Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Subaru Near-Infrared Imaging of Herbig Ae Stars
We report results of H-band (? = 1.65?m) imaging observationsof young intermediate-mass stars using the Subaru 8.2-m Telescope withthe adaptive optics AO36 and the infrared coronagraph CIAO. The targetsconsist of 16 Herbig Ae/Be stars (15 Herbig Ae stars and one Herbig Bestar) and four additional main-sequence stars with infrared excesses.Five protoplanetary disks have been spatially resolved around the HerbigAe stars with ages of 2-8 Myr. The resolved disks have outer radii ofseveral 100 AU, and their surface brightnesses range from 10 magarcsec-2 to 18 mag arcsec-2. The images revealvarious morphologies in optically thick disks: spiral arms around ABAur, a banana-split structure with an outer arm for HD 142527, a compactcircumprimary disk of HD 150193, a faint discontinuous ring around HD163296, and an unstructured face-on disk of HD 169142. The detection ofan optically thick disk in scattered light implies that it is verticallyflared, and intercepts stellar light at least in the outer region wherethose images were obtained. However, the surface brightnessdistribution, the resolved structure, and other observationalcharacteristics suggest that the disks are unlikely to be continuouslyflared young disks with small grains well mixed with gas. The detectionrate and the disk brightness do not correlate with the stellar age andthe disk mass, but there is a tendency that the brightest disks arestill surrounded by long-lived envelopes (AB Aur, HD 100546, HD 142527).The significant diversity of the disk structure can be attributed to themultiplicity and the initial condition of the local star-formingenvironments. The detections of companion candidates around our targetedstars are also reported.
| Analysis of the Pico Dos Dias Survey Herbig Ae/Be Candidates
A large sample of Herbig Ae/Be (HAeBe) candidates, distributed indifferent Galactic regions south to declination +30°, wereidentified by the Pico dos Dias Survey (a search for young stellarobjects based on IRAS colors). Most of the candidates are nearby orassociated with star-forming clouds, but several others are consideredisolated objects. Aiming to verify the young nature of 93 HAeBecandidates, we searched for additional information that could be usefulto confirm if they are pre-main-sequence (PMS) stars or evolved objects,which coincidentally show similar IRAS colors. By adopting a spectralindex that is related to the amount of infrared excess and the shape ofthe spectral energy distribution, we have classified the sampleaccording to three groups, which are analyzed on the basis of (1)circumstellar luminosity; (2) spatial distribution; (3) opticalpolarization; (4) near-infrared colors; (5) stellar parameters (mass,age, effective temperature); and (5) intensity of emission lines. Ouranalysis indicates that only 76% of the studied sample, mainly the groupwith intermediate to low levels of circumstellar emission, can be moreconfidently considered PMS stars. The nature of the remaining stars,which are in the other group that contains the highest levels ofinfrared excess, remains to be confirmed. They share the samecharacteristics of evolved objects, requiring complementary studies inorder to correctly classify them. At least seven objects showcharacteristics typical of post-asymptotic giant branch orproto-planetary nebulae.Based on observations made at the Observatório do Pico dosDias/LNA (Brazil).
| On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars
Context: Based on their spectral energy distribution, Herbig stars havebeen categorized into two observational groups, reflecting their overalldisk structure: group I members have disks with a high degree of flaringas opposed to their group II counterparts. Literature results show thatthe structure of the disk is a strong function of the disk mass in?m-sized dust grains. Aims: We investigate the 5-35 ?mSpitzer IRS spectra of a sample of 13 group I sources and 20 group IIsources. We focus on the continuum emission to study the underlying diskgeometry. Methods: We have determined the [30/13.5] and [13.5/7]continuum flux ratios. The 7-?m flux excess with respect to thestellar photosphere is measured, as a marker for the strength of thenear-IR emission produced by the hot inner disk. We have compared ourdata to the spectra produced by self-consistent passive-disk models, forwhich the same quantities were derived. Results: We confirm theresults by Meijer et al. (2008, A&A, 492, 451) that the differencesin continuum emission between group I and II sources can largely beexplained by a difference in amount of small dust grains. However, wereport a strong correlation between the [30/13.5] and [13.5/7] fluxratios for Meeus group II sources. Moreover, the [30/13.5] flux ratiodecreases with increasing 7-?m excess for all targets in the sample.To explain these correlations with the models, we need to introduce anartificial scaling factor for the inner disk height. In roughly 50% ofthe Herbig Ae/Be stars in our sample, the inner disk must be inflated bya factor 2 to 3 beyond what hydrostatic calculations predict. Conclusions: The total disk mass in small dust grains determines thedegree of flaring. We conclude, however, that for any given disk mass insmall dust grains, the shadowing of the outer (tens of AU) disk isdetermined by the scale height of the inner disk (~1 AU). The inner diskpartially obscures the outer disk, reducing the disk surfacetemperature. Here, for the first time, we prove these effectsobservationally.
| Mid-Infrared Size Survey of Young Stellar Objects: Description of Keck Segment-Tilting Experiment and Basic Results
The mid-infrared properties of pre-planetary disks are sensitive to thetemperature and flaring profiles of disks for the regions where planetformation is expected to occur. In order to constrain theories of planetformation, we have carried out a mid-infrared (λ = 10.7 μm)size survey of young stellar objects using the segmented Keck telescopein a novel configuration. We introduced a customized pattern of tilts toindividual mirror segments to allow efficient sparse-apertureinterferometry, allowing full aperture synthesis imaging with highercalibration precision than traditional imaging. In contrast to previoussurveys on smaller telescopes and with poorer calibration precision, wefind that most objects in our sample are partially resolved. Here, wepresent the main observational results of our survey of five embeddedmassive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Orisystem, and five emission-line objects of uncertain classification. Theobserved mid-infrared sizes do not obey the size-luminosity relationfound at near-infrared wavelengths and a companion paper will providefurther modeling analysis of this sample. In addition, we report imagingresults for a few of the most resolved objects, including complexemission around embedded massive protostars, the photoevaporatingcircumbinary disk around MWC 361A, and the subarcsecond binaries T Tau,FU Ori, and MWC 1080.
| Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Context: Recently, evidence for the presence of weak magnetic fields inHerbig Ae/Be stars has been found in several studies. Aims: Weseek to expand the sample of intermediate-mass pre-main sequence starswith circular polarization data to measure their magnetic fields, and todetermine whether magnetic field properties in these stars arecorrelated with mass-accretion rate, disk inclination, companions,silicates, PAHs, or show a correlation with age and X-ray emission asexpected for the decay of a remnant dynamo. Methods:Spectropolarimetric observations of 21 Herbig Ae/Be stars and six debrisdisk stars have been obtained at the European Southern Observatory withFORS 1 mounted on the 8 m Kueyen telescope of the VLT. With the GRISM600B in the wavelength range 3250-6215 Å we were able to cover allhydrogen Balmer lines from Hβ to the Balmer jump. In allobservations a slit width of 0.4 arcsec was used to obtain a spectralresolving power of R ≈ 2000. Results: Among the 21 HerbigAe/Be stars studied, new detections of a magnetic field were achieved insix stars. For three Herbig Ae/Be stars, we confirm previous magneticfield detections. The largest longitudinal magnetic field, =-454±42 G, was detected in the Herbig Ae/Be star HD 101412 usinghydrogen lines. No field detection at a significance level of 3σwas achieved in stars with debris disks. Our study does not indicate anycorrelation of the strength of the longitudinal magnetic field with diskorientation, disk geometry, or the presence of a companion. We also donot see any simple dependence on the mass-accretion rate. However, it islikely that the range of observed field values qualitatively supportsthe expectations from magnetospheric accretion models giving support fordipole-like field geometries. Both the magnetic field strength and theX-ray emission show hints of a decline with age in the range of ~2-14Myr probed by our sample, supporting a dynamo mechanism that decays withage. However, our study of rotation does not show any obvious trend ofthe strength of the longitudinal magnetic field with rotation period.Furthermore, the stars seem to obey the universal power-law relationbetween magnetic flux and X-ray luminosity established for the Sun andmain-sequence active dwarf stars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes 077.C-0521(A) and 081.C-0410(A)).
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| The Alignment of the Polarization of Herbig Ae/Be Stars with the Interstellar Magnetic Field
We present a study of the correlation between the direction of thesymmetry axis of the circumstellar material around intermediate massyoung stellar objects and that of the interstellar magnetic field. Weuse CCD polarimetric data on 100 Herbig Ae/Be stars. A large numberof them show intrinsic polarization, which indicates that theircircumstellar envelopes are not spherical. The interstellar magneticfield direction is estimated from the polarization of field stars. Thereis an alignment between the position angle of the Herbig Ae/Be starpolarization and that of the field stars for the most polarized objects.This may be an evidence that the ambient interstellar magnetic fieldplays a role in shaping the circumstellar material around young stars ofintermediate mass and/or in defining their angular momentum axis.Based on observations made at the Observatório do Pico dos Dias,Brazil, operated by the Laboratório Nacional deAstrofísica.
| Circumstellar disks around Herbig Be stars
Aims. Our goal is to investigate the properties of the circumstellardisks around intermediate mass stars to determine their occurrence,lifetime and evolution. Methods: We completed a search forcircumstellar disks around Herbig Be stars using the NRAO Very LargeArray (VLA) and the IRAM Plateau de Bure (PdB) interferometers. Thusfar, we have observed 6 objects with 4 successful detections. Theresults towards 3 of these stars (R Mon, MWC 1080, MWC 137) werepresented elsewhere. We present our new VLA and PdBI data for the threeobjects MWC 297, Z CMa, and LKHα 215. We constructed the SED fromnear-IR to centimeter wavelengths by adding our millimeter andcentimeter data to the available data at other wavelengths, mainlySpitzer images. The entire SED was fitted using a disk+envelope model.In addition, we compiled all the disk millimeter observations in theliterature and completed a statistical analysis of all the data. Results: We show that the disk mass is usually only a small percentage(less than 10%) of the mass of the entire envelope in HBe stars. For thedisks, there are large source-to-source variations. Two disks in oursample, R Mon and Z CMa, have similar sizes and masses to those found inT Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are,however, smaller (r_out < 100 AU). We did not detect the diskstowards MWC 137 and LkHα 215 at millimeter wavelengths, whichlimits the mass and the size of the possible circumstellar disks. Conclusions: A comparison between our data and previous results for TTauri and Herbig Ae stars indicates that although massive disks (~0.1M_ȯ) are found in young objects (~104 yr), the masses ofthe disks around Herbig Be stars are usually 5-10 times lower than thosearound lower mass stars. We propose that disk photoevaporation isresponsible for this behavior. In Herbig Be stars, the UV radiationdisperses the gas in the outer disk on a timescale of a few105 yr. Once the outer part of the disk has vanished, theentire gaseous disk is photoevaporated on a very short timescale(~105 yr) and only a small, dusty disk consisting of largegrains remains.
| Spatially Resolved Spectroscopy of Sub-AU-Sized Regions of T Tauri and Herbig Ae/Be Disks
We present spatially resolved near-IR spectroscopic observations of 15young stars. Using a grism spectrometer behind the Keck interferometer,we obtained an angular resolution of a few milliarcseconds and aspectral resolution of 230, enabling probes of both gas and dust in theinner disks surrounding the target stars. We find that the angular sizeof the near-IR emission typically increases with wavelength, indicatinghot, presumably gaseous material within the dust sublimation radius. Ourdata also clearly indicate Brγ emission arising from hot hydrogengas, and suggest the presence of water vapor and carbon monoxide gas inthe inner disks of several objects. This gaseous emission is morecompact than the dust continuum emission in all cases. We constructsimple physical models of the inner disk and fit them to our data toconstrain the spatial distribution and temperature of dust and gasemission components.
| Spectropolarimetric Observations of Herbig Ae/Be Stars. II. Comparison of Spectropolarimetric Surveys: Haebe, Be and Other Emission-Line Stars
The polarization of light across individual spectral lines containsinformation about the circumstellar environment on very small spatialscales. We have obtained a large number of high-precision,high-resolution spectropolarimetric observations of Herbig Ae/Be,Classical Be and other emission-line stars collected on 117 nights ofobservations with the Hi-Resolution Visible spectropolarimeter at aresolution of R = 13, 000 on the 3.67 m Advanced Electro-Optical Systemtelescope. We also have many observations from the ESPaDOnSspectropolarimeter at a resolution of R = 68, 000 on the 3.6 mCanada-France-Hawaii Telescope. In roughly ~2/3 of the so-called "windy"or "disky" Herbig Ae/Be stars, the detected Hα linearpolarization varies from our typical detection threshold near 0.1% toover 2%. In all but one HAe/Be star, the detected polarization effect isnot coincident with the Hα emission peak but isdetected in and around the obvious absorptive part of the line profile.The qu-loops are dominated by the polarization in this absorptiveregion. In several stars, the polarization varies in time mostly in theabsorptive component and is not necessarily tied to correspondingvariations in intensity. This is a new result not seen at lowerresolution. In the Be and emission-line stars, ten out of a sample of 30show a typical broad depolarization effect but four of these ten showweaker effects only visible at high resolution. Another five of 30 showsmaller amplitude, more complex signatures. Six stars of alternateclassification showed large amplitude (1%-3%) absorptive polarizationeffects. These detections are largely inconsistent with the traditionaldisk-scattering and depolarization models.
| New X-ray detections of Herbig stars
Context: The interpretation of X-ray detections from Herbig Ae/Be starsis disputed as it is not clear whether these intermediate-mass pre-mainsequence stars are able to drive a dynamo and ensuing phenomena ofmagnetic activity. Alternative X-ray production mechanisms, related tostellar winds, star-disk magnetospheres, or unresolved late-type T Tauristar companions have been proposed. Aims: The companionhypothesis can be tested by resolving Herbig stars in X-rays from theirknown visual secondaries. Furthermore, their global X-ray properties(such as detection rate, luminosity, temperature, variability) may giveclues to the emission mechanism by comparison to other types of stars,e.g. similar-age but lower-mass T Tauri stars, similar-mass but moreevolved main-sequence A- and B-type stars, and with respect to modelpredictions. Methods: In a series of papers we have beeninvestigating high-resolution X-ray Chandra images of Herbig Ae/Be andmain-sequence B-type stars where known close visual companions arespatially separated from the primaries. Results: Here we reporton six as yet unpublished Chandra exposures from our X-ray survey ofHerbig stars. The target list comprises six Herbig stars with known coolcompanions, and three other A/B-type stars that are serendipitously inthe Chandra field-of-view. In this sample we record a detection rate of100%; i.e. all A/B-type stars display X-ray emission at levels oflog(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms thatHAeBes have hotter and/or more absorbed X-ray emitting plasma than moreevolved B-type stars. Conclusions: Radiative winds are ruled outas an exclusive emission mechanism on the basis of the high X-raytemperatures. Confirming earlier results, the X-ray properties of HerbigAe/Be stars are not vastly different from those of their late-typecompanion stars (if such are known). The diagnostics provided by thepresently available data leave it open whether the hard X-ray emissionof Herbig stars is due to young age or indicative of further coronallyactive low-mass companion stars. In the latter case, our detectionstatistics imply a high fraction of higher order multiple systems amongHerbig stars.
| A Tale of Two Herbig Ae Stars, MWC 275 and AB Aurigae: Comprehensive Models for Spectral Energy Distribution and Interferometry
We present comprehensive models for the Herbig Ae stars MWC 275 and ABAur that aim to explain their spectral energy distribution (from UV tomillimeter) and long-baseline interferometry (from near-infrared tomillimeter) simultaneously. Data from the literature, combined with newmid-infrared (MIR) interferometry from the Keck Segment TiltingExperiment, are modeled using an axisymmetric Monte Carlo radiativetransfer code. Models in which most of the near-infrared (NIR) emissionarises from a dust rim fail to fit the NIR spectral energy distribution(SED) and sub-milliarcsecond NIR CHARA interferometry. Following recentwork, we include an additional gas emission component with similar sizescale to the dust rim, inside the sublimation radius, to fit the NIR SEDand long-baseline NIR interferometry on MWC 275 and AB Aur. In theabsence of shielding of starlight by gas, we show that the gas-dusttransition region in these YSOs will have to contain highly refractorydust, sublimating at ~1850 K. Despite having nearly identical structurein the thermal NIR, the outer disks of MWC 275 and AB Aur differsubstantially. In contrast to the AB Aur disk, MWC 275 lacks smallgrains in the disk atmosphere capable of producing significant 10-20μm emission beyond ~7 AU, forcing the outer regions into the``shadow'' of the inner disk.
| A parameter study of self-consistent disk models around Herbig AeBe stars
We present a parameter study of self-consistent models of protoplanetarydisks around Herbig AeBe stars. We use the code developed by Dullemondand Dominik, which solves the 2D radiative transfer problem including aniteration for the vertical hydrostatic structure of the disk. This gridof models will be used for several studies on disk emission andmineralogy in follow-up papers. In this paper we take a first look onthe new models, compare them with previous modeling attempts and focuson the effects of various parameters on the overall structure of the SEDthat leads to the classification of Herbig AeBe stars into two groups,with a flaring (group I) or self-shadowed (group II) SED. We find thatthe parameter of overriding importance to the SED is the total mass ingrains smaller than 25 μm, confirming the earlier results byDullemond and Dominik. All other parameters studied have only minorinfluences, and will alter the SED type only in borderline cases. Wefind that there is no natural dichotomy between group I and II. From amodeling point of view, the transition is a continuous function of thesmall dust mass. We also show that moderate grain growth producesspectra with weak or no 10 μm feature, both for flaring (Group I) andnon-flaring (Group II) sources. The fact that sources with weak featureshave been found mostly in Group I sources is therefore surprising andmust be due to observational biases or evolutionary effects.
| Spectroscopic metallicities of Vega-like stars
Aims. We aim to determine the metallicities of 113 Southern HemisphereVega-like candidate stars in relation to the giant exoplanet host groupand field stars. Methods: We applied two spectroscopic methods ofabundance determinations: equivalent width measurements together withthe ATLAS9 model atmospheres and the WIDTH9 program, and a comparison ofobserved spectra with a grid of synthetic spectra. Results: For theVega-like group, the metallicities are indistinguishable from those offield stars not known to be associated with planets or disks. Thisresult is quite different from the metallicities of giant exoplanet hoststars, which are metal-rich in comparison to field stars.Tables 1-4 are only available in electronic form at http://www.aanda.org
| PAH Emission from Herbig Ae/Be Stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) starsobtained with the Infrared Spectrograph on Spitzer. All but one of theHerbig stars show emission from PAHs, and seven of the spectra show PAHemission, but no silicate emission at 10 μm. The central wavelengthsof the 6.2, 7.7-8.2, and 11.3 μm emission features decrease withstellar temperature, indicating that the PAHs are less photoprocessed incooler radiation fields. The apparent low level of photoprocessing inHAeBe stars, relative to other PAH emission sources, implies that thePAHs are newly exposed to the UV-optical radiation fields from theirhost stars. HAeBe stars show a variety of PAH emission intensities andionization fractions but a narrow range of PAH spectral classificationsbased on positions of major PAH feature centers. This may indicate that,regardless of their locations relative to the stars, the PAH moleculesare altered by the same physical processes in the protoplanetary disksof intermediate-mass stars. Analysis of the mid-IR SEDs indicates thatour sample likely includes both radially flared and moreflattened/settled disk systems, but we do not see the expectedcorrelation of overall PAH emission with disk geometry. We suggest thatthe strength of PAH emission from HAeBe stars may depend not only on thedegree of radial flaring but also on the abundance of PAHs inilluminated regions of the disks and possibly on the vertical structureof the inner disk as well.
| Is the silicate emission feature only influenced by grain size?
The flattening of the 10 μm silicate emission feature observed in thespectra of T Tauri and Herbig Ae/Be stars is usually interpreted as anindicator of grain growth. We show in this paper that a similarbehaviour of the feature shape occurs when the porosity of compositegrains varies. We modelled the fluffy aggregates with inclusions ofdifferent sizes with multi-layered spheres consisting of amorphouscarbon, amorphous silicate, and vacuum. We also found that the inclusionof crystalline silicates in composite porous particles can lead to ashift of the known resonances and production of new ones.Table 1 is only available in electronic form at http://www.aanda.org
| Characterization of the magnetic field of the Herbig Be star HD200775
The origin of the magnetic fields observed in some intermediate-mass andhigh-mass main-sequence stars is still a matter of vigorous debate. Thefavoured hypothesis is a fossil field origin, in which the observedfields are the condensed remnants of magnetic fields present in theoriginal molecular cloud from which the stars formed. According to thistheory a few per cent of the pre-main-sequence (PMS) Herbig Ae/Be starshould be magnetic with a magnetic topology similar to that ofmain-sequence intermediate-mass stars.After our recent discovery of four magnetic Herbig stars, we havedecided to study in detail one of them, HD200775, to determine if itsmagnetic topology is similar to that of the main-sequence magneticstars. With this aim, we monitored this star in Stokes I and V over morethan 2yr, using the new spectropolarimeters ESPaDOnS atCanada-France-Hawaii Telescope (CFHT), and Narval at Bernard LyotTelescope (TBL).By analysing the intensity spectrum we find that HD200775 is adouble-lined spectroscopic binary system, whose secondary seems similar,in temperature, to the primary. We have carefully compared the observedspectrum to a synthetic one, and we found no evidence of abundanceanomalies in its spectrum. We infer the luminosity ratio of thecomponents from the Stokes I profiles. Then, using the temperature andluminosity of HD200775 found in the literature, we estimate the age, themass and the radius of both components from their HR diagram positions.From our measurements of the radial velocities of both stars wedetermine the ephemeris and the orbital parameters of the system.A Stokes V Zeeman signature is clearly visible in most of theleast-squares deconvolution profiles and varies on a time-scale on theorder of 1d. We have fitted the 30 profiles simultaneously, using a?2 minimization method, with a centred and adecentred-dipole model. The best-fitting model is obtained with areduced ?2 = 1.0 and provides a rotation period of 4.3281+/- 0.0010d, an inclination angle of 60° +/- 11° and a magneticobliquity angle ? = 125° +/- 8°. The polar strength of themagnetic dipole field is 1000 +/- 150G, which is decentred by 0.05 +/-0.04R* from the centre of the star. The derived magneticfield model is qualitatively identical to those commonly observed in theAp/Bp stars.Our determination of the inclination of the rotation axis leads to aradius of the primary which is smaller than that derived from the HRdiagram position. This can be explained by a larger intrinsic luminosityof the secondary relative to the primary, due to a larger circumstellarextinction of the secondary relative to the primary.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique of France and the University of Hawaii.E-mail: evelyne.alecian@rmc.ca
| The Inner Radius of T Tauri Disks Estimated from Near-Infrared Interferometry: The Importance of Scattered Light
For young Herbig AeBe stars, near-infrared interferometric measurementshave revealed a correlation between the luminosity of the central objectand the position of the disk inner rim. This correlation breaks down forthe cooler T Tauri stars, a fact often interpreted in terms of diskswith larger inner radii. In most cases, the conversion between theobserved interferometric visibility and the calculated disk inner radiuswas done with a crude disk emission model. Here we examine how the useof models that neglect scattered light can lead to an overestimation ofthe disk sizes. To do so, synthetic disk images (and visibilities) arecalculated with a full treatment of the radiative transfer. The relativecontributions of thermal emission and scattered light are compared. Wefind that the latter cannot be neglected for cool stars. For furthercomparison, the model visibilities are also converted into inner diskradii using the same simple disk models as found in the literature. Wefind that reliable inner radii can only be estimated for Herbig Ae/Bestars with these models. However, they lead to a systematicoverestimation of the disk size, by a factor of 2-3, for T Tauri stars.We suggest that including scattered light in the models is a simple (andsufficient) explanation of the current interferometric measurements of TTauri stars.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| A search for strong, ordered magnetic fields in Herbig Ae/Be stars
The origin of magnetic fields in intermediate- and high-mass stars isfundamentally a mystery. Clues towards solving this basic astrophysicalproblem can likely be found at the pre-main-sequence (PMS) evolutionarystage. With this work, we perform the largest and most sensitive searchfor magnetic fields in PMS Herbig Ae/Be (HAeBe) stars. We seek todetermine whether strong, ordered magnetic fields, similar to those ofmain-sequence Ap/Bp stars, can be detected in these objects, and if so,to determine the intensities, geometrical characteristics, andstatistical incidence of such fields. 68 observations of 50 HAeBe starshave been obtained in circularly polarized light using the FORS1spectropolarimeter at the ESO VLT. An analysis of both Balmer andmetallic lines reveals the possible presence of weak longitudinalmagnetic fields in photospheric lines of two HAeBe stars, HD 101412 andBF Ori. Results for two additional stars, CPD-53 295 and HD 36112, aresuggestive of the presence of magnetic fields, but no firm conclusionscan be drawn based on the available data. The intensity of thelongitudinal fields detected in HD 101412 and BF Ori suggest that theycorrespond to globally ordered magnetic fields with surface intensitiesof order 1 kG. On the other hand, no magnetic field is detected in 4other HAeBe stars in our sample in which magnetic fields had previouslybeen confirmed. Monte Carlo simulations of the longitudinal fieldmeasurements of the undetected stars allow us to place an upper limit ofabout 300 G on the general presence of aligned magnetic dipole magneticfields, and of about 500 G on perpendicular dipole fields. Taking intoaccount the results of our survey and other published results, we findthat the observed bulk incidence of magnetic HAeBe stars in our sampleis 8-12 per cent, in good agreement with that of magnetic main-sequencestars of similar masses. We also find that the rms longitudinal fieldintensity of magnetically detected HAeBe stars is about 200 G, similarto that of Ap stars and consistent with magnetic flux conservationduring stellar evolution. These results are all in agreement with thehypothesis that the magnetic fields of main-sequence Ap/Bp stars arefossils, which already exist within the stars at the PMS stage. Finally,we explore the ability of our new magnetic data to constrainmagnetospheric accretion in Herbig Ae/Be stars, showing that ourmagnetic data are not consistent with the general occurrence in HAeBestars of magnetospheric accretion as described by the theories ofKönigl and Shu et al..Based on observations from the ESO telescopes at the La Silla ParanalObservatory under programme ID 072.C-0447, DDT-272.C-5063, 074.C-0442.E-mail: wade-g@rmc.ca
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| Optical spectroscopy of close companions to nearby Herbig Ae/Be and T Tauri stars
We present VLT-FORS2 optical (5700-9400 Å) spectroscopy of close(r < 1.5´´) companions to three nearby (d < 200 pc)Herbig Ae/Be stars (HD 144432, HD 150193, KK Oph) and one T Tauri star(S CrA). We report the detection of Li I (6707 Å) in absorptionand emission lines (Hα, Ca II triplet) in the spectra of thecompanions. Our observations strongly suggest that the companions arephysically associated pre-main-sequence stars. The spectral type derivedfor the companions is K5Ve for HD 144432 B, F9Ve for HD 150193 B, andG6Ve for KK Oph B. S CrA A and B were observed simultaneously. Thespatially resolved spectra indicate that S CrA A (primary, north) is a Gstar and that S CrA B (secondary, south) is a K star. Using photometryfrom the literature and estimations of the R and I magnitude derivedfrom the spectra, we localized primaries and companions in the HRdiagram, derived their masses and assuming coevality constrained the ageof the systems. KK Oph B (7 Myr) and S CrA B (2 Myr) are activelyaccreting T Tauri stars and are very likely surrounded by disks. HD150193 B (10 Myr) and HD 144432 B (8 Myr) are weak-line T Tauri stars.Three of the four systems studied (HD 144432, HD 150193, KK Oph) haveages >7 Myr. These systems retained their disks for a longer timethan typical of a young star. Our results suggest that binarity may be akey issue in understanding the lifetime of disks.Based on observations collected at the European Southern Observatory,Chile (program ID 075.C-0395(A)). Figure 3 is only available inelectronic form at http://www.aanda.org
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| The magnetic field in the photospheric and circumstellar components of Herbig Ae stars
Aims:We investigate the photospheric and circumstellar (CS) magneticfield components separately in seven Herbig Ae stars. Methods:The study is based on low-resolution (R˜ 2000 and 4000)spectropolarimetric data collected from 2003 to 2005 at the Very LargeTelescope (ESO, Chile) with the multi-mode instrument FORS1. Results: We show that the spectropolarimetric results strongly dependon the level of CS contribution to the stellar spectra. We have improvedthe determination accuracy of magnetic fields up to the 7? levelin the two Herbig Ae stars HD 139614 and HD 144432, observed in 2005when these objects were at a low level state in their CS activity. Wehave established that at a higher level state of CS activity thepolarisation signatures are mainly related to the CS matter. Thepresence of CS polarisation signatures formed in the stellar windsupports the assumption that the magnetic centrifuge is one of the mainmechanisms of wind acceleration. Conclusions: .We conclude thatthe most effective way to investigate the magnetism of Herbig Ae starsis to monitor their spectropolarimetric behaviour at different stages ofCS activity. Obviously, higher-resolution spectropolarimetricobservations would extend the sample of spectral lines which can be usedto measure magnetic fields at different levels in the stellar atmosphereand CS envelope. These observations will give a more complete insightinto the magnetic topology in Herbig Ae stars.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO programme Nos. 072.D-0377, 073.D-0464, 074.C-0463,and 075.D-0507).
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| 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.
| C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features
Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) featurestowards young low-mass (T Tauri) stars and compare them with surveys ofintermediate mass (Herbig Ae/Be) stars. The presence and strength of thePAH features are interpreted with disk radiative transfer modelsexploring the PAH feature dependence on the incident UV radiation, PAHabundance and disk parameters. Methods: .Spitzer Space Telescope5-35 μm spectra of 54 pre-main sequence stars with disks wereobtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars withunknown spectral type. Results: .Compact PAH emission is detectedtowards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2μm PAH feature is detected in all of these sources, as is the 6.2μm PAH feature for the 4 sources for which short wavelength data areavailable. However, the 7.7 and 8.6 μm features appear strongly inonly 1 of these 4 sources. Based on the 11.2 μm feature, PAH emissionis observed towards at least 3 T Tauri stars, with 14 tentativedetections, resulting in a lower limit to the PAH detection rate of 8%.The lowest mass source with PAH emission in our sample is TCha with a spectral type G8. All 4 sources in our sample withevidence for dust holes in their inner disk show PAH emission,increasing the feature/continuum ratio. Typical 11.2 μm lineintensities are an order of magnitude lower than those observed for themore massive Herbig Ae/Be stars. Measured line fluxes indicate PAHabundances that are factors of 10-100 lower than standard interstellarvalues. Conversely, PAH features from disks exposed to stars withT_eff≤ 4200 K without enhanced UV are predicted to be below thecurrent detection limit, even for high PAH abundances. Disk modelingshows that the 6.2 and 11.2 μm features are the best PAH tracers forT Tauri stars, whereas the 7.7 and 8.6 μm bands have low feature overcontinuum ratios due to the strongly rising silicate emission.
| 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.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Few Skewed Disks Found in First Closure-Phase Survey of Herbig Ae/Be Stars
Using the three-telescope IOTA interferometer on Mount Hopkins, wereport results from the first near-infrared (λ=1.65 μm)closure-phase survey of young stellar objects (YSOs). These closurephases allow us to unambiguously detect departures from centrosymmetry(i.e., skew) in the emission pattern from YSO disks on the scale of ~4mas, expected from generic ``flared disk'' models. Six of 14 targetsshowed small, yet statistically significant nonzero closure phases, withlargest values from the young binary system MWC 361-A and the(pre-main-sequence?) Be star HD 45677. Our observations are quitesensitive to the vertical structure of the inner disk, and we confrontthe predictions of the ``puffed-up inner wall'' models of Dullemond,Dominik, & Natta (DDN). Our data support disk models with curvedinner rims because the expected emission appears symmetricallydistributed around the star over a wide range of inclination angles. Incontrast, our results are incompatible with the models possessingvertical inner walls because they predict extreme skewness (i.e., largeclosure phases) from the near-IR disk emission that is not seen in ourdata. In addition, we also present the discovery of mysterious H-band``halos'' (~5%-10% of light on scales 0.01"-0.50") around a few objects,a preliminary ``parametric imaging'' study for HD 45677, and the firstastrometric orbit for the young binary MWC 361-A.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Scorpius |
| Ascensió Recta: | 16h06m57.96s |
| Declinació: | -27°43'09.8" |
| Magnitud Aparent: | 8.195 |
| Distancia: | 252.525 parsecs |
| Moviment propi RA: | -11.7 |
| Moviment propi Dec: | -23.7 |
| B-T magnitude: | 8.599 |
| V-T magnitude: | 8.229 |
Catàlegs i designacions:
|
