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ASAS Photometry of ROSAT Sources. I. Periodic Variable Stars Coincident with Bright Sources from the ROSAT All Sky Survey
Photometric data from the ASAS - South (declination less than 29°)survey have been used for identification of bright stars located nearthe sources from the ROSAT All Sky Survey Bright Source Catalog (RBSC).In total 6028 stars brighter than 12.5 mag in I- or V-bands have beenselected and analyzed for periodicity. Altogether 2302 variable starshave been found with periods ranging from 0.137 d to 193 d. Most ofthese stars have X-ray emission of coronal origin with a few cataclysmicbinaries and early type stars with colliding winds. Whenever it waspossible we collected data available in the literature to verify periodsand to classify variable objects.The catalog includes 1936 stars (1233 new) considered to be variable dueto presence of spots (rotationally variable), 127 detached eclipsingbinary stars (33 new), 124 contact binaries (11 new), 96 eclipsing starswith deformed components (19 new), 13 ellipsoidal variables (4 new), 5miscellaneous variables and one pulsating RR Lyr type star (blended withan eclipsing binary). More than 70% of new variable stars haveamplitudes smaller than 0.1 mag, but for ASAS 063656-0521.0 we havefound the largest known amplitude of brightness variations due to thepresence of spots (up to ?V=0.8 mag). The table with the compileddata and figures with light curves can be downloaded from Acta AstronomicaArchive.
| A Magellan MIKE and Spitzer MIPS Study of 1.5-1.0 M sun Stars in Scorpius-Centaurus
We obtained Spitzer Space Telescope Multiband Imaging Photometer forSpitzer (MIPS) 24 ?m and 70 ?m observations of 182 nearby,Hipparcos F- and G-type common proper motion single and binary systemsin the nearest OB association, Scorpius-Centaurus. We also obtainedMagellan/MIKE R ~ 50,000 visual spectra at 3500-10500 Å for 181candidate ScoCen stars in single and binary systems. Combining our MIPSobservations with those of other ScoCen stars in the literature, weestimate 24 ?m F+G-type disk fractions of 9/27 (33% ± 11%),21/67 (31% ± 7%), and 25/71 (35% ± 7%) for Upper Scorpius(~10 Myr), Upper Centaurus Lupus (~15 Myr), and Lower Centaurus Crux(~17 Myr), respectively. We confirm previous IRAS and MIPS excessdetections and present new discoveries of 41 protoplanetary and debrisdisk systems, with fractional infrared luminosities ranging from LIR/L * = 10-5 to10-2 and grain temperatures ranging from Tgr = 40-300 K. We searched for an increase in 24 ?m excessat an age of 15-20 Myr, consistent with the onset of debris productionpredicted by coagulation N-body simulations of outer planetary systems.We found such an increase around 1.5 M sun stars butdiscovered a decrease in the 24 ?m excess around 1.0 M sunstars. We additionally discovered that the 24 ?m excess around 1.0 Msun stars is larger than predicted by self-stirred models.Finally, we found a weak anti-correlation between fractional infraredluminosity (L IR/L *) and chromospheric activity(R'HK), that may be the result of differences in stellarproperties, such as mass, luminosity, and/or winds.
| 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.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| 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.
| Photometric Variability Properties of 21 T Tauri and Related Stars from AAVSO Visual Observations
T Tauri variables are sun-like stars in various stages of their birth.We have analyzed long-term AAVSO visual observations of 21 T Tauri andrelated stars, using Fourier and self-correlation techniques. Thisfollows our previous study of eleven such stars in JAAVSO 35, 290(2006). Only a few of the variables showed periodic behavior, butself-correlation analysis makes it possible to construct a "variabilityprofile" - amount of variability versus time scale - for all the stars,not just the periodic ones. For some of the periodic variables, we havestudied the long-term behavior of the periods and amplitudes: T Cha andHT Lup appear to be rotating variables with stable periods less than 10days; RU Lup, UX Ori, and TU Phe appear to show transient cycles oftypically 50-500 days, probably arising in the accretion disc. R CrA hasa stable 66-day period, which would be unusually long for a rotationperiod; its cause is not clear. We also discuss interesting but spuriouslow-amplitude one-year and one-month periodicities which occur in a fewof the stars. Finally: we comment on the star AQ Dra, an RR Lyrae star,originally classified as a T Tauri star with a 5.5-day period.
| 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.
| Rotational velocities of nearby young stars
Context. Stellar rotation is a crucial parameter driving stellarmagnetism, activity and mixing of chemical elements. Measuringrotational velocities of young stars can give additional insight in theinitial conditions of the star formation process. Furthermore, theevolution of stellar rotation is coupled to the evolution ofcircumstellar disks. Disk-braking mechanisms are believed to beresponsible for rotational deceleration during the accretion phase, androtational spin-up during the contraction phase after decoupling fromthe disk for fast rotators arriving at the ZAMS. On the ZAMS, stars getrotationally braked by solar-type winds. Aims: We investigate theprojected rotational velocities v sin i of a sample of young stars withrespect to the stellar mass and disk evolutionary state to search forpossible indications of disk-braking mechanisms. Furthermore, we searchfor signs of rotational spin-up of stars that have already decoupledfrom their circumstellar disks. Methods: We analyse the stellarspectra of 220 nearby (mostly <100 pc) young (2-600 Myr) stars fortheir v sin i, stellar age, H? emission, and accretion rates. Thestars have been observed with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile. The spectra have beencross-correlated with appropriate theoretical templates. We build a newcalibration to be able to derive v sin i values from thecross-correlated spectra. Stellar ages are estimated from the Li Iequivalent width at 6708 Å. The equivalent width and width at 10%height of the H? emission are measured to identify accretors andused to estimate accretion rates dot{M}_acc. The v sin i is thenanalysed with respect to the evolutionary state of the circumstellardisks to search for indications of disk-braking mechanisms in accretors. Results: We find that the broad v sin i distribution of ourtargets extends to rotation velocities of up to more than 100 kms-1 and peaks at a value of 7.8 ± 1.2 km s-1, and that 70% of our stars show v sin i < 30 kms-1. Furthermore, we can find indications for disk-braking inaccretors and rotational spin-up of stars which are decoupled from theirdisks. In addition, we show that a number of young stars are suitablefor precise radial-velocity measurements for planet-search surveys.Based upon observations with FEROS at the 2.2 m MPG/ESO telescope andHARPS at the 3.6 m telescope in La Silla, Chile.
| 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
| Large-Scale CO Maps of the Lupus Molecular Cloud Complex
Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3transitions toward three members of the Lupus Molecular CloudComplex—Lupus I, III, and IV—trace the column density andtemperature of the molecular gas. Comparison with IR extinction mapsfrom the c2d project requires most of the gas to have a temperature of8-10 K. Estimates of the cloud mass from 13CO emission areroughly consistent with most previous estimates, while the line widthsare higher, around 2 km s-1. CO 4-3 emission is foundthroughout Lupus I, indicating widespread dense gas, and toward LupusIII and IV. Enhanced line widths at the NW end and along the edge of theB 228 ridge in Lupus I, and a coherent velocity gradient across theridge, are consistent with interaction between the molecular cloud andan expanding H I shell from the Upper-Scorpius subgroup of the Sco-CenOB Association. Lupus III is dominated by the effects of two HAe/Bestars, and shows no sign of external influence. Slightly warmer gasaround the core of Lupus IV and a low line width suggest heating by theUpper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an H Ishell.
| 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
| Physical structure and dust reprocessing in a sample of HH jets
Context: Stellar jets are an essential ingredient of the star formationprocess and a wealth of information can be derived from theircharacteristic emission-line spectra. Aims: We investigate thephysical structure and dust reprocessing in the shocks along the beam ofa number of classical Herbig-Haro (HH) jets in the Orion and Lupusmolecular clouds (HH 111, HH 1/2, HH 83, HH 24 M/A/E/C, and Sz68).Parameters describing plasma conditions, as well as dust content, arederived as a function of distance from the source and, for HH 111, ofgas velocity. Methods: Spectral diagnostic techniques are appliedto obtain the jet physical conditions (the electron and total density,ne and n_H, the ionisation fraction, x_e, and thetemperature, T_e) from the ratios of selected forbidden lines. Thepresence of dust grains is investigated by estimating the gas-phaseabundance of calcium with respect to its solar value. Results: Wefind the electron density varies between 0.05-4×103cm-3, the ionisation fraction xe from 0.01-0.7,the temperature ranges between 0.6-3×104 K, and thehydrogen density between 0.01-6×104 cm-3.Interestingly, in the HH 111 jet, n_e, x_e, and Te peak inthe high velocity interval (HVI) of the strongest working surfaces,confirming a prediction from shocks models. Calcium turns out to bedepleted with respect to its solar value, but its gas-phase abundance ishigher than estimates for the interstellar medium in Orion. Thedepletion is high (up to 80%) along the low-excited jets, while low orno depletion is measured in those jets which show higher excitationconditions. Moreover, for HH 111 the depletion is lower in the HVI ofthe fastest shock. Conclusions: Our results confirm the shockstructure predicted by models and indicate that shocks occurring alongjets, and presumably those present in the launch zone, only partiallydestroy dust grains and that the efficiency of dust reprocessingstrongly depends on shock velocity. However, the high Ca gas-phaseabundance estimated in some of the knots, is not well understood interms of existing models of dust reprocessing in shocks, and indicatesthat the dust must have been partially reprocessed in the region wherethe flow originates.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO programmes 60.C-0398(A). Appendix A is only availablein electronic form at http://www.aanda.org
| Herbig-Haro Objects in the Lupus I and III Molecular Clouds
We performed a deep search for Herbig-Haro (HH) objects toward the LupusI and III clouds, covering a sky area of ~ 1 and ~ 0.5 deg2,respectively. In total, 11 new HH objects, HH 981--991, are discovered.The HH objects both in Lupus I and in Lupus III tend to be concentratedin small areas. The HH objects detected in Lupus I are located in aregion of radius 0.26 pc near the young star Sz 68. The abundance of HHobjects shows that this region of the cloud is active in on-going starformation. HH objects in the Lup III cloud are concentrated in thecentral part of the cloud around the Herbig Ae/Be stars HR 5999 and6000. HH 981 and 982 in Lupus I are probably driven by the young browndwarf SSTc2d J154457.9-342340 which has a mass of 50 MJ . HH990 and 991 in Lup III align well with the HH 600 jet emanating from thelow-mass star Par-Lup3-4, and are probably excited by this low-mass starof spectral type M5. High proper motions for HH 228 W, E, and E2 aremeasured, which confirms that they are excited by the young star Th 28.In contrast, HH 78 exhibits no measurable proper motion in the time spanof 18 years, indicating that HH 78 is unlikely part of the HH 228 flow.The HH objects in Lup I and III are generally weak in terms ofbrightness and dimension in comparison to HH objects we detected withthe same technique in the R CrA and Cha I clouds. Through a comparisonwith the survey results from the Spitzer c2d program, we find that ouroptical survey is more sensitive, in terms of detection rate, than theSpitzer IRAC survey to high-velocity outflows in the Lup I and IIIclouds.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Starspots
Starspots are created by local magnetic fields on the surfaces of stars,just as sunspots. Their fields are strong enough to suppress theoverturning convective motion and thus block or redirect the flow ofenergy from the stellar interior outwards to the surface andconsequently appear as locally cool and therefore dark regions againstan otherwise bright photosphere (Biermann in Astronomische Nachrichten264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots areobservable tracers of the yet unknown internal dynamo activity and allowa glimpse into the complex internal stellar magnetic field structure.Starspots also enable the precise measurement of stellar rotation whichis among the key ingredients for the expected internal magnetictopology. But whether starspots are just blown-up sunspot analogs, we donot know yet. This article is an attempt to review our current knowledgeof starspots. A comparison of a white-light image of the Sun (G2V, 5Gyr) with a Doppler image of a young solar-like star (EK Draconis;G1.5V, age 100 Myr, rotation 10 × ? Sun) and witha mean-field dynamo simulation suggests that starspots can be ofsignificantly different appearance and cannot be explained with ascaling of the solar model, even for a star of same mass and effectivetemperature. Starspots, their surface location and migration pattern,and their link with the stellar dynamo and its internal energytransport, may have far reaching impact also for our understanding oflow-mass stellar evolution and formation. Emphasis is given in thisreview to their importance as activity tracers in particular in thelight of more and more precise exoplanet detections around solar-like,and therefore likely spotted, host stars.
| A new population of cool stars and brown dwarfs in the Lupus clouds
Context: Most studies of the stellar and substellar populations ofstar-forming regions rely on using the signatures of accretion,outflows, disks, or activity characterizing the early stages of stellarevolution. However, these signatures rapidly decay with time. Aims: We present the results of a wide-area study of the stellarpopulation of clouds in the Lupus star-forming region. When combinedwith 2MASS photometry, our data allow us to fit the spectral energydistributions of over 150 000 sources and identify possible new membersbased on their photospheric fluxes, independent of any display of thesignposts of youth. Methods: We used the Wide Field Imager (WFI)at the La Silla 2.2 m telescope to image an area of more than 6 squaredegrees in the Lupus 1, 3 and 4 clouds in the R_C, I_C, and z_WFI bands,selected so as to overlap with the areas observed in the Spitzer LegacyProgram “From molecular cores to planet-forming disks”. Wecomplement our data with 2MASS photometry to sample the spectral energydistribution from 0.6 ?m to 2.2 ?m. We validate our method on thecensus of known members of the Lupus clouds, for which spectroscopicclassification is available. The temperatures derived for cool objectsare generally accurate, with most of the exceptions attributed toveiling, strong emission lines at short wavelengths, near-infraredexcess, variability, or the presence of close companions. Results: Considering that the dereddened fluxes of most cool (T_eff< 3500 K) young stellar objects at the distance of Lupus occupy a gapbetween those typical both of field cool dwarfs and of backgroundgiants, we identify a new population of cool members of Lupus 1 and 3.The approximately 130 new members are only moderately concentratedtoward the densest clouds, they appear to have ages in the same range asthe known members, and very few show the infrared excess caused by warmdisks. This population is absent in Lupus 4. Conclusions: Thisnew population of Lupus members seems to be composed of stars and browndwarfs that have lost their inner disks on a timescale of a few Myr orless. Almost all these objects are in low extinction regions. Wespeculate that dissipation of unshielded disks caused by nearby O starsor fast collapse of the pre-(sub)stellar cores triggered by the passageof old supernova shocks may have led to disk properties and evolutionarypaths very different from those resulting from the more quiescentenvironment provided by dense molecular clouds.Based on observations carried out at the ESO/La Silla, Chile underproposals numbers 69.C-0260(A) and 71.C-0259(A).Tables 4 and 9 are only available in electronic form athttp://www.aanda.org
| Probing dust grain evolution in IM Lupi's circumstellar disc. Multi-wavelength observations and modelling of the dust disc
Aims. We present a panchromatic study, involving a multiple techniqueapproach, of the circumstellar disc surrounding the T Tauri star IM Lupi(Sz 82). Methods: We have undertaken a comprehensive observationalstudy of IM Lupi using photometry, spectroscopy, millimetreinterferometry and multi-wavelength imaging. For the first time, thedisc is resolved from optical and near-infrared wavelengths in scatteredlight, to the millimetre regime in thermal emission. Our data-set, inconjunction with existing photometric data, provides an extensivecoverage of the spectral energy distribution, including a detailedspectrum of the silicate emission bands. We have performed asimultaneous modelling of the various observations, using the radiativetransfer code MCFOST, and analysed a grid of models over a largefraction of the parameter space via Bayesian inference. Results: Wehave constructed a model that can reproduce all of the observations ofthe disc. Our analysis illustrates the importance of combining a widerange of observations in order to fully constrain the disc model, witheach observation providing a strong constraint only on some aspects ofthe disc structure and dust content. Quantitative evidence of dustevolution in the disc is obtained: grain growth up to millimetre-sizedparticles, vertical stratification of dust grains with micrometricgrains close to the disc surface and larger grains which have settledtowards the disc midplane, and possibly the formation of fluffyaggregates and/or ice mantles around grains.
| The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed with IRAC and MIPS
We present c2d Spitzer IRAC observations of the Lupus I, III, and IVdark clouds and discuss them in combination with optical, near-infrared,and c2d MIPS data. With the Spitzer data, the new sample contains 159stars, 4 times larger than the previous one. It is dominated by low- andvery low mass stars, and it is complete down to M~0.1 Msolar.We find 30%-40% binaries with separations between 100 and 2000 AU withno apparent effect on the disk properties of the members. A largemajority of the objects are Class II or III objects, with only 20 (12%)Class I or flat-spectrum sources. The disk sample is complete down to``debris''-like systems in stars as small as M~0.2 Msolar andincludes substellar objects with larger IR excesses. The disk fractionin Lupus is 70%-80%, consistent with an age of 1-2 Myr. However, theyoung population contains 20% optically thick accretion disks and 40%relatively less flared disks. A growing variety of inner disk structuresis found for larger inner disk clearings for equal disk masses. LupusIII is the most centrally populated and rich, followed by Lupus I with afilamentary structure and by Lupus IV, where a very high density corewith little star formation activity has been found. We estimate starformation rates in Lupus of 2-10 Msolar Myr-1 andstar formation efficiencies of a few percent, apparently correlated withthe associated cloud masses.
| Towards the main sequence: detailed analysis of weak line and post-T Tauri stars
A detailed study was performed for a sample of low-masspre-main-sequence (PMS) stars, previously identified as weak-line TTauri stars, which are compared to members of the Tucanae and HorologiumAssociations. Aiming to verify if there is any pattern of abundanceswhen comparing the young stars at different phases, we selected objectsin the range from 1 to 100 Myr, which covers most of PMS evolution.High-resolution optical spectra were acquired at European SouthernObservatory and Observatório do Pico dos Dias. The stellarfundamental parameters effective temperature and gravity were calculatedby excitation and ionization equilibria of iron absorption lines.Chemical abundances were obtained via equivalent width calculations andspectral synthesis for 44 per cent of the sample, which showsmetallicities within 0.5 dex solar. A classification was developed basedon equivalent width of LiI 6708 Å and Hα lines and spectraltypes of the studied stars. This classification allowed a separation ofthe sample into categories that correspond to different evolutive stagesin the PMS. The position of these stars in the Hertzsprung-Russelldiagram was also inspected in order to estimate their ages and masses.Among the studied objects, it was verified that our sample actuallycontains seven weak-line T Tauri stars, three are Classical T Tauri, 12are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequencestars. An estimation of circumstellar luminosity was obtained using adisc model to reproduce the observed spectral energy distribution. Mostof the stars show low levels of circumstellar emission, corresponding toless than 30 per cent of the total emission.
| The Evolution of the Multiplicity of Embedded Protostars. I. Sample Properties and Binary Detections
We present the observational results of a near-infrared survey of alarge sample of Class I protostars designed to determine the Class Ibinary separation distribution from ~100 AU to ~5000 AU. We haveselected targets from a new sample of 267 nearby candidate Class Iobjects. This sample is well understood, consists of mostly Class Iyoung stellar objects (YSOs) within 1 kpc, has targets selected from thewhole sky, and is not biased by previous studies of star formation. Wehave observed 189 Class I YSOs north of ? = -40° at the H, K,and L' bands, with a median angular resolution of 0farcs33 at L'. Wedetermine our detection limit for close binary companions by observingartificial binaries. We choose a contrast limit and an outer detectionlimit to minimize contamination and to ensure that a candidate companionis gravitationally bound. Our survey uses observations at the L' ratherthan the K band for the detection of binary companions since there isless scattered light and better seeing at L'. This paper presents thepositions of our targets, the near-IR photometry of sources detected inour fields at L', as well as the observed properties of the 89 detectedcompanions (73 of which are newly discovered). Although we have chosencontrast and separation limits to minimize contamination, we expect thatthere are about six stars identified as binary companions that are dueto contamination. Finder charts at L' for each field are shown tofacilitate future studies of these objects.The Infrared Telescope Facility is operated by the University of Hawaiiunder Cooperative Agreement no. NCC 5-538 with the National Aeronauticsand Space Administration, Science Mission Directorate, PlanetaryAstronomy Program. The United Kingdom Infrared Telescope is operated bythe Joint Astronomy Centre on behalf of the Science and TechnologyFacilities Council of the U.K. Based in part on data collected at theSubaru Telescope, which is operated by the National AstronomicalObservatory of Japan.
| Interacting jets from binary protostars
Aims: We investigate potential models that could explain why multipleproto-stellar systems predominantly show single jets. During theirformation, stars most frequently produce energetic outflows and jets.However, binary jets have only been observed in a very small number ofsystems. Methods: We model numerically 3D binary jets for variousoutflow parameters. We also model the propagation of jets from aspecific source, namely L1551 IRS 5, known to have two jets, usingrecent observations as constraints for simulations with a new MHD code.We examine their morphology and dynamics, and produce synthetic emissionmaps. Results: We find that the two jets interfere up to thestage where one of them is almost destroyed or engulfed into the secondone. We are able to reproduce some of the observational features ofL1551 such as the bending of the secondary jet. Conclusions:While the effects of orbital motion are negligible over the jetsdynamical timeline, their interaction has significant impact on theirmorphology. If the jets are not strictly parallel, as in most observedcases, we show that the magnetic field can help the collimation andrefocusing of both of the two jets.
| X-Winds in Action
The interaction of accretion disks with the magnetospheres of youngstars can produce X-winds and funnel flows. With the assumption of axialsymmetry and steady state flow, the problem can be formulated in termsof quantities that are conserved along streamlines, such as theBernoulli integral (BI), plus a partial differential equation (PDE),called the Grad-Shafranov equation (GSE), that governs the distributionof streamlines in the meridional plane. The GSE plus BI yields a PDE ofmixed type, elliptic before critical surfaces where the flow speedequals certain characteristic wave speeds are crossed and hyperbolicafterward. The computational difficulties are exacerbated by thelocations of the critical surfaces not being known in advance. Toovercome these obstacles, we consider a variational principle by whichthe GSE can be attacked by extremizing an action integral, with allother conserved quantities of the problem explicitly included as part ofthe overall formulation. To simplify actual applications we adopt thecold limit of a negligibly small ratio of the sound speed to the speedof Keplerian rotation in the disk where the X-wind is launched. We alsoignore the obstructing effects of any magnetic fields that might threada disk approximated to be infinitesimally thin. We then introduce trialfunctions with adjustable coefficients to minimize the variations thatgive the GSE. We tabulate the resulting coefficients so that otherworkers can have analytic forms to reconstruct X-wind solutions forvarious astronomical, cosmochemical, and meteoritical applications.
| Spatially extended polycyclic aromatic hydrocarbons in circumstellar disks around T Tauri and Herbig Ae stars
Aims.Our aim is to determine the presence and location of the emissionfrom polycyclic aromatic hydrocarbons (PAHs) towards low andintermediate mass young stars with disks using large aperturetelescopes. Methods: VLT-VISIR N-band spectra and VLT-ISAAC andVLT-NACO L-band spectra of 29 sources are presented, spectrallyresolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features.Spatial-extent profiles of the features and the continuum emission havebeen derived and used to associate the PAH emission with the disks. Theresults are discussed in the context of recent PAH-emission disk models. Results: The 3.3, 8.6, and 11.2 μm PAH features are detected towarda small fraction of the T Tauri stars, with typical upper limits between1 × 10-15 and 5 × 10-17 Wm-2. All 11.2 μm detections from a previous Spitzer surveyare confirmed with (tentative) 3.3 μm detections, and both the 8.6and the 11.2 μm features are detected in all PAH sources. For 6detections, the spatial extent of the PAH features is confined to scalestypically smaller than 0.12-0.34'', consistent with the radii of 12-60AU disks at their distances (typically 150 pc). For 3 additionalsources, WL 16, HD 100546, and TY CrA, one or more of the PAH featuresare more extended than the hot dust continuum of the disk, whereas forOph IRS 48, the size of the resolved PAH emission is confirmed assmaller than for the large grains. For HD 100546, the 3.3 μm emissionis confined to a small radial extent of 12±3 AU, most likelyassociated with the outer rim of the gap in this disk. Gaps with radiiout to 10-30 AU may also affect the observed PAH extent for othersources. For both Herbig Ae and T Tauri stars, the small measuredextents of the 8.6 and 11.2 μm features are consistent with larger(≥100 carbon atoms) PAHs.Based on observations obtained at the European Southern Observatory,Paranal, Chile, within the observing programs 164.I-0605 (ISAAC May2002), 074.C-0413 (NACO, March/April 2005), 075.C-0420 (ISAAC August2005), 077.C-0668 (VISIR/ISAAC April/May 2006). Appendix A is onlyavailable in electronic form at http://www.aanda.org
| The Spitzer c2d Survey of Weak-Line T Tauri Stars. II. New Constraints on the Timescale for Planet Building
One of the central goals of the Spitzer Legacy Project ``From Cores toDisks'' (c2d) is to determine the frequency of circumstellar disksaround weak-line T Tauri stars (WTTSs) and to study the properties andevolutionary status of these disks. Here we present a census of disksfor a sample of over 230 WTTSs located in the c2d IRAC and MIPS maps ofthe Ophiuchus, Lupus, and Perseus Molecular Clouds. We find that ~20% ofthe WTTSs in a magnitude-limited subsample have IR excesses at IRACwavelengths. These disks frequencies are ~3-6 times larger than thatrecently found for a sample of relatively isolated WTTSs located outsidethe highest extinction regions covered by the c2d maps. The diskfractions we find are more consistent with those obtained in recentSpitzer studies of WTTSs in young clusters such as IC 348 and Tr 37.According to their location in the H-R diagram, the WTTSs with excessesin our sample are among the younger part of the age distribution. Still,up to ~50% of the apparently youngest stars in the sample show noevidence of IR excess, suggesting that the circumstellar disks of asizable fraction of pre-main-sequence stars dissipate in a timescale of~1 Myr. We also find that none of the stars in our sample apparentlyolder than ~10 Myr have detectable circumstellar disks at wavelengths<24 ?m. The WTTS disks in our sample exhibit a wide range ofproperties (SED morphology, inner radius,Ldisk/L*, etc.) that bridge the gaps observedbetween the CTTSs and the debris disk regimes.
| The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IV. Lupus Observed with MIPS
We present maps of 7.78 deg2 of the Lupus molecular cloudcomplex at 24, 70, and 160 ?m. They were made with the Spitzer SpaceTelescope Multiband Imaging Photometer for Spitzer (MIPS) instrument aspart of the Spitzer Legacy Program ``From Molecular Cores toPlanet-Forming Disks'' (c2d). The maps cover three separate regions inLupus, denoted I, III, and IV. We discuss the c2d pipeline and how ourdata processing differs from it. We compare source counts in the threeregions with two other data sets and predicted star counts from theWainscoat model. This comparison shows the contribution from backgroundgalaxies in Lupus I. We also create two color-magnitude diagrams usingthe 2MASS and MIPS data. From these results, we can identify backgroundgalaxies and distinguish them from probable young stellar objects. Thesources in our catalogs are classified based on their spectral energydistribution (SED) from 2MASS and Spitzer wavelengths to create a sampleof young stellar object candidates. From 2MASS data, we createextinction maps for each region and note a strong correspondence betweenthe extinction and the 160 ?m emission. The masses we derived in eachLupus cloud from our extinction maps are compared to masses estimatedfrom 13CO and C18 O and found to be similar to ourextinction masses in some regions, but significantly different inothers. Finally, based on our color-magnitude diagrams, we selected 12of our reddest candidate young stellar objects for individualdiscussion. Five of the 12 appear to be newly discovered YSOs.
| 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.
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| Takýmyýldýz: | Kurt |
| Sað Açýklýk: | 15h45m12.86s |
| Yükselim: | -34°17'30.6" |
| Görünürdeki Parlaklýk: | 10.351 |
| özdevim Sað Açýklýk: | -18.3 |
| özdevim Yükselim: | -19.6 |
| B-T magnitude: | 11.866 |
| V-T magnitude: | 10.477 |
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