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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.
| Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating β Cephei stars
Seismic modelling of the β Cephei stars promises major advances inour understanding of the physics of early B-type stars on (or close to)the main sequence. However, a precise knowledge of their physicalparameters and metallicity is a prerequisite for correct modeidentification and inferences regarding their internal structure. Herewe present the results of a detailed NLTE abundance study of nine primetargets for theoretical modelling: γ Peg,δ Cet, ν Eri,β CMa, ξ1 CMa,V836 Cen, V2052 Oph,β Cep and DD (12) Lac (hereafter 12Lac). The following chemical elements are considered: He, C,N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis isbased on a large number of time-resolved, high-resolution opticalspectra covering in most cases the entire oscillation cycle of thestars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars,three of which have severe constraints on their equatorial rotationalvelocity, Ω R, from seismic or line-profile variation studies:β Cep (Ω R ~ 26 km s-1),V2052 Oph (Ω R ~ 56 km s-1),δ Cet (Ω R < 28 km s-1) andξ1 CMa (Ω R sin i 10 kms-1). The existence of core-processed material at the surfaceof such largely unevolved, slowly-rotating objects is not predicted bycurrent evolutionary models including rotation. We draw attention to thefact that three stars in this subsample have a detected magnetic fieldand briefly discuss recent theoretical work pointing to the occurrenceof diffusion effects in β Cephei stars possibly capable of alteringthe nitrogen surface abundance. On the other hand, the abundances of allthe other chemical elements considered are, within the errors,indistinguishable from the values found for OB dwarfs in the solarneighbourhood. Despite the mild nitrogen excess observed in someobjects, we thus find no evidence for a significantly higherphotospheric metal content in the studied β Cephei stars comparedto non-pulsating B-type stars of similar characteristics.
| Attempts to measure the magnetic field of the pulsating B star ν Eridani
We report on attempts to measure the magnetic field of the pulsating Bstar ν Eridani with the Musicos spectropolarimeter attached to the 2m telescope at the Pic du Midi, France. This object is one of the mostextensively studied stars for pulsation modes, and the existence of amagnetic field was suggested from the inequality of the frequencyseparations of a triplet in the stars' oscillation spectrum. We showthat the inferred 5-10 kG field was not present during our observations,which cover about one year. We discuss the influence of the strongpulsations on the analysis of the magnetic field strength and set anupper limit to the effective longitudinal field strength and to thefield strength for a dipolar configuration. We also find that theobserved wind line variability is caused by the pulsations.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| Dynamo-generated magnetic fields at the surface of a massive star
Spruit has shown that an astrophysical dynamo can operate in thenon-convective material of a differentially rotating star as a result ofa particular instability in the magnetic field (the Tayler instability).By assuming that the dynamo operates in a state of marginal instability,Spruit has obtained formulae which predict the equilibrium strengths ofazimuthal and radial field components in terms of local physicalquantities. Here, we apply Spruit's formulae to our previously publishedmodels of rotating massive stars in order to estimate Tayler dynamofield strengths. There are no free parameters in Spruit's formulae. Inour models of 10- and 50-Msolar stars on the zero-age mainsequence, we find internal azimuthal fields of up to 1 MG, and internalradial components of a few kG. Evolved models contain weaker fields. Inorder to obtain estimates of the field strength at the stellar surface,we examine the conditions under which the Tayler dynamo fields aresubject to magnetic buoyancy. We find that conditions for Taylerinstability overlap with those for buoyancy at intermediate to highmagnetic latitudes. This suggests that fields emerge at the surface of amassive star between magnetic latitudes of about 45° and the poles.We attempt to estimate the strength of the field which emerges at thesurface of a massive star. Although these estimates are very rough, wefind that the surface field strengths overlap with values which havebeen reported recently for line-of-sight fields in several O and Bstars.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| The orbit of the close spectroscopic binary \varepsilon Lup and the intrinsic variability of its early B-type components
We subjected 106 new high-resolution spectra of the double-linedspectroscopic close binary \varepsilon Lup, obtained in a time-span of17 days from two different observatories, to a detailed study of orbitaland intrinsic variations. We derived accurate values of the orbitalparameters. We refined the sidereal orbital period to 4.55970d days andthe eccentricity to e=0.277. By adding old radial velocities, wediscovered the presence of apsidal motion with a period of the rotationof apses of about 430 years. Such a value agrees with theoreticalexpectations. Additional data is needed to confirm and refine thisvalue. Our dataset did not allow us to derive the orbit of the thirdbody, which is known to orbit the close system in 64 years. We presentthe secondary of \varepsilon Lup as a new β Cephei variable, whilethe primary is a β Cephei suspect. A first detailed analysis ofline-profile variations of both primary and secondary led to detectionof one pulsation frequency near 10.36 c d-1 in thevariability of the secondary, while no clear periodicity was found inthe primary, although low-amplitude periodicities are still suspected.The limited accuracy and extent of our dataset did not allow any furtheranalysis, such as mode-identification.
| PSR B1929+10 revisited in X-rays
We performed timing and spectral analyses for PSR B1929+10, one of theoldest (~107 years) of the ordinary pulsars detected inX-rays, using archival ROSAT, ASCA and RXTE data. Pulsed emission wasdetected at a more than five sigma level for the combined ROSAT PSPC-Band previously unpublished HRI data. Our pulse profile is in agreementwith that obtained by Yancopoulos et al. (1994, ApJ, 429, 832, ROSATPSPC-B) but now with better statistics. The pulsed fraction in the ROSATX-ray band is 0.25 ± 0.04. The pulsed signal has also beendetected in the ASCA GIS data (0.5-5 keV) with a similar pulsed fractionof 0.36 ± 0.11. No significant timing signal is found in the RXTEPCA data (>2~keV). We found that the combined ROSAT PSPC-B and ASCAGIS spectrum can satisfactorily be described by a power-law as well asby a double black-body model but not by a single black-body model orblack-body plus power law model. Fitting the combined ROSAT/ASCA 0.1-10keV spectrum by a power-law model we obtain a photon index α of2.54 ± 0.12 and a neutral hydrogen column density NHtowards the source of 9.8-1.0+1.4 ×1020 cm-2. For a double black-body fit our resultsare T1 = 2.0-0.05+0.05 ×106~K, T2 = 6.9-0.35+0.23× 106 K and NH =4.4-1.1+2.1 × 1020cm-2. In both cases the derived value of NH ishigher than that adopted in earlier works, but our result is fullyconsistent with the larger distance estimate of 331 ± 10 pc fromparallax measurements combined with the hydrogen distributionmeasurements in the direction to the pulsar.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Disentangling component spectra of κ Scorpii, a spectroscopic binary with a pulsating primary. II. Interpretation of the line-profile variability
We analyse the complex short-term SiIII line-profile variability of the spectroscopic binary β Cep star κ Scorpii after orbit subtraction, before and after spectral disentangling. We refine the known oscillation frequency of the star: f1=4.99922 c d-1 and detect2f1. Variability is also found at frequencies nearf2≃ 4.85 c d-1 and f3≃ 5.69c d-1 or their aliases. These frequencies are not significantif we consider the spectra alone, but they survive our selection afterthe consideration that they were derived previously from independentground-based and space photometry by different teams. Moreover, we finddominant variability in the equivalent width with a frequency in theinterval [0.22,0.30] c d-1 which we interpret as therotational frequency frot of the star. The complex windowfunction does not allow us to determine definite values forf2, f3, frot. The variability withf1 is interpreted as a prograde non-radial oscillation modewith spherical wavenumbers (ℓ,m)=(2,-1) or (1,-1). The additionalfrequencies are explained in terms of rotational modulation superposedto the main oscillation. We also point out that we cannot disprove thevariability in κ Scorpii to originate from co-rotating structures. KOREL disentangling preserves the large-amplitude line-profile variability but its performance for complex low-amplitude variability remains to bestudied in detail.Based on observations obtained with the Coudé ÉchelleSpectrograph on the ESO CAT telescope and with the CORALIEéchelle spectrograph on the 1.2-m Euler Swiss telescope, bothsituated at La Silla, Chile.
| Asteroseismology with robotic telescopes
Asteroseismology explores the interior of pulsating stars by analysingtheir normal mode spectrum. The detection of a sufficient number ofpulsation modes for seismic modelling of main sequence variablesrequires large quantities of high-precision time resolved photometry.Robotic telescopes have become an asset for asteroseismology because oftheir stable instrumentation, cost- and time-efficient operation and thepotentially large amounts of observing time available. We illustratethese points by presenting selected results on several types ofpulsating variables, such as δ Scuti stars (main sequence andpre-main sequence), γ Doradus stars, rapidly oscillating Ap starsand β Cephei stars, thereby briefly reviewing recent successstories of asteroseismic studies of main sequence stars.
| Discovery of weak magnetic fields in early B-type stars
We summarize recent results of magnetic measurements of three brightearly B-type stars, β Cep, ζ Cas, and V2052 Oph, which werefound to be oblique rotators with a weak magnetic dipole field withtypical strength of a few hundred Gauss. From stellar wind studies wecould derive their rotational periods very accurately, and match thestellar wind with the magnetic phase. From model atmosphere fits wederive the angles of rotational and magnetic axis. All three stars showsome chemical abundance anomaly, presumably associated with the magneticproperties. The stars are also pulsating variables. This is of highasteroseismological interest, since these are the only early-type starsknown with observationally determined rotation, pulsation and magneticproperties.
| Statistics of the Instability Strip of β Cephei Stars
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
| Rotation, pulsations and magnetic field in V 2052 Ophiuchi: A new He-strong star
V 2052 Oph is a beta Cep star with v sin i ~ 60 kms-1. The behavior of its stellar wind was studied in theultraviolet wavelength region with the IUE satellite. It revealedperiodic variations in the equivalent widths (EW) of the resonance linesof wind-sensitive ions with a period of 3.638833 d, which is identifiedas the rotational period. These variations are typical of magneticstars. Therefore time-resolved circular spectropolarimetric observationswere obtained with the Musicos échelle spectropolarimeter at the2-m Télescope Bernard Lyot (TBL) to search for a magnetic fieldin the star. Stokes V patterns were observed, the inclination andmagnetic angles were derived and a value was determined for the polarmagnetic field (250 +/- 190 G) using an oblique rotator dipole model.The spectroscopic information was used to search for periodicity inline-profile variations (lpv), radial velocity and minimum intensitycurves. Multiperiodicity was found, corresponding to radial (f_1=7.15 cd-1) and non-radial (f_2=6.82 c d-1, l=3 or 4)pulsation modes. The rotation period is also detected in rotationallymodulated observables because of the magnetic poles passing through theobserver's line of sight. We determined the stellar parameters of thestar, which was found to be chemically peculiar, in particularHe-enriched. This makes V 2052 Oph the first magneticHe-strong early B star with known pulsational properties.Based on observations obtained with the MuSiCoS spectropolarimeter atthe Observatoire du Pic du Midi, France, and by the InternationalUltraviolet Explorer, collected at NASA Goddard Space Flight Center andVillafranca Satellite Tracking Station of the European Space Agency,retrieved from the INES database.}\fnmsep\thanks{Table \ref{tabmagV} isonly available in electronic form athttp://www.edpsciences.org
| Grey extinction in the solar neighbourhood?
Some of the close O and B dwarfs appear to be fainter than indicated bytheir Hipparcos distances, intrinsic absolute magnitudes attributed totheir spectral types, and estimated selective interstellar extinction.This discrepancy is explained in the paper by the grey (neutral)interstellar extinction in the visual range of spectrum. The measure ofsuch an effect is related to discrete features of the interstellarmatter.
| 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
| Discovery of a magnetic field in the Slowly Pulsating B star zeta Cassiopeiae
zeta Cas is a B2 IV star with vsin i = 17 kms-1. Time-resolved circular spectropolarimetric observationsof zeta Cas obtained in 2001 and 2002 with theMusicos échelle spectropolarimeter at the 2 m TélescopeBernard Lyot (TBL) show a sinusoidally varying longitudinal magneticfield with a strength between 10 G and -46 G for the averagedline-of-sight component, corresponding toB_pol=335+120-65 G. The period corresponds veryaccurately with the 5.37045 day period as derived from stellar windvariations observed in the ultraviolet. The epoch of the positivemaximum field corresponds in phase with the maximum emission in the UVwind lines. This gives compelling evidence for a magnetic rotator modelfor this star, with an unambiguous rotation period of 5.37 days. Wesearched for periodicity in line-profile variations (lpv), radialvelocity and minimum intensity curves in the ~ 400 optical spectra. Wefound a non-radial pulsation mode with l=2 +/- 1 at the frequency f=0.64c d-1. From this periodicity and from stellar parametersderived from model fits, we propose to classify zetaCas as a Slowly Pulsating B (SPB) star. This is the thirddetection of a magnetic field in an early B-type pulsating star and thefirst one in a SPB star.Based on observations obtained using the Musicos spectropolarimeter atthe Observatoire du Pic du Midi, France, and on INES data from theInternational Ultraviolet Explorer (IUE) satellite.
| Detection of Diffuse Interstellar Bands in the Magellanic Clouds
With the Ultraviolet Visual Echelle Spectrograph mounted at the VeryLarge Telescope, we have observed at unprecedented spectral resolutionthe absorption spectrum toward reddened stars in the Magellanic Cloudsover the wavelength range of 3500-10500 Å. This range covers thestrong transitions associated with neutral and charged large carbonmolecules of varying sizes and structures. We report the first detectionof diffuse interstellar bands (DIBs) at 5780 and 5797 Å in theSmall Magellanic Cloud and the variation of the 6284 Å DIB towardseveral targets in the Large Magellanic Cloud. The variation of DIBs inthe Magellanic Clouds compared with Galactic targets may be governed bya combination of the different chemical processes prevailing inlow-metallicity regions and the local environmental conditions. Based onobservations made at the European Southern Observatory, Paranal, Chile(ESO program 67.C-0281).
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| A highly sensitive search for magnetic fields in B, A and F stars
Circular spectropolarimetric observations of 74 stars were obtained inan attempt to detect magnetic fields via the longitudinal Zeeman effectin their spectral lines. The sample observed includes 22 normal B, A andF stars, four emission-line B and A stars, 25 Am stars, 10 HgMn stars,two lambda Boo stars and 11 magnetic Ap stars. Using the Least-SquaresDeconvolution multi-line analysis approach (Donati et al.\cite{donati97etal}), high precision Stokes I and V mean signatures wereextracted from each spectrum. We find absolutely no evidence formagnetic fields in the normal, Am and HgMn stars, with upper limits onlongitudinal field measurements usually considerably smaller than anypreviously obtained for these objects. We conclude that if any magneticfields exist in the photospheres of these stars, these fields are notordered as in the magnetic Ap stars, nor do they resemble the fields ofactive late-type stars. We also detect for the first time a field in theA2pSr star HD 108945 and make new precise measurements of longitudinalfields in five previously known magnetic Ap stars, but do not detectfields in five other stars classified as Ap SrCrEu. We also report newresults for several binary systems, including a new vsin i for therapidly rotating secondary of the Am-delta Del SB2 HD 110951. Based onobservations obtained using the MuSiCoS spectropolarimeter on theBernard Lyot telescope, l'Observatoire du Pic du Midi, France.
| Profiles of blue and infrared diffuse interstellar bands
The paper presents a survey of profiles of reasonably strong diffuseinterstellar bands (DIBs) based on the extensive set of high-resolutionspectra acquired with the aid of echelle spectrographs installed at the2-m Terskol, 2-m Pic du Midi and 1-m SAO telescopes. The surveyeddiffuse interstellar bands cover the spectral ranges of blue andnear-infrared, i.e the DIBs not surveyed by Krełowski &Schmidt. The possible modifications caused by stellar and telluric linesare discussed. The very broad features such as 4430 are not discussedbecause the shapes of their profiles, extracted from echelle spectra,are very uncertain. The signal-to-noise (S/N) ratios of the spectra arenot high enough to enable discussion of the profiles of numerous weakinterstellar features discovered recently.
| 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}).
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Long-term period and amplitude variations in beta Cephei stars.
Not Available
| On correlations between diffuse interstellar bands
One way to better apprehend the problem of diffuse interstellar bands(DIBs) is to search for correlations between the bands in a large sampleof spectra towards various lines of sight: a strict correlation mayimply that a common carrier is at the origin of the bands, whereas anon-correlation means that different species are involved. We proposethis observational test for 10 DIBs collected in up to 62 Galactic linesof sight. Strong DIBs do not strictly correlate, and sometimes thecorrelation is very poor. Only one example of a strict correlation hasbeen found in our sample between the DIBs at 6614 and 6196 Ä, thatcould signify a single carrier for those two bands. The general absenceof strict correlations is discussed in the context of molecular carriersfor the DIBs.
| On the relation between diffuse interstellar bands and simple molecular species
We present observations of the major diffuse interstellar bands (DIBs)at 5780 and 5797 Ä as well as literature data and our ownobservations of the violet lines of CH and CH(+) , in the lines of sighttoward some 70 stars representing various degrees of the interstellarreddening. The correlations are shown and discussed in the context ofindicators such as far-UV extinction parameters and neutral molecularabundances. The results show that the DIBs in question (lambda lambda5797 and 5780) both probably form in diffuse cloud interiors, in arelated regime where CH and H_2 form. The ratio of the two DIBscorrelates with CH abundance, confirming that the lambda 5797 carrier isfavoured in enhanced molecular gas regions over the lambda 5780 carrier.The ratio of the two DIBs correlates poorly with CH(+) abundance. Ourcompilation of observational data also suggests that the DIB ratio maybe equally useful as a cloud type indicator as is R_V, the ratio oftotal to selective extinction, and much more readily observed. Based onobservations obtained at the Russian Special Astrophysical Observatory(SAO), Terskol Observatory (TER), Canada France Hawaii Telescope (CFHT),European Southern Observatory (ESO), Observatoire de Haute-Provence(OHP)
| On the normal spectral energy distribution of stars: Spectral types O9-B5
The normal energy distributions for fifteen spectral subtypes from O9 toB5 for luminosity classes V, IV, and III are derived. Threephotometrically uniform catalogs served as the source of thespectrophotometric data used. Synthetic color indices for all spectraltypes are calculated using the energy distribution curves obtained.Comparison of these indices with the expected normal color indicessuggests that the energy distributions derived are reliable.
| 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.
| Diffuse Interstellar Bands: Physical Conditions that Facilitate the Formation or Preservation of Their Carriers
This paper suggests that the formation and/or preservation of thecarriers of at least some of the diffuse interstellar bands (DIBs)depends on the level of ionization of certain interstellar atoms, suchas potassium and calcium inside H I clouds. The spectral lines of theseelements are apparently well correlated with the narrow diffuse bands,such as 5797 or 6379 Angstroms. Thus, the physical conditions thatfacilitate the growing abundance of neutral alkali atoms also facilitatethe formation or preservation of the carriers of the narrow diffusebands. The broad features, such as 5780 or 6284 Angstroms, apparentlyoriginate in different ionization conditions.
| On the relation between the carriers of diffuse interstellar bands and simple molecular species
The interstellar clouds composed mostly of neutral hydrogen (H I clouds)show evident differences in the pattern of interstellar absorptions: inthe shapes of the extinction curves, the strengths of the diffuseinterstellar bands, and the intensities of the features originating insimple molecular species. It is well known that the ratio of the twoprominent diffuse bands (5780 and 5797 A) varies strongly from cloud tocloud; our data suggest that this phenomenon is related to theintensities of the absorption features of interstellar molecules such asCH(+) or CH. However, the intensity ratio of the diffuse bands canchange gradually, suggesting a 'smooth' variation of the carrierabundances, while the strength ratios of molecular features and thecolor excesses change rather stepwise when the diffuse band ratioreaches a certain value.
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