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The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?
We report the discovery of a medium-strength (~0.5 kG) magnetic field onthe young, massive star τ Sco (B0.2V), which becomes thethird-hottest magnetic star known. Circularly polarized Zeemansignatures are clearly detected in observations collected mostly withthe ESPaDOnS spectropolarimeter, recently installed on the 3.6-mCanada-France-Hawaii Telescope; temporal variability is also clearlyestablished in the polarimetry, and can be unambiguously attributed torotational modulation with a period close to 41 d. Archival ultraviolet(UV) spectra confirm that this modulation repeats over time-scales ofdecades, and refine the rotation period to 41.033 +/- 0.002 d.Despite the slow rotation rate of τ Sco, we none the less succeed inreconstructing the large-scale structure of its magnetic topology. Wefind that the magnetic structure is unusually complex for a hot star,with significant power in spherical-harmonic modes of degree up to 5.The surface topology is dominated by a potential field, although amoderate toroidal component is probably present. We fail to detectintrinsic temporal variability of the magnetic structure over the 1.5-yrperiod of our spectropolarimetric observations (in agreement with thestable temporal variations of the UV spectra), and infer that anydifferential surface rotation must be very small.The topology of the extended magnetic field that we derive from thephotospheric magnetic maps is also more complex than a global dipole,and features in particular a significantly warped torus of closedmagnetic loops encircling the star (tilted at about 90° to therotation axis), with additional, smaller, networks of closed-fieldlines. This topology appears to be consistent with the exceptional X-rayproperties of τ Sco and also provides a natural explanation of thevariability observed in wind-formed UV lines. Although we cannotcompletely rule out the possibility that the field is produced throughdynamo processes of an exotic kind, we conclude that its magnetic fieldis most probably a fossil remnant from the star formation stage.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Science de l'Univers of the Centre National de laRecherche Scientifique of France, and the University of Hawaii.E-mail: donati@ast.obs-mip.fr (J-FD); idh@star.ucl.ac.uk (IDH);mmj@st-andrews.ac.uk (MMJ); petit@ast.obs-mip.fr (PP);claude.catala@obspm.fr (CC); jlandstr@uwo.ca (JDL);jean-claude.bouret@oamp.fr (J-CB); evelyne.alecian@obspm.fr (EA);jrb3@st-andrews.ac.uk (JRB); forveill@cfht.hawaii.edu (TF);fpaletou@ast.obs-mip.fr (FP); manset@cfht.hawaii.edu (NM)
| Rotational and Cyclical Variability in γ Cassiopeia
γ Cas is an unusual classical Be star for which the optical-bandand hard X-ray fluxes vary on a variety of timescales. We report resultsof a 9 yr monitoring effort on this star with a robotic ground-based(APT) telescope in the B, V filter system, as well as simultaneousobservations in 2004 November with this instrument and the RXTE. Ourobservations disclosed no correlated optical response to the rapid X-rayflares in this star, nor did the star show any sustained flux changesany time during two monitored nights in either wavelength regime.Consistent with an earlier study by Robinson et al., optical lightcurves obtained in our new APT program revealed that γ Casundergoes ~3% amplitude cycles with lengths of 50-91 days. Ourobservations in 2004 showed a similar optical cycle. Over the 9 days wemonitored the star with the RXTE, the X-ray flux varied in phase withits optical cycle and with an amplitude predicted from the data inRobinson et al. In general, the amplitude of the V magnitude cycles are30%-40% larger than the corresponding B amplitude, suggesting that theproduction site of the cycles is circumstellar. The cycle lengthsconstantly change and can damp or grow on timescales as short as 13days. We have also discovered a coherent period of 1.21581+/-0.00004days in all our data, which appears consistent only with rotation. Thefull amplitude of this variation is 0.0060 in both filters, and,surprisingly, its waveform is almost sawtooth in shape. This variationis likely to originate on the star's surface. This circumstance hints atthe existence of a strong magnetic field with a complex topology and apossible heterogeneous surface distribution of metals.
| Multicomponent radiatively driven stellar winds. IV. On the helium decoupling in the wind of σ Orionis E
We study the possibility of the helium decoupling in the stellar wind ofσ Ori E. To obtain reliable wind parameters for this star we firstcalculate NLTE wind model and derive wind mass-loss rate and terminalvelocity. Using corresponding force multipliers we study the possibilityof helium decoupling. We find that helium decoupling is not possible forrealistic values of helium charge (calculated from NLTE wind models).Helium decoupling only seems possible for a very low helium charge. Thereason for this behavior is the strong coupling between helium andhydrogen. We also find that frictional heating becomes important in theouter parts of the wind of σ Ori E due to the collisions betweensome heavier elements and the passive components - hydrogen and helium.For a metallicity ten times lower than the solar one, both hydrogen andhelium decouple from the metals and may fall back onto the stellarsurface. However, this does not explain the observed chemicalpeculiarity since both these components decouple together from theabsorbing ions. Although we do not include the effects of the magneticfield into our models, we argue that the presence of a magnetic fieldwill likely not significantly modify the derived results because in suchcase model equations describe the motion parallel to the magnetic field.
| Remarkable non-dipolar magnetic field of the Bp star HD 137509
The southern magnetic Bp star HD 137509 exhibits complex rotationalmodulation of the longitudinal field and other magnetic observables.Interpretation of this magnetic variability in the framework of thelow-order multipolar field models suggests a very strong quadrupolarcomponent to dominate the surface field topology of HD 137509. I haveexamined the high-quality VLT/UVES spectra of HD 137509 and discoveredresolved Zeeman split components in some of the spectral lines. Theinferred mean surface field modulus, < B >=29 kG, agrees with themultipolar model predictions. This confirms the presence of an extremelystrong non-dipolar magnetic field in HD 137509 and establishes this staras the object with the second-largest field among magnetic chemicallypeculiar stars.
| Magnetic field models for HD 116458 and HD 126515
We have modeled the magnetic fields of the slowly rotating stars HD116458 and HD 126515 using the “magnetic charge” technique.HD 116458 has a small angle between its rotation axis and dipole axis(β = 12°), whereas this angle is large for HD 126515 (β =86°). Both stars can be described with a decentered-dipole model,with the respective displacements being r = 0.07 and r = 0.24 in unitsof the stellar radius. The decentered-dipole model is able tosatisfactorily explain the phase relations for the effective field, Be(P), and the mean surface field, B s(P), for both stars, along with thefact that the B e(P) phase relation for HD 126515 is anharmonic. Wediscuss the role of systematic measurement errors possibly resultingfrom instrumental or methodical effects in one or both of the phaserelations. The displacement of the dipole probably reflects realasymmetry of the stellar field structure, and is not due to measurementerrors. Using both phase relations, B e(P) and B s(P), in the modelingconsiderably reduces the influence of the nonuniform distribution ofchemical elements on the stellar surface.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| 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).
| A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar system
``Cherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc).
| Some Comments on the Magnetic Braking of CP Stars
The low rotation velocities of magnetic CP stars are discussed.Arguments against the involvement of the magnetic field in the loss ofangular momentum are given: (1) the fields are not strong enough inyoung stars in the stage of evolution prior to the main sequence; (2)there is no significant statistical correlation between the magneticfield strength and the rotation period of CP stars; (3) stars with shortperiods have the highest fields; (4) a substantial number of stars withvery low magnetic fields (B e P>25 days, which form 12% of the total,probably lie at the edge of the velocity distribution for low massstars. All of these properties conflict with the hypothesis of magneticbraking of CP stars.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| On the behavior of the Cii 4267.261, 6578.052 and 6582.882 Å lines in chemically peculiar and standard stars
With the aim of investigating the possible particular behavior of carbonin a sample of chemically peculiar stars of the main sequence withoutturning to modeling, we performed spectroscopic observations of threeimportant and usually prominent single ionized carbon lines: 4267.261,6578.052 and 6582.882 Å. In addition, we observed a large numberof standard stars in order to define a kind of normality strip, usefulfor comparing the observed trend for the peculiar stars. We paidparticular attention to the problem of the determination of fundamentalatmospheric parameters, especially for the chemically peculiar stars forwhich the abundance anomalies change the flux distribution in such a waythat the classical photometric methods to infer effective temperaturesand gravities parameter cannot be applied. Regarding CP stars, we founda normal carbon abundance in Hg-Mn, Si (with some exceptions) and Hestrong stars. He weak stars are normal too, but with a large spread outof the data around the mean value. A more complicated behavior has beennoted in the group of SrCrEu stars: four out of seven show a strongoverabundance, being the others normal.
| On the Periods of the Magnetic CP Stars
An HR diagram annotated to show several ranges of photometericallydetermined periods has been constructed for the magnetic CP stars whoseperiods have been determined by the author and his collaborators. Thedistribution of periods reflects both the initial conditions as well asthe subsequent stellar histories. Since the stellar magnetic field doesnot penetrate the convective core, eventually a shear zone near thecore-radiative envelope boundary may develop which produces turbulenceand modifies the field. Many, but not all, of the most rapidly rotatingmCP stars are close to the ZAMS and some of the least rapidly rotatingmCP stars are the furthest from the ZAMS.
| Wind circulation in selected rotating magnetic early-B stars
The rotating magnetic B stars are a class of variables consisting ofHe-strong and some beta Cep stars which have oblique dipolar magneticfields. Such stars develop co-rotating, torus-shaped clouds bychanneling wind particles from their magnetic poles to circumstellarregions centered around the plane of their magnetic equators. Therotation of the cloud-star complex permits the study of absorptions fromthe cloud as it occults the star. In this paper we describe aquantitative analysis of archival IUE data to map the properties ofthese clouds over four stars (HD 184927, sigma Ori E, beta Cep, and HR6684). By computing spectral synthesis models for these stars, we findthat only beta Cep has a solar-like metallicity. Our analysis also showsthat the metal composition across the surfaces of all these stars is atleast approximately homogeneous. Using the Hubeny code CIRCUS, wedemonstrate that the periodic variations of broad-band ultravioletcontinuum fluxes can be explained fully by the absorptions of theco-rotating cloud. We show next that among selected lines, those arisingfrom low-excitation states are selectively affected by cloud absorptionand turbulence. Our analysis also quantifies the cloud temperatures andcolumn densities required to match the absorptions of a number of weakto moderate strength resonance lines. These temperatures increase withthe ionization potential of the parent ions of these various lines, aresult which is consistent with radiative equilibrium models in whichtemperature increases with proximity to the star's surface. Althoughthese attributes appear stable from one epoch to another, dynamicprocesses are nonetheless at work. Both the strengths and widths ofresonance lines at occultation phases indicate the presence of aturbulence in the cloud which increases inwards. The spectroscopichallmark of this stellar class is the presence of strong C IV and N Vresonance line absorptions at occultation phases and of redshiftedemissions of these lines at magnetic pole-on phases. The emissions havecharacteristics which seem most compatible with their generation byhigh-energy shocks at the wind-cloud interface, as predicted recently byBabel (\cite{bab}).
| Observing roAp Stars with WET: A Primer
We give an extensive primer on roAp stars -- introducing them, puttingthem in context and explaining terminology and jargon, and giving athorough discussion of what is known and not known about them. Thisprovides a good understanding of the kind of science WET could extractfrom these stars. We also discuss the many potential pitfalls andproblems in high-precision photometry. Finally, we suggest a WETcampaign for the roAp star HR 1217.
| Doppler-Zeeman Mapping of the Rapidly Rotating Magnetic CP Star HD37776
We present the results of our analysis of magnetic-field configurationand abundance anomalies on the surface of the rapidly rotating,chemically peculiar helium-strong variable B2 V star HD37776 withunresolved Zeeman components of spectral lines. Simultaneous inversionof the observed Stokes I and V profiles, which realizes the method ofDoppler-Zeeman mapping (Vasilchenko et al. 1996), has been applied forthe first time. Spectroscopic observations were carried out with theMain stellar spectrograph of the 6-m Special Astrophysical Observatorytelescope equipped with a Zeeman analyzer and a CCD array, which allowedspectra in right- and left-hand circularly polarized light to be takensimultaneously at a signal-to-noise ratio S/N > 200 (Romanyuk et al.1999). The profile width of winged spectral lines (reaching 5 A) isdetermined by Zeeman line splitting; however, the observed Zeemancomponents are blurred and unresolved because of the rapid stellarrotation. When solving the inverse problem, we sought for themagnetic-field configuration in the form of a combination of arbitrarilyoriented dipole, quadrupole, and octupole placed at the stellar center.The observed Stokes I and V profiles for eight spectral lines of He,OII, AlIII, SiIII, and FeIII averaged over the visible stellar surfacewere used as input data. We constructed a model of the magnetic fieldfrom the condition of coincidence of magnetic maps obtained fromdifferent lines of different chemical elements and from the condition ofa minimum profile residual. This model is a combination of centeredcoaxial dipole and quadrupole with the dominant quadrupole component at30 deg < i < 50 deg, beta = 40 deg, and a maximum surface fieldstrength H_s = 60 kG. A comparison of our abundance maps with the fieldconfiguration shows that the He concentration is at a maximum in theregions of maximum radial field, while the maximum concentrations of O,Al, Si, and Fe coincide with the regions of maximum tangential field.
| 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.
| The Second Extreme Ultraviolet Explorer Right Angle Program Catalog
We present the detection of 235 extreme ultraviolet sources, of which169 are new detections, using the Extreme Ultraviolet Explorer's (EUVE)Right Angle Program (RAP) data. This catalog includes observations sincethe first EUVE RAP catalog (1994 January) and covers 17% of the sky. TheEUVE RAP uses the all-sky survey telescopes (also known as``scanners''), mounted at right angles to the Deep Survey andspectrometer instruments, to obtain photometric data in four wavelengthbands centered at ~100 Å (Lexan/B), ~200 Å (Al/Ti/C), ~400Å (Ti/Sb/Al), and ~550 Å (Sn/SiO). This allows the RAP toaccumulate data serendipitously during pointed spectroscopicobservations. The long exposure times possible with RAP observationsprovide much greater sensitivity than the all-sky survey. We presentEUVE source count rates and probable source identifications from theavailable catalogs and literature. The source distribution is similar toprevious extreme ultraviolet (EUV) catalogs with 2% early-type stars,45% late-type stars, 8% white dwarfs, 6% extragalactic, 24% with no firmclassification, and 15% with no optical identification. We also present36 detections of early-type stars that are probably the result ofnon-EUV radiation. We have detected stellar flares from approximately 12sources, including: EUVE J0008+208, M4 star G32-6 (EUVE J0016+198), anew source EUVE J0202+105, EUVE J0213+368, RS CVn V711 Tau (EUVEJ0336+005), BY Draconis type variable V837 Tau (EUVE J0336+259), the newK5 binary EUVE J0725-004, EUVE J1147+050, EUVE J1148-374, EUVE J1334-083(EQ Vir), EUVE J1438-432 (WT 486/487), EUVE J1808+297, and the M5.5estar G208-45 (EUVE J1953+444). We present sample light curves for thebrighter sources.
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| Fractionated stellar wind and the H/He abundance anomalies in BP stars
Radiatively driven winds occur in all main sequence stars (Babel\cite{Bab1}, \cite{Bab2}). However, due to the weak coupling between thephoton absorbing metals and the inert elements H and He, the wind in thelow temperature domain is fractionated: He decouples from the wind atT_eff < 25 000 K, and below 17 000 K even H. The decoupled elementsfall back to the surface of the star thus creating overabundances andabundance stratifications. These anomalies, however, become manifestonly if atmospheric turbulence is suppressed (say by magnetic fields).In order to prove the validity of the described scenario, all B_p starsfor which reliable fundamental parameters exist, are discussed on thebasis of the (augmented) (g, T_eff)-diagram of Babel (\cite{Bab2}). Itis shown that the fractionation process is able to explain the observedtemperature sequence of He-rich and He-poor stars, additionally toclassical diffusion processes. A necessary condition is that a magneticfield is present. This explains why only a fraction of B stars exhibitsH/He anomalies. While classical diffusion operates in the quiet zones(no wind) of a star, fractionation takes place above the wind bases.
| Properties of He-rich stars. II. CNO abundances and projected rotational velocities
We present an abundance analysis of light elements in the most massivechemically peculiar (CP) stars, He-rich stars. The analysis is based onboth low and high-resolution observations collected at ESO, La Silla,Chile in the optical region and includes 6 standard and 21 He-richstars. Light element abundances display a diverse pattern fromunder-solar up to above-solar values. Carbon is found underabundant inthe hottest He-rich stars and normal in the coolest ones, according tothe LTE model predictions. The distribution of projected rotationalvelocities shows a significant excess of slow rotators, no He-rich starshaving vsin i > 130 km s(-1) . Based on observations collected at theEuropean Southern Observatory, La Silla, Chile (programmes 7-043 and7-010 of periods 49 and 50 resp.)
| Modelling of magnetic fields of CP stars. II. Analysis of longitudinal field, crossover, and quadratic field observations
In recent years, the introduction and systematic application of newdiagnostic techniques has enormously increased the opportunities toinvestigate magnetic fields of chemically peculiar (CP) stars. Toapproach the problem of modelling these fields, in previous papers weset up a theory aimed at describing the magnetic configuration due tothe superposition of a dipole with an arbitrary quadrupole. The presentwork is a first application of this theory to spectro-polarimetricobservations of Stokes I and V. We have attempted to model nine magneticCP stars by analysing their curves of longitudinal field, crossover andquadratic field. We found that the classical dipolar model is adequatein only one case, while in six cases it should definitely be ruled out.For two stars a specific dipole plus quadrupole model has beenrecovered.
| Light element abundances in He-rich stars
We present an abundance analysis of light elements in He-rich stars. Theanalysis is based on both low and high resolution observations collectedat ESO, La Silla, Chile in the optical region and includes 6 standardsand 21 He-rich stars. Light-element abundances display a diversepattern: they range from under-solar up to above-solar values.
| On the near infrared variability of chemically peculiar stars
Some CP stars have recently been discovered by Catalano et al. to bevariable also in the near infrared, although with smaller amplitudesthan in the visible. Hence an observational campaign was started inwhich the infrared light variability of a number of CP2 and CP4 starshas been investigated at the ESO-La Silla Observatory in the bands J, H,and K. As a general result, infrared variations show the same behaviorin all three filters but amplitudes are smaller than in the visible.
| Age variation of helium abundance in He-rich stars.
Not Available
| Spectrum and magnetic variations of the remarkable helium-strong star HD 37776. I. Observations and data reduction.
Not Available
| Photometry from the HIPPARCOS Catalogue: Constant MCP Stars, Comparison and Check Stars
Photometry from the Hipparcos catalogue is used to verify the constancyof four magnetic CP stars, as well as the comparison and the check starsused for variability studies of normal and chemically peculiar B and Astars with the Four College Automated Photoelectric Telescope;variability in these stars can produce spurious results. A few of thecomparison stars are found to be variable and should be replaced forfuture differential photometric studies.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
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Observation and Astrometry data
| Constellation: | Orion |
| Right ascension: | 05h40m56.37s |
| Declination: | -01°30'25.9" |
| Apparent magnitude: | 6.986 |
| Distance: | 510.204 parsecs |
| Proper motion RA: | 4.2 |
| Proper motion Dec: | 1.2 |
| B-T magnitude: | 6.814 |
| V-T magnitude: | 6.972 |
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