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Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars
Context: .The X-ray emission from B-type main-sequence stars is alongstanding mystery in stellar coronal research. Since there is notheory at hand that explains intrinsic X-ray emission fromintermediate-mass main-sequence stars, the observations have often beeninterpreted in terms of (unknown) late-type magnetically activecompanion stars. Aims: .Resolving the hypothesized companionsrequires high spatial resolution observations in the infrared and inX-rays. We use Chandra imaging observations to spatially resolve asample of main-sequence B-type stars with recently discovered companionsat arcsecond separation. Methods: .Our strategy is to search forX-ray emission at the position of both the B-type primary and the faintcompanion. Results: .We find that all spatially resolvedcompanions are X-ray emitters, but seven out of eleven intermediate-massstars are also X-ray sources. If this emission is interpreted in termsof additional sub-arcsecond or spectroscopic companions, this implies ahigh multiplicity of B-type stars. Firm results on B star multiplicitypending, the alternative, that B stars produce intrinsic X-rays, cannotbe discarded. An appropriate scenario would be a magnetically confinedwind, as suggested for the X-ray emission of the magnetic Ap star IQAur. However, the only Ap star in the Chandra sample is not detected inX-rays, and therefore does not support this picture.
| A rigidly rotating magnetosphere model for circumstellar emission from magnetic OB stars
We present a semi-analytical approach for modelling circumstellaremission from rotating hot stars with a strong dipole magnetic fieldtilted at an arbitrary angle to the rotation axis. By assuming therigid-field limit in which material driven (e.g. in a wind outflow) fromthe star is forced to remain in strict rigid-body corotation, we areable to solve for the effective centrifugal-plus-gravitational potentialalong each field line, and thereby identify the location of potentialminima where material is prone to accumulate. Applying basic scalingsfor the surface mass flux of a radiatively driven stellar wind, wecalculate the circumstellar density distribution that obtains onceejected plasma settles into hydrostatic stratification along fieldlines. The resulting accumulation surface resembles a rigidly rotating,warped disc, tilted such that its average surface normal lies betweenthe rotation and magnetic axes. Using a simple model of the plasmaemissivity, we calculate time-resolved synthetic line spectra for thedisc. Initial comparisons show an encouraging level of correspondencewith the observed rotational phase variations of Balmer-line emissionprofiles from magnetic Bp stars such as σ Ori E.
| Chandra HETGS Multiphase Spectroscopy of the Young Magnetic O Star θ1 Orionis C
We report on four Chandra grating observations of the oblique magneticrotator θ1 Ori C (O5.5 V), covering a wide range ofviewing angles with respect to the star's 1060 G dipole magnetic field.We employ line-width and centroid analyses to study the dynamics of theX-ray-emitting plasma in the circumstellar environment, as well asline-ratio diagnostics to constrain the spatial location, and globalspectral modeling to constrain the temperature distribution andabundances of the very hot plasma. We investigate these diagnostics as afunction of viewing angle and analyze them in conjunction with new MHDsimulations of the magnetically channeled wind shock mechanism onθ1 Ori C. This model fits all the data surprisinglywell, predicting the temperature, luminosity, and occultation of theX-ray-emitting plasma with rotation phase.
| New Perspectives on the X-Ray Emission of HD 104237 and Other Nearby Herbig Ae/Be Stars from XMM-Newton and Chandra
The origin of the X-ray emission from Herbig Ae/Be stars is not yetknown. These intermediate-mass pre-main-sequence stars lie on radiativetracks and are not expected to emit X-rays via solar-like magneticprocesses, nor are their winds powerful enough to produce X-rays byradiative wind shocks as in more massive O-type stars. The emissioncould originate in unseen low-mass companions, or it may be intrinsic tothe Herbig stars themselves if they still have primordial magneticfields or can sustain magnetic activity via a nonsolar dynamo. Wepresent new X-ray observations of the nearby Herbig Ae star HD 104237(=DX Cha) with XMM-Newton, whose objective is to clarify the origin ofthe emission. Several X-ray emission lines are clearly visible in theCCD spectra, including the high-temperature Fe Kα complex. Theemission can be accurately modeled as a multitemperature thermal plasmawith cool (kT<1 keV) and hot (kT>~3 keV) components. The presenceof a hot component is compelling evidence that the X-rays originate inmagnetically confined plasma, either in the Herbig star itself or in thecorona of an as yet unseen late-type companion. The X-ray temperaturesand luminosity (logLX=30.5 ergs s-1) are withinthe range expected for a T Tauri companion, but high-resolution Chandraand Hubble Space Telescope images constrain the separation of a putativecompanion to less than 1". We place these new results into broadercontext by comparing the X-ray and bolometric luminosities of a sampleof nearby Herbig stars with those of T Tauri stars and classicalmain-sequence Be stars. We also test the predictions of a model thatattributes the X-ray emission of Herbig stars to magnetic activity thatis sustained by a shear-powered dynamo.
| A three-dimensional model for the radio emission of magnetic chemically peculiar stars
In this paper we present a three-dimensional numerical model for theradio emission of Magnetic Chemically Peculiar stars, on the hypothesisthat energetic electrons emit by the gyrosynchrotron mechanism. For thisclass of radio stars, characterized by a mainly dipolar magnetic fieldwhose axis is tilted with respect to the rotational axis, the geometryof the magnetosphere and its deformation due to the stellar rotation aredetermined. The radio emitting region is determined by the physicalconditions of the magnetosphere and of the stellar wind. Free-freeabsorption by the thermal plasma trapped in the inner magnetosphere isalso considered. Several free parameters are involved in the model, suchas the size of the emitting region, the energy spectrum and the numberdensity of the emitting electrons, and the characteristics of the plasmain the inner magnetosphere. By solving the equation of radiativetransfer, along a path parallel to the line of sight, the radiobrightness distribution and the total flux density as a function ofstellar rotation are computed. As the model is applied to simulate theobserved 5 GHz lightcurves of HD 37479 andHD 37017, several possible magnetosphereconfigurations are found. After simulations at other frequencies, inspite of the large number of parameters involved in the modeling, twosolutions in the case of HD 37479 and only onesolution in the case of HD 37017 match the observedspectral indices. The results of our simulations agree with themagnetically confined wind-shock model in a rotating magnetosphere. TheX-ray emission from the inner magnetosphere is also computed, and foundto be consistent with the observations.Appendices A and B are only available in electronic form athttp://www.edpsciences.org
| Discovery of X-ray flaring on the magnetic Bp-star σ Ori E
We report the detection of an X-ray flare on the Bp star σ Ori Ewith the ROSAT high resolution imager (HRI). The flare is shown to havelikely occurred on the early-type star, rather than on an hypothesizedlate-type companion. We derive flare parameters such as total energyrelease, coarse estimates of size and density, and also presentarguments for a magnetic origin of the flare. We place our observationsin the context of a magnetic character of Bp-type stars and speculate ona common physical basis and connection between Bp and Be stars.
| Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster
During a survey of field strengths in upper main sequence stars in openclusters, we observed the star NGC 2244-334 in the Rosette Nebulacluster and discovered an extraordinarily large mean longitudinal fieldof about -9 kG, the second largest longitudinal field known in anon-degenerate star. This star appears to be a typical Ap He-wk (Si)star of about 4 Mȯ. Spectrum synthesis using a linesynthesis code incorporating the effects of the strong magnetic fieldindicates that He is underabundant by about 1.5 dex, and C, O and Mg byabout 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex,and Cr and Ti are nearly 2 dex overabundant.Cluster membership for this star is secure, so its age is about 2×106 yr, which is less than 3% of its main sequence lifetime.This star is one of the very youngest magnetic upper main sequence starswith a well-determined age, and confirms that both magnetic fields andstrong chemical peculiarity can appear in stars which are both extremelyyoung and very close to the ZAMS.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 70.D-0352 and 270.D-5032.
| 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
| A Magnetically Torqued Disk Model for Be Stars
Despite extensive study, the mechanisms by which Be star disks acquirehigh densities and angular momentum while displaying variability on manytimescales are still far from clear. In this paper, we discuss howmagnetic torquing may help explain disk formation with the observedquasi-Keplerian (as opposed to expanding) velocity structure and theirvariability. We focus on the effects of the rapid rotation of Be stars,considering the regime where centrifugal forces provide the dominantradial support of the disk material. Using a kinematic description ofthe angular velocity, vφ(r), in the disk and aparametric model of an aligned field with a strength B(r), we developanalytic expressions for the disk properties that allow us to estimatethe stellar surface field strength necessary to create such a disk for arange of stars on the main sequence. The fields required to form a diskare compared with the bounds previously derived from photosphericlimiting conditions. The model explains why disks are most common formain-sequence stars at about spectral class B2 V. The earlier type starswith very fast and high-density winds would require unacceptably strongsurface fields (>103 G) to form torqued disks, while the lateB stars (with their low mass-loss rates) tend to form disks that produceonly small fluxes in the dominant Be diagnostics. For stars at B2 V theaverage surface field required is about 300 G. The predicted disksprovide an intrinsic polarization and a flux at Hα comparable toobservations. The radial extent of our dense quasi-Keplerian disks iscompatible with typical estimates. We also discuss whether the effect onfield containment of the time-dependent accumulation of matter in theflux tubes/disk can help explain some of the observed variability of Bestar disks.
| Dynamical Simulations of Magnetically Channeled Line-driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow
We present numerical magnetohydrodynamic (MHD) simulations of the effectof stellar dipole magnetic fields on line-driven wind outflows from hot,luminous stars. Unlike previous fixed-field analyses, the simulationshere take full account of the dynamical competition between field andflow and thus apply to a full range of magnetic field strength andwithin both closed and open magnetic topologies. A key result is thatthe overall degree to which the wind is influenced by the field dependslargely on a single, dimensionless ``wind magnetic confinementparameter'' η*(=B2eqR2*/Mv&infy;), which characterizes the ratio between magneticfield energy density and kinetic energy density of the wind. For weakconfinement, η*<=1, the field is fully opened by thewind outflow, but nonetheless, for confinements as small asη*=1/10 it can have a significant back-influence inenhancing the density and reducing the flow speed near the magneticequator. For stronger confinement, η*>1, the magneticfield remains closed over a limited range of latitude and height aboutthe equatorial surface, but eventually is opened into a nearly radialconfiguration at large radii. Within closed loops, the flow is channeledtoward loop tops into shock collisions that are strong enough to producehard X-rays, with the stagnated material then pulled by gravity backonto the star in quite complex and variable inflow patterns. Within openfield flow, the equatorial channeling leads to oblique shocks that areagain strong enough to produce X-rays and also lead to a thin, dense,slowly outflowing ``disk'' at the magnetic equator. The polar flow ischaracterized by a faster-than-radial expansion that is more gradualthan anticipated in previous one-dimensional flow tube analyses andleads to a much more modest increase in terminal speed (less than 30%),consistent with observational constraints. Overall, the results hereprovide a dynamical groundwork for interpreting many types ofobservations-e.g., UV line profile variability, redshifted absorption oremission features, enhanced density-squared emission, and X-rayemission-that might be associated with perturbation of hot-star winds bysurface magnetic fields.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| The magnetic field and wind confinement of β Cephei: new clues for interpreting the Be phenomenon?
In this paper, we use the very recent spectropolarimetric observationsof β Cep collected by Henrichs et al. and propose for this star aconsistent model of the large-scale magnetic field and of the associatedmagnetically confined wind and circumstellar environment. Are-examination of the fundamental parameters of β Cep in the lightof the Hipparcos parallax indicates that this star is most likely a12-Msolar star with a radius of 7Rsolar, effectivetemperature of 26000K and age of 12Myr, viewed with an inclination ofthe rotation axis of about 60°. Using two different modellingstrategies, we obtain that the magnetic field of β Cep can beapproximately described as a dipole with a polar strength of 360\pm 30\hphantom{0} G, the axisof symmetry of which is tilted with respect to the rotation axis byabout 85\circ\pm10\circ.Although one of the weakest detected to date, this magnetic field isstrong enough to magnetically confine the wind of β Cep up to adistance of about 8 to 9R*. We find that both the X-ray luminosity andvariability of β Cep can be explained within the framework of themagnetically confined wind-shock model of Babel & Montmerle, inwhich the stellar-wind streams from both magnetic hemispheres collidewith each other in the magnetic equatorial plane, producing a strongshock, an extended post-shock region and a high-density cooling disc.By studying the stability of the cooling disc, we obtain that fieldlines can support the increasing disc weight for no more than a monthbefore they become significantly elongated in an effort to equilibratethe gravitational plus centrifugal force, thereby generating strongfield gradients across the disc. The associated current sheet eventuallytears, forcing the field to reconnect through resistive diffusion andthe disc plasma to collapse towards the star. We propose that thiscollapse is the cause for the recurrent Be episodes of β Cep, andfinally discuss the applicability of this model to He peculiar,classical Be and normal non-supergiant B stars.
| Are Stellar Rotational Axes Distributed Randomly?
Stellar line widths yield values of Vsini, but the equatorial rotationalvelocities, V, cannot be determined for individual stars withoutknowledge of their inclinations, i, relative to the lines of sight. Forlarge numbers of stars we usually assume random orientations ofrotational axes to derive mean values of V, but we wonder whether thatassumption is valid. Individual inclinations can be derived only inspecial cases, such as for eclipsing binaries where they are close to90° or for chromospherically active late-type dwarfs or spotted(e.g., Ap) stars where we have independent information about therotational periods. We consider recent data on 102 Ap stars for whichCatalano & Renson compiled rotational periods from the literatureand Abt & Morrell (primarily) obtained measures of Vsini. We findthat the rotational axes are oriented randomly within the measuringerrors. We searched for possible dependence of the inclinations onGalactic latitude or longitude, and found no dependence.
| 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
| NLTE calculations of neutral helium lines in the wind of the Herbig Ae star AB Aurigae
In order to improve our knowledge about the structure of the winds ofthe Herbig Ae/Be stars and the mechanisms that drive them, we performeda NLTE analysis of the formation of helium lines in the wind of AB Aur,the prototype of the whole class. These lines are indeed expected toform in the densest layers of the envelope, at the very base of thewind, and therefore to provide constraints on these regions where thewind originates. A large observational data set is now available whichwe have extensively used to deduce strong constraints on the physicsthat prevail in the inner part of the wind. Among these data, weparticularly used those from the MUSICOS 1996 campaign, which show thatthe He I D3 (lambda 5876 Ä) line is made of two distinctscomponents: a blue-shifted emission component, and a redshiftedcomponent alternatively seen as an absorption or an emission. Theclassical model for AB Aur's wind (Bouret & Catala 1998; Catala& Kunasz 1987) fails to reproduce the observed profile. We find thatthe observations can be explained by a two component model in which thebase of the wind includes dense and hot clumps of outflowing material,while the bulk of the wind is still described by the model by Bouret& Catala (1998). This clumpy structure can result from the presenceof co-rotating interaction regions which form where fast streams in thewind overtake slow streams. Our results show that a blue-shiftedemission is formed in such conditions, and can be rotationally modulatedas observed, due to fast and slow streams alternating on the line ofsight as the star rotates. Our model can also explain the occasionalpresence of an absorption component near rest wavelength, as formed inthe bulk of the extended chromosphere. Concerning the red emissioncomponent, which is occasionally observed instead of the absorption one,we argue that it may arise from polar downflows.
| Mapping the contours of the Local bubble: preliminary results
We present preliminary results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,taking advantage of Hipparcos stellar distances. Equivalent widths ofthe NaI D-line doublet at 5890 Å are presented for thelines-of-sight towards some 143 new target stars lying within 300 pc ofthe Sun. Using these data which were obtained at the Observatoire deHaute Provence, together with previously published NaI absorptionmeasurements towards a further 313 nearby targets, we present absorptionmaps of the distribution of neutral gas in the local interstellar mediumas viewed from 3 different galactic projections. In particular, thesemaps reveal the Local Bubble region as a low neutral densityinterstellar cavity in the galactic plane with radii between 65-250 pcthat is surrounded by a (dense) neutral gas boundary (or ``wall''). Wehave compared our iso-column contours with the contours derived bySnowden et al. (\cite{snowden98}) using ROSAT soft X-ray emission data.Consistency in the global dimensions derived for both sets of contoursis found for the case of a million degree hot LB plasma of emissivity0.0023 cm(-6) pc with an electron density of 0.005 cm(-2) . We havedetected only one relatively dense accumulation of cold, neutral gaswithin 60 pc of the Sun that surrounds the star delta Cyg, and note thatthe nearest molecular cloud complex of MBM 12 probably resides at thevery edge of the Local Bubble at a distance of ~ 90 pc. Our observationsmay also explain the very different physical properties of the columnsof interstellar gas in the line-of-sight to the two hot stars epsilonCMa and beta CMa as being due to their locations with respect to theBubble contours. Finally, in the meridian plane the LB cavity is foundto be elongated perpendicularly to the Gould's Belt plane, possiblybeing ``squeezed'' by the expanding shells of the Sco-Cen andPerseus-Taurus OB associations. Tables 1 and 2 are also available inelectronic form at the CDS (Strasbourg) via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| High-energy phenomena in magnetic CP stars as revealed by their X-ray and radio emission
Before 1985, attempts to detect radio or X-ray emission from Magnetic CP(MCP) stars were either fruitless or ambiguous. However, more successfulresults have been obtained in the last dozen years: (i) Radio emissionhas now been detected from appr. 35 MCP stars of the Helium-peculiar andSilicon-strong subclasses, with a functional dependence of radioluminosity L(R) proportional to T_(eff)^(7) H_(s)^(1.2) P_(rot)^(-0.6),where T(eff) is the effective temperature, H_(s) is the surface magneticfield strength, and P_(rot) is the rotational period; rotationalmodulation of the radio emission has also been observed for several MCPstars. All of this evidence suggests that it is the MCP starsthemselves, not close companions, that are responsible for the radioemission; (ii) The X-ray emission properties of MCP stars are howeverstill poorly characterized: although a moderate number (appr. 18) haveby now been associated with X-ray sources, the lack of a clearcorrelation of this X-ray emission with other stellar parameters hasmade it difficult to rule out the close companion hypothesis.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Effective temperatures of AP stars
A new method of determination of the effective temperatures of Ap starsis proposed. The method is based on the fact that the slopes of theenergy distribution in the Balmer continuum near the Balmer jump for``normal" main sequence stars and chemically peculiar stars with thesame Teff are identical. The effective temperaturecalibration is based on a sample of main sequence stars with well knowntemperatures (\cite[Sokolov 1995]{sokolov}). It is shown that theeffective temperatures of Ap stars are derived by this method in goodagreement with those derived by the infrared flux method and by themethod of \cite[Stepien & Dominiczak (1989)]{stepien}. On the otherhand, the comparison of obtained Teff with Teffderived from the color index (B2-G) of Geneva photometry shows a largescatter of the points, nevertheless there are no systematicaldifferences between two sets of the data.
| The observed periods of AP and BP stars
A catalogue of all the periods up to now proposed for the variations ofCP2, CP3, and CP4 stars is presented. The main identifiers (HD and HR),the proper name, the variable-star name, and the spectral type andpeculiarity are given for each star as far as the coordinates at 2000.0and the visual magnitude. The nature of the observed variations (light,spectrum, magnetic field, etc.) is presented in a codified way. Thecatalogue is arranged in three tables: the bulk of the data, i.e. thosereferring to CP2, CP3, and CP4 stars, are given in Table 1, while thedata concerning He-strong stars are given in Table 2 and those foreclipsing or ellipsoidal variables are collected in Table 3. Notes arealso provided at the end of each table, mainly about duplicities. Thecatalogue contains data on 364 CP stars and is updated to 1996, October31. This research has made use of the SIMBAD database, operated at CDS,Strasbourg, France.
| Chemically peculiar stars in the field of NGC 2244
Low-resolution long-slit spectra of reference stars, including MKKstandard stars and well-known chemically peculiar stars, are used todevelop a spectroscopic method for the detection of the 5200 {Angstroms} flux depression in CP stars. This new method is shown to beas sensitive a detection tool as the photometrical techniques, andprovides a higher resolution view of the excess blocking. Application tostars in the field of NGC 2244 allows us to estimate and eliminatereddening effects. CP stars detected in this field include two members(# 334 and # 276) of the very young stellar group NGC 2244 (age ~ 3 x10(6) yr) and two or three foreground stars (# 381, # 625 and maybe #629). # 334 and # 625 are strongly peculiar. Based on observationsobtained at the Observatoire du Haute--Provence (OHP), France
| Do SI stars undergo any rotational braking?
The old question of rotational braking of Ap Si stars is revisited onthe empirical side, taking advantage of the recent Hipparcos results.Field stars with various evolutionary states are considered, and it isshown that the loose correlation between their rotational period andtheir surface gravity is entirely compatible with conservation ofangular momentum. No evidence is found for any loss of angular momentumon the Main Sequence, which confirms earlier results based on lessreliable estimates of surface gravity. The importance of reliable,fundamental T_eff determinations of Bp and Ap stars is emphasized. Basedon data from the ESA Hipparcos satellite
| ASCA X-Ray Spectroscopy of the Unusual B0 V Star tau Scorpii
We have obtained a high-quality ASCA spectrum of the MK standard B0 Vstar tau Sco in order to test the standard wind-shock picture of OB starX-ray production. The fluxes in three line complexes from ionsindicative of hot plasma---Mg+10, Si+12, and S+14---are measured, and wealso present a global spectral fit using a fairly standardmultitemperature, optically thin, collisional equilibrium model. We wereable to achieve a statistically good fit, but only by using the MeKaLplasma emission code (Mewe, Kaastra, & Liedahl) and fixing theelemental abundances at the photospheric values as determined by opticalspectroscopy. The parameters of the model are T1 = 7 MK, EM1 = 3.5 x1054 cm-3, T2 = 12 MK, EM2 = 8.1 x 1053 cm-3, T3 > 27 MK, EM3 >3.0 x 1053 cm-3. The quantity of material with temperature in excess of107 K on tau Sco is comparable to that with temperature in excess of 106K on most other early B stars. The data cannot be explained by thestandard line-force instability wind-shock mechanism. However, moreunusual shock mechanisms involving magnetically confined wind shocks orinteractions between infalling matter and the ambient stellar windcannot be ruled out. Alternately, a dynamo driven by differentialrotation could be powering coronal plasma. If magnetic fields areinvolved in any way, then the star's extreme youth could play a role.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| X-ray emission from Ap-Bp stars: a magnetically confined wind-shock model for IQ Aur.
We present the results of a ROSAT-PSPC pointed observation on the hotA0p star IQ Aur (HD 34452). The X-ray luminosity obtained isL_X_=4.0x10^29^erg/s (logL_X_/L_Bol_=~-6.9) and the plasma temperatureis T_X_=0.29keV. Since A and B stars have no known magnetic activity,the only possibility is to invoke a wind origin for this emission. On IQAur, the expected mass loss rate driven by stellar radiation is small,around 10^-10^-10^-11^Msun_/yr withvinfinity_=800km/s, so that the kinetic energy flux of thewind is not much larger than the observed X-ray emission. This impliesthat the X-ray emission from IQ Aur must arise from a very efficientprocess. These constraints, together with the large observed magneticfield of IQ Aur, lead us to propose that the confinement of the wind bythe magnetic field leads to a collision from the wind components of thetwo hemispheres in the closed magnetosphere, leading to a strong shock.In this model the magnetic field confines the wind and also affects themass loss rate. We propose a self-consistent approach for the X-rayemission of IQ Aur, using radiatively driven wind models based on thestellar parameters of IQ Aur and including the effect of magneticconfinement of the wind. We also model the whole postshock region. Weshow that our shock model is able to satisfy the constraints on theobserved X-ray luminosity and temperature. The model also leads to theformation of a disk at the magnetic equator corotating up tor<~4R_*_. We show that ambipolar diffusion of hydrogen in the disk orcurrent sheet formation due to equipartition between the disk and themagnetic field might play a significant role in emptying themagnetosphere. We discuss the interplay between mass loss and particlediffusion. Our computations suggest that the onset of a wind on IQ Auris very recent, so that the abundance anomalies at the stellar surfacehave not yet been removed by the outflow, or that the wind exhibitstransient phases due to the mutual feedback between the wind andabundance anomalies. Finally, we point out that our wind-shock modelprovides a very convenient framework to explain the radio emission ofAp-Bp stars. It has been shown that the emission mechanism is opticallythick gyrosynchrotron, but instead of (or in addition to) the previouslyinvoked acceleration by magnetic reconnection in current sheets, wepropose that the electrons are accelerated by second-order Fermiacceleration mechanism ("stochastic" acceleration) often invoked for theacceleration of solar flare flare particles. We show that electronsaccelerated by the wind shock easily reach the required energies forradio emission in the GHz band throughout the magnetosphere. Given thesuccess of our model in explaining IQ Aur, we think that it has a fairlygeneral application to magnetic Ap-Bp stars. The wind-shock model alsoprovides a unified explanation for both the X-rays and the radioemission from these stars.
| Photoionization resonances of SiII in stellar spectra.
We have studied the influence of the Si^+^ photoionization on stellaratmospheres. Recent calculations using the R-matrix code providedetailed cross-sections with autoionization resonances for the first 50energy levels of Si^+^. These photoionization cross-sections areincluded in the calculation of the far ultraviolet synthetic spectrum ofA and late B-type stars. The opacity from the six lowest excited statesof Si^+^ provides observable features in the emergent spectrum of asolar-composition A0 star. The photoionization from higher states ofSi^+^ should be included when modeling hotter or silicon-rich stars.These new cross-sections increase dramatically the opacity in the far UVspectrum of Ap Si stars and allow to reproduce the most characteristicUV features of these stars, as illustrated by a comparison with IUE datafor the hot Ap Si star HD 34452. Broad features in its spectrum aredefinitely assigned to Si ii autoionization resonances, and the fluxdeficiency below 130nm is well matched with a predicted continuum edgenear this wavelength. The successful stellar spectrum synthesis brings astrong support to the R-matrix photoionization calculations, whichcannot otherwise be compared to laboratory data.
| The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Linear polarimetry of AP stars. V. A general catalogue of measurements.
A systematic program of broadband linear polarimetry, bearing on 55 Apstars, has been developed during the 4 last years, at the Pic du MidiObservatory. While separate data have been already published, we presentin this paper a complete catalogue of our observational material,including more than 400 measurements. We complement these data withanother 100 measurements, obtained previously by other authors, so as toget a synthetic view of the phenomenon. Most of the observations havebeen dedicated to a small number (15) of stars, which show conspicuouschanges of the linear polarization, so that it is possible to knowaccurately the time variation of the Stokes parameters: we expect thatthese new data will really improve our knowledge of the magneticconfiguration, after a proper analysis which is currently beingdeveloped. For the other 40 stars, the polarization is either too small,or strongly contaminated by the interstellar polarization, so thatbroadband polarimetry is not very effective. Anyway, this firstsystematic investigation on the linear polarization of Ap stars will bea useful starting point for future measurements which should be madewith higher spectral resolution. Finally, our measurements have providednew determinations of the rotation period for several stars.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
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| Takýmyýldýz: | Arabaci |
| Sað Açýklýk: | 05h19m00.00s |
| Yükselim: | +33°44'54.0" |
| Görünürdeki Parlaklýk: | 5.41 |
| Uzaklýk: | 137.363 parsek |
| özdevim Sað Açýklýk: | 15.1 |
| özdevim Yükselim: | -28.9 |
| B-T magnitude: | 5.154 |
| V-T magnitude: | 5.343 |
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