Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| Spectrally Dispersed K-Band Interferometric Observations of Herbig Ae/Be Sources: Inner Disk Temperature Profiles
We use spectrally dispersed near-IR interferometry data to constrain thetemperature profiles of sub-AU-sized regions of 11 Herbig Ae/Be sources.We find that a single-temperature ring does not reproduce the data well.Rather, models incorporating radial temperature gradients are preferred.These gradients may arise in a dusty disk, or may reflect separate gasand dust components with different temperatures and spatialdistributions. Comparison of our models with broadband spectral energydistributions suggests the latter explanation. The data support the viewthat the near-IR emission of Herbig Ae/Be sources arises from hotcircumstellar dust and gas in sub-AU-sized disk regions. Intriguingly,our derived temperature gradients appear systematically steeper fordisks around higher mass stars. It is not clear, however, whether thisreflects trends in relative dust/gas contributions or gradients withinindividual components.
| Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
Context: .Low-mass stars form with disks in which the coagulation ofgrains may eventually lead to the formation of planets. It is not knownwhen and where grain growth occurs, as models that explain theobservations are often degenerate. A way to break this degeneracy is toresolve the sources under study. Aims: .Our aim is to findevidence for the existence of grains of millimetre sizes in disks aroundT Tauri stars, implying grain growth. Methods: .The AustraliaTelescope Compact Array (ATCA) was used to observe 15 southern T Tauristars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm.The five Lupus sources were also observed with the SubMillimeter Array(SMA) at 1.4 mm. Our new data are complemented with data from theliterature to determine the slopes of the spectral energy distributionsin the millimetre regime. Results: .Ten sources were detected atbetter than 3σ with the ATCA, with σ ≈ 1-2 mJy, and allsources that were observed with the SMA were detected at better than15σ, with σ ≈ 4 mJy. Six of the sources in our sample areresolved to physical radii of ~100 AU. Assuming that the emission fromsuch large disks is predominantly optically thin, the millimetre slopecan be related directly to the opacity index. For the other sources, theopacity indices are lower limits. Four out of six resolved sources haveopacity indices ⪉1, indicating grain growth to millimetre sizes andlarger. The masses of the disks range from <0.01 to 0.08 M_ȯ,which is comparable to the minimum mass solar nebula. A tentativecorrelation is found between the millimetre slope and the strength andshape of the 10-μm silicate feature, indicating that grain growthoccurs on similar (short) timescales in both the inner and outerdisk.
| Mid-infrared imaging of the circumstellar dust around three Herbig Ae stars: HD 135344, CQ Tau, and HD 163296
Aims.Planet formation has been known for many years to be tied to thespatial distribution of gas and dust in disks around young stars. Toconstrain planet formation models, imaging observations ofprotoplanetary disks are required. Methods: . Given this, we haveundertaken a mid-infrared imaging survey of Herbig Ae stars, which arepre-main sequence stars of intermediate mass still surrounded by a largeamount of circumstellar material. The observations were made at awavelength of 20.5 μm with the CAMIRAS camera mounted at theCassegrain focus of the Canada France Hawaii Telescope. Results: .We report the observations of three stars, HD 135344, CQ Tau, and HD163296. The circumstellar material around the three objects is spatiallyresolved. The extensions feature a disk-like shape. The images providedirect information on two key parameters of the disk: its inclinationand its outer radius. The outer radius is found to be quite differentfrom the one deduced from disk models, which is only constrained byfitting the Spectral Energy Distribution of the object. Other parametersof the disk, such as flaring and dust mass have been deduced fromfitting both the observed extension and the spectral energy distributionwith sophisticated disk models. Conclusions: .Our results showhow important imaging data are to tighten constraints on the disk modelparameters.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| Resolving and Probing the Circumstellar Disk of the Herbig Ae Star MWC 480 at λ = 1.4 mm: Evolved Dust?
We present high-resolution 0.45"×0.32" observations from the BIMAarray toward the Herbig Ae system MWC 480 in the λ=1.4 mm dustcontinuum. We resolve a circumstellar disk of radius ~170 AU andconstrain the disk parameters by comparing the observations to flat-diskmodels. These results show that the typical fit parameters of the disk,such as the mass, MD~0.04-0.18 Msolar, and thesurface density power-law index, p=0.5 or 1, are comparable to those ofthe lower mass T Tauri stars. The dust in the MWC 480 disk can bemodeled as processed dust material (β~0.8), similar to the HerbigAe star CQ Tau disk; the fitted disk parameters are also consistent withless evolved dust (β~1.2). The possibility of grain growth in theMWC 480 circumstellar disk is supported by the acceptable fits withβ~0.8. The surface density power-law profiles of p=0.5 and p=1 canbe easily fitted to the MWC 480 disk; however, a surface densitypower-law profile similar to the minimum mass solar nebula model p=1.5is ruled out at an 80% confidence level.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Large dust grains in the inner region of circumstellar disks
Context. Simple geometrical ring models account well for near-infraredinterferometric observations of dusty disks surrounding pre-mainsequence stars of intermediate mass. Such models demonstrate that thedust distribution in these disks has an inner hole and puffed-up inneredge consistent with theoretical expectations. Aims. In thispaper, we reanalyze the available interferometric observations of sixintermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC758, V1295 Aql and AB Aur) in the framework of a more detailed physicalmodel of the inner region of the dusty disk. Our aim is to verifywhether the model will allow us to constrain the disk and dustproperties. Methods. Observed visibilities from the literature arecompared with theoretical visibilities from our model. With theassumption that silicates are the most refractory dust species, ourmodel computes self-consistently the shape and emission of the inneredge of the dusty disk (and hence its visibilities for giveninterferometer configurations). The only free parameters in our modelare the inner disk orientation and the size of the dust grains. Results.In all objects with the exception of AB Aur, ourself-consistent models reproduce both the interferometric results andthe near-infrared spectral energy distribution. In four cases, grainslarger than ~1.2 μm, and possibly much larger are either required byor consistent with the observations. The inclination of the inner diskis found to be always larger than ~30°, and in at least two objectsmuch larger.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
We present adaptive-optics high-angular resolution (~0.1 arcsec)spectroscopic observations in the 3 μm region of eight well-knownHerbig Ae/Be stars with circumstellar disks. We detected the aromaticemission feature at 3.3 μm for four out of six of our objects withflared disks (HD 169142, HD 97048, HD 100453, HD 100546), someadditional features at 3.4 and 3.46 μm, and strong diamond featuresat 3.43 and 3.53 μm in two of our flared objects (HD 100546 and HD97048 respectively). We also detected hydrogen recombination line at3.74 μm in practically all the objects. The emission in thepolycyclic aromatic hydrocarbon (PAHs) feature at 3.3 μm and in theadditional carbon dust features in the 3.4-3.5 μm region is, for thefirst time, spatially resolved in all the sources where the features aredetected. The full-width at half-maximum sizes that we derive aretypical of emission arising in a circumstellar disk. On the other hand,the continuum emission remains unresolved, with the exception of HD97048 where it is marginally resolved. We compared the observed spatialdistribution of the 3.3 μm PAH feature and the adjacent continuum tothe predictions of a disk model that includes transiently heated smallgrains and PAHs, in addition to large grains in thermal equilibrium(Habart et al. 2004a). The model predicts that, as observed, the 3.3μm PAH emission feature is significantly broader than that of theadjacent continuum and also that about 50% of its integrated intensitycomes from a radius R< 30 AU. We find that the predicted brightnessprofiles reproduce the observed ones very well. This proves beyond doubtthat the energetic 3.3 μm PAH emission feature takes its origin inthe inner disk regions.
| On the Submillimeter Opacity of Protoplanetary Disks
Solid particles with the composition of interstellar dust and power-lawsize distribution dn/da~a-p (for a<=amax withamax>~3λ and 3~3 mm will result in β(1 mm)<~1. Grain growth cannaturally account for β~1 observed for protoplanetary disks,provided that amax>~3λ.
| Large dust particles in disks around T Tauri stars
We present 7-mm continuum observations of 14 low-mass pre-main-sequencestars in the Taurus-Auriga star-forming region obtained with the VeryLarge Array with ~1.''5 resolution and ~0.3 mJy rms sensitivity. For 10objects, the circumstellar emission has been spatially resolved. Thelarge outer disk radii derived suggest that the emission at thiswavelength is mostly optically thin. The millimetre spectral energydistributions are characterised by spectral indicesαmm= 2.3 to 3.2. After accounting for contributionsfrom free-free emission and corrections for optical depth, we determinedust opacity indices β in the range 0.5 to 1.6, which suggest thatmillimetre-sized dust aggregates are present in the circumstellar disks.Four of the sources with β>1 may be consistent withsubmicron-sized dust as found in the interstellar medium. Our findingsindicate that dust grain growth to millimetre-sized particles iscompleted within less than 1 Myr for the majority of circumstellardisks.
| Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAMPlateau de Bure interferometer in the isotopologues of CO, and incontinuum at 3 and 1.3 mm. These observations allow us to trace thestructure of the circumstellar material of AB Aur inregions where optical and IR imaging is impossible because of theemission from the star. These images reveal that the environment of ABAur is widely different from the proto-planetary disks that surround TTauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 inseveral aspects. Instead of being centrally peaked, the continuumemission is dominated by a bright, asymmetric (spiral-like) feature atabout 140 AU from the central star. Little emission is associated withthe star itself. The molecular emission shows that AB Aur is surroundedby a very extended flattened structure ("disk"), which is rotatingaround the star. Bright molecular emission is also found towards thecontinuum "spiral". The large-scale molecular structure suggests the ABAur disk is inclined between 23 and 43 degrees, but the strong asymmetryof the continuum and molecular emission prevents an accuratedetermination of the inclination of the inner parts. Analysis of theemission in terms of a Keplerian disk provides a reasonable fit to thedata, but fails to give a consistent picture because the inclinationsdetermined from 12CO J~= 2~→ 1, 13CO J~=2~→ 1, 13CO J~= 1~→ 0 and C18O J~=1~→ 0 do not agree. The mass predicted for the central star in suchKeplerian models is in the range 0.9 to 1.2 Mȯ, muchsmaller than the expected 2.2 Mȯ from the spectral typeof AB Aur. Better and more consistent fits to the 13CO J~=2~→ 1, 13CO J~= 1~→ 0 data are obtained by relaxingthe Keplerian hypothesis. We find significant non-Keplerian motion, witha best fit exponent for the rotation velocity law of 0.41 ± 0.01,but no evidence for radial motion. The disk has an inner hole about 70AU in radius. The disk is warm and shows no evidence of depletion of CO.The dust properties suggest that the dust is less evolved than intypical T Tauri disks. Both the spiral-like feature and the departurefrom purely Keplerian motion indicates the AB Aur disk is not inquasi-equilibrium. Disk self-gravity is insufficient to create theperturbation. This behavior may be related either to an early phase ofstar formation in which the Keplerian regime is not yet fullyestablished and/or to a disturbance of yet unknown origin. An alternate,but unproven, possibility is that of a low mass companion located about40 AU from AB Aur.
| Dust distribution in protoplanetary disks. Vertical settling and radial migration
We present the results of a three dimensional, locally isothermal,non-self-gravitating SPH code which models protoplanetary disks with twofluids: gas and dust. We ran simulations of a 1 M_ȯ star surroundedby a 0.01 M_ȯ disk comprising 99% gas and 1% dust in mass andextending from 0.5 to ˜300 AU. The grain size ranges from10-6 m to 10 m for the low resolution (˜25 000 SPHparticles) simulations and from 10-4 m to 10 cm for the highresolution (˜160 000 SPH particles) simulations. Dust grains areslowed down by the sub-Keplerian gas and lose angular momentum, forcingthem to migrate towards the central star and settle to the midplane. Thegas drag efficiency varies according to the grain size, with the largerbodies being weakly influenced and following marginally perturbedKeplerian orbits, while smaller grains are strongly coupled to the gas.For intermediate sized grains, the drag force decouples the dust andgas, allowing the dust to preferentially migrate radially andefficiently settle to the midplane. The resulting dust distributions foreach grain size will indicate, when grain growth is added, the regionswhen planets are likely to form.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| An ISO-LWS two-colour diagram of Herbig Ae/Be stars
In this paper, we present and discuss an infrared two-colour diagrambuilt with the 60, 100, and 170 μm photometry of the whole sample ofHerbig Ae/Be stars observed by the spectrometers on board of ISOsatellite. An overview of this diagram reveals a certain degree ofhomogeneity in the behaviour of these stars and their IR-emittingenvironments, with some exceptions. In particular, we account for theobjects located to the left of the blackbody line. In addition, theinfrared colours obtained with ISO gave us the opportunity to comparewith the IRAS measurements, which generally appear in good agreement.Finally, a simple spherically symmetric model of pre-ZAMS circumstellarenvironment is used to obtain the two-colour diagram, as a diagnostictool complementary to the best-fit of the spectra in investigating thedistribution of matter around these stars.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| The 90-110 μm dust feature in low to intermediate mass protostars: Calcite?
We present ISO spectra between 60 and 180 μm of 32 protostars of lowto intermediate mass. About half of the spectra present a dust featurebetween ~90 and ~110 μm. We describe the observationalcharacteristics of this feature, which seems to be due to one singlecarrier. In Class 0 sources the feature peaks around 100 μm, while inAeBe stars it peaks around 95 μm. The feature peak position seems tomostly depend on the temperature of the dust of the source, suggestingreprocessing of the dust. We present arguments for the identification ofthe observed feature as due to calcite, and estimate that about 10% to30% of elemental Ca is locked up in it. Therefore, calcite seems to beformed relatively easily around protostars despite the observation thaton Earth it needs aqueous solutions. This rises the question of whetherconditions simulating liquid water are common around forming stars andwhat creates them.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) with the participation of ISAS andNASA.
| Search for 17 μm H2 Pure Rotational Emission from Circumstellar Disks
We report the negative detection of the S(1) pure rotational lineemission of molecular hydrogen at 17.035 μm for four young stars, HD163296, MWC 863, CQ Tau, and LkCa 15, for which Infrared SpaceObservatory (ISO) observations detected the S(1) emission, with theCooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m SubaruTelescope. We did not detect the line emission toward the central starnor within 15'' of the central star along the slit for any ofthe present targets. Upper limits of the present observations are muchsmaller than previous line flux estimates based on ISO Short WavelengthSpectrometer ( SWS) observations, irrespective of the intrinsic linewidth. The present results indicate that the emission detected by theISO SWS is not concentrated within the disk. Our detection limit of theH2 S(1) emission corresponds to upper limits of the diskmasses of (0.72-3.8)×10-4 Msolar within aradius of ~30-45 AU for the optically thin emission from the gas of 150K. The upper limits of the disk masses are significantly lower thanthose of the warm molecular hydrogen mass predicted by the model (Chiang& Goldreich 1997), suggesting that the optically thick emission fromdust dominates in the radiation from the disks in the mid-infraredwavelength. We point out that detectability of the H2emission in young stars depends on the evolution of the disks,particularly in the sedimentation and growth of dust grains in the disk.
| Resolved Inner Disks around Herbig Ae/Be Stars
We have observed 14 Herbig Ae/Be (HAEBE) sources with the long-baselinenear-IR Palomar Testbed Interferometer. All except two sources areresolved at 2.2 μm, with angular sizes generally <~5 mas. Wedetermine the size scales and orientations of the 2.2 μm emissionusing various models: uniform disks, Gaussians, uniform rings, flataccretion disks with inner holes, and flared disks with puffed-up innerrims. Although it is difficult to distinguish different radialdistributions, we are able to place firm constraints on the inclinationsof most sources; seven objects display significantly inclinedmorphologies. The inner disk inclinations derived from our near-IR dataare generally compatible with the outer disk geometries inferred frommillimeter interferometric observations, implying that HAEBE disks arenot significantly warped. Using the derived inner disk sizes andinclinations, we compute the spectral energy distributions (SEDs) fortwo simple physical disk models and compare these with observed SEDscompiled from the literature and new near-IR photometry. Whilegeometrically flat accretion disk models are consistent with the datafor the earliest spectral types in our sample (MWC 297, V1685 Cyg, andMWC 1080), the later type sources are explained better through modelsincorporating puffed-up inner disk walls. The different inner diskgeometries may indicate different accretion mechanisms for early- andlate-type HAEBE stars.
| Variable Star Network: World Center for Transient Object Astronomy and Variable Stars
Variable Star Network (VSNET) is a global professional-amateur networkof researchers in variable stars and related objects, particularly intransient objects, such as cataclysmic variables, black-hole binaries,supernovae, and gamma-ray bursts. The VSNET has been playing apioneering role in establishing the field of transient object astronomy,by effectively incorporating modern advances in observational astronomyand global electronic networks, as well as collaborative progress intheoretical astronomy and astronomical computing. The VSNET is now oneof the best-featured global networks in this field of astronomy. Wereview the historical progress, design concept, associated technology,and a wealth of scientific achievements powered by VSNET.
| Spectral Analysis and Classification of Herbig Ae/Be Stars
We present an analysis of the optical spectra of 75 early-typeemission-line stars, many of which have been classified previously asHerbig Ae/Be (HAeBe) stars. Accurate spectral types were derived for 58members of the sample; high continuum veiling, contamination bynonphotospheric absorption features, or a composite binary spectrumprevented accurate spectral typing for the rest. Approximately half ofour sample exhibited [O I] λ6300 forbidden-line emission down toour detection limit of 0.1 Å equivalent width; a third of thesample exhibited Fe II emission (multiplet 42). A subset of 11 of theHAeBe sample showed abnormally strong Fe II absorption; 75% of thissubset are confirmed UX Ori objects. Combining our spectral typingresults with photometry from the literature, we confirm previousfindings of high values of total-to-selective extinction(RV~5) in our larger sample, suggesting significant graingrowth in the environments of HAeBe stars. With this high value ofRV, the vast majority of HAeBe stars appear younger than withthe standard RV=3.1 extinction law and are more consistentwith being pre-main-sequence objects.
| A search for evolved dust in Herbig Ae stars
We present observations of six isolated, pre-main-sequence, intermediatemass stars selected for shallow spectra at submillimeter wavelengths at1.3, 2.6, 7.0, and 36 millimeters from the IRAM PdBI and the VLA. Weanalyze the new observations of these stars (HD 34282, HD 35187, HD142666, HD 143006, HD 150193, HD 163296) together with similarobservations of three additional stars from the literature (CQ Tau, UXOri, TW Hya), in the context of self-consistent irradiated disk models.Our aim is to constrain the wavelength dependence of the dust opacityand the total dust mass in the disks. The shallow wavelength dependenceof the opacity is confirmed and for a few stars extended tosignificantly longer wavelengths. For any plausible dust properties,this requires grain growth from interstellar sizes to maximum sizes ofat least a few millimeters, and very likely to several centimeters ormore. For four of the stars (HD 34282, HD 163296, CQ Tau, TW Hya), themillimeter emission has been spatially resolved, and the large diskradii (>100 AU) rule out that high optical depths play a role. Themass of dust that has been processed into large grains is substantial,and in some cases implies a disk mass comparable to the mass of thecentral star.
| Modeling the Continuum Emission from the Circumstellar Environment of Herbig Ae/Be Stars
This paper discusses a model for the continuum emission of the HerbigAe/Be stars in the light of an updated set of observational dataspanning 5 orders of magnitude in wavelength and including thelow-resolution spectra obtained with the Short Wavelength Spectrometerand Long Wavelength Spectrometer on board the Infrared Space Observatory(ISO). The model is used to reproduce the continuum emission of the 36Herbig Ae/Be stars included in the list by Thé and coworkers andobserved by ISO. The circumstellar matter responsible for the observedspectral energy distributions has been investigated by comparing the setof the observations with the model spectra computed for differentpossible distributions of circumstellar matter. Cases have beenconsidered with the circumstellar regions partially evacuated along thepolar axis by the action of the stellar wind, a phenomenon that is quitecommon in these pre-main-sequence objects. The inclusion of the polarcavities indirectly allows geometries in which a small-scale disklikestructure around the central star is present. The possible coexistenceof two different density profiles, in the inner and the outer region ofthe envelope, respectively, has been also considered. The comparison ofthe computed models with the observed spectral energy distributionsselects the parameter values in such a way that the larger dust grainsare preferentially associated with the later spectral types. We findthat 17 objects are reasonably fitted, eight of which with a purelyspherical model and the remaining nine with the inclusion of the polarcavities. For 10 further objects the fit is worse, and for the remainingnine, almost all associated to IR companions, our model is clearlyinappropriate. A linear relationship is suggested between the logarithmof the initial density n0 and the exponent p of the power lawn(r)=n0(R*/r)p adopted for thecircumstellar density distribution.
| A search for H2 around pre-main-sequence stars
We present the results of a search for pure rotational molecularhydrogen emission from two pre-main-sequence stars, AB Aur and CQ Tau.Observations were made using MICHELLE, the mid-infrared echellespectrometer at the United Kingdom Infrared Telescope. We found someevidence for emission in the J= 4 -> 2 line in AB Aur, but no J= 3-> 1 line from either star. We derive upper limits on line flux whichare significantly smaller than previous line flux estimates based on ISOobservations. This suggests that the emission detected by ISO isextended on scales of at least 6 arcsec, and does not come from the discas previously thought.
| Photometric and Polarimetric Activity of RZ Psc
We present the results of synchronous photometric and polarimetric U BVRI observations of the irregular variable star RZ Psc, acquired at theCrimean Astrophysical Observatory in 1989 2002. The star’sphotometric behavior is characterized by short, sporadic Algol-likedimmings. We observed only one deep minimum, with a V amplitude of about1.5m, during the entire observation time. During this minimum, thestar’s linear polarization reached 3.5%. Comparisons withpolarization observations of RZ Psc during another deep minimum in 1989show that the two minima can be described by the samepolarization-brightness relation, testifying to an eclipsing nature forthe minima. This provides evidence that the optical characteristics ofthe flattened circumstellar dust envelope that gives rise to thestar’s intrinsic polarization have remained virtually unchangedover the last 13 years. We argue that the origin of this stability isthe presence of a large dust-free cavity in the central region of thecircumstellar dust disk of RZ Psc. The cavity could be associated withbinarity of the star or the formation of a planetary system, with mostof the dust in the central region of the disk being transformed intolarge bodies—planetesimals and planets.
| Large grains in the disk of CQ Tau
We present 7 mm observations of the dusty disk surrounding the 10 Myrold 1.5 Msun pre-main-sequence star CQ Tauri obtained at theVery Large Array with 0.8 arcsec resolution and 0.1 mJy rms sensitivity.These observations resolve the 7 mm emission in approximately thenorth-south direction, confirming previous results obtained with lowerresolution. We use a two-layer flared disk model to interpret theobserved fluxes from 7 mm to 1.3 mm together with the resolved 7 mmstructure. We find that the disk radius is constrained to the range 100to 300 AU, depending on the steepness of the disk surface densitydistribution. The power law index of the dust opacity coefficient, beta, is constrained to be 0.5 to 0.7. Since the models indicate that thedisk is optically thin at millimeter wavelengths for radii greater than8 AU, the contribution of an optically thick region to the emission isless than 10%. This implies that high optical depth or complex diskgeometry cannot be the cause of the observed shallow millimeter spectralindex. Instead, the new analysis supports the earlier suggestion thatdust particles in the disk have grown to sizes as large as a fewcentimeters. The dust in the CQ Tauri system appears to be evolved muchlike that in the TW Hydra system, a well-studied pre-main-sequence starof similar age and lower mass. The survival of gas-rich disks withincomplete grain evolution at such old ages deserves furtherinvestigations.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Телец |
| Прямое восхождение: | 05h35m58.46s |
| Склонение: | +24°44'54.1" |
| Видимая звёздная величина: | 10.591 |
| Собственное движение RA: | 1.6 |
| Собственное движение Dec: | -26 |
| B-T magnitude: | 11.443 |
| V-T magnitude: | 10.662 |
Каталоги и обозначения:
|
