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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.
| Einigen uberarbeitete und neue Vergleissternkarten.
Not Available
| 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.
| 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.
| Pre-Main-Sequence A-type stars
Young A-type stars in the pre-main-sequence (PMS) evolutionary phase areparticularly interesting objects since they cover the mass range(1.5-4 Mȯ) which is most sensitive to the internalconditions inherited from the protostellar phase. In particular, theyundergo a process of thermal relaxation from which they emerge as fullyradiative objects contracting towards the Main Sequence. A-type starsalso show intense surface activity (including winds, accretion,pulsations) whose origin is still not completely understood, andinfrared excesses related to the presence of circumstellar disks andenvelopes. Disks display significant evolution in the dust properties,likely signalling the occurrence of protoplanetary growth. Finally,A-type stars are generally found in multiple systems and smallaggregates with lower mass companions.
| Optical Properties of the Circumstellar Dust around Stars with Aperiodic Fadings
Multicolor observations of 21 rapid, irregular variable stars withAlgol-like fadings and of R Coronae Borealis are used to derive theoptical extinction coefficients of the circumstellar dust associatedwith these objects, θ(λ). We used more than 3600 uniformmulti-color brightness measurements obtained in a rigorous U BV R systemat the Terskol High-altitude Observing Station. The mean extinctioncoefficient θ for the circumstellar dust for this sample of starscoincides almost exactly with the interstellar extinction coefficient,θis, but is somewhat higher in the ultraviolet. We suggest anexplanation for this difference.
| 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.
| Comparative morphology of ALIVARS and RCB type stars
Some conclusions were drawn from the analysis of the following specificfeatures of the rapid Algol-Like Irregular VARiable Stars (ALIVARS): 1)general mode of variability for ALIVARS and R CrB, 2) long periods ofnearly constant brightness, 3) specific "color - V-magnitude"dependence,4) Anticorrelation between V brightness and degree ofpolarization, 5) identity of the optical properties of the RCB andALIVARS circumstellar dust, 6) cyclic light variations at normalbrighness, 7) connection between cyclic light variations and the onsetof drastic light fadings. We found that ALIVARS have some morphologicalfeatures more in common with evolved RCB-type stars rather than withyoung Herbig Ae/Be stars. Moreover, the results of the analysis of theALIVARS evolution characteristic - a) space distribution of individualALIVARS and star formation regions, b) loci on the H-R diagram, c)equivalent widths of hydrogen absorption lines, d) relative hydrogendeficiency of two ALIVARS investigated (70% of the solar abundance for V351 Ori and 30% for RZ Psc), e) mean group parameter Vsini of ALIVARS,f) luminosity of ALIVARS �alpha emission envelopes- unambiguously point that ALIVARS are not so young as Herbig Ae/Bestars and they have left the main sequence. Thus, we have encounteredthe fact of stellar mimicry: ALIVARS tend to look out as young stars,having at that time features of rather evolved stars. Relying on suchfindings, we conclude the same ALIVARS should not be further regarded asyoung stars of the HAEBES group.
| 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
| New observations of the pulsating PMS star V351 Ori
We present new precise photoelectric observations of thePre-Main-Sequence delta Scuti star, V351 Ori. These new data show thatV351 Ori pulsates in a mixture of several radial modes (at least four).The comparison between observations and detailed pulsational models,allows us to provide independent constraints on the mass and luminosityof the star. The predicted distance is 210 pc, indicating that V351 Oriis much closer than the Orion star forming region. With an inferred massof ~ 1.8 M_sun and an uncertain evolutionary stage, V351 Ori representsan excellent candidate for future asteroseismological studies that willassess whether it is a young PMS star ( ~ 6 Myr) or an evolved object (~ 1 Gyr) leaving the main-sequence.
| The far-UV spectrum of T Tauri stars - I. The relevance of the IUE Newly Extracted Spectra
The far-UV spectrum of the T Tauri stars (TTSs) provides important cluesabout the structure of the stellar atmospheres, winds and accretionshocks. The IUE (International Ultraviolet Explorer) Final Archivecontains the most complete data base for such studies. A new extractionsystem, the IUE Newly Extracted Spectra (ines), has been developed toovercome the disadvantages of the extraction system used in the IUEFinal Archive, the Signal Weighted Extraction Technique (swet). We havecompared the ines spectra of the whole sample of TTSs in the far-UVrange (1200-2000Å) with the swet low-resolution spectra availablein the IUE Final Archive. Although in most of the cases there is a goodagreement between both samples, an important enhancement of the inesline fluxes with respect to the swet line fluxes is reported forparticular spectra. The line fluxes are enhanced by as much as a factorof ~2.5 in some objects, which is significant for variability studies ofTTSs because the variations of the UV lines are typically of this order.The emission-measure distributions built to study the atmospheres ofthese stars are based on the UV emission line fluxes, so the new systemis susceptible to introduce changes in these models. Moreover, thenon-linear enhancement of the ines line fluxes produces variations indiagnostic line ratios usually taken as temperature and density tracersin late-type stars. These line ratios can vary by as much as a factor of3 when the ines data are compared with the swet, with the subsequentvariation of the physical parameters derived from them.
| Do Herbig-Ae stars pulsate ?
Not Available
| 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.
| 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
| The impact of UV observations in the studies of pre-main sequence stars.
Not Available
| 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.
| The FUOR characteristics of the PMS star BN Orionis inferred from new spectroscopic and photometric observations
BN Ori is a young emission-line star with a peculiar light curve. Duringthe first half of this century the star showed strong irregularbrightness- variations, similar to those of Herbig Ae stars. In thecurrent half of the century the light curve resembles that of a FUOri-object (FUOR, after Ambartsumian 1971). It can be characterised byan initial large-scale rise in brightness followed by a gradual decayover a period of about 15 years. From various photometric patrolprogrammes we concluded that the star remained at the same brightnesslevel for the last 30 years. Between 1991 and 1995 the spectrum of BNOri was intensively observed and was found to exhibit some uniquepecularities. We have analysed the visual spectra obtained with the 6 mBTA telescope of the SAO, the 60 cm and 48 cm telescopes at Mt.Maidanak, the 1.4 m CAT and the 1.5 m telescope of ESO and theUV-spectra obtained with the IUE in 1984 and 1986. The spectra showcertain similarities with those of classical FUORs, such as alarge-scale thermal stratification, with Balmer lines showing A6-A7spectral type wings, while other lines are typical for late F-typestars. However, in contrast to classical FUORs, BN Ori is not of highluminosity-class. On the other hand the spectrum also shows similaritieswith those of Herbig A7e stars, although these stars have much strongerabsorption lines in the UV from their outer shell and theircircumstellar dust excesses are much larger than for BN Ori. Also, therotation rate v sin i of BN Ori is between 180 and 220 km s^{-1}, whichis typical for A-type stars of 2-5 Modot. This may indicatethat BN Ori is a fast rotating FUOR with an intermediate-mass precursoron which the outburst had a different effect than in the case of theclassical FUORs (which are rotating slower and have a low-mass T Tauristar as precursor). In the BN Ori outburst most of the massive gas- anddust shell, characteristic for Herbig Ae stars, seems to have beenremoved, except for the Hα- and Mg ii-emission region close to thephotosphere. The disappearance of the circumstellar dust shell mayexplain the drastic variability-change in the light curve of BN Ori. Themass-accretion rates of BN Ori and several Herbig A7e stars (derivedfrom the analysis of their UV-spectra in terms of disc-accretion)suggest that the FUOR outburst in BN Ori was due to a thermal runaway\cite[(Bell 1994)]{bel94} in its inner accretion-disc which wastriggered by a modest increase in the accretion rate. Based onobservations collected at/with: Mt. Maidanak Observatory, Samarkand,Uzbekistan, Special Astrophysical Observatory, Russian Academy ofSciences, Karachai-Cherkessia, Zelenchuk region, Nizhnii-Arkhyz,European Southern Observatory, La Silla, Chile, Kitt Peak NationalObservatory, Tucson AZ, U.S.A., Dutch Astronomical Station Ausserbinn,Switzerland, International Ultraviolet Explorer.
| The luminosity of the Halpha_-emission envelopes of variable Is(A)-type stars.
We present the results of the analysis of the emission spectra ofvariable Is(A)-type stars. Sixteen stars have been observed in 1988-1992at the 6-m telescope of Special Astrophysical Observaroty, RussianAcademy of Sciences (SAO RAS) with the middle-resolution (R=~3000)echelle spectrograph ``Zebra". Equivalent widths of theHalpha_ and Hbeta_ (if available) emissioncomponents were obtained, the underlying profile of the correspondingabsorption lines were taken into account. The luminosities of theemission envelopes in the Halpha_ and Hbeta_ lineswere then calculated. The main result of the investigation is that theluminosity of the emission envelope appears to depend on the effectivetemperature of the star.
| Determination of lg G of several variable Herbig Ae/Be stars.
Echelle spectrogrammes of 19 variable Herbig Ae/Be stars were obtainedin 1988-1992 in the wavelength interval 4000-7000Å with thespectral complex ``Zebra" at the 6-m telescope of Special AstrophysicalObservatory, Russian Academy of Sciences (SAO RAS). Kurucz's models wereemployed for the analysis of the hydrogen absorption line profiles. Forall programme and standard stars lg(g) gravity parameters were obtained.The location of the stars on the "lg(g)-T_eff_" diagram does not agreewith the hypothesis that the studied objects are pre-main sequencestars.
| The continuum spectrum of BM Ori.
Not Available
| Application of Stromgren Photometry to the Study of Very Young Herbig Ae/Be Stars
In an ESO collaborative effort entitled ``Long Term Photometry ofVariables'', the Stromgren photometric system has been used, amongothers, for the study of pre-main sequence Herbig Ae/Be (HAeBe) stars.The results of this study, revealing important properties of these veryyoung stars, are discussed.
| Orientation of circumstellar disks and the statistics of H alpha profiles of Ae/Be Herbig stars
Not Available
| Hα emission from pre-main sequence stars.
We present the results of an Hα emission line observational studyof a sample of 75 pre-main sequence stars consisting of 17 HAEBE, 47 CTTand 11 WTT stars. Approximately half of the stars are faint, with abrightness in the V band >13. The spectra have been taken atresolutions =~30km/s and =~80km/s. The data shown provide information onequivalent width, radial velocity, variability and line profile.
| A new catalogue of members and candidate members of the Herbig Ae/Be (HAEBE) stellar group
A new up-to-date catalogue of Herbig Ae/Be (HAEBE) stars and relatedobjects is certainly needed, for both well-seasoned researchers and, inparticular, for new investigators starting to study the many interestingastrophysical properties of these very young objects. We present a briefdiscussion of the current observational characteristics that distinguishthis class from their main sequence counterparts. The HAEBE and relatedstars are listed in five tables, containing 287 objects. Table 1contains all Ae and Be stars which historically are recognized as trueHAEBE stars or potential candidate members. Table 2 gives the stars ofspectral type Fe, and emission line stars with very uncertain or unknownspectral type. In Table 3 are given all known Extreme Emission LineObjects (EELOs), of which most have not been identified to belong to anyspecific group. Table 4a and b list other Bep or B[e] stars with strongIR-excess and unknown spectral type. Table 5 contains the non-emissionline possible young objects. Furthermore, Table 6 contains 35 starsrejected from former published lists of HAEBE stars. In these tables weare including coordinates, spectral types, visual magnitudes, ranges inphotometric variability and references of several key publicationsrelated to each object. Relevant remarks, such as the presence of anebula in the vicinity of an object, are also given.
| The excess infrared emission of Herbig Ae/Be stars - Disks or envelopes?
It is suggested that the near-IR emission in many Herbig Ae/Be starsarises in surrounding dusty envelopes, rather than circumstellar disks.It is shown that disks around Ae/Be stars are likely to remain opticallythick at the required accretion rates. It is proposed that the IRexcesses of many Ae/Be stars originate in surrounding dust nebulaeinstead of circumstellar disks. It is suggested that the near-IRemission of the envelope is enhanced by the same processes that produceanomalous strong continuum emission at temperatures of about 1000 K inreflection nebulae surrounding hot stars. This near-IR emission could bedue to small grains transiently heated by UV photons. The dust envelopescould be associated with the primary star or a nearby companion star.Some Ae/Be stars show evidence for the 3.3-6.3-micron emission featuresseen in reflection nebulae around hot stars, which lends further supportto this suggestion.
| Periodic phenomena in Ae/Be Herbig stars light curves. I - Light curves classification and digital analysis methods. II - Results and probable interpretation for selected stars
Digital analysis methods are used to study the light curves of 78 HerbigAe/Be stars. The mean epoch for a star is 5 yr. A morphologicalclassification of the light curves is proposed. The digital analysismethod is briefly described. To estimate the reliability of the cyclicphenomena discovered in the present study, a large number of them areexamined. Three types of cyclic phenomena are identified and discussed.These phenomena are interpreted in terms of Keplerian rotation ofvarious structures in accretion disks and formations. The hypothesis ofgiant protocomets is suggested to explain P(0) periods. Short-periodcyclicity is associated with the variation of hot compact shell shapesmodulated by rotation.
| Longterm Photometry of Variables at ESO - Part Two - the Second Data Catalogue 1986-1990
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102...79S&db_key=AST
| The prophology of Is(A) irregular variables. II. The location on the H-R diagram.
Not Available
| The morphology of Is(A) irregular variables. I. The multicolour photometry of the stars in Orion and Taurus.
Not Available
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Observation and Astrometry data
| Constellation: | Orion |
| Right ascension: | 05h36m29.35s |
| Declination: | +06°50'02.2" |
| Apparent magnitude: | 9.62 |
| Proper motion RA: | -0.1 |
| Proper motion Dec: | 2.7 |
| B-T magnitude: | 10.124 |
| V-T magnitude: | 9.662 |
Catalogs and designations:
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