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Публикации по объекту
Rotation effects in classical T Tauri stars
Surface temperature inhomogeneities in classical T Tauri stars (CTTS)induced by magnetic activity andmass accretion lead torotationalmodulation of both photometric and spectroscopic parameters ofthese stars. Using the extended photometric catalogue byGrankin et al.,we have derived the periods and amplitudes of the rotational modulationof brightness and color for 31 CTTS; for six of them, the periods havebeen revealed for the first time. The inclinations of the rotation axisand equatorial rotational velocities of CTTS have been determined. Weshow that the known periods of brightness variations for some of theCTTS are not the axial rotation periods but are the Keplerian periodsnear the inner boundary of the dusty disk. We have found that theangular velocity of CTTS with a mass of 0.3-3 M &sun; in theTaurus-Auriga complex remains constant in the age range 1-10 Myr. CTTSon radiative evolutionary tracks rotate faster than completelyconvective CTTS. The specific angular momentum of CTTS depends on theabsolute luminosity in the H α line.
| A Far-ultraviolet Atlas of Low-resolution Hubble Space Telescope Spectra of T Tauri Stars
We present a far-ultraviolet (FUV) spectral atlas consisting of spectraof 91 pre-main-sequence stars. Most stars in this sample were observedwith the Space Telescope Imaging Spectrograph and Advanced Camera forSurveys on the Hubble Space Telescope (HST). A few archival spectra fromthe International Ultraviolet Explorer and the Goddard High ResolutionSpectrograph on HST are included for completeness. We find strongcorrelations among the O I λ1304 triplet, the Si IVλλ1394/1403 doublet, the C IV λ1549 doublet, andthe He II λ1640 line luminosities. For classical T Tauri stars(CTTSs), we also find strong correlations between these lines and theaccretion luminosity, suggesting that these lines form in processesrelated to accretion. These FUV line fluxes and X-ray luminositycorrelate loosely with large scatters. The FUV emission also correlateswell with Hα, Hβ, and Ca II K line luminosities. Thesecorrelations between FUV and optical diagnostics can be used to obtainrough estimates of FUV line fluxes from optical observations. Molecularhydrogen (H2) emission is generally present in the spectra ofactively accreting CTTSs but not the weak-lined T Tauri stars that arenot accreting. The presence of H2 emission in the spectrum ofHD 98800 N suggests that the disk should be classified as activelyaccreting rather than a debris disk. We discuss the importance of FUVradiation, including the hydrogen Lyα line, on thephotoevaporation of exoplanet atmospheres. We find that the Ca II/C IVflux ratios for more evolved stars are lower than those for less evolvedaccretors, indicating preferential depletion of refractory metals intodust grains.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute.STScI is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This work alsocontains results from Chandra projects 09200763 and 10200804 supportedby SAO grants GO8-9024X and GO9-0020B to the University of Colorado.This work was supported in part by NASA Swift grants NNX09AL59G andNNX10AK88G, and Smithsonian Institution Chandra grants GO8-9024X,GO9-0020B, GO0-11042X, and GO1-12031X to the University of Colorado.
| A proper motion study of the Lupus clouds using Virtual Observatory tools
Context. The Lupus dark cloud complex is a well-known, nearby low-massstar-forming region, probably associated with the Gould Belt. In recentyears, the number of stellar and substellar Lupus candidate members hasbeen remarkably increased thanks to the Cores to Disks (c2d) SpitzerLegacy Program and other studies. However, most of these newlydiscovered objects still lack confirmation that they belong to the darkclouds. Aims: By using available kinematical information, we testthe membership of the new Lupus candidate members proposed by the c2dprogram and by a complementary optical survey. We also investigate therelationship between the proper motions and other properties of theobjects, in order to get some clues about their formation and earlyevolution. Methods: We compiled a list of members and possiblemembers of Lupus 1, 3, and 4, together with all available information ontheir spectral types, disks, and physical parameters. Using VirtualObservatory tools, we cross-matched this list with the availableastrometric catalogues to get proper motions for our objects. Our finalsample contains sources with magnitudes I < 16 mag and estimatedmasses ≳ 0.1 M&sun;. Results: According to thekinematic information, our sources can be divided into two main groups.The first one contains sources with higher proper motions in agreementwith other Gould Belt populations and with spatial distribution, opticaland near-infrared colours, and disk composition consistent with theseobjects belonging to the Lupus clouds. In the second group, sources havelower proper motions with random orientations, and they are mostlylocated outside the cloud cores, making their association with the Lupuscomplex more doubtful. We investigate the properties of the higherproper motion group, but cannot find any correlations with spatiallocation, binarity, the presence of a circumstellar disk, or withphysical properties such as effective temperature, luminosity, mass, orage. Conclusions: We conclude that the lower proper motion groupprobably represents a background population or mixture of populationsunrelated to the Lupus clouds. The higher proper motion group, on theother hand, has properties consistent with it being a genuine populationof the Lupus star-forming region. More accurate proper motions and/orradial velocity information are required for a more detailed study ofthe kinematic properties of the Lupus stellar members.Tables 2-6 and Appendix are only available in electronic form at http://www.aanda.org
| A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models
We present an analysis of Spitzer Infrared Spectrograph observations ofH2O, OH, HCN, C2H2, and CO2emission, and Keck-NIRSPEC observations of CO emission, from a diversesample of T Tauri and Herbig Ae/Be circumstellar disks. We find thatdetections and strengths of most mid-IR molecular emission features arecorrelated with each other, suggesting a common origin and similarexcitation conditions for this mid-infrared line forest. Aside from theremarkable differences in molecular line strengths between T Tauri,Herbig Ae/Be, and transitional disks discussed in Pontoppidan et al., wenote that the line detection efficiency is anti-correlated with the13/30 ?m spectral slope, which is a measure of the degree of grainsettling in the disk atmosphere. We also note a correlation betweendetection efficiency and H? equivalent width, and tentatively withaccretion rate, suggesting that accretional heating contributes to lineexcitation. If detected, H2O line fluxes are correlated withthe mid-IR continuum flux, and other co-varying system parameters, suchas L sstarf. However, significant sample variation,especially in molecular line ratios, remains, and its origin has yet tobe explained. Local thermal equilibrium (LTE) models of theH2O emission show that line strength is primarily related tothe best-fit emitting area, and this accounts for most source-to-sourcevariation in H2O emitted flux. Best-fit temperatures andcolumn densities cover only a small range of parameter space, near~1018 cm-2 and 450 K for all sources, suggesting ahigh abundance of H2O in many planet-forming regions. Othermolecules have a range of excitation temperatures from ~500to1500 K,also consistent with an origin in planet-forming regions. We findmolecular ratios relative to water of ~10-3 for allmolecules, with the exception of CO, for which n(CO)/n(H2O) ~1. However, LTE fitting caveats and differences in the waythermo-chemical modeling results are reported make comparisons with suchmodels difficult, and highlight the need for additional observationscoupled with the use of line-generating radiative transfer codes.
| Single peaked CO emission line profiles from the inner regions of protoplanetary disks
Context. Protoplanetary disks generally exhibit strong line emissionfrom the CO fundamental v = 1-0 ro-vibrational band around 4.7 ?m.The lines are usually interpreted as being formed in the Keplerian disk,as opposed to other kinematic components of the young stellar system. Aims: This paper investigates a set of disks that show COemission line profiles characterized by a single, narrow peak and abroad base extending to >50 km s-1, not readily explainedby just Keplerian motions of gas in the inner disk. Methods: Highresolution (R = 105) M-band spectroscopy has been obtainedusing CRIRES at the Very Large Telescope in order to fully resolvefundamental ro-vibrational CO emission line profiles around 4.7 ?m. Results: Line profiles with a narrow peak and broad wings arefound for 8 disks among a sample of ~50 disks around T Tauri stars withCO emission. The lines are very symmetric, have high line/continuumratios and have central velocity shifts of < 5 km s-1relative to the stellar radial velocity. The disks in this subsample areaccreting onto their central stars at high rates relative to the parentsample. All 8 disks show CO emission lines from the v = 2 vibrationalstate and 4/8 disks show emission up to v = 4. Excitation analyses ofthe integrated line fluxes reveal a significant difference betweentypical rotational (~300-800 K) and vibrational (~1700 K) temperatures,suggesting that the lines are excited, at least in part, byUV-fluorescence. For at least one source, the narrow and broadcomponents show different excitation temperatures, but generally the twocomponent fits have similar central velocities and temperature. Analysisof their spatial distribution shows that the lines are formed within afew AU of the central star. Conclusions: It is concluded thatthese broad centrally peaked line profiles are inconsistent with thedouble peaked profiles expected from just an inclined disk in Keplerianrotation. Models in which the low velocity emission arises from largedisk radii are excluded based on the small spatial distribution.Alternative non-Keplerian line formation mechanisms are discussed,including thermally and magnetically launched winds and funnel flows.The most likely interpretation is that the broad-based centrally peakedline profiles originate from a combination of emission from the innerpart (
| Young Stellar Groups and Their Most Massive Stars
We analyze the masses and spatial distributions of 14 young stellargroups in Taurus, Lupus3, ChaI, and IC348. These nearby groups, whichtypically contain 20-40 members, have membership catalogs complete to~0.02 M sun, and are sufficiently young that their locationsshould be similar to where they formed. These groups show fiveproperties seen in clusters having many more stars and much greatersurface density of stars: (1) a broad range of masses, (2) aconcentration of the most massive star toward the center of the group,(3) an association of the most massive star with a high surface densityof lower mass stars, (4) a correlation of the mass of the most massivestar with the total mass of the group, and (5) the distribution of alarge fraction of the mass in a small fraction of the stars.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. I. Detection Rates
We present a Spitzer InfraRed Spectrometer search for 10-36 ?mmolecular emission from a large sample of protoplanetary disks,including lines from H2O, OH, C2H2,HCN, and CO2. This paper describes the sample and dataprocessing and derives the detection rate of mid-infrared molecularemission as a function of stellar mass. The sample covers a range ofspectral type from early M to A, and is supplemented by archival spectraof disks around A and B stars. It is drawn from a variety of nearbystar-forming regions, including Ophiuchus, Lupus, and Chamaeleon.Spectra showing strong emission lines are used to identify which linesare the best tracers of various physical and chemical conditions withinthe disks. In total, we identify 22 T Tauri stars with strongmid-infrared H2O emission. Integrated water lineluminosities, where water vapor is detected, range from 5 ×10-4 to 9 × 10-3 L sun, likelymaking water the dominant line coolant of inner disk surfaces inclassical T Tauri stars. None of the five transitional disks in thesample show detectable gaseous molecular emission with Spitzer upperlimits at the 1% level in terms of line-to-continuum ratios (apart fromH2), but the sample is too small to conclude whether this isa general property of transitional disks. We find a strong dependence ondetection rate with spectral type; no disks around our sample of 25 Aand B stars were found to exhibit water emission, down to 1%-2%line-to-continuum ratios, in the mid-infrared, while more than half ofdisks around late-type stars (M-G) show sufficiently intense wateremission to be detected by Spitzer, with a detection rate approaching2/3 for disks around K stars. Some Herbig Ae/Be stars show tentativeH2O/OH emission features beyond 20 ?m at the 1%-2% level,however, and one of them shows CO2 in emission. We argue thatthe observed differences between T Tauri disks and Herbig Ae/Be disksare due to a difference in excitation and/or chemistry depending onspectral type and suggest that photochemistry may be playing animportant role in the observable characteristics of mid-infraredmolecular line emission from protoplanetary disks.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. V. Spectral decomposition
Context. Dust particles evolve in size and lattice structure inprotoplanetary disks, due to coagulation, fragmentation andcrystallization, and are radially and vertically mixed in disks due toturbulent diffusion and wind/radiation pressure forces. Aims:This paper aims at determining the mineralogical composition and sizedistribution of the dust grains in planet forming regions of disksaround a statistical sample of 58 T Tauri stars observed withSpitzer/IRS as part of the Cores to Disks (c2d) Legacy Program. Methods: We present a spectral decomposition model, named“B2C”, that reproduces the IRS spectra over the fullspectral range (5-35 ?m). The model assumes two dust populations: awarm component responsible for the 10 ?m emission arising from thedisk inner regions (?1 AU) and a colder component responsible forthe 20-30 ?m emission, arising from more distant regions (?10AU). The fitting strategy relies on a random exploration of parameterspace coupled with a Bayesian inference method. Results: We showevidence for a significant size distribution flattening in theatmospheres of disks compared to the typical MRN distribution, providingan explanation for the usual flat, boxy 10 ?m feature profilegenerally observed in T Tauri star spectra. We reexamine thecrystallinity paradox, observationally identified by Olofsson et al.(2009 , A&A, 507, 327), and we find a simultaneous enrichment of thecrystallinity in both the warm and cold regions, while grain sizes inboth components are uncorrelated. We show that flat disks tend to havelarger grains than flared disk. Finally our modeling results do not showevidence for any correlations between the crystallinity and either thestar spectral type, or the X-ray luminosity (for a subset of thesample). Conclusions: The size distribution flattening maysuggests that grain coagulation is a slightly more effective processthan fragmentation (helped by turbulent diffusion) in disk atmospheres,and that this imbalance may last over most of the T Tauri phase. Thisresult may also point toward small grain depletion via strong stellarwinds or radiation pressure in the upper layers of disk. The nonnegligible cold crystallinity fractions suggests efficient radial mixingprocesses in order to distribute crystalline grains at large distancesfrom the central object, along with possible nebular shocks in outerregions of disks that can thermally anneal amorphous grains.Appendix A is only available in electronic form at http://www.aanda.org
| On the origin of [NeII] 12.81 ?m emission from pre-main sequence stars: Disks, jets, and accretion
Context. Extreme-ultraviolet (EUV) and X-ray photons from classical TTauri stars are powerful ionization and heating agents that drive diskchemistry, disk instabilities, and photoevaporative flows. Themid-infrared fine-structure line of [Ne ii] at 12.81 ?m has beenproposed to trace gas in disk surface layers heated and ionized bystellar X-ray and EUV radiation. Aims: We aim at locating theorigin of [Ne ii] line emission in circumstellar environments bystudying distributions of [Ne ii] emission and correlating the inferred[Ne ii] luminosities, L[Ne II], with stellar andcircumstellar disk parameters. Methods: We have conducted a studyof [Ne ii] line emission based on a sample of 92 pre-main sequence starsmostly belonging to the infrared Class II, but including 13 accretingtransition disk objects, and also 14 objects that drive known jets andoutflows. Results: We find several significant correlationsbetween L[Ne II] and stellar parameters, in particularLX and the wind mass loss rate, dot{M}_loss. Mostcorrelations are, however, strongly dominated by systematic scatter ofunknown origin. While there is a positive correlation between L[NeII] and LX, the stellar mass accretion rate,dot{M}_acc, induces a correlation only if we combine the largelydifferent subsets of jet sources and stars without jets. Our resultsindeed suggest that L[Ne II] is bi-modally distributed, withseparate distributions for the two subsamples. The jet sources showsystematically higher L[Ne II], by 1-2 orders of magnitudewith respect to objects without jets. Jet-driving stars also tend toshow higher mass accretion rates. We therefore hypothesize that thetrend with dot{M}_acc only reflects a trend with dot{M}_loss that ismore physically relevant for [Ne ii] emission. Conclusions: The[Ne ii] luminosities measured for objects without known outflows andjets are found to agree with simplified calculations of [Ne ii] emissionfrom disk surface layers if the measured stellar X-rays are responsiblefor heating and ionizing the gas. The large scatter in L[NeII] may be introduced by variations of disk properties and theirradiation spectrum, as previously suggested. If these additionalfactors can be sufficiently well constrained, then the [Ne ii] 12.81?m line should be an important diagnostic for disk surface ionizationand heating, at least in the inner disk region. This applies inparticular to transition disks also included in our sample. Thesystematically enhanced [Ne ii] flux from jet sources clearly suggests arole for the jets themselves, as previously demonstrated by a spatiallyresolved observation of the outflow system in the T Tau triple.Complete Tables 1-4 are only available in electronic form at http://www.aanda.org
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars. II. A Survey
We have used archival CCD data, Fourier analysis, and self-correlationanalysis to study the photometric time variability of 162 T Tauri starsand related objects, including Herbig Ae/Be stars. Many show periodicvariability, presumably due to rotation of a spotted star in most cases.For the nonperiodic stars, we have estimated an upper limit to theperiodic variability. We have also analyzed 26 stars suspected to haveperiods longer than about 10 days. Of these 26 stars, at least 14 haveperiods significantly longer than 10 days; their variability may be dueto processes in the disk, or the effect of a companion, since most (butnot all) T Tauri stars have rotational periods less than this value. Fora few of the rotational variables, namely AA Tau, DK Tau, DL Tau, DNTau, GK Tau, GM Aur, Rox 29, V1121 Oph, V410 Tau, and V649 Ori, we havefound long-term variability of the amplitude, presumably due to activitycycles; the time scales are 1500-4000 days.
| Grain growth across protoplanetary discs: 10 ?m silicate feature versus millimetre slope
Context. Young stars are formed with dusty discs around them. The dustgrains in the disc are originally of the same size as interstellar dust,i.e., of the order of 0.1 ?m. Models predict that these grains willgrow in size through coagulation. Observations of the silicate featuresaround 10 and 20 ?m are consistent with growth from submicron tomicron sizes in selected sources whereas the slope of the spectralenergy distribution (SED) at mm and cm wavelengths traces growth up tomm sizes and larger. Aims: We here look for a correlation betweenthese two grain growth indicators. Methods: A large sample ofT-Tauri and Herbig-Ae/Be stars, spread over the star-forming regions inChamaeleon, Lupus, Serpens, Corona Australis, and the Gum nebula inVela, was observed with the Spitzer Space Telescope at 5-13 ?m, and asubsample was observed with the SMA, ATCA, CARMA, and VLA at mmwavelengths. We complement this subsample with data from the literatureto maximise the overlap between ?m and mm observations and search forcorrelations in the grain-growth signatures. Synthetic spectra areproduced to determine which processes may produce the dust evolutionobserved in protoplanetary discs. Results: Dust disc masses inthe range <1 to 7×10-4 M_&sun; are obtained. Themajority of the sources have a mm spectral slope consistent with graingrowth. There is a tentative correlation between the strength and theshape of the 10-?m silicate feature and the slope of the SED between1 and 3 mm. The observed sources seem to be grouped per star-formingregion in the 10-?m-feature vs. mm-slope diagram. The modellingresults show that, if only the maximum grain size is increased, firstthe 10-?m feature becomes flatter and subsequently the mm slopebecomes shallower. To explain the sources with the shallowest mm slopes,a grain size distribution shallower than that of the interstellar mediumis required. Furthermore, the strongest 10-?m features can only beexplained with bright (L ~ 6 L_&sun;), hot (Teff = 4000 K)central stars. Settling of larger grains towards the disc midplaneresults in a stronger 10-?m feature, but has a very limited effect onthe mm slope. Conclusions: A tentative correlation between thestrength of the 10-?m feature and the mm slope is found, which wouldimply that the inner and outer disc evolve simultaneously. Dust with amass dominated by large, ~mm-sized, grains is required to explain theshallowest mm slopes. Other processes besides grain growth, such as theclearing of an inner disc by binary interaction, may also be responsiblefor the removal of small grains. Observations with future telescopeswith larger bandwidths or collecting areas are required to provide thenecessary statistics to study these processes of disc and dustevolution.Appendices are only available in electronic form at http://www.aanda.org
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A2
| C2D Spitzer-IRS spectra of disks around T Tauri stars. IV. Crystalline silicates
Aims: Dust grains in the planet-forming regions around young stars areexpected to be heavily processed due to coagulation, fragmentation, andcrystallization. This paper focuses on the crystalline silicate dustgrains in protoplanetary disks for a statistically significant number ofTTauri stars (96). Methods: As part of the cores to disks (c2d)legacy program, we obtained more than a hundred Spitzer/IRS spectra ofTTauri stars, over a spectral range of 5-35 μm where many silicateamorphous and crystalline solid-state features are present. At thesewavelengths, observations probe the upper layers of accretion disks upto distances of a dozen AU from the central object. Results: Morethan 3/4 of our objects show at least one crystalline silicate emissionfeature that can be essentially attributed to Mg-rich silicates. TheFe-rich crystalline silicates are largely absent in the c2d IRS spectra.The strength and detection frequency of the crystalline features seen atλ > 20 μm correlate with each other, while they are largelyuncorrelated with the observational properties of the amorphous silicate10 μm feature. This supports the idea that the IRS spectraessentially probe two independent disk regions: a warm zone (≤1 AU)emitting at λ ~ 10 μm and a much colder region emitting atλ > 20 μm (≤10 AU). We identify a crystallinity paradox,as the long-wavelength (λ > 20 μm) crystalline silicatefeatures are detected 3.5 times more frequently (~55% vs. ~15%) than thecrystalline features arising from much warmer disk regions (λ ~10μm). This suggests that the disk has an inhomogeneous dustcomposition within ~10 AU. The analysis of the shape and strength ofboth the amorphous 10 μm feature and the crystalline feature around23 μm provides evidence for the prevalence of μm-sized (amorphousand crystalline) grains in upper layers of disks. Conclusions:The abundant crystalline silicates found far from their presumedformation regions suggest efficient outward radial transport mechanismsin the disks around TTauri stars. The presence of μm-sized grains indisk atmospheres, despite the short timescales for settling to themidplane, suggests efficient (turbulent) vertical diffusion, probablyaccompanied by grain-grain fragmentation to balance the expectedefficient growth. In this scenario, the depletion of submicron-sizedgrains in the upper layers of the disks points toward removal mechanismssuch as stellar winds or radiation pressure.Tables [see full textsee full textsee full textsee full text], [see fulltextsee full textsee full textsee full text] and Figs. [see full textseefull textsee full textsee full text]-[see full textsee full textsee fulltextsee full text] are only available in electronic form athttp://www.aanda.org
| Variability of the transitional T Tauri star T Chamaeleontis
Context: It is known that for solar-mass stars planet formation beginsin a circumstellar disc. The study of transitional objects exhibitingclear signs of evolution in their discs, such as the growth of dustparticles and beginning of disc dispersal, is fundamental tounderstanding the processes governing dust-grain coagulation and theonset of planet formation. Aims: We attempt to characterise thephysical properties of T Chamaeleontis, a transitional T Tauri starshowing UX Ori-type variability, and of its associated disc, and probethe possible effects of disc-clearing processes. Methods:Different spectral diagnostics were examined, based on a rich collectionof optical high- and low-resolution spectra. The cross-correlationtechnique was used to determine radial and projected rotationalvelocities, shape changes of photospheric lines were analysed viabisector-method applied to the cross-correlation profile, and theequivalent widths of both the Li i ?6708 Å photosphericabsorption and the most prominent emission lines (e.g., H?,H? and [Oi] 6300 Å) were measured. Variability in the mainemission features was inspected by means of line-profile correlationmatrices. Available optical and near-infrared photometry combined withinfrared data from public catalogues was used to construct the spectralenergy distribution (SED) and infer basic stellar and disc properties.Results: Remarkable variability on timescale of days in the mainemission lines, H? changing from pure emission to nearlyphotospheric absorption, is correlated with variations in visualextinction of over three magnitudes, while the photospheric absorptionspectrum shows no major changes. The strength of emission in H?and H? is highly variable and well correlated with that of the [Oi]lines. The structure of the H? line-profile also varies on a dailytime-span, while the absence of continuum veiling suggests very low orno mass accretion. Variations of up to nearly 10 km s-1 inthe radial velocity of the star are measured on analogous timescales,but with no apparent periodicity. SED modelling confirms the existenceof a gap in the disc. Conclusions: Variable circumstellarextinction is inferred to be responsible for the conspicuous variationsobserved in the stellar continuum flux and for concomitant changes inthe emission features by contrast effect. Clumpy structures,incorporating large dust grains and orbiting the star within a fewtenths of AU, obscure episodically the star and, eventually, part of theinner circumstellar zone, while the bulk of the hydrogen-lineemitting-zone and outer low-density wind region traced by the [Oi]remain unaffected. In agreement with this scenario, the detected radialvelocity changes are also explainable in terms of clumpy materialtransiting and partially obscuring the star.Based on observations obtained at the European Southern Observatoryat La Silla, Chile in program 63.I-0045(A); 65.I-0089(A); 66.C-0616(A);67.C-0155(A); 67.C-0155(B); 68.C-0292(A); 69.C-0207(A); 70.C-0163(A);073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F).Tables 1, 2 and 4, and Figs. 21-32 are only available in electronic formvia http://www.aanda.org
| Photometric and polarimetric clues to the circumstellar environment of RY Lupi
Aims: We investigate the stellar and circumstellar properties of thebright southern T Tauri star RY Lup, a G-type star showing type IIIvariability. Methods: We report simultaneous BV polarimetric andUBV photometric observations obtained during 12 consecutive nights onthe 1.0 m and 50 cm telescopes of the European Southern Observatory atLa Silla. We compare these data to models. Results: Thepolarization is high (≈3.0%) when the star is faint and red (V ≈12.0, B-V ≈ 1.3), and it is low (≈0.5%) when it is bright andbluer (V ≈ 11.0, B-V ≈ 1.1). The photometric and polarimetricvariations share a common period of 3.75 d. Irregular light variations,larger at shorter wavelengths, are also superposed on the cyclicvariations and may be due to processes different than the one producingthe periodic variations. The linear polarization is produced by dustscattering in an asymmetric (flat) circumstellar envelope. Thephotometric and polarimetric variations can be explained with an almostedge-on circumstellar disk that is warped close to the star, where itinteracts with the star's magnetosphere. The inhomogeneous disk mattercontained in the warp corotates with the star and partially occults itduring part of the rotation period, which explains the dips inluminosity and the accompanying increase in polarization. All theinformation available on RY Lup is consistent with a system comprising aG8 star surrounded by an edge-on disk, and we find that the mass of RYLup is M_star/M&sun;= 1.71 ± 0.43, while its age is(1.2± 0.4) × 107 {yr}.Based on observations collected at the European Southern Observatory inLa Silla, Chile.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. III. [Ne II], [Fe I], and H2 Gas-Phase Lines
We present a survey of mid-infrared gas-phase lines toward a sample of76 circumstellar disks around low-mass pre-main-sequence stars from theSpitzer ``Cores to Disks'' legacy program. We report the firstdetections of [Ne II] and [Fe I] toward classical T Tauri stars in ~20%and ~9% of our sources, respectively. The observed [Ne II] line fluxesand upper limits are consistent with [Ne II] excitation in an X-rayirradiated disk around stars with X-ray luminositiesLX=1029-1031 erg s-1. [Fe I]is detected at ~10-5 to 10-4 Lsolar,but no [S I] or [Fe II] is detected down to ~10-6Lsolar. The [Fe I] detections indicate the presence ofgas-rich disks with masses of >~0.1 MJ. No H20-0 S(0) and S(1) disk emission is detected, except for S(1) toward onesource. These data give upper limits on the warm (T~100-200 K) gas massof a few Jovian masses, consistent with recent T Tauri disk models thatinclude gas heating by stellar radiation. Compact disk emission of hot(T>~500 K) gas is observed through the H2 0-0 S(2) and/orS(3) lines toward ~8% of our sources. The line fluxes are, however,higher by more than an order of magnitude than those predicted by recentdisk models, even when X-ray and excess UV radiation are included. The[Ne II]/H2 0-0 S(2) ratios for these sources are similarlylower than predicted, consistent with the presence of an additional hotmolecular gas component not included in current disk models. Obliqueshocks of stellar winds interacting with the disk can explain manyaspects of the hot gas emission but are inconsistent with thenondetection of [S I] and [Fe II] lines.
| Pre-main sequence spectroscopic binaries suitable for VLTI observations
Context: A severe problem for research in star-formation is that themasses of young stars are almost always estimated from evolutionarytracks alone. Since the tracks published by different groups differ, itis often only possible to give a rough estimate of the masses of youngstars. It is thus crucial to test and calibrate the tracks. Up to now,only a few tests of the tracks could be carried out. However, it is nowpossible with the VLTI to set constrains on the tracks by determiningthe masses of many young binary stars precisely. Aims: In order to usethe VLTI efficiently, a first step is to find suitable targets, which isthe purpose of this work. Given the distance of nearby star-formingregions, suitable VLTI targets are binaries with orbital periods betweenat least 50 days and a few years. Although a number of surveys fordetecting spectroscopic binaries have been carried out, most of thebinaries found so far have periods that are too short. Methods: We thussurveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρOphiuci star-forming regions in order to search for spectroscopicbinaries with periods longer than 50 days, which are suitable for theVLTI observations. Results: As a result of the 8 year campaign, wediscovered 8 binaries with orbital periods longer than 50 days. Amongstthe newly discovered long-period binaries is CS Cha, which is one of thefew classical T Tauri stars with a circumbinary disk. The survey islimited to objects with masses higher than 0.1 to 0.2 M_ȯ forperiods between 1 and 8 years. Conclusions: We find that the frequencyof binaries with orbital periods ≤3000 days is of 20±5%. Thefrequency of long and short period pre-main sequence spectroscopicbinaries is about the same as for stars in the solar neighbourhood. Intotal 14 young binaries are now known that are suitable for massdetermination with the VLTI.based on observations obtained at the European Southern Observatory atLa Silla, Chile in program 62.I-0418(A); 63.I-0096(A); 64.I-0294(A);65.I-0012(A); 67.C-0155(A); 68.C-0292(A); 68.C-0561(A); 69.C-0207(A);70.C-0163(A); 073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F). Tables 2,3, 5, 6, 8, 10, 12-20 are only available in electronic form athttp://www.aanda.org
| Searching for gas-rich disks around T Tauri stars in Lupus
Aims.We characterize the molecular environment of classical T Tauristars in Lupus observed with the Spitzer Space Telescope in our searchfor gas-rich disks toward these sources. Methods: Submillimeterobservations of 12CO, 13CO, and C18O inthe J=3-2 and 2-1 lines were obtained with the James Clerk MaxwellTelescope toward a sample of 21 T Tauri stars with disks in the Lupusmolecular clouds. Pointings at the sources and at selected off-positionsare presented in order to disentangle material associated with disksfrom ambient cloud material. Results: One source, IM Lup (Sz 82), wasfound with the double-peaked 12CO and 13COprofiles characteristic of a large rotating gas disk. The inclination ofthe disk is found to be ~20°, with an outer radius of 400-700 AU.For most other sources, including GQ Lup with its substellar companion,the single-dish 12CO lines are dominated by extended cloudemission with a complex velocity structure. No evidence of molecularoutflows is found. Due to dense circumstellar material, compactC18O emission was detected toward only two sources. Futuresearches for gas-rich disks in Lupus should either use interferometersor perform very deep single-dish integrations in dense gas tracers toseparate the cloud and disk emission.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| 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.
| The short period multiplicity among T Tauri stars
We present the results of high-resolution spectroscopic observationscarried out over three years aimed at estimating the short-period(P_orb<100 days) binary frequency of a sample of T Tauri stars inOph-Sco, Cha, Lup, CrA star forming regions (SFRs), already observedwith high angular resolution techniques by Ghez et al.(\cite{Ghezetal93}) and by Ghez et al. (\cite{Ghezetal97}) to detectwider components. When combining all four SFRs, the short-period binaryfrequency is indistinguishable from that found by Duquennoy & Mayor(\cite{DuqMay91}) for the solar-type field stars which is alsoconsistent with the previous result obtained by Mathieu(\cite{Mathieu92}, \cite{Mathieu94}). When Oph-Sco is analyzedseparately, it seems that there is an excess of short-period binaries ofa factor 2-2.5. On the contrary, short-period binary systems seem to beabsent in the sample containing stars in Cha/Lup/CrA. Such a trend wasequally found by Mathieu (\cite{Mathieu92}) in Taurus. An excess ofspectroscopic systems among the components of visual multiple systems isalso observed.Based on observations collected with the Swiss Euler Telescope and 1.5-mESO, proposal 63.I-0112.
| The photosphere and veiling spectrum of T Tauri stars
Using recent high signal-to-noise and high spectral resolution (R ~ 60000) observations with VLT/UVES, we have analysed the photosphere andveiling spectrum of five T Tauri stars. With a grid of 1-dimensionalplane-parallel hydrostatic model atmospheres from the MARCS consortiumwe have determined their atmospheric properties, calculated syntheticspectra and determined the spectrum of the veiling continuum. Ouranalysis of the veiling spectrum supports the view that veiling can berepresented by a combination of continuum sources. However, for the moststrongly accreting stars we find a broad region around 5300 Åwhere the derived level of veiling is consistently higher than expectedfrom continuum sources.Based on observations collected at the European Southern Observatory,Chile (proposal 65.I-0404).
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Study of Spectroscopic Variability of T Tau Stars
Not Available
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Occultation of young stellar objects by circumstellar disks. I. Theoretical expectations and preliminary comparison with observations
The hypothesis that partial occultation of young stars by theircircumstellar disks is responsible for UX Ori and T Tauri Type IIIphotometric variability is quantitatively studied. We constructaccretion disk models in which reprocessing of stellar and accretionluminosity leads to flaring of the disk (assumed to be in verticalhydrostatic equilibrium), and compute the probability to observe thestar through the disk atmosphere. The mass accretion rate is found toprimarily determine the range of view angles over which the star isfully or partially obscured by the disk. For average disk/starparameters relevant to active T Tauri stars, the probability ofobserving occultation events is ~ 0.15, while it is typically 0.2 forparameters relevant to UX Ori. A preliminary analysis of availablephotometric data confirms these estimates and allows us to uncover insome stars cyclic variability on a time-scale of weeks to years that maytentatively be attributed to disk inhomogeneities or warps. Theseresults suggest that direct observational study of the disk structuremay be possible for a sizable fraction of young stellar objects.
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Наблюдательные данные и астрометрия
| Созвездие: | Волк |
| Прямое восхождение: | 15h59m28.40s |
| Склонение: | -40°21'51.3" |
| Видимая звёздная величина: | 11.173 |
| Собственное движение RA: | -6.6 |
| Собственное движение Dec: | -23.8 |
| B-T magnitude: | 13.08 |
| V-T magnitude: | 11.331 |
Каталоги и обозначения:
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