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Magnetic Doppler imaging of ?2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields
Context. Strong organized magnetic fields have been studied in the uppermain sequence chemically peculiar stars for more than half a century.However, only recently have observational methods and numericaltechniques become sufficiently mature to allow us to record andinterpret high-resolution four Stokes parameter spectra, leading to thefirst assumption-free magnetic field models of these stars. Aims:Here we present a detailed magnetic Doppler imaging analysis of thespectropolarimetric observations of the prototypical magnetic Ap star?2 CVn. This is the second star for which the magneticfield topology and horizontal chemical abundance inhomogeneities havebeen inferred directly from phase-resolved observations of line profilesin all four Stokes parameters, free from the traditional assumption of alow-order multipolar field geometry. Methods: We interpret therotational modulation of the circular and linear polarization profilesof the strong Fe II and Cr II lines in the spectra of ?2 CVn recorded with the MuSiCoS spectropolarimeter. The surface abundance distributions of the two chemical elements and a full vector map of the stellar magnetic field are reconstructed in a self-consistent inversion using our state-of-the-art magnetic Doppler imaging code Invers10. Results: We succeeded in reproducing most of the details of the availablespectropolarimetric observations of ?2 CVn with amagnetic map which combines a global dipolar-like field topology withlocalized spots of higher field intensity. We demonstrate that thesesmall-scale magnetic structures are inevitably required to fit thelinear polarization spectra; however, their presence cannot be inferredfrom the Stokes I and V observations alone. We also found high-contrastsurface distributions of Fe and Cr, with both elements showing abundanceminima in the region of weaker and topologically simpler magnetic field. Conclusions: Our magnetic Doppler imaging analysis of?2 CVn and previous results for 53 Cam support the viewthat the upper main sequence stars can harbour fairly complex surfacemagnetic fields which resemble oblique dipoles only at the largestspatial scales. Spectra in all four Stokes parameters are absolutelyessential to unveil and meaningfully characterize this field complexityin Ap stars. We therefore suggest that understanding magnetism of starsin other parts of the H-R diagram is similarly incomplete withoutinvestigation of their linear polarization spectra.Based on data obtained using the Télescope Bernard Lyot atObservatoire du Pic du Midi.
| Magnetic Doppler imaging of the roAp star HD 24712
Aims: We present the first magnetic Doppler images of a rapidlyoscillating Ap (roAp) star. Methods: We deduce information aboutmagnetic field geometry and abundance distributions of a number ofchemical elements on the surface of the hitherto best studied roAp star,HD 24712, using the magnetic Doppler imaging (MDI) code, invers10, whichallows us to reconstruct simultaneously and consistently the magneticfield geometry and elemental abundance distributions on a stellarsurface. For this purpose we analyse time series spectra obtained inStokes I and V parameters with the SOFIN polarimeter at the NordicOptical Telescope and recover surface abundance structures of sixteendifferent chemical elements, respectively ions, including Mg, Ca, Sc,Ti, Cr, Fe, Co, Ni, Y, La, Ce, Pr, Nd, Gd, Tb, and Dy. For the rareearth elements (REE) Pr and Nd separate maps were obtained using linesof the first and the second ionization stage. Results: We findand confirm a clear dipolar structure of the surface magnetic field andan unexpected correlation of elemental abundance with respect to thisfield: one group of elements accumulates solely where the positivemagnetic pole is visible, whereas the other group avoids this region andis enhanced where the magnetic equatorial region dominates the visiblestellar surface. We also observe relative shifts of abundanceenhancement- or depletion regions between the various elementsexhibiting otherwise similar behaviour.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
| Mid-IR observations of circumstellar disks. Part III. A mixed sample of PMS stars and Vega-type objects
We present new mid-infrared spectra of 15 targets (1 FU Orionis object,4 Herbig Ae stars, 5 T Tauri stars, and 5 Vega-type stars), obtainedwith the TIMMI2 camera at La Silla Observatory (ESO). Three targets aremembers of the ? Pic moving group (HD 155 555,HD 181 296, and HD 319 139). PAHbands are observed towards the T Tauri star HD 34 700and the Herbig Ae star PDS 144 N. For HD 34700, the band profiles indicate processed PAHs. The spectrumof the Vega-type object ? Corvi (HD 109 085), forwhich a resolved disk at sub-mm wavelengths is known, appears stellarbetween 8-13 ?m, but a small excess emission was reported by Spitzerobservations. Similarly, no indication of circumstellar matter atmid-infrared wavelengths is found towards the Vega-like starsHD 3003, HD 80 951, HD181 296, and, surprisingly, the T Tauri system HD 155555. The silicate emission features of the remaining eightsources are modelled with a mixture of silicates of different grainsizes and composition. Unprocessed dust dominates FUOri, HD 143 006, and CD-43344. Large amorphous grains are the main dust component aroundHD 190 073, HD 319 139,KK Oph, and PDS 144 S. Both smallgrains and crystalline dust is found for the Vega-type HD 123356, with a dominance of small amorphous grains. We show thatthe infrared emission of the binary HD 123 356 isdominated by its late-type secondary, but optical spectroscopy is stillrequired to confirm the age of the system and the spectral class of thecompanion. For most targets, this is their first mid-infraredspectroscopic observation. We investigate trends between stellar, disk,and silicate properties and confirm correlations identified in previousstudies. Several objects present an exciting potential for follow-uphigh-resolution disk studies.Based on observations made at the European Southern Observatory,obtained under program IDs 076.C-0634(A) and 077.C-0054(A),during TIMMI2 technical time as well as collected from the ESO/ST-ECFScience Archive Facility.
| Near-Infrared Photometric Survey of Herbig Ae/Be Candidate Stars
We report near-infrared photometric measurements of 35 Herbig Ae/Becandidate stars obtained with direct imaging and aperture photometry.Observations were made through the broadband J, H, and K' filters, witheach source imaged in at least one of the wavebands. We achievedsubarcsecond angular resolution for all observations, providing us withthe opportunity to search for close binary candidates and extendedstructure. The imaging revealed five newly identified binary candidatesand one previously resolved T Tauri binary among the target sources withseparations of lsim2farcs5. Separate photometry is provided for each ofthe binary candidate stars. We detect one extended source that has beenidentified as a protoplanetary nebula. Comparing our magnitudes to pastmeasurements yields significant differences for some sources, possiblyindicating photometric variability. H-band finding charts for all of oursources are provided to aid follow-up high-resolution imaging.
| 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.
| Tracing the potential planet-forming regions around seven pre-main-sequence stars
Aims: We investigate the nature of the innermost regions with radii ofseveral AUs of seven circumstellar disks around pre-main-sequence stars,T Tauri stars in particular. Our object sample contains disks apparentlyat various stages of their evolution. Both single stars and spatiallyresolved binaries are considered. In particular, we search for innerdisk gaps as proposed for several young stellar objects (YSOs). Whenanalyzing the underlying dust population in the atmosphere ofcircumstellar disks, the shape of the 10 ?m feature shouldadditionally be investigated. Methods: We performedinterferometric observations in N band (8-13 ?m) with theMid-Infrared Interferometric Instrument (MIDI) at the Very LargeTelescope Interferometer (VLTI) using baseline lengths of between 54 mand 127 m. The data analysis is based on radiative-transfer simulationsusing the Monte Carlo code MC3D by modeling simultaneously the spectralenergy distribution (SED), N band spectra, and interferometricvisibilities. Correlated and uncorrelated N band spectra are compared toinvestigate the radial distribution of the dust composition of the diskatmosphere. Results: Spatially resolved mid-infrared (MIR)emission was detected in all objects. For four objects (DR Tau, RU Lup,S CrA N, and S CrA S), the observed N band visibilities andcorresponding SEDs could be simultaneously simulated using aparameterized active disk-model. For the more evolved objects of oursample, HD 72106 and HBC 639, a purely passive disk-model provides theclosest fit. The visibilities inferred for the source RU Lup allow thepresence of an inner disk gap. For the YSO GW Ori, one of two visibilitymeasurements could not be simulated by our modeling approach. Alluncorrelated spectra reveal the 10 ?m silicate emission feature. Incontrast to this, some correlated spectra of the observations of themore evolved objects do not show this feature, indicating a lack ofsmall silicates in the inner versus the outer regions of these disks. Weconclude from this observational result that more evolved dust grainscan be found in the more central disk regions.Based on observations made with Telescopes of the European Organisationfor Astronomical Research in the Southern Hemisphere (ESO) at theParanal Observatory, Chile, under the programs 074.C-0342(A),075.C-0064(A,B), 075.C-0413(A,B), and 076.C-0356(A).Appendix A is only available in electronic form at http://www.aanda.org
| 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.
| Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106
The recently discovered magnetic Herbig Ae and Be stars may providequalitatively new information about the formation and evolution ofmagnetic Ap and Bp stars. We have performed a detailed investigation ofone particularly interesting binary system with a Herbig Ae secondaryand a late B-type primary possessing a strong, globally ordered magneticfield. 20 high-resolution Stokes V spectra of the system were obtainedwith the ESPaDOnS instrument mounted on the Canada-France-HawaiiTelescope. In these observations we see clear evidence for a magneticfield in the primary, but no evidence for a magnetic field in thesecondary. A detailed abundance analysis was performed for both stars,revealing strong chemical peculiarities in the primary and normalchemical abundances in the secondary. The primary is stronglyoverabundant in Si, Cr and other iron-peak elements, as well as Nd, andunderabundant in He. The primary therefore appears to be a very young Bpstar. In this context, line profile variations of the primary suggestnon-uniform lateral distributions of surface abundances. Interpretingthe 0.63995 +/- 0.00009 d variation period of the Stokes I and Vprofiles as the rotational period of the star, we have modelled themagnetic field geometry and the surface abundance distributions of Si,Ti, Cr and Fe using magnetic Doppler imaging. We derive a dipolargeometry of the surface magnetic field, with a polar strengthBd = 1230 G and an obliquity ? = 57°. Thedistributions Ti, Cr and Fe are all qualitatively similar, with anelongated patch of enhanced abundance situated near the positivemagnetic pole. The Si distribution is somewhat different, and itsrelationship to the magnetic field geometry less clear.
| A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-massstars that exhibit an 8 μm excess in either the Galactic LegacyInfrared Mid-Plane Survey Extraordinaire or the Mid-Course SpaceExperiment using high-resolution optical spectra to identify starssurrounded by warm circumstellar dust. From these data we determineprojected stellar rotational velocities and estimate stellar effectivetemperatures for the sample. We estimate stellar ages from thesetemperatures, parallactic distances, and evolutionary models. Using MIPS[24] measurements and stellar parameters we determine the nature of theinfrared excess for 19 GLIMPSE stars. We find that 15 stars exhibitHα emission and four exhibit Hα absorption. Assuming thatthe mid-IR excesses arise in circumstellar disks, we use the Hαfluxes to model and estimate the relative contributions of dust andfree-free emission. Six stars exhibit Hα fluxes that implyfree-free emission can plausibly explain the infrared excess at [24].These stars are candidate classical Be stars. Nine stars exhibitHα emission, but their Hα fluxes are insufficient to explainthe infrared excesses at [24], suggesting the presence of acircumstellar dust component. After the removal of the free-freecomponent in these sources, we determine probable disk dust temperaturesof Tdisk~=300-800 K and fractional infrared luminosities ofLIR/L*~=10-3. These nine stars may bepre-main-sequence stars with transitional disks undergoing diskclearing. Three of the four sources showing Hα absorption exhibitcircumstellar disk temperatures ~=300-400 K,LIR/L*~=10-3, IR colors K-[24]<3.3,and are warm debris disk candidates. One of the four Hα absorptionsources has K-[24]>3.3 implying an optically thick outer disk and isa transition disk candidate.
| Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
Chemically peculiar A stars (Ap) are extreme examples of the interactionof atomic element diffusion processes with magnetic fields in stellaratmospheres. The rapidly oscillating Ap stars provide a means forstudying these processes in three dimensions and are at the same timeimportant for studying the pulsation excitation mechanism in A stars. Aspart of the first comprehensive, uniform, high-resolution spectroscopicsurvey of Ap stars, which we are conducting in the Southern hemispherewith the Michigan Spectral Catalogues as the basis of target selection,we report here the discovery of 17 new magnetic Ap stars havingspectroscopically resolved Zeeman components from which we derivemagnetic field moduli in the range 3-30kG. Among these are (1) thecurrent second strongest known magnetic A star, (2) a double-lined Apbinary with a magnetic component and (3) an A star with particularlypeculiar and variable abundances. Polarimetry of these stars is neededto constrain their field geometries and to determine their rotationperiods. We have also obtained an additional measurement of the magneticfield of the Ap star HD92499.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 078.D-0080(A), 078.D-0192(A),072.D-0138(A).E-mail: lmfreyhammer@uclan.ac.uk
| The characteristics of the IR emission features in the spectra of Herbig Ae stars: evidence for chemical evolution
Context: Infrared (IR) spectra provide a prime tool to study thecharacteristics of polycyclic aromatic hydrocarbon (PAH) molecules inregions of star formation. Herbig Ae/Be stars are a class of youngpre-main sequence stellar objects of intermediate mass. They are knownto have varying amounts of natal cloud material still present in theirdirect vicinity. Aims: We characterise the IR emission bands, due tofluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars andlink observed variations to spatial aspects of the mid-IR emission. Methods: We analysed two PAH dominated spectra from a sample of 15Herbig Ae/Be stars observed with the Spitzer Space Telescope. Results:We derived profiles of the major PAH bands by subtracting appropriatecontinua. The shape and the measured band characteristics showpronounced variations between the two Spitzer spectra investigated.Those variations parallel those found between three infrared spaceobservatory (ISO) spectra of other, well-studied, Herbig Ae/Be stars.The derived profiles are compared to those from a broad sample ofsources, including reflection nebulae, planetary nebulae, H II regions,young stellar objects, evolved stars and galaxies. The Spitzer and ISOspectra exhibit characteristics commonly interpreted respectively asinterstellar matter-like (ISM), non-ISM-like, or a combination of thetwo. Conclusions: We argue that the PAH emission detected from thesources exhibiting a combination of ISM-like and non-ISM-likecharacteristics indicates the presence of two dissimilar, spatiallyseparated, PAH families. As the shape of the individual PAH bandprofiles reflects the composition of the PAH molecules involved, thisdemonstrates that PAHs in subsequent, evolutionary linked stages of starformation are different from those in the general ISM, implying activechemistry. None of the detected PAH emission can be associated with the(unresolved) disk and is thus associated with the circumstellar (natal)cloud. This implies that chemical changes may already occur in the(collapsing?) natal cloud and not necessarily in the disk.
| 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
| 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
| The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSEsurveys. Of these, 183 are either OB stars earlier than B8 in which theexcess plausibly arises from a thermal bremsstrahlung component orevolved stars in which the excess may be explained by an atmosphericdust component. The remaining 47 stars have spectral classifications B8or later and appear to be main-sequence or late pre-main-sequenceobjects harboring circumstellar disks. Six of the 47 stars exhibitmultiple signatures characteristic of pre-main-sequence circumstellardisks, including emission lines, near-IR K-band excesses, and X-rayemission. Approximately one-third of the remaining 41 sources haveemission lines suggesting relative youth. We modeled the excesses in 26stars having two or more measurements in excess of the expectedphotospheres as single-component blackbodies. We determine probable disktemperatures and fractional IR luminosities in the range 191K
| The magnetic field of the pre-main sequence Herbig Ae star HD 190073
Context: .The general context of this paper is the study of magneticfields in the pre-main sequence intermediate mass Herbig Ae/Be stars.Magnetic fields are likely to play an important role in pre-mainsequence evolution at these masses, in particular in controlling thegains and losses of stellar angular momentum. Aims: .Theparticular aim of this paper is to announce the detection of astructured magnetic field in the Herbig Ae star HD 190073, and todiscuss various scenarii for the geometry of the star, its environmentand its magnetic field. Methods: .We have used the ESPaDOnSspectropolarimeter at CFHT in 2005 and 2006 to obtain high-resolution,high signal-to-noise circular polarization spectra which demonstrateunambiguously the presence of a magnetic field in the photosphere ofthis star. Results: .Nine circular polarization spectra wereobtained, each one showing a clear Zeeman signature. This signature issuggestive of a magnetic field structured on large scales. Thesignature, which corresponds to a longitudinal magnetic field of74± 10 G, does not vary detectably on a one-year timeframe,indicating either an azimuthally symmetric field, a zero inclinationangle between the rotation axis and the line of sight, or a very longrotation period. The optical spectrum of HD 190073 exhibits a largenumber of emission lines. We discuss the formation of these emissionlines in the framework of a model involving a turbulent heated region atthe base of the stellar wind, possibly powered by magnetic accretion. Conclusions: .This magnetic detection contributes an important newobservational discovery which will aid our understanding of stellarmagnetism at intermediate masses.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.
| Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars?
We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at theCFHT, of magnetic fields in the young A-type stars HD 101412, V380 Oriand HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-mainsequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physicallyassociated with a HAeBe star. Remarkably, evidence of surface abundancespots is detected in the spectra of HD 72106A. The magnetic fields ofthese objects display intensities of order 1 kG at the photosphericlevel, are ordered on global scales, and appear in approximately 10% ofthe stars studied. Based on these properties, the detected stars maywell represent pre-main sequence progenitors of the magnetic Ap/Bpstars. The low masses inferred for these objects (2.6, 2.8 and 2.4M_ȯ) represent additional contradictions to the hypothesis ofHubrig et al. (2000, ApJ, 539, 352), who claim that magnetic fieldsappear in intermediate-mass stars only after 30% of their main sequenceevolution is complete. Finally, we fail to confirm claims by Hubrig etal. (2004, A&A, 428, L1) of magnetic fields in the Herbig Ae star HD139614.
| Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars ?
Not Available
| 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).
| 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.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| ICCD Speckle Observations of Binary Stars. XIII. Measurements During 1989- 1994 From the Cerro Tololo 4 M Telescope
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....111..936H&db_key=AST
| Search for T Tauri Stars Based on the IRAS Point Source Catalog. II.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2146T&db_key=AST
| Candidate OH/IR stars in the outer parts of our Galaxy
The nature of a sample of IRAS candidate OH/IR stars outside the solarcircle is investigated with additional information from near infraredphotometry and radio OH(1612 MHz) measurements. It is shown that thesample is contaminated by a very large fraction (about 55 percent) ofobjects not on the Asymptotic-Giant Branch (AGB) and also by a smallfraction of carbon-rich AGB stars (about 10 percent). In contradictionto Habing (1988), we conclude that there is no thick disc of OH/IR starsin the outer Galaxy and no strict cutoff of the thin disc. The smallnumber of OH/IR stars in the outer Galaxy are most likely of a high mass(2-3 solar masses) based on their luminosity estimates in excess of 10exp 4 solar luminosities. We also discuss the differences in appearanceof the OH/IR stars in the outer Galaxy with those from the innerregions. We believe that the objects of larger galactic radii have lowerdust-mass-loss rates and that this is caused by a lower metallicity ofthese stars.
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| A search for T Tauri stars based on the IRAS point source catalog.
The results of the first part of a survey for new T Tauri stars, withspectroscopic and photometric observations of about 100 stars, arepresented. A list of candidates has been selected by means of the IRASPoint Source Catalog, based on appropriate far-infrared colors. Coudespectra of the selected objects in the 655-673 nm wavelength range,which includes H-alpha and the resonance Li I line, have been obtained.Thirty-three new T Tauri stars, and a number of other interestingobjects, like early type premain-sequence stars and Li-rich K giants,have been detected. Several new isolated T Tauri were found, includingHen 1, which may be the T Tauri star with the highest galactic latitudeknown, if its nature is confirmed.
| 1612 MHz OH survey of IRAS point sources. I - Observations made at Dwingeloo, Effelsberg and Parkes
The data from a large sky survey are presented including a northernpilot study and a detailed southern study in which detections are biasedtoward the most evolved sources and distant sources. Both areinvestigated at the 1612-MHz transition of OH to take advantage of thestrongest line for AGB stars with optically thick dust shells. The IRASsources are chosen by considering their IR colors related to fluxes at12, 25, and 60 microns. Observations are reported for 2703 IRAS pointsources at the 1612-MHz transition, and 738 OH/IR stars are detected.The survey identifies 597 of the sources as previously unidentified, and95 percent of the OH profiles observed have twin-peak masercharacteristics which are related to emission from expandingcircumstellar shells. The other 5 percent of the sources are concludedto be transition objects between OH/IR stars and planetary nebulae.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | מפרש |
| התרוממות ימנית: | 08h29m34.89s |
| סירוב: | -38°36'21.3" |
| גודל גלוי: | 8.607 |
| מרחק: | 288.184 פארסק |
| תנועה נכונה: | -7.2 |
| תנועה נכונה: | 12.7 |
| B-T magnitude: | 8.624 |
| V-T magnitude: | 8.609 |
קטלוגים וכינוים:
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