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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
| X-ray and optical bursts and flares in YSOs: results from a 5-day XMM-Newton monitoring campaign of L1551
We present the results of a five-day monitoring campaign with XMM-Newtonof six X-ray bright young stellar objects (YSOs) in the star-formingcomplex L1551 in Taurus. All stars present significant variability onthe five-day time scale. Modulation of the light curve on time scalescomparable with the star's rotational period appeared to be present inthe case of one weak-lined T Tauri star. Significant spectral variationsbetween the 2000 and the 2004 observations were detected in the(unresolved) classical T Tauri binary system XZ Tau: a hot plasmacomponent which was present in the X-ray spectrum in 2000 hadsignificantly weakened in 2004. As XZ Tau N was undergoing a strongoptical outburst in 2000, which had terminated since then, we speculateon the possible relationship between episodic, burst accretion, andX-ray heating. The transition object HL Tau underwent a strong flarewith a complex temperature evolution, which is indicative of an eventconfined within a very large magnetic structure (few stellar radii),similar to the ones found in YSOs in the Orion Nebula Cluster.
| ISOCAM observations of the L1551 star formation region
The results of a deep mid-IR ISOCAM survey of theL1551 dark molecular cloud are presented. The aim ofthis survey is a search for new YSO (Young Stellar Object) candidates,using two broad-band filters centred at 6.7 and 14.3 μm. Although tworegions close to the centre of L1551 had to beavoided due to saturation problems, 96 sources were detected in total(76 sources at 6.7 μm and 44 sources at 14.3 μm). Using the 24sources detected in both filters, 14 were found to have intrinsic mid-IRexcess at 14.3 μm and were therefore classified as YSO candidates.Using additional observations in B, V, I, J, H and K obtained from theground, most candidates detected at these wavelengths were confirmed tohave mid-IR excess at 6.7 μm as well, and three additional YSOcandidates were found. Prior to this survey only three YSOs were knownin the observed region (avoiding L1551 IRS 5/NE and HL/XZTau). This survey reveals 15 new YSO candidates, althoughseveral of these are uncertain due to their extended nature either inthe mid-IR or in the optical/near-IR observations. Two of the sourceswith mid-IR excess are previously known YSOs, one is a brown dwarf (MHO5) and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seenedge on.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/420/945
| An XMM-Newton-based X-ray survey of pre-main sequence stellar emission in the L1551 star-forming complex
We present a study of the X-ray sources present in the nearby L1551 starforming region, based on a deep XMM-Newton observation complemented withChandra data for the brightest sources. Most known pre-main sequencestars in the region have been detected, most of them with sufficientstatistics to allow a detailed study of the temporal and spectralcharacteristics of their X-ray emission. Significant temporal (andspectral) variability on both short and long time scales is visible formost of the stars. In particular XZ Tau shows large-amplitude variationson time scales of several hours with large changes in the interveningabsorption, suggestive of the X-ray emission being eclipsed by theaccretion stream (and thus of the X-ray emission being partly or totallyaccretion-induced). The coronal metal abundance of the WTTS sources isclustered around Z =~ 0.2, while the CTTS sources span almost twoorders of magnitudes in coronal Z, even though the photosphericabundance of all stars in the L1551 is likely to be very similar. Someindividual elements (notably Ne) appear to be systematically enhancedwith respect to Fe in the WTTS stars. The significant differencesbetween the spectral and temporal characteristics of the CTTS and WTTSpopulations suggest that a different emission mechanism is (at leastpartly) responsible for the X-ray emission of the two types of stars.
| X-Rays from the Vicinity of the Protostar L1551 IRS 5: Reflection or Fast Shocks?
We present new Chandra X-Ray Observatory observations of the L1551molecular cloud in Taurus. We find a compact, but slightly resolved,X-ray source, displaced westward from the IRS 5 binary protostar byabout 50-100 AU, which coincides with the base of the HH 154protostellar jet. The column density of material lying in front of theX-ray source is more than an order of magnitude lower than the columndensity toward L1551 IRS 5. Thus, it is highly improbable that theX-rays come directly from the embedded protobinary. It is possible,however, that X-rays produced by one or both members of the IRS 5 binaryescape through the outflow cavity and are scattered into the line ofsight by a dense infalling envelope or material in the outflow.Constraints on the physical properties of the scattering medium arediscussed. It is also possible that the X-rays are produced in situ byfast shocks at the base of the HH 154 jet. We consider several possiblegeometries for such shocks. The radiating plasma may be located behindstanding shocks, where a wide-angle wind from one member of the IRS 5binary is collimated into a jet, or behind shocks formed on the axis ofthe outflow, where winds from each member of the binary collide or wherethese winds impact a tilted circumbinary disk. To produce the observedX-ray luminosity and plasma temperature, shock velocities larger than350 km s-1 are required, with a preshock hydrogen density of(1-10)×103 cm-3. The implied postshockcooling length is around 800 AU, close to the observed length of thebright near-infrared [Fe II] emission in the inner portion of the HH 154jet.
| Duplicity in Hubble Space Telescope Guide Stars: Fine Guidance Sensor Serendipitous Survey Results
Data from the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS)interferometers, covering 22 months of guide-star acquisitionoperations, have been analyzed for evidence of stellar duplicity. Thedata comprise a survey of observed guide stars, all of which are takenfrom the HST Guide Star Catalog, ranging in magnitude from 9 to 14. Thesurvey results cover a parameter space for the newly found doubles, forthe fainter stars, which are of smaller limiting angular separationsthan in any previous surveys. The normal HST engineering telemetry datafrom 13,979 acquisitions on 4882 stars have been processed. The FGSguidance data can reveal duplicity with separations ranging fromapproximately 30 mas, for the brighter stars, with small magnitudedifferences, up to the neighborhood of 500 mas, and in some cases to1000 mas. The fraction of guide stars indicating duplicity is a functionof the statistical criteria used but is over 5% at a very high level ofconfidence. It is possible that if some of the brighter and closer pairscould be identified as nearby, then their orbital motions would be rapidenough to allow a mass and distance determination on a timescale of adecade if followed with ground-based interferometric and spectroscopicinstruments. A brief catalog of doubles is given, nearly all of whichare of certain duplicity. Information for accessing on-line catalogs oflarge numbers of stars with lesser, but nevertheless strong,probabilities of duplicity and also for the solutions for duplicity fromall acquisitions is provided.
| Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
| X-Ray-emitting T Tauri Stars in the L1551 Cloud
Low-mass pre-main-sequence stars in the nearby Lynds 1551 star-formingcloud are studied with the ROSAT and ASCA X-ray satellites. An 8 ksROSAT image reveals 38 sources including seven well-known T Tauri stars,two likely new weak-lined T Tauri stars, five potential new weak-lined TTauri stars, one young B9 star, and the remaining sources are unrelatedto the cloud or poorly identified. A 40 ks ASCA image of the clouddetects seven of the ROSAT sources. Spectral fitting of the brighterX-ray-emitting stars suggests the emission is produced in either amulti-temperature plasma, with temperatures near 0.2 and 1 keV, or asingle-temperature plasma with low metal abundances. XZ Tau, a youngclassical T Tauri star, is much stronger in ASCA than ROSATobservations, showing a harder (1.5-2.0 keV) component. Timing analysisreveals all but one of the T Tauri stars are variable on timescalesranging from 1 hr to 1 year. A powerful flare, emitting 3 x 10^34^ ergswithin a 40 minute rise and fall, was observed by ASCA on the weak-linedT Tauri star V826 Tau. The event was preceded and followed by constantquiescent X-ray emission. The extreme classical T Tauri star XZ Tau wasalso caught during both high and low states, varying by a factor of 15between the ASCA and ROSAT observations. Neither of the luminousinfrared embedded protostars L1551 IRS 5 or L1551NE were detected byROSAT or ASCA.
| 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 search for weak H-alpha emission line pre-main-sequence stars
A spectroscopic survey of 177 stars brighter than about 12.5 in R nearfour dark clouds in an effort to find pre-main-sequence (PMS) stars withweak H-alpha emission is reported. A number of such stars have beenfound previously in X-ray images of star formation clouds. Six newcandidate PMS stars with EW (H-alpha) between 0.4 and 4.2 A, in additionto four X-ray discovered stars, are found in the L1551 and L1529 cloudsin the Taurus-Auriga complex. The total number of pre-main-sequencestars in these clouds appears to be about two times the number of strongemission line T Tauri stars.
| Interstellar extinction in the Taurus dark clouds. II
The results of photoelectric photometry of 89 stars in the Vilniusseven-color system in the area of the Taurus dark clouds withcoordinates (1950) 4h16m-4h33m, +16 deg-+20 deg are presented.Photometric spectral types, absolute magnitudes, color excesses,interstellar extinctions and distances of the stars are determined. Thedistance of the dark nebula is found to be 140 pc and is in goodagreement with the distance determined by Straizys and Meistas (1980)for the dark nebula Khavtassi 286, 278. The average extinction in theinvestigated area is of the order of 1.4.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 04h31m25.14s |
| Deklináció: | +18°16'16.7" |
| Vizuális fényesség: | 10.322 |
| RA sajátmozgás: | -38.2 |
| Dec sajátmozgás: | -10.9 |
| B-T magnitude: | 11.238 |
| V-T magnitude: | 10.398 |
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