthe disk.
| Emission lines from rotating proto-stellar jets with variable velocity profiles. I. Three-dimensional numerical simulation of the non-magnetic case
Using the Yguazú-a three-dimensional hydrodynamic code, we havecomputed a set of numerical simulations of heavy, supersonic,radiatively cooling jets including variabilities in both the ejectiondirection (precession) and the jet velocity (intermittence). In order toinvestigate the effects of jet rotation on the shape of the lineprofiles, we also introduce an initial toroidal rotation velocityprofile, in agreement with some recent observational evidence found injets from T Tauri stars which seems to support the presence of arotation velocity pattern inside the jet beam, near the jet productionregion. Since the Yguazú-a code includes an atomic/ionic network,we are able to compute the emission coefficients for several emissionlines, and we generate line profiles for the Hα, [OI]λ6300, [S II]λ6716 and [N II]λ6548 lines. Usinginitial parameters that are suitable for the DG Tau microjet, we showthat the computed radial velocity shift for the medium-velocitycomponent of the line profile as a function of distance from the jetaxis is strikingly similar for rotating and non-rotating jet models.These findings lead us to put forward some caveats on the interpretationof the observed radial velocity distribution from a few outflows fromyoung stellar objects, and we claim that these data should not bedirectly used as a doubtless confirmation of the magnetocentrifugal windacceleration models.
| Studies of pre-main-sequence stars with integral field spectrographs
We review the contribution of integral field spectroscopy (IFS) topre-main-sequence star studies. These studies are mostly synoptic innature reflecting the use of this technique in addressing difficult andcomplex objects. Physical diagnostics were derived from IFS data such as(atomic, molecular) gas excitation and pre-shock densities, gasmorphology and spectra from close binaries. Models for the sources aredirectly tested: shock models (planar, bow-shock) andmagneto-hydrodynamics jet models. Future contributions to the field areaddressed. Surveys of mass loss across age and mass spectrum and in thenearby Orion Nebula cluster emerge as the most scientifically promising.
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Keck Interferometer Observations of Classical and Weak-line T Tauri Stars
We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GMAur, LkCa 15, RW Aur, and V830 Tau, using long baseline infraredinterferometry at K band (2.2 μm) from the Keck Interferometer. Thetarget sources have a range of mass accretion rates and excessnear-infrared emission. The interferometer is most sensitive to extendedemission on characteristic size scales of 1-5 mas. All sources showevidence for resolved K-band emission on these scales, although a few ofthe sources are marginally consistent with being unresolved. Wecalculate the infrared excess based on fitting stellar photospheremodels to the optical photometry and estimate the physical size of theemission region using simple geometric models for the sources with asignificant infrared excess. Assuming that the K-band-resolved emissiontraces the inner edge of the dust disk, we compare the measuredcharacteristic sizes to predicted dust sublimation radii and find thatthe models require a range of dust sublimation temperatures and possiblyoptical depths within the inner rim to match the measured radii.
| 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.
| 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.
| The extreme T Tauri star RW Aur: accretion and outflow variability
We present an analysis of the classical T Tauri star RW Aur A, based on77 echelle spectra obtained at Lick Observatory over a decade ofobservations. RW Aur, which has a higher than average mass accretionrate among T Tauri stars, exhibits permitted (Hα, Hβ, Ca II,He I, NaD) and forbidden ([OI]6300 Å) emission lines with strongvariability. The permitted lines display multiple periodicities over theyears, often with variable accretion (redshifted) and outflow(blueshifted) absorption components, implying that both processes areactive and changing in this system. The broad components of thedifferent emission lines exhibit correlated behavior, indicating acommon origin for all of them. We compute simple magnetosphericaccretion and disk-wind Hα, Hβ and NaD line profiles for RWAur. The observed Balmer emission lines do not have magnetosphericaccretion line profiles. Our modeling indicates that the windcontribution to these line profiles is very important and must be takeninto account. Our results indicate that the Hα, Hβ and NaDobserved line profiles of RW Aur are better reproduced by collimateddisk-winds starting from a small region near the disk inner radius.Calculations were performed in a region extending out to 100R*. Within this volume, extended winds originating over manystellar radii along the disk are not able to reproduce the three linessimultaneously. Strongly open-angled winds also generate profiles thatdo not look like the observed ones. We also see evidence that theoutflow process is highly dynamic - the low- and high-velocitycomponents of the [OI](6300 Å) line vary independently ontimescales of days. The apparent disappearance from December 1999 toDecember 2000 of the [OI](6300 Å) low velocity component, which isthought to come from the disk-wind, shows that the the slow wind canexhibit dramatic variability on timescales of months (placing limits onhow extended it can be). There is no comprehensive explanation yet forthe behavior of RW Aur, which may in part be due to complications thatwould be introduced if it is actually a close binary.
| Whims of an Accreting Young Brown Dwarf: Exploring the Emission-Line Variability of 2MASSW J1207334-393254
We report the first comprehensive study of emission-line variability inan accreting young brown dwarf. We have collected 14 high-resolutionoptical spectra of 2MASSW J1207334-393254 (M8), a likely member of thenearby 8 million year old TW Hydrae association with a recentlyidentified planetary mass companion, in three observing runs between2005 January-March on the Magellan Clay telescope. These spectra show avariety of emission lines that are commonly seen in classical T Tauristars. The Hα line, in particular, shows dramatic changes in shapeand intensity in our data set, on timescales of both several weeks andseveral hours. In spectra from late January, the line is relatively weakand only slightly asymmetric. Spectra from mid and late March showintense, broad (10% width ~280 km s-1), and asymmetricHα emission, indicative of ongoing disk accretion. Based onempirical diagnostics, we estimate that the accretion rate could havechanged by a factor of 5-10 over ~6 weeks in this brown dwarf, which maybe in the final stages of accreting from its disk. March spectra alsoreveal significant ``quasi-periodic'' changes in the Hα lineprofile over the course of a night, from clearly double-peaked to nearlysymmetric. These nightly profile changes, roughly consistent with thebrown dwarf's rotation period, could be the result of a redshiftedabsorption feature coming into and out of our line of sight; when theprofile is double-peaked we may be looking into an accretion column,flowing from the inner disk edge-on to the central object, indicatingthat the accretion is probably channeled along the magnetic field lines.Our findings provide strong support for the magnetospheric accretionscenario, and thus for the existence of large-scale magnetic fields, inthe substellar regime. We also present the first high-resolution opticalspectrum of SSSPM J1102-3431 (M8.5), which has recently been identifiedas another likely substellar member of the TW Hydrae association. Itsemission lines are relatively narrow and fairly symmetric, suggestingthat it is accreting only very weakly, if at all.
| 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.
| 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.
| The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs
Using the largest high-resolution spectroscopic sample to date of young,very low mass stars and brown dwarfs, we investigate disk accretion inobjects ranging from just above the hydrogen-burning limit all the wayto nearly planetary masses. Our 82 targets span spectral types from M5to M9.5, or masses from 0.15 Msolar down to about 15jupiters. They are confirmed members of the ρ Ophiuchus, Taurus,Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius, and TW Hydraestar-forming regions and young clusters, with ages from <1 to ~10Myr. The sample contains 41 brown dwarfs (spectral types >=M6.5). Wehave previously presented high-resolution optical spectra for roughlyhalf the sample; the rest are new. This is a close to complete survey ofall confirmed brown dwarfs known so far in the regions examined, exceptin ρ Oph and IC 348 (where we are limited by a combination ofextinction and distance). We find that (1) classical T Tauri-like diskaccretion persists in the substellar domain down to nearly thedeuterium-burning limit; (2) while an Hα 10% width >~200 kms-1 is our prime accretion diagnostic (following our previouswork), permitted emission lines of Ca II, O I, and He I are also goodaccretion indicators, just as in classical T Tauri stars (we cautionagainst a blind use of Hα width alone, since inclination androtation effects on the line are especially important at the lowaccretion rates in very low mass objects); (3) the Ca II λ8662line flux is an excellent quantitative measure of the accretion rate invery low mass stars and brown dwarfs (as in higher mass classical TTauri Stars), correlating remarkably well with the M˙ obtained fromveiling and Hα modeling; (4) the accretion rate diminishes rapidlywith mass-our measurements support previous suggestions thatM˙~M2* (albeit with considerable scatter)and extend this correlation to the entire range of substellar masses;(5) the fraction of very low mass stellar and substellar accretorsdecreases substantially with age, as in higher mass stars; (6) at anygiven age, the fraction of very low mass stellar and substellaraccretors is comparable to the accretor fraction in higher mass stars;and (7) a number of our sources with infrared excesses arising fromdusty disks do not evince measurable accretion signatures, with theincidence of such a mismatch increasing with age: this implies thatdisks in the low-mass regime can persist beyond the main accretion phaseand parallels the transition from the classical to post-T Tauri stage inmore massive stars. These strong similarities at young ages, betweenhigher mass stars on the one hand and low-mass bodies close to and belowthe hydrogen-burning limit on the other, are consistent with a commonformation mechanism in the two mass regimes.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Cochero |
| Ascensión Recta: | 05h07m49.56s |
| Declinación: | +30°24'05.2" |
| Magnitud Aparente: | 10.371 |
| Movimiento Propio en Ascensión Recta: | 6.2 |
| Movimiento Propio en Declinación: | -23.5 |
| B-T magnitude: | 10.963 |
| V-T magnitude: | 10.42 |
Catálogos y designaciones:
|
