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Spectral properties of X-ray bright variable sources in the Taurus molecular cloud
Aims.We analyze 19 bright variable X-ray sources detected in theXMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST), tocharacterize the variations with time of their coronal properties and toderive information on the X-ray emitting structures. Methods: Weperformed time-resolved spectroscopy of the EPIC PN and MOS spectra ofthe XEST sources, using a model with one or two thermal components, andwe used the time evolution of the temperatures and emission measuresduring the decay phase of flares to derive the size of the flaringloops. Results: The light curves of the selected sources showdifferent types of variability: flares, long-lasting decay or risethrough the whole observation, and slow modulation or complex flare-likevariability. Spectral analysis shows typical quiescent plasmatemperatures of ~5-10 MK and ~15-35 MK; the cool component generallyremains constant, while the observed flux changes are due to variationsof the hot component. During flares the plasma reaches temperatures upto 100 MK and luminosities up to ~1031 erg s-1.Loop sizes inferred from flare analysis are generally smaller than orcomparable to the stellar radius.Table [see full textsee full text] is only available in electronic format http://www.aanda.org
| The Calar Alto lunar occultation program: update and new results
We present an update of the lunar occultation program which is routinelycarried out in the near-IR at the Calar Alto Observatory. A total of 350events were recorded since our last report (Fors et al. 2004, A&A,419, 285). In the course of eight runs we have observed, among others,late-type giants, T-Tauri stars, and infrared sources. Noteworthy was apassage of the Moon close to the galactic center, which produced a largenumber of events during just a few hours in July 2004. Results includethe determinations of the angular diameter of RZ Ari,and the projected separations and brightness ratios for one triple and13 binary stars, almost all of which representing first time detections.Projected separations range from 0farcs09 to 0farcs007. We provide aquantitative analysis of the performance achieved in our observations interms of angular resolution and sensitivity, which reach about 0farcs003and K ≈8.5 mag, respectively. We also present a statisticaldiscussion of our sample, and in particular of the frequency ofdetection of binaries among field stars.
| Very cold cores in the Taurus Molecular Ring as seen by ISO
Three prominent cold objects of the Taurus MolecularRing (TMR) were revealed by our ISOPHOT 200μm map of the south-eastern part of the Heiles Cloud2 (HCL 2) cloud complex. One corresponds tothe cyanopolyyne peak region of the TMC-1 ridge,(TMC-1 CP), one is the HCL2-Ecloud, and one which we call HCL2-ES lies south ofTMC-1. The 200/100 μm colour temperatures and column densities of thethree ISOPHOT cold clouds are ≈12 K, and 1.2 ± 0.7 ×1022 cm-2 respectively, as calculated fromISO/IRAS surface brightnesses. As Nagoya-4 m C18 (1-0)spectra show, these are dense molecular clouds with N(H)>1021 cm-2 column density peaks. The ISOPHOT 200μm surface brightness is well correlated with the C18Oline intensity (corr. coef.≈ 70%). The large dust particle emissivityis found to be increased in the prototypical very dense core TMC-1 CP.As the low linewidths (Δ v=0.8 ± 0.2 km s-1)indicate, the level of turbulent energy density is 50% lower in thesethree clouds than in other clouds of HCL2. Dense cores were identifiedinside the C18O clouds by NH3 measurements withthe Effelsberg-100 m telescope and Nobeyama-45 mH13CO+ data. The density of the dense cores isn≥1.1 × 105 cm-3, and their kinetictemperatures are ⪉10 K, in good agreement with the FIR results. Thetotal molecular gas mass in the gravitationally bound cloud cores ofTMC-1 CP and HCL2-E is about 21 Mȯ and 8Mȯ respectively. The cores, TMC-1 CPb and HCL2-Eab areassociated with 3 low mass YSO candidate 2MASS point sources, while 35other low mass YSO candidates are seen elsewhere in TMR south, which weconsider as evidence for ongoing low mass star formation.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| A Large-Scale Objective-Prism and X-Ray Survey in Taurus-Auriga
We present the results of a moderately deep, wide-field optical/X-raysurvey extending over ~60 deg in the Taurus-Auriga molecular cloudcomplex. Our observations are sensitive enough to allow us to detectlower mass and older pre-main-sequence stars that could have been missedin previous surveys. We identify four new T Tauri stars, two in the areaof L1500 and L1503, one in L1538, and one in L1544. X-rays were detectedfrom three near-IR sources, one of them a very reddened Class II T Tauristar. Four X-ray sources have no known optical/IR known counterpart;they could be very reddened pre-main-sequence stars. Our findings areconsistent with the idea that little (if any) star formation has takenplace in L1537 and L1538. Although we found one slightly older T Tauristar (~3-4 Myr old), our survey yields no evidence for any significantnumber of T Tauri stars, regardless of mass and age, outside the maingroups of young stars in Taurus. We conclude that the M stars comprisingthe low-mass end of the ROSAT All-Sky Survey (RASS) source populationmust be older than ~10 Myr and most probably unrelated to the ongoingprocess of star formation in Taurus. Instead, the bulk of the RASSsources seen toward regions like Taurus seem to trace the history ofstar formation over a longer period in the solar vicinity. Researchreported herein based on observations made with the 1 m Schmidttelescope at Observatorio Nacional de Llano del Hato, Venezuela,operated by the Centro de Investigaciones de Astronomía (CIDA).
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| A spectroscopic survey of the Taurus-Auriga dark clouds for pre-main-sequence stars having CA II H, K emission
Stars having the Ca II H, K lines in emission were the object of asearch in a large area of the Tau-Aur clouds to about B = 15.0 that ledto the rediscovery of essentially all 51 T Tauri stars (TTSs); H, Kemission was detected in about 20 other faint red stars, which slitspectrograms and multicolor photometry show to be low luminosity K and Mstars with weak H-alpha emission. The location of the H, K emissionstars in an H-R diagram shows them to be mixed among the conventionalTTSs, so that they appear to be simply TTSs whose H-alpha emission istoo faint to be detected by conventional surveys. These results stressthe notion that, if initial conditions are similar and if premainsequence ages usually assigned are correct, then emission line intensitydoes not decay with time in any simple general way.
| Magnetic field structure in the Taurus dark cloud
Optical and infrared polarimetry of sources in the direction of theTaurus cloud are obtained in order to study the magnetic field in thiscloud and its possible role in the cloud's evolution. Most of the starsare background giant stars whose light shines through the cloud and ispolarized by the cloud material. The transverse component of themagnetic field, as delineated by the polarization vectors, is generallyperpendicular to the galactic plane, and the stratified structure of thecloud could be due to the effect of the magnetic field during the earlystages of collapse. Three of the 13 embedded stars are stronglypolarized with position angles nearly perpendicular to those of nearbyfield stars. The polarization of these stars is most likely intrinsic,and the direction of polarization indicates that the materialsurrounding these stars may be magnetic i.e., that the magnetic field isfrozen in this material.
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
| Interstellar extinction in the dark Taurus clouds. I
The results of photoelectric photometry of 74 stars in the Vilniusseven-color system in the area of Taurus dark clouds with coordinates(1950) 4h20m-4h48m +24.5 deg to +27 deg are presented. Photometricspectral types, absolute magnitudes, color excesses, interstellarextinctions and distances of the stars are determined. The dark cloudKhavtassi 286, 278 and the surrounding absorbing nebulae are found toextend from 140 to 175 pc from the sun. The average interstellarextinction on both sides of the dark cloud is of the order of 1.5m. Noevidence of the existence of several absorbing clouds situated atvarious distances is found.
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