Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
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.
| 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.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
| Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains
We present 8-13 μm low-resolution spectra (R~100) of eight low-massprotostellar objects (``proplyds'') in the Orion Nebula using the LongWavelength Spectrometer at the W. M. Keck Observatory. All but one ofthe sources in our sample show strong circumstellar silicate emission,with profiles that are qualitatively similar to those seen in some TTauri and Herbig Ae/Be stars. The silicate profile in all cases issignificantly flattened compared with the profile for typicalinterstellar dust, suggesting that the dominant emitting grains aresignificantly larger than those found in the interstellar medium. The11.3-to-9.8 μm flux ratio-often used as an indicator of graingrowth-is in the 0.8-1.0 range for all of our targets, indicating thatthe typical grain size is around a few microns in the surface layers ofthe attendant circumstellar disk for each object. Furthermore, thesilicate profiles show some evidence of crystalline features, as seen inother young stellar objects. The results of our analysis show that thegrains in the photoevaporating protostellar disks of Orion haveundergone significant growth and perhaps some annealing, suggesting thatgrain evolution for these objects is not qualitatively different fromother young stellar objects.
| c2d Spitzer IRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth
Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7intermediate-mass Herbig Ae stars with circumstellar disks were obtainedusing the Spitzer Space Telescope as part of the c2d IRS survey. Thiswork complements prior spectroscopic studies of silicate infraredemission from disks, which were focused on intermediate-mass stars, withobservations of solar mass stars limited primarily to the 10 μmregion. The observed 10 and 20 μm silicate feature strengths/shapesare consistent with source-to-source variations in grain size. A largefraction of the features are weak and flat, consistent with micron-sizedgrains indicating fast grain growth (from 0.1 to 1.0 μm in radius).In addition, approximately half of the T Tauri star spectra showcrystalline silicate features near 28 and 33 μm, indicatingsignificant processing when compared to interstellar grains. A fewsources show large 10-to-20 μm ratios and require even larger grainsemitting at 20 μm than at 10 μm. This size difference may arisefrom the difference in the depth into the disk probed by the twosilicate emission bands in disks where dust settling has occurred. The10 μm feature strength versus shape trend is not correlated with ageor Hα equivalent width, suggesting that some amount of turbulentmixing and regeneration of small grains is occurring. The strengthversus shape trend is related to spectral type, however, with M starsshowing significantly flatter 10 μm features (larger grain sizes)than A/B stars. The connection between spectral type and grain size isinterpreted in terms of the variation in the silicate emission radius asa function of stellar luminosity, but could also be indicative of otherspectral-type-dependent factors (e.g., X-rays, UV radiation, andstellar/disk winds).
| Influence of stellar wind on the long-term variability of the Hα emission line. The case of the Herbig Ae-star HD 31648
The results of high-resolution long-term spectral monitoring in theregion of the emission Ha line of the Herbig Ae star HD 31648 arereported. The variability in the observed profile is shown to be causedmainly by a change in the parameters of outflowing circumstellar gasover a time scale of about 3 years.
| A Keplerian Disk around the Herbig Ae Star HD 169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in 1.3 mm continuum emission and 12CO J=2-1 lineemission at ~1.5" resolution that reveal a circumstellar disk. Thecontinuum emission is centered on the star position and resolved, and itprovides a mass estimate of ~0.02 Msolar for the disk. The COimages show patterns in position and velocity that are well matched by adisk in Keplerian rotation with low inclination to the line of sight. Weuse radiative transfer calculations based on a flared, passive diskmodel to constrain the disk parameters by comparison to the spectralline emission. The derived disk radius is 235 AU, and the inclination is13°. The model also necessitates modest depletion of the COmolecules, similar to that found in Keplerian disks around T Tauristars.
| Resolving the inner dust disks surrounding LkCa 15 and MWC 480 at mm wavelengths
Aims.We constrain the dust distribution and its properties (temperature,emissivity) in inner proto-planetary disks Methods: .We performedsub-arcsecond high-sensitivity interferometric observations of thethermal dust emission at 1.4 mm and 2.8 mm in the disks surrounding LkCa15 and MWC 480, with the new 750 m baselines of the IRAM PdBI array.This provides a linear resolution of ~60 AU at the distance ofTaurus. Results: .We report the existence of a cavity of ~50 AUradius in the inner disk of LkCa 15. Whereas LkCa 15 emission isoptically thin, the optically thick core of MWC 480 is resolved at 1.4mm with a radius of ~35 AU, constraining the dust temperature. In MWC480, the dust emission is coming from a colder layer than the COemission, most likely the disk mid-plane. Conclusions: .Theseobservations provide direct evidence of an inner cavity around LkCa 15.Such a cavity most probably results from the tidal disturbance createdby a low-mass companion or large planet at ~30 AU from the star. Theseresults suggest that planetary system formation is already at work inLkCa 15. They also indicate that the classical steady-state viscous diskmodel is too simplistic a description of the inner 50 AU of"proto-planetary" disks and that the disk evolution is coupled to theplanet formation process. The MWC 480 results indicate that a properestimate of the dust temperature and size of the optically thick coreare essential for determining the dust emissivity index β.
| A compact dusty disk around the Herbig Ae star HR 5999 resolved with VLTI / MIDI
Aims.We have used mid-infrared long-baseline interferometry to resolvethe circumstellar material around the Herbig Ae star HR 5999, providingthe first direct measurement of its angular size, and to deriveconstraints on the spatial distribution of the dust. Methods:.MIDI at the VLTI was used to obtain a set of ten spectrally dispersed(8-13 μm) interferometric measurements of HR 5999 at differentprojected baseline lengths and position angles. To derive constraints onthe geometrical distribution of the dust, we compared ourinterferometric measurements to 2D, frequency-dependent radiationtransfer simulations of circumstellar disks and envelopes. Results: .The derived visibility values between 0.5 and 0.9 show thatthe mid-infrared emission from HR 5999 is clearly resolved. Thecharacteristic size of the emission region depends on the projectedbaseline length and position angle, and it ranges between 5-15milliarcsec (Gauss FWHM), corresponding to remarkably small physicalsizes of 1-3 AU. For disk models with radial power-law densitydistributions, the relatively weak but very extended emission from outerdisk regions ( 3 AU) leads to model visibilities that aresignificantly lower than the observed visibilities, making these modelsinconsistent with the MIDI data. Disk models in which the density istruncated at outer radii of 2 - 3 AU, on the other hand, provide goodagreement with the data. Conclusions: .A satisfactory fit to theobserved MIDI visibilities of HR 5999 is found with a model of ageometrically thin disk that is truncated at 2.6 AU and seen under aninclination angle of 58degr (i.e. closer to an edge-on view than to aface-on view). Neither models of a geometrically thin disk seen nearlyedge-on, nor models of spherical dust shells can achieve agreementbetween the observed and predicted visibilities. The reason why the diskis so compact remains unclear; we speculate that it has been truncatedby a close binary companion.
| 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.
| Analysis of the dust evolution in the circumstellar disks of T Tauri stars
Aims.We present a compositional analysis of 8{-}13 μ m spectra of 32young stellar objects (YSOs). Our sample consists of 5 intermediate-massstars and 27 low-mass stars. Although some previous studies give reasonsfor the similarity between the dust in circumstellar disks of T Tauristars and Herbig Ae/Be stars, a quantitative comparison has been lackingso far. Therefore, we include a discussion of the results of the 10μm spectroscopic survey of van Boekel et al. (2005, A&A, 437,189), who focus on Herbig Ae/Be stars, the higher mass counterparts of TTauri stars, and draw comparisons to this and other studies. Methods: .While the spectra of our 32 objects and first scientificresults have already been published elsewhere we perform a more detailedanalysis of the 10 μ m silicate feature. In our analysis weassume that this emission feature can be represented by a linearsuperposition of the wavelength-dependent opacity κ_abs(λ)describing the optical properties of silicate grains with differentchemical composition, structure, and grain size. Determining an adequatefitting equation is another goal of this study. Using a restrictednumber of fitting parameters, we investigate which silicate species arenecessary for the compositional fitting. Particles, with radii of 0.1μm- and 1.5 μm and consisting of amorphous olivine and pyroxene,forsterite, enstatite, and quartz are considered. Only compact,homogeneous dust grains are used in the presented fitting procedures. Inthis context we show that acceptable fitting results can also beachieved if emission properties of porous silicate grains are consideredinstead. Results: .Our analysis shows - in terms of the propertiesof the circumstellar dust-like crystallinity - T Tauri systems are acontinuation of HAeBe systems at their lower mass end. However, a weakcorrelation between grain growth and stellar luminosity could be found,in contrast to HAeBe systems.
| Tidal stripping and disk kinematics in the RW Aurigae system
We present interferometric maps of the RW Aur system obtained with theIRAM Plateau de Bure Interferometer in CO(J=2-1), CO(J=1-0), and nearbycontinuum. The sub-arcsecond angular resolution (0.89''×0.58'')and high-sensitivity reached at 1.3 mm enable us to resolve threemolecular structures: (1) an optically thick disk around RW Aur A inrotation about the optical jet axis, (2) a disturbed asymmetric peakaround RW Aur B, (3) a 600 AU-long “arm” of materialtrailing from the RW Aur A disk. Comparison with Keplerian modelsindicates that the RW Aur A disk is the smallest detected so far arounda T Tauri star (radius 40-57 AU) and that the CO emitting layer at theouter edge is warmer than the dust ({Tout}≃ 60-100 K)and relatively thick ({Nwarm}≃0.1-10×1022cm-2). The morphology andkinematics of the detected features strongly suggest that we arewitnessing tidal stripping of the primary disk by the recent fly-by ofRW Aur B. We speculate that tidal dissipation might explain the warmergas temperatures in the RW Aur A disk compared with typical T Tauristars, and perhaps play a role in its elevated accretion rate. We alsofind that the rotation sense of the RW Aur A disk is opposite totransverse velocity shifts in the optical jet reported by Woitas et al.(2005, A&A, 432, 149). We argue that these transverse shifts arelikely to represent only upper limits to the true jet rotation speed.The limits remain consistent with current models of MHD launching fromthe disk.
| 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.
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| 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.
| 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.
| 8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
Silicate features arising from material around pre-main-sequence starsare useful probes of the star and planet formation process. In order toinvestigate possible connections between dust processing and diskproperties, 8-13 μm spectra of 34 young stars, exhibiting a range ofcircumstellar environments and including spectral types A-M, wereobtained using the Long Wavelength Spectrometer at the W. M. KeckObservatory. The broad 9.7 μm amorphous silicate (SiO stretching)feature that dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older ``debris'' disk systems for both low- andintermediate-mass stars. This is similar to the evolutionary patternseen in Infrared Space Observatory (ISO) observations ofhigh/intermediate-mass young stellar objects (YSOs). The peak wavelengthand FWHM are centered about 9.7 and ~2.3 μm, respectively,corresponding to amorphous olivine, with a larger spread in FWHM forembedded sources and in peak wavelength for disks. In a few of ourobjects that have been previously identified as class I low-mass YSOs,the observed silicate feature is more complex, with absorption near 9.5μm and emission peaking around 10 μm. Although most of theemission spectra show broad classical features attributed to amorphoussilicates, small variations in the shape/strength may be linked to dustprocessing, including grain growth and/or silicate crystallization. Forsome of the Herbig Ae stars in the sample, the broad emission featurehas an additional bump near 11.3 μm, similar to the emission fromcrystalline forsterite seen in comets and the debris disk βPictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Aestar, HD 179218, clearly show strong, narrow emission near 11.3 μm.We study quantitatively the evidence for evolutionary trends in the 8-13μm spectra through a variety of spectral shape diagnostics. Based onthe lack of correlation between these diagnostics and broadband infraredluminosity characteristics for silicate emission sources, we concludethat although spectral signatures of dust processing are present, theycannot be connected clearly to disk evolutionary stage (for opticallythick disks) or optical depth (for optically thin disks). Thediagnostics of silicate absorption features (other than the centralwavelength of the feature), however, are tightly correlated with opticaldepth and thus do not probe silicate grain properties.
| 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.
| Spectroscopic behaviour of the unusual Ae star HD 190073
The results of high-resolution spectroscopy of the peculiar Ae star HD190073 obtained within the framework of a cooperative observingprogramme in 1994-2002 are presented. The temporal behaviour of theHα, Hβ, Hγ, Hδ, He i λ 5876 Å, Na iD and other circumstellar line profiles are investigated in detail.Special attention has been paid to the analysis of the deepmulticomponent blueshifted Ca ii H and K absorption lines. It has beenfound that the fine structure of their profiles is variable ontimescales from months to decades. The analysis of the circumstellarspectrum of HD 190073, rich in shell-like and emission lines with narrowabsorption cores, allows us to conclude that all absorption lines andcores are likely to be of photospheric origin. The emission lines arevariable in time and demonstrate signs of a stellar wind as well as adense equatorial circumstellar disk. As a preliminary hypothesis, wepropose that a global magnetic field of a specific topology can beresponsible for the formation of stable latitudinal stratification ofthe outflowing gas resulting in appearance of the complex structure ofthe Ca ii H and K line profiles. We emphasise that a measurement of thestellar magnetic field and an investigation of its detailedconfiguration would be an important step in understanding the nature ofHD 190073.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| 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.
| Close binary companions of the HAeBe stars LkHα 198, Elias 1, HK Ori and V380 Ori
We present diffraction-limited bispectrum speckle interferometryobservations of four well-known Herbig Ae/Be (HAeBe) stars,LkHα 198, Elias 1,HK Ori and V380 Ori. For two ofthese, LkHα 198 and Elias 1, we present the first unambiguousdetection of close companions. The plane of the orbit of the newLkHα 198 companion appears to be significantly inclined to theplane of the circumprimary disk, as inferred from the orientation of theoutflow. We show that the Elias 1 companion may be a convective star,and suggest that it could therefore be the true origin of the X-rayemission from this object. In the cases of HK Ori and V380 Ori, wepresent new measurements of the relative positions of already-knowncompanions, indicating orbital motion. For HK Ori, photometricmeasurements of the brightness of the individual components in fourbands allowed us to decompose the system spectral energy distribution(SED) into the two separate component SEDs. The primary exhibits astrong infrared excess which suggests the presence of circumstellarmaterial, whereas the companion can be modelled as a naked photosphere.The infrared excess of HK Ori A was found to contribute around twothirds of the total emission from this component, suggesting thataccretion power contributes significantly to the flux. Submillimetreconstraints mean that the circumstellar disk cannot be particularlymassive, whilst the near-infrared data indicates a high accretion rate.Either the disk lifetime is very short, or the disk must be seen in anoutburst phase.Based on observations performed with the 6 m telescope of the SpecialAstrophysical Observatory, Russia, the 2.2 m ESO/MPG telescope at LaSilla, and with the NASA/ESA Hubble Space Telescope, obtained from thedata archive at the Space Telescope Institute. STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555.
| 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.
| 3-14 Micron Spectroscopy of Comets C/2002 O4 (Hönig), C/2002 V1 (NEAT), C/2002 X5 (Kudo-Fujikawa), C/2002 Y1 (Juels-Holvorcem), and 69P/Taylor and the Relationships among Grain Temperature, Silicate Band Strength, and Structure among Comet Families
We report 3-13 μm spectroscopy of four comets observed between 2002August and 2003 February: C/2002 O4 (Hönig) on 2002 August 1,C/2002 V1 (NEAT) on 2003 January 9 and 10, C/2002 X5 (Kudo-Fujikawa) on2003 January 9 and 10, and C/2002 Y1 (Juels-Holvorcem) on 2003 February20. In addition, we include data obtained much earlier on 69P/Taylor(1998 February 9) but not previously published. For comets Taylor,Hönig, NEAT, and Kudo-Fujikawa, the silicate emission band wasdetected, being approximately 23%, 12%, 15%, and 10%, respectively,above the continuum. The data for comet Juels-Holvorcem were ofinsufficient quality to detect the presence of a silicate band ofcomparable strength to the other three objects, and we place an upperlimit of 24% on this feature. The silicate features in both NEAT andKudo-Fujikawa contained structure indicating the presence of crystallinematerial. The shape of the silicate feature at a projected distance of1900 km from the nucleus of Kudo-Fujukawa was nearly identical to thatcentered on the nucleus, indicating that the grain size population hadnot been measurably modified by the time it had reached that distance.Combining these data with those of other comets, we confirm thecorrelation between silicate band strength and grain temperature ofGehrz & Ney and Williams and coworkers for dynamically new andlong-period comets, but the majority of Jupiter family objects maydeviate from this relation. Despite the weakness of the silicate band inKudo-Fujikawa, its structure resembles the bands seen in dynamically newand long-period objects with substantially stronger features. Thelimited data available on Jupiter family objects suggest that they mayhave silicate bands that are slightly different from the former objects.Finally, when compared to the silicate emission bands observed inpre-main-sequence stars, the dynamically new and long-period comets mostclosely resemble the more evolved stellar systems, while the limiteddata (in quantity and quality) on Jupiter family objects seem to suggestthat these have spectra more like the less evolved stars. Higher qualitydata on a larger number of Jupiter family objects are needed to confirm(or reject) this trend.
| High-Resolution 4.7 Micron Keck/NIRSPEC Spectroscopy of the CO Emission from the Disks Surrounding Herbig Ae Stars
We explore the high-resolution (λ/Δλ=25,000Δv=12 km s-1) M-band (4.7-5.1 μm) spectra of severaldisk-dominated Herbig Ae (HAe) systems: AB Aur, MWC 758, MWC 480, HD163296, and VV Ser. All five objects show 12CO v=1-0 emissionlines up to J=42, but there is little or no evidence of moderate-J,v=2-1 transitions despite their similar excitation energies. AB Aurshows 13CO emission as well. The line/continuum ratios andintensity profiles are well correlated with inclination, and they tracecollisionally driven emission from the inner disk(Rth<~0.5-1 AU) as well as resonance fluorescence to muchlarger radii (Rhν<~50-100 AU for J<~10). Thetemperature, density, and radiation field profiles required to fit theCO emission are in good agreement with models of HAe disks derived fromtheir spectral energy distributions. High-resolution and high dynamicrange infrared spectroscopy of CO, and future observations of lessabundant species, thus provide direct access to the physicochemicalproperties and surface structure of disks in regions where planetformation likely occurs.
| An Assessment of Dynamical Mass Constraints on Pre-Main-Sequence Evolutionary Tracks
We have assembled a database of stars having both masses determined frommeasured orbital dynamics and sufficient spectral and photometricinformation for their placement on a theoretical H-R diagram. Our sampleconsists of 115 low-mass (M<2.0 Msolar) stars, 27pre-main-sequence and 88 main-sequence. We use a variety of availablepre-main-sequence evolutionary calculations to test the consistency ofpredicted stellar masses with dynamically determined masses. Despitesubstantial improvements in model physics over the past decade, largesystematic discrepancies still exist between empirical and theoreticallyderived masses. For main-sequence stars, all models considered predictmasses consistent with dynamical values above 1.2 Msolar andsome models predict consistent masses at solar or slightly lower masses,but no models predict consistent masses below 0.5 Msolar,with all models systematically underpredicting such low masses by5%-20%. The failure at low masses stems from the poor match of mostmodels to the empirical main sequence below temperatures of 3800 K, atwhich molecules become the dominant source of opacity and convection isthe dominant mode of energy transport. For the pre-main-sequence samplewe find similar trends. There is generally good agreement betweenpredicted and dynamical masses above 1.2 Msolar for allmodels. Below 1.2 Msolar and down to 0.3 Msolar(the lowest mass testable), most evolutionary models systematicallyunderpredict the dynamically determined masses by 10%-30%, on average,with the Lyon group models predicting marginally consistent masses inthe mean, although with large scatter. Over all mass ranges, theusefulness of dynamical mass constraints for pre-main-sequence stars isin many cases limited by the random errors caused by poorly determinedluminosities and especially temperatures of young stars. Adopting awarmer-than-dwarf temperature scale would help reconcile the systematicpre-main-sequence offset at the lowest masses, but the case for this isnot compelling, given the similar warm offset at older ages between mostsets of tracks and the empirical main sequence. Over all age ranges, thesystematic discrepancies between track-predicted and dynamicallydetermined masses appear to be dominated by inaccuracies in thetreatment of convection and in the adopted opacities.
| 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.
| Long-Term Spectral Variability of the Herbig Ae-Star HD 179218
The results of high-resolution long-term spectral monitoring of HerbigAe star HD 179218 in the region of emission H line and the sodiumresonance doublet Na I D are presented. The received data show theexistence of short-term variability (about 10 days) and long-termwave-like variability of equivalent width EW, intensity of circumstellar(CS) emission I and parameters of H emission profile. The analysis ofthese data allows us to suppose the existence of the global variabilityof parameters of accretion disk.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Кочијаш |
| Ректацензија: | 04h58m46.27s |
| Deклинација: | +29°50'37.0" |
| Apparent магнитуда: | 7.74 |
| Даљина: | 131.234 parsecs |
| Proper motion RA: | 5.5 |
| Proper motion Dec: | -25.4 |
| B-T magnitude: | 7.991 |
| V-T magnitude: | 7.761 |
Каталог и designations:
|
