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The XMM-Newton extended survey of the Taurus molecular cloud (XEST)
Context: The Taurus Molecular Cloud (TMC) is the nearest largestar-forming region, prototypical for the distributed mode of low-massstar formation. Pre-main sequence stars are luminous X-ray sources,probably mostly owing to magnetic energy release. Aims: The XMM-NewtonExtended Survey of the Taurus Molecular Cloud (XEST) presented in thispaper surveys the most populated ≈5 square degrees of the TMC, usingthe XMM-Newton X-ray observatory to study the thermal structure,variability, and long-term evolution of hot plasma, to investigate themagnetic dynamo, and to search for new potential members of theassociation. Many targets are also studied in the optical, andhigh-resolution X-ray grating spectroscopy has been obtained forselected bright sources. Methods: The X-ray spectra have beencoherently analyzed with two different thermal models (2-componentthermal model, and a continuous emission measure distribution model). Wepresent overall correlations with fundamental stellar parameters thatwere derived from the previous literature. A few detections from Chandraobservations have been added. Results: The present overview paperintroduces the project and provides the basic results from the X-rayanalysis of all sources detected in the XEST survey. Comprehensivetables summarize the stellar properties of all targets surveyed. Thesurvey goes deeper than previous X-ray surveys of Taurus by about anorder of magnitude and for the first time systematically accesses veryfaint and strongly absorbed TMC objects. We find a detection rate of 85%and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), andidentify about half of the surveyed protostars and brown dwarfs.Overall, 136 out of 169 surveyed stellar systems are detected. Wedescribe an X-ray luminosity vs. mass correlation, discuss thedistribution of X-ray-to-bolometric luminosity ratios, and show evidencefor lower X-ray luminosities in CTTS compared to WTTS. Detailed analysis(e.g., variability, rotation-activity relations, influence of accretionon X-rays) will be discussed in a series of accompanying papers.Table 3, Full Tables 4-11 and Appendices A and B are only available inelectronic form at http://www.aanda.org Full Table [see full text] isonly 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/468/353
| Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust near the Diffuse/Dense Cloud Interface
Observations of interstellar linear polarization in the spectral range0.35-2.2 μm are presented for several stars reddened by dust in theTaurus region. Combined with a previously published study by Whittet etal., these results represent the most comprehensive data set availableon the spectral dependence of interstellar polarization in this nearbydark cloud (a total of 27 sight lines). Extinction data for these andother reddened stars in Taurus are assembled for the same spectralrange, combining published photometry and spectral classifications withphotometry from the Two Micron All Sky Survey. The polarization andextinction curves are characterized in terms of the parametersλmax (the wavelength of maximum polarization) andRV (the ratio of total to selective extinction),respectively. The data are used to investigate in detail the question ofwhether the optical properties of the dust change systematically as afunction of environment, considering stars observed throughprogressively more opaque (and thus progressively denser) regions of thecloud. At low visual extinctions (03, real changes in grainproperties occur, characterized by observed RVvalues in the range 3.5-4.0. A simple model for the development ofRV with AV suggests thatRV may approach values of 4.5 or more in thedensest regions of the cloud. The transition between ``normal''extinction and ``dense cloud'' extinction occurs at AV~3.2, avalue coincident with the threshold extinction above whichH2O-ice is detected on grains within the cloud. Changes inRV are thus either a direct consequence ofmantle growth or occur under closely similar physical conditions. Dustin Taurus appears to be in a different evolutionary state compared withother nearby dark clouds, such as ρ Oph, in which coagulation is thedominant physical process.
| Determining the extinction through dark clouds
We discuss the problem of a consistent determination of the visualextinction to objects in and behind a dark cloud. The use ofnear-infrared colours is discussed, in particular the H-K colours. Weconcentrate on the uncertainties, with emphasis on the extinction lawand on the intrinsic colours of the young stars embedded in a darkcloud. The cases of the two intensively studied nearby star-formingclouds, Taurus and Ophiuchus, are addressed.
| Ices and extinction through the Taurus and Ophiuchus clouds
A detailed intercomparison is made between published observations of H_2O and CO ices towards the Taurus and Ophiuchus dark clouds. The columndensities of the ices are intercompared, and each compared to the visualextinction through the clouds, A_v. It is neither clear that the twoclouds have different or well defined threshold extinctions for thesurvival of ice mantles, nor that, for each of the clouds, thethresholds for the survival of CO and H_2 O mantles are different. Theinclusion of new objects in Taurus (Teixeira et al. 1998) introduces alarge scatter in the relations between those quantities relative toresults obtained by previous authors. Lines-of-sight towards deeplyembedded Young Stellar Objects in Taurus appear to show an enhancementin the amount of water-ice relative to lines-of-sight towards fieldstars behind that cloud. While for A_v < 14 mag there is a tightcorrelation between the water-ice column density, N_s(H_2 O), and A_v,the inclusion of those new objects reveals a discontinuity in therelation between N_s(H_2 O) and A_v. The interpretation of thisdiscontinuity is discussed.
| Grain Alignment in the Taurus Dark Cloud
Variations in the polarization efficiency (p/A) of interstellar grainsas a function of environment place vital constraints on models for themechanism of alignment. In this Letter, polarimetric observations ofbackground field stars are used to investigate alignment in the TaurusDark Cloud for extinctions in the magnitude range 0 < AK < 2.5 (0< AV < 25). Results show a strong systematic trend in polarizationefficiency with extinction, well represented by a power law p/A ~A-0.56. A number of possible interpretations of this result arediscussed. Assuming magnetic alignment of the grains, the observed trendmay be influenced by such factors as small-scale magnetic fieldstructure, variation of magnetic field strength and coupling of gas anddust temperatures as functions of density, and systematic changes in theefficiency of suprathermal spin as a function of grain surfaceproperties and H/H2 fraction within the dark cloud.
| High-Resolution Studies of Solid CO in the Taurus Dark Cloud: Characterizing the Ices in Quiescent Clouds
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...455..234C
| A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud
This paper presents optical spectrophotometry of field stars projectedon the Taurus-Auriga molecular cloud. We derive extinction laws forheavily reddened stars in this sample and show that the reddening lawthrough the dark cloud is nearly identical to the standard law forlambda-lambda(3600-6100) and AV approximately less than 3mag. Our spectroscopic parallaxes suggest a distance of 140 +/- 10 pcfor the northern portion of the cloud and show no compelling evidencefor a substantial variation in distance across the leading edge of thecloud.
| A Search for the 13175-A Infrared Diffuse Band in Dense Environments
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.268..705A
| Chemical transitions for interstellar C2 and CN in cloud envelopes
Observations were made of absorption from CH, C2, and CN towardmoderately reddened stars in Sco, OB2, Ceo OB3, and Taurus/Auriga. Forthese directions, most of the reddening is associated with a singlecloud complex, for example, the rho Ophiuchus molecular cloud, and as aresult, the observations probe moderately dense material. When combinedwith avaliable data for nearby directions, the survey provides the basisfor a comprehensive analysis of the chemistry for these species. Thechemical transitions affecting C2 and CN in cloud envelopes wereanalyzed. The depth into a cloud at which a transition takes place wascharacterized by tauuv, the grain optical depth at 1000 A.One transition at tauuv approx. = 2, which arises from, theconversion of C(+) into CO, affects the chemistries for both moleculesbecause of the key role this ion plays. A second one involvingproduction terms in the CN chemistry occurs at tauuv ofapprox. = 3; neutral reactions which C2 and CH is more important atlarger values for tauuv. The transition fromphotodissociation to chemical destruction takes place attauuv approx. = 4.5 for C2 and CN. The observational data forstars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemicalrate equations containing the most important production and destructionmechanisms. Because the sample of stars in Sco OB2 includes sight lineswith Av ranging from 1-4 mag, sight lines dominated byphotochemistry could be analyzed separately from those controlled bygas-phase destruction. The analysis yielded values for two poorly knownrate constants for reactions involved in the production of CN; thereactions are C2 + N yields CN + C and C(+) + NH yields all products.The other directions were analyzed with the inferred values. Thepredicted column densities for C2 and CN agree with the observed valuesto better than 50%, and in most instances 20%. When combining theestimates for density and temperature derived from chemical modeling andmolecular excitation for a specific cloud, such as the rho Ophiuchusmolecular cloud, the portion of the cloud envelope probed by C2 and CNabsorption was found to be in pressure equilibrium.
| Grain Mantles in the Taurus Dark Cloud
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.263..749S&db_key=AST
| Systematic variations in the wavelength dependence of interstellar linear polarization
New observations of the wavelength dependence of interstellar linearpolarization, p(lambda), which were performed to investigate theinfluence of the environment on the effective size distribution of thealigned polarizing particles, are presented. Optical and IR measurementswere obtained simultaneously in eight photometric passbands between Uand K, giving a coherent data set for a total of 105 reddened stars. Thecontention that variations of K and lambda-max are correlated isconfirmed, and the empirical linear relationship between K andlambda-max found by Wilking et al. (1980, 1982, 1983) is revised to K +0.01 +/-0.05 + (1.66 +/-0.09)lambda-max. There is some cosmic scatter ofthe data about this line. The same linear dependence of K on l-max seenoverall provides a consistent representation of the systematicpolarization in changes within individual regions with rather differingenvironments. It is argued that the grain size distribution in denseregions is modified by coagulation which removes the smaller particleswithout major modification of the larger ones.
| Diffuse interstellar bands in the Taurus dark clouds
This paper presents strengths and profiles of seven diffuse interstellarbands measured in a group of field stars situated behind the Taurus darkcloud complex. Comparison with the current 'band family' theories showsthat, within a single complex, the relative proportions of differentfamily carriers can vary in rather close unions: although correlationswith extinction are poor in all cases, some pairs of bands arenonetheless strongly interrelated. Band strengths relative to reddeningare investigated and found to be reasonably well fitted by a model inwhich all of the DIB production takes place in a surface layer of thecloud. These results are compared with existing data for the Rho Ophcloud. Band production efficiency varies in the same way in bothregions, falling rapidly with increasing E(B-V) to a limiting value afactor of 3-10 below the norm for the diffuse ISM. Comparison of grainproperties and the UV radiation field in the two cases shows that therelative weakness of the diffuse bands in dark clouds is driven bychanging grain properties rather than by species or other ionizedmolecules.
| Infrared spectroscopy of dust in the Taurus dark clouds - Ice and silicates
Low-resolution spectra of the 3 micron water-ice features of 22 starsand the 10 micron silicate dust features of 16 stars are presented forstars in the direction of the extensive dark cloud complex in Taurus,including both dust-embedded objects and background field stars seenthrough the cloud. A very close linear correlation is found between thepeak optical depth in the 3 micron feature and visual extinction, A(v),for field stars. The detection of ice in all cases where A(v) exceeds athreshold value of 3.3 + or - 0.1 mag serves as a basis for models ofvolatile mantle growth on grains in the dark cloud environment. Thesilicate feature is poorly correlated with A(v), and its relativeweakness per unit A(v) in the spectra of field stars with strong icebands may reflect the dilution of silicates as a fraction of total grainmass as mantles accrete in the dark cloud.
| On the ratio of total-to-selective extinction in the Taurus dark cloud complex
UBVRI and JHK photometry, as well as spectral classifications arepresented for seven reddened early-type field stars that are observedthrough the Taurus dark cloud complex. The ratio of total-to-selectiveextinction is derived for each star by the color-difference method. Forsix stars with absolute magnitudes in violet of more than 1.7 and lessthan 3.2 mag, a normal ratio R of total-to-selective extinction of about3.1 is found. The mildly anomalous R value of about 3.5 for thewell-studied star HD 29647 was also confirmed. The results providefurther evidence that the interstellar extinction law in the Taurus darkcloud complex is basically normal for lines of sight with absolutemagnitudes in violet of less than 3 mag.
| 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.
| Polarimetric investigation of background stars in the region of T and RY Tau
Electropolarimetric observations of 97 background stars in the TTauri/RY Tauri region, obtained with the 40-cm Cassegrain telescope ofthe Biurakan Astrophysical Observatory in December 1982, are reportedand interpreted in terms of local-magnetic-field effects oncometary-nebula bending. The data are presented in a table, and thedependence of polarization position angle on declination is establishedin a graph and attributed to a continuous variation in the direction ofthe magnetic field. From the magnitude of the effect, however, it isconcluded that the observed bending of the cometary nebulae in theregion results from the combined influence of the local magnetic momentand the magnetic moment of the star itself (as proposed by Vardanian,1983) rather than from the local moment alone.
| 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.
| 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.
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