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Observations of Herbig Ae Disks with Nulling Interferometry
We present the results of 10 μm nulling interferometric observationsof 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 mtelescopes. A portion of the observations was completed with theadaptive secondary at the MMT. We have conclusively spatially resolved 3of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter tworecently resolved using adaptive optics in combination with nullinginterferometry. For the resolved objects we find that the 10 μmemitting regions have a spatial extent of 15-30 AU in diameter. We alsohave some evidence for resolved emission surrounding an additional twostars (V892 Tau and R CrA). For those objects in our study with mid-IRSEDs in the classification of Meeus and coworkers, we find that thegroup I objects (those with constant to increasing mid-IR flux) are morelikely to be resolved, within our limited sample. This trend is evidentin correlations in the inferred disk sizes versus the submillimeter SEDslope and disk size versus fractional infrared luminosity of thesystems. We explore the spatial distribution and orientation of the warmdust in the resolved systems and constrain physical models that areconsistent with their observational signatures.The results presented here made use of the of MMT Observatory, a jointlyoperated facility of the University of Arizona and the SmithsonianInstitution. This paper also includes data gathered with the 6.5 mMagellan Telescopes located at Las Campanas Observatory, Chile.
| 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.
| Optical spectroscopy of close companions to nearby Herbig Ae/Be and T Tauri stars
We present VLT-FORS2 optical (5700-9400 Å) spectroscopy of close(r < 1.5´´) companions to three nearby (d < 200 pc)Herbig Ae/Be stars (HD 144432, HD 150193, KK Oph) and one T Tauri star(S CrA). We report the detection of Li I (6707 Å) in absorptionand emission lines (Hα, Ca II triplet) in the spectra of thecompanions. Our observations strongly suggest that the companions arephysically associated pre-main-sequence stars. The spectral type derivedfor the companions is K5Ve for HD 144432 B, F9Ve for HD 150193 B, andG6Ve for KK Oph B. S CrA A and B were observed simultaneously. Thespatially resolved spectra indicate that S CrA A (primary, north) is a Gstar and that S CrA B (secondary, south) is a K star. Using photometryfrom the literature and estimations of the R and I magnitude derivedfrom the spectra, we localized primaries and companions in the HRdiagram, derived their masses and assuming coevality constrained the ageof the systems. KK Oph B (7 Myr) and S CrA B (2 Myr) are activelyaccreting T Tauri stars and are very likely surrounded by disks. HD150193 B (10 Myr) and HD 144432 B (8 Myr) are weak-line T Tauri stars.Three of the four systems studied (HD 144432, HD 150193, KK Oph) haveages >7 Myr. These systems retained their disks for a longer timethan typical of a young star. Our results suggest that binarity may be akey issue in understanding the lifetime of disks.Based on observations collected at the European Southern Observatory,Chile (program ID 075.C-0395(A)). Figure 3 is only available inelectronic form at http://www.aanda.org
| A brown dwarf desert for intermediate mass stars in Scorpius OB2?
We present JHKS observations of 22 intermediate-mass stars inthe Scorpius-Centaurus OB association, obtained with the NAOS/CONICAsystem at the ESO Very Large Telescope. This survey was performed todetermine the status of (sub)stellar candidate companions of Sco OB2member stars of spectral type A and late-B. The distinction betweencompanions and background stars is made on the basis of a comparison toisochrones and additional statistical arguments. We are sensitive tocompanions with an angular separation of 0.1''-11'' (13-1430 AU) and thedetection limit is K_S=17 mag. We detect 62 stellar components of which18 turn out to be physical companions, 11 candidate companions, and 33background stars. Three of the 18 confirmed companions were previouslyundocumented as such. The companion masses are in the range 0.03{M}_ȯ ≤ M ≤ 1.19 {M}_ȯ, corresponding to mass ratios0.06 ≤ q ≤ 0.55. We include in our sample a subset of 9 targetswith multi-color ADONIS observations from Kouwenhoven et al. (2005,A&A, 430, 137). In the ADONIS survey secondaries with KS< 12 mag were classified as companions; those with KS >12 mag as background stars. The multi-color analysis in this paperdemonstrates that the simple K_S=12 mag criterion correctly classifiesthe secondaries in 80% of the cases. We reanalyse the total sample(i.e. NAOS/CONICA and ADONIS) and conclude that of the 176 secondaries,25 are physical companions, 55 are candidate companions, and 96 arebackground stars. Although we are sensitive (and complete) to browndwarf companions as faint as K_S=14 mag in the semi-major axis range130-520 AU, we detect only one, corresponding to a brown dwarf companionfraction of 0.5 ± 0.5% (M ⪆ 30 {M_J}). However, the number ofbrown dwarfs is consistent with an extrapolation of the (stellar)companion mass distribution into the brown dwarf regime. This indicatesthat the physical mechanism for the formation of brown dwarf companionsaround intermediate mass stars is similar to that of stellar companions,and that the embryo ejection mechanism does not need to be invoked inorder to explain the small number of brown dwarf companions amongintermediate mass stars in the Sco OB2 association.Based on observations collected at the European Southern Observatory,Chile. Program 073.D-0354(A). Appendix A is only available in electronicform at http://www.aanda.org
| A VLT/NACO survey for triple and quadruple systems among visual pre-main sequence binaries
Aims.This paper describes a systematic search for high-ordermultiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods: .We conducted an Adaptive Optics survey of a sample of 58 PMSwide binaries from various star-forming regions, which include 52 TTauri systems with mostly K- and M-type primaries, with the NIRinstrument NACO at the VLT. Results: .Of these 52 systems, 7 arefound to be triple (2 new) and 7 quadruple (1 new). The new closecompanions are most likely physically bound based on their probabilityof chance projection and, for some of them, on their position on acolor-color diagram. The corresponding degree of multiplicity among widebinaries (number of triples and quadruples divided by the number ofsystems) is 26.9 ± 7.2% in the projected separation range ~0.07arcsec -12'', with the largest contribution from the Taurus-Aurigacloud. We also found that this degree of multiplicity is twice in Tauruscompared to Ophiuchus and Chamaeleon for which the same number ofsources are present in our sample. Considering a restricted samplecomposed of systems at distance 140-190 pc, the degree of multiplicityis 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30binaries, 5 triples, 6 quadruples). The observed frequency agrees withresults from previous multiplicity surveys within the uncertainties,although a significant overabundance of quadruple systems compared totriple systems is apparent. Tentatively including the spectroscopicpairs in our restricted sample and comparing the multiplicity fractionsto those measured for solar-type main-sequence stars in the solarneighborhood leads to the conclusion that both the ratio of triples tobinaries and the ratio of quadruples to triples seems to be in excessamong young stars. Most of the current numerical simulations of multiplestar formation, and especially smoothed particles hydrodynamicssimulations, over-predict the fraction of high-order multiplicity whencompared to our results. The circumstellar properties around theindividual components of our high-order multiple systems tend to favormixed systems (i.e. systems including components of wTTS and cTTS type),which is in general agreement with previous studies of disks inbinaries, with the exception of Taurus, where we find a preponderance ofsimilar type of components among the multiples studied.
| 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.
| On the origin of the X-ray emission from Herbig Ae/Be stars
Context: .Herbig Ae/Be stars are fully radiative and not expected tosupport dynamo action analogous to their convective lower-masscounterparts, the T Tauri stars. Alternative X-ray productionmechanisms, related to stellar winds or star-disk magnetospheres havebeen proposed, but their X-ray emission has remained a mystery. Aims: .A study of Herbig Ae/Be stars' global X-ray properties (such asdetection rate, luminosity, temperature, variability), helps toconstrain the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: .We performed a systematic search forChandra archival observations of Herbig Ae/Be stars. The superiorspatial resolution of this satellite with respect to previous X-rayinstrumentation has allowed us to also examine the possible role oflate-type companions in generating the observed X-rays. Results:.In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-rayemitting faint companions or other unrelated X-ray bright objects within10''. The detection fraction of Herbig Ae/Be stars is 76%, but decreasesto 35% if all emission is attributed to further known and unresolvedcompanions. The spectral analysis confirms the high X-ray temperatures( 20 MK) and large range of fractional X-ray luminosities(log{L_x/L_*}) of this class derived from earlier studies of individualobjects. Conclusions: .Radiative winds are ruled out as anemission mechanism on the basis of the high temperatures. The X-rayproperties of Herbig Ae/Be stars are not vastly different from those oftheir late-type companion stars (if such are known), nor from otheryoung late-type stars used for comparison. Therefore, either a similarkind of process takes place in both classes of objects, or there must beas yet undiscovered companion stars.
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Spectroscopic analysis of southern B and Be stars
Spectroscopic monitoring of 141 southern field B-type stars, 114 of themknown to exhibit the Be phenomenon, allowed the estimation of theirprojected rotational velocities, effective temperatures and superficialgravities from both line and equivalent width fitting procedures.Stellar ages, masses and bolometric luminosities were derived frominternal structure models. Without taking into account the effects ofgravity darkening, we note the occurrence of the Be phenomenon in laterstages of main-sequence phase.
| 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.
| 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.
| Optical polarimetry of infrared excess stars
We present UBRVI polarimetry measurements for a group of 38 IRASinfrared excess stars and complement these observations with V-band datataken from the literature for 87 additional objects. After correctingthe observed values by the interstellar contribution, we find that 48%of the analyzed sample has polarization excess. In addition, thepolarization of these stars may correlate with infrared color excesses,particularly at 60 and 100 μm. We caution, however, that poor IRASdata quality at longer wavelengths affects this correlation. We analyzethe wavelength dependence of the linear polarization of 15 polarizedobjects in relation to Serkowski's empirical interstellar law. We findthat for 6 to 7 objects (depending on the interstellar model) themeasured polarization differs significantly from the empiricalinterstellar law, suggesting an intrinsic origin. We analyze thepolarimetry distribution of IRAS infrared excess objects in relation tothe Exoplanet host stars (i.e., stars associated with at least onelikely planetary mass object). The corresponding polarimetrydistributions are different within a high confidence level. Finally, wecompare the metallicity distributions of F and G IRAS infrared excess,Exoplanet host and field main sequence stars, and find that F-G IRASinfrared excess objects have metallicities quite similar (although notidentical) to field main sequence stars and significantly different fromthe Exoplanet host group.
| 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.
| 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 imaging polarimetry of dusty young stars
We have carried out JHK polarimetric observations of 11 dusty youngstars, by using the polarimeter module IRPOL2 with the near-infraredcamera UIST on the 3.8-m United Kingdom Infrared Telescope (UKIRT). Oursample targeted systems for which UKIRT-resolvable discs had beenpredicted by model fits to their spectral energy distributions. Ourobservations have confirmed the presence of extended polarized emissionaround TW Hya and around HD 169142. HD 150193 and HD 142666 show thelargest polarization values among our sample, but no extended structurewas resolved. By combining our observations with Hubble Space Telescope(HST) coronographic data from the literature, we derive the J- andH-band intrinsic polarization radial dependences of the disc of TW Hya.We find the polarizing efficiency of the disc is higher at H than at J,and we confirm that the J- and H-band percentage polarizations arereasonably constant with radius in the region between 0.9 and 1.3arcsecfrom the star. We find that the objects for which we have detectedextended polarizations are those for which previous modelling hassuggested the presence of flared discs, which are predicted to bebrighter than flat discs and thus would be easier to detectpolarimetrically.
| 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.
| 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.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| [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.
| The Disk Atmospheres of Three Herbig Ae/Be Stars
We present infrared (IR) spectrophotometry (R~=180) of three HerbigAe/Be stars surrounded by possible protoplanetary disks: HD 150193, HD100546, and HD 179218. We construct a mid-IR spectral energydistributions (SEDs) for each object by using 7.6-13.2 μm HIFOGSspectra, 2.4-45 μm spectrophotometry from the Infrared SpaceObservatory Short-Wavelength Spectrometer, the 12, 25, 60, and 100 μmphotometric points from IRAS, and for HD 179218, photometric bolometricdata points from the Mount Lemmon Observing Facility. The SEDs aremodeled by using an expanded version of the Chiang & Goldreichtwo-layer, radiative and hydrostatic equilibrium, passive disk. Thisexpanded version includes the emission from Mg-pure crystalline olivine(forsterite) grains in the disk surface layer. Each of the three objectsstudied vary in the amount of crystals evident from theirspectrophotometry. HD 150193 contains no crystals, while HD 100546 andHD 179218, respectively, show evidence of having crystalline silicatesin the surface layers of their disks. We find that the inner region ofHD 100546 has a 37% higher crystalline-to-amorphous silicate ratio inits inner disk region (<=5 AU) than in the outer disk region, whilethe inner disk region of HD 179218 has a 84% highercrystalline-to-amorphous silicate ratio in its inner disk region (<=5AU) than in the outer region. All three objects are best fitted using agrain-size distribution power law that falls as a-3.5. HD150193 is best fitted by a small disk (~5 AU in radius), while HD 100546and HD 179218 are best fitted by larger disks (~150 AU in radius).Furthermore, HD 100546's disk flares larger than those of HD 150193 (25%more at 5 AU) and HD 179218 (80% more at 5 AU). We discuss theimplications of our results and compare them with other modelingefforts.
| 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.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| The primordial binary population. I. A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey withthe aim to detect close companions to Hipparcos members in the threesubgroups of the nearby OB association Sco OB2: Upper Scorpius (US),Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We havetargeted 199 A-type and late B-type stars in the KS band, anda subset also in the J and H band. We find 151 stellar components otherthan the target stars. A brightness criterion is used to separate thesecomponents into 77 background stars and 74 candidate physical companionstars. Out of these 74 candidate companions, 41 have not been reportedbefore (14 in US; 13 in UCL; 14 in LCC). The angular separation betweenprimaries and observed companion stars ranges from 0.22'' to 12.4''. Atthe mean distance of Sco OB2 (130 pc) this corresponds to a projectedseparation of 28.6 AU to 1612 AU. Absolute magnitudes are derived forall primaries and observed companions using the parallax andinterstellar extinction for each star individually. For each object wederive the mass from KS, assuming an age of 5 Myr for the USsubgroup, and 20 Myr for the UCL and LCC subgroups. Companion starmasses range from 0.10 Mȯ to 3.0 Mȯ. Themass ratio distribution follows f(q) = q-Γ withΓ=0.33, which excludes random pairing. No close (ρ ≤3.75'') companion stars or background stars are found in the magnituderange 12 mag≤ KS ≤ 14 mag. The lack of stars withthese properties cannot be explained by low-number statistics, and mayimply a lower limit on the companion mass of 0.1Mȯ. Close stellar components with KS >14mag are observed. If these components are very low-mass companion stars,a gap in the companion mass distribution might be present. The smallnumber of close low-mass companion stars could support theembryo-ejection formation scenario for brown dwarfs. Our findings arecompared with and complementary to visual, spectroscopic, andastrometric data on binarity in Sco OB2. We find an overall companionstar fraction of 0.52 in this association. This is a lower limit sincethe data from the observations and from literature are hampered byobservational biases and selection effects. This paper is the first steptoward our goal to derive the primordial binary population in Sco OB2.Full Table 1 is only available in electronic form athttp://www.edpsciences.orgBased on observations collected with the ADONIS instrument at theEuropean Southern Observatory, La Silla, Chile (65.H-0568(A) and67.D-0220(A)).
| 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
| SiO Maser Survey of IRAS Sources in the Inner Galactic Disk
We surveyed 401 color-selected IRAS sources in the galactic disk in theSiO J = 1—0, v = 1 and 2 maser lines at 43GHz, resulting in 254(239 new) detections. The observed sources lie mostly in a strip of theinner galactic disk with boundaries -10° < l < 40° and |b|< 3°. This survey provides radial velocities of inner-disk starsfor which optical measurements cannot be made due to interstellarextinction. The SiO l‑v diagram in the area -10° < l <40° exhibits fewer objects coincident with the molecular ringfeature than does the OH 1612MHz source l ‑ v map, indicating aslight difference of stellar type between SiO and OH emitting stars.After identifying all of the SiO detected sources in the 2MASSnear-infrared catalog, we computed their luminosity distances based onthe infrared fluxes. We then mapped these objects onto the firstquadrant of the galactic plane. Combining the distances with the SiOradial velocities, we obtained a pattern speed for SiO maser sources,ΩP = 21(±13)km s-1 kpc-1,between the distances 1 and 5.5kpc, without the use of any dynamicalmodels. The increase of the pattern speed toward the galactic center (upto 60 km s-1 kpc-1 between the distances, 5.5 and7kpc) suggests the presence of two pattern speeds in the Galaxy.
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Osservazione e dati astrometrici
| Costellazione: | Ofiuco |
| Ascensione retta: | 16h40m17.92s |
| Declinazione: | -23°53'45.2" |
| Magnitudine apparente: | 8.889 |
| Distanza: | 150.15 parsec |
| Moto proprio RA: | -6 |
| Moto proprio Dec: | -17 |
| B-T magnitude: | 9.492 |
| V-T magnitude: | 8.939 |
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