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The Disk Around CoKu Tauri/4: Circumbinary, Not Transitional
CoKu Tau/4 has been labeled as one of the very few known transition diskobjects-disks around young stars that have their inner disks cleared ofdust, arguably as a result of planetary formation. We reportaperture-masking interferometry and adaptive optics imaging observationsshowing that CoKu Tau/4 is in fact a near-equal binary star of projectedseparation ~53 mas (~8 AU). The spectral energy distribution of the diskis then naturally explained by the inner truncation of the disk throughgravitational interactions with the binary star system. We discuss thepossibility that such ``unseen'' binary companions could cause othercircumbinary disks to be labeled as transitional.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| A Case Study of Low-Mass Star Formation
This article synthesizes observational data from an extensive programaimed toward a comprehensive understanding of star formation in alow-mass star-forming molecular cloud. New observations and publisheddata spanning from the centimeter wave band to the near-infrared revealthe high- and low-density molecular gas, dust, and pre-main-sequencestars in L1551. The total cloud mass of ~160 Msolar containedwithin 0.9 pc has a dynamical timescale, tdyn=1.1 Myr.Thirty-five pre-main-sequence stars with masses from ~0.1 to 1.5Msolar are selected to be members of the L1551 associationconstituting a total of 22+/-5 Msolar of stellar mass. Theobserved star formation efficiency, SFE=12%, while the total efficiency,SFEtot, is estimated to fall between 9% and 15%. L1551appears to have been forming stars for several tdyn, with therate of star formation increasing with time. Star formation has likelyprogressed from east to west, and there is clear evidence that anotherstar or stellar system will form in the high column density region tothe northwest of L1551 IRS 5. High-resolution, wide-field maps of L1551in CO isotopologue emission display the structure of the molecular cloudat 1600 AU physical resolution. The 13CO emission clearlyreveals the disruption of the ambient cloud by outflows in the line coreand traces the interface between regions of outflow and quiescent gas inthe line wings. Kinetic energy from outflows is being deposited backinto the cloud on a physical scale λpeak~0.05 pc at arate, E˙input~0.05 Lsolar. The remainingenergy afforded by the full mechanical luminosity of outflow in L1551destroys the cloud or is otherwise lost to the greater interstellarmedium. The C18O emission is optically thin and traces wellthe turbulent velocity structure of the cloud. The total turbulentenergy is close to what is expected from virial equilibrium. Theturbulent velocities exist primarily on small scales in the cloud, andthe energy spectrum of turbulent fluctuations, E(k)~k-β,is derived by various methods to have β~1-2. The turbulentdissipation rate estimated using the results of current numericalsimulations is E˙diss~E˙input. This studyreveals that stellar feedback is a significant factor in the evolutionof the L1551 cloud.
| On the Diversity of the Taurus Transitional Disks: UX Tauri A and LkCa 15
The recently recognized class of ``transitional disk'' systems consistsof young stars with optically thick outer disks but inner disks whichare mostly devoid of small dust grains. Here we introduce a furtherclass of ``pre-transitional disks'' with significant near-infraredexcesses which indicate the presence of an optically thick inner diskseparated from an optically thick outer disk; thus, the spectral energydistributions of pre-transitional disks suggest the incipientdevelopment of disk gaps rather than inner holes. In UX Tau A, ouranalysis of the Spitzer IRS spectrum finds that the near-infraredexcess is produced by an inner optically thick disk and that a gap of~56 AU is present. The Spitzer IRS spectrum of LkCa 15 is suggestive ofa gap of ~46 AU, confirming previous millimeter imaging. In addition, UXTau A contains crystalline silicates in its disk at radii >~ 56 AUwhich poses a challenge to our understanding of the production of thiscrystalline material. In contrast, LkCa 15's silicates are amorphous andpristine. UX Tau A and LkCa 15 increase our knowledge of the diversityof dust clearing in low-mass star formation.
| The evolution of stars in the Taurus-Auriga T association
In a recent study, individual parallaxes were determined for many starsof the Taurus-Auriga T association that are members of the same movinggroup. We use these new parallaxes to re-address the issue of therelationship between classical T Tauri stars (CTTSs) and weak-emissionline T Tauri stars (WTTSs). With the available spectroscopic andphotometric information for 72 individual stars or stellar systems amongthe Taurus-Auriga objects with known parallaxes, we derived reliablephotospheric luminosities, mainly from the Ic magnitude ofthese objects. We then studied the mass and age distributions of thestellar sample, using pre-main sequence evolutionary models to determinethe basic properties of the stellar sample. Statistical tests and MonteCarlo simulations were then applied to studying the properties of thetwo T Tauri subclasses. We find that the probability of CTTS and WTTSsamples being drawn from the same parental age and mass distributions islow; CTTSs are, on average, younger than WTTSs. They are also lessmassive, but this is due to selection effects. The observed mass and agedistributions of both T Tauri subclasses can be understood in theframework of a simple disk evolution model, assuming that the CTTSsevolve into WTTSs when their disks are fully accreted by the stars.According to this empirical model, the average disk lifetime inTaurus-Auriga is 4 × 10^6~(M_*/M_ȯ)0.75 yr.Table 4 is only available in electronic form at http://www.aanda.org
| Demographics of transition objects
The unusual properties of transition objects (young stars with anoptically thin inner disc surrounded by an optically thick outer disc)suggest that significant disc evolution has occurred in these systems.We explore the nature of these systems by examining their demographics,specifically their stellar accretion rates and disc massesMdisc compared to those of accreting T Tauri stars ofcomparable age. We find that the transition objects in Taurus occupy arestricted region of the versus Mdisc plane. Compared tonon-transition single stars in Taurus, they have stellar accretion ratesthat are typically ~10 times lower at the same disc mass and median discmasses approximately four times larger. These properties are anticipatedby several proposed planet formation theories and suggest that theformation of Jovian mass planets may play a significant role inexplaining the origin of at least some transition objects. Consideringtransition objects as a distinct demographic group among accreting TTauri stars leads to a tighter relationship between disc masses andstellar accretion rates, with a slope between the two quantities that isclose to the value of unity expected in simple theories of discaccretion.
| 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.
| The lithium-rotation correlation for WTTS in Taurus–Auriga
Surface lithium abundance and rotation velocity can serve as powerfuland mutually complementary diagnostics of interior structure of stars.So far, the processes responsible for the lithium depletion duringpre-main sequence evolution are still poorly understood. We investigatewhether a correlation exists between equivalent widths of Li (EW(Li))and rotation period (Prot) for weak-line T Tauri stars(WTTSs). We find that rapidly rotating stars have lower EW(Li) and thefast burning of Li begins at the phase when star’s Protevolves towards 3 days among 0.9Mȯ to1.4Mȯ WTTSs in Taurus–Auriga. Our results supportthe conclusion by Piau and Turch-Chiéze about a model for lithiumdepletion with age of the star and by Bouvier et al. in relation torotation evolution. The turn over of the curve for the correlationbetween EW(Li) and Prot is at the phase of zero-age mainsequence (ZAMS). The EW(Li) decreases with decreasing Protbefore the star reaches the ZAMS, while it decreases with increasingProt (decreasing rotation velocity) for young low-mass mainsequence stars. This result could be explained as an age effect of Lidepletion and the rapid rotation does not inhibit Li destruction amonglow-mass PMS stars.
| Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
Context: .T Tauri stars exhibit variability on all timescales, whoseorigin is still debated. Aims: .We investigate the long termvariability of CTTs over up to 20 years, characterize it from a set ofstatistical parameters and discuss its origin. Methods: .Wepresent a unique, homogeneous database of photometric measurements forClassical T Tauri stars extending up to 20 years. The database containsmore than 21 000 UBVR observations of 72 CTTs. All the data werecollected within the framework of the ROTOR-program at Mount MaidanakObservatory (Uzbekistan) and together they constitute the longesthomogeneous, accurate record of TTS variability ever assembled. Wecharacterize the long term photometric variations of 49 CTTs withsufficient data to allow a robust statistical analysis and propose anempirical classification scheme. Results: .Several patterns oflong term photometric variability are identified. The most commonpattern, exhibited by a group of 15 stars which includes T Tau itself,consists of low level variability (Δ V≤0.4 mag) with nosignificant changes occurring from season to season over many years. Arelated subgroup of 22 stars exhibits a similar stable long termvariability pattern, though with larger amplitudes (up to ΔV≃1.6 mag). Besides these representative groups, we identify threesmaller groups of 3-5 stars each which have distinctive photometricproperties. Conclusions: .The long term variability of most CTTsis fairly stable and merely reflects shorter term variability due tocold and hot surface spots. Only a small fraction of CTTs undergosignificant brightness changes on the long term (months, years), whichprobably arise from slowly varying circumstellar extinction.
| Mid-Infrared Observations of T Tauri Stars: Probing the Star-Disk Connection in Rotational Evolution
We present mid-IR N-band (λeff=10.2 μm) photometryof a carefully selected sample of T Tauri stars thought to be singlefrom the Taurus-Auriga molecular cloud. Infrared excesses in these starsare generally attributed to circumstellar dust disks. Combiningobservations at 2.16 (Ks band) and 10.2 μm (N band) weprobe a region in the circumstellar dust disk from a few stellar radiithrough the terrestrial planet zone (0.02-1.0 AU). By analyzing thedistribution of the (Ks-N) color index with respect topreviously measured photometric rotation periods we investigate whatrole circumstellar disks play in the rotational evolution of the centralstar. The resulting positive correlation between these two variables isconsistent with the notion that a star-disk interaction facilitates theregulation of angular momentum during the T Tauri stage. We alsodemonstrate how including nonsingle stars in such an analysis weakensany correlation in the relation between (Ks-N) color andperiod. To further understand disk properties we also present spectralenergy distributions for a few objects with new ground-based M-band(λeff=4.8 μm) and Q-band (λeff=20μm) data and compare them to a geometrically thin, optically thickdisk model.
| A kinematic study of the Taurus-Auriga T association
Aims.This is the first paper in a series dedicated to investigating thekinematic properties of nearby associations of young stellar objects.Here we study the Taurus-Auriga association, with the primary objectiveof deriving kinematic parallaxes for individual members of this low-massstar-forming region. Methods: .We took advantage of a recentlypublished catalog of proper motions for pre-main sequence stars, whichwe supplemented with radial velocities from various sources found in theCDS databases. We searched for stars of the Taurus-Auriga region thatshare the same space velocity, using a modified convergent point methodthat we tested with extensive Monte Carlo simulations. Results:.Among the sample of 217 Taurus-Auriga stars with known proper motions,we identify 94 pre-main sequence stars that are probable members of thesame moving group and several additional candidates whose pre-mainsequence evolutionary status needs to be confirmed. We derive individualparallaxes for the 67 moving group members with known radial velocitiesand give tentative parallaxes for other members based on the averagespatial velocity of the group. The Hertzsprung-Russell diagram for themoving group members and a discussion of their masses and ages arepresented in a companion paper.
| 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.
| A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus
We present mid-infrared spectra of T Tauri stars in the Taurusstar-forming region obtained with the Spitzer Infrared Spectrograph(IRS). For the first time, the 5-36 μm spectra of a large sample of TTauri stars belonging to the same star-forming region is studied,revealing details of the mid-infrared excess due to dust incircumstellar disks. We analyze common features and differences in themid-IR spectra based on disk structure, dust grain properties, and thepresence of companions. Our analysis encompasses spectral energydistributions from the optical to the far-infrared, a morphologicalsequence based on the IRS spectra, and spectral indices in IRS wavebands representative of continuum emission. By comparing the observedspectra to a grid of accretion disk models, we infer some basic diskproperties for our sample of T Tauri stars and find additional evidencefor dust settling.
| 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.
| Analysis of AAVSO Visual Measurements of T Tauri Variable Stars
T Tauri stars are stars in the final stages of birth, in which accretionfrom a circumstellar disc of gas and dust is still taking place. By somedefinitions, T Tauri stars include only GKM types; in this paper, weinclude higher-mass pre-mainsequence variables as well. AAVSO observershave made tens of thousands of visual measurements of T Tauri stars, butmost of the measurements were never validated because their scientificvalue was not clear. We have used Fourier and selfcorrelation techniquesto analyze AAVSO visual measurements of eleven T Tauri stars, namely ABAur, RW Aur, SV Cep, R CrA, S CrA, RY Lup, R Mon, UX Tau, BP Tau, DLTau, and WW Vul. We have compared the results to those obtained fromlong-term CCD measurements of the same stars. Using our methods ofanalysis, it is possible to detect periods, even if the amplitude isonly a few hundredths of a magnitude, or to set upper limits, as smallas 0.01 magnitude, on any periodic component to the variability. Foreach star, periodic or not, we have determined the variability"profile"-the relation between time scale and amount of variability. Weconclude that the AAVSO visual measurements of T Tauri stars havedefinite scientific value. It would therefore be desirable to validatethe visual measurements of other T Tauri stars.
| Rotational periods of T Tauri stars in Taurus-Auriga, south of Taurus-Auriga, and in MBM12
Context: .The ROSAT All-Sky Survey detected many young objects outsideany known star forming region. Their formation is yet unclear.Aims: .In order to improve the knowledge about these X-ray brightobjects we aimed at measuring their rotational properties, which arefundamental stellar parameters, and at comparing them to young objectsinside molecular clouds. Methods: .We monitored photometricvariations of 5 T Tauri stars in MBM12 and of 26 young objects in theTaurus-Auriga molecular cloud and south of it. Among the 26 youngobjects there are 17 weak-line T Tauri stars, 7 zero age main-sequencestars and 2 of unknown type. In addition, 2 main-sequence K-type starswere observed, and one comparison star turned out to be an eclipsingbinary. Results: .We found periodic variations for most of thetargets. The measured periods of the T Tauri stars range from 0.57 to7.4 days. The photometric variation can be ascribed to rotationalmodulation caused by spots. For a few of the periodic variables, changesof the light curve profile within several weeks are reported. For onestar such changes have been observed in data taken two years apart. Theexceptions are two eclipsing systems. One so far unknown system - GSC2.2N3022313162 - shows a light curve with full phase coverage having bothprimary and secondary minima well resolved. It has an orbital period of0.59075 days. From our spectroscopic observations we conclude that it isa main sequence star of spectral type F2 ± 4. We further comparedthe off-cloud weak-line T Tauri stars to the weak-line T Tauri starsinside the molecular cloud in terms of rotational period distribution.Statistical analysis of the two samples shows that both groups arelikely to have the same period distribution.
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Evidence for J- and H-Band Excess in Classical T Tauri Stars and the Implications for Disk Structure and Estimated Ages
We argue that classical T Tauri stars (CTTSs) possess significantnonphotospheric excess in the J and H bands (1.25 and 1.66 μm,respectively). We first show that normalizing the spectral energydistributions (SEDs) of CTTSs to the J band leads to a poor fit of theoptical fluxes (which are systematically overestimated), whilenormalizing the SEDs to the IC band (0.8 μm) produces abetter fit to the optical bands and in many cases reveals the presenceof a considerable excess at J and H. Near-infrared spectroscopic veilingmeasurements from the literature support this result. We find that J-and H-band excesses correlate well with the K-band (2.2 μm) excessand that the J-K and H-K colors of the excess emission are consistentwith that of a blackbody at the dust sublimation temperature (~1500-2000K). We propose that this near-IR excess originates at a hot inner rim,analogous to those suggested to explain the ``near-IR bump'' in the SEDsof Herbig Ae/Be stars. To test our hypothesis, we use the modelpresented by Dullemond and coworkers to fit the photometry data between0.5 and 24 μm of 10 CTTSs associated with the Chamaeleon II molecularcloud. We find that simple models that include luminosities calculatedfrom IC-band magnitudes and an inner rim may account for thereported J- and H-band excesses. The models that best fit the data arethose in which the inner radius of the disk is larger than expected fora rim in thermal equilibrium with the photospheric radiation fieldalone. In particular, we find that large inner rims are necessary toaccount for the mid-infrared fluxes (3.6-8.0 μm) obtained by theSpitzer Space Telescope (Spitzer). The large radius could be explainedif, as proposed by D'Alessio and colleagues, the UV radiation from theaccretion shock significantly affects the sizes of the inner holes indisks around CTTSs. Finally, we argue that deriving the stellarluminosities of CTTSs by making bolometric corrections to the J-bandfluxes, which is the ``standard'' procedure for obtaining CTTSluminosities, systematically overestimates these luminosities. Theoverestimated luminosities translate into underestimated ages when thestars are placed in the H-R diagram. Thus, the results presented hereinhave important implications for the dissipation timescale of inneraccretion disks.
| 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.
| Einigen uberarbeitete und neue Vergleissternkarten.
Not Available
| 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.
| Testing Protoplanetary Disk Alignment in Young Binaries
We present K-band (2.2 μm) imaging polarimetry that resolves 19 TTauri binary and multiple systems in the Taurus-Auriga andScorpius-Ophiuchus star-forming regions. We observed systems withprojected separations 1.5"-7.2" (~200-1000 AU) in order to determine therelative orientation of the circumstellar disks in each binary system.Scattered light from these disks is polarized, allowing us to deduce theposition angle of the disk on the sky from the position angle ofpolarization even though our observations do not resolve the disksthemselves. We detected measurable polarization (typically 0.5%-2%, withtypical uncertainty 0.1%) from both stars in 14 of the systems observed.In eight of the nine binary systems, the two stars' polarizationposition angles are within 30° of each other, inconsistent withrandom orientations. In contrast, the five triple and quadruple systemsappear to have random disk orientations when comparing the polarizationposition angles of the widest pair in the system; the close pairs areunresolved in all but one system. Our observations suggest that disks inwide (200-1000 AU) binaries are aligned with each other within<~20° but not perfectly coplanar. However, we cannot conclusivelyrule out random relative disk orientations if the observed polarizationsare significantly contaminated by interstellar polarization. Even in thepresence of interstellar polarization our observations securely excludecoplanar disks. These results provide constraints on possible binaryformation mechanisms if the observed orientations are primordial. On theother hand, models of disk-binary interactions indicate that the disksmay have had time to decrease their relative inclinations sinceformation. If the common orientation of the disks in these binaries is atracer of the binary orbital plane, then our results also havesignificance for the stability of planetary orbits, suggesting thatplanetary systems in wide binaries should be stable over 109yr timescales.
| The ɛ Chamaeleontis Young Stellar Group and the Characterization of Sparse Stellar Clusters
We present the outcomes of a Chandra X-Ray Observatory snapshot studyof five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linkedwith the Oph-Sco-Cen association (OSCA). Optical photometric andspectroscopic follow-up was conducted for the HD 104237 field. Theprincipal result is the discovery of a compact group ofpre-main-sequence (PMS) stars associated with HD 104237 and itscodistant, comoving B9 neighbor ɛ Chamaeleontis AB. We name thegroup after the most massive member. The group has five confirmedstellar systems ranging from spectral type B9 to M5, including aremarkably high degree of multiplicity for HD 104237 itself. The HD104237 system is at least a quintet, with four low-mass PMS companionsin nonhierarchical orbits within a projected separation of 1500 AU ofthe HAeBe primary. Two of the low-mass members of the group are activelyaccreting classical T Tauri stars. The Chandra observations alsoincrease the census of companions for two of the other four HAeBe stars,HD 141569 and HD 150193, and identify several additional new members ofthe OSCA. We discuss this work in light of several theoretical issues:the origin of X-rays from HAeBe stars; the uneventful dynamical historyof the high-multiplicity HD 104237 system; and the origin of the ɛCha group and other OSCA outlying groups in the context of turbulentgiant molecular clouds. Together with the similar η Cha cluster, wepaint a portrait of sparse stellar clusters dominated byintermediate-mass stars 5-10 Myr after their formation.
| Near-Infrared, Adaptive Optics Observations of the T Tauri Multiple-Star System
With high angular resolution, near-infrared observations of the youngstellar object T Tauri at the end of 2002, we show that, contrary toprevious reports, none of the three infrared components of T Tau seem tocoincide with the compact radio source that has apparently been ejectedrecently from the system (Loinard, Rodríguez, &Rodríguez). The compact radio source and the infrared object ithas previously been identified with, T Tau Sb, have distinct paths thatdeparted from orbital or uniform motion between 1997 and 2000, perhapsindicating that their interaction led to the ejection of the radiosource. The path that T Tau Sb took between 1997 and 2003 may indicatethat this star is still bound to the presumably more massive southerncomponent, T Tau Sa. The radio source (if indeed distinct from T Tau Sb)is absent from our near-infrared images and must therefore be fainterthan K=10.2 (if located within 100 mas of T Tau Sb, as the radio datawould imply), still consistent with an identity as a low-mass star orsubstellar object.
| A fourth component in the young multiple system V 773 Tauri
I report on a new component in the pre-main sequence multiple systemV 773 Tauri. This second visual companion, V 773 TauC, with a projected separation of ~0farcs2 has been detected usingspeckle interferometry in the near-infrared. Repeated observations from1996 to 2002 show significant orbital motion and thus confirm thecharacter of the new companion as a gravitationally bound star. Togetherwith the two components of the spectroscopic binary V 773 Tau A and thepreviously known visual companion V 773 Tau B, the V 773 Tau systemappears as a young ``mini-cluster'' of four T Tauri stars within asphere of a radius less than 100 AU. V 773 Tau A, B and C form a triplesystem that is not hierarchic, but is apparently stable despite of this.The brightness of V 773 Tau C has probably increased over the lastyears, which may explain its non-detection in previous binary surveys.Based on observations collected at the German-Spanish AstronomicalCenter on Calar Alto, Spain.
| Natural Coronagraphic Observations of the Eclipsing T Tauri System KH 15D: Evidence of Accretion and Bipolar Outflow in a Weak-Line T Tauri Star
We present high-resolution (R~44,000) UV-Visual Echelle Spectrographspectra of the eclipsing pre-main-sequence star KH 15D covering thewavelength range 4780-6810 Å obtained at three phases: out ofeclipse, near minimum light, and during egress. The system evidentlyacts as a natural coronagraph, enhancing the contrast relative to thecontinuum of hydrogen and forbidden emission lines during eclipse. Atmaximum light, the Hα equivalent width was ~2 Å and theprofile showed broad wings and a deep central absorption. During egress,the equivalent width was much higher (~70 Å) and the broad wings,which extend to +/-300 km s-1, were prominent. During eclipsetotality, the equivalent width was less than during egress (~40 Å)and the high-velocity wings were much weaker. Hβ showed a somewhatdifferent behavior, revealing only the blueshifted portion of thehigh-velocity component during eclipse and egress. [O I]λλ6300, 6363 lines are easily seen both out of eclipse andwhen the photosphere is obscured and exhibit little or no flux variationwith the eclipse phase. Our interpretation is that KH 15D, althoughclearly a weak-line T Tauri star by the usual criteria, is stillaccreting matter from a circumstellar disk and has a well-collimatedbipolar jet. As the knife-edge of the occulting matter passes across theclose stellar environment, it is evidently revealing structure in themagnetosphere of this pre-main-sequence star with unprecedented spatialresolution. We also show that there is only a small, perhaps marginallysignificant change in the velocity of the K7 star between the maximumlight and egress phases probed here.Based on UV-Visual Echelle Spectrograph observations collected at theEuropean Southern Observatory's Very Large Telescope via Director'sDiscretionary Time, within the observing program P267.C-5736.
| Comments on Inferences of Star Formation Histories and Birth Lines
Palla & Stahler have recently argued that star formation in Taurusand other nearby molecular clouds extends over a period of at least 10Myr, implying quasi-static cloud evolution and star formation. Theirconclusions contradict other recent results indicating that molecularclouds are transient objects and star formation proceeds rapidly. ThePalla & Stahler picture implies that most molecular clouds shouldhave extremely low rates of star formation and that in such inactivestages, the stellar initial mass function (IMF) should be stronglyskewed toward producing stars with masses >~1Msolarneither prediction is supported by observations. I show that the Palla& Stahler conclusions for Taurus depend almost entirely on a smallnumber of stars with masses >~1Msolar the lower mass starsshow no evidence for such an extended period of star formation. Ifurther show that most of the stars apparently older than 10 Myr in thedirection of Taurus are probably foreground nonmembers. I also presentbirth line calculations that support the idea that the ages of the starswith masses >~1Msolar have been systematicallyoverestimated because birth line age corrections have beenunderestimated; such birth lines would eliminate the need to postulateskewed IMFs. The simplest and most robust explanation of currentobservations characterizing the vast majority of young stars inmolecular clouds is that cloud and star formation is rapid and dynamic.
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