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Resim Yükleyin
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Ýlgili Makaleler
Evidence for Accretion in the High-Resolution X-Ray Spectrum of the T Tauri Star System Hen 3-600
We present high-resolution X-ray spectra of the nearby multiple T Tauristar (TTS) system Hen 3-600, obtained with the High Energy TransmissionGrating Spectrograph (HETGS) aboard the Chandra X-Ray Observatory. Bothprincipal binary components of Hen 3-600 (A and B, separation 1.4") weredetected in the zeroth-order Chandra HETGS X-ray image. Hen 3-600-A, thecomponent with a large mid-infrared excess, is a factor of ~2-3 fainterin X-rays than Hen 3-600-B, due to a large flare at Hen 3-600-B duringour observation. The dispersed X-ray spectra of the two primarycomponents overlap significantly, so spectral analysis was performedprimarily on the first-order spectrum of the combined A + B system, withanalysis of the individual dispersed spectra limited to regions wherethe contributions of A and B can be disentangled via cross-dispersionprofile fitting. This analysis results in two lines of evidenceindicating that the X-ray emission from Hen 3-600 A + B is derived, inpart, from accretion processes. (1) The line ratios of He-like O VII inthe spectrum of Hen 3-600 A + B indicate that the characteristic densityof its X-ray-emitting plasma is significantly larger than those ofcoronally active main-sequence and pre-main-sequence stars. (2) Asignificant component of low-temperature (2-3 MK) plasma is present inthe Hen 3-600 A + B spectrum; this ``soft excess'' appears somewhatstronger in component A. These results for Hen 3-600 A + B areconsistent with, although less pronounced than, results obtained fromX-ray grating spectroscopy of more rapidly accreting TTS systems.Indeed, all of the emission signatures of Hen 3-600 A + B that arepotential diagnostics of accretion activity-from its high-resolutionX-ray spectrum, through its UV excess and Hα emission-linestrengths, to its weak near-infrared excess-suggest that its components(and component A in particular) represent a transition phase betweenrapidly accreting, classical T Tauri stars and nonaccreting, weak-linedT Tauri stars.
| Extended Wind in Jetless Classical T Tauri Star TW Hya
We have conducted a spectro-astrometric (SA) analysis of high spectralresolution data of the near-infrared He I λ10830 and Paγlines in the nearby classical T Tauri star TW Hya. We find clearposition offsets associated with the blueshifted absorption part of theHe I λ10830 P Cygni profile. The derived spatial features extendup to 50 mas (2.8 AU at TW Hya distance) in two opposite directions. Byusing simple exploratory models we show that this feature cannot beproduced by the same stellar wind that produces the P Cygni profile.Instead, we are able to reproduce the observed blueshifted SA profilewith emission from a disk wind. The production of SA artifacts throughinstrumental effects was examined. Artifact models have difficulties infitting both the PSF and the angular scale of the observed positionspectra offsets, suggesting that the signal may be real.
| An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| Debris Disks in NGC 2547
We have surveyed the 30 Myr old cluster NGC 2547 for planetary debrisdisks using Spitzer. At 4.5-8 μm we are sensitive to the photosphericlevel down to mid-M stars (0.2 Msolar), and at 24 μm toearly G stars (1.2 Msolar). We find only two to four starswith excesses at 8 μm out of ~400-500 cluster members, resulting inan excess fraction <~1% at this wavelength. By contrast, the excessfraction at 24 μm is ~40% (for B-F types). Out of four late-typestars with excesses at 8 μm two marginal ones are consistent withasteroid-like debris disks. Among stars with strong 8 μm excesses oneis possibly from a transitional disk, while another one can be a resultof a catastrophic collision. Our survey demonstrates that the inner0.1-1 AU parts of disks around solar-type stars clear out verythoroughly by 30 Myr of age. Comparing with the much slower decay ofexcesses at 24 and 70 μm, disks clear from the inside out, of order10 Myr for the inner zones probed at 8 μm, compared with 100 or moremegayears for those probed with the two longer wavelengths.This paper includes data gathered with the 6.5 m Magellan telescopeslocated at Las Campanas Observatory, Chile.
| High-Resolution Spectroscopy of [Ne II] Emission from TW Hydrae
We present high-resolution echelle spectra of [Ne II] 12.81 μmemission from the classical T Tauri star (CTTS) TW Hya, obtained withMICHELLE on Gemini North. The line is centered at the stellar radialvelocity and has an intrinsic FWHM of 21+/-4 km s-1. The linewidth is broader than other narrow emission lines typically associatedwith the disk around TW Hya. If formed in a disk, the line broadeningcould result from turbulence in a warm disk atmosphere, Keplerianrotation at an average distance of 0.1 AU from the star, or aphotoevaporative flow from the optically thin region of the disk. Weplace upper limits on the [Ne II] emission flux from the CTTSs DP Tauand BP Tau.
| High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region
We present multiwavelength, high spatial resolution imaging of the IRS 7region in the R Corona Australis molecular cloud. Our observationsinclude 1.1 mm continuum and HCO+ J=3-->2 images from theSubmillimeter Array (SMA), 12CO J=3-->2 outflow maps fromthe DesertStar heterodyne array receiver on the Heinrich Hertz Telescope(HHT), 450 and 850 μm continuum images from SCUBA, and archivalSpitzer IRAC and MIPS 24 μm images. The accurate astrometry of theIRAC images allow us to identify IRS 7 with the centimeter source VLA10W (IRS 7A) and the X-ray source XW. The SMA 1.1 mm imagereveals two compact continuum sources that are also distinguishable at450 μm. SMA 1 coincides with X-ray source CXOU J190156.4-365728 andVLA centimeter source 10E (IRS 7B) and is seen in the IRAC and MIPSimages. SMA 2 has no infrared counterpart but coincides with centimetersource VLA 9. Spectral energy distributions constructed from SMA, SCUBA,and Spitzer data yield bolometric temperatures of 83 K for SMA 1 and<=70 K for SMA 2. These temperatures along with the submillimeter tototal luminosity ratios indicate that SMA 2 is a Class 0 protostar,while SMA 1 is a Class 0/Class I transitional object (L=17+/-6Lsolar). The 12CO J=3-->2 outflow map shows onemajor and possibly several smaller outflows centered on the IRS 7region, with masses and energetics consistent with previous work. Weidentify the Class 0 source SMA 2/VLA 9 as the main driver of thisoutflow. The complex and clumpy spatial and velocity distribution of theHCO+ J=3-->2 emission is not consistent with either bulkrotation, or any known molecular outflow activity.
| Discovery of an M9.5 Candidate Brown Dwarf in the TW Hydrae Association: DENIS J124514.1-442907
We report the discovery of a fifth candidate substellar system in the~5-10 Myr TW Hydrae association: DENIS J124514.1-442907. This object hasa NIR spectrum remarkably similar to that of 2MASS J1139511-315921, aknown TW Hydrae brown dwarf, with low surface gravity features such as atriangular-shaped H band, deep H2O absorption, weak alkalilines, and weak hydride bands. We find an optical spectral type of M9.5and estimate a mass of <~24 MJup, assuming an age of ~5-10Myr. While the measured proper motion for DENIS J124514.1-442907 isinconclusive as a test for membership, its position in the sky iscoincident with the TW Hydrae association. A more accurate proper-motionmeasurement, higher resolution spectroscopy for radial velocity, and aparallax measurement are needed to derive the true space motion and toconfirm its membership.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. This Letter includes data gathered with the 6.5 m MagellanTelescopes located at Las Campanas Observatory, Chile.
| The Trigonometric Parallax of the Brown Dwarf Planetary System 2MASSW J1207334-393254
We have measured a trigonometric parallax to the young brown dwarf2MASSW J1207334-393254. The distance (54.0+3.2-2.8pc) and space motion confirm membership in the TW Hydrae Association.The primary is a ~25 MJup brown dwarf. We discuss the``planetary mass'' secondary, which is certainly below thedeuterium-burning limit but whose colors and absolute magnitudes posechallenges to our current understanding of planetary-mass objects.
| A Direct Distance and Luminosity Determination for a Self-luminous Giant Exoplanet: The Trigonometric Parallax to 2MASSW J1207334-393254Ab
We present the first trigonometric parallax and distance for a youngplanetary mass object. A likely TW Hya cluster member, 2MASSWJ1207334-393254Ab (hereafter 2M1207Ab), is an M8 brown dwarf with amid-to-late L-type planetary mass companion. Recent observations ofspectral variability have uncovered clear signs of disk accretion andoutflow, constraining the age of the system to <10 Myr. Because ofits late spectral type and the clearly youthful nature of the system,2M1207b is very likely a planetary mass object. We have measured thefirst accurate distance and luminosity for a self-luminous planetarymass object. Our parallax measurements are accurate to <2 mas (1σ) for 2M1207Ab. With 11 total epochs of data taken from 2006January through 2007 April (475 images for 2M1207Ab), we determine adistance of 58.8+/-7.0 pc (17.0+2.3-1.8 mas, 1.28σ) to 2M1207Ab and a calculated luminosity of(0.68-2.2)×10-5 Lsolar for 2M1207b. Hence,2M1207Ab is a clear member of the TW Hya cluster in terms of itsdistance, proper motions, and youthful nature. However, as previouslynoted by Mohanty and coworkers, 2M1207b's luminosity appears lowcompared to its temperature according to evolutionary models.
| The X-ray soft excess in classical T Tauri stars
Aims.We study an anomaly in the X-ray flux (or luminosity) ratio betweenthe O vii λλ21.6-22.1 triplet and the O viii Lyαline seen in classical T Tauri stars (CTTS). This ratio is unusuallyhigh when compared with ratios for main-sequence and non-accreting TTauri stars (Telleschi et al. 2007c, A&A, 468, 443). We comparethese samples to identify the source of the excess. A sample of recentlydiscovered X-ray stars with a soft component attributed to jet emissionis also considered. Methods: We discuss data obtained from theXMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) project,complemented by data from the published literature. We also present datafrom the CTTS RU Lup. Results: All CTTS in the sample show an anomalousO vii/O viii flux ratio when compared with WTTS or MS stars. The anomalyis due to an excess of cool, O vii emitting material rather than adeficiency of hotter plasma. The excess plasma must therefore havetemperatures of ⪉2 MK. This soft excess does not correlate with UVexcesses of CTTS, but seems to be related to the stellar X-rayluminosity. The spectra of the jet-driving TTS do not fit into thiscontext. Conclusions: The soft excess depends both on the presence ofaccretion streams in CTTS and on magnetic activity. The gas may beshock-heated near the surface, although it may also be heated in themagnetospheric accretion funnels. The soft component of the jet-drivingsources is unlikely to be due to the same process.
| Coronal abundances of X-ray bright pre-main sequence stars in the Taurus molecular cloud
Aims.We studied the thermal properties and chemical composition of theX-ray emitting plasma of a sample of bright members of the TaurusMolecular Cloud to investigate possible differences among classical andweak-lined T Tauri stars, and possible dependences of the abundances onthe stellar activity level and/or on the presence ofaccretion/circumstellar material. Methods: We used medium-resolutionX-ray spectra obtained with the sensitive EPIC/PN camera in order toanalyze the possible sample. The PN spectra of 20 bright (LX~ 1030-1031 erg s-1) Taurus members,with at least ~ 4500 counts, were fitted using thermal models ofoptically thin plasma with two components and variable abundances of O,Ne, Mg, Si, S, Ar, Ca, and Fe. Extensive preliminary investigations wereemployed to study the performances of the PN detectors regardingabundance determinations, and finally to check the results of thefittings. Results: We found that the observed X-ray emission of thestudied stars can be attributed to coronal plasma having similar thermalproperties and chemical composition both in the classical and in theweak-lined T Tauri stars. The results of the fittings did not reveal acorrelation between the abundance patterns and activity oraccretion/disk presence. The iron abundance of these active stars issignificantly lower than the solar photospheric value (~ 0.2 solar). Anindication of slightly different coronal properties in stars withdifferent spectral type is found in this study. G-type and early K-typestars have, on average, slightly higher Fe abundances (Fe ~ 0.24 solar)than stars with later spectral type (Fe ~ 0.15 solar), confirmingprevious findings from high-resolution X-ray spectroscopy. Stars of theformer group are also found to have, on average, hotter coronae.
| Dust sedimentation in protoplanetary disks with polycyclic aromatic hydrocarbons
Context: Dust sedimentation is known to affect the infrared spectra andimages of disks. In particular the far-infrared emission may be reducedby strong sedimentation. However, dust grains of different sizessediment to different depths in the disk. Spectral features of onespecies may thus be enhanced, while those of other species may besuppressed. Aims: Polycyclic aromatic hydrocarbons (PAHs) are among thesmallest “dust grains”. We investigate how the presence ofPAHs in protoplanetary disks affects the disk's spectral energydistribution (SED) and feature strengths when the thermal (large) grainsare allowed to sediment. Methods: We calculate the verticaldistribution of dust grains for both the PAH “dust” and thethermal dust grains. We include vertical settling and vertical mixingvia turbulence. The results are inserted into a Monte-Carlo radiativetransfer code to compute the SEDs. Results: For high turbulence thesedimentation barely affects the spectrum. For low turbulence, however,the PAHs still stay well-mixed in the disk's surface layer, while the0.1 μm size grains sediment deep into the disk. This stronglyenhances the PAH features relative to the continuum (by factors of 2 to10), while the far-infrared flux is reduced. This predicts that sourceswith weak far-infrared flux have stronger PAH features, which is - atleast among Herbig Ae stars - opposite to what is observed, suggestingthat sedimentation is not the only factor responsible for the weak mid-to far-infrared excess in some disks. We speculate that coagulationmight be a solution, reducing both the mid- to far-infrared flux and thePAH features.
| X-rays from RU Lupi: accretion and winds in classical T Tauri stars
Context: Low-mass stars are known to exhibit strong X-ray emissionduring their early evolutionary stages. This also applies to classical TTauri stars (CTTS), whose X-ray emission differs from that ofmain-sequence stars in a number of aspects. Aims: We study the specificcase of RU Lup, a well known accreting and wind-driving CTTS. Incomparison with other bright CTTS we study possible signatures ofaccretion and winds in their X-ray emission. Methods: Using threeXMM-Newton observations of RU Lup, we investigate its X-ray propertiesand their generating mechanisms. High-resolution X-ray spectra of RU Lupand other CTTS are compared to main-sequence stars. We examine thepresence of a cool plasma excess and enhanced plasma density in relationto X-rays from accretion shocks and investigate anomalous strong X-rayabsorption and its connection to winds or circumstellar material. Results: We find three distinguishable levels of activity among theobservations of RU Lup. While no large flares are present, thisvariability is clearly of magnetic origin due to the correspondingplasma temperatures of around 30 MK; in contrast the cool plasmacomponent at 2-3 MK is quite stable over a month, resulting in a drop ofaverage plasma temperature from 35 MK down to 10 MK. Density analysiswith the O VII triplet indicates high densities in the cool plasma,suggesting accretion shocks to be a significant contributor to the softX-ray emission. No strong overall metal depletion is observed, with Nebeing more abundant than Fe, that is at solar value, and especially O.Excess emission at 6.4 keV during the more active phase suggest thepresence of iron fluorescence. Additionally RU Lup exhibits anextraordinary strong X-ray absorption, incompatible with estimatesobtained at optical and UV wavelengths. Comparing spectra from a sampleof main-sequence stars with those of accreting stars we find an excessof cool plasma as evidenced by lower O VIII/O VII line ratios in allaccreting stars. High density plasma appears to be only present inlow-mass CTTS, while accreting stars with intermediate masses(≳2~Mȯ) have lower densities. Conclusions: In allinvestigated CTTS the characteristics of the cooler X-ray emittingplasma are influenced by the accretion process. We suspect differentaccretion rates and amounts of funnelling, possibly linked to stellarmass and radius, to be mainly responsible for the different propertiesof their cool plasma component. The exceptional X-ray absorption in RULup and other CTTS is probably related to the accretion flows and anoptically transparent wind emanating from the star or the disk.
| η Chamaeleontis: abnormal initial mass function or dynamical evolution?
Context: η Chamaeleontis is a unique young (~9 Myr) association with18 systems concentrated in a radius of ≈35 arcmin, i.e. 1 pc at thecluster distance of 97 pc. No other members have been found up to 1.5degrees from the cluster centre. The cluster mass function is consistentwith the IMF of other rich young open clusters in the higher mass rangebut shows a clear deficit of low mass stars and brown dwarfs with noobjects below 0.1 Mȯ. Aims: The aim of this paper is totest whether this peculiar mass function could result from dynamicalevolution despite the young age of the cluster. Methods: We performedN-body numerical calculations starting with a log-normal IMF anddifferent initial conditions in terms of number of systems and clusterradius using the code NBODY3. We simulated the cluster dynamicalevolution over 10 Myr and compared the results to the observations. Results: We found that it is possible to reproduce η Cha whenstarting with a very compact configuration (with N_init=40 and R_0=0.005pc) which suggests that the IMF of the association might not beabnormal. The high initial density might also explain the deficit ofwide binaries that is observed in the cluster.
| Light-induced disassembly of dusty bodies in inner protoplanetary discs: implications for the formation of planets
Laboratory experiments show that a solid-state greenhouse effect incombination with thermophoresis can efficiently erode a dust bed in alow-pressure gaseous environment. The surface of an illuminated, lightabsorbing dusty body is cooler than the dust below the surface(solid-state greenhouse effect). This temperature gradient leads to adirected momentum transfer between gas and dust particles and the dustparticles are subject to a force towards the surface (thermophoresis).If the thermophoretic force is stronger than gravity and cohesion, dustparticles are ejected. Applied to protoplanetary discs, dusty bodiessmaller than several kilometres in size which are closer to a star than~0.4 au are subject to a rapid and complete disassembly to submillimetresize dust aggregates by this process. While an inward-drifting dustybody is destroyed, the generated dust is not lost for the disc bysublimation or subsequent accretion on to the star but can bereprocessed by photophoresis or radiation pressure. Planetesimals cannotoriginate through aggregation of dust inside the erosion zone. Ifobjects larger than several kilometres already exist, they prevail andfurther grow by collecting dust from disassembled smaller bodies. Thepile-up of solids in a confined inner region of the disc, in general,boosts the formation of planets. Erosion is possible in even stronglygas-depleted inner regions as observed for TW Hya. Reprocessing of dustthrough light-induced erosion offers one possible explanation for growthof large cores of gas-poor giant planets in a gas-starved region asrecently found around HD 149026b.
| XMM-Newton observation of the classical T Tauri star SU Aurigae and the surrounding field
Aims. We investigate the properties of the X-ray emitting plasma of theclassical T Tauri star SU Aurigae and of other sources in the field ofview. Methods: We use XMM-Newton to obtain a high-resolution RGSspectrum of SU Aur as well as EPIC imaging data and low-resolutionspectra of the star and of other X-ray sources in the surrounding field.We reconstruct the emission measure distribution of SU Aur from the RGSspectrum using a line-based method, and we perform multi-temperaturefits of the MOS spectra of the strongest sources both for the fullobservation and for selected time intervals to study their spectralvariability. Results: The emission from SU Aur is highly variable,showing three flares during the observation. The MOS spectra indicate avery hot corona, with significant emissivity up to ~40 MK in quiescence,and temperatures up to 140 MK during flares. The emission measuredistribution derived from the RGS spectrum peaks at log T = 7.0; anycontribution to the X-ray luminosity from cool plasma (T˜ 2 MK)cannot exceed 5% of the total emission. Abundances are ~0.3-0.6 solarwith the exception of Mg and Ne that are solar. Spatial analysis of thefull EPIC field results in the detection of 104 X-ray sources, 6 ofwhich are associated with the known Taurus-Auriga members in the fieldof view (including SU Aur). Conclusions: The characteristics of theX-ray emission of SU Aur are very similar to those of young activelate-type stars, with a very hot corona and flares, suggesting magneticactivity as the origin of most of the X-ray emission, rather thanaccretion.Table 2 is only available in electronic form at http://www.aanda.org
| Soft X-ray emissions of Si IX in Procyon
A detailed analysis of emission lines of carbon-like silicon revealsthat some ratios of n = 3 → 2 line intensities are sensitive to theelectron density, ne. The ratio between two groups of 3d→ 2p transition lines of 55.246 Å and 55.346 Å providesa good diagnostic of ne because of the combinedcharacteristic of sensitivity to electron density and relativeinsensitivity to temperature. From this ratio, a lower limit of theelectron density of 0.6 × 108 cm‑3 wasset for Procyon, which is consistent with the values constrained by C Vand Si X emission lines. Significant discrepancies were found betweentheoretical predictions and observations for the 3s → 2p linesrelative to 3d → 2p lines in Procyon, recently measured using theChandra high-resolution transmission grating instrument. The differenceof more than a factor of 3, cannot be explained by uncertainties ofatomic data. Ness and co-workers also suggested that the effect ofopacity appeared not to be a major factor for the discrepancy. For the3s → 2p line at 61.611 Å, present work indicates that thelarge discrepancy may be from the contamination of a S VIII line at61.645 Å. For lines at 61.702 and 61.846 Å, we suggest thatthe discrepancies may be attributed to contaminations by currentlyyet-unknown spectral lines.
| Molecular hydrogen emission from discs in the η Chamaeleontis cluster
Discs in the 6 Myr old cluster η Chamaeleontis were searched foremission from hot H2. Around the M3 star ECHA J0843.3-7905,we detect circumstellar gas orbiting at ~2 au. If the gas is ultravioletexcited, the ro-vibrational line traces a hot gas layer supported by adisc of mass ~0.03Msolar, similar to the minimum mass solarnebula. Such a gas reservoir at 6 Myr would promote the formation andthe inwards migration of gas giant planets.
| The Disk and Environment of a Young Vega Analog: HD 169142
We trace the disk of HD 169142 (A8 Ve) from 0.57" to 1.4" (~80-200 AUprojected distance) in 1.1 μm scattered light with HST NICMOScoronagraphy. The azimuthally symmetric disk has a peak azimuthallymedianed surface brightness (SB) of ~5 mJy arcsec-2 at 0.57"from the star, and drops ~r-3. This radial SB profile isconsistent with the presence of spatially resolved PAH emission and aMeeus group I IR SED only if the inner disk is either substantiallyflatter than the outer disk or partially devoid of material. Analysis ofnew HST ACS FUV imagery in tandem with archival IUE data indicatesM˙acc<=10-9 Msolaryr-1. We estimate the age of HD 169142 to be6+6-3 Myr by identifying 2MASS 18242929-2946559,located 9.3" to the southwest, as a 130 mas separation weak-line T Tauribinary that is comoving with HD 169142 at the 4 σ confidencelevel. We find no evidence for any additional stellar companion ineither the ACS or Chandra ACIS-S data at r<=1''. HD169142 has previously been interpreted as a slowly rotating, chemicallypeculiar star. However, by combining the disk inclination and vsini fromthe literature, we find that the star has vequatorial~240 kms-1, making it a rapid rotator, similar to Altair or Vega.The UV data for HD 169142 are consistent with gravity darkening, whilethe X-ray luminosity and spectrum resembles early F stars at the age ofthe β Pictoris moving group, rather than mid-A stars. In thiscontext, spectral features previously interpreted as evidence forchemical peculiarity are more likely to reflect the presence of a strongphotospheric latitudinal temperature gradient. With such a gradient, HD169142 should closely resemble Vega at the epoch of central diskclearing.
| On X-Ray Optical Depth in the Coronae of Active Stars
We have investigated the optical thickness of the coronal plasma throughthe analysis of high-resolution X-ray spectra of a large sample ofactive stars observed with the High Energy Transmission GratingSpectrometer on Chandra. In particular, we probed for the presence ofsignificant resonant scattering in the strong Lyman series lines arisingfrom hydrogen-like oxygen and neon ions. The active RS CVn-type binariesII Peg and IM Peg and the single M dwarf EV Lac show significant opticaldepth. For these active coronae, the Lyα/Lyβ ratios aresignificantly depleted as compared with theoretical predictions and withthe same ratios observed in similar active stars. Interpreting thesedecrements in terms of resonance scattering of line photons out of theline of sight, we are able to derive an estimate for the typical size ofcoronal structures, and from these we also derive estimates of coronalfilling factors. For all three sources we find that both the photon pathlength as a fraction of the stellar radius and the implied surfacefilling factors are very small and amount to a few percent at most. Themeasured Lyα/Lyβ ratios are in good agreement with APEDtheoretical predictions, thus indicating negligible optical depth, forthe other sources in our sample. We discuss the implications for coronalstructuring and heating flux requirements. For the stellar sample as awhole, the data suggest increasing quenching of Lyα relative toLyβ as a function of both LX/Lbol and thedensity-sensitive Mg XI forbidden-to-intercombination line ratio, asmight generally be expected.
| Hubble Space Telescope Advanced Camera for Surveys Coronagraphic Observations of the Dust Surrounding HD 100546
We present HST ACS coronagraphic observations of HD 100546, a B9.5 star,103 pc away from the Sun, taken in the F435W, F606W, and F814W bands.Scattered light is detected up to 14" from the star. The observationsare consistent with the presence of an extended flattened nebula withthe same inclination as the inner disk. The well-known ``spiral arms''are clearly observed and trail the rotating disk material. Weaker armsnever before reported are also seen. The interarm space becomesbrighter, but the structures become more neutral in color at longerwavelengths, which is not consistent with models that assume that theyare due to the effects of a warped disk. Along the major disk axis, thecolors of the scattered light relative to the star areΔ(F435W-F606W)~0.0-0.2 mag and Δ(F435W-F814W)~0.5-1 mag. Toexplain these colors, we explore the role of asymmetric scattering,reddening, and large minimum sizes on ISM-like grains. We conclude thateach of these hypotheses by itself cannot explain the colors. The diskcolors are similar to those derived for Kuiper Belt objects, suggestingthat the same processes responsible for their colors may be at workhere. We argue that we are observing only the geometrically thick,optically thin envelope of the disk, while the optically thick diskresponsible for the far-IR emission is undetected. The observed spiralarms are then structures on this envelope. The colors indicate that theextended nebulosity is not a remnant of the infalling envelope butreprocessed disk material.
| Probing the Dust and Gas in the Transitional Disk of CS Cha with Spitzer
Here we present the Spitzer IRS spectrum of CS Cha, a member of the ~2Myr old Chamaeleon star-forming region, which reveals an optically thickcircumstellar disk truncated at ~43 AU, the largest hole modeled in atransitional disk to date. Within this inner hole,~5×10-5 lunar masses of dust are located in a smalloptically thin inner region that extends from 0.1 to 1 AU. In addition,the disk of CS Cha has bigger grain sizes and more settling than thepreviously modeled transitional disks DM Tau, GM Aur, and CoKu Tau/4,suggesting that CS Cha is in a more advanced state of dust evolution.The Spitzer IRS spectrum also shows [Ne II] 12.81 μm fine-structureemission with a luminosity of 1.3×1029 ergss-1, indicating that optically thin gas is present in this~43 AU hole, in agreement with Hα measurements and a UV excessthat indicate that CS Cha is still accreting 1.2×10-8Msolar yr-1. We do not find a correlation of the[Ne II] flux with LX; however, there is a possiblecorrelation with M˙, which if confirmed would suggest that EUVfluxes due to accretion are the main agent for formation of the [Ne II]line.
| Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dustemission from their inner 15-50 AU. All four stars have F-G spectraltype and were uncovered as part of the Spitzer Space Telescope ``Coresto Disks'' Legacy Program Infrared Spectrograph (IRS) first-look surveyof ~100 pre-main-sequence stars. Modeling of the spectral energydistributions indicates a reduction in dust density by factors of100-1000 from disk radii between ~0.4 and 15-50 AU but with massivegas-rich disks at larger radii. This large contrast between the innerand outer disk has led us to use the term ``cold disks'' to distinguishthese unusual systems. However, hot dust [(0.02-0.2)Mmoon] isstill present close to the central star (R<=0.8 AU). We introduce the30 μm/13 μm flux density ratio as a new diagnostic for identifyingcold disks. The mechanisms for dust clearing over such large gaps arediscussed. Although rare, cold disks are likely in transition from anoptically thick to an optically thin state and so offer excellentlaboratories for the study of planet formation.
| HD 98800: A 10 Myr Old Transition Disk
We present the mid-infrared spectrum, obtained with the Spitzer InfraredSpectrograph (IRS), of HD 98800, a quadruple star system located in the10 Myr old TW Hydrae association. It has a known mid-infrared excessthat arises from a circumbinary disk around the B components of thesystem. The IRS spectrum confirms that the disk around HD 98800Bdisplays no excess emission below about 5.5 μm, implying an opticallythick disk wall at 5.9 AU and an inner, cleared-out region; however,some optically thin dust, consisting mainly of 3 μm-sized silicatedust grains, orbits the binary in a ring between 1.5 and 2 AU. Thepeculiar structure and apparent lack of gas in the HD 98800B disksuggests that this system is likely already at the debris disks stage,with a tidally truncated circumbinary disk of larger dust particles andan inner, second-generation dust ring, possibly held up by theresonances of a planet. The unusually large infrared excess can beexplained by gravitational perturbations of the Aa+Ab pair puffing upthe outer dust ring and causing frequent collisions among the largerparticles.
| The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars(CTTSs). We combine these data with one previous field determination inorder to compare our observed field strengths with the field strengthspredicted by magnetospheric accretion models. We use literature data onthe stellar mass, radius, rotation period, and disk accretion rate topredict the field strength that should be present on each of our starsaccording to these magnetospheric accretion models. We show that ourmeasured field values do not correlate with the field strengthspredicted by simple magnetospheric accretion theory. We also use ourfield strength measurements and literature X-ray luminosity data to testa recent relationship expressing X-ray luminosity as a function ofsurface magnetic flux derived from various solar feature andmain-sequence star measurements. We find that the T Tauri stars we haveobserved have weaker than expected X-ray emission by over an order ofmagnitude on average using this relationship. We suggest the cause forthis is actually a result of the very strong fields on these stars,which decreases the efficiency with which gas motions in the photospherecan tangle magnetic flux tubes in the corona.
| High spatial resolution mid-infrared observations of the low-mass young star TW Hydrae
Aims.We want to improve knowledge of the structure of the inner few AUof the circumstellar disk around the nearby T Tauri star TWHya. Earlier studies have suggested the existence of a largeinner hole, possibly caused by interactions with a growing protoplanet. Methods: We used interferometric observations in the N-band obtainedwith the MIDI instrument on the Very Large Telescope Interferometer,together with 10 μm spectra recorded by the infrared satelliteSpitzer. The fact that we were able to determine N-band correlatedfluxes and visibilities for this comparatively faint source shows thatmid-infrared interferometry can be applied to a large number of low-massyoung stellar objects. Results: The mid-infrared spectra obtained withSpitzer reveal emission lines from H I (6-5), H I (7-6), and [Ne II] andshow that over 90% of the dust we see in this wavelength regime isamorphous. According to the correlated flux measured with MIDI, most ofthe crystalline material is in the inner, unresolved part of the disk,about 1 AU in radius. The visibilities exclude the existence of a verylarge (3{-}4 AU radius) inner hole in the circumstellar disk of TW Hya,which was required in earlier models. We propose instead a geometry ofthe inner disk where an inner hole still exists, but at a much reducedradius, with the transition from zero to full disk height between 0.5and 0.8 AU, and with an optically thin distribution of dust inside. Sucha model can comply with SED and mid-infrared visibilities, as well aswith visibility and extended emission observed in the near-infrared at 2μm. If a massive planet was the reason for this inner hole, as hasbeen speculated, its orbit would have to be closer to the star than 0.3AU. Alternatively, we may be witnessing the end of the accretion phaseand an early phase of an inward-out dispersal of the circumstellar disk.Based on observations with the Very Large TelescopeInterferometer (VLTI, proposal 075.C-0014).
| An Inner Hole in the Disk around TW Hydrae Resolved in 7 mm Dust Emission
We present Very Large Array observations at 7 mm wavelength that resolvethe dust emission structure in the disk around the young star TW Hydraeat the scale of the ~4 AU (0.16") radius inner hole inferred fromspectral energy distribution modeling. These high-resolution datadirectly confirm the presence of an inner hole in the dust disk andreveal a high-brightness ring that we associate with the directlyilluminated inner edge of the disk. The clearing of the inner diskplausibly results from the dynamical effects of a giant planet information. In an appendix, we develop an analytical framework for theinterpretation of visibility curves from power-law disk models withinner holes.
| Mass and Temperature of the TWA 7 Debris Disk
We present photometric detections of dust emission at 850 and 450 μmaround the pre-main-sequence M1 dwarf TWA 7 using the SCUBA camera onthe James Clerk Maxwell Telescope. These data confirm the presence of acold dust disk around TWA 7, a member of the TW Hydrae Association(TWA). Based on the 850 μm flux, we estimate the mass of the disk tobe 18 Mlunar (0.2 M⊕) assuming a massopacity of 1.7 cm2 g-1 with a temperature of 45 K.This makes the TWA 7 disk (d=55 pc) an order of magnitude more massivethan the disk reported around AU Microscopii (GL 803), the closest (9.9pc) debris disk detected around an M dwarf. This is consistent with TWA7 being slightly younger than AU Mic. We find that the mid-IR andsubmillimeter data require the disk to be comprised of dust at a rangeof temperatures. A model in which the dust is at a single radius fromthe star, with a range of temperatures according to grain size, is aseffective at fitting the emission spectrum as a model in which the dustis of uniform size, but has a range of temperatures according todistance. We discuss this disk in the context of known disks in the TWAand around low-mass stars; a comparison of masses of disks in the TWAreveals no trend in mass or evolutionary state (gas-rich vs. debris) asa function of spectral type.
| Detection of [Ne II] Emission from Young Circumstellar Disks
We report the detection of [Ne II] emission at 12.81 μm in four outof the six optically thick dust disks observed as part of the FEPSSpitzer Legacy program. In addition, we detect a H I (7-6) emission lineat 12.37 μm from the source RX J1852.3-3700. Detections of [Ne II]lines are favored by low mid-infrared excess emission. Both stellarX-rays and extreme ultraviolet (EUV) photons can sufficiently ionize thedisk surface to reproduce the observed line fluxes, suggesting thatemission from Ne+ originates in the hot disk atmosphere. Onthe other hand, the H I (7-6) line is not associated with the gas inthe disk surface, and magnetospheric accretion flows can account for atmost ~30% of the observed flux. We conclude that accretion shock regionsand/or the stellar corona could contribute to most of the H I (7-6)emission. Finally, we discuss the observations necessary to identifywhether stellar X-rays or EUV photons are the dominant ionizationmechanism for Ne atoms. Because the observed [Ne II] emission probesvery small amounts of gas in the disk surface (~10-6MJ) we suggest using this gas line to determine the presenceor absence of gas in more evolved circumstellar disks.
| Structure and Dust Composition of the TW Hya Disc
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| Takýmyýldýz: | Su Yilani |
| Sað Açýklýk: | 11h01m51.89s |
| Yükselim: | -34°42'17.2" |
| Görünürdeki Parlaklýk: | 11.274 |
| özdevim Sað Açýklýk: | -73.4 |
| özdevim Yükselim: | -17.5 |
| B-T magnitude: | 12.091 |
| V-T magnitude: | 11.342 |
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