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Публикации по объекту
Hubble Space Telescope Advanced Camera for Surveys Coronagraphic Observations of the Dust Surrounding HD 100546
We present HST ACS coronagraphic observations of HD 100546, a B9.5 star,103 pc away from the Sun, taken in the F435W, F606W, and F814W bands.Scattered light is detected up to 14" from the star. The observationsare consistent with the presence of an extended flattened nebula withthe same inclination as the inner disk. The well-known ``spiral arms''are clearly observed and trail the rotating disk material. Weaker armsnever before reported are also seen. The interarm space becomesbrighter, but the structures become more neutral in color at longerwavelengths, which is not consistent with models that assume that theyare due to the effects of a warped disk. Along the major disk axis, thecolors of the scattered light relative to the star areΔ(F435W-F606W)~0.0-0.2 mag and Δ(F435W-F814W)~0.5-1 mag. Toexplain these colors, we explore the role of asymmetric scattering,reddening, and large minimum sizes on ISM-like grains. We conclude thateach of these hypotheses by itself cannot explain the colors. The diskcolors are similar to those derived for Kuiper Belt objects, suggestingthat the same processes responsible for their colors may be at workhere. We argue that we are observing only the geometrically thick,optically thin envelope of the disk, while the optically thick diskresponsible for the far-IR emission is undetected. The observed spiralarms are then structures on this envelope. The colors indicate that theextended nebulosity is not a remnant of the infalling envelope butreprocessed disk material.
| 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.
| 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.
| The Asymmetric Thermal Emission of the Protoplanetary Disk Surrounding HD 142527 Seen by Subaru/COMICS
Mid-infrared (MIR) images of the Herbig Ae star HD 142527 were obtainedat 18.8 and 24.5 μm with the Subaru/COMICS. Bright extended arclikeemission (outer disk) is recognized at r=0.85" together with a strongcentral source (inner disk) and a gap around r=0.6" in both images. Thethermal emission on the eastern side is much brighter than that on thewestern side in the MIR. We estimate the dust size to be a few micronsfrom the observed color of the extended emission and the distance fromthe star. The dust temperature T and the optical depth τ of theMIR-emitting dust are also derived from the two images as T=82+/-1 K,τ=0.052+/-0.001 for the eastern side and T=85+/-3 K,τ=0.018+/-0.001 for the western side. The observed asymmetry in thebrightness can be attributed to the difference in the optical depth ofthe MIR-emitting dust. To account for the present observations, wepropose an inclined disk model, in which the outer disk is inclinedalong the east-west direction with the eastern side being on the farside while the inner rim of the outer disk on the eastern side isdirectly exposed to us. The proposed model can successfully account forthe MIR observations as well as the near-infrared images of thescattering light, in which the asymmetry is seen in the opposite senseand in which the forward scattering light (near side-western side) isbrighter.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| 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.
| Near-Infrared Images of Protoplanetary Disk Surrounding HD 142527
We discovered a unique morphology in a disk around the Herbig Ae star HD142527 by near-infrared (H and K bands) adaptive optics imagingobservations. The almost face-on disk consists of two bright arcs facingone another along the east-west direction (banana-split structure) andone spiral arm extending to the north from the western arc. The easternarc is located at ~100-400 AU in radius from the star, and the westernone is detected at ~150-490 AU. The stellar position is displaced fromthe center of the disk by about 20 AU to the north, and also from thecenter of the inner hole. The two arcs show an asymmetry in their sizeand brightness; the larger western arc is brighter than the east one byabout 2 mag. The morphology of the disk, consisting of a banana-splitstructure and a spiral arm, most likely suggests the presence of anunseen eccentric binary and a recent stellar encounter.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| First determination of the (re)crystallization activation energy of an irradiated olivine-type silicate
To study the evolution of silicate dust in different astrophysicalenvironments we simulate, in the laboratory, interstellar andcircumstellar ion irradiation and thermal annealing processes. Anexperimental protocol that follows different steps in the dustlife-cycle was developed. Using the silicate 10 μm band as anindicator, the evolution of the structural properties of anion-irradiated olivine-type silicate sample, as a function oftemperature, is investigated and an activation energy forcrystallization is determined. The obtained value of {E_a}/k = 41 700± 2400 K is in good agreement with previous determinations of theactivation energies of crystallization reported for non-ion-irradiated,amorphous silicates. This implies that the crystallization process isindependent of the history of the dust. In particular, the defectconcentration due to irradiation appears not to play a major role instimulating, or hindering, crystallization at a given temperature. Thisactivation energy is an important thermodynamical parameter that must beused in theoretical models which aim to explain the dust evolution fromits place of birth in late type stars to its incorporation into youngstellar environments, proto-stellar discs and proto-planetary systemsafter long passage through the interstellar medium.
| The shape of the inner rim in proto-planetary disks
This paper discusses the properties of the inner puffed-up rim thatforms in circumstellar disks when dust evaporates. We argue that the rimshape is controlled by a fundamental property of circumstellar disks,namely their very large vertical density gradient, through thedependence of grain evaporation temperature on gas density. As a result,the bright side of the rim is curved, rather than vertical, as expectedwhen a constant evaporation temperature is assumed. We have computed anumber of rim models that take into account this effect in aself-consistent way. The results show that the curved rim (as thevertical rim) emits most of its radiation in the near and mid-IR, andprovides a simple explanation for the observed values of the near-IRexcess (the “3 μm bump” of Herbig Ae stars). Contrary tothe vertical rim, for curved rims the near-IR excess does not dependmuch on the inclination, being maximum for face-on objects. We thencomputed synthetic images of the curved rim seen under differentinclinations; face-on rims are seen as bright, centrally symmetric ringson the sky; increasing the inclination, the rim takes an ellipticalshape, with one side brighter than the other.
| 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.
| 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.
| Recent astrophysical results from the VLTI.
Not Available
| 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.
| 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.
| 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 building blocks of planets within the `terrestrial' region of protoplanetary disks
Our Solar System was formed from a cloud of gas and dust. Most of thedust mass is contained in amorphous silicates, yet crystalline silicatesare abundant throughout the Solar System, reflecting the thermal andchemical alteration of solids during planet formation. (Even primitivebodies such as comets contain crystalline silicates.) Little is knownabout the evolution of the dust that forms Earth-like planets. Here wereport spatially resolved detections and compositional analyses of thesebuilding blocks in the innermost two astronomical units of threeproto-planetary disks. We find the dust in these regions to be highlycrystallized, more so than any other dust observed in young stars untilnow. In addition, the outer region of one star has equal amounts ofpyroxene and olivine, whereas the inner regions are dominated byolivine. The spectral shape of the inner-disk spectra shows surprisingsimilarity with Solar System comets. Radial-mixing models naturallyexplain this resemblance as well as the gradient in chemicalcomposition. Our observations imply that silicates crystallize beforeany terrestrial planets are formed, consistent with the composition ofmeteorites in the Solar System.
| 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
| ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks
Observations of interstellar gas-phase and solid-state species in the2.4 200 m range obtained with the spectrometers on board the InfraredSpace Observatory (ISO) are reviewed. Lines and bands caused by ices,polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms andmolecules (in particular H2, CO, H2O, OH, andCO2) are summarized and their diagnostic capabilitiesillustrated. The results are discussed in the context of the physicaland chemical evolution of star-forming regions, includingphoton-dominated regions, shocks, protostellar envelopes, and disksaround young stars.
| Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI
We present the first long baseline mid-infrared interferometricobservations of the circumstellar disks surrounding Herbig Ae/Be stars.The observations were obtained using the mid-infrared interferometricinstrument MIDI at the European Southern Observatory (ESO) Very LargeTelescope Interferometer VLTI on Cerro Paranal. The 102 m baseline givenby the telescopes UT1 and UT3 was employed, which provides a maximumfull spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10μm. The interferometric signal was spectrally dispersed at aresolution of 30, giving spectrally resolved visibility information from8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars andresolved all objects. The warm dust disk of HD 100546 could even beresolved in single-telescope imaging. Characteristic dimensions of theemitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10μm sizes of our sample stars correlate with the slope of the 10-25μm infrared spectrum in the sense that the reddest objects are thelargest ones. Such a correlation would be consistent with a differentgeometry in terms of flaring or flat (self-shadowed) disks for sourceswith strong or moderate mid-infrared excess, respectively. We comparethe observed spectrally resolved visibilities with predictions based onexisting models of passive centrally irradiated hydrostatic disks madeto fit the SEDs of the observed stars. We find broad qualitativeagreement of the spectral shape of visibilities corresponding to thesemodels with our observations. Quantitatively, there are discrepanciesthat show the need for a next step in modelling of circumstellar disks,satisfying both the spatial constraints such as are now available fromthe MIDI observations and the flux constraints from the SEDs in aconsistent way.Based on observations made with the Very Large Telescope Interferometerat Paranal Observatory.
| 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.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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).
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Understanding the spectra of isolated Herbig stars in the frame of a passive disk model
We discuss spectral energy distributions of a sample of Herbig Ae/Bestars in the context of a passive irradiated disk model. The data havebeen presented earlier by Meeus et al. (\cite{2001A&A...365..476M}),and preliminary interpretations of these data were given in that paper.While the spectra of Herbig Ae stars all show similarities, there issignificant variation between the spectra, in particular in the shape ofthe mid-IR rise and in the presence or absence of a silicate feature. Weexplore the hypothesis that all these different spectra can beinterpreted as pure disk spectra without additional components. Usingthe model of Dullemond et al. (\cite{2001ApJ...560..957D}) we deduce thedisk parameters of a number of the sources, and find that for a largefraction of investigated sources, satisfactory fits can be obtained. Thederived model parameters show that some group Ia sources can only be fitwith radially increasing surface densities, indicating the presence ofdepleted inner disk regions. The steep-sloped SEDs of group IIa sourcescan be fit with very compact disks, probably representing disks withcollapsed outer regions. The largest difficulties arise from sourcesthat do not show significant silicate emission features. Our attempts toexplain these objects with a pure geometric effect are only partiallysuccessful. It seems that these stars indeed require a strong depletionof small silicate grains.
| The absence of the 10 mu m silicate feature in the isolated Herbig Ae star HD 100453
We analyse the optical and IR spectra, as well as the spectral energydistribution (UV to mm) of the candidate Herbig Ae star HD 100453. Thisstar is particular, as it shows an energy distribution similar to thatof other isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them,it does not have a silicate emission feature at 10 mu m, as is shown inMeeus et al. (\cite{meeus2001}). We confirm the HAEBE nature of HD100453 through an analysis of its optical spectrum and derived locationin the H-R diagram. The IR spectrum of HD 100453 is modelled by anoptically thin radiative transfer code, from which we derive constraintson the composition, grain-size and temperature distribution of thecircumstellar dust. We show that it is both possible to explain the lackof the silicate feature as (1) a grain-size effect - lack of smallsilicate grains, and (2) a temperature effect - lack of small, hotsilicates, as proposed by Dullemmond et al. (\cite{dullemond2001}), anddiscuss both possibilities. Finally, we show that the latter possibilityis the more preferable.
| Nanodiamonds around HD 97048 and Elias 1
We present an analysis of ISO-SWS observations of the Herbig Ae/Be starsHD 97048 and Elias 1. Besides the well-known family of IR emission bandsat 3.3, 6.2, ``7.7'', 8.6 and 11.2 mu m these objects show strong,peculiar emission features at 3.43 and 3.53 mu m. The latter twofeatures show pronounced substructure which is very similar in the twosources. Comparison of the spectra of HD 97048 and Elias 1 withlaboratory spectra of H-terminated diamond surfaces show excellent andvery convincing agreement in peak position and spectral detail (Guilloiset al. \cite{Gui99}). The position of the 3.53 mu m band indicates atemperature of ~ 1000 K. An analysis of the radiative energy budgetmakes us conclude that the diamond carrier of the 3.53 mu m feature hastypical sizes of 1-10 nm for HD 97048. A fit of the 3.53 mu m featurewith a theoretical, calculated profile indicates that the emittingdiamonds in HD 97048 see a FUV flux of 5.8 x 10-3[W/cm2]. The derived diamond mass, 1.5 x 10-10Msun, is only a tiny fraction of the total circumstellar dustmass and corresponds to only about 1 parts per billion relative tohydrogen. We discuss the origin of the diamond around these Herbig Ae/Bestars and conclude that most likely they are formed in situ. Theimplications for the nanodiamonds discovered in meteorites are alsodiscussed.
| Steps toward interstellar silicate mineralogy. VI. Dependence of crystalline olivine IR spectra on iron content and particle shape
Crystalline olivines are an important component of silicate dustparticles in space. ISO observations revealed the presence ofcrystalline silicates in comets, protoplanetary accretion disks, andoutflows from evolved stars. For the interpretation of astronomicalspectra, the relevant material data at a variety of temperatures andover a broad wavelength range, are urgently needed. In contrast to thisneed, optical properties of the astronomically interesting olivines arescarcely available at present. In order to close this gap, we studiedthe optical properties of three minerals of the olivine group byreflection spectroscopy on single crystals in the infrared spectralrange. We measured the iron endmember (fayalite,Fe2SiO4), an Mg-rich olivine(Mg1.9Fe0.1SiO4), and the magnesiumendmember (forsterite, Mg2SiO4) of the(Mg,Fe)2SiO4 series. For a direct comparison withastronomical observations, we present calculated mass absorptioncoefficients in the Rayleigh limit for different shapes and varying ironcontent of the dust particles. The laboratory data together with a setof ISO data for envelopes around evolved stars (Molster 2000) are usedto constrain the properties of circumstellar silicates. We find thatessentially all band positions are shifted to larger wavelengths withincreasing iron content. The particle shape influences verysignificantly the strong bands such as the B1u:nu3mode that appears as the ``11.4 mu m'' band of forsterite, whereas e.g.the two FIR modes longward of 40 mu m remain practically unaffected bythe particle shape but shift due to increasing iron content. Thecomparison with the band positions in ISO spectra points to the presenceof olivine crystals strongly elongated along the crystallographicc-axis. In addition, we apply the calculated mass absorptioncoefficients to evaluate transmission measurements of particles embeddedin a matrix - a technique which is frequently used in laboratoryastrophysics. All data shown in this paper will be made available indigital form via the electronic database http://www.astro.uni-jena.de.
| Detection of the unusual 3.5 mu m feature in the Herbig Be star MWC 297
We present spectroscopic observations of MWC 297 with medium spectralresolution in the 2.1-4.1 mu m region, that show the unusual emissionband at 3.53 mu m as well as gaseous emission lines of the Brackett,Pfund and Humphrey series of hydrogen. A unique aspect of the measured3.53 mu m emission band is the fact that, within our detection limit, itis not accompanied by the 3.3 and 3.4 mu m emission band. We suggestthat the 3.53 mu m feature could be emitted by highly dehydrogenizedcarbonaceous dust particles that were processed under the influence ofthe strong radiation field of the central star.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Волк |
| Прямое восхождение: | 15h56m41.89s |
| Склонение: | -42°19'23.3" |
| Видимая звёздная величина: | 8.348 |
| Расстояние: | 198.413 парсек |
| Собственное движение RA: | -13.9 |
| Собственное движение Dec: | -25.5 |
| B-T magnitude: | 9.203 |
| V-T magnitude: | 8.419 |
Каталоги и обозначения:
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