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Публикации по объекту
Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium
Although they are relatively different in band shape, infrared featuresaround 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 andin the absorption spectra of various dense clouds have both beenattributed to diamondoid molecules/particles. This assignment is basedmainly on infrared spectra of hydrogenated diamond thin films and ofdiamond nanocrystals of known average size. Here we present an analysisof the astrophysical implications of recently reported solid-state2.5-12.5 μm spectra of individual diamondoid molecules, up to thesize of hexamantane (C26H30). These spectraprovide the first experimental measurements of the infrared frequenciesof this class of molecules. In addition, laboratory gas-phase infraredemission spectra of the three smallest members of the diamondoid familyare reported, as well as theoretical spectra for some larger species.The present data set allows us to relate spectral signatures to themolecular size and structure. The spectra of tetrahedral diamondoids arefound to be qualitatively different from those of lower symmetryspecies, which possibly explains the differences between theastrophysical emission and absorption spectra. Interestingly, the 3.53μm band is clearly observed in the spectra of these small moleculardiamondoids, whereas previous studies on nanodiamond particles foundthis band only for species larger than ~50 nm. Our results support theassignment of the 3.43 and 3.53 μm emission features in HD 97048 andElias 1 to diamondoids of a few nanometers in size as well as thesuggestion that smaller diamondoid molecules contribute to the 3.47μm interstellar absorption band.
| Molecular Hydrogen Emission from Protoplanetary Disks. II. Effects of X-Ray Irradiation and Dust Evolution
Detailed models for the density and temperature profiles of gas and dustin protoplanetary disks are constructed by taking into account X-ray andUV irradiation from a central T Tauri star, as well as dust size growthand settling toward the disk midplane. The spatial and sizedistributions of dust grains are numerically computed by solving thecoagulation equation for settling dust particles, with the result thatthe mass and total surface area of dust grains per unit volume of thegas in the disks are very small, except at the midplane. TheH2 level populations and line emission are calculated usingthe derived physical structure of the disks. X-ray irradiation is thedominant heating source of the gas in the inner disk and in the surfacelayer, while the UV heating dominates otherwise. If the central star hasstrong X-ray and weak UV radiation, the H2 level populationsare controlled by X-ray pumping, and the X-ray-induced transition linescould be observable. If the UV irradiation is strong, the levelpopulations are controlled by thermal collisions or UV pumping,depending on the dust properties. As the dust particles evolve in thedisks, the gas temperature at the disk surface drops because the grainphotoelectric heating becomes less efficient. This makes the levelpopulations change from LTE to non-LTE distributions, which results inchanges to the line ratios. Our results suggest that dust evolution inprotoplanetary disks could be observable through the H2 lineratios. The emission lines are strong from disks irradiated by strong UVand X-rays and possessing small dust grains; such disks will be goodtargets in which to observe H2 emission.
| HD 97048's Circumstellar Environment as Revealed by a Hubble Space Telescope ACS Coronagraphic Study of Disk Candidate Stars
We present the results of a coronagraphic scattered-light imaging surveyof six young disk candidate stars using the Hubble Space Telescope Advanced Camera for Surveys. The observations made use of the 1.8"occulting spot through the F606W (broad V) filter. Circumstellarmaterial was imaged around HD 97048, a Herbig Ae/Be star located in theChamaeleon I dark cloud at a distance of 180 pc. The material is seenbetween ~2" (360 AU) and ~4" (720 AU) from the star in all directions. A V-band azimuthally averaged radial surface brightness profile peaks atr=2'' with a value of 19.6+/-0.2 mag arcsec-2 andsmoothly decreases with projected distance from the star asI~r-3.3+/-0.5. An integrated flux of 16.8+/-0.1 mag ismeasured between 2" and 4", corresponding to a scattered-lightfractional luminosity lower limit ofLsca/L*>8.4×10-4. Filamentarystructure resembling spiral arms similar to that seen in Herbig Ae/Bedisks is observed. Such structure has been attributed to the influenceof orbiting planets or stellar encounters. Average surface brightnessupper limits are determined for the five nondetections: HD 34282, HD139450, HD 158643, HD 159492, and HD 195627. Possible reasons for thenondetections are disks that are too faint or disks hidden by theocculter.
| Young Stellar Groups around Herbig Ae/Be Stars: A Low-Mass YSO Census
We present near-IR and mid-IR observations of eight embedded youngstellar groups around Herbig Ae/Be stars (HAEBEs) using archived SpitzerIRAC data and 2MASS data. These young stellar groups are nearby (<=1kpc) and still embedded within their molecular clouds. In order toidentify the young stellar objects in our sample, we use the color-colordiagram of J-[3.6] versus Ks-[4.5]. The Spitzer images of oursample show that the groups around HAEBEs, spectral types earlier thanB8, are usually associated with bright infrared nebulosity. Within this,there are normally 10-50 young stars distributed close to the HAEBEs(<1 pc). Not only are there young stars around the HAEBEs, there arealso young stellar populations throughout the whole cloud, some of whichare distributed and some of which are clumped. The groups around theHAEBEs are substructures of the large young population within themolecular cloud. The sizes of groups are also comparable with thosesubstructures seen in massive clusters. Young stars in groups aroundHAEBEs have generally larger SED slopes compared to those outside, whichsuggests that the young stars in groups are probably younger than thedistributed systems. This might imply that there is usually a higher andmore continuous star-forming rate in groups, that the formation ofgroups initiates later, or that low-mass stars in groups form slowerthan those outside. Finally, there is no obvious trend between the SEDslopes and the distance to the HAEBEs for those young stars within thegroups. This suggests that the clustering of young stars dominates overthe effect of massive stars on the low-mass young stars at the scale ofour study.
| Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado Diamonds
The first complete infrared FTIR absorption spectra of carbonado diamondconfirm the interstellar origin of this most enigmatic diamond. Allprevious attempts failed to measure the absorption of carbonado diamondin the most important IR range of 1000-1300 cm-1 (10.00-7.69μm) because of silica inclusions. In our investigation, KBr pelletswere made from crushed silica-free carbonado diamond, and thin sectionswere also prepared. The 100-1000 times brighter synchrotron infraredradiation permits a greater spatial resolution. Inclusions and porespaces were avoided and/or sources of chemical contamination wereremoved. The FTIR spectra of carbonado diamond mostly depict thepresence of single nitrogen impurities and hydrogen. The lack ofidentifiable nitrogen aggregates in the infrared spectra, the presenceof features related to hydrocarbon stretch bonds, and the resemblance ofthe spectra to CVD and presolar diamonds indicate that carbonadodiamonds formed in a hydrogen-rich interstellar environment. This isconsistent with carbonado diamond being sintered and porous, withextremely reduced metals, metal alloys, carbides, and nitrides, lightcarbon isotopes, surfaces with glassy melt-like patinas, deformationlamellae, and a complete absence of primary, terrestrial mineralinclusions. The 2.6-3.8 billion year old fragmented body was ofasteroidal proportions.
| Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds
We have obtained new mid-infrared observations of 65 ClassI/flat-spectrum (FS) objects in the Perseus, Taurus, Chamaeleon I andII, ρ Ophiuchi, and Serpens dark clouds. These objects represent asubset of the young stellar objects (YSOs) from our previousnear-infrared multiplicity surveys. We detected 45 out of 48 (94%) ofthe single sources, 16 out of 16 (100%) of the primary components, and12 out of 16 (75%) of the secondary/triple components of thebinary/multiple objects surveyed. One target, IRS 34, a 0.31" separationFS binary, remains unresolved at near-infrared wavelengths. Thecomposite spectral energy distributions for all of our sample YSOs areeither Class I or FS, and in 15 out of 16 multiple systems at least oneof the individual components displays a Class I or FS spectral index.However, the occurrence of mixed pairings, such as FS with Class I, FSwith Class II, and, in one case, an FS with a Class III (Cha I T33B), issuprisingly frequent. Such behavior is not consistent with that ofmultiple systems among T Tauri stars (TTSs), where the companion of aclassical TTS also tends to be a classical TTS, although other mixedpairings have been previously observed among Class II YSOs. Based on ananalysis of the spectral indices of the individual binary components,there appears to be a higher proportion of mixed Class I/FS systems(65%-80%) than that of mixed classical and weak-lined TTSs (25%-40%),demonstrating that the envelopes of Class I/FS systems are rapidlyevolving during this evolutionary phase. In general, the individualbinary/multiple components suffer very similar extinctions,Av, suggesting that most of the line-of-sight material iseither in the foreground of the molecular cloud or circumbinary. Wereport the discovery of a steep spectral index secondary companion toISO-Cha I 97, detected for the first time via our mid-infraredobservations. In our previous near-infrared imaging survey ofbinary/multiple Class I and FS YSOs, ISO-Cha I 97 appeared to be single.With a spectral index of α>=+3.9, the secondary component ofthis system is a member of a rare class of very steep spectral indexYSOs, those with α>+3. Only three such objects have previouslybeen reported, all of which are either Class 0 or Class I.
| Mid-infrared imaging of the circumstellar dust around three Herbig Ae stars: HD 135344, CQ Tau, and HD 163296
Aims.Planet formation has been known for many years to be tied to thespatial distribution of gas and dust in disks around young stars. Toconstrain planet formation models, imaging observations ofprotoplanetary disks are required. Methods: . Given this, we haveundertaken a mid-infrared imaging survey of Herbig Ae stars, which arepre-main sequence stars of intermediate mass still surrounded by a largeamount of circumstellar material. The observations were made at awavelength of 20.5 μm with the CAMIRAS camera mounted at theCassegrain focus of the Canada France Hawaii Telescope. Results: .We report the observations of three stars, HD 135344, CQ Tau, and HD163296. The circumstellar material around the three objects is spatiallyresolved. The extensions feature a disk-like shape. The images providedirect information on two key parameters of the disk: its inclinationand its outer radius. The outer radius is found to be quite differentfrom the one deduced from disk models, which is only constrained byfitting the Spectral Energy Distribution of the object. Other parametersof the disk, such as flaring and dust mass have been deduced fromfitting both the observed extension and the spectral energy distributionwith sophisticated disk models. Conclusions: .Our results showhow important imaging data are to tighten constraints on the disk modelparameters.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| Diamonds and polycyclic aromatic hydrocarbons in the circumstellar environment of the Herbig Ae/Be star Elias 1
We report long-slit spectroscopic observations of the Herbig Ae/Be starElias 1 in the 3.2-3.6 μm region covering the C-H stretch emissionfeatures of hydrogen-terminated diamonds and polycyclic aromatichydrocarbons (PAHs). The data were recorded at the United KingdomInfrared Telescope using the UKIRT Imaging Spectrometer (UIST) and yieldinformation on the profiles and intensities of the bands as a functionof offset along the NS and EW axes centred on the close binary. Thediamond and nearby infrared continuum emission arises from a symmetricalinner core region (<=0.34 arcsec or 48 au). The 3.3-μm PAHemission is extended along the EW axis up to ca. 100 au each side of thestar. This result supports a suggestion of Haas, Leinert & Richichiof an EW oriented bipolar nebula in Elias 1.
| Deep Imaging Surveys of Star-forming Clouds. IV. The Meek and the Mighty: Outflows from Young Stars in Chamaeleon I
We present a survey of shocks and outflows in the Chamaeleon Istar-forming complex using Hα, [S II], and SDSS i' images obtainedfrom the ground, an i' image obtained with the Hubble Space Telescope,and 4.5 μm images obtained with the Infrared Array Camera (IRAC) onthe Spitzer Space Telescope. We find new Herbig-Haro (HH) objects andextensions to the previously cataloged shocks that trace parts of atleast 20 distinct outflows from young stars. Some HH objects mark thepresence of giant outflows, the largest of which is powered by Cha-MMS1and associated with HH 49/50 near the Ced 110 region. Other large flowsare powered by Cha-MMS2 in the Ced 112 region and the IRN in the Ced 111region. Although some shocks exhibit infrared emission in the IRACbands, most notably HH 49/50 (the ``tornado''), most outflows in the ChaI clouds are not detected in the Spitzer IRAC bands. This result isconsistent with the general lack of extensive 2.12 μm H2emission from Cha I.
| Anatomy of a Flaring Proto-Planetary Disk Around a Young Intermediate-Mass Star
Although planets are being discovered around stars more massive than theSun, information about the proto-planetary disks where such planets havebuilt up is sparse. We have imaged mid-infrared emission from polycyclicaromatic hydrocarbons at the surface of the disk surrounding the youngintermediate-mass star HD 97048 and characterized the disk. The disk isin an early stage of evolution, as indicated by its large content ofdust and its hydrostatic flared geometry, indicative of the presence ofa large amount of gas that is well mixed with dust and gravitationallystable. The disk is a precursor of debris disks found aroundmore-evolved A stars such as β-Pictoris and provides the rareopportunity to witness the conditions prevailing before (or during)planet formation.
| 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.
| 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.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| Spatially resolved PAH emission in the inner disks of Herbig Ae/Be stars
We present adaptive-optics high-angular resolution (~0.1 arcsec)spectroscopic observations in the 3 μm region of eight well-knownHerbig Ae/Be stars with circumstellar disks. We detected the aromaticemission feature at 3.3 μm for four out of six of our objects withflared disks (HD 169142, HD 97048, HD 100453, HD 100546), someadditional features at 3.4 and 3.46 μm, and strong diamond featuresat 3.43 and 3.53 μm in two of our flared objects (HD 100546 and HD97048 respectively). We also detected hydrogen recombination line at3.74 μm in practically all the objects. The emission in thepolycyclic aromatic hydrocarbon (PAHs) feature at 3.3 μm and in theadditional carbon dust features in the 3.4-3.5 μm region is, for thefirst time, spatially resolved in all the sources where the features aredetected. The full-width at half-maximum sizes that we derive aretypical of emission arising in a circumstellar disk. On the other hand,the continuum emission remains unresolved, with the exception of HD97048 where it is marginally resolved. We compared the observed spatialdistribution of the 3.3 μm PAH feature and the adjacent continuum tothe predictions of a disk model that includes transiently heated smallgrains and PAHs, in addition to large grains in thermal equilibrium(Habart et al. 2004a). The model predicts that, as observed, the 3.3μm PAH emission feature is significantly broader than that of theadjacent continuum and also that about 50% of its integrated intensitycomes from a radius R< 30 AU. We find that the predicted brightnessprofiles reproduce the observed ones very well. This proves beyond doubtthat the energetic 3.3 μm PAH emission feature takes its origin inthe inner disk regions.
| Resolving the disk rotation of HD 97048 and HD 100546 in the [O I] 6300 Å line: evidence for a giant planet orbiting HD 100546
Aims.We intend to spatially and spectrally resolve the [O i] emissionregion in two nearby Herbig stars.Methods.We present high-resolution(λ/Δλ = 80 000) VLT/UVES echelle spectra of the [Oi] 6300 Å line in the Herbig Ae/Be stars HD97048 and HD 100546. Apart from thespectral signature, also the spatial extent of the [O i] emission regionis investigated. For both stars, we have obtained spectra with the slitpositioned at different position angles on the sky.Results.The [O i]emission region of HD 100546 appears to be coinciding with the dustdisk, its major axis located at 150±11° east of north. The SEpart of the disk moves towards the observer, while the NW side isredshifted. The [O i] emission region rotates counterclockwise aroundthe central star. For HD 97048, the position angle of the emissionregion is 160±19° east of north, which is the firstdetermination of this angle in the literature. The southern parts of thedisk are blueshifted, the northern side moves away from us. Our datasupport the idea that a gap is present at 10 AU in the disk of HD100546. Such a gap is likely planet-induced. We estimate the mass andorbital radius of this hypothetical companion responsible for this gapto be 20~M_Jupiter and 6.5 AU respectively.Conclusions.Based on temporalchanges in the [O i] line profile, we conclude that inhomogeneities arepresent in the [O i] emission region of HD 100546. These "clumps" couldbe in resonance with the suggested companion, orbiting the central starin about 11 yr. If confirmed, these observations could point to theexistence of an object straddling the line between giant planet andbrown dwarf in a system as young as 10 million years.
| The Darkest Bright Star: Chandra X-Ray Observations of Vega
We present X-ray observations of Vega obtained with the Chandra HighResolution Camera and Advanced CCD Imaging Spectrometer. After a totalof 29 ks of observation with Chandra, X-rays from Vega remainundetected. We derive upper limits to the X-ray luminosity of Vega as afunction of temperature over the range of 105-107K and find a 99.7% upper limit as low as ~2×1025 ergss-1 at T=106.2 K. We also compare these new deeperobservations with the limit derived from a reanalysis of ROSAT PSPCdata. Our X-ray luminosity limit for Vega is still greater thanpredictions of post-Herbig Ae phase X-rays from the shear dynamo modelproposed by Tout & Pringle for a Vega age of 350 Myr. If the age ofVega is closer to 100 Myr, as suggested by some indicators, our X-raylimit is then similar to Tout-Pringle model predictions. Current X-rayobservations of Vega are therefore unable to discriminate betweendifferent scenarios explaining the X-ray activity of the convectivelystable Herbig Ae/Be stars. Further progress is more likely to beachieved through X-ray observations of younger main-sequence early-typeA stars, whose conjectured residual post-Herbig Ae phase X-ray activitywould be significantly higher.
| Nanodiamond Dust and the Far-Ultraviolet Quasar Break
We explore the possibility that the steepening observed shortward of1000 Å in the energy distribution of quasars may result fromabsorption by dust, being either intrinsic to the quasar environment orintergalactic. We find that a dust extinction curve consisting ofnanodiamonds, composed of terrestrial cubic diamonds or with surfaceimpurities as found in carbonaceous chondrite meteorites, such asAllende, is successful in reproducing the sharp break observed. Theintergalactic dust model is partially successful in explaining the shapeof the composite energy distribution but must be discarded in the end,as the amount of crystalline dust required is unreasonable and wouldimply an improbable fine-tuning among the dust formation processes. Thealternative intrinsic dust model requires a mixture of both cubicdiamonds and Allende nanodiamonds and provides a better fit of the UVbreak. The gas column densities implied are of the order 1020cm-2, assuming solar metallicity for carbon and fulldepletion of carbon into dust. The absorption only occurs in theultraviolet and is totally negligible in the visible. The minimum dustmass required is of the order ~0.003r2pcMsolar, where rpc is the distance in parsecsbetween the dust screen and the continuum source. The intrinsic dustmodel reproduces the flux rise observed around 660 Å in key quasarspectra quite well. We present indirect evidence of a shallow continuumbreak near 670 Å (18.5 eV), which would be intrinsic to the quasarcontinuum.
| 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.
| A cold globule with a Class 0/I embedded source
Infrared observations of the dark cloud DC 303.8-14.2 and the embeddedpoint source IRAS 13036-7644 have been made with the ISOPHOT instrumentaboard ISO at 7.7 μm, 60 μm, 100 μm and 200 μm. We havecompared the properties of dust in DC 303.8-14.2 and the ThumbprintNebula (TPN), a morphologically similar globule but without starformation. The dust temperature at the center of DC 303.8-14.2 has aminimum of 14.6 ± 1 K, similar to that in the TPN. A comparisonof far-infrared radial optical depth distributions between these cloudsat angular scales of ~1.5'-3' shows no difference. The observationssuggest that there is a bright rim of 7.7 μm emission that peaks justoutside the optical bright rim of the cloud, indicating a halo of verysmall dust particles or PAHs around the cloud. The bolometric luminosityof the IRAS source is estimated to be about 1.0 Lȯ. Theheating of the cloud by the IRAS source is negligible; the thermalstructure of the cloud is dominated by external heating. We have studiedthe evolutionary status of the IRAS source and find it to be locatedbetween Class 0 and Class I, at a late accretion phase. The source wasdetected at 7.7 μm and we suggest that this emission is due to the7.7 μm UIR band. We also give a compilation of small molecular cloudswhose extended FIR emission has been mapped with ISO.
| [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.
| VISIR, a taste of scientific potential.
Not Available
| 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.
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Chemical analysis of 24 dusty (pre-)main-sequence stars
We have analysed the chemical photospheric composition of 24 HerbigAe/Be and Vega-type stars in search for the λ Bootis phenomenon.We present the results of the elemental abundances of the sample stars.Some of the stars were never before studied spectroscopically at opticalwavelengths. We have determined the projected rotational velocities ofour sample stars. Furthermore, we discuss stars that depict a(selective) depletion pattern in detail. HD 4881 andHD 139614 seem to display an overall deficiency.AB Aur and possibly HD 126367 havesubsolar values for the iron abundance, but are almost solar in silicon.HD 100546 is the only clear λ Bootis star inour sample.Appendix is only available in electronic form athttp://www.edpsciences.org
| 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
| Diamonds in HD 97048: A Closer Look
We present adaptive optics high angular resolution (~0.1") spectroscopicobservations in the 3 μm region of the Herbig Ae/Be star HD 97048.For the first time, we spatially resolve the emission in the diamondfeatures at 3.43 and 3.53 μm and in the adjacent continuum. Usingboth the intensity profiles along the slit and reconstructedtwo-dimensional images of the object, we derive FWHM sizes consistentwith the predictions for a circumstellar disk seen pole-on. The diamondemission originates in the inner region (R<~15 AU) of the disk.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Хамелеон |
| Прямое восхождение: | 11h08m03.31s |
| Склонение: | -77°39'17.5" |
| Видимая звёздная величина: | 8.455 |
| Расстояние: | 175.439 парсек |
| Собственное движение RA: | -20.7 |
| Собственное движение Dec: | -0.2 |
| B-T magnitude: | 8.852 |
| V-T magnitude: | 8.488 |
Каталоги и обозначения:
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