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A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| [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.
| CO emission from discs around isolated HAeBe and Vega-excess stars
We describe results from a survey for J = 3-2 12CO emissionfrom visible stars classified as having an infrared excess. The line isclearly detected in 21 objects, and significant molecular gas(>=10-3 Jupiter masses) is found to be common in targetswith infrared excesses >=0.01 (>=56 per cent of objects), but rarefor those with smaller excesses (~10 per cent of objects).A simple geometrical argument based on the infrared excess implies thatdisc opening angles are typically >=12° for objects with detectedCO; within this angle, the disc is optically thick to stellar radiationand shields the CO from photodissociation. Two or three CO discs have anunusually low infrared excess (<=0.01), implying the shielding discis physically very thin (<=1°).Around 50 per cent of the detected line profiles are double-peaked,while many of the rest have significantly broadened lines, attributed todiscs in Keplerian rotation. Simple model fits to the line profilesindicate outer radii in the range 30-300 au, larger than found throughfitting continuum SEDs, but similar to the sizes of debris discs aroundmain-sequence stars. As many as five have outer radii smaller than theSolar System (50 au), with a further four showing evidence of gas in thedisc at radii smaller than 20 au. The outer disc radius is independentof the stellar spectral type (from K through to B9), but there isevidence of a correlation between radius and total dust mass. Also themean disc size appears to decrease with time: discs around stars of age3-7 Myr have a mean radius ~210 au, whereas discs of age 7-20 Myr are afactor of three smaller. This shows that a significant mass of gas (atleast 2 M⊕) exists beyond the region of planetformation for up to ~7 Myr, and may remain for a further ~10Myr withinthis region.The only bona fide debris disc with detected CO is HD9672; this shows adouble-peaked CO profile and is the most compact gas disc observed, witha modelled outer radius of 17 au. In the case of HD141569, detailedmodelling of the line profile indicates gas may lie in two rings, withradii of 90 and 250 au, similar to the dust structure seen in scatteredlight and the mid-infrared. In both AB Aur and HD163296 we also findthat the sizes of the molecular disc and the dust scattering disc aresimilar; this suggests that the molecular gas and small dust grains areclosely co-located.
| 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.
| 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.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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.
| 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).
| 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
| 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.
| 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.
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| Analysis of correlations between polarimetric and photometric characteristics of young stars. A new approach to the problem after eleven years' study
We present the results of the investigation of correlations between thepolarimetric and photometric characteristics of a sample (496 objects)of young Herbig Ae/Be (HAEBE) stars and T Tauri (TT) stars. It is shownthat, for 85% of the sample stars there is a general relation betweenthe degree of optical polarization and the infrared colour index(V-L)_obs and the colour excess E(V-L) due to the contribution of acircumstellar dust shell. Polarimetric data were also compared with thevalue of vsin i to search for a possible correlation between thepolarization and an inclination of circumstellar disks. Polarimetricdata as well as IR excesses are considered and compared for differentsubgroups of young stars namely: HAEBE and TT stars with Algol-likeminima of brightness (26 objects), Vega-type stars and post HAEBE stars(114 objects) and young solar-type stars (58 objects). For statisticalpurposes the data for young stars were compared with those collected fordifferent groups of evolved objects such as: classical Be stars (~300objects), Mira Ceti stars (39 objects), early-type supergiants from theSerkowski et al. (\cite{serk}) catalogue (120 objects) and main sequence(MS) stars within 50 pc from the Sun from the Leroy (\cite{leroy})catalogue (68 objects). The value of polarization is discussed incontext with the stages of evolution of circumstellar shells which wereestablished by comparison of spectral energy distribution in the far IR(using the IRAS data). It is shown that most young stars havestatistically larger value of polarization in comparison with the starswhich are on a stage of evolution close to MS. We are able to contendthat the changes in polarimetric behaviour of young stars are connectedwith evolution of their circumstellar shells. Appendices 1 to 5 are onlyavailable in electronic form at http://www.edpsciences.org
| 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
| Do Herbig-Ae stars pulsate ?
Not Available
| Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections
We present the results of high-resolution spectroscopic, low-resolutionspectrophotometric and spectropolarimetric and broad-band multicolourobservations of four B-type stars (HD 4881, 5839, 224648 and 179218) andtwo A-type stars (HD 32509 and 184761) with strong far-infrared (IR)excesses. The excess in HD 184761, which is located at a distance of 65pc from the Sun, was recognized for the first time. Double-peakedHα emission line profiles are found in HD 4881 and HD 5839, whileHD 184761, HD 224648 and HD 32509 display no emission in Hα. Theremarkable variations observed in the Hα profile of HD 179218 arealso observed in some classical Be and Herbig Ae/Be stars. An intrinsiccomponent of polarization is clearly present in HD 179218, only aninterstellar component is detected in HD 4881 and HD 224648, and HD184761 was found to be unpolarized. Improved effective temperatures forall six objects were derived. Parallaxes measured by the Hipparcossatellite were used to determine positions of the stars in the HRdiagram. HD 4881 and HD 5839 are an order of magnitude more luminousthan main-sequence stars of similar temperatures and are most likelynewly discovered classical Be stars. Study of the high-resolution IRASmaps and modelling of the spectral energy distributions of HD 4881, HD5839 and HD 224648 suggest that the observed large IR excesses arecaused by radiation from circumstellar dust rather than free--freeradiation or infrared cirrus, so they may be higher mass counterparts ofbeta Pictoris stars. HD 32509, HD 224648 and HD 184761, which have verysmall near-IR excesses, are probably young main-sequence stars. HD179218, which exhibits the largest near- and far-IR excess in thesample, is an isolated pre-main-sequence Herbig Be star.
| The Instability Strip for Pre-main-sequence Stars
We investigate the pulsational properties of pre-main-sequence (PMS)stars by means of linear and nonlinear calculations. The equilibriummodels were taken from models evolved from the protostellar birthline tothe zero-age main sequence for masses in the range 1-4 M_solar. Thenonlinear analysis allows us to define the instability strip of PMSstars in the H-R diagram. These models are used to constrain theinternal structure of young stars and to test evolutionary models. Wecompare our results with observations of the best case of a pulsatingyoung star, HR 5999, and we also identify possible candidates forpulsational variability among known Herbig Ae/Be stars that are locatedwithin or close to the instability strip boundaries.
| 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.
| Polarimetry of southern peculiar early-type stars
We present and discuss optical polarimetry of a large group (60 objects)of peculiar early-type stars (Herbig Ae/Be stars and candidate members,B[e] stars, extreme emission line objects etc.). Most were taken from``A new catalogue of members and candidate members of Herbig Ae/Bestellar group'' \cite[The et al. (1994)]{the2}. For 40 out of the 60objects polarization was measured for the first time. On the basis ofthe data we draw conclusions about the circumstellar shell configurationand orientation for several objects. Polarimetric variability ondifferent time-scales (minutes \ldots hours \ldots days) wasinvestigated, as well as comparison with earlier measurements toinvestigate variability over longer time-scales. No circularpolarization was detected to a 5sigma level in most of the investigatedstars. Polarimetric data obtained here as well as data available fromthe literature were analyzed from the point of investigating theevolutionary status of the objects. The relationship between thepolarimetric and photometric data was also investigated. Tables 6 to 22are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsarc.u-strasbg.fr/Abstract.html
| An ultraviolet, optical and infrared study of Herbig Ae/Be stars
We have selected a list of 45 Herbig Ae/Be-type candidates on the baseof their IRAS colors and their spectral types. We propose the presenceof a broad infrared excess as a defining criterion for these stars,rather than the detection of circumstellar nebulosity. In this way, ourselection also includes more evolved young stars, that are no longerembedded in their star-forming region. A few objects in our sample arewell-known Herbig Ae/Be stars, others are new. New optical andnear-infrared photometric observations, as well as ultraviolet ones, arepresented. The position of the objects in several color-color diagrams,as well as their de-reddened energy distributions, permit a reliableclassification. Three objects probably are binaries with a coolsecondary, 9 appear to be related to the Vega-type stars and 33 objectscan be classified as genuine Herbig Ae/Be stars. The majority of theHerbig Ae/Be stars have a dusty environment consisting of a distinct hotand cool component. These isolated Herbig Ae/Be stars suggest anevolution from embedded Herbig Ae/Be stars to beta Pictoris-likemain-sequence stars, an evolution in which planet formation may play animportant role. Based on observations obtained at the European SouthernObservatory, La Silla, Chile; and at the Swiss Telescope, La Silla,Chile; and at the Swiss Telescope, Hochalpine ForschungsstationJungfraujoch, Switzerland, and with the International UltravioletExplorer (IUE)
| HIPPARCOS photometry of Herbig Ae/Be stars
The photometric behaviour of a sample of 44 Herbig Ae/Be (HAeBe)candidate stars was studied using a uniform set of optical photometryobtained by the Hipparcos mission. Astrophysical parameters (distance,temperature, luminosity, mass, age) of this sample of stars were derivedas well by combining the astrometric data provided by Hipparcos withdata from literature. Our main conclusions can be summarized as follows:(1) More than 65% of all HAeBe stars show photometric variations with anamplitude larger than 0\fm05; (2) HAeBes with a spectral type earlierthan A0 only show moderate (amplitude < 0\fm5) variations, whereasthose of later spectral type can (but not necessarily have to) showvariations of more than 2\fm5. We explain this behaviour as being due tothe fact that stars with lower masses become optically visible, andhence recognizable as Herbig Ae stars, while still contracting towardsthe zero-age main sequence (ZAMS), whereas their more massivecounterparts only become optically visible after having reached theZAMS; (3) The Herbig stars with the smallest infrared excesses do notshow large photometric variations. This can be understood by identifyingthe stars with lower infrared excesses with the more evolved objects inour sample; (4) No correlation between the level of photometricvariability and the stellar v sin i could be found. If the largephotometric variations are due to variable amounts of extinction by dustclouds in the equatorial plane of the system, the evolutionary effectsprobably disturb the expected correlation between the two. Based on datafrom the Hipparcos astrometry satellite.
| 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.
| A study of the candidates for Herbig Ae/Be stars HD 35929 and HD 203024.
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| The β Pictoris phenomenon among Herbig Ae/Be stars. UV and optical high dispersion spectra.
We present a survey of high dispersion UV and optical spectra of HerbigAe/Be (HAeBe) and related stars. We find accreting, circumstellar gasover the velocity range +100 to +400km/s, and absorption profilessimilar to those seen toward β Pic, in 36% of the 33 HAeBe starswith IUE data as well as in 3 non-emission B stars. We also findevidence of accretion in 7 HAeBe stars with optical data only. Lineprofile variability appears ubiquitous. As a group, the stars withaccreting gas signatures have higher vsini than the stars withoutflowing material, and tend to exhibit large amplitude (>=1^m^)optical light variations. All of the program stars with polarimetricvariations that are anti-correlated with the optical light, previouslyinterpreted as the signature of a dust disk viewed close to equator-on,also show spectral signatures of accreting gas. These data imply thataccretion activity in HAeBe stars is preferentially observed when theline of sight transits the circumstellar dust disk. Our data imply thatthe spectroscopic signatures of accreting circumstellar material seen inβ Pic are not unique to that object, but instead are consistentwith interpretation of β Pic as a comparatively young A star withits associated circumstellar disk.
| Infrared excesses in A-type stars
Not Available
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
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