Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| Silicate Dust in Evolved Protoplanetary Disks: Growth, Sedimentation, and Accretion
We present the Spitzer IRS spectra for 33 young stars in Tr 37 and NGC7160. The sample includes the high- and intermediate-mass stars withMIPS 24 μm excess, the only known active accretor in the 12 Myr oldcluster NGC 7160, and 19 low-mass stars with disks in the 4 Myr oldcluster Tr 37. We examine the 10 μm silicate feature, present in thewhole sample of low-mass stars and in three of the high- andintermediate-mass targets, and we find that PAH emission is detectableonly in the Herbig Be star. We analyze the composition and size of thewarm photospheric silicate grains by fitting the 10 μm silicatefeature and study the possible correlations between the silicatecharacteristics and the stellar and disk properties (age, SED slope,accretion rate, and spectral type). We find indications of dust settlingwith age and of the effect of turbulent enrichment of the diskatmosphere with large grains. Crystalline grains are only smallcontributors to the total silicate mass in all disks and do not seem tocorrelate with any other property, except maybe binarity. We alsoobserve that spectra with very weak silicate emission are at least 3times more frequent among M stars than among earlier spectral types,which may be evidence of inner disk evolution. Finally, we find thatfive of the high- and intermediate-mass stars have SEDs and IRS spectraconsistent with debris disk models involving planet formation, whichcould indicate debris disk formation at ages as early as 4 Myr.
| Optical spectroscopy of close companions to nearby Herbig Ae/Be and T Tauri stars
We present VLT-FORS2 optical (5700-9400 Å) spectroscopy of close(r < 1.5´´) companions to three nearby (d < 200 pc)Herbig Ae/Be stars (HD 144432, HD 150193, KK Oph) and one T Tauri star(S CrA). We report the detection of Li I (6707 Å) in absorptionand emission lines (Hα, Ca II triplet) in the spectra of thecompanions. Our observations strongly suggest that the companions arephysically associated pre-main-sequence stars. The spectral type derivedfor the companions is K5Ve for HD 144432 B, F9Ve for HD 150193 B, andG6Ve for KK Oph B. S CrA A and B were observed simultaneously. Thespatially resolved spectra indicate that S CrA A (primary, north) is a Gstar and that S CrA B (secondary, south) is a K star. Using photometryfrom the literature and estimations of the R and I magnitude derivedfrom the spectra, we localized primaries and companions in the HRdiagram, derived their masses and assuming coevality constrained the ageof the systems. KK Oph B (7 Myr) and S CrA B (2 Myr) are activelyaccreting T Tauri stars and are very likely surrounded by disks. HD150193 B (10 Myr) and HD 144432 B (8 Myr) are weak-line T Tauri stars.Three of the four systems studied (HD 144432, HD 150193, KK Oph) haveages >7 Myr. These systems retained their disks for a longer timethan typical of a young star. Our results suggest that binarity may be akey issue in understanding the lifetime of disks.Based on observations collected at the European Southern Observatory,Chile (program ID 075.C-0395(A)). Figure 3 is only available inelectronic form at http://www.aanda.org
| 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.
| Stellar parameters and evidence of circumstellar activity for a sample of Herbig Ae/Be stars
Aims.We investigate evidence of accretion in a sample of 15 Herbig Ae/Bestars to determine whether these events originate in a remnant gaseousstructure from the primordial cloud (rich in hydrogen) or in ametal-rich body (like comets in our Solar System). During such analysiswe also determine precise stellar parameters for this sample ofstars. Methods: .The stars were observed using high resolutionspectroscopy (R = 48 000). A synthetic photospheric spectrum wasconstructed and then subtracted from the observed one to obtain thecircumstellar component. An iterative procedure was applied to find thestellar parameters that were used to build the synthetic photosphericspectrum. Results: .Evidence of circumstellar activity were foundin four stars: HD 100546, HD 142666, HD 144432 and HD 145718. Thepresence of redshifted absorption features only in the Balmer linesimplies that the accreting material is hydrogen-rich, excluding thepossibility that the accretion events might have been created bycomet-like bodies. We determined effective temperature, surface gravity,metallicity and the projected rotational velocity for the stars in oursample.
| 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.
| VLT/NACO adaptive optics imaging of the Herbig Ae star HD 100453
We report the detection of a companion candidate to the Herbig Ae starHD 100453, using the NACO instrument at the VLT UT4. The separationbetween the companion candidate and HD 100453 is 1.6 arcsec, and theirbrightness contrast is 5.1 mag at Ks and 4.2 mag atBrγ. We discuss the possible nature of the newly detected starbased on near-infrared photometry. The photometric data are consistentwith a 0.3 M_ȯ companion to HD 100453, corresponding to a spectraltype of M3-M5. This detection could have important implications for thestructure of the disk around HD 100453.
| 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.
| Mid-IR observations of circumstellar disks. I. Pre-main sequence objects
We present new N-band photometry and spectroscopy for a sample of eightpre-main sequence stars including T Tauri, Herbig Ae/Be stars and FU Oriobjects using the ESO TIMMI2 camera at the La Silla observatory (Chile).For some objects this is their first N-band spectroscopic observationever. The FU Ori stars V 346 Nor, V 883 Ori and Z CMa show a broadabsorption band which we attribute to silicates, while for BBW 76 wefind silicate emission. A comparison with ISO-SWS spectra of V 346 Norand Z CMa taken in 1996/1997 reveals no differences in spectral shape.All T Tauri and Herbig Ae/Be stars possess N-band emission features. Wemodel the emission spectra with a mixture of silicates consisting ofdifferent grain sizes and composition. The Herbig Ae star HD 34282 showsstrong features of PAHs but none of silicate, while the emissionspectrum of the Herbig Ae star HD 72106 resembles those of solar-systemcomets and known Herbig sources of evolved dust. We demonstrate that HD72106 is host to highly processed silicates and find evidence forenstatite, which is not common in young objects. Evolved dust is alsoseen in the T Tauri stars HD 98800 and MP Mus. We further detected MPMus at 1200 μm with the bolometer array SIMBA at the SEST in LaSilla. The findings of our analysis are given in the context of previousdust studies of young stellar objects.Based on observations collected at the European Southern Observatory, LaSilla, Chile (69.C-0073, 70.C-0468, 71.C-0001, 73.C-0372).
| The primordial binary population. I. A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey withthe aim to detect close companions to Hipparcos members in the threesubgroups of the nearby OB association Sco OB2: Upper Scorpius (US),Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We havetargeted 199 A-type and late B-type stars in the KS band, anda subset also in the J and H band. We find 151 stellar components otherthan the target stars. A brightness criterion is used to separate thesecomponents into 77 background stars and 74 candidate physical companionstars. Out of these 74 candidate companions, 41 have not been reportedbefore (14 in US; 13 in UCL; 14 in LCC). The angular separation betweenprimaries and observed companion stars ranges from 0.22'' to 12.4''. Atthe mean distance of Sco OB2 (130 pc) this corresponds to a projectedseparation of 28.6 AU to 1612 AU. Absolute magnitudes are derived forall primaries and observed companions using the parallax andinterstellar extinction for each star individually. For each object wederive the mass from KS, assuming an age of 5 Myr for the USsubgroup, and 20 Myr for the UCL and LCC subgroups. Companion starmasses range from 0.10 Mȯ to 3.0 Mȯ. Themass ratio distribution follows f(q) = q-Γ withΓ=0.33, which excludes random pairing. No close (ρ ≤3.75'') companion stars or background stars are found in the magnituderange 12 mag≤ KS ≤ 14 mag. The lack of stars withthese properties cannot be explained by low-number statistics, and mayimply a lower limit on the companion mass of 0.1Mȯ. Close stellar components with KS >14mag are observed. If these components are very low-mass companion stars,a gap in the companion mass distribution might be present. The smallnumber of close low-mass companion stars could support theembryo-ejection formation scenario for brown dwarfs. Our findings arecompared with and complementary to visual, spectroscopic, andastrometric data on binarity in Sco OB2. We find an overall companionstar fraction of 0.52 in this association. This is a lower limit sincethe data from the observations and from literature are hampered byobservational biases and selection effects. This paper is the first steptoward our goal to derive the primordial binary population in Sco OB2.Full Table 1 is only available in electronic form athttp://www.edpsciences.orgBased on observations collected with the ADONIS instrument at theEuropean Southern Observatory, La Silla, Chile (65.H-0568(A) and67.D-0220(A)).
| 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
| Tentative detection of micron-sized forsterite grains in the proto-planetary disk surrounding HD 100453
We have re-analyzed the ISO-SWS spectrum of the intermediate masspre-main-sequence star HD 100453. We confirm the weakness of the 10μm amorphous silicate band. We have found strong indications of thepresence of a crystalline silicate emission at 34 μm, which hadescaped detection so far due to severe glitches in the data. The 23.5and 28.5 μm forsterite bands have not been detected. This resultindicates that the absence of crystalline silicate features atwavelengths shorter than 30 μm does not prove the absence ofcrystalline silicate material in the circumstellar disks of young stars.The 34 μm feature can be fitted well with crystalline forsteritegrains with an average size of at least 2 μm, but the precise size isuncertain due to the poor data quality. The temperature of these grainsmust be 110 K or below, or otherwise the 23.5 and 28.5 micron featureswould be visible as well. Ground-based 10 μm spectra show prominentemission from Polycyclic Aromatic Hydrocarbons and a weak silicate bandwhose shape suggests that it is dominated by a few micron size grains,of unknown lattice structure. The absence of any significant forsteritestructure in the 10 micron region limits the mass of warm forsteritegrains with sizes less than a few microns to ≤ 2×10-4 of the mass of cold forsterite. Forsterite may bepresent in the warm regions, but then must be contained in even largergrains. The absence of 10 μm silicate emission in some Herbig Aestars is therefore due to the removal of small grains by dust growthprocesses.Based on observations obtained at the European Southern Observatory(ESO), La Silla, and on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and UK) and with the participation ofISAS and NASA.
| 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.
| 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.
| Does the 3.3 Micron Polycyclic Aromatic Hydrocarbon Emission Feature Require Ultraviolet Excitation?
Unidentified infrared bands (UIBs) have been observed in virtually everydusty astrophysical environment investigated. The UIB carrier must beabundant and ubiquitous. Strong evidence points to polycyclic aromatichydrocarbons as likely candidates, but the identification is notcomplete. Additional diagnostics are needed to further constrain the UIBcarrier, such as probing excitation sources ranging from UV-strong toUV-weak to determine the ``band gap'' of the UIB carrier. Observationsand models suggest that the UIBs can be found in sources with weak UVfields. To that end, we present new results of observing the 3.3 μmspectral region in six stars embedded in reflection nebulae and in sixVega-like stars. These objects have effective temperatures ranging from3500 to 12,000 K. Their environments include dust that should berelatively unprocessed (reflection nebulae) and dust that has mostlikely undergone significant processing (Vega-like) by the embeddedillumination source. Together with data from the literature, we have asample of 27 sight lines. Our analysis suggests that neither thestrength of the UV field impinging on the dust nor the effectivetemperature of the star is the determining factor in whether the 3.3μm UIB emission is present in an object. We found three detections ofthe 3.3 μm emission band, one in a Vega-type object, one in a HerbigAe/Be object, and one in a reflection nebula, and all with disks. Therole of disk geometry is likely to be important in revealing orobscuring the photodissociation regions from which the UIB emissionarises.
| The Environment of the Optically Brightest Herbig Ae Star, HD 104237
We investigate the environment of the nearest Herbig Ae star, HD 104237,with a multiwavelength combination of optical coronagraphic, near-IR,and mid-IR imaging supported by optical, UV, and far-ultravioletspectroscopy. We confirm the presence of T Tauri stars associated withthe Herbig Ae star HD 104237, noted by Feigelson et al. We find that twoof the stars within 15" of HD 104237 have IR excesses, potentiallyindicating the presence of circumstellar disks, in addition to theHerbig Ae star itself. We derive a new spectral type of A7.5Ve-A8Ve forHD 104237 and find log(L/Lsolar)=1.39. With these data, HD104237 has an age of t~5 Myr, in agreement with the estimates for theother members of the association. HD 104237 is still actively accreting,with a conspicuous UV/far-UV excess seen down to 1040 Å, and isdriving a bipolar microjet termed HH 669. This makes it the second,older Herbig Ae star now known to have a microjet. The presence of themicrojet enables us to constrain the circumstellar disk to r<=0.6"(70 AU) with an inclination angle of i=18deg+14-11from pole-on. The absence of a spatially extended continuum andfluorescent H2 emission near Lyα is in agreement withthe prediction of shadowed disk models for the IR spectral energydistribution. With the high spatial density of disks in this group ofstars, proximity, and minimal reddening, HD 104237 and its companionsshould serve as ideal laboratories for probing the comparative evolutionof planetary systems.Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA Contract NAS5-26555.Based on observations made with ESO's TIMMI2 camera on La Silla, Chile, under program ID 71.C-0438.Based on observations made with the ESO VLT and the Near-IR Adaptive Optics System+Conica, under program ID 71.C-0143. Based on observations made under the ON-ESO agreement for the joint operation of the 1.52 m ESO telescope.Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University underNASA contract NAS5-32985.
| 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).
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| 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
| 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.
| 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
| Processing of silicate dust grains in Herbig Ae/Be systems
We have analysed the 10 mu m spectral region of a sample of Herbig Ae/Be(HAEBE) stars. The spectra are dominated by a broad emission featurecaused by warm amorphous silicates, and by polycyclic aromatichydrocarbons. In HD 163296 we find aliphatic carbonaceous dust, thefirst detection of this material in a HAEBE star. The silicate bandshows a large variation in shape, due to variable contributions of threecomponents: (i) a broad shoulder at 8.6 mu m; (ii) a broad maximum at9.8 mu m; and (iii) a narrow feature with a broad underlying continuumat 11.3 mu m. From detailed modeling these features can be identifiedwith silica (SiO2), sub-micrometer sized amorphous olivinegrains and micrometer sized amorphous olivine grains in combination withforsterite (Mg2SiO4), respectively. Typical massfractions are 5 to 10 per cent of crystalline over amorphous olivine,and a few per cent of silica compared to the olivines. The detection ofsilica in emission implies that this material is heated by thermalcontact with other solids that have a high absorptivity at optical tonear-IR wavelengths. The observed change in peak position of thesilicate band in HAEBE stars from 9.7 mu m to 11.3 mu m is dominated byan increase in average grain size, while changes in composition playonly a minor rôle. The HAEBE stars, beta Pic and the solar systemcomet Halley form a sequence of increasing crystallinity. We find thatthe abundance of SiO2 tends to increase with increasingcrystallinity. This is consistent with the compositional changesexpected from thermal annealing of amorphous grains in the inner regionsof the disk. We confirm earlier studies that the timescale forcrystallisation of silicates in disks is longer than that ofcoagulation. Our results indicate that the processes that governed grainprocessing in the proto-solar nebula, are also at work in HAEBE stars.
| ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: Towards an understanding of dust processing
We present Infrared Space Observatory (ISO) spectra of fourteen isolatedHerbig Ae/Be (HAEBE) stars, to study the characteristics of theircircumstellar dust. These spectra show large star-to-star differences,in the emission features of both carbon-rich and oxygen-rich dustgrains. The IR spectra were combined with photometric data ranging fromthe UV through the optical into the sub-mm region. We defined two keygroups, based upon the spectral shape of the infrared region. Thederived results can be summarized as follows: (1) the continuum of theIR to sub-mm region of all stars can be reconstructed by the sum of apower-law and a cool component, which can be represented by a blackbody. Possible locations for these components are an optically thick,geometrically thin disc (power-law component) and an optically thinflared region (black body); (2) all stars have a substantial amount ofcold dust around them, independent of the amount of mid-IR excess theyshow; (3) also the near-IR excess is unrelated to the mid-IR excess,indicating different composition/location of the emitting material; (4)remarkably, some sources lack the silicate bands; (5) apart fromamorphous silicates, we find evidence for crystalline silicates inseveral stars, some of which are new detections; (6) PAH bands arepresent in at least 50% of our sample, and their appearance is slightlydifferent from PAHs in the ISM; (7) PAH bands are, with one exception,not present in sources which only show a power-law continuum in the IR;their presence is unrelated to the presence of the silicate bands; (8)the dust in HAEBE stars shows strong evidence for coagulation; this dustprocessing is unrelated to any of the central star properties (such asage, spectral type and activity). Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
| 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.
| An ultraviolet, optical and infrared study of Herbig Ae/Be stars
We have selected a list of 45 Herbig Ae/Be-type candidates on the baseof their IRAS colors and their spectral types. We propose the presenceof a broad infrared excess as a defining criterion for these stars,rather than the detection of circumstellar nebulosity. In this way, ourselection also includes more evolved young stars, that are no longerembedded in their star-forming region. A few objects in our sample arewell-known Herbig Ae/Be stars, others are new. New optical andnear-infrared photometric observations, as well as ultraviolet ones, arepresented. The position of the objects in several color-color diagrams,as well as their de-reddened energy distributions, permit a reliableclassification. Three objects probably are binaries with a coolsecondary, 9 appear to be related to the Vega-type stars and 33 objectscan be classified as genuine Herbig Ae/Be stars. The majority of theHerbig Ae/Be stars have a dusty environment consisting of a distinct hotand cool component. These isolated Herbig Ae/Be stars suggest anevolution from embedded Herbig Ae/Be stars to beta Pictoris-likemain-sequence stars, an evolution in which planet formation may play animportant role. Based on observations obtained at the European SouthernObservatory, La Silla, Chile; and at the Swiss Telescope, La Silla,Chile; and at the Swiss Telescope, Hochalpine ForschungsstationJungfraujoch, Switzerland, and with the International UltravioletExplorer (IUE)
| Near infrared photometry of IRAS sources with colours like planetary nebulae. III
We present the near infrared photometry of a new sample of 225 IRASsources, many of them previously unidentified in the literature,selected because their far infrared colours are similar to those shownby known planetary nebulae. The results obtained are used to establishthe main source of near infrared emission. Combining this informationwith the far infrared IRAS data and a few additional criteria wedetermine the nature and evolutionary stage of all the sources observedso far, including those for which near infrared photometry waspreviously reported in Papers I and II. Among the unidentified IRASsources in our sample we find only a small percentage of planetarynebulae, many of them very young and dusty, showing peculiar nearinfrared colours. Most of the new objects observed in the near infraredare identified as transition objects in the previous stages of thestellar evolution. Among them, we find heavily obscured late-AGB stars,early post-AGB stars still obscured by thick circumstellar envelopeswhich are probably the true progenitors of planetary nebulae, and asignificant fraction of stars with bright optical counterparts showinglittle or no near infrared excess, which we associate with highlyevolved post-AGB stars with low mass progenitors, which may never becomeplanetary nebulae. In addition, we also find a small percentage of youngstellar objects, as well as a few Seyfert galaxies. We conclude that, inmost cases, based on near infrared data alone, it is not possible togive a confident classification of the unidentified IRAS source.However, the near infrared is shown to be a powerful tool, speciallywhen dealing with objects which are heavily obscured in the optical. Inthis case, the detection of the near infrared counterpart is the onlyway in which we can extend the study of these sources to other spectralranges and may be crucial to understand the short-lived phase whichprecedes the formation of a new planetary nebula. Based on observationscollected at the European Southern Observatory, La Silla (Chile) and atthe Spanish Observatorio del Teide, Tenerife, Spain. Table 6 is onlyavailable electronically at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Erbogan |
| Sað Açýklýk: | 11h33m05.58s |
| Yükselim: | -54°19'28.5" |
| Görünürdeki Parlaklýk: | 7.795 |
| Uzaklýk: | 111.483 parsek |
| özdevim Sað Açýklýk: | -37.9 |
| özdevim Yükselim: | -3.9 |
| B-T magnitude: | 8.15 |
| V-T magnitude: | 7.825 |
Kataloglar ve belirtme:
|
