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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.
| 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.
| Some implications of the introduction of scattered starlight in the spectrum of reddened stars
This paper presents new investigations on coherent scattering in theforward direction (orders of magnitude; conservation of energy;dependence of scattered light on geometry and wavelength), and on howscattered light contamination in the spectrum of reddened stars ispossibly related to as yet unexplained observations (the diminution ofthe 2200 Å bump when the obscuring material is close to the star,the difference between Hipparcos and photometric distances). This paperthen goes on to discuss the fit of the extinction curve, a possible roleof extinction by the gas in the far-UV, and the reasons of theinadequacy of the Fitzpatrick and Massa [ApJSS, 72 (1990) 163] fit.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud
We investigate a sample of 18 Large Magellanic CloudHerbig Ae/Be candidate stars looking at their (1) spectral types (2)brightness variability mechanism and (3) near infra-red JHK emission. Wefind that the majority of the target stars have Hα emission, areof spectral type early- to mid-B and lack strong JHK excess emission.Their Balmer decrements are found to be similar to that of Galactic Bestars in general. Their erratic brightness variability is evaluated byusing the observed optical color excess and the color gradient from thelight curves and is subsequently interpreted as being due to variabledust obscuration or variable bf-ff emission from circumstellar ionizedgas. For approximately half of the target stars in our sample the typeof variability seems to be dissimilar to the mechanism involvingbound-free and free-free emission, but could be interpreted as caused byvariable dust obscuration, as we have proposed in earlier studies. It istherefore suggested that they are pre-main sequence objects, despite thefact that they nearly all lack thermal dust emission in the nearinfra-red; mid/far infra-red observations for these objects arewarranted. One star is observed to have JHK excess emission and aninspection of its 7.5 year MACHO light curve confirms its erraticphotometric behavior. The object displays deep photometric minima with aquasi-period of 191.3 days, as generally seen in the Galactic pre-mainsequence subgroup of the UX Orionis stars.Based on observations collected at ESO, La Silla.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is 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/431/773
| Spectral and Photoelectric Studies of the Herbig Ae/Be Star HD 259431
Spectral and photoelectric (ubvy, H, H) observations of the Herbig Ae/Bestar HD 259431 are reported. It is found that as its brightness fades,this star becomes bluer in the Paschen continuum and the intensity andequivalent width of the hydrogen emission lines increase. The spectralobservations reveal significant variations in the intensity of the Mg II4481 Å photospheric absorption line. A rise and fall in theluminosity by 0m.04 within a period of 5-7 minutes was recorded. Radicalvariations in the H lineshape ("double" "P Cyg") and flare activity arenot only observed in this star, but also in a number of HAEBE stars. Itis suggested that flare activity may initiate a change in the velocitygradient at the base of the wind and, thereby, induce "double P Cyg" or"P Cyg single" transitions. The nonradial pulsations of this star arealso discussed.
| Spectral Analysis and Classification of Herbig Ae/Be Stars
We present an analysis of the optical spectra of 75 early-typeemission-line stars, many of which have been classified previously asHerbig Ae/Be (HAeBe) stars. Accurate spectral types were derived for 58members of the sample; high continuum veiling, contamination bynonphotospheric absorption features, or a composite binary spectrumprevented accurate spectral typing for the rest. Approximately half ofour sample exhibited [O I] λ6300 forbidden-line emission down toour detection limit of 0.1 Å equivalent width; a third of thesample exhibited Fe II emission (multiplet 42). A subset of 11 of theHAeBe sample showed abnormally strong Fe II absorption; 75% of thissubset are confirmed UX Ori objects. Combining our spectral typingresults with photometry from the literature, we confirm previousfindings of high values of total-to-selective extinction(RV~5) in our larger sample, suggesting significant graingrowth in the environments of HAeBe stars. With this high value ofRV, the vast majority of HAeBe stars appear younger than withthe standard RV=3.1 extinction law and are more consistentwith being pre-main-sequence objects.
| Far Ultraviolet Spectroscopy of HD 76534
We present the first Far Ultraviolet Spectroscopic Explorer (FUSE)spectrum of HD 76534, a Herbig Be star. Here we focus on the analysis ofthe H2 absorption lines which allow to quantify the gaseouscontent along the line of sight. This analysis evidences large amountsof cold and warm H2 toward the star. We demonstrate that theH2 is bound to HD 76534 and we argue that it is likelydistributed in a circumstellar envelope. In addition, the present studygives clues for the existence of common properties for the circumstellarH2 of Herbig Ae/Be stars.
| Optical interferometry in astronomy
Here I review the current state of the field of optical stellarinterferometry, concentrating on ground-based work although a briefreport of space interferometry missions is included. We pause both toreflect on decades of immense progress in the field as well as toprepare for a new generation of large interferometers just now beingcommissioned (most notably, the CHARA, Keck and VLT Interferometers).First, this review summarizes the basic principles behind stellarinterferometry needed by the lay-physicist and general astronomer tounderstand the scientific potential as well as technical challenges ofinterferometry. Next, the basic design principles of practicalinterferometers are discussed, using the experience of past and existingfacilities to illustrate important points. Here there is significantdiscussion of current trends in the field, including the new facilitiesunder construction and advanced technologies being debuted. This decadehas seen the influence of stellar interferometry extend beyond classicalregimes of stellar diameters and binary orbits to new areas such asmapping the accretion discs around young stars, novel calibration of thecepheid period-luminosity relation, and imaging of stellar surfaces. Thethird section is devoted to the major scientific results frominterferometry, grouped into natural categories reflecting these currentdevelopments. Lastly, I consider the future of interferometry,highlighting the kinds of new science promised by the interferometerscoming on-line in the next few years. I also discuss the longer-termfuture of optical interferometry, including the prospects for spaceinterferometry and the possibilities of large-scale ground-basedprojects. Critical technological developments are still needed to makethese projects attractive and affordable.
| Photometric Activity of LkHα 215
Results of photoelectric observations in the Strömgren system ofthe Herbig Ae/Be star LkH 215 are presented. It is found that this starundergoes bursts. Two types of bursts are observed. The amplitude of thefirst type of burst increases with decreasing wavelength. In the secondtype of burst, the brightness amplitude has a maximum in the V band.
| 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.
| Far UV spectroscopy of the circumstellar environment of the Herbig Be stars HD 259431 and HD 250550
We present an analysis of FUSE spectra of HD 259431 and HD 250550, twoyoung Herbig Be stars. Numerous absorption lines of H_2 are seen in thespectrum of each star, revealing large amounts of this gas on the linesof sight. In addition, absorption lines from atomic species in differentionization and excitation states are also identified. We demonstratethat all these species probe the close circumstellar environment aroundthe stars, with two or even three temperature components in HD 259431,as revealed by molecular gas analysis. Although favouring the flareddisk scenario, our results do not rule out the possibility that weactually detect CS envelopes/halos around these stars.
| 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.
| On the Interferometric Sizes of Young Stellar Objects
Long-baseline optical interferometers can now detect and resolve hotdust emission thought to arise at the inner edge of circumstellar disksaround young stellar objects (YSOs). We argue that the near-infraredsizes being measured are closely related to the radius at which dust issublimated by the stellar radiation field. We consider how realisticdust optical properties and gas opacity dramatically affect thepredicted location of this dust destruction radius, an exerciseroutinely done in other contexts but so far neglected in the analysis ofnear-infrared sizes of YSOs. We also present the accumulated literatureof near-infrared YSO sizes in the form of a size-luminosity diagram andcompare with theoretical expectations. We find evidence that large(>~1.0 μm) dust grains predominate in the inner disks of T Tauriand Herbig Ae/Be stars, under the assumption that the innermost gaseousdisks are optically thin at visible wavelengths.
| Testing Models of Low-Excitation Photodissociation Regions with Far-Infrared Observations of Reflection Nebulae
This paper presents Kuiper Airborne Observatory observations of thephotodissociation regions (PDRs) in nine reflection nebulae. Theseobservations include the far-infrared atomic fine-structure lines of [OI] 63 and 145 μm, [C II] 158 μm, and [Si II] 35 μm and theadjacent far-infrared continuum to these lines. Our analysis of thesefar-infrared observations provides estimates of the physical conditionsin each reflection nebula. In our sample of reflection nebulae, thestellar effective temperatures are 10,000-30,000 K, the gas densitiesare 4×102-2×104 cm-3, thegas temperatures are 200-690 K, and the incident far-ultravioletintensities are 300-8100 times the ambient interstellar radiation fieldstrength (1.2×10-4 ergs cm-2 s-1sr-1). Our observations are compared with current theory forlow-excitation PDRs. The [C II] 158 μm to [O I] 63 μm line ratiodecreases with increasing incident far-ultraviolet intensity. This trendis due in part to a positive correlation of gas density with incidentfar-ultraviolet intensity. We show that this correlation arises from abalance of pressure between the H II region and the surrounding PDR. The[O I] 145 to 63 μm line ratio is higher (greater than 0.1) thanpredicted and is insensitive to variations in incident far-ultravioletintensity and gas density. The stellar temperature has little effect onthe heating efficiency that primarily had the value3×10-3, within a factor of 2. This result agrees with amodel that modifies the photoelectric heating theory to account forcolor temperature effects and predicts that the heating efficiencieswould vary by less than a factor of 3 with the color temperature of theilluminating field. In addition to the single-pointing observations, an[O I] 63 μm scan was done across the molecular ridge of one of oursample reflection nebulae, NGC 1977. The result appears to supportprevious suggestions that the ionization front of this well-studied PDRis not purely edge-on.
| Polarimetric Studies of Stars with an Infrared Emission Excess
The results of polarimetric and IR (IRAS) observations of 24 B-A-F starsare given. Intrinsic polarization of the light from 11 of the 24 starsis observed. The degree of polarization for the other 13 stars is withinthe measurement errors. Two-color diagrams are also constructed. From acomparison of the degree of polarization with the color index on thetwo-color diagrams it is seen that 8 of these 13 stars probably are ofthe Vega type, while 5 are stars with gasdust shells and/ordiskshells. It is shown that 6 of the aforementioned 11 starswith intrinsic polarization evidently are stars with gasdustshells and/or diskshells, while 5 of them (also including No. 24)are of the Vega type. It is also shown that the IR emission from 10 ofthe stars corresponds to a power-law distribution F . This fact may beexplained both by freefree transitions of electrons and bythermal emission from dust grains in circumstellar gasdust shells(disks).
| Detecting Embedded Intermediate-Mass Stars Using Mid-Infrared and H I 21 Centimeter Emission
A technique is presented to detect distant, young embedded B stars (andpossible clusters) from their imprint on the surrounding interstellarmedium. H I 21 cm and 12CO (J=1-0) line data from theCanadian Galactic Plane Survey are combined with mid-infrared imagesfrom the Midcourse Space Experiment (MSX) Galactic plane survey todetect the photodissociation regions (PDRs) and molecular gas associatedwith the embedded intermediate-mass stars. The technique has beenapplied to a section of the Galaxy covered by the 12CO FiveCollege Radio Astronomy Observatory outer Galaxy survey(102.5d
| The Dusty Circumstellar Environments of Ae/Be Protoplanetary Disk Candidates
We have obtained high spatial resolution mid-IR observations of fourintermediate-mass pre-main-sequence stars: HD 259431, AS 310, LkHα234, and MWC 1080. Our observations reveal the morphology of thecircumstellar (500-10,000 AU) environments of these young stars. Weresolve companion stars and complex diffuse emission from dust that canaccount for on average 50% of the IR emission detected in previouslow-resolution observations. We calculate new spectral indices for theseobjects and show that they differ from those expected from simplegeometrically flat disk models. We also show mid-IR spectral energydistributions that reveal the presence of silicate emission andabsorption features for several objects. Finally, temperature mapsconstructed from our observations suggest the presence of transientlyheated grains, and optical depth maps point to the locations of the mostembedded objects.
| 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.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
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| Spatially Resolved Circumstellar Structure of Herbig AE/BE Stars in the Near-Infrared
We have conducted the first systematic study of Herbig Ae/Be stars usingthe technique of long baseline stellar interferometry in thenear-infrared, with the objective of characterizing the distribution andproperties of the circumstellar dust responsible for the excessnear-infrared fluxes from these systems. The observations for this workhave been conducted at the Infrared Optical Telescope Array (IOTA). Theprincipal result of this paper is that the interferometer resolves thesource of infrared excess in 11 of the 15 systems surveyed. A newbinary, MWC 361-A, has been detected interferometrically for the firsttime. The visibility data for all the sources has been interpretedwithin the context of four simple models which represent a range ofplausible representations for the brightness distribution of the sourceof excess emission: a Gaussian, a narrow uniform ring, a flat blackbodydisk with a single temperature power law, and an infrared companion. Wefind that the characteristic sizes of the near-infrared emitting regionsare larger than previously thought (0.5-5.9 AU, as given by the FWHM ofthe Gaussian intensity). A further major result of this paper is thatthe sizes measured, when combined with the observed spectral energydistributions, essentially rule out accretion disk models represented byblackbody disks with the canonical T(r)~r-3/4 law. We alsofind that, within the range observed in this study, none of the sources(except the new binary) shows varying visibilities as the orientation ofthe interferometer baseline changes. This is the expected behavior forsources which appear circularly symmetric on the sky, and for thesources with the largest baseline position angle coverage (AB Aur, MWC1080-A) asymmetric brightness distributions (such as inclined disks orbinaries) become highly unlikely. Taken as an ensemble, with no clearevidence in favor of axisymmetric structure, the observations favor theinterpretation that the circumstellar dust is distributed in sphericalenvelopes (the Gaussian model) or thin shells (the ring model). Thisinterpretation is also supported by the result that the measured sizes,combined with the excess near-infrared fluxes, imply emission of finiteoptical depth, as required by the fact that the central stars areoptically visible. The measured sizes and brightnesses do not correlatestrongly with the luminosity of the central star. Moreover, in twocases, the same excess is observed from circumstellar structures thatdiffer in size by more than a factor of 2 and surround essentiallyidentical stars. Therefore, different physical mechanisms for thenear-infrared emission may be at work in different cases, oralternatively, a single underlying mechanism with the property that thesame infrared excess is produced on very different physical scales.
| GasDust Shells around Some Early-Type Stars with an IR Excess (of Emission)
The results of an investigation of IR (IRAS) observations of 58OBAF stars of different luminosity classes, whichare mainly members of various associations, are presented. The colorindices of these stars are determined and two-color diagrams areconstructed. The emission excesses at 12 and 25 mm (E 12 and E 25) arealso compared with the absorption A1640 of UV radiation. It is concludedthat 24 stars (of the 58 investigated) are disk systems of the Vegatype, to which Vega = N 53 also belongs. Eight known stars of the Vegatype are also given in the figures for comparison. The remaining 34stars may have gasdust shells and/or shelldisks. The IRemission excesses of the 34 investigated stars and 11 comparison stars(eight of them are Be-Ae stars) are evidently due both to thermalemission from grains and to the emission from freefreetransitions of electrons in the gasdust shells of these stars.
| K-Band Spectroscopy of Luminous Young Stellar Objects
We present spectroscopy from 2.0 to 2.33 μm of 32 luminous youngstellar objects (YSOs), which are presumed to be precursors of HerbigAe/Be stars. From these stars, Brγ, H2, CO, He I, andFe II were found in emission with detection rates of 97%, 34%, 22%, 9%,and 3%, respectively. We compare the spectral features with those ofHerbig Ae/Be stars in the literature to investigate the spectralbehavior of intermediate- to high-mass YSOs and to search for theirrelations to the spectral energy distributions (SEDs). H2emission is detected only in Class I SEDs with particularly largespectral indices. The detection of H2 emission is related tothe degree of the dispersal of circumstellar envelopes, whereH2 molecules are probably excited by shocks from outflows. Onthe other hand, Brγ emission, which is generally thought to occurin stellar winds close to the stars, does not depend on the SEDs. Thisindicates that stellar wind from luminous YSOs does not change much fromthe embedded phase to the optically visible phase. CO emission is alsoindependent of the SEDs, but the detection rate is much lower than thatof Brγ emission. Probably, more specific physical conditionsregarding circumstellar disks and stellar radiation are necessary for COemission to take place.
| 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
| Investigation of Herbig Ae/Be Stars in the Near-Infrared with a Long-Baseline Interferometer
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| Dust around Herbig Ae/Be Stars: Porous, Cometary-Like Grains?
Herbig Ae/Be (HAeBe) stars are pre-main sequence objects of moderatemass representing the early evolutionary stages of β Pic-typestars. The objects of both kinds are surrounded by dust disks/shells,and the dust is believed to be continually replenished by comet-likebodies orbiting close to the stars. As the dust particles in the βPictoris disk are likely to be as fluffy as cometary dust is, it isreasonable to suppose that the grains around HAeBe stars could be porousas well. This brings up the question, which we try to answer here, ofhow the grain porosity affects the observational manifestations of thedust shells of HAeBe stars: the excess infrared emission, anomalousextinction in the ultraviolet, and the peculiar behavior of colorindices and linear polarization during deep brightness minima observedfor about 25% of HAeBe stars (UX Ori, WW Vul, etc). We have performednumerical Monte Carlo simulations of polarized radiation transfer inspheroidal inhomogeneous shells for three different models ofcircumstellar dust grains. In addition to the observational datamentioned above, intensity scans and polarization maps of the objectshave been constructed in view of possible future observations. It isfound that the porosity of circumstellar grains weakly affects thespectral energy distribution (when the grain model does not includeparticles with quite different optical properties) and does not showpronounced effects in the intensity and polarization maps for all grainmodels considered. On the other hand, the porosity normally reducessingle-scattering albedo of particles, and therefore, fluffy grainsexplain the spectral energy distributions of UX Ori-like stars and theirtracks in the color-magnitude diagrams considered in combination muchbetter than compact grains do. However, highly porous, cometary-likegrains are unlikely to be abundant in the shells as they produce too lowlinear polarization to account for the observations of the HAeBe 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.
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