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Star formation in Cometary Globule 1: the second generation
Context. Cometary Globule 1 (CG 1) is the archetype Cometary Globule inthe Gum nebula. Aims: We attempt to discover stars possiblyembedded in the globule head of the Gum nebula and to map distributionof its ISM. Methods: We analyse C18O spectral lineobservations, NIR spectrosopy, narrow and broad band NIR imaging, andstellar photometry to determine the structure of the CG 1 head and theextinction of stars in its direction. Results: A young stellarobject (YSO) associated with a bright NIR nebulosity and a molecularhydrogen object (MHO 1411, a probable obscured HH-object) werediscovered in the globule. Molecular hydrogen and Br? lineemission is seen in the direction of the YSO. The observed maximumoptical extinction in the globule head is 9.2m. The peakN(H2) column density and the total mass derived from theextinction are 9.0 × 1021 cm-2 and 16.7M_? (d/300 pc)2. The C18O emission in theglobule head is detected in a 1.5 arcmin by 4´ area with a sharpmaximum SW of the YSO. Three regions can be discerned in C18Oline velocity and excitation temperature. Because of variations in theC18O excitation temperature the integrated C18Oline emission does not follow the optical extinction. It is argued thatthe changes in the C18O excitation temperatures are caused byradiative heating by NX Pup and interaction of the YSO with the parentcloud. No indication of a strong molecular outflow from the YSO isevident in the molecular line data. The IRAS point source 07178-4429located in the CG 1 head resolves into two sources in the HIRES enhancedIRAS images. The 12 and 25 ?m emission originates mainly in the starNX Puppis and the 60 and 100 ?m emission in the YSO.Based partly on observations collected at the European SouthernObservatory, La Silla, Chile and partly obtained from the ESO/ST-ECFScience Archive Facility.Appendices A, B and C are only available inelectronic form at http://www.aanda.org
| Dust Evolution in Protoplanetary Disks Around Herbig Ae/Be Stars—the Spitzer View
In this paper, we present mid-infrared spectra of a comprehensive set ofHerbig Ae/Be stars observed with the Spitzer Space Telescope. Thesignal-to-noise ratio of these spectra is very high, ranging betweenabout a hundred and several hundreds. During the analysis of these datawe tested the validity of standardized protoplanetary dust models andstudied grain growth and crystal formation. On the basis of the analyzedspectra, the major constituents of protoplanetary dust around HerbigAe/Be stars are amorphous silicates with olivine and pyroxenestoichiometry, crystalline forsterite, and enstatite and silica. Noother solid-state features, indicating other abundant dust species, arepresent in the Spitzer spectra. Deviations of the synthetic spectra fromthe observations are most likely related to grain shape effects anduncertainties in the iron content of the dust grains. Our analysisrevealed that larger grains are more abundant in the disk atmosphere offlatter disks than in that of flared disks, indicating that grain growthand sedimentation decrease the disk flaring. We did not find, however,correlations between the value of crystallinity and any of theinvestigated system parameters. Our analysis shows that enstatite ismore concentrated toward the warm inner disk than forsterite, incontrast to predictions of equilibrium condensation models. None of thethree crystal formation mechanisms proposed so far can alone explain allour findings. It is very likely that all three play at least some rolein the formation of crystalline silicates.
| Spitzer's View on Aromatic and Aliphatic Hydrocarbon Emission in Herbig Ae Stars
The chemistry of astronomical hydrocarbons, responsible for thewell-known infrared emission features detected in a wide variety oftargets, remains enigmatic. Here we focus on the group of youngintermediate-mass Herbig Ae stars. We have analyzed the aliphatic andpolycyclic aromatic hydrocarbon (PAH) emission features in the infraredspectra of a sample of 53 Herbig Ae stars, obtained with the InfraredSpectrograph aboard the Spitzer Space Telescope. We confirm that thePAH-to-stellar luminosity ratio is higher in targets with a flared dustdisk. However, a few sources with a flattened dust disk still showrelatively strong PAH emission. Since PAH molecules trace the gas disk,this indicates that gas disks may still be flared, while the dust diskhas settled due to grain growth. There are indications that the strengthof the 11.3 ?m feature also depends on dust disk structure, withflattened disks being less bright in this feature. We confirm that theCC bond features at 6.2 and 7.8 ?m shift to redder wavelengths withdecreasing stellar effective temperature. Moreover, we show that thisredshift is accompanied by a relative increase of aliphatic CH emissionand a decrease of the aromatic 8.6 ?m CH feature strength. Cool starsin our sample are surrounded by hydrocarbons with a highaliphatic/aromatic CH ratio and a low aromatic CH/CC ratio, and viceversa for the hot stars. We conclude that, while the overall hydrocarbonemission strength depends on the dust disk's geometry, the relativedifferences seen in the IR emission features in disks around Herbig Aestars are mainly due to chemical differences of the hydrocarbonmolecules induced by the stellar UV field. Strong UV flux reduces thealiphatic component and emphasizes the spectral signature of thearomatic molecules in the IR spectra.
| Star Formation in the Cometary Globule Ori I-2
We investigate the young stellar population in and near the cometaryglobule Ori I-2. The analysis is based on deep Nordic Optical TelescopeR-band and H? images, JCMT SCUBA 450 and 850 ?m images combinedwith near-infrared Two Micron All Sky Survey (2MASS) photometry andmid-infrared archival Spitzer images obtained with the Infrared ArrayCamera (IRAC; 3.6, 4.5, 5.8, and 8 ?m), and MIPS (24 and 70 ?m)instruments. We identify a total of 125 sources within the 5'×5'region imaged by the IRAC. Of these sources, 87 are detected in theR-band image and 51 are detected in the 2MASS. The detailed physicalproperties of the sources are explored using a combination ofnear/mid-infrared color-color diagrams, graybody fitting of spectralenergy distributions (SEDs) and an online SED fitting tool that uses alibrary of two-dimensional radiation transfer based accretion models ofyoung stellar objects with disks. Ori I-2 shows clear evidence oftriggered star formation with four young low-luminositypre-main-sequence (PMS) stars embedded in the globule. At least two,possibly as many as four, additional low-mass PMS objects werediscovered in the field which are probably part of the young ?Orionis cluster. Among the PMS stars which have formed in the globule,MIR-54 is a young, deeply embedded Class 0/I object; MIR-51 and 52 areyoung Class II sources, while MIR-89 is a more evolved, heavilyextincted Class II object with its apparent colors mimicking a Class 0/Iobject. The Class 0/I object MIR-54 coincides with a previously knownIRAS source and is a strong submillimeter source. It is most likely thesource for the molecular outflow and the large parsec-scale Herbig-Haro(HH) flow. However, the nearby Class II source, MIR-52, which is stronga H? emission line star, also appears to drive an outflowapproximately aligned with the outflow from MIR-54, and because of theproximity of the two outflows, either star could contribute. MIR-89appears to excite a low-excitation HH object, HH 992, discovered for thefirst time in this study.
| Molecular hydrogen in the circumstellar environments of Herbig Ae/Be stars probed by FUSE
Context: Molecular hydrogen (H2) gas is the most abundant molecule inthe circumstellar (CS) environments of young stars. It is thus a keyelement in our understanding of the evolution of pre-main sequence starsand their environments towards the main sequence. Aims: At the presenttime, little is known about the gas as compared to the dust in theenvironments of young stars. We thus observed molecular hydrogen arounda sample of pre-main sequence stars in order to better characterizetheir gaseous CS environments. Methods: The FUSE (Far UltravioletSpectroscopic Explorer) spectral range offers access to electronictransitions of H2. We analyzed the FUSE spectra of a sample of HerbigAe/Be stars (HAeBes) covering a broad spectral range (from F4 to B2),including the main-sequence A5 star β Pictoris. To better diagnosethe origin of the detected molecular gas and its excitation conditions,we used a model of a photodissociation region. Results: Our analysisdemonstrates that the excitation of H2 is clearly different around mostof the HAeBes compared to the interstellar medium. Moreover, thecharacteristics of H2 around Herbig Ae and Be stars give evidence fordifferent excitation mechanisms. For the most massive stars of oursample (B8 to B2 type), the excitation diagrams are reproduced well by amodel of photodissociation regions (PDR). Our results favor aninterpretation in terms of large CS envelopes, remnants of the molecularclouds in which the stars were formed. On the other hand, the group ofAe stars (later than B9 type) known to possess disks is moreinhomogeneous. In most cases, when CS H2 is detected, the lines of sightdo not pass through the disks. The excitation conditions of H2 around Aestars cannot be reproduced by PDR models and correspond to warm and/orhot excited media very close to the stars. In addition, no clearcorrelation has been found between the ages of the stars and the amountof circumstellar H2. Our results suggest structural differences betweenHerbig Ae and Be star environments. Herbig Be stars do evolve fasterthan Ae stars, and consequently, most Herbig Be stars are younger thanAe ones at the time we observe them. It is thus more likely to findremnants of their parent cloud around them.
| 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
| Evolution of Emission-Line Activity in Intermediate-Mass Young Stars
We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars.Together with the multiepoch spectroscopic and photometric data compiledfor a large sample of these stars and ages estimated for individualstars by using pre-main-sequence evolutionary tracks, we have studiedthe evolution of emission-line activity in them. We find that, onaverage, the Hα emission line strength decreases with increasingstellar age in Herbig Ae/Be stars, indicating that the accretionactivity gradually declines during the pre-main-sequence phase. Thiswould hint at a relatively long-lived (a few Myr) process beingresponsible for the cessation of accretion in Herbig Ae/Be stars. Wealso find that the accretion activity in these stars drops substantiallyby ~3 Myr. This is comparable to the timescale in which mostintermediate-mass stars are thought to lose their inner disks,suggesting that inner disks in intermediate-mass stars are dissipatedrapidly after the accretion activity has fallen below a certain level.We further find a relatively tight correlation between strength of theemission line and near-infrared excess due to inner disks in HerbigAe/Be stars, indicating that the disks around Herbig Ae/Be stars cannotbe entirely passive. We suggest that this correlation can be understoodwithin the framework of the puffed-up inner rim disk models if theradiation from the accretion shock is also responsible for the diskheating.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| 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.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| 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.
| Unusual Color Variability of Eruptive Stars
We substantiate the conclusion that the unusual color variability foundpreviously in some eruptive stars is typical of a broad class ofnonstationary objects, manifests itself over a wide temperature range(from B0 to K 3), and can be regarded as a new type of stellarvariability.
| Close binary companions of the HAeBe stars LkHα 198, Elias 1, HK Ori and V380 Ori
We present diffraction-limited bispectrum speckle interferometryobservations of four well-known Herbig Ae/Be (HAeBe) stars,LkHα 198, Elias 1,HK Ori and V380 Ori. For two ofthese, LkHα 198 and Elias 1, we present the first unambiguousdetection of close companions. The plane of the orbit of the newLkHα 198 companion appears to be significantly inclined to theplane of the circumprimary disk, as inferred from the orientation of theoutflow. We show that the Elias 1 companion may be a convective star,and suggest that it could therefore be the true origin of the X-rayemission from this object. In the cases of HK Ori and V380 Ori, wepresent new measurements of the relative positions of already-knowncompanions, indicating orbital motion. For HK Ori, photometricmeasurements of the brightness of the individual components in fourbands allowed us to decompose the system spectral energy distribution(SED) into the two separate component SEDs. The primary exhibits astrong infrared excess which suggests the presence of circumstellarmaterial, whereas the companion can be modelled as a naked photosphere.The infrared excess of HK Ori A was found to contribute around twothirds of the total emission from this component, suggesting thataccretion power contributes significantly to the flux. Submillimetreconstraints mean that the circumstellar disk cannot be particularlymassive, whilst the near-infrared data indicates a high accretion rate.Either the disk lifetime is very short, or the disk must be seen in anoutburst phase.Based on observations performed with the 6 m telescope of the SpecialAstrophysical Observatory, Russia, the 2.2 m ESO/MPG telescope at LaSilla, and with the NASA/ESA Hubble Space Telescope, obtained from thedata archive at the Space Telescope Institute. STScI is operated by theassociation of Universities for Research in Astronomy, Inc. under theNASA contract NAS 5-26555.
| CO observations of Southern molecular clouds. Outflows from young stellar objects GRV 8 and GRV 16
12CO (1-0) observations of two Southern dark clouds (globules)associated with cometary nebulae GRV 8 (a biconical nebula) and GRV 16(a conelike nebula) are presented. GRV 8 shows an outflow from thecentral part of the nebula (where in 2MASS images a star is located,which is perhaps responsible for this outflow); however, both lobes ofthe outflow are redshifted with a velocity of +1.95 km/s with respect tothe molecular cloud. The two opposite redshifted lobes are a rather rarephenomenon that could be explained by the presence of a double starinstead of a single one as the engine responsible for the outflow. Thetwo lobes are almost parallel to the axis of symmetry of the biconicalnebula. In the case of the conelike nebula GRV 16 we observe a bipolaroutflow, where the eastern blueshifted lobe has a velocity of -4 km/swith respect to the molecular cloud, and the western redshifted one hasa velocity +2.5 km/s. The outflow has a direction almost coinciding withthe axis of symmetry of the conelike nebula. The star associated withthe conelike nebula is responsible for this outflow.
| On Rotation of an Isolated Globule
During CO observations of new Southern objects with the 15-m SEST mmtelescope (Cerro La Silla, Chile) we have found that the globuleconnected with the object CLN127-128 rotates with an angular velocity4.3 · 10-14 s-1, which corresponds to the velocity of extremelyfast rotating globules. The object CLN127-128 is a chain of three stars;two of them are connected with bright nebulae, and the third is asuspected Herbig Ae/Be star. All three stars are bright in near IR,which is specific for the existence of circumstellar shells (or disks)around them. The specific angular momentum of the globule confirms thatit is in virial equilibrium. Besides the finding of a rotating globule,CO observations suggest the presence of a blue-shifted outflow fromCLN127-128 with a velocity of -1.1 km/s (in the system connected withthe globule).
| The Herbig Ae/Be Star V586 Ori
Studies of the Herbig Ae/Be star V586 Ori based on photoelectricobservations in the Strömgren system are reported. It is found thatas the brightness decreases, the Balmer discontinuity index initiallyincreases and then begins to drop. As the star fades, the indexdecreases. We have previously found similar variations in the indicesand C for UX Ori, RR Tau, and V351 Ori. It is shown that V586 Ori hasthe characteristics of a shell-star.
| The ɛ Chamaeleontis Young Stellar Group and the Characterization of Sparse Stellar Clusters
We present the outcomes of a Chandra X-Ray Observatory snapshot studyof five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linkedwith the Oph-Sco-Cen association (OSCA). Optical photometric andspectroscopic follow-up was conducted for the HD 104237 field. Theprincipal result is the discovery of a compact group ofpre-main-sequence (PMS) stars associated with HD 104237 and itscodistant, comoving B9 neighbor ɛ Chamaeleontis AB. We name thegroup after the most massive member. The group has five confirmedstellar systems ranging from spectral type B9 to M5, including aremarkably high degree of multiplicity for HD 104237 itself. The HD104237 system is at least a quintet, with four low-mass PMS companionsin nonhierarchical orbits within a projected separation of 1500 AU ofthe HAeBe primary. Two of the low-mass members of the group are activelyaccreting classical T Tauri stars. The Chandra observations alsoincrease the census of companions for two of the other four HAeBe stars,HD 141569 and HD 150193, and identify several additional new members ofthe OSCA. We discuss this work in light of several theoretical issues:the origin of X-rays from HAeBe stars; the uneventful dynamical historyof the high-multiplicity HD 104237 system; and the origin of the ɛCha group and other OSCA outlying groups in the context of turbulentgiant molecular clouds. Together with the similar η Cha cluster, wepaint a portrait of sparse stellar clusters dominated byintermediate-mass stars 5-10 Myr after their formation.
| 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
| 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.
| 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
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude 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.
| Porous Dust Grains in the Shells of Herbig Ae/Be Stars
The transfer of polarized radiation in inhomogeneous circumstellarshells with a spheroidal spatial distribution of porous dust particlesis computed. The grains are modeled by an MRN mixture of silicate andgraphite particles. The optical properties of porous particles(considered separately in the Appendix) are computed by using effectivemedium theory and Mie theory. The following observationalcharacteristics have been computed for WW Vul, a typical Herbig Ae starwith Algol-like minima: the spectral energy distribution from theultraviolet to the far infrared, the color-magnitude diagrams, thewavelength dependence of linear polarization, and the shell brightnessdistribution. The effect of grain porosity on the results is considered.It has been found that only moderate particle porosity (the volumefraction of matter is f ~ 0.5) can explain available observational datain terms of the approach used. Since radiation pressure must rapidlysweep submicron-sized grains out of the vicinity of Herbig Ae/Be stars,we briefly discuss how particle porosity can affect this process.
| 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
| Pulsation in two Herbig Ae stars: HD 35929 and V351 Ori
New photometric observations of seven intermediate mass pre-mainsequence delta Scuti candidates are presented. The periods and pulsationmodes are derived for two of these stars, namely HD 35929 and V351 Ori.The comparison between observations and nonlinear pulsational modelsallows us to provide some initial constraints on their mass andevolutionary state. As an illustration we discuss the use of periods toidentify the mode of pulsation in these two stars and to have anindependent estimate of their distances. Based on observations carriedout at the European Southern Observatory, La Silla, Chile underproposals number 62-I-0533, 63-I-0053
| 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.
| Do Herbig-Ae stars pulsate ?
Not Available
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| A search for spectroscopic binaries among Herbig Ae/Be stars
We present the results of a spectroscopic survey of binaries among 42bright (m_V<11) Herbig Ae/Be stars in both hemispheres. Radialvelocity variations were found in 7 targets, 4 are new spectroscopicbinaries. The Li I 6 708 Angstroms absorption line (absent feature insimple HAeBe stars spectra) indicates the presence of a cooler companionin 6 HAeBe spectrum binaries, 4 of which are new detections. Few starsclassified as possible Herbig Ae/Be stars are not confirmed as such.While for short-period (P<100 days) spectroscopic binaries, theobserved binary frequency is 10%, the true spectroscopic binaryfrequency for Herbig Ae/Be stars may be as high as 35%. Based onobservations collected at the European Southern Observatory (ESO), LaSilla, Chile and at the Observatoire de Haute--Provence (OHP),Saint--Michel l'Observatoire, France. Table 1 only available inelectronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
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