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Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in theformation of low- and intermediate-mass stars in OB associations (theλ Ori region, Ori OB1, and Lac OB1 associations). We find thatthe classical T Tauri stars and Herbig Ae/Be stars tend to line upbetween luminous O stars and bright-rimmed or comet-shaped clouds; thecloser to a cloud the progressively younger they are. Our positional andchronological study lends support to the validity of theradiation-driven implosion mechanism, where the Lyman continuum photonsfrom a luminous O star create expanding ionization fronts to evaporateand compress nearby clouds into bright-rimmed or comet-shaped clouds.Implosive pressure then causes dense clumps to collapse, prompting theformation of low-mass stars on the cloud surface (i.e., the bright rim)and intermediate-mass stars somewhat deeper in the cloud. These starsare a signpost of current star formation; no young stars are seenleading the ionization fronts further into the cloud. Young stars inbright-rimmed or comet-shaped clouds are likely to have been formed bytriggering, which would result in an age spread of several megayearsbetween the member stars or star groups formed in the sequence.
| 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.
| A 10 μm spectroscopic survey of Herbig Ae star disks: Grain growth and crystallization
We present spectroscopic observations of a large sample of Herbig Aestars in the 10 μm spectral region. We perform compositional fits ofthe spectra based on properties of homogeneous as well as inhomogeneousspherical particles, and derive the mineralogy and typical grain sizesof the dust responsible for the 10 μm emission. Several trends arereported that can constrain theoretical models of dust processing inthese systems: i) none of the sources consists of fully pristine dustcomparable to that found in the interstellar medium; ii) all sourceswith a high fraction of crystalline silicates are dominated by largegrains; iii) the disks around more massive stars (M 2.5{M}ȯ, L 60 {L}ȯ) have a higherfraction of crystalline silicates than those around lower mass stars,iv) in the subset of lower mass stars (M 2.5 {M}ȯ)there is no correlation between stellar parameters and the derivedcrystallinity of the dust. The correlation between the shape andstrength of the 10 micron silicate feature reported by van Boekel et al.(2003) is reconfirmed with this larger sample. The evidence presented inthis paper is combined with that of other studies to present a likelyscenario of dust processing in Herbig Ae systems. We conclude that thepresent data favour a scenario in which the crystalline silicates areproduced in the innermost regions of the disk, close to the star, andtransported outward to the regions where they can be detected by meansof 10 micron spectroscopy. Additionally, we conclude that the finalcrystallinity of these disks is reached very soon after active accretionhas stopped.
| [O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation
We present high spectral-resolution optical spectra of 49 Herbig Ae/Bestars in a search for the [O i] 6300 Å line. The vast majority ofthe stars in our sample show narrow ({FWHM} < 100 km s-1)emission lines, centered on the stellar radial velocity. In only threesources is the feature much broader ( 400 km s-1), andstrongly blueshifted (-200 km s-1) compared to the stellarradial velocity. Some stars in our sample show double-peaked lineprofiles, with peak-to-peak separations of 10 km s-1. Thepresence and strength of the [O i] line emission appears to becorrelated with the far-infrared energy distribution of each source:stars with a strong excess at 60 μm have in general stronger [O i]emission than stars with weaker 60 μm excesses. We interpret thesenarrow [O i] 6300 Å line profiles as arising in the surface layersof the protoplanetary disks surrounding Herbig Ae/Be stars. A simplemodel for [O i] 6300 Å line emission due to the photodissociationof OH molecules shows that our results are in quantitative agreementwith that expected from the emission of a flared disk if the fractionalOH abundance is 5 × 10-7.
| 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 Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| 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
| 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.
| A Catalog of Young Stellar Groups and Clusters within 1 Kiloparsec of the Sun
We present a catalog of near-infrared surveys of young (<~ a few106 yr) stellar groups and clusters within 1 kpc from theSun, based on an extensive search of the literature from the past tenyears. We find 143 surveys from 69 published articles, covering 73different regions. The number distribution of stars in a region has amedian of 28 and a mean of 100. About 80% of the stars are in clusterswith at least 100 members. By a rough classification of the groups andclusters based on the number of their associated stars, we show thatmost of the stars form in large clusters. The spatial distribution ofcataloged regions in the Galactic plane shows a relative lack ofobserved stellar groups and clusters in the range270°
| 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
| The Spatial Distribution of the λ Orionis Pre-Main-Sequence Population
The λ Ori star-forming region presents a snapshot of amoderate-mass giant molecular cloud 1-2 Myr after cloud disruption by OBstars, with the OB stars, the low-mass stellar population, remnantmolecular clouds, and the dispersed gas all still present. We have usedoptical photometry and multiobject spectroscopy for lithium absorptionto identify 266 pre-main-sequence stars in 8 deg2 of theregion. We also present new Strömgren photometry for the massivestars, from which we derive a distance of 450 pc and a turnoff age of6-7 Myr. Using these parameters and pre-main-sequence evolutionarymodels, we map the star formation history of the low-mass stars. We findthat low-mass star formation started throughout the region at about thesame time as the birth of the massive stars, and thereafter the birthrate accelerated. Within the last 1-2 Myr star formation ceased in thecenter of the star-forming region, near the concentration of OB stars,while it continues in dark clouds 20 pc away. We suggest that asupernova 1-2 Myr ago destroyed the molecular cloud core from which theOB stars formed, but it did not terminate star formation in more distantreaches of the giant molecular cloud. We find no secure evidence fortriggered or sequential star formation in the outer molecular clouds.The global star formation of the λ Ori region has generated thefield initial mass function, but local star formation in subregionsshows large deviations from the expected ratio of high- to low-massstars.
| 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.
| Multi-wavelength imaging of the peculiar Vela Molecular Ridge nebula BBW 192E
We present the first images of the nebula BBW 192E at near- andmid-infrared wavelengths as well as a 1.3 mm continuum map. The nebulaBBW 192E is associated with the IRAS point source 08513-4201 which has aluminosity of about 1400 L_sun and a strongly rising spectral energydistribution towards mid-infrared wavelengths. The infrared images showa pronounced bipolar nebula, which is offset by about 10arcsec from theknown optical emission, as well as several point sources. We interpretthis morphology as evidence for an inclined disk-like structure andscattered light emerging from the lobes. This is supported by ournear-infrared imaging polarimetry at sub-arcsecond resolution whichfurthermore indicates that the central energy source is seen directly atnear-infrared wavelengths. At 1.3 millimetre, we detected a cometaryshaped source in the dust continuum radiation, 151arcsec x 55arcsec insize, with a total mass of 180 M_sun as well as average hydrogen columnand number densities of 4.5x1022 cm-2 and2.6x105 cm-3, respectively. This dust cloud isalso responsible for the strong spatial variation of the extinctionacross the nebula. We discuss the physical properties of the infraredpoint sources and conclude that some might be young, low-mass pre-mainsequence stars. The main energy source of the nebula is an embeddedintermediate-mass young stellar object. These observations are among thefirst infrared detections of a disk-like system associated with abipolar nebula surrounding an intermediate-mass young stellar object.Based on observations collected at the European Southern Observatory, LaSilla, Chile (Proposal-IDs: 57.B-0392, 52.7-0086, 57.D-0260, 58.D-0213,and 63.I-0173)
| Dust Emission from Herbig AE/BE Stars: Evidence for Disks and Envelopes
Infrared and millimeter-wave emission from Herbig Ae/Be stars hasproduced conflicting conclusions regarding the dust geometry in theseobjects. We show that the compact dimensions of themillimeter-wave-emitting regions are a decisive indication for disks.But a disk cannot explain the spectral energy distribution unless it isembedded in an extended envelope that (1) dominates the IR emission and(2) provides additional disk heating on top of the direct stellarradiation. Detailed radiative transfer calculations based on thesimplest model for envelope-embedded disks successfully fit the datafrom UV to millimeter wavelengths and show that the disks have centralholes. This model also resolves naturally some puzzling results of IRimaging.
| 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.
| Observations of recently recognized candidate Herbig Ae/Be stars
The results of multicolor photometric and low-resolution spectroscopicobservations of 9 Herbig Ae/Be candidate stars are reported. This sampleincludes two newly recognized objects, MQ Cas and BD+11degr829 , whichwere found by means of cross-correlation of the IRAS Point SourceCatalogue and the catalogue of the galactic early-type emission-linestars (Wackerling \cite{wack}). Near-IR excesses were detected in twostars (AS 116 and BD+11degr829 ) for the first time. Algol-typevariability, which is not common in Herbig Ae/Be stars, was detected inMQ Cas and V1012 Ori. Spectral types are determined for MQ Cas, GSC1811-0767, HDE 244604, BD+11degr829 , V1012 Ori, AS116, AS117, and HDE290380 from low-resolution spectroscopy. Analysis of our and previouslypublished data suggests that 8 of the 9 objects are pre-main-sequencestars, while the last one, Hen 938, is more likely a B[e] supergiant.Based on observations collected at the Astrophysical National Laboratory(LNA -- Brazil), the South--African Astronomical Observatory, the DarkSky Observatory (USA), and the Tien--Shan Observatory (Kazakhstan)
| 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.
| The Incidence of lambda Boötis Stars via an Extension of the MK Spectral Classification System to Very Young A-Type Stars
In this paper we introduce an extension to the MK system of spectralclassification that allows the precise classification ofpre-main-sequence (PMS) A-type stars, including most Herbig Ae stars.This classification scheme characterizes the star by a standard MK type,the presence and strength of emission and/or shell lines, and thestrength of the Balmer decrement. It can be used to summarize temporalspectral changes in PMS A-type stars, and to search for peculiar types.We have used this scheme to classify 38 Herbig Ae stars as well as 22PMS stars in the young open clusters NGC 2264, NGC 7160, and IC 348. Wehave also used this extended system to search for lambda Boötisstars among PMS A-type stars. We have found one definite lambdaBoötis star among the Ae stars, and one marginal lambda Boötisstar in NGC 2264, yielding statistics not significantly different fromthose of the lambda Boötis stars in the field. This, in addition toother considerations from previous studies, leads us to conclude thatthe lambda Boötis mechanism is operable from very early ages (PMS)to well into the main-sequence life (a few times 10^8 yr) of only 2%-3%of A-type stars.
| A search for clustering around Herbig Ae/Be stars. II. Atlas of the observed sources
We present large field infrared images of a sample of 45 Herbig Ae/Bestars. Stellar parameters, such as age and luminosity, have been derivedfor all of them in a consistent way. The images have been used toidentify stellar groups or clusters associated with the Herbig Ae/Bestar. The results presented in this paper form the database for a studyof clustering around intermediate mass stars (\cite[Testi et al.1998]{Tea98a}). Based on observations collected at the TIRGO(Gornergrat, Switzerland) operated by the CAISMI--CNR, Firenze, Italy,and at the NOT (La Palma, Canary Islands) operated by the Nordic OpticalTelescope Scientific Association (Denmark, Finland, Norway, Sweden).
| Photoelectric Photometry of Herbig Ae/be and Related Stars in the Vilnius Photometric System
A catalog of photoelectric photometry of 62 Herbig Ae/Be and relatedstars in the Vilnius system is presented. It contains stars down to V =12 mag located mainly in the northern hemisphere and observed in theperiod of 1994--1996.
| The Evolutionary Status of UX Orionis-Type Stars
We present measurements of 1.3 mm continuum emission from a sample of UXOri stars (UXOrs). The UXOrs are pre-main-sequence stars, typically ofintermediate mass, and they are distinguished from otherpre-main-sequence stars by their large photometric and polarimetricvariations that are thought to be due to variable extinction bycircumstellar dust. Transient optical and UV redshifted absorption linesare a second defining characteristic, and have been interpreted in termsof the disruption of infalling cometary bodies. Our new millimeterfluxes are used to derive masses of circumstellar dust, MCSD. We combinethese measurements with data from the literature to examine a sample of30 pre-main-sequence systems with spectral types F0-B9, i.e., Herbig Aestars, about half of which display UXOr characteristics. We find nosystematic differences in MCSD between the two groups of stars.Moreover, no statistically significant correlation between MCSD andstellar age is found, and the amplitude of photometric variabilityappears to be independent of age. We propose that UXOr phenomena do notcharacterize a more evolved environment. They are probably common to themajority of stars in our sample, but are observed only when the line ofsight is close to the equatorial plane of an aspheric circumstellarnebula.
| A High-Resolution Study of Gas and Dust around Young Intermediate-Mass Stars: Evidence for Circumstellar Disks in Herbig AE Systems
As part of a long-term program of observations to search for andcharacterize disks of gas and dust around intermediate-mass counterpartsto solar-mass T Tauri stars, we have probed the environments of sevenpre-main-sequence stars of spectral type Ae using millimeter-wavecontinuum and molecular line aperture synthesis imaging. In each case weidentify a compact region of thermal continuum emission centered on thestar. Upper limits to radii are in the range 200-300 AU for five membersof our sample, and 680 AU for the distant source HD 245185. We identifyan elongated continuum source around HD 163296, with a semimajor axis of110 AU. Adopting relatively high values for dust grain opacities, weobtain minimum masses of circumstellar dust and gas in the range0.005-0.034 Mȯ for the seven sources, assuming that the observedcontinuum emission is optically thin. We detect molecular line emissionfrom gas regions centered on four of the stars. Two of these regions arespatially resolved and are found to be elongated, with semimajor axes of310 and 450 AU for HD 163296 and AB Aur, respectively. Ordered velocitygradients along the major axes of both of these structures pointstrongly to the presence of orbiting material in disklikeconfigurations, and we argue that the nebular environments of our entiresample include substantial disk components.
| Observational Overview of Young Intermediate-Mass Objects: Herbig Ae/Be Stars
Not Available
| Infrared excesses in A-type stars
Not Available
| Orientation of circumstellar disks and the statistics of H alpha profiles of Ae/Be Herbig stars
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Observation and Astrometry data
| Constellation: | Ωρίων |
| Right ascension: | 05h35m09.61s |
| Declination: | +10°01'51.5" |
| Apparent magnitude: | 9.958 |
| Proper motion RA: | 3 |
| Proper motion Dec: | -1.3 |
| B-T magnitude: | 10.048 |
| V-T magnitude: | 9.966 |
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