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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.
| Lick Northern Proper Motion Program. III. Lick NPM2 Catalog
The Lick Northern Proper Motion (NPM) program, a two-epoch (1947-1988)photographic survey of the northern two-thirds of the sky(δ>~-23deg), has measured absolute proper motions,on an inertial system defined by distant galaxies, for 378,360 stars inthe magnitude range 8<~B<~18. The 1993 NPM1 Catalog contains148,940 stars in 899 fields outside the Milky Way's zone of avoidance.The 2003 NPM2 Catalog contains 232,062 stars in the remaining 347 NPMfields near the plane of the Milky Way. This paper describes the NPM2star selection, plate measurements, astrometric and photometric datareductions, and catalog compilation. The NPM2 Catalog contains 122,806faint (B>=14) anonymous stars for astrometry and Galactic studies,91,648 bright (B<14) positional reference stars, and 34,868 ``specialstars'' chosen for astrophysical interest. The NPM2 proper motions areon the ICRS system, via Tycho-2 stars, to an accuracy of 0.6 masyr-1 in each field. The rms proper-motion precision is 6 masyr-1. Positional errors average 80 mas at the mean plateepoch 1968, and 200 mas at the NPM2 catalog epoch 2000. NPM2photographic photometry errors average 0.18 mag in B and 0.20 mag inB-V. The NPM2 Catalog and the updated (to J2000.0) NPM1 Catalog areavailable at the CDS Strasbourg data center and on the NPM Web site. TheNPM2 Catalog completes the Lick Northern Proper Motion program after ahalf-century of work by three generations of Lick Observatoryastronomers. The NPM Catalogs will serve as a database for research inGalactic structure, stellar kinematics, and astrometry.
| The 76th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1406 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Photometric observations of weak-line T Tauri stars . II. WTTS in Taurus-Auriga, Orion and Scorpius OB2-2
We present uvby-$beta a photometry of 116 X-ray flux-selected activestars in the directions of the Orion (40), Taurus-Auriga (58) andScorpius OB2-2 (18) star forming regions. Additionally, we give near IRJHK photometry of 20 active stars in the Taurus-Auriga direction. Theprogram stars were selected from the R_\odotsat All Sky Survey andEINSTEIN X-ray surveys and are spectroscopically confirmed weak-line TTauri stars and weak-line T Tauri star candidates. The photometryconfirms the young nature of the program stars and also indicates that asignificant fraction of the sample could be foreground objects. The datagiven here probably represent the largest homogeneous uvby-beta a$photometric sample of new WTTS and WTTS candidates. Many objects in thesample are observed photometrically for the first time. Based onobservations collected at the Observatorio Astronómico Nacionalin Sierra San Pedro Mártir, Baja California, México.Tables 1-4 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A study of Li-rich stars discovered by ROSAT in Taurus-Auriga
In recent years, large numbers of lithium-rich stars were discoverednear several nearby star forming regions (SFRs). We present a detailedstudy of those stars discovered in and near the central region of theTaurus-Auriga T Tauri association, based on high-resolution echellespectroscopy and proper motion data. We find that about 60 per cent ofour sample can be regarded as pre-main sequence (PMS) stars, while theremaining stars likely are foreground zero-age main sequence (ZAMS)stars. We conclude that the PMS stars are likely associated with theTaurus-Auriga SFR, while the ZAMS stars may represent a population ofsomewhat older Gould Belt stars. The fraction of ZAMS stars in theTaurus-Auriga sample studied in this work is larger than in a similarsample in the Lupus SFR, and we argue that this may be explained by thespatial structure of the Gould Belt and the Sun's location within it.Based on observations obtained at Observatoire de Haute Provence. Someof the observations reported here were obtained with the Multiple MirrorTelescope, a joint facility of the Smithsonian Institution and theUniversity of Arizona.
| Multiplicity of T Tauri stars in Taurus after ROSAT
We surveyed a sample of 75 T Tauri stars in the Taurus star formingregion for companions. These stars were discovered with the help ofROSAT. The separation range covered is 0.13'' to 13'', where the lowerlimit is given by the diffraction limit of the telescope and the upperlimit by confusion with background stars. Combined with the results ofthe preceding survey by Leinert et al. (\cite{Leinert93}), we now havesurveyed a sample of 178 young stars in Taurus, 63 classical, 106weak-line, and 9 unclassified T Tauri stars. Within this sample, we find68 binaries, 9 triples, and 3 quadruples. After corrections to accountfor confusion with background stars and for a bias induced through X-rayselection, we count 74 binaries or multiples with a total of 85companions in 174 systems. This corresponds to a degree of multiplicity(number of binaries or multiples divided by number of systems) of(42.5+/- 4.9)%, or to a duplicity, measured by the number of companionsper system, of (48.9+/- 5.3)%, which is higher by a factor of (1.93+/-0.26) compared to solar-type main-sequence stars. We find no differencein duplicity between classical and weak-line T Tauri stars. There is adifference between close and wide pairs in the sense that close pairshave a flat distribution of flux ratios, while the flux ratios of widepairs are peaked towards small values.
| Radio Emission from ROSAT-discovered Young Stars in and around Taurus-Auriga
An 8.4 GHz VLA survey of 91 recently discovered lithium-rich late-typestars from the ROSAT All-Sky Survey and pointed observations ispresented. These objects lie in the vicinity of the Taurus-Aurigastar-forming region (d ~= 140 pc); however, some are dispersed nearly 30deg from known active star-forming cloud cores. This sample represents aspatially complete, flux-limited population of X-ray-bright young starsboth within and away from the primary Tau-Aur stellar nurseries. Of the91 sources, 29 are detected in this radio survey with a sensitivitylimit of ~0.15 mJy. If they are at the distance of the star-formingclouds, we find that 32% of widely distributed young stars with LX >=5 x 1028 ergs s-1 have radio luminosity densities in excess of 3.5 x1015 ergs s-1 Hz-1. This detection rate, the ranges of radio and X-rayluminosities, and the LR/LX ratios are consistent with known youngweak-lined T Tauri stars (ages ~106 yr) that reside within the Taurusmolecular clouds, but they are considerably higher than a zero-agemain-sequence population such as the Pleiades (age ~=7 x 107 yr). Theradio properties thus support the pre-main-sequence classification ofthe stars. They fitted well among other active young stars on theempirical LR versus LX diagram, implying that solar-type gyrosynchrotronactivity is the radio emission mechanism.
| New proper motions of pre-main sequence stars in Taurus-Auriga
We present proper motions of 72 T Tauri stars located in the centralregion of Taurus-Auriga (Tau-Aur). These proper motions are taken from anew proper motion catalogue called STARNET. Our sample comprises 17classical T Tauri stars (CTTS) and 55 weak-line T Tauri stars (WTTS),most of the latter discovered by ROSAT. 53 stars had no proper motionmeasurement before. Kinematically, 62 of these stars are members of theassociation. A velocity dispersion of less than 2-3km/s is found whichis dominated by the errors of the proper motions. This velocitydispersion correlates with a spread in distances. Furthermore we presentproper motions of 58 stars located in a region just south of the Taurusmolecular clouds and compare the kinematics of the youngest stars inthis sample (younger than 3.5x10^7^yrs) with the kinematics of thepre-main sequence stars (PMS) in the Taurus-Auriga association. From acomparison of the space velocities we find that the stars in the centralregion of Tau-Aur are kinematically different from the stars in thesouthern part. Among the stars with large proper motions far off theTaurus mean motion we find 2 Pleiades candidates and 7 possible Pleiadesrunaway stars.
| COYOTES IV: the rotational periods of low-mass Post-T Tauri stars in Taurus.
We monitored the light variations of 58 weak-line T Tauri stars inTaurus, recently discovered in the X-ray wavelength range during theROSAT All-Sky Survey. We derive photometric periods for 18 stars, allbut one being ascribed to rotational modulation by stellar spots. Theexception is a 37.6d period assigned to the orbital motion of a newpre-main sequence spectroscopic binary. Two thirds of the stars in oursample have an age larger than 10Myr and up to 40Myr, thus filling theobservational gap that previously existed between T Tauri stars onconvective tracks and ZAMS dwarfs for the determination of therotational evolution of young low-mass stars. The rotational periods arefound to range from 0.5 to 7.5-days, most periods being shorter than5-days. This result provides direct evidence for the spin up of solartype stars as they contract on pre-main sequence radiative tracks, aspredicted by recent models of angular momentum evolution. The paucity oflong periods (P>=5d, i.e., V_eq_<=10km/s) in the sample of post-TTauri stars leaves, however, the origin of the numerous slow rotatorsobserved in young clusters an open issue.
| New weak-line T Tauri stars in Taurus-Auriga.
On the basis of the ROSAT All-Sky-Survey, a study of the Taurus-Aurigastar forming region has been performed in order to search for hithertoundiscovered TTauri stars. Our study covers an area of about 280 squaredegrees, located between 4^h^ and 5^h^ in right ascension and between15deg and 34deg in declination. Identification of ROSAT All-Sky Surveysources in this area by means of optical spectroscopy revealed 2 newclassical T Tauri stars (CTTS) and 66 new weak-line-T Tauri stars (WTTS)with Wlambda_(Hα)<=10A. Additional pointed ROSATobservations led to the identification of 6 more WTTS and 2 CTTS, givinga total of 76 new T Tauri stars. The large area of our study, ascompared with previous works, allows us to study the spatialdistribution of WTTS in this star forming region. We find the WTTS ofour survey to be distributed over the whole region investigated. Thereis a noticeable decline of the surface density from south to northwithin our study area, but the spatial distribution extends mostprobably beyond our study region. No clustering towards the populationof TTauri stars known prior to ROSAT in Taurus-Auriga could be observed.We suggest that the WTTS found in our study might in part be somewhatolder than the previously known TTauri stars in Taurus-Auriga, and thattheir broad spatial distribution is due to the typical velocitydispersion of a few km/s measured for Taurus TTauri stars, in which casefor some of our WTTS an age on the order of 10^7^years would be requiredfor reaching the observed distances from the Taurus dark clouds. Weestimate a WTTS/CTTS ratio of about 6 within our study area, butconclude that because of the different spatial distribution of WTTS andCTTS this ratio will be most probably significantly larger for a moreextended area.
| Search for T Tauri Stars Based on the IRAS Point Source Catalog. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2146T&db_key=AST
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 04h45m51.30s |
| Deklináció: | +15°55'49.7" |
| Vizuális fényesség: | 9.37 |
| RA sajátmozgás: | 12.1 |
| Dec sajátmozgás: | -17.7 |
| B-T magnitude: | 10.298 |
| V-T magnitude: | 9.447 |
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