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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.
| The photopolarimetric activity and circumstellar environment of the T Tauri star CO Ori
Not Available
| 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.
| 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.
| Structure of the inner regions of the circumstellar gas envelopes of young hot stars. I. The isolated Ae Herbig star WW Vul
Results of simultaneous spectral and photometric monitoring of the AeHerbig star WW Vul in the neighborhoods of the Ha line and the sodiumNaI D resonance doublet are reported. It is shown that the spectralvariability of the star is caused mainly by the anisotropic disk wind,whose high velocity component forms in the inner region of the accretiondisk. The circumstellar gas in footpoint of the wind shows thevariability of the density and velocity, that is in good agreement withthe results of modeling of an accretion and outflows around young starscontrolled by the stellar and/or disk magnetic field. An analysis of thevariability of the parameters of the Ha emission line also showed thatthe density of the gas in the inner region of the accretion disk variesover a time scale exceeding 10 years.
| 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).
| Near-Infrared and the Inner Regions of Protoplanetary Disks
We examine the ``puffed-up inner disk'' model proposed by Dullemond,Dominik, & Natta for explaining the near-IR excess radiation fromHerbig Ae/Be stars. Detailed model computations show that the observednear-IR excess requires more hot dust than is contained in the puffed-updisk rim. The rim can produce the observed near-IR excess only if itsdust has perfectly gray opacity, but such dust is in conflict with theobserved 10 μm spectral feature. We find that a compact (~10 AU),tenuous (τV<~0.4), dusty halo around the disk innerregions contains enough dust to readily explain the observations.Furthermore, this model also resolves the puzzling relationship noted byMonnier & Millan-Gabet between luminosity and the interferometricinner radii of disks.
| 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.
| Unusual Color Variability of Eruptive Stars
We substantiate the conclusion that the unusual color variability foundpreviously in some eruptive stars is typical of a broad class ofnonstationary objects, manifests itself over a wide temperature range(from B0 to K 3), and can be regarded as a new type of stellarvariability.
| [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.
| 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 Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disksizes using the 85 m baseline Keck Interferometer. Targets were chosento span the maximum range of stellar properties to probe the disk sizedependence on luminosity and effective temperature. For most targets,the measured near-infrared sizes (ranging from 0.2 to 4 AU) support asimple disk model possessing a central optically thin (dust-free)cavity, ringed by hot dust emitting at the expected sublimationtemperatures (Ts~1000-1500 K). Furthermore, we find a tightcorrelation of disk size with source luminosity R~L1/2 for Aeand late Be systems (valid over more than two decades in luminosity),confirming earlier suggestions based on lower quality data.Interestingly, the inferred dust-free inner cavities of the highestluminosity sources (Herbig B0-B3 stars) are undersized compared topredictions of the ``optically thin cavity'' model, likely because ofoptically thick gas within the inner AU.
| 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.
| PAHs in circumstellar disks around Herbig Ae/Be stars
We investigate the presence and properties of PAHs on the surface ofcircumstellar disks around Herbig Ae/Be stars by comparing thepredictions of disk models with observations. We present results of aradiation transfer code for disks heated by the central star, inhydrostatic equilibrium in the vertical direction (flared disks). Thedust is a mixture of large grains in thermal equilibrium, transientlyheated small grains and PAHs. Special attention is given to theinfluence of the stellar, disk and PAH properties on the strength of thePAH emission lines and their spatial distribution. The models predict aninfrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 μmclearly visible above the continuum, and with some of them very strong.The PAH emission, spatially extended, comes mostly from the outer diskregion (R100 AU) while the continuum emission at similarwavelengths, mostly due to warm large grains, is confined to theinnermost disk regions (R few AU). We compare the model results toinfrared observations from ISO and ground-based telescopes of somethirty Herbig Ae/Be stars. Most of the observed PAH features in objectswith spectral type later than B9 are well described by our disk modelsand we discuss in some detail the PAH characteristics one can derivefrom the existing data. Objects with strong radiation field (generallyearlier than about B9) have the 3.3 μm feature (often the only oneobserved) much weaker than predicted, and we discuss possibleexplanations (dissipation of the disk, photoevaporation or modificationof the PAH properties).Appendix is only available in electronic form athttp://www.edpsciences.org
| ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be starsfor which spectroscopic data are available in the ISO data archive. Ourquantitative analysis of these spectra focuses on the emission bands at3.3, 6.2, ``7.7'', 8.6 and 11.2 micron, linked to polycyclic aromatichydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5micron, the amorphous 10 micron silicate band and the crystallinesilicate band at 11.3 micron. We have detected PAH emission in 57% ofthe Herbig stars in our sample. Although for most of these sources thePAH spectra are similar, there are clear examples of differences in thePAH spectra within our sample which can be explained by differences inPAH size, chemistry and/or ionization. Amorphous silicate emission wasdetected in the spectra of 52% of the sample stars, amorphous silicateabsorption in 13%. We have detected crystalline silicate emission in 11stars (24% of our sample), of which four (9%) also display strong PAHemission. We have classified the sample sources according to thestrength of their mid-IR energy distribution. The systems with strongermid-infared (20-100 μm) excesses relative to their near-infrared (1-5μm) excess display significantly more PAH emission than those withweaker mid-infrared excesses. There are no pronounced differences in thebehaviour of the silicate feature between the two groups. This providesstrong observational support for the disk models by \citet{dullemond01},in which systems with a flaring disk geometry display a strongmid-infrared excess, whereas those with disks that are strongly shadowedby the puffed-up inner rim of the disk only display modest amounts ofmid-infrared emission. Since the silicates are expected to be producedmainly in the warm inner disk regions, no large differences in silicatebehaviour are expected between the two groups. In contrast to this, thePAH emission is expected to be produced mainly in the part of the diskatmosphere that is directly exposed to radiation from the central star.In this model, self-shadowed disks should display weaker PAH emissionthan flared disks, consistent with our observations.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) and with the participation of ISASand NASA.Tables 1, 3-6 and Appendix are only available in electronic form athttp://www.edpsciences.org
| Correlation between grain growth and disk geometry in Herbig Ae/Be systems
We have calculated the (sub-)mm spectral indices of 26 Herbig Ae/Bestars, for which we can determine the infrared spectral energydistribution (SED). We find a clear correlation between the strength ofthe ratio of the near- to mid-infrared excess of these sources, and theslope of the (sub-)mm energy distribution. Based on earliermulti-dimensional modeling of disks around Herbig Ae stars, we interpretthis as a correlation between the geometry of the disk (flared orself-shadowed) and the size of the grains: self-shadowed disks have, onaverage, larger grains than their flared counterparts. These datasuggest that the geometry of a young stellar disk evolves from flared toself-shadowed.
| Optical Properties of the Circumstellar Dust around Stars with Aperiodic Fadings
Multicolor observations of 21 rapid, irregular variable stars withAlgol-like fadings and of R Coronae Borealis are used to derive theoptical extinction coefficients of the circumstellar dust associatedwith these objects, θ(λ). We used more than 3600 uniformmulti-color brightness measurements obtained in a rigorous U BV R systemat the Terskol High-altitude Observing Station. The mean extinctioncoefficient θ for the circumstellar dust for this sample of starscoincides almost exactly with the interstellar extinction coefficient,θis, but is somewhat higher in the ultraviolet. We suggest anexplanation for this difference.
| Dynamics of the circumstellar gas in the Herbig Ae stars BF Orionis, SV Cephei, WW Vulpeculae and XY Persei
We present high resolution (λ/Δ λ = 49 000)échelle spectra of the intermediate mass, pre-main sequence starsBF Ori, SV Cep, WWWul and XY Per. The spectra cover the range3800-5900 Å and monitor the stars on time scales of months anddays. All spectra show a large number of Balmer and metallic lines withvariable blueshifted and redshifted absorption features superimposed tothe photospheric stellar spectra. Synthetic Kurucz models are used toestimate rotational velocities, effective temperatures and gravities ofthe stars. The best photospheric models are subtracted from eachobserved spectrum to determine the variable absorption features due tothe circumstellar gas; those features are characterized in terms oftheir velocity, v, dispersion velocity, Δ v, and residualabsorption, Rmax. The absorption components detected in eachspectrum can be grouped by their similar radial velocities and areinterpreted as the signature of the dynamical evolution of gaseousclumps with, in most cases, solar-like chemical composition. Thisinfalling and outflowing gas has similar properties to the circumstellargas observed in UX Ori, emphasizing the need for detailed theoreticalmodels, probably in the framework of the magnetospheric accretionscenario, to understand the complex environment in Herbig Ae (HAe)stars. WW Vul is unusual because, in addition to infalling andoutflowing gas with properties similar to those observed in the otherstars, it shows also transient absorption features in metallic lineswith no obvious counterparts in the hydrogen lines. This could, inprinciple, suggest the presence of CS gas clouds with enhancedmetallicity around WW Vul. The existence of such a metal-rich gascomponent, however, needs to be confirmed by further observations and amore quantitative analysis.Tables \ref{master_table_bfori}-\ref{master_table_xyper} are onlyavailable in electronic form at http://www.edpsciences.org
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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.
| 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.
| The Herbig Ae/Be Star V586 Ori
Studies of the Herbig Ae/Be star V586 Ori based on photoelectricobservations in the Strömgren system are reported. It is found thatas the brightness decreases, the Balmer discontinuity index initiallyincreases and then begins to drop. As the star fades, the indexdecreases. We have previously found similar variations in the indicesand C for UX Ori, RR Tau, and V351 Ori. It is shown that V586 Ori hasthe characteristics of a shell-star.
| Search for Accretion Events of Circumstellar Matter in UX Ori Stars
During some Algol-like minima flare-like events in the optical lightcurves of UX Ori-type stars were observed. Flare-like events withamplitudes 0.15--1.0 mag in the l V passband sometimes were observedafter the minimum of the star brightness. After the flare the starreturns to the initial state. UV-excesses with an amplitude of {\msfU--B} = 0.15--0.20 mag sometimes appear near the minimum brightness, butnot simultaneously with the optical flare. We suggest that theseflare-like phenomena indicate the fall of a large amount ofcircumstellar matter onto the young star.
| Galactic environment and the 10-μm silicate feature of young stellar objects
Disc and sphere dust models are used to fit 8-13 μm flux spectra of19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The13 non-photospheric low-mass YSOs in quiescent environments and the fiveHerbig AeBe stars have mean disc temperature indices of 0.4, indicatingthat the emission arises from optically thin layers above a flaredoptically thick disc; 10 out of 14 of the low-mass YSO and four out offive of the Herbig AeBe features contain an optically thin silicateemission component. The radius of the peak 10-μm emission for nineout of the 13 low-mass YSOs is 10-130 au, and three out of the fiveHerbig AeBe stars are 10-30 au in size. In contrast, the five YSOs fromdisrupted molecular clouds that have been shaped by expanding supernovaremnants have temperature indices of between 0.3 and 0.8; four out ofthe five are optically thick and three out of the five have radii <~2au. The photosphere-like continuum of Taurus-Elias 18 could be fittedonly with truncated optically thick models, implying the presence of avoid between the >500 K and cold (<~100 K) foreground dust.Silicates surrounding low-mass YSOs in quiescent molecular clouds aresimilar to those in the Trapezium region of the Orion Nebula except whenAV<~ 2 mag. In the low-AV case and in low-massYSOs in disrupted molecular clouds the silicates are similar tocircumstellar dust around the evolved star μ Cephei.
| Photometric and Polarimetric Activity of the Herbig Ae Star VX Cas
We present the results of our simultaneous photometric and polarimetricobservations of the Herbig Ae/Be star VX Cas acquired in 1987 2001. Thestar belongs to the UX Ori subtype of young variable stars and exhibitsa rather low level of photometric activity: only six Algol-like minimawith amplitudes ΔV>1m were recorded in 15 years ofobservations. Two of these minima, in 1998 and 2001, were the deepest inthe history of the star’s photometric studies, with V amplitudesof about 2m. In each case, the dimming was accompanied by an increase inthe linear polarization in agreement with the law expected for variablecircumstellar extinction. The highest V polarization was about 5%.Observations of VX Cas in the deep minima revealed a turnover of thecolor tracks, typical of stars of this type and due to an increasedcontribution from radiation scattered in the circumstellar disk. Weseparated the observed polarization of VX Cas into interstellar (P is)and intrinsic (P in) components. Their position angles differ byapproximately 60°, with P is dominating in the bright state and P indominating during the deep minima. The competition of these twopolarization components leads to changes in both the degree and positionangle of the polarization during the star’s brightness variations.Generally speaking, in terms of the behavior of the brightness, colorindices, and linear polarization, VX Cas is similar to other UX Oristars studied by us earlier. A number of episodes of photometric andpolarimetric activity suggest that, in their motion along highlyeccentric orbits, circumstellar gas and dust clouds can enter the closevicinity of the star (and be disrupted there).
| ISO Observations of the Infrared Continuum of the Herbig Ae/Be Stars
We present a study of the whole sample of the Herbig Ae/Be starsobserved with the spectrometers on board of the Infrared SpaceObservatory (ISO/ESA). These objects have been studied not only by meansof their infrared continuum emission but also with respect to all theavailable photometric data, collected from the optical region to theradio wavelengths. The global spectral energy distributions (SEDs) havebeen compared with the results of radiative transfer calculations, thathave been made in the framework of a circumstellar (CS) model. Theresults of the selection of the best models allow us to infer on therelationship between the geometry of the CS matter distribution and theevolutionary stage of these objects.
| Anomalous Behavior of the Linear Polarization of the Herbig Ae Star WW Vul during the 1997 Minimum
In the course of photopolarimetric monitoring of the Herbig Ae star WWVul, an unusual variation of linear polarization upon energence from adeep minimum was recorded, which cannot be explained by the standardmechanism for such objects (UX Ori stars): a variable contribution ofradiation scattered by circumstellar dust. An analysis of possiblecauses of the anomalous behavior of polarization shows that it was mostlikely due to the presence of aligned, aspherical dust grains in thedust cloud crossing the line of sight at that time.
| On the simultaneous optical and near-infrared variability of pre-main sequence stars
For a complete understanding of the physical processes causing thephotometric variability of pre-main sequence systems, simultaneousoptical and near-IR observations are required to disentangle theemission from the stars and that from their associated circumstellardisks. Data of this sort are extremely rare and little systematic workhas been reported to date. The work presented in this paper is asystematic attempt in this direction. It presents an analysis of thesimultaneous optical and near-IR photometric variability of 18 HerbigAe/Be and T Tauri stars which were observed in October 98 by the EXPORTcollaboration. The time difference between the UBVRI and JHKmeasurements is less than 1 hour in ~50% of the data and the largestdifference is around 2 hours in only ~10% of the data. Twelve starsappear to show a correlation between the optical and near-IR variabilitytrends, which suggests a common physical origin such as spots and/orvariable extinction. The optical and near-IR variability is uncorrelatedin the rest of the objects, which suggests it originates in distinctlydifferent regions. In general, the optical variability qualitativelyfollows the predictions of starspots or variable extinction. As far asthe near-IR is concerned, the simultaneity of the observationsdemonstrates that for most objects the flux is largely produced by theircircumstellar disks and, consequently, in many cases the near-IRfluctuations must be attributed to structural variations of such disksproducing variations of their thermal emission and/or scattered light.The observed near-IR changes of up to around 1 mag on timescales of 1-2days provide interesting challenges for understanding the mechanismsgenerating such remarkable variabilities, an issue insufficientlyinvestigated until now but one which deserves further theoretical andmodeling efforts.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Лисичка |
| Прямое восхождение: | 19h25m58.75s |
| Склонение: | +21°12'31.3" |
| Видимая звёздная величина: | 10.692 |
| Собственное движение RA: | 1.4 |
| Собственное движение Dec: | -6.9 |
| B-T magnitude: | 11.049 |
| V-T magnitude: | 10.722 |
Каталоги и обозначения:
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