Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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.
| 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.
| Terrestrial planet formation surrounding close binary stars
Most stars reside in binary/multiple star systems; however, previousmodels of planet formation have studied growth of bodies orbiting anisolated single star. Disk material has been observed around bothcomponents of some young close binary star systems. Additionally, it hasbeen shown that if planets form at the right places within such disks,they can remain dynamically stable for very long times. Herein, wenumerically simulate the late stages of terrestrial planet growth incircumbinary disks around ‘close’ binary star systems withstellar separations 0.05 AU⩽a⩽0.4 AU andbinary eccentricities 0⩽e⩽0.8. In each simulation,the sum of the masses of the two stars is 1 M, and giantplanets are included. The initial disk of planetary embryos is the sameas that used for simulating the late stages of terrestrial planetformation within our Solar System by Chambers [Chambers, J.E., 2001.Icarus 152, 205–224], and around each individual component of theα Centauri AB binary star system by Quintana et al. [Quintana,E.V., Lissauer, J.J., Chambers, J.E., Duncan, M.J., 2002. Astrophys. J.576, 982–996]. Multiple simulations are performed for each binarystar system under study, and our results are statistically compared to aset of planet formation simulations in theSun–Jupiter–Saturn system that begin with essentially thesame initial disk of protoplanets. The planetary systems formed aroundbinaries with apastron distancesQ≡a(1+e)≲0.2 AU arevery similar to those around single stars, whereas those with largermaximum separations tend to be sparcer, with fewer planets, especiallyinterior to 1 AU. We also provide formulae that can be used to scaleresults of planetary accretion simulations to various systems withdifferent total stellar mass, disk sizes, and planetesimal masses anddensities.
| 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.
| 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.
| 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).
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| A New Classification Scheme for T Tauri Light Curves
Based on many years of observational data from a photometric database onyoung stars, we propose a new classification scheme for the light curvesof classical T Tauri stars. Our analysis of master light-curve shapesfor 28 classical T Tauri stars is used to distinguish up to fivelight-curve types. The proposed scheme suggests a qualitativeinterpretation in terms of interaction of the central star with itscircumstellar accretion disk.
| Constraints on the ionizing flux emitted by T Tauri stars
We present the results of an analysis of ultraviolet observations of TTauri stars (TTs). By analysing emission measures taken from theliterature, we derive rates of ionizing photons from the chromospheresof five classical TTs in the range ~1041-1044photon s-1, although these values are subject to largeuncertainties. We propose that the HeII/CIV line ratio can be used as areddening-independent indicator of the hardness of the ultravioletspectrum emitted by TTs. By studying this line ratio in a much largersample of objects, we find evidence for an ionizing flux which does notdecrease, and may even increase, as TTs evolve. This implies that asignificant fraction of the ionizing flux from TTs is not powered by theaccretion of disc material on to the central object, and we discuss thesignificance of this result and its implications for models of discevolution. The presence of a significant ionizing flux in the laterstages of circumstellar disc evolution provides an important newconstraint on disc photoevaporation models.
| Submillimeter Array Observations of Disks in the SR 24 Multiple Star System
We present high-resolution aperture synthesis images from theSubmillimeter Array of the 225 GHz (1.3 mm) continuum and12CO J=2-1 line emission from the disks around components ofthe hierarchical triple system SR 24, located in the Ophiuchusstar-forming region. The most widely separated component, SR 24 S (witha projected semimajor axis a=832 AU), has a circumstellar disk withproperties typical of those around single T Tauri stars. The binary SR24 N (a=32 AU) is undetected in the continuum but has strong, resolvedCO emission that likely originates in a circumbinary disk with a centralgap. The data constrain the total disk mass in the SR 24 N system to be<~10-3 Msolar and indicate that the depletionof CO onto dust grains is not more than 100 times larger than the meanvalue in the interstellar medium. The SR 24 N disk is unusual in that itis only detected in line emission. It is possible that other low-massdisks around binaries and single stars may have been missed insingle-dish continuum surveys.The Submillimeter Array is a joint project between the SmithsonianAstrophysical Observatory and the Academia Sinica Institute of Astronomyand Astrophysics, and is funded by the Smithsonian Institution and theAcademia Sinica.
| Polarimetric Variations of Binary Stars. VI. Orbit-Induced Variations in the Pre-Main-Sequence Binary AK Scorpii
We present simultaneous UBV polarimetric and photometric observations ofthe pre-main-sequence binary AK Sco, obtained over 12 nights, slightlyless than the orbital period of 13.6 days. The polarization is a sum ofinterstellar and intrinsic polarization, with a significant intrinsicpolarization of 1% at 5250 Å, indicating the presence ofcircumstellar matter distributed in an asymmetric geometry. Thepolarization and its position angle are clearly variable on timescalesof hours and nights in all three wavelengths, with a behavior related tothe orbital motion. The variations have the highest amplitudes seen sofar for pre-main-sequence binaries (~1% and ~30°) and are sinusoidalwith periods similar to the orbital period and half of it. Thepolarization variations are generally correlated with the photometricones: when the star gets fainter, it also gets redder, and itspolarization increases. The (B-V, V) color-magnitude diagram exhibits aratio of total to selective absorption R=4.3, higher than in normalinterstellar clouds (R=3.1). The interpretation of the simultaneousphotometric and polarimetric observations is that a cloud ofcircumstellar matter passes in front of the star, decreasing the amountof direct, unpolarized light and hence increasing the contribution ofscattered (blue) light. We show that the large amplitude of thepolarization variations cannot be reproduced with a single-scatteringmodel and axially symmetric circumbinary or circumstellar disks.Based on observations made with the ESO telescopes at the La SillaObservatory.
| Polarimetric line profiles for scattering off rotating disks
We predict polarimetric line profiles for scattering off rotating disksusing a Monte Carlo technique. We have discovered that there is a markeddifference between scattering of line emission by a disk that reachesthe stellar surface, and a disk with an inner hole. For the case with aninner hole, we find single position-angle rotations, similar to thosepredicted by analytic models. For the case of an undisrupted disk, wefind double rotations in the position angle - an effect not reportedbefore. We show that this new effect is due to the finite-sized starinteracting with the disk's rotational velocity field. Since a gradualincrease of the hole size transforms the double rotations smoothly backinto single ones - as the line emission object approaches that of apoint source - our models demonstrate the diagnostic potential of linepolarimetry in determining not only the disk inclination, but also thesize of the disk inner hole. Our models are generic, and relevant to anyline emitting object that is embedded in a rotating disk. Examples are:Herbig Ae stars, T Tauri stars, other young stars, early-type stars withdisks, post-AGB stars, cataclysmic variables and other binary systems,as well as extra-galactic objects, such as the disks aroundsuper-massive black holes.
| The pre-main sequence spectroscopic binary UZ Tau East: Improved orbital parameters and accretion phase dependence
We present radial-velocity measurements obtained using high- andintermediate-resolution spectroscopic observations of the classical TTauri star UZ Tau East from 1994 to 1996. We also provide measurementsof Hα equivalent widths and optical veiling. Combining ourradial-velocity data with those recently reported by Prato et al.(2002), we improve the orbital elements for this spectroscopic binary.The orbital period is 18.979±0.007 days and the eccentricity ise=0.14. We find variability in the Hα emission andveiling, signposts of accretion, but at periastron passage the accretionis not as clearly enhanced as in the case of the binary DQ Tau. Thedifference in the behaviour of these two binaries is consistent with thehydrodynamical models of accretion from circumbinary disks because UZTau East has lower eccentricity than DQ Tau. It seems that enhancedperiastron accretion may occur only in systems with very higheccentricity (e>0.5).Based on observations made with the Isaac Newton Telescope, operated onthe island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias, the ESO 3.6-m telescope at La SillaObservatory in Chile, and the Shane 3-m telescope at Lick observatory inCalifornia.
| A photometric pilot study on Sonneberg archival patrol plates. How many ``constant'' stars are in fact long-term variables?
The light curves of 216 arbitrarly chosen field stars and of 23 knownvariables in the Aur/Tau/Ori region were derived (7.8 m ≤ B ≤12.2 m) from scanned, blue-sensitive archival patrol plates, covering atotal of 34 years (1961-1995). We achieved a photometric accuracy of0.07 ... 0.12 mag in spite of rather unfavourable locations of moststars near the plate borders. 17 field stars turned out to be variables,most of them with time scales of 1000-8000 days in the form of slowwaves with amplitudes between 0.1 and 0.3 mag, i.e. below the thresholdof traditional variable searches on photographic plates. About 50% ofthese new long-term variables exhibit drifts indicating periodic orerratic variability at much longer time scales than covered here. Forthe 23 known variables we achieved improvements in their periods andamplitudes and detected long-term variations (drifts, waves) in about50% of them. The above fraction of low-amplitude long-term variablesamong field stars implies that a total of about 45 000 new variablesshould be detectable in the Sonneberg patrol plate archive. They willrepresent a new, hitherto not investigated population of variable starswith a possibly significant impact on our understanding of the stellarinterior and evolution.Tables 2 and 3 are only available in electronic form athttp://www.edpsciences.org
| Heating Protoplanetary Disk Atmospheres
We calculate the thermal-chemical structure of the gaseous atmospheresof the inner disks of T Tauri stars, starting from the density and dusttemperature distributions derived by D'Alessio and coworkers in 1999. Asa result of processes such as X-ray irradiation or mechanical heating ofthe surface layers, the gas temperature at the very top of the diskatmosphere in the neighborhood of 1 AU is of the order of 5000 K. Deepdown, it drops rapidly into the range of the dust temperature, i.e.,several hundred degrees kelvin. In between these upper hot and lowercool layers, there is a transition zone with gas temperatures in therange 500-2000 K. The thickness and location of this warm region dependon the strength of the surface heating. This region also manifests thebasic chemical transitions of H to H2 andC+ and C to CO. It is remarkable that even thoughthe H2 transition begins first (higher up), it does not go tocompletion until after CO does. Consequently, there is a reasonablythick layer of warm CO that is predominantly atomic H. Thisthermal-chemical structure is favorable to the excitation of thefundamental and overtone bands of CO because of the large ratecoefficients for vibrational excitation in H+CO asopposed to H2+CO collisions. Thisconclusion is supported by the recent observations of the fundamentalband transitions in most T Tauri stars. We also argue that layeredatmospheres of inner T Tauri disks may play an important role inunderstanding the observations of H2 UV fluorescence pumpedfrom excited vibrational levels of that molecule. Possible candidatesfor surface heating include the interaction of a wind with the upperlayers of the disk and dissipation of hydromagnetic waves generated bymechanical disturbances close to the midplane, e.g., by theBalbus-Hawley instability. Detailed modeling of the observations has thepotential to reveal the nature of the mechanical surface heating that wemodel phenomenologically in these calculations and to help explain thenature of the gas in protoplanetary disks.
| Disk Wind in Young Binaries and the Origin of the Cyclic Activity of Young Stars
We present the results of our numerical simulations of the cyclicbrightness modulation in young binary systems with eccentric orbits andlow-mass secondary components. We suggest that the binary componentsaccrete matter from the remnants of the protostellar cloud, with themain accretor (according to current models) being the low-masscomponent. The brightness variations of the primary are attributable tothe periodic extinction variations on the line of sight caused by thedisk wind from the secondary and by the common envelope produced by thiswind. The distribution of matter in the envelope was calculated in theballistic approximation. When calculating the optical effects producedby the dust component of the disk wind, we adopted the dust-to-gas massratio of 1: 100 characteristic of the interstellar medium and theoptical parameters of the circumstellar dust typical of young stars. Ourcalculations show that the theoretical light curves for binaries withelliptical orbits exhibit a wider variety of shapes than those forbinaries with circular orbits. In this case, the parameters of thephotometric minima (their depth, duration, and shape of the light curve)depend not only on the disk-wind parameters and the orbital inclinationof the binary to the line of sight, but also on the longitude of theperiastron. We investigate the modulation of the scattered radiationfrom the common envelope with orbital phase in the single-scatteringapproximation. The modulation amplitude is shown to be at a maximum whenthe system is seen edge-on and to be also nonzero in binaries seenpole-on. We discuss possible applications of the theory to young stellarobjects. In particular, several model light curves have been found to besimilar to those of candidate FU Orionis stars (FUORs).
| The Mass Accretion Rates of Intermediate-Mass T Tauri Stars
We present Hubble Space Telescope ultraviolet spectra and supportingground-based data for a sample of nine intermediate-mass T Tauri stars(IMTTSs; 1.5-4 Msolar). The targets belong to threestar-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZOri, P2441, and V1044 Ori in the Ori OB1c association surrounding theOrion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring aroundλ Ori. The supporting ground-based observations include nearlysimultaneous UBV(R I)C photometry, 6 Å resolutionspectra covering the range 3900-7000 Å, optical echelleobservations in the range 5800-8600 Å, and K-band near-infraredspectra. We use these data to determine improved spectral types andreddening corrections and to obtain physical parameters of the targets.We find that an extinction law with a weak 2175 Å feature but highvalues of AUV/AV is required to explain thesimultaneous optical-UV data; the reddening laws for two B-type starslocated behind the Taurus clouds, HD 29647 and HD 283809, meet theseproperties. We argue that reddening laws with these characteristics maywell be representative of cold, dense molecular clouds. Spectral energydistributions and emission-line profiles of the IMTTSs are consistentwith expectations from magnetospheric accretion models. We compare oursimultaneous optical-UV data with predictions from accretion shockmodels to get accretion luminosities and mass accretion rates (M) forthe targets. We find that the average mass accretion rate for IMTTSs is~3×10-8 Msolar yr-1, a factor of~5 higher than that for their low-mass counterparts. The new data extendthe correlation between M and stellar mass to the intermediate-massrange. Since the IMTTSs are evolutionary descendants of the Herbig Ae/Bestars, our results put limits to the mass accretion rates of theirdisks. We present luminosities of the UV lines of highly ionized metalsand show that they are well above the saturation limit for magneticallyactive cool stars but correlate strongly with accretion luminosity,indicating that they are powered by accretion, in agreement withprevious claims but using a sample in which reddening and accretionluminosities have been determined self-consistently. Finally, we findthat the relation between accretion luminosity and Brγ luminosityfound for low-mass T Tauri stars extends to the intermediate-massregime.
| 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).
| The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga
We present an astrometric study of the candidates of T Tauri stars (TTS)and non-TTS X-ray sources around Tau-Aur, based on the HipparcosCatalogue and the ACT Reference Catalogue. The ROSAT selected X-raysources are found to be a mixed population. A few of them areassociated with the Tau-Aur or Orion Star Forming Regions (SFR). Some,with distances similar to that of Tau-Aur but with discrepant propermotions, are probable or sure Pleiades super-cluster members or otherlate type young active stars with unresolved nature, more likely tooriginate in rapidly moving cloudlets, or else having originated fromdifferent sites other than Tau-Aur and moved to the present locations. Agood many of the non-TTS X-ray sources are considered as Hyades clustermembers. Some TTS candidates could be foreground pre-main sequencestars or actually young dwarfs not yet depleted of their Lithium. Underthe hypothesis that the sources we studied are representative of theROSAT selected TTS candidates discovered in the outskirts of the Tau-Aurregion, we conclude that only up to one third of the weak-line TTScandidates could be expected to be physically associated with theTau-Aur association. Along with the parallax and proper motion analysisof the non-TTS X-ray sources around the Tau-Aur SFR, our result suggeststhat the vast majority of the young active X-ray sources within anangular diameter of about 30 ° of the Tau-Aur SFR, belong to fourmain subgroups that are spatially separate.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Evidence for grain growth in T Tauri disks
In this article we present the results from mid-infrared spectroscopy ofa sample of 14 T Tauri stars with silicate emission. The qualitativeanalysis of the spectra reveals a correlation between the strength ofthe silicate feature and its shape similar to the one which was foundrecently for the more massive Herbig Ae/Be stars by van Boekel et al.(2003). The comparison with theoretical spectra of amorphous olivine([Mg,Fe]2SiO4) with different grain sizes suggeststhat this correlation is indicating grain growth in the disks of T Tauristars. Similar mechanisms of grain processing appear to be effective inboth groups of young stars.Based on observations made with the ESO 3.6m Telescope at the La SillaObservatory under program ID 68.D-0537(A), 69.C-0268 and 70.C-0544.
| On the evolution of giant protoplanets forming in circumbinary discs
We present the results of hydrodynamic simulations of Jovian massprotoplanets that form in circumbinary discs. The simulations follow theorbital evolution of the binary plus protoplanet system acting undertheir mutual gravitational forces, and forces exerted by the viscouscircumbinary disc. The evolution involves the clearing of the innercircumbinary disc initially, so that the binary plus protoplanet systemorbits within a low density cavity. Continued interaction between discand protoplanet causes inward migration of the planet towards the innerbinary. Subsequent evolution can take three distinct paths: (i) theprotoplanet enters the 4:1 mean motion resonance with the binary, but isgravitationally scattered through a close encounter with the secondarystar; (ii) the protoplanet enters the 4:1 mean motion resonance, theresonance breaks, and the planet remains in a stable orbit just outsidethe resonance; (iii) when the binary has initial eccentricityebin>= 0.2, the disc becomes eccentric, leading to astalling of the planet migration, and the formation of a stablecircumbinary planet.These results have implications for a number of issues in the study ofextrasolar planets. The ejection of protoplanets in close binary systemsprovides a source of `free-floating planets', which have been discoveredrecently. The formation of a large, tidally truncated cavity may providean observational signature of circumbinary planets during formation. Theexistence of protoplanets orbiting stably just outside a mean motionresonance (4:1) in the simulations indicate that such sites may harbourplanets in binary star systems, and these could potentially be observed.Finally, the formation of stable circumbinary planets in eccentricbinary systems indicates that circumbinary planets may not be uncommon.
| T Tauri Stars
The observable characteristics of young solar-mass stars (T Tauri stars)are reviewed. Modern theoretical concepts and models of these stars arediscussed briefly.
| The pre-main sequence spectroscopic binary AK Scorpii revisited
We present an analysis of 32 high-resolution echelle spectra of thepre-main sequence spectroscopic binary AK Sco obtained during 1998 and2000, as well as a total of 72 photoelectric radial-velocityobservations from the period 1986-1994. These data allow considerableimprovement of the period and other orbital parameters of AK Sco. Ouranalysis also includes eight series of photometric observations in theuvby and Geneva seven-color systems from 1987, 1989, 1990, 1992, 1994and 1997. No eclipses or other periodic variations are seen in thephotometry, but the well-determined HIPPARCOS parallax allows us toconstrain the orbital inclination of the system to the range 65 degr< i < 70 degr, leading to the following physical parameters forthe two near-identical stars: M =1.35 +/- 0.07 \msun, R =1.59 +/- 0.35\rsun, and vsin i =18.5 +/- 1.0 \kmsn.Disk models have been fit to the spectral energy distribution of AK Scofrom 350 nm to 1100 mu m. The above stellar parameters permit aconsistent solution with an inner rim temperature of 1250 K, instead ofthe usual 1500 K corresponding to the dust evaporation temperature.Dynamical effects due to tidal interaction of the binary system aresupposed to be responsible for pushing the inner disk radius outwards.Combining simultaneous photometric and spectroscopic data sets allows usto compute the dust obscuration in front of each star at several pointsover the orbit. The results demonstrate the existence of substructure atscales of just a single stellar diameter, and also that one side of theorbit is more heavily obscured than the other.The spectrum of AK Sco exhibits emission and absorption lines that showsubstantial variety and variability in shape. The accretion-relatedlines may show both outflow and infall signatures. The system displaysvariations at the binary orbital period in both the photospheric andaccretion-related line intensities and equivalent widths, although withappreciable scatter. The periodic variations in the blue and red wing of\hbeta are almost 180° out of phase.We find no evidence of enhanced accretion near the periastron passage inAK Sco as expected theoretically and observed previously in DQ Tau, asimilarly young binary system with a mass ratio near unity and aneccentric orbit. The \hal equivalent width displays rather smoothvariations at the stellar period, peaking around phases 0.6-0.7, faraway from periastron where theory expects the maximum accretion rate tooccur.Based on observations obtained at the 0.5-m (SAT) and 1.54-m Danishtelescopes and the Swiss 70-cm, Swiss Euler 1.2-m and ESO 1.52-mtelescopes at ESO, La Silla, Chile.The complete version of Table 3 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/1037
| Peculiarities of the UV Continuum Energy Distribution for T Tauri Stars
In the UV spectra of BP Tau, GW Ori, T Tau, and RY Tau obtained with theHubble Space Telescope, we detected an inflection near 2000 Angstrems inthe F^clambda(lambda) curve that describes the continuumenergy distribution. The inflection probably stems from the fact thatthe UV continuum in these stars consists of two components: the emissionfrom an optically thick gas with T < 8000 K and the emission from agas with a much higher temperature. The total luminosity of the hotcomponent is much lower than that of the cool component, but the hot-gasradiation dominates at lambda < 1800 Angstrems. Previously, otherauthors have drawn a similar conclusion for several young stars fromlow-resolution IUE spectra. However, we show that the short-wavelengthcontinuum is determined from these spectra with large errors. We alsoshow that, for three of the stars studied (BP Tau, GW Ori, and T Tau),the accretion-shock radiation cannot account for the observed dependenceF^clambda(lambda) in the ultraviolet. We argue that more than90% of the emission continuum in BP Tau at lambda > 2000 Angstremsoriginates not in the accretion shock but in the inner accretion disk.Previously, a similar conclusion was reached for six more classical TTau stars. Therefore, we believe that the high-temperature continuum canbe associated with the radiation from the disk chromosphere. However, itmay well be that the stellar chromosphere is its source.
| Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars
We report the results of a high-resolution spectroscopic survey for COfundamental emission from T Tauri stars. CO fundamental emission isfrequently detected, with the likely origin of the emission in thecircumstellar disk. An initial assessment of the line profiles indicatesthat the emission region includes the equivalent of the terrestrialplanet region of our solar system, a result that suggests the utility ofCO fundamental emission as a probe of disks at planet formationdistances. Since fundamental emission is detected frequently from bothclose binary and apparently single stars, it appears that both lowcolumn density regions, such as disk gaps, and temperature inversionregions in disk atmospheres can produce significant emission. Theestimated excitation temperature of the emitting gas is unexpectedlywarm for the disk radii that they appear to probe. Thus, the surfacegaseous component of inner disks may be significantly warmer than thesurface dust component. We also detect CO emission from a transitional TTauri star. Because fundamental emission from CO and its isotopes issensitive to a wide range of gas masses, including masses<
| Polarimetric Variations of Binary Stars. V. Pre-Main-Sequence Spectroscopic Binaries Located in Ophiuchus and Scorpius
We present polarimetric observations of seven pre-main-sequence (PMS)spectroscopic binaries located in the ρ Ophiuchus and Upper Scorpiusstar-forming regions (SFRs). The average observed polarizations at 7660Å are between 0.5% and 3.5%. After estimates of the interstellarpolarization are removed, all binaries have an intrinsic polarizationabove 0.4%, even though most of them do not present other evidences forcircumstellar dust. Two binaries, NTTS 162814-2427 and NTTS162819-2423S, present high levels of intrinsic polarization between 1.5%and 2.1%, in agreement with the fact that other observations(photometry, spectroscopy) indicate the presence of circumstellar dust.Tests reveal that all seven PMS binaries have a statistically variableor possibly variable polarization. Combining these results with ourprevious sample of binaries located in the Taurus, Auriga, and OrionSFRs, 68% of the binaries have an intrinsic polarization above 0.5%, and90% of the binaries are polarimetrically variable or possibly variable.NTTS 160814-1857, 162814-2427, and 162819-2423S are clearlypolarimetrically variable. The first two also exhibit phase-lockedvariations over ~10 and ~40 orbits, respectively. Statistically, NTTS160905-1859 is possibly variable, but it shows periodic variations notdetected by the statistical tests; those variations are not phasedlocked and only present for short intervals of time. The amplitudes ofthe variations reach a few tenths of a percent, greater than for thepreviously studied PMS binaries located in the Taurus, Orion, and AurigaSFRs. The high-eccentricity system NTTS 162814-2427 showssingle-periodic variations, in agreement with our previous numericalsimulations. We compare the observations with some of our numericalsimulations and also show that an analysis of the periodic polarimetricvariations with the Brown, McLean, & Emslie (BME) formalism to findthe orbital inclination is for the moment premature: nonperiodic eventsintroduce stochastic noise that partially masks the periodic variationsand prevents the BME formalism from finding a reasonable estimate of theinclination.Based in part on observations collected with the 2 m Bernard-Lyottelescope, operated by INSU/CNRS and Pic-du-Midi Observatory (CNRS USR5026). Financial support for the observations at Pic-du-Midi wasprovided by the Programme National de Physique Stellaire of CNRS/INSU,France.
| Spectroscopic diagnostics of UV power and accretion in T Tauri stars
It is known that in the upper atmospheres of the Sun and some late-typestars there is a systematic relationship between the optically thintotal radiated power and the power emitted by single spectral lines.Using recently derived emission-measure distributions from IUE spectrafor BP Tau, CV Cha, RY Tau, RU Lupi and GW Ori, we demonstrate that thisis also true for classical T Tauri stars (CTTSs). As in the solar caseit is found that the CIV resonance doublet at 1548 Å is also themost accurate indicator of the total radiated power from the atmospheresof CTTSs. Since the total radiated-power density in CTTSs exceeds thatof the Sun by over three orders of magnitude we derive new analyticexpressions that can be used to estimate the values for these stars. Wealso discuss the implications of these results with regard to theinfluence or absence of accretion in this sample of stars and suggestthat the method can be used to infer properties of the geometricalstructure of the emission regions. As a demonstration case we also usearchived HST-GHRS data to estimate the total radiative losses in the UVemitting region of BP Tau. We find values of 4.57 × 109erg cm-2 s-1 and 5.11 × 1032 ergs-1 dependent on the geometry of the emission region. Theseresults are several orders of magnitude larger than would be expected ifthe UV emission came primarily from an atmosphere covered in solar-likeactive regions and are closer to values associated with solar flares.They lead to luminosity estimates of 0.07 and 0.13 Lsolar,respectively, which are in broad agreement with results obtained fromtheoretical accretion shock models. Taken together they suggest thataccretion may well be the dominant contributor to the UV emission in BPTau.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Орион |
| Ректацензија: | 05h29m08.39s |
| Deклинација: | +11°52'12.7" |
| Apparent магнитуда: | 9.876 |
| Proper motion RA: | -0.6 |
| Proper motion Dec: | 0.2 |
| B-T magnitude: | 11.079 |
| V-T magnitude: | 9.976 |
Каталог и designations:
|
INHABITED SKY
