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Far-Ultraviolet H2 Emission from Circumstellar Disks
We analyze the far-ultraviolet (FUV) spectra of 33 classical T Tauristars (CTTS), including 20 new spectra obtained with the Advanced Camerafor Surveys Solar Blind Channel (ACS/SBC) on the Hubble Space Telescope.Of the sources, 28 are in the ~1 Myr old Taurus-Auriga complex or OrionMolecular Cloud, 4 in the 8-10 Myr old Orion OB1a complex, and 1, TWHya, in the 10 Myr old TW Hydrae Association. We also obtained FUVACS/SBC spectra of 10 non-accreting sources surrounded by debris diskswith ages between 10 and 125 Myr. We use a feature in the FUV spectradue mostly to electron impact excitation of H2 to study theevolution of the gas in the inner disk. We find that the H2feature is absent in non-accreting sources, but is detected in thespectra of CTTS and correlates with accretion luminosity. Since allyoung stars have active chromospheres which produce strong X-ray and UVemission capable of exciting H2 in the disk, the fact thatthe non-accreting sources show no H2 emission implies thatthe H2 gas in the inner disk has dissipated in thenon-accreting sources, although dust (and possibly gas) remains atlarger radii. Using the flux at 1600 Å, we estimate that thecolumn density of H2 left in the inner regions of the debrisdisks in our sample is less than ~3 × 10-6 gcm-2, 9 orders of magnitude below the surface density of theminimum mass solar nebula at 1 AU.
| Near infrared polarimetry of a sample of YSOs
Aims: Our goal is to study the physical properties of the circumstellarenvironment of young stellar objetcs (YSOs). In particular, thedetermination of the scattering mechanism can help us to constrain theoptical depth of the disk and/or envelope in the near infrared. Methods: We used the IAGPOL imaging polarimeter along with the CamIVinfrared camera at the LNA observatory to obtain near infraredpolarimetry measurements in the H band of a sample of optically visibleYSOs, namely, eleven T Tauri stars and eight Herbig Ae/Be stars. Anindependent determination of the disk (or jet) orientation was obtainedfor twelve objects from the literature. The circumstellar optical depthcould then be estimated by comparing the integrated polarizationposition angle (PA) with the direction of the major axis of the diskprojected onto the plane of the sky. Optically thin disks have, ingeneral, a polarization PA that is perpendicular to the disk plane. Incontrast, optically thick disks have polarization PAs parallel to thedisks. Results: Among the T Tauri stars, three are consistentwith having optically thin disks (AS 353A,RY Tau and UY Aur) and five withoptically thick disks (V536 Aql, DGTau, DO Tau, HL Tau andLkHα 358). Among the Herbig Ae/Be stars, twostars exhibit evidence of optically thin disks (Hen3-1191 and VV Ser) and two of opticallythick disks (PDS 453 and MWC 297).Our results seem consistent with optically thick disks at near infraredbands, which are more likely to be associated with younger YSOs.Marginal evidence of polarization reversal is found in RYTau, RY Ori, WW Vul, andUY Aur. In the first three cases, this feature can beassociated with the UXOR phenomenon. Correlations with the IRAS colorsand the spectral index yielded evidence of an evolutionary segregationin which the disks tend to be optically thin when they are older.Based on observations obtained at the Observatório do Pico dosDias, LNA/MCT, Itajubá, Brazil.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| The photopolarimetric activity and circumstellar environment of the T Tauri star CO Ori
Not Available
| 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.
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| A VLT/NACO survey for triple and quadruple systems among visual pre-main sequence binaries
Aims.This paper describes a systematic search for high-ordermultiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods: .We conducted an Adaptive Optics survey of a sample of 58 PMSwide binaries from various star-forming regions, which include 52 TTauri systems with mostly K- and M-type primaries, with the NIRinstrument NACO at the VLT. Results: .Of these 52 systems, 7 arefound to be triple (2 new) and 7 quadruple (1 new). The new closecompanions are most likely physically bound based on their probabilityof chance projection and, for some of them, on their position on acolor-color diagram. The corresponding degree of multiplicity among widebinaries (number of triples and quadruples divided by the number ofsystems) is 26.9 ± 7.2% in the projected separation range ~0.07arcsec -12'', with the largest contribution from the Taurus-Aurigacloud. We also found that this degree of multiplicity is twice in Tauruscompared to Ophiuchus and Chamaeleon for which the same number ofsources are present in our sample. Considering a restricted samplecomposed of systems at distance 140-190 pc, the degree of multiplicityis 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30binaries, 5 triples, 6 quadruples). The observed frequency agrees withresults from previous multiplicity surveys within the uncertainties,although a significant overabundance of quadruple systems compared totriple systems is apparent. Tentatively including the spectroscopicpairs in our restricted sample and comparing the multiplicity fractionsto those measured for solar-type main-sequence stars in the solarneighborhood leads to the conclusion that both the ratio of triples tobinaries and the ratio of quadruples to triples seems to be in excessamong young stars. Most of the current numerical simulations of multiplestar formation, and especially smoothed particles hydrodynamicssimulations, over-predict the fraction of high-order multiplicity whencompared to our results. The circumstellar properties around theindividual components of our high-order multiple systems tend to favormixed systems (i.e. systems including components of wTTS and cTTS type),which is in general agreement with previous studies of disks inbinaries, with the exception of Taurus, where we find a preponderance ofsimilar type of components among the multiples studied.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars
T Tauri stars are variable stars that are in an early phase ofevolution, in which accretion and contraction to the main sequence arestill taking place. Their photometric variability is complex; it takesplace on a variety of timescales, due to a variety of physicalprocesses. Periodic variability occurs due to rotation and the presenceof cool or hot spots on the star. It may also occur due to periodicobscuration of the star by inhomogeneities in the still presentaccretion disk. But the periodicity may be masked by other forms ofvariability, or by time variation in the cool or hot spots, or theobscuring inhomogeneities. For other types of variable stars,self-correlation has proven to be a useful adjunct to Fourier analysisfor studying semiregular variability; it determines the cycle-to-cyclebehavior of the star, averaged over all the data. We have therefore usedit to investigate the photometric variability of about 30 T Tauri starsusing existing data. It has provided useful information about periodsand their coherence, about the amplitude of the periodic variation, orits upper limit, and about the ``profile'' of the amplitude-timescalebehavior. In most cases, it has confirmed periods previously determinedby Fourier analysis, but in some cases it has suggested that thepreviously determined period is spurious.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 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.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| 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.
| 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
| 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.
| Optical Properties of the Circumstellar Dust around Stars with Aperiodic Fadings
Multicolor observations of 21 rapid, irregular variable stars withAlgol-like fadings and of R Coronae Borealis are used to derive theoptical extinction coefficients of the circumstellar dust associatedwith these objects, θ(λ). We used more than 3600 uniformmulti-color brightness measurements obtained in a rigorous U BV R systemat the Terskol High-altitude Observing Station. The mean extinctioncoefficient θ for the circumstellar dust for this sample of starscoincides almost exactly with the interstellar extinction coefficient,θis, but is somewhat higher in the ultraviolet. We suggest anexplanation for this difference.
| 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
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Periodic Variability in the Pre-Main-Sequence Object CB 34V
A photometric study of the G-type pre-main-sequence star CB 34V wasundertaken at Van Vleck Observatory during the 2001-2002 observingseason. The star remained in its bright state throughout the 5 monthmonitoring period, showing no evidence of the large amplitudevariability that originally drew attention to it. However, it didexhibit periodic variability with an amplitude of ~0.6 mag. A highlysignificant peak is present in the periodogram at 1.186 days, butanalysis of the light curve suggests that the actual rotation period ofthe star is twice that-2.372 days. Apparently, the star had similarspots on opposite hemispheres during this time and we view it close toequator-on. This circumstance allows us to place additional constraintson the radius and luminosity of the star. The new data argue against itsbeing an FU Orionis star and in favor of its being either a UX Orionisstar or an object (similar to KH 15D) that is eclipsed episodically bycircumstellar matter.
| Intrinsic Properties of the Young Stellar Object SU Aurigae
Intensive long-term photometric observations of the archetypical youngG2 IIIe stellar object SU Aurigae are analyzed to determine many of itsphotometric and physical properties. Combining nearly 2000Strömgren uvby measures obtained using the 0.8 m Four CollegeAutomatic Photoelectric Telescope, previously published photometry, andrecently obtained high-resolution echelle spectra has led to thedetermination of its effective temperature, surface gravity, luminosity,mass, age, rotation period, and absolute radius. Since the disk of SUAur is seen at a high inclination angle (i.e., nearly edge-on), thiscomplicates the observations with significant(ΔV~Δy<~0.80 mag), and apparently random, drops inobserved mean light. These are possibly due to the transits ofprotoplanetary bodies, protocomets, or associated accretion halos. Inthis study, an effort has been made to separate the contributions of thecircumstellar disk from the intrinsic properties of the stellar coreitself. Furthermore, photometry has been simultaneously obtained for thenearby young A0 Vpe stellar companion AB Aurigae. Analysis has shownthat SU Aur and AB Aur are a coeval proper-motion pair. Also includedare certain photometric and physical properties of AB Aur.
| Analysis of HST Ultraviolet Spectra for T Tauri Stars: Estimating the Interstellar Extinction and the Contribution from an Accretion Shock to the Emission-Continuum Formation
Not Available
| Circumstellar disks around Herbig Ae/Be stars: Polarization, outflows and binary orbits
The geometrical relationship between the distribution of circumstellarmatter, observed optical linear polarization, outflows and binaryorbital plane in Herbig Ae/Be stars is investigated. Optical linearpolarization measurements carried out for a number of Herbig Ae/Be starsthat are either known to be in binary systems and/or have bipolar jetsare presented in this paper. Available information on the positionangles of polarization, outflows and binary companions for Herbig Ae/Bestars is compiled and analysed for any possible correlations. In ~85% ofthe sources the outflow position angle is within 30deg ofbeing parallel or perpendicular to the polarization position angle. In~81% of the sources the binary position angle is within 30degof being parallel or perpendicular to the polarization position angle.Out of 15 sources with bipolar outflows, 10 sources have the binaryposition angle within 30deg of being perpendicular to theoutflow position angle. These results favour those binary formationmechanisms in which the binary components and the disks aroundindividual stars or circumbinary disks are coplanar.
| On the simultaneous optical and near-infrared variability of pre-main sequence stars
For a complete understanding of the physical processes causing thephotometric variability of pre-main sequence systems, simultaneousoptical and near-IR observations are required to disentangle theemission from the stars and that from their associated circumstellardisks. Data of this sort are extremely rare and little systematic workhas been reported to date. The work presented in this paper is asystematic attempt in this direction. It presents an analysis of thesimultaneous optical and near-IR photometric variability of 18 HerbigAe/Be and T Tauri stars which were observed in October 98 by the EXPORTcollaboration. The time difference between the UBVRI and JHKmeasurements is less than 1 hour in ~50% of the data and the largestdifference is around 2 hours in only ~10% of the data. Twelve starsappear to show a correlation between the optical and near-IR variabilitytrends, which suggests a common physical origin such as spots and/orvariable extinction. The optical and near-IR variability is uncorrelatedin the rest of the objects, which suggests it originates in distinctlydifferent regions. In general, the optical variability qualitativelyfollows the predictions of starspots or variable extinction. As far asthe near-IR is concerned, the simultaneity of the observationsdemonstrates that for most objects the flux is largely produced by theircircumstellar disks and, consequently, in many cases the near-IRfluctuations must be attributed to structural variations of such disksproducing variations of their thermal emission and/or scattered light.The observed near-IR changes of up to around 1 mag on timescales of 1-2days provide interesting challenges for understanding the mechanismsgenerating such remarkable variabilities, an issue insufficientlyinvestigated until now but one which deserves further theoretical andmodeling efforts.
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/379/564
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| The Spatial Distribution of the λ Orionis Pre-Main-Sequence Population
The λ Ori star-forming region presents a snapshot of amoderate-mass giant molecular cloud 1-2 Myr after cloud disruption by OBstars, with the OB stars, the low-mass stellar population, remnantmolecular clouds, and the dispersed gas all still present. We have usedoptical photometry and multiobject spectroscopy for lithium absorptionto identify 266 pre-main-sequence stars in 8 deg2 of theregion. We also present new Strömgren photometry for the massivestars, from which we derive a distance of 450 pc and a turnoff age of6-7 Myr. Using these parameters and pre-main-sequence evolutionarymodels, we map the star formation history of the low-mass stars. We findthat low-mass star formation started throughout the region at about thesame time as the birth of the massive stars, and thereafter the birthrate accelerated. Within the last 1-2 Myr star formation ceased in thecenter of the star-forming region, near the concentration of OB stars,while it continues in dark clouds 20 pc away. We suggest that asupernova 1-2 Myr ago destroyed the molecular cloud core from which theOB stars formed, but it did not terminate star formation in more distantreaches of the giant molecular cloud. We find no secure evidence fortriggered or sequential star formation in the outer molecular clouds.The global star formation of the λ Ori region has generated thefield initial mass function, but local star formation in subregionsshows large deviations from the expected ratio of high- to low-massstars.
| EXPORT: Near-IR observations of Vega-type and pre-main sequence stars
We present near-IR JHK photometric data of a sample of 58 main-sequence,mainly Vega-type, and pre-main sequence stars. The data were takenduring four observing runs in the period May 1998 to January 1999 andform part of a coordinated effort with simultaneous optical spectroscopyand photo-polarimetry. The near-IR colors of the MS stars correspond inmost cases to photospheric colors, although noticeable reddening ispresent towards a few objects, and these stars show no brightnessvariability within the observational errors. On the other hand, the PMSstars show near-IR excesses and variability consistent with previousdata. Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110
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