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New magnetic field measurements of ? Cephei stars and slowly pulsating B stars
We present the results of the continuation of our magnetic survey withFORS 1 at the VLT of a sample of B-type stars consisting of confirmed orcandidate ? Cephei stars and Slowly Pulsating B (hereafter SPB)stars, along with a small number of normal B-type stars. A weak meanlongitudinal magnetic field of the order of a few hundred Gauss wasdetected in three ? Cephei stars and two stars suspected to be ?Cephei stars, in five SPB stars and eight stars suspected to be SPBstars. Additionally, a longitudinal magnetic field at a level largerthan 3? has been diagnosed in two normal B-type stars, thenitrogen-rich early B-type star HD 52089 and in the B5 IV star HD153716. Roughly one third of ? Cephei stars have detected magneticfields: Out of 13 ? Cephei stars studied to date with FORS 1, fourstars possess weak magnetic fields, and out of the sample of sixsuspected ? Cephei stars two show a weak magnetic field. Thefraction of magnetic SPBs and candidate SPBs is found to be higher:Roughly half of the 34 SPB stars have been found to be magnetic andamong the 16 candidate SPBs eight stars possess magnetic fields. In anattempt to understand why only a fraction of pulsating stars exhibitmagnetic fields, we studied the position of magnetic and non-magneticpulsating stars in the H-R diagram. We find that their domains in theH-R diagram largely overlap, and no clear picture emerges as to thepossible evolution of the magnetic field across the main sequence. It ispossible that stronger fields tend to be found in stars with lowerpulsating frequencies and smaller pulsating amplitudes. A somewhatsimilar trend is found if we consider a correlation between the fieldstrength and the v sin i-values, i.e. stronger magnetic fields tend tobe found in more slowly rotating stars.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes 078.D-0140(A), 078.D-0330(A),079.D-0241(A), and 080.D-0383(A)).
| Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| 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.
| The slowly pulsating B stars HD 34798 and HD 45284.
We discuss the period analysis of two variable mid-B stars that havebeen observed in the Geneva photometric system between 1981 and 1990.Both HD 34798 and HD 45284 are multiperiodic g-mode pulsators thatbelong to the group of slowly pulsating B stars. The power spectrum ofHD 34798 is characterized by several modes which have about equal power.This star belongs to a visual binary, the other component of which is asingly periodical CP star. Quasi-simultaneous observations of both starsare discussed and turn out to be helpful to illustrate the multiperiodicbehavior of HD 34798. Three oscillation modes with a nearly stableamplitude are found; the remaining variability may be due to two veryclose oscillation modes, or, alternatively, to one mode with anintrinsically variable amplitude. Three oscillation frequencies can bedistinguished clearly in the power spectrum of HD 45284. The beatperiods for this star are the shortest for any SPB detected so far, andtherefore make HD 45284 an excellent candidate for verification of ourresults and for asteroseimological follow-up.
| HD 37151: A new 'slowly pulsating B star'
We report the discovery of non-radial pusations with at least 5 periodsin the B8V star HD 37151, a probable member of the Orion OB1association. This result is based on 465 photometric measurements in 7colors spanning 12 years. In addition, 30 high-resolution spectra weretaken in the Mg II lambda 4481 region: they show a small projectedrotational velocity and slight variations of the line profiles where atleast two out of the five photometric periods can be identified.Although this star was once classified B8Vp(Si), all available data showit is a normal star instead. Therefore, this object in not anotherexample (after ET And) of a pulsating silicon star, but entirely matchesthe characteristics of Waelkens' 'Slowly Pulsating B stars' (SPB). Itoffers the interest of being the second coolest member of this class,helping to define empirically the cool border of the related instabilitydomain. The ratio of color to visual amplitudes A(sub(u-v)/Avof the known SPB stars is nicely correlated with Teff, ingood agreement with theoretical expectations for non-radial pulsationswith order less than or = 2.
| The Opacity Mechanism in B-Type Stars - Part Two - Excitation of High-Order G-Modes in Main Sequence Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.265..588D&db_key=AST
| Photometric variability of mid-B stars
An extended systematic study was carried out of photometric variationsof B-type main sequence stars. The campaign was performed using theseven-color instrument at La Silla. The survey covered 30 stars andshowed that the objects could be typified by the 53 Per variables. Theobserved variabilities may arise from pulsations in nonradial g-modeswith high radial wavenumbers. The motions detected occur in the outerstellar layers, i.e., in the second He ionization zone, a situation withsignificance for classifications of variable OB stars and supergiants,as opposed to mid-B stars. Another 10 yr of observations would berequired to derive the frequency spectra with an accuracy as high asthat available for white dwarfs.
| Optical observations of ultraviolet objects. I - Spectral classification of 103 stars /l = 200-275 deg/
Results are presented of a program of spectral classification of 103stars originally selected as ultraviolet objects from TD-1 satellitephotometry with the S2/68 experiment. Most of the objects appear to bespectroscopically normal stars; the method of selection yielded a sampleof relatively unreddened B stars at distances up to about 2 kpc. Thisresult is compared with recent studies of the spatial distribution ofinterstellar extinction in the same regions of the sky.
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Monoceros |
| Ascensió Recta: | 06h26m13.17s |
| Declinació: | -07°21'41.2" |
| Magnitud Aparent: | 7.369 |
| Distancia: | 331.126 parsecs |
| Moviment propi RA: | -3.7 |
| Moviment propi Dec: | 0.2 |
| B-T magnitude: | 7.247 |
| V-T magnitude: | 7.359 |
Catàlegs i designacions:
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