Contents
Images
Upload your image
DSS Images Other Images
Related articles
Where are the hot ion lines in classical T Tauri stars formed?
Context: Classical T Tauri stars (hereafter CTTS) show a plethora of in-and outflow signatures in a variety of wavelength bands. Aims: In orderto constrain gas velocities and temperatures, we analyse the emission inthe hot ion lines. Methods: We use all available archival FUSE spectraof CTTS to measure the widths, fluxes and shifts of the detected hot ionlines and complement these data with HST/GHRS and HST/STIS data. Wepresent theoretical estimates of the temperatures reached in possibleemission models such as jets, winds, disks and accretion funnels andlook for correlations with X-ray lines and absorption properties. Results: We find line shifts in the range from -170 km s-1 to+100 km s-1. Most linewidths exceed the stellar rotationalbroadening. Those CTTS with blue-shifted lines also show excessabsorption in X-rays. CTTS can be distinguished from main sequence(hereafter MS) stars by their large ratio of the O VII to O VIluminosities. Conclusions: No single emission mechanism can be foundfor all objects. The properties of those stars with blue-shifted linesare compatible with an origin in a shock-heated dust-depleted outflow.
| Tracing the origins of permitted emission lines in RU Lupi down to AU scales
Context: Most of the observed emission lines and continuum excess fromyoung accreting low mass stars (Classical T Tauri stars - CTTSs) takeplace in the star-disk or inner disk region. These regions have acomplex emission topology that is still largely unknown. Aims: In thispaper the magnetospheric accretion and inner wind contributions to theobserved permitted He and H near infrared (NIR) lines of the brightsouthern CTTS RU Lupi are investigated for the first time. Methods:Previous optical observations of RU Lupi have shown a strong Hαprofile, due to the emission from a wind in the line wings, and amicro-jet detected in forbidden lines. We extend this analysis to NIRlines through the seeing-limited, high spectral-resolution spectra takenwith VLT/ISAAC, and narrow-band imaging and low spectral-resolutionspectroscopy acquired with the adaptive optics (AO) aided telescopeVLT/NACO. Using spectro-astrometric analysis we investigate the presenceof extended emission down to very low spatial scales (a few AU). Results: The HeI λ10830 line presents a P Cygni profile whoseabsorption feature indicates the presence of an inner stellar wind.Moreover, the spectro-astrometric analysis proves the presence of anextended emission superimposed on the absorption feature and likelycoming from the micro-jet detected in the optical. In contrast, theorigin of the hydrogen Paschen and Brackett lines is difficult toaddress. We tried to explain the observed line profiles and flux ratioswith both accretion and wind models showing the limits of bothapproaches. The lack of a spectro-astrometric signal indicates that theHI emission is either compact or symmetric. Our analysis confirms thesensitivity of the HeI line to the faint extended emission regions inthe close proximity of the star.Based on observations collected at the European Southern Observatory,Chile (ESO Programme 71.C-0703).
| Spatially extended polycyclic aromatic hydrocarbons in circumstellar disks around T Tauri and Herbig Ae stars
Aims.Our aim is to determine the presence and location of the emissionfrom polycyclic aromatic hydrocarbons (PAHs) towards low andintermediate mass young stars with disks using large aperturetelescopes. Methods: VLT-VISIR N-band spectra and VLT-ISAAC andVLT-NACO L-band spectra of 29 sources are presented, spectrallyresolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features.Spatial-extent profiles of the features and the continuum emission havebeen derived and used to associate the PAH emission with the disks. Theresults are discussed in the context of recent PAH-emission disk models. Results: The 3.3, 8.6, and 11.2 μm PAH features are detected towarda small fraction of the T Tauri stars, with typical upper limits between1 × 10-15 and 5 × 10-17 Wm-2. All 11.2 μm detections from a previous Spitzer surveyare confirmed with (tentative) 3.3 μm detections, and both the 8.6and the 11.2 μm features are detected in all PAH sources. For 6detections, the spatial extent of the PAH features is confined to scalestypically smaller than 0.12-0.34'', consistent with the radii of 12-60AU disks at their distances (typically 150 pc). For 3 additionalsources, WL 16, HD 100546, and TY CrA, one or more of the PAH featuresare more extended than the hot dust continuum of the disk, whereas forOph IRS 48, the size of the resolved PAH emission is confirmed assmaller than for the large grains. For HD 100546, the 3.3 μm emissionis confined to a small radial extent of 12±3 AU, most likelyassociated with the outer rim of the gap in this disk. Gaps with radiiout to 10-30 AU may also affect the observed PAH extent for othersources. For both Herbig Ae and T Tauri stars, the small measuredextents of the 8.6 and 11.2 μm features are consistent with larger(≥100 carbon atoms) PAHs.Based on observations obtained at the European Southern Observatory,Paranal, Chile, within the observing programs 164.I-0605 (ISAAC May2002), 074.C-0413 (NACO, March/April 2005), 075.C-0420 (ISAAC August2005), 077.C-0668 (VISIR/ISAAC April/May 2006). Appendix A is onlyavailable in electronic form at http://www.aanda.org
| The X-ray soft excess in classical T Tauri stars
Aims.We study an anomaly in the X-ray flux (or luminosity) ratio betweenthe O vii λλ21.6-22.1 triplet and the O viii Lyαline seen in classical T Tauri stars (CTTS). This ratio is unusuallyhigh when compared with ratios for main-sequence and non-accreting TTauri stars (Telleschi et al. 2007c, A&A, 468, 443). We comparethese samples to identify the source of the excess. A sample of recentlydiscovered X-ray stars with a soft component attributed to jet emissionis also considered. Methods: We discuss data obtained from theXMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) project,complemented by data from the published literature. We also present datafrom the CTTS RU Lup. Results: All CTTS in the sample show an anomalousO vii/O viii flux ratio when compared with WTTS or MS stars. The anomalyis due to an excess of cool, O vii emitting material rather than adeficiency of hotter plasma. The excess plasma must therefore havetemperatures of ⪉2 MK. This soft excess does not correlate with UVexcesses of CTTS, but seems to be related to the stellar X-rayluminosity. The spectra of the jet-driving TTS do not fit into thiscontext. Conclusions: The soft excess depends both on the presence ofaccretion streams in CTTS and on magnetic activity. The gas may beshock-heated near the surface, although it may also be heated in themagnetospheric accretion funnels. The soft component of the jet-drivingsources is unlikely to be due to the same process.
| X-rays from RU Lupi: accretion and winds in classical T Tauri stars
Context: Low-mass stars are known to exhibit strong X-ray emissionduring their early evolutionary stages. This also applies to classical TTauri stars (CTTS), whose X-ray emission differs from that ofmain-sequence stars in a number of aspects. Aims: We study the specificcase of RU Lup, a well known accreting and wind-driving CTTS. Incomparison with other bright CTTS we study possible signatures ofaccretion and winds in their X-ray emission. Methods: Using threeXMM-Newton observations of RU Lup, we investigate its X-ray propertiesand their generating mechanisms. High-resolution X-ray spectra of RU Lupand other CTTS are compared to main-sequence stars. We examine thepresence of a cool plasma excess and enhanced plasma density in relationto X-rays from accretion shocks and investigate anomalous strong X-rayabsorption and its connection to winds or circumstellar material. Results: We find three distinguishable levels of activity among theobservations of RU Lup. While no large flares are present, thisvariability is clearly of magnetic origin due to the correspondingplasma temperatures of around 30 MK; in contrast the cool plasmacomponent at 2-3 MK is quite stable over a month, resulting in a drop ofaverage plasma temperature from 35 MK down to 10 MK. Density analysiswith the O VII triplet indicates high densities in the cool plasma,suggesting accretion shocks to be a significant contributor to the softX-ray emission. No strong overall metal depletion is observed, with Nebeing more abundant than Fe, that is at solar value, and especially O.Excess emission at 6.4 keV during the more active phase suggest thepresence of iron fluorescence. Additionally RU Lup exhibits anextraordinary strong X-ray absorption, incompatible with estimatesobtained at optical and UV wavelengths. Comparing spectra from a sampleof main-sequence stars with those of accreting stars we find an excessof cool plasma as evidenced by lower O VIII/O VII line ratios in allaccreting stars. High density plasma appears to be only present inlow-mass CTTS, while accreting stars with intermediate masses(≳2~Mȯ) have lower densities. Conclusions: In allinvestigated CTTS the characteristics of the cooler X-ray emittingplasma are influenced by the accretion process. We suspect differentaccretion rates and amounts of funnelling, possibly linked to stellarmass and radius, to be mainly responsible for the different propertiesof their cool plasma component. The exceptional X-ray absorption in RULup and other CTTS is probably related to the accretion flows and anoptically transparent wind emanating from the star or the disk.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H2 Gas-Phase Lines
We present a survey of mid-infrared gas-phase lines toward a sample of76 circumstellar disks around low-mass pre-main-sequence stars from theSpitzer ``Cores to Disks'' legacy program. We report the firstdetections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20%and ~9% of our sources, respectively. The observed [Ne II] line fluxesand upper limits are consistent with [Ne II] excitation in an X-rayirradiated disk around stars with X-ray luminositiesLX=1029-1031 erg s-1. [Fe I]is detected at ~10-5 to 10-4 Lsolar,but no [S I] or [Fe II] is detected down to ~10-6Lsolar. The [Fe I] detections indicate the presence ofgas-rich disks with masses of >~0.1 MJ. No H20-0 S(0) and S(1) disk emission is detected, except for S(1) toward onesource. These data give upper limits on the warm (T~100-200 K) gas massof a few Jovian masses, consistent with recent T Tauri disk models thatinclude gas heating by stellar radiation. Compact disk emission of hot(T>~500 K) gas is observed through the H2 0-0 S(2) and/orS(3) lines toward ~8% of our sources. The line fluxes are, however,higher by more than an order of magnitude than those predicted by recentdisk models, even when X-ray and excess UV radiation are included. The[Ne II]/H2 0-0 S(2) ratios for these sources are similarlylower than predicted, consistent with the presence of an additional hotmolecular gas component not included in current disk models. Obliqueshocks of stellar winds interacting with the disk can explain manyaspects of the hot gas emission but are inconsistent with thenondetection of [S I] and [Fe II] lines.
| Pre-main sequence spectroscopic binaries suitable for VLTI observations
Context: A severe problem for research in star-formation is that themasses of young stars are almost always estimated from evolutionarytracks alone. Since the tracks published by different groups differ, itis often only possible to give a rough estimate of the masses of youngstars. It is thus crucial to test and calibrate the tracks. Up to now,only a few tests of the tracks could be carried out. However, it is nowpossible with the VLTI to set constrains on the tracks by determiningthe masses of many young binary stars precisely. Aims: In order to usethe VLTI efficiently, a first step is to find suitable targets, which isthe purpose of this work. Given the distance of nearby star-formingregions, suitable VLTI targets are binaries with orbital periods betweenat least 50 days and a few years. Although a number of surveys fordetecting spectroscopic binaries have been carried out, most of thebinaries found so far have periods that are too short. Methods: We thussurveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρOphiuci star-forming regions in order to search for spectroscopicbinaries with periods longer than 50 days, which are suitable for theVLTI observations. Results: As a result of the 8 year campaign, wediscovered 8 binaries with orbital periods longer than 50 days. Amongstthe newly discovered long-period binaries is CS Cha, which is one of thefew classical T Tauri stars with a circumbinary disk. The survey islimited to objects with masses higher than 0.1 to 0.2 M_ȯ forperiods between 1 and 8 years. Conclusions: We find that the frequencyof binaries with orbital periods ≤3000 days is of 20±5%. Thefrequency of long and short period pre-main sequence spectroscopicbinaries is about the same as for stars in the solar neighbourhood. Intotal 14 young binaries are now known that are suitable for massdetermination with the VLTI.based on observations obtained at the European Southern Observatory atLa Silla, Chile in program 62.I-0418(A); 63.I-0096(A); 64.I-0294(A);65.I-0012(A); 67.C-0155(A); 68.C-0292(A); 68.C-0561(A); 69.C-0207(A);70.C-0163(A); 073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F). Tables 2,3, 5, 6, 8, 10, 12-20 are only available in electronic form athttp://www.aanda.org
| How Hot is the Wind from TW Hydrae?
It has recently been suggested that the winds from classical T Tauristars in general, and the wind from TW Hya in particular, reachestemperatures of 300,000 K while maintaining a mass-loss rate of~10-11 Msolar yr-1 or larger. Ifconfirmed, this would place strong new requirements on wind launchingand heating models. We therefore reexamine spectra from the SpaceTelescope Imaging Spectrograph aboard the Hubble Space Telescope andspectra from the Far Ultraviolet Spectroscopic Explorer satellite in aneffort to better constrain the maximum temperature in the wind of TWHya. We find clear evidence for a wind in the C II doublet at 1037Å and in the C II multiplet at 1335 Å. We find no windabsorption in the C IV λ1550 doublet observed at the same time asthe C II λ1335 line or in observations of O VI observedsimultaneously with the C II λ1037 line. The presence or absenceof C III wind absorption is ambiguous. The clear lack of a wind in the CIV line argues that the wind from TW Hya does not reach the 100,000 Kcharacteristic formation temperature of this line. We therefore arguethat the available evidence suggests that the wind from TW Hya, andperhaps all classical T Tauri stars, reaches a maximum temperature inthe range of 10,000-30,000 K.This work is based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555. These observations are associatedwith program GTO-7718. This work is also based on observations made withthe NASA/CNES/CSA Far Ultraviolet Spectroscopic Explorer, which isoperated for NASA by the Johns Hopkins University under NASA contractNAS5-32985.
| Evidence of Stellar-driven Outflows from the Classical T Tauri Star RY Tau
RY Tau is a rapidly rotating classical T Tauri star observed close toedge-on. The combination of new HST/STIS observations obtained in 2001with HST/GHRS archive data from 1993 has allowed us to get, for thefirst time, information on the thermal structure and velocity of thewind. The repeated observations of the Si III] and C III] lines show alack of changes with time on the blue side of the profile (dominated bythe wind contribution). Very high temperature plasma(logTe=4.8) is detected at densities of9.5<=logne(cm-3)<=10.2 associated with thewind. The emitting volumes are ~(0.35 Rsolar)3,suggesting a stellar origin. The wind kinematics derived from theprofiles (Si III], C III], and [O II]) does not satisfy the theoreticalpredictions of MHD centrifugally driven disk winds. The profiles'asymmetry, large velocity dispersions, and small variability as well asthe small emitting volumes are best explained if the wind is produced bythe contributions of several outflows from atmospheric open-fieldstructures like those observed in the Sun.
| Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
Context: .Low-mass stars form with disks in which the coagulation ofgrains may eventually lead to the formation of planets. It is not knownwhen and where grain growth occurs, as models that explain theobservations are often degenerate. A way to break this degeneracy is toresolve the sources under study. Aims: .Our aim is to findevidence for the existence of grains of millimetre sizes in disks aroundT Tauri stars, implying grain growth. Methods: .The AustraliaTelescope Compact Array (ATCA) was used to observe 15 southern T Tauristars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm.The five Lupus sources were also observed with the SubMillimeter Array(SMA) at 1.4 mm. Our new data are complemented with data from theliterature to determine the slopes of the spectral energy distributionsin the millimetre regime. Results: .Ten sources were detected atbetter than 3σ with the ATCA, with σ ≈ 1-2 mJy, and allsources that were observed with the SMA were detected at better than15σ, with σ ≈ 4 mJy. Six of the sources in our sample areresolved to physical radii of ~100 AU. Assuming that the emission fromsuch large disks is predominantly optically thin, the millimetre slopecan be related directly to the opacity index. For the other sources, theopacity indices are lower limits. Four out of six resolved sources haveopacity indices ⪉1, indicating grain growth to millimetre sizes andlarger. The masses of the disks range from <0.01 to 0.08 M_ȯ,which is comparable to the minimum mass solar nebula. A tentativecorrelation is found between the millimetre slope and the strength andshape of the 10-μm silicate feature, indicating that grain growthoccurs on similar (short) timescales in both the inner and outerdisk.
| Searching for gas-rich disks around T Tauri stars in Lupus
Aims.We characterize the molecular environment of classical T Tauristars in Lupus observed with the Spitzer Space Telescope in our searchfor gas-rich disks toward these sources. Methods: Submillimeterobservations of 12CO, 13CO, and C18O inthe J=3-2 and 2-1 lines were obtained with the James Clerk MaxwellTelescope toward a sample of 21 T Tauri stars with disks in the Lupusmolecular clouds. Pointings at the sources and at selected off-positionsare presented in order to disentangle material associated with disksfrom ambient cloud material. Results: One source, IM Lup (Sz 82), wasfound with the double-peaked 12CO and 13COprofiles characteristic of a large rotating gas disk. The inclination ofthe disk is found to be ~20°, with an outer radius of 400-700 AU.For most other sources, including GQ Lup with its substellar companion,the single-dish 12CO lines are dominated by extended cloudemission with a complex velocity structure. No evidence of molecularoutflows is found. Due to dense circumstellar material, compactC18O emission was detected toward only two sources. Futuresearches for gas-rich disks in Lupus should either use interferometersor perform very deep single-dish integrations in dense gas tracers toseparate the cloud and disk emission.
| 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.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features
Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) featurestowards young low-mass (T Tauri) stars and compare them with surveys ofintermediate mass (Herbig Ae/Be) stars. The presence and strength of thePAH features are interpreted with disk radiative transfer modelsexploring the PAH feature dependence on the incident UV radiation, PAHabundance and disk parameters. Methods: .Spitzer Space Telescope5-35 μm spectra of 54 pre-main sequence stars with disks wereobtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars withunknown spectral type. Results: .Compact PAH emission is detectedtowards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2μm PAH feature is detected in all of these sources, as is the 6.2μm PAH feature for the 4 sources for which short wavelength data areavailable. However, the 7.7 and 8.6 μm features appear strongly inonly 1 of these 4 sources. Based on the 11.2 μm feature, PAH emissionis observed towards at least 3 T Tauri stars, with 14 tentativedetections, resulting in a lower limit to the PAH detection rate of 8%.The lowest mass source with PAH emission in our sample is TCha with a spectral type G8. All 4 sources in our sample withevidence for dust holes in their inner disk show PAH emission,increasing the feature/continuum ratio. Typical 11.2 μm lineintensities are an order of magnitude lower than those observed for themore massive Herbig Ae/Be stars. Measured line fluxes indicate PAHabundances that are factors of 10-100 lower than standard interstellarvalues. Conversely, PAH features from disks exposed to stars withT_eff≤ 4200 K without enhanced UV are predicted to be below thecurrent detection limit, even for high PAH abundances. Disk modelingshows that the 6.2 and 11.2 μm features are the best PAH tracers forT Tauri stars, whereas the 7.7 and 8.6 μm bands have low feature overcontinuum ratios due to the strongly rising silicate emission.
| 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.
| On the formation of Hα line emission around classical T Tauri stars
We present radiative transfer models of the circumstellar environment ofclassical T Tauri stars, concentrating on the formation of the Hαemission. The wide varieties of line profiles seen in observations areindicative of both inflow and outflow, and we therefore employ acircumstellar structure that includes both magnetospheric accretion anda disc wind. We perform systematic investigations of the modelparameters for the wind and the magnetosphere to search for possiblegeometrical and physical conditions which lead to the types of profilesseen in observations. We find that the hybrid models can reproduce thewide range of profile types seen in observations, and that the mostcommon profile types observed occupy a large volume of parameter space.Conversely, the most infrequently observed profile morphologies requirea very specific set of models parameters. We find our model profiles areconsistent with the canonical value of the mass-loss rate tomass-accretion rate ratio (μ = 0.1) found in earliermagnetohydrodynamic calculations and observations, but the models with0.05 < μ < 0.2 are still in accord with observed Hαprofiles. We investigate the wind contribution to the line profile as afunction of model parameters, and examine the reliability of Hα asa mass-accretion diagnostic. Finally, we examine the Hαspectroscopic classification used by Reipurth et al., and discuss thebasic physical conditions that are required to reproduce the profiles ineach classified type.
| The Origins of Fluorescent H2 Emission From T Tauri Stars
We survey fluorescent H2 emission in HST STIS spectra of theclassical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DGTau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of thosesources we detect between 41 and 209 narrow H2 emissionlines, most of which are pumped by strong Lyα emission.H2 emission is not detected from the WTTS V410 Tau. Thefluorescent H2 emission appears to be common to circumstellarenvironments around all CTTSs, but high spectral and spatial resolutionSTIS observations reveal diverse phenomenon. Blueshifted H2emission detected from RU Lupi, T Tau, and DG Tau is consistent with anorigin in an outflow. The H2 emission from TW Hya, DF Tau,and V836 Tau is centered at the radial velocity of the star and isconsistent with an origin in a warm disk surface. The H 2lines from RU Lupi, DF Tau, and T Tau also have excess blueshiftedH2 emission that extends to as much as -100 kms-1. The strength of this blueshifted component from DF Tauand T Tau depends on the upper level of the transition. In all cases,the small aperture and attenuation of H2 emission by stellarwinds restricts the H2 emission to be formed close to thestar. In the observation of RU Lupi, the Lyα emission and theH2 emission that is blueshifted by 15 km s-1 areextended to the SW by ~0.07", although the faster H2 gas thatextends to ~100 km s-1 is not spatially extended. We alsofind a small reservoir of H 2 emission from TW Hya and DF Tauconsistent with an excitation temperature of ~2.5×104K.
| The SPITZER c2d Survey of Weak-Line T Tauri Stars. I. Initial Results
Using the Spitzer Space Telescope, we have observed 90 weak-line andclassical T Tauri stars in the vicinity of the Ophiuchus, Lupus,Chamaeleon, and Taurus star-forming regions as part of the Cores toDisks (c2d) Spitzer Legacy project. In addition to the Spitzer data, wehave obtained contemporaneous optical photometry to assist inconstructing spectral energy distributions. These objects werespecifically chosen as solar-type young stars with low levels ofHα emission, strong X-ray emission, and lithium absorption, i.e.,weak-line T Tauri stars, most of which were undetected in the mid- tofar-IR by the IRAS survey. Weak-line T Tauri stars are potentiallyextremely important objects in determining the timescale over which diskevolution may take place. Our objective is to determine whether theseyoung stars are diskless or have remnant disks that are below thedetection threshold of previous infrared missions. We find that only5/83 weak-line T Tauri stars have detectable excess emission between 3.6and 70 μm, which would indicate the presence of dust from the innerfew tenths of an AU out to the planet-forming regions a few tens of AUfrom the star. Of these sources, two have small excesses at 24 μmconsistent with optically thin disks; the others have optically thickdisks already detected by previous IR surveys. All of the sevenclassical T Tauri stars show excess emission at 24 and 70 μm althoughtheir properties vary at shorter wavelengths. Our initial results showthat disks are rare among young stars selected for their weak Hαemission.
| On the detection of artefacts in spectro-astrometry
We demonstrate that artificial bipolar structure can be detected usingspectro-astrometry when the point spread function (PSF) of a pointsource suffers distortion in a relatively wide slit. Spectro-astrometryis a technique which allows us to probe the spatial structure ofastronomical sources on milliarcsec (mas) scales making it possible todetect close binaries and to study the geometry and kinematics ofoutflowing gas on scales much smaller than the seeing or the diffractionlimit of the telescope. It is demonstrated that a distorted PSF, causedby tracking errors of the telescope or unstable active optics during anexposure, can induce artificial signals which may be misinterpreted as areal spectro-astrometric signal. Using simulations, we show that thesemay be minimized by using a narrow slit relative to the seeing. Spectrashould be obtained at antiparallel slit position angles (e.g. 0° and180°) for comparison in order to allow artificial signatures to beidentified.
| 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).
| Light scattering by moving dust grains in the immediate vicinity of young stars
We consider the problem of the distortion of the photospheric spectrumfor a young star as its light is scattered in the inner accretion diskin the dust grain evaporation region. In T Tauri stars, this region isat a distance of the order of several stellar radii and is involved inthe large-scale motions of matter with velocities of 100 km s-1 orhigher. The light scattering in such a medium causes the frequency ofthe scattered radiation to be shifted due to the Doppler effect. Weanalyze the influence of this effect on the absorption line profiles inthe spectra of T Tauri stars using classical results of the theory ofradiative transfer. We consider two models of a scattering medium: (i) ahomogeneous cylindrical surface and (ii) a cylindrical surface with anazimuth-dependent height (such conditions take place during theaccretion of matter onto a star with an oblique magnetic dipole). Weshow that in the first case, the scattering of the photosphericradiation causes the absorption lines to broaden. If the motion of thecircumstellar matter in the dust evaporation region is characterized bytwo velocity components, then the line profile of the scatteredradiation is asymmetric, with the pattern of the asymmetry depending onthe direction of the radial velocity. In the second case, the scatteredradiation can cause periodic shifts of the absorption line centroid,which can be perceived by an observer as periodic radial-velocityvariations in the star. We suggest that precisely this effect isresponsible for the low-amplitude radial-velocity variations withperiods close to the stellar rotation periods that have recently beenfound in some of the T Tauri stars.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Weak-line T Tauri stars: circumstellar disks and companions. I. Spectral energy distributions and infrared excesses
We have analysed new infrared photometric data taken by the InfraredSpace Observatory, in combination with 2MASS data where available, of 12weak-line T Tauri stars (WTTS), belonging to different molecular clouds.A control study of 6 classical T Tauri stars (CTTS) for comparison withIRAS data has also been made. WTTS traditionally are not expected tohave any circumstellar material, but we have found that 11 of the WTTSstudied, have a spectral energy distribution that suggests the presenceof reprocessing circumstellar disks, albeit half of them with apparentinner "dust holes". The spectral energy distribution curves of all thestars in our sample show infrared excess in all cases except one. Wehave also searched for companions to these stars and studied theirpossible contributions to the infrared emission. We find that 10 of theWTTS with excess infrared emission have companions that may contributemeasurably and, in some cases significantly, at infrared wavelengths. Inthe case of the binary systems, the amount of excess due to thecompanion is still unclear but it is unlikely they are the only sourceof all the observed excess. We give estimations of the excesses andmasses of the stars and also find their ages and radii to be inagreement with those expected for T Tauri stars. Our results suggestthere is a continuum in infrared excess properties from CTTS to WTTSrather than a clear differentiation based on a somewhat arbitraryEW(Hα) criterion.
| 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.
| 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.
| The Loopy Ultraviolet Line Profiles of RU Lupi: Accretion, Outflows, and Fluorescence
We present far-ultraviolet (FUV) spectra of the classical T Tauri starRU Lup covering the 912-1710 Å spectral range, as observed by theHubble Space Telescope STIS and the Far Ultraviolet SpectroscopicExplorer satellite. We use these spectra, which are rich in emission andabsorption lines, to probe both the accreting and outflowing gas.Absorption in the Lyα profile constrains the extinction toAV~0.07 mag, which we confirm with other diagnostics. Weestimate a mass accretion rate of (5+/-2)×10-8Msolar yr-1 using the optical-NUV accretioncontinuum. The accreting gas is also detected in bright, broad lines ofC IV, Si IV, and N V, which all show complex structures across the lineprofile. Many other emission lines, including those of H2 andFe II, are pumped by Lyα. RU Lup's spectrum varies significantlyin the FUV; our STIS observations occurred when RU Lup was brighter thanseveral other observations in the FUV, possibly because of a high massaccretion rate.
| T Tauri stellar magnetic fields: HeI measurements
We present measurements of the longitudinal magnetic field in thecircumstellar environment of seven classical T Tauri stars. Themeasurements are based on high-resolution circular spectropolarimetry ofthe HeIλ5876 emission line, which is thought to form in accretionstreams controlled by a stellar magnetosphere. We detect magnetic fieldsin BP Tau, DF Tau and DN Tau, and detect statistically significantfields in GM Aur and RW Aur A at one epoch but not at others. We detectno field for DG Tau and GG Tau, with the caveat that these objects wereobserved at one epoch only. Our measurements for BP Tau and DF Tau areconsistent, in terms of both sign and magnitude, with previous studies,suggesting that the characteristics of T Tauri magnetospheres arepersistent over several years. We observed the magnetic field of BP Tauto decline monotonically over three nights, and have detected a peakfield of 4 kG in this object, the highest magnetic field yet observed ina T Tauri star. We combine our observations with results from theliterature in order to perform a statistical analysis of themagnetospheric fields in BP Tau and DF Tau. Assuming a dipolar field, wedetermine a polar field of ~3 kG and a dipole offset of 40° for BPTau, while the field of DF Tau is consistent with a polar field of ~-4.5kG and a dipole offset of 10°. We conclude that many classical TTauri stars have circumstellar magnetic fields that are both strongenough and sufficiently globally ordered to sustain large-scalemagnetospheric accretion flows.
| 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.
| Emission-line profile modelling of structured T Tauri magnetospheres
We present hydrogen emission-line profile models of magnetosphericaccretion on to classical T Tauri stars. The models are computed underthe Sobolev approximation using the three-dimensional Monte Carloradiative-transfer code TORUS. We have calculated four illustrativemodels in which the accretion flows are confined to azimuthal curtains -a geometry predicted by magnetohydrodynamic simulations. Properties ofthe line profile variability of our models are discussed, with referenceto dynamic spectra and cross-correlation images. We find that some grosscharacteristics of the observed line profile variability are reproducedby our models, although in general the level of variability predicted islarger than that observed. We conclude that this excessive variabilityprobably excludes dynamical simulations that predict accretion flowswith low degrees of axisymmetry.
| The close T Tauri binary V 4046 Sagittarii
We have collected high-resolution (R ≈ 60 000) VLT-UVES spectrogramsof the close T Tauri binary V 4046 Sgr from 3500-6750Å at different phases of its 2.4 day orbital period. The highquality of these spectra allows us to present an improved ephemeris ofthe system. To model the photospheric absorption line spectrum wecalculate synthetic spectra for the observed phases of the system. Thesesynthetic spectra are used to determine veiling levels, and to extractemission line profiles that are undistorted by photospheric absorptionlines. We find that the shapes of the strong emission lines of H and CaII H & K all vary periodically with phase. A weak veiling continuumis superimposed on the stellar absorption line spectra. The Ca II H& K emission lines are composed of two narrow emission componentsthat closely follow the stellar orbital motion, and we speculate thatthese lines are formed in global chromospheric networks on the stars.Also the Balmer lines have similar narrow ``stellar" components,possibly also chromospheric. However, in addition there are extendedwings on each side of the line centers of all H lines, which changedramatically in shape with the orbital phase. We find that the shape andvelocity changes of the wing components are consistent with twoconcentrations of gas moving at high velocity but co-rotating with thestars. These concentrations move with a projected velocity of 80 kms-1 around the center of mass. They are located well insidethe edge of the circumbinary disk, and also inside the co-linearLagrangian points of this binary. With this concept we obtain a verygood agreement between calculated and observed line profiles of H8, H9and H10 as a function of phase. In some recent calculations of masstransfer from circumbinary disks to close binaries in circular motion,accumulations of gas in co-rotation with the stars develop, resulting instructures which are similar to what we have found from ourobservations. We also investigate the cause of periodic photometricvariations observed by others.Based on observations collected at the European Southern Observatory,Chile (proposal 69.C-0481).
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | おおかみ座 |
| Right ascension: | 15h56m42.31s |
| Declination: | -37°49'15.5" |
| Apparent magnitude: | 11.547 |
| Proper motion RA: | -10.9 |
| Proper motion Dec: | -26 |
| B-T magnitude: | 11.839 |
| V-T magnitude: | 11.572 |
Catalogs and designations:
|
INHABITED SKY
