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Recent results on Pre-main sequence delta Scuti stars. .
Intermediate mass Pre-main sequence stars (1.5 Mȯ< M< 5Mȯ) cross the instability strip on their way tothe main sequence. They are therefore expected to be pulsating in asimilar way as the delta Scuti stars. In this contribution we presentthe status of the observational studies of pulsations in these starswith special emphasis on recent results from our group. The prospectsfor future investigations of these objects from the ground and fromspace are discussed.
| Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| Detection of 75+ pulsation frequencies in the δ Scuti star FG Virginis
Extensive photometric multisite campaigns of the δ Scuti variableFG Vir are presented. For the years 2003 and 2004, 926 h of photometryat the millimag precision level were obtained. The combinations withearlier campaigns lead to excellent frequency resolution and highsignal/noise. A multifrequency analysis yields 79 frequencies. Thisrepresents a new record for this type of star. The modes discoveredearlier were confirmed. Pulsation occurs over a wide frequency band from5.7 to 44.3 c/d with amplitudes of 0.2 mmag or larger. Within this wideband the frequencies are not distributed at random, but tend to clusterin groups. A similar feature is seen in the power spectrum of theresiduals after 79 frequencies are prewhitened. This indicates that manyadditional modes are excited. The interpretation is supported by ahistogram of the photometric amplitudes, which shows an increase ofmodes with small amplitudes. The old question of the "missing modes" maybe answered now: the large number of detected frequencies as well as thelarge number of additional frequencies suggested by the power spectrumof the residuals confirms the theoretical prediction of a large numberof excited modes. FG Vir shows a number of frequency combinations of thedominant mode at 12.7162 c/d (m = 0) with other modes of relatively highphotometric amplitudes. The amplitudes of the frequency sums are higherthan those of the differences. A second mode (20.2878 c/d) also showscombinations. This mode of azimuthal order m = -1 is coupled with twoother modes of m = +1.
| 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.
| The Herbig Ae/Be Star V586 Ori
Studies of the Herbig Ae/Be star V586 Ori based on photoelectricobservations in the Strömgren system are reported. It is found thatas the brightness decreases, the Balmer discontinuity index initiallyincreases and then begins to drop. As the star fades, the indexdecreases. We have previously found similar variations in the indicesand C for UX Ori, RR Tau, and V351 Ori. It is shown that V586 Ori hasthe characteristics of a shell-star.
| Fundamental parameters and evolutionary state of the Herbig Ae star candidate HD 35929
HD 35929 is a pre-main-sequence Herbig Ae star candidate that exhibitsemission in the Hα line and a weak IR excess. Although itsluminosity type was estimated as IV from low-resolution spectroscopy,the insignificant HIPPARCOS parallax (0.88±0.93 mas) contradictedthe low luminosity. To resolve this discrepancy, we took 3high-resolution optical spectra of the star (region 5200-6915 Å, R60 000) and used an archival one (4000-6800 Å, R 48000). Our analysis of the spectral and photometric information showsthat HD 35929 is an F2 III star with Teff=6880±100 K,log g = 3.3±0.1, log L/Lȯ=1.7±0.2, v sini =70±5 km s-1, M=2.3±0.2 Mȯ, amild metal deficit [Fe/H]=-0.2±0.1, and a weak interstellarreddening (AV 0.1 mag) at a distance D = 345±60pc. These results confirm an earlier suggestion by Marconi et al.(\cite{marc00}) that HD 35929 is located within the instability strip.At the same time, we argue that it is not a young object, but rather apost-main-sequence giant. The fundamental parameters and IR excess of HD35929 are similar to those of HD 19993, an emission-line late A-typegiant recently discovered by us (Miroshnichenko et al. \cite{mir03}).These objects might represent a group of evolved intermediate-mass starswith an enhanced mass loss.
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| V351 Ori as a Target for the WET
We present time series observations of the Herbig Ae star V351 Ori. Thisstar presents light variations on short time scale (a few hours),typical of the delta Scuti pulsation. The new data are brieflydescribed and the possibility to observe V351 Ori with the WET isdiscussed.
| Investigation of 131 Herbig Ae/Be Candidate Stars
We present a new catalog of 108 Herbig Ae/Be candidate stars identifiedin the Pico dos Dias Survey, together with 19 previously knowncandidates and four objects selected from the IRAS Faint Source Catalog.These 131 stars were observed with low- and/or medium-resolutionspectroscopy, and we complement these data with high-resolution spectraof 39 stars. The objects present a great variety of Hα lineprofiles and were separated according to them. Our study suggests thatmost of the time a Herbig Ae/Be star will present a double peak Hαline profile. Correlations among different physical parameters, such asspectral type and vsini with Hα line profiles were searched. Wefound no correlation among Hα line profiles and spectral type orvsini except for stars with P Cygni profiles, where there is acorrelation with vsini. We also use preliminary spectral energydistributions to seek for any influence of the circumstellar medium inthe Hα line profiles. The presence of [O I] and [S II] forbiddenlines is used together with the Hα line profiles and thesepreliminary spectral energy distributions to discuss the circumstellarenvironment of the Herbig Ae/Be candidates. The distribution of thedetected [O I] and [S II] forbidden lines among different spectral typespoints to a significantly higher occurrence of these lines among Bstars, whereas the distribution among different Hα profile typesindicates that forbidden lines are evenly distributed among eachHα line-profile type. Combining the distance estimates of theHerbig candidates with the knowledge of the interstellar mediumdistribution, we have found that 84 candidates can be associated withsome of the more conspicuous SFRs, being in the right direction and at acompatible distance. As a further means of checking the properties ofthe HAeBe candidates, as well as their present evolutionary status, thederived luminosities and effective temperatures of the stars withpossible association to the star-forming regions and/or Hipparcosdistances were plotted together with a set of pre-main-sequenceevolutionary tracks on an HR diagram. A set of 14 stars were found outof their expected positions in the HR diagram.Based on observations made at the Obsevatório do Pico dosDias/LNA (Brazil), ESO (Chile), and the Lick Observatory.
| Multisite observations of the PMS delta Scuti star V351 Ori
We present the results of multisite observations spanning two years onthe pre-main-sequence (PMS) star V351 Ori. A total of around 180 hoursof observations over 29 nights have been collected, allowing us tomeasure five different periodicities, most likely related to the deltaScuti variability of V351 Ori. Comparison with the predictions of linearnonadiabatic radial pulsation models put stringent constraints on thestellar parameters and indicate that the distance to V351 Ori isintermediate between the lower limit measured by Hipparcos (210 pc) andthat of the Orion Nebula (450 pc). However, radial pulsation models areunable to reproduce all of the observed frequencies with a single choiceof (M*, L*, and Teff), suggesting thepresence of additional nonradial modes.Based on observations collected at the: Loiano Observatory (Italy), SanPedro Martir Observatory (México), OGS at Teide Observatory(Spain), JKT at Roque de los muchachos Observatory (Spain), BeijingAstronomical Observatory (China), SARA Observatory at Kitt Peak, whichis owned and operated by the Southeastern Association for Research inAstronomy.Table \ref{jou} in only available in electronic form athttp://www.edpsciences.org
| Oscillations in the PMS delta Scuti star V346 Ori
We report the discovery of periodicities in the light curve of thePre-Main Sequence (PMS) Herbig Ae star V346 Ori. We interpret variationsin the light curve of the star as the superposition of at least twoperiodic signals that result from delta Scuti-type stellaroscillations. The computation of linear non-adiabatic radial pulsationmodels for PMS stars reproduces these periods for a 1.55 Msunstar with Teff = 7410 K and log L/Lsun = 0.74pulsating in the fundamental and in the second overtone. If ourassumption of radial oscillations is indeed correct, the star's locationon the HR diagram places it at a distance significantly smaller thanthat of the Orion star forming region, hence raising questions regardingthe evolutionary status of V346 Ori. Observations with better timesampling and spanning a longer period of time are needed if the latterquestion is to be addressed fully.Tables 1 and 2 are 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/399/271
| Short-period line profile and light variations in the Herbig Ae star V351 Orionis
We present 318 high-dispersion echelle spectra and 450UBVRCIC observations of the pre-main-sequence starV351 Orionis. These data were obtained simultaneously over a two-weekperiod. The star is unusual in that it no longer shows the frequentsharp fadings that occurred prior to 1986. We find featurescharacteristic of Herbig Ae stars: Hα emission, sharp and broadNaII absorption components and weak HeI absorption. The Hα profileis strongly variable with a time-scale of 1d and normally is an inverseP Cygni profile, but on occasions overt emission disappears altogether.The equivalent widths of the sharp NaII absorption lines have decreasedquite markedly since 1995, indicating a decrease in the column densityof gas in the line of sight. We also detect very weak forbidden [OI]emission. There is no evidence for veiling of the star, but a possible1.0-d or 0.5-d period in the equivalent widths of the lines couldindicate the presence of an accretion hotspot. The star is of particularinterest because it is pulsating. We detect at least two periods in thelight variation and a further period from the radial velocityvariations. Attempts at photometric mode identification were notsuccessful, but from the morphology of the line profiles we can identifyone mode as non-radial. In addition, we see on some nights severalresidual absorption subfeatures moving across the line profile from blueto red. These are almost certainly caused by one or more modes of highdegree (l~8).
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
We present time series observations of intermediate mass PMS starsbelonging to the young star cluster IC 348. The new data reveal that ayoung member of the cluster, H254, undergoes periodic light variationswith delta Scuti-like characteristics. This occurrence provides anunambiguous evidence confirming the prediction that intermediate-masspre-main sequence (PMS) stars should experience this transientinstability during their approach to the main-sequence. On the basis ofthe measured frequency f=7.406 d-1, we are able to constrainthe intrinsic stellar parameters of H254 by means of linear, nonadiabatic, radial pulsation models. The range of the resultingluminosity and effective temperature permitted by the models is narrowerthan the observational values. In particular, the pulsation analysisallows to derive an independent estimate of the distance to IC 348 ofabout 320 pc. Further observations could either confirm the monoperiodicnature of H254 or reveal the presence of other frequencies. Based onobservations collected at the Loiano Observatory, Italy and the SanPedro Martir Observatory, México.
| 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.
| Pulsation in the pre-main-sequence Herbig Ae star HD 142666
Asteroseismology of pre-main-sequence δ Scuti stars has thepotential not only to provide unprecedented constraints on models ofthese stars, but also to allow for the possibility of detectingevolutionary period changes, thus providing a direct measure of thepre-main-sequence evolutionary time-scale. In the last two years, thepublished number of such stars known has doubled from four to eight.Searches are now being conducted amongst the Herbig Ae stars, which areconsidered to be excellent candidates. We announce the discovery ofδ Scuti pulsation in one Herbig Ae star, HD 142666, which lieswithin Marconi & Palla's theoretically predicted instability stripfor pre-main-sequence stars, making this the ninth knownpre-main-sequence δ Scuti star. We also demonstrate a lack ofδ Scuti pulsation in another such star, HD 142527.
| Aufsuchkarten fur intrinsic Variable Stars in Brno.
Not Available
| 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.
| On the delta Scuti variability in the pre-main sequence Herbig Ae star HR 5999
A reanalysis of photometric data for the bright, southern Herbig Ae starHR 5999 obtained by Praderie et al. (\cite{r17}) at ESO in 1985 showsdelta Scuti pulsation with the same period, 4.812 d-1, andsemi-amplitude, 6 mmag, found by Kurtz & Marang (\cite{r10}) in dataobtained from SAAO in 1994. HR 5999 is, therefore, a potential candidateto test Breger & Pamyatnykh's (\cite{r4}) predictions ofevolutionary period changes 10 to 100 times faster in pre-main sequencedelta Scuti stars than in post-main sequence stars. We argue that thelow frequency, low amplitude alpha 2 CVn variability of HR6000 with a period near 2 d, and its long-term variability are nohindrance to using it as a comparison star for study of the delta Scutivariability of HR 5999. With a separation of 44 arcsec between HR 5999and HR 6000, and similar brightnesses for the two stars, we urge the useof small telescopes with CCD photometers to obtain the long-term datanecessary for the study of period change. We point out that this can bedone under less-than-ideal photometric conditions and urban lightingwith CCD photometers on small telescopes which are widely available atsmall observatories, at universities and at the home observatories ofserious amateur astronomers.
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| Do Herbig-Ae stars pulsate ?
Not Available
| Spectral Features of RZ Psc, a Cool Star with Algol-like Brightness Minima
We have determined the physical (T eff, logg, ζ) and kinematic (V esini,V r ) parameters and abundances for 14 chemical elements in theatmosphere of the “antiflare” variable RZ Psc, usingmedium-resolution spectra obtained with the Coudé spectrographsof the 6-m telescope of the Special Astrophysical Observatory and theCrimean Astrophysical Observatory 2.6-m Shain telescope. The chemicalcomposition of the star is characterized by a slight metal deficiency;however, the iron and calcium abundances are consistent with the solarvalues within the errors. We also detected a peculiar dip (depression)of the continuum level near the Hα line. Assuming that thisdepression and the photospheric Hα line have independent origins,we calculated the hydrogen abundance X in the atmosphere of RZ Psc. Theresulting value X=0.70 (of the solar value) implies a relativedeficiency of hydrogen. Together with the spatial location of the star,these properties provide evidence that RZ Psc is an evolved star, mostlikely belonging to population II.
| 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
| 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 dense gas-dust cloud near the A-e star DD Serpentis
We present results of the medium resolution spectral observations of avariable A-e star DD Ser. In 1984-1986 the star was found to have anormal double emission Hα -line with equivalent width of emissioncomponent EW Hα_ {e}=13.3+/-0.5 Ä. In 1991 a drastic Hα-line profile, as well as photometric variations, were detected.Analysis of the reconstructed profile of the emission line absorptioncomponent implies that a dense gas-dust cloud has most likely emerged onthe line of sight at the moment of the spectral observations.Quantitative characteristics of the absorption profile variation aregiven.
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