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Warm Gas in the Inner Disks around Young Intermediate-Mass Stars
The characterization of gas in the inner disks around young stars is ofparticular interest because of its connection to planet formation. Inorder to study the gas in inner disks, we have obtained high-resolutionK- and M-band spectroscopy of 14 intermediate-mass young stars. Insources that have optically thick inner disks, i.e., E(K-L)>1, ourdetection rate of the rovibrational CO transitions is 100%, and the gasis thermally excited. Of the five sources that do not have opticallythick inner disks, we only detect the rovibrational CO transitions fromHD 141569. In this case, we show that the gas is excited by UVfluorescence and that the inner disk is devoid of gas and dust. Wediscuss the plausibility of the various scenarios for forming this innerhole. Our modeling of the UV-fluoresced gas suggests an additionalmethod by which to search for and/or place stringent limits on gas indust-depleted regions in disks around Herbig Ae/Be stars.
| 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.
| A Sensitive Survey for Water Maser Emission Toward Bok Globules Using the Robledo 70 m Antenna
We report on the most sensitive water maser survey toward Bok globulesto date, performed using NASA's 70 m antenna at Robledo de Chavela(Spain). We observed 207 positions within the Clemens and Barvainiscatalog with a higher probability of harboring a young star, using asselection criteria the presence of radio continuum emission (fromsubmillimeter to centimeter wavelengths), geometric centers of molecularoutflows, peaks in maps of high-density gas tracers (NH3 orCS), and IRAS point sources. We have obtained seven maser detections,six of which (in CB 34, CB 54, CB 65, CB 101, CB 199, and CB 232) arereported for the first time here. Most of the water masers we detectedare likely to be associated with young stellar objects (YSOs), exceptfor CB 101 (probably an evolved object) and CB 65 (uncertain nature).The water maser in CB 199 shows a relatively high shift (~=30 kms-1) of its velocity centroid with respect to the cloudvelocity, which is unusual for low-mass YSOs. We speculate thathigh-velocity masers in this kind of object could be related to episodesof energetic mass loss in close binaries. Alternatively, the maser in CB199 could be pumped by a protoplanetary or a young planetary nebula. CB232 is the smallest Bok globule (~=0.6 pc) known to be associated withwater maser emission, although it would be superseded by the cases of CB65 (~=0.3 pc) and CB 199 (~=0.5 pc) if their association with YSOs isconfirmed. All our selection criteria have statistically compatibledetection rates, except for IRAS sources, which tend to be somewhatworse predictors for the presence of maser emission.
| 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.
| Near-Infrared and the Inner Regions of Protoplanetary Disks
We examine the ``puffed-up inner disk'' model proposed by Dullemond,Dominik, & Natta for explaining the near-IR excess radiation fromHerbig Ae/Be stars. Detailed model computations show that the observednear-IR excess requires more hot dust than is contained in the puffed-updisk rim. The rim can produce the observed near-IR excess only if itsdust has perfectly gray opacity, but such dust is in conflict with theobserved 10 μm spectral feature. We find that a compact (~10 AU),tenuous (τV<~0.4), dusty halo around the disk innerregions contains enough dust to readily explain the observations.Furthermore, this model also resolves the puzzling relationship noted byMonnier & Millan-Gabet between luminosity and the interferometricinner radii of disks.
| 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.
| Spectroscopic behaviour of the unusual Ae star HD 190073
The results of high-resolution spectroscopy of the peculiar Ae star HD190073 obtained within the framework of a cooperative observingprogramme in 1994-2002 are presented. The temporal behaviour of theHα, Hβ, Hγ, Hδ, He i λ 5876 Å, Na iD and other circumstellar line profiles are investigated in detail.Special attention has been paid to the analysis of the deepmulticomponent blueshifted Ca ii H and K absorption lines. It has beenfound that the fine structure of their profiles is variable ontimescales from months to decades. The analysis of the circumstellarspectrum of HD 190073, rich in shell-like and emission lines with narrowabsorption cores, allows us to conclude that all absorption lines andcores are likely to be of photospheric origin. The emission lines arevariable in time and demonstrate signs of a stellar wind as well as adense equatorial circumstellar disk. As a preliminary hypothesis, wepropose that a global magnetic field of a specific topology can beresponsible for the formation of stable latitudinal stratification ofthe outflowing gas resulting in appearance of the complex structure ofthe Ca ii H and K line profiles. We emphasise that a measurement of thestellar magnetic field and an investigation of its detailedconfiguration would be an important step in understanding the nature ofHD 190073.
| 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.
| X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264
Images of the NGC 2264 star-forming region, which we have acquired withthe XMM-Newton spacecraft, reveal strong X-ray emission from threedeeply embedded (AV>=10 mag) young stellar objects in thevicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz& Grasdalen's infrared source (RNO-EW = IRS 2). Thermal plasmamodels for the brightest source in X-rays, located 11" southwest ofAllen's star, yield a quasi-steady luminosity ofLX=1033 ergs s-1 and an extraordinarilyhigh X-ray temperature of 100 MK. The high temperature is consistentwith the presence of emission lines of Fe XXV and Fe XXVI at photonenergies of 6.7 and 6.9 keV, respectively. An even higher temperature ofnearly ~140 MK was observed during the rise phase of a powerfulimpulsive X-ray flare of another young star in the IRS 2 region.Moderate-resolution near-infrared (1-4 μm) spectra of the embeddedobjects, obtained at the NASA Infrared Telescope Facility, exhibit deepwater ice absorption bands, as well as a variety of emission andabsorption features of H I, CO, and both neutral and ionized metals.
| Chemical analysis of 24 dusty (pre-)main-sequence stars
We have analysed the chemical photospheric composition of 24 HerbigAe/Be and Vega-type stars in search for the λ Bootis phenomenon.We present the results of the elemental abundances of the sample stars.Some of the stars were never before studied spectroscopically at opticalwavelengths. We have determined the projected rotational velocities ofour sample stars. Furthermore, we discuss stars that depict a(selective) depletion pattern in detail. HD 4881 andHD 139614 seem to display an overall deficiency.AB Aur and possibly HD 126367 havesubsolar values for the iron abundance, but are almost solar in silicon.HD 100546 is the only clear λ Bootis star inour sample.Appendix is only available in electronic form athttp://www.edpsciences.org
| PAHs in circumstellar disks around Herbig Ae/Be stars
We investigate the presence and properties of PAHs on the surface ofcircumstellar disks around Herbig Ae/Be stars by comparing thepredictions of disk models with observations. We present results of aradiation transfer code for disks heated by the central star, inhydrostatic equilibrium in the vertical direction (flared disks). Thedust is a mixture of large grains in thermal equilibrium, transientlyheated small grains and PAHs. Special attention is given to theinfluence of the stellar, disk and PAH properties on the strength of thePAH emission lines and their spatial distribution. The models predict aninfrared SED showing PAH features at 3.3, 6.2, 7.7, and 11.3 μmclearly visible above the continuum, and with some of them very strong.The PAH emission, spatially extended, comes mostly from the outer diskregion (R100 AU) while the continuum emission at similarwavelengths, mostly due to warm large grains, is confined to theinnermost disk regions (R few AU). We compare the model results toinfrared observations from ISO and ground-based telescopes of somethirty Herbig Ae/Be stars. Most of the observed PAH features in objectswith spectral type later than B9 are well described by our disk modelsand we discuss in some detail the PAH characteristics one can derivefrom the existing data. Objects with strong radiation field (generallyearlier than about B9) have the 3.3 μm feature (often the only oneobserved) much weaker than predicted, and we discuss possibleexplanations (dissipation of the disk, photoevaporation or modificationof the PAH properties).Appendix is only available in electronic form athttp://www.edpsciences.org
| A study of high velocity molecular outflows with an up-to-date sample
A statistical study of the properties of molecular outflows is performedbased on an up-to-date sample. 391 outflows were identified in publishedarticles or preprints before February 28, 2003. The parameters ofposition, morphology, mass, energy, outflow dynamics and central sourceluminosity are presented for each outflow source. Outflow lobe polarityis known for all the sources, and 84% are found to be bipolar. Thesources are divided into low mass and high mass groups according toeither the available bolometric luminosity of the central source or theoutflow mass. The pace of discovery of outflows over the past sevenyears has increased much more rapidly than in previous periods. Surveysfor outflows are still continuing. The number of high-mass outflowsdetected (139) has considerably increased, showing that they arecommonly associated with massive as well as low mass stars. Energeticmass ejection may be a common aspect of the formation of high mass aswell as low mass stars. Outflow masses are correlated strongly withbolometric luminosity of the center sources, which was obtained for thefirst time. There are also correlations between the central sourceluminosity and the parameters of mechanical luminosity and the thrust orforce necessary to drive the outflow. The results show that flow mass,momentum and energy depend on the nature of the central source. Despitetheir similarity, there are differences between the high mass and lowmass outflows. Low mass outflows are more collimated than high massoutflows. On average, the mass of high mass sources can be more than twoorders of magnitude larger than those of low mass outflows. The relationbetween flow mass and dynamical time appears to differ for the two typesof outflows. Low mass sources make up 90% of outflows associated with HHobjects while high mass outflows make up 61% of the sources associatedwith H_2O masers. Sources with characteristics of collapse or infallcomprise 12% of the entire outflow sample. The spatial distribution ofthe outflow sources in the Galaxy is presented and the local occurrencerate is compared with the stellar birth rate.Tables 1a and 1b are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/503
| 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 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.
| Diffuse interstellar bands in the spectra of Herbig Ae/Be stars and related objects
We report a detection of weak diffuse interstellar bands (DIBs) in theclose vicinity of 6 Herbig Ae/Be pre-main sequence stars. A commonfeature of these objects is the presence of a dusty shell where DIBcarriers are apparently formed/destroyed. The possible influence ofultraviolet flux on carriers of diffuse interstellar bands is discussedas well as DIB to DIB intensity ratios in the spectra of the programstars and in the general interstellar medium.Based on data collected at the 2 m telescope operated on IC AMERObservatory (Terskol, Russia).
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| 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
| The Near-Infrared Study of Large Bok Globules
We present results from a near-infrared study of CB 3, CB 34, CB 39 andCB 54 as a part of an ongoing project, which is set to determine how Bokglobules form, evolve and disperse. Our observations have revealed thepresence of active star formation in CB 3, CB 34 and CB 54, by thediscovery of new knots in the H2 1-0 S(1) emission lineimages. We have discovered no indication of star formation in the Bokglobule CB 39 which may suggest that this globule is in an early stageof evolution. Meanwhile the globules CB 3, CB 34 and CB 54 are obviouslyin their later stages of evolution.
| FUV spectroscopic study of the circumstellar environment of the Herbig Be star HD 250550.
We present FUSE observations of the Herbig Ae/Be star HD 250550, a wellknown analog to the prototype of the whole class, AB Aurigae. Previousoptical and UV (IUE) observations showed that HD 250550 possesses astrong stellar wind and a dense chromosphere. Quite unexpectedly, theFUSE spectra of HD 250550 show only a faint emission feature at 977Å, and emission from O VI resonance lines is barely visible,contrary to what was observed for AB Aurigae by FUSE. Several absorptionfeatures from molecular hydrogen are observed and show that H2 isthermalized up to J=3 and its radial velocity is identical to that ofthe surrounding molecular cloud's velocity. Similar velocities aremeasured on absorption features arising from excited levels of atomicspecies like N I, Cl I and Cl II, P II, Fe II and Fe III. This stronglyfavours a circumstellar origin for these gazeous components. Besides,the spectra also contains several other absorption features ofinterstellar origin (Ar I, Fe II, ...) as demonsrated by the lowerradial velocities and excitation temperatures. Our results reveal acomplex circumstellar environment with unxepected characteritics, andsuggest that quite a large part of the original molecular cloud thatcollapsed to form the star is still present, though its spatialdistribution is still unknown. Based on observations made with theNASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operatedfor NASA by the Johns Hopkins University under NASA contract NAS5-32985.
| 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.
| 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
| 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
| Photometric activity and rotational velocities of UX Ori stars and related objects.
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
| Survey of bipolar outflows and methanol masers in the C(32) S (2-1) and C(34) S (2-1) lines in the Northern sky
A survey of 158 sources (bipolar outflows and methanol masers) wascarried out in the CS(2-1) and C(34) S(2-1) lines to measure and comparedensities in a large number of bipolar outflows and in Class I and ClassII methanol masers, both associated with and unrelated to bipolaroutflows. The statistical characteristics of the regions, forming ClassI methanol masers differ from those both in the centres of bipolaroutflows and in the regions forming Class II methanol masers. It ispossible that physical conditions in bipolar outflows are closer tophysical conditions of Class II methanol masers. Figures i and ii areonly available in electronic form at http://www.edpsciences.com
| A Photometric Catalog of Herbig AE/BE Stars and Discussion of the Nature and Cause of the Variations of UX Orionis Stars
UBVR photometric monitoring of Herbig Ae/Be stars and some relatedobjects has been carried out at Maidanak Observatory in Uzbekistan since1983. More than 71,000 observations of about 230 stars have beenobtained and are made available for anonymous ftp. Virtually all HerbigAe/Be stars observed are irregular variables (called ``UXors'' after UXOri), but there is a wide range of amplitudes from barely detectable tomore than 4 mag in V. Our data confirm the results of previous studies,which indicate that large-amplitude variability is confined to starswith spectral types later than B8. The distribution of variabilityranges is quite similar to what is seen in classical T Tauri stars. Acareful search has failed to reveal any evidence for periodic variationsup to 30 days, which can be interpreted as rotation periods. This is aclear distinction between the light variations of low-mass and high-masspre-main-sequence stars. The Herbig Ae/Be stars evidently do not possesseither the large, stable cool spots or persistent hot spots associatedwith strong surface magnetic fields and magnetically funneled accretionin classical T Tauri stars. A wide variety of shapes, timescales, andamplitudes exists, but the most common behavior is well illustrated bythe light curve of LkHα 234. There are two principal components:(1) irregular variations on timescales of days around a mean brightnesslevel that changes on a much longer timescale (typically years),sometimes in a quasi-cyclic fashion, and (2) occasional episodes of deepminima, occurring at irregular intervals but more frequently near thelow points of the brightness cycles. Our data suggest that many T Tauristars of K0 and earlier spectral type share the same variabilitycharacteristics as Herbig Ae/Be stars and should be regarded as UXors.Two FU Orionis stars (``FUors''), FU Ori and V1515 Cyg, also have recentlight curves that are similar, in some respects, to UXors. The mostdeveloped model to account for the variations of some large-amplitudeUXors involves variable obscuration by circumstellar dust clumpsorbiting the star in a disk viewed nearly edge-on. However, there areproblems in extending this model to the entire class, which lead us topropose an alternative mechanism, i.e., unsteady accretion. Evidencefavoring the accretion model over the obscuration model is presented. Itis suggested that the thermal instability mechanism responsible foroutbursts in interacting binary system disks, and possibly FUors, may bethe cause of the deep minima in UXors.
| 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
| The onset of cluster formation around Herbig Ae/Be stars
The large body of near infrared observations presented in Testi et al. (te{Tea97}; te{Tea98}) are analysed with the aim of characterizing theyoung stellar clusters surrounding Herbig Ae/Be stars. The resultsconfirm the tendency of early Be stars to be surrounded by denseclusters of lower mass ``companions'', while Ae stars are never found tobe associated with conspicuous groups. The transition between thedifferent environments appears to occur smoothly from Ae to Be starswithout a sharp threshold. No correlation of the richness of the stellargroups detected is found with the galactic position or the age of thecentral Herbig Ae/Be star. The stellar volume densities estimated forthe groups surrounding pre-main-sequence stars of intermediate mass showthe transition from the low density aggregates of T Tauri stars and thedense clusters around massive stars. Only the most massive stars (10-20M_sun) are found to be associated with dense ( ~ 10(3 pc(-3)) ) stellarclusters. This is exactly the mass regime at which the conventionalaccretion scenario for isolated star formation faces theoreticalproblems. Thus our findings strongly supports the idea that theformation of high-mass stars is influenced by dynamical interaction in ayoung cluster environment.
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Osservazione e dati astrometrici
| Costellazione: | Orione |
| Ascensione retta: | 06h01m59.99s |
| Declinazione: | +16°30'56.7" |
| Magnitudine apparente: | 9.592 |
| Moto proprio RA: | 0.1 |
| Moto proprio Dec: | -7.9 |
| B-T magnitude: | 9.609 |
| V-T magnitude: | 9.594 |
Cataloghi e designazioni:
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