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The nature of mid-infrared excesses from hot dust around Sun-like stars
Aims. Studies of the debris disk phenomenon have shown that most systemsare analogous to the Edgeworth-Kuiper Belt (EKB). However a rare subsetof sun-like stars possess dust which lies, in contrast, in theterrestrial planet region. In this study we aim to determine how manysources with apparent mid-infrared excess are truly hosts of warm dust,and investigate where the dust in these systems must lie. Methods: Weobserved using ground-based mid-infrared imaging with TIMMI2, VISIR andMICHELLE a sample of FGK main sequence stars previously reported to havehot dust. A new modelling approach was developed to determine theconstraints that can be set on the radial extent of excess emission insuch observations by demonstrating how the detectability of a disk of agiven flux as a fraction of the total flux from the system(F{disk}/F{total}) depends primarily on the ratioof disk radius to PSF width and on the uncertainty on that PSF width. Results: We confirm the presence of warm dust around three of thecandidates; η Corvi, HD 145263 and HD 202406. For η Corvimodelling constrains the dust to lie in regions smaller than 3.5 AU.The modelling constrains the dust to regions smaller than 80-100 AU forHD 145263 and HD 202406, with SED fitting suggesting the dust lies at afew tens of AU. Of two alternative models for the η Corvi excessemission, we find that a model with one hot dust component at less than0.164 arcsec (<3 AU) (combined with the known submm dust populationat 150 AU) fits all the data better at the 2.6σ level than analternative model with two populations of dust emitting in themid-infrared: hot dust at less than 0.19 arcsec (<3.5 AU) and amid-temperature component at 0.66 arcsec (12 AU). We identify severalsystems which have a companion (HD 65277 and HD 79873) or backgroundobject (HD 53246, HD 123356 and HD 128400) responsible for theirmid-infrared excess, and for three other systems we were able to ruleout a point-like mid-infrared source near the star at the level ofexcess observed in lower resolution observations (HD 12039, HD 69830 andHD 191089). Conclusions: Hot dust sources are either young and possiblyprimordial or transitional in their emission, or have relatively smallradius steady-state planetesimal belts, or they are old and luminouswith transient emission. High resolution imaging can be used toconstrain the location of the disk and help to discriminate betweendifferent models of disk emission. For some small disks, interferometryis needed to resolve the disk location.
| 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.
| 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.
| Crystalline Silicate Emission in the Protostellar Binary Serpens SVS 20
We present spatially resolved mid-infrared spectroscopy of the ClassI/flat-spectrum protostellar binary system SVS 20 in the Serpens cloudcore. The spectra were obtained with the mid-infrared instrument T-ReCSon Gemini South. SVS 20-South, the more luminous of the two sources,exhibits a mid-infrared emission spectrum peaking near 11.3 μm, whileSVS 20-North exhibits a shallow amorphous silicate absorption spectrumwith a peak optical depth of τ~0.3. After removal of theline-of-sight extinction by the molecular common envelope, the``protostar-only'' spectra are found to be dominated by strong amorphousolivine emission peaking near 10 μm. We also find evidence foremission from crystalline forsterite and enstatite associated with bothSVS 20-S and SVS 20-N. The presence of crystalline silicate in such ayoung binary system indicates that the grain processing found in moreevolved Herbig Ae/Be and T Tauri pre-main-sequence stars likely beginsat a relatively young evolutionary stage, while mass accretion is stillongoing.
| 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).
| Crystalline Silicate Feature of the Vega-like Star HD 145263
We have observed the 8-13 μm spectrum (R~250) of the Vega-like starcandidate HD 145263 using Subaru/COMICS. The spectrum of HD 145263 showsthe broad trapezoidal silicate feature with the shoulders at 9.3 and11.44 μm, indicating the presence of crystalline silicate grains.This detection implies that crystalline silicate may also be commonlypresent around Vega-like stars. The 11.44 μm feature is slightlyshifted to a longer wavelength compared to the usual 11.2-3 μmcrystalline forsterite feature detected toward Herbig Ae/Be stars and TTauri stars. Although the peak shift due to the effects of the grainsize cannot be ruled out, we suggest that Fe-bearing crystalline olivineexplains the observed peak wavelength fairly well. Fe-bearing silicatesare commonly found in meteorites and most interplanetary dust particles,which originate from planetesimal-like asteroids. According to studiesof meteorites, Fe-bearing silicate must have been formed in asteroidalplanetesimals, supporting the scenario that dust grains around Vega-likestars are of planetesimal origin, if the observed 11.44 μm peak isdue to Fe-bearing silicates.ID="FN1"> 1Based on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan.
| 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
| Discovery of Reflection Nebulosity around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars revealreflection nebulosities, five of which were previously unknown. Thenebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, andHD 49662 resemble that of the Pleiades, indicating an interstellarorigin for dust grains. The reflection nebulosity around HD 123160 has adouble-arm morphology, but no disklike feature is seen as close as 2.5"from the star in K-band adaptive optics data. We demonstrate that auniform density dust cloud surrounding HD 23362, HD 23680, and HD 123160can account for the observed 12-100 μm spectral energy distributions.For HD 4881, HD 26676, and HD 49662, an additional emission source, suchas from a circumstellar disk or nonequilibrium grain heating, isrequired to fit the 12-25 μm data. These results indicate that insome cases, particularly for Vega-like stars located beyond the LocalBubble (>100 pc), the dust responsible for excess thermal emissionmay originate from the interstellar medium rather than from a planetarydebris system.
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| SCUBA photometry of candidate Vega-like sources
New SCUBA measurements at millimetre wavelengths are presented for asample of Vega-like stars. Six stars were detected, while sensitiveupper limits were obtained for a further 11 sources. Most of the sampleselected from a recent catalogue of Vega-like stars have infraredexcesses similar to those of the prototype Vega-like stars α Lyrand α PsA. Their IR-submm spectral indices are steep, indicatingthat the submm emission from the discs is dominated by grains which aresmaller than the wavelength of observation and that only small grainsexist in those dusty discs. HD 98800 has an IR-submillimetre spectralindex of less than two, which suggests that grains have grown to morethan 0.3mm in size. Hipparcos parallax data for HD 42137 and HD 123160suggest that these two stars are giants rather than dwarfs, similar tothe situation previously found for HD 233517. Dust masses, or upperlimits, were derived for the sample; these indicate that most of thesources do not have as much dust as Herbig Ae/Be or T Tauri stars, butare likely to have dust masses comparable to those of the prototypeVega-like stars.
| Optical, infrared and millimetre-wave properties of Vega-like systems - IV. Observations of a new sample of candidate Vega-like sources
Photometric observations at optical and near-infrared wavelengths arepresented for members of a new sample of candidate Vega-like systems, ormain sequence stars with excess infrared emission due to circumstellardust. The observations are combined with IRAS fluxes to define thespectral energy distributions of the sources. Most of the sources showonly photospheric emission at near-IR wavelengths, indicating a lack ofhot (~1000K) dust. Mid-infrared spectra are presented for four sourcesfrom the sample. One of them, HD 150193, shows strong silicate emission,while another, HD 176363, was not detected. The spectra of two starsfrom our previous sample of Vega-like sources both show UIR-bandemission, attributed to hydrocarbon materials. Detailed comparisons ofthe optical and IRAS positions suggest that in some cases the IRASsource is not physically associated with the visible star. Alternativeassociations are suggested for several of these sources. Fractionalexcess luminosities are derived from the observed spectral energydistributions. The values found are comparable to those measuredpreviously for other Vega-like sources.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| Occultations of stars by solar system objects. VIII - Occultations of catalog stars by asteroids, planets, Titan, and Triton in 1990 and 1991
Predictions are given for occultations of catalog stars by asteroids,planets, Titan, and Triton in 1990 and 1991. The predictions are basedon a computerized comparison of the occulting bodies' ephemerides andnine major star catalogs. The search is complete for all numberedasteroids whose angular diameters exceed 0.08 arcsec during the searchyears. Preliminary ground tracks are shown for the more favorableoccultations.
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