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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.
| Maximum stellar mass versus cluster membership number revisited
We have made a new compilation of observations of maximum stellar massversus cluster membership number from the literature, which we analysefor consistency with the predictions of a simple random drawinghypothesis for stellar mass selection in clusters. Previously, Weidnerand Kroupa have suggested that the maximum stellar mass is lower, inlow-mass clusters, than would be expected on the basis of randomdrawing, and have pointed out that this could have importantimplications for steepening the integrated galactic initial massfunction (IGIMF) at high masses. Our compilation demonstrates how theobserved distribution in the plane of maximum stellar mass versusmembership number is affected by the method of target selection; inparticular, rather low n clusters with large maximum stellar masses areabundant in observational data sets that specifically seek clusters inthe environs of high-mass stars. Although we do not consider ourcompilation to be either complete or unbiased, we discuss the method bywhich such data should be statistically analysed. Our very provisionalconclusion is that the data are not indicating any striking deviationfrom the expectations of random drawing.
| 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.
| On the difference between Herbig Ae and Herbig Be stars
We present linear spectropolarimetric data for eight Herbig Be and fourHerbig Ae stars at Hα, Hβ and Hγ. Changes in the linearpolarization are detected across all Balmer lines for a large fractionof the observed objects, confirming that the small-scale regionssurrounding these objects are flattened (i.e. disc-like). Furthermore,all objects with detections show similar characteristics at the threespectral lines, despite differences in transition probability andoptical depth going from Hα to Hγ. A large fraction of earlyHerbig Be stars (B0-B3) observed show line-depolarization effects.However, the early Herbig Ae stars (A0-A2), observed for comparison,show intrinsic line-polarization signatures. Our data suggest that thepopular magnetic accretion scenario for T Tauri objects may be extendedto Herbig Ae stars, but that it may not be extended to early Herbig Bestars, for which the available data are consistent with disc accretion.
| The Complex Structure of the Mg II λλ 2795.523, 2802.698 Å Regions of 64 Be Stars
Here we consider the presence of absorption components shifted to theviolet or red side of the main spectral line (satellite or discreteabsorption components, i.e., SACs or DACs) in the regions of the Mg IIresonance lines in Be stars as well as their kinematicalcharacteristics. Namely, our objective is to check whether there existsa common physical structure for the atmospheric regions creating SACs orDACs of the Mg II resonance lines. In order to do this, a statisticalstudy of the Mg II λλ 2795.523, 2802.698 Å lines inthe spectra of 64 Be stars of all spectral subtypes and luminosityclasses was performed. We found that the atmospherical absorptionregions where the Mg II resonance lines originated may be formed ofseveral independent density layers of matter that rotate with differentvelocities. It was also attempted to separate SACs and DACs according tolow or high radial velocity. The emission lines were detected only inthe earliest and latest spectral subtypes.
| 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
| A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Polarized standard stars
Not Available
| Fundamental parameters of Be stars located in the seismology fields of COROT
In preparation for the COROT space mission, we determined thefundamental parameters (spectral type, temperature, gravity, V sin i) ofthe Be stars observable by COROT in its seismology fields (64 Be stars).We applied a careful and detailed modeling of the stellar spectra,taking into account the veiling caused by the envelope, as well as thegravitational darkening and stellar flattening due to rapid rotation.Evolutionary tracks for fast rotators were used to derive stellar massesand ages. The derived parameters will be used to select Be stars assecondary targets (i.e. observed for 5 consecutive months) and short-runtargets of the COROT mission. Furthermore, we note that the main part ofour stellar sample falls in the second half of the main sequence lifetime, and that in most cases the luminosity class of Be stars isinaccurate in characterizing their evolutionary status.
| On the binarity of Herbig Ae/Be stars
We present high-resolution spectro-astrometry of a sample of 28 HerbigAe/Be and three F-type pre-main-sequence stars. The spectro-astrometry,which is essentially the study of unresolved features in long-slitspectra, is shown from both empirical and simulated data to be capableof detecting binary companions that are fainter by up to 6mag atseparations larger than ~0.1arcsec. The nine targets that werepreviously known to be binary are all detected. In addition, we reportthe discovery of six new binaries and present five further possiblebinaries. The resulting binary fraction is 68 +/- 11 per cent. Thisoverall binary fraction is the largest reported for any observed sampleof Herbig Ae/Be stars, presumably because of the exquisite sensitivityof spectro-astrometry for detecting binary systems. The data hint thatthe binary frequency of the Herbig Be stars is larger than that of theHerbig Ae stars. The Appendix presents model simulations to assess thecapabilities of spectro-astrometry and reinforces the empiricalfindings. Most spectro-astrometric signatures in this sample of HerbigAe/Be stars can be explained by the presence of a binary system. Twoobjects, HD 87643 and Z CMa, display evidence for asymmetric outflows.Finally, the position angles of the binary systems have been comparedwith available orientations of the circumprimary disc and these appearto be coplanar. The alignment between the circumprimary discs and thebinary systems strongly suggests that the formation of binaries withintermediate-mass primaries is due to fragmentation as the alternative,stellar capture, does not naturally predict aligned discs. The alignmentextends to the most massive B-type stars in our sample. This leads us toconclude that formation mechanisms that do result in massive stars, butpredict random angles between the binaries and the circumprimary discs,such as stellar collisions, are also ruled out for the same reason.
| Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars
We present Hα spectropolarimetry observations of a sample of 10bright T Tauri stars, supplemented with new Herbig Ae/Be star data. Achange in the linear polarization across Hα is detected in most ofthe T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret interms of a compact source of line photons that is scattered off arotating accretion disc. We find consistency between the position angle(PA) of the polarization and those of imaged disc PAs from infrared andmillimetre imaging and interferometry studies, probing much largerscales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find thepolarization PA to be perpendicular to the imaged disc, which isexpected for single scattering. On the other hand, the polarization PAaligns with the outer disc PA for the T Tauri stars DR Tau and SU Aurand FU Ori, conforming to the case of multiple scattering. Thisdifference can be explained if the inner discs of Herbig Ae stars areoptically thin, whilst those around our T Tauri stars and FU Ori areoptically thick. Furthermore, we develop a novel technique that combinesknown inclination angles and our recent Monte Carlo models to constrainthe inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, weconsider the connection of the inner disc structure with the orientationof the magnetic field in the foreground interstellar medium: for FU Oriand DR Tau, we infer an alignment of the stellar axis and the largermagnetic field direction.
| 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.
| A quest for PMS candidate stars at low metallicity: Variable HAe/Be and Be stars in the Small Magellanic Cloud
We report the discovery of 5 new Herbig Ae/Be candidate stars in theSmall Magellanic Cloud in addition to the 2 reported in Beaulieu et al.(\cite{Beaulieu01}). We discuss these 7 HAeBe candidate stars in termsof (1) their irregular photometric variability, (2) their near infraredemission, (3) their Hα emission and (4) their spectral type. Onestar has the typical photometric behaviour that is observed only amongPre-Main Sequence UX Orionis type stars. The objects are more luminousthan Galactic HAeBe stars and Large Magellanic Cloud HAeBe candidates ofthe same spectral type.The stars were discovered in a systematic search for variable stars in asubset of the EROS2 database consisting of 115 612 stars in a field of24x 24 arcmin in the Small Magellanic Cloud. In total we discovered 504variable stars. After classifying the different objects according totheir type of variability, we concentrate on 7 blue objects withirregular photometric behaviour. We cross-identified these objects withemission line catalogues from Simbad and JHK photometry from 2MASS. Theanalysis is supplemented with obtained narrow and broad band imaging. Wediscuss their variability in terms of dust obscuration and bound-freeand free-free emission. We estimate the influence of metallicity on thecircumstellar dust emission from pre-main sequence stars.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue 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/399/141
| Probing the circumstellar structure of Herbig Ae/Be stars
We present Hα spectropolarimetry observations of a sample of 23Herbig Ae/Be stars. A change in the linear polarization across Hαis detected in a large fraction of the objects, which indicates that theregions around Herbig stars are flattened (disc-like) on small scales. Asecond outcome of our study is that the spectropolarimetric signaturesfor the Ae stars differ from those of the Herbig Be stars, withcharacteristics changing from depolarization across Hα in theHerbig Be stars, to line polarizations in the Ae group. The frequency ofdepolarizations detected in the Herbig Be stars (seven out of 12) isparticularly interesting as, by analogy with classical Be stars, it maybe the best evidence to date that the higher-mass Herbig stars aresurrounded by flattened structures. For the Herbig Ae stars, nine out of11 show a line polarization effect that can be understood in terms of acompact Hα emission that is itself polarized by a rotatingdisc-like circumstellar medium. The spectropolarimetric differencebetween the Herbig Be and Ae stars may be the first indication thatthere is a transition in the Hertzsprung-Russell diagram from magneticaccretion at spectral type A to disc accretion at spectral type B.Alternatively, the interior polarized line emission apparent in the Aestars may be masked in the Herbig Be stars owing to their higher levelsof Hα emission.
| Circumstellar disks around Herbig Ae/Be stars: Polarization, outflows and binary orbits
The geometrical relationship between the distribution of circumstellarmatter, observed optical linear polarization, outflows and binaryorbital plane in Herbig Ae/Be stars is investigated. Optical linearpolarization measurements carried out for a number of Herbig Ae/Be starsthat are either known to be in binary systems and/or have bipolar jetsare presented in this paper. Available information on the positionangles of polarization, outflows and binary companions for Herbig Ae/Bestars is compiled and analysed for any possible correlations. In ~85% ofthe sources the outflow position angle is within 30deg ofbeing parallel or perpendicular to the polarization position angle. In~81% of the sources the binary position angle is within 30degof being parallel or perpendicular to the polarization position angle.Out of 15 sources with bipolar outflows, 10 sources have the binaryposition angle within 30deg of being perpendicular to theoutflow position angle. These results favour those binary formationmechanisms in which the binary components and the disks aroundindividual stars or circumbinary disks are coplanar.
| The history of mass dispersal around Herbig Ae/Be stars
We present a systematic study of the material surroundingintermediate-mass stars. Our sample includes 34 Herbig Ae/Be (HAEBE)stars of different ages and luminosities. This is a quite completerepresentation of the whole class of HAEBE stars and consequently, ourconclusions should have a solid statistical meaning. In addition, wehave observed 2 intermediate-mass protostars and included published dataon 15 protostellar objects in order to determine the evolution of thecircumstellar material in the early stages of stellar evolution. All theHAEBE stars have been classified according with the three Types alreadydefined in Fuente et al. (\cite{fuen98}): Type I stars are immersed in adense clump and have associated bipolar outflows, their ages are ~ 0.1Myr; Type II stars are still immersed in the molecular cloud though notin a dense clump, their ages are between ~ a few 0.1 to ~ a few Myr;Type III stars have completely dispersed the surrounding material andare located in a cavity of the molecular cloud, their ages are >1Myr. Our observations are used to reconstruct the evolution of thecircumstellar material around intermediate-mass stars and investigatethe mass dispersal mechanisms at the different stages of the stellarevolution. Our results can be summarized as follows: intermediate-massstars disperse >=90% of the mass of the parent clump during theprotostellar phase. During this phase, the energetic outflows sweep outthe gas and dust forming a biconical cavity while the equatorialmaterial is infalling to feed the circumstellar disk and eventually theprotostar. In this way, the density structure of the parent clumpremains well described by a density law n~ r\beta with -2=1 Myr. Since the outflowdeclines and the stars are still too cold to generate UV photons,stellar winds are expected to be the only dispersal mechanism at work.In 1 Myr an early-type star (B0-B5) and in >=1 to 10 Myr a late-typestar (later than B6) meets the ZAMS. Now the star is hot enough toproduce UV photons and starts excavating the molecular cloud.Significant differences exist between early-type and late-type stars atthis evolutionary stage. Only early-type stars are able to create large(R>0.08 pc) cavities in the molecular cloud, producing a dramaticchange in the morphology of the region. This difference is easilyunderstood if photodissociation plays an important role in the massdispersal around these objects.
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| The stellar composition of the star formation region CMa R1 - II. Spectroscopic and photometric observations of nine young stars
We present new high- and low-resolution spectroscopic and photometricdata of nine members of the young association CMa R1. All the stars havecircumstellar dust at some distance, as could be expected from theirassociation with reflection nebulosity. Four stars (HD52721, HD53367,LkHα220 and LkHα218) show Hα emission and we arguethat they are Herbig Be stars with discs. Our photometric andspectroscopic observations of these stars reveal new characteristics oftheir variability. We present first interpretations of the variabilityof HD52721, HD53367 and the two LkHα stars in terms of a partiallyeclipsing binary, a magnetic activity cycle and circumstellar dustvariations, respectively. The remaining five stars show no clearindications of Hα emission in their spectra, although theirspectral types and ages are comparable with those of HD52721 andHD53367. This indicates that the presence of a disc around a star in CMaR1 may depend on the environment of the star. In particular we find thatall Hα emission stars are located at or outside the arc-shapedborder of the Hii region, which suggests that the stars inside the archave lost their discs through evaporation by UV photons from nearby Ostars, or from the nearby (<25pc) supernova, about 1Myr ago.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| On the Variability of O4-B5 Luminosity Class III-V Stars
We investigate the Hipparcos Satellite photometry of O4-B5 luminosityclass III-V stars. Some for which further study is desirable areidentified. These stars in general are more variable than cooler stars
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| Analysis of correlations between polarimetric and photometric characteristics of young stars. A new approach to the problem after eleven years' study
We present the results of the investigation of correlations between thepolarimetric and photometric characteristics of a sample (496 objects)of young Herbig Ae/Be (HAEBE) stars and T Tauri (TT) stars. It is shownthat, for 85% of the sample stars there is a general relation betweenthe degree of optical polarization and the infrared colour index(V-L)_obs and the colour excess E(V-L) due to the contribution of acircumstellar dust shell. Polarimetric data were also compared with thevalue of vsin i to search for a possible correlation between thepolarization and an inclination of circumstellar disks. Polarimetricdata as well as IR excesses are considered and compared for differentsubgroups of young stars namely: HAEBE and TT stars with Algol-likeminima of brightness (26 objects), Vega-type stars and post HAEBE stars(114 objects) and young solar-type stars (58 objects). For statisticalpurposes the data for young stars were compared with those collected fordifferent groups of evolved objects such as: classical Be stars (~300objects), Mira Ceti stars (39 objects), early-type supergiants from theSerkowski et al. (\cite{serk}) catalogue (120 objects) and main sequence(MS) stars within 50 pc from the Sun from the Leroy (\cite{leroy})catalogue (68 objects). The value of polarization is discussed incontext with the stages of evolution of circumstellar shells which wereestablished by comparison of spectral energy distribution in the far IR(using the IRAS data). It is shown that most young stars havestatistically larger value of polarization in comparison with the starswhich are on a stage of evolution close to MS. We are able to contendthat the changes in polarimetric behaviour of young stars are connectedwith evolution of their circumstellar shells. Appendices 1 to 5 are onlyavailable in electronic form at http://www.edpsciences.org
| 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.
| The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
| The stellar composition of the star formation region CMa R1 - I. Results from new photometric and spectroscopic classifications
A new photometric and spectroscopic survey of the star formation region(SFR) CMa R1 is described. In a sample of 165 stars brighter than 13thmag, 88 stars were found to be probable members of the SFR. They aredefined as early-type stars with E(B-V)>=0.16mag, which correspondsto a distance of about 1kpc. 74 of the probable members are B stars. 19stars are possibly associated with an IRAS point source. We derive amost probable distance of 1050+/-150pc to the association. It appearsthat about 80 candidate members are pre-main-sequence stars with ageslower than 6 million years, while the main sequence extends over6.0-7.6mag, which is consistent with star formation starting about 8million years ago and continuing until at least half a million yearsago. Two bright B stars in the association (GU CMa and FZ CMa) seem tobe much older and probably do not originate from the same star formationepisode. The star formation efficiency appears to increase roughlymonotonically with time up to half a million years ago. From our data,we conclude that only a minor fraction of the stars has been createdthrough the scenario suggested by Herbst & Assousa, in which themembers of CMa R1 form by compression of ambient material by a supernovashock wave. An extensive search for candidate members with Hαemission did not reveal new Herbig Ae/Be candidates, so that the numberof stars in this class seems to be limited to four: Z CMa, LkHα218, LkHα 220 and possibly HD 53367.
| The Cepheid distance to M96 and the Hubble constant
HST WFPC2 observations of Cepheids in M96 (NGC 3368) are used to find adistance to that galaxy of 11.2+/-1.0Mpc. This estimate is based on acalibration of the Cepheid period-luminosity relation in the LargeMagellanic Cloud, and includes a correction for the difference inmetallicity between the two systems. There are good reasons forbelieving M96 is at the same distance as four E/S0 galaxies in the Leo-Igroup, and hence we calibrate secondary distance indicators based on theearly-type galaxies, namely the fundamental plane andsurface-brightness-fluctuation method. Also the Type Ia supernova 1998buoccurred in M96 itself and is used to calibrate the SN Ia distancescale. These methods reach to recession velocities of greater than5000kms-1 and can therefore allow us to evaluate the Hubbleconstant without reference to the peculiar velocity of M96 itself. Infact, these indicators agree well between themselves and hence we findH0=67+/-7kms-1Mpc-1 where the quotederror includes estimates of potential systematic effects.
| Hα spectropolarimetry of B[e] and Herbig Be stars
We present the results of medium-resolution (Delta v ~ 60 km s^-1)spectropolarimetric observations across H alpha of a sample of B[e] andHerbig Be objects. A change in linear polarization across H alpha isdetected in a large fraction of the objects, with characteristicsranging from simple depolarization in a couple of Herbig Be stars, tomore complex behaviour in the probable post-main-sequence B[e] stars. Halpha in the spectra of HD 37806 and 50138 each consist of adouble-peaked polarized line and a superposed unpolarized singleemission peak, suggesting two distinct line-forming regions. Multipleobservations of HD 45677 allow for the separation of electron and dustscattering effects for the first time: the difference between derivedintrinsic polarization angles of the two components indicates that thedust scattering region is clumpy. Two unexpected results are thenon-detections of H alpha polarization changes in omicron Ori, wheredepolarization has previously been detected, and in MWC 297, whichexhibits source elongation at radio wavelengths. In omicron Ori timevariability is probably responsible such that the electron scatteringdisc of this star was much weakened at the time of observation. Twohypotheses are advanced that might explain the MWC 297 result. Thegeneral findings are that roughly half of the observed Herbig Be starsshow polarization changes across Hα, implying immediately thattheir ionized envelopes are not spherically symmetric. This pattern, ifconfirmed by observations of a larger sample, could indicate that thenon-detection rate is simply a consequence of sampling randomly orientedcircumstellar discs able to scatter starlight within a few stellarradii. The deduced alignment of the disc of HD 53367 with the localinterstellar magnetic field suggests an orderly star formation processin which the star `remembers' the larger scale magnetic field direction.The stars classified as B[e] stars all show startling polarizationchanges across Hα. The details in each case are different, but thewidely accepted concept of dense Hα-emitting equatorial discsaround these objects is supported.
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