Contents
Images
Upload your image
DSS Images Other Images
Related articles
Stable magnetic fields in stellar interiors
We investigate the 50-year old hypothesis that the magnetic fields ofthe Ap stars are stable equilibria that have survived in these starssince their formation. With numerical simulations we find that stablemagnetic field configurations indeed appear to exist under theconditions in the radiative interior of a star. Confirming a hypothesisby Prendergast (1956, ApJ, 123, 498), the configurations have roughlyequal poloidal and toroidal field strengths. We find that tori of suchtwisted fields can form as remnants of the decay of an unstable randominitial field. In agreement with observations, the appearance at thesurface is an approximate dipole with smaller contributions from highermultipoles, and the surface field strength can increase with the age ofthe star. The results of this paper were summarised by Braithwaite &Spruit (2004, Nature, 431, 891).
| Simulations of Core Convection in Rotating A-Type Stars: Magnetic Dynamo Action
Core convection and dynamo activity deep within rotating A-type stars of2 Msolar are studied with three-dimensional nonlinearsimulations. Our modeling considers the inner 30% by radius of suchstars, thus capturing within a spherical domain the convective core anda modest portion of the surrounding radiative envelope. Themagnetohydrodynamic (MHD) equations are solved using the anelasticspherical harmonic (ASH) code to examine turbulent flows and magneticfields, both of which exhibit intricate time dependence. By introducingsmall seed magnetic fields into our progenitor hydrodynamic modelsrotating at 1 and 4 times the solar rate, we assess here how thevigorous convection can amplify those fields and sustain them againstohmic decay. Dynamo action is indeed realized, ultimately yieldingmagnetic fields that possess energy densities comparable to that of theflows. Such magnetism reduces the differential rotation obtained in theprogenitors, partly by Maxwell stresses that transport angular momentumpoleward and oppose the Reynolds stresses in the latitudinal balance. Incontrast, in the radial direction we find that the Maxwell and Reynoldsstresses may act together to transport angular momentum. The centralcolumns of slow rotation established in the progenitors are weakened,with the differential rotation waxing and waning in strength as thesimulations evolve. We assess the morphology of the flows and magneticfields, their complex temporal variations, and the manner in whichdynamo action is sustained. Differential rotation and helical convectionare both found to play roles in giving rise to the magnetic fields. Themagnetism is dominated by strong fluctuating fields throughout the core,with the axisymmetric (mean) fields there relatively weak. Thefluctuating magnetic fields decrease rapidly with radius in the regionof overshooting, and the mean toroidal fields less so due to stretchingby rotational shear.
| Model of the Magnetic Field of HD 187474
A model is constructed for the magnetic field of the star HD 187474,which has a very long axial rotation period P = 2345d. It turns out thatthe structure of the magnetic field is best described by a model of adisplaced (Δα = 0.1) dipole inclined to the axis of rotationby an angle β = 24°. The star is inclined to the line of sightby an angle i = 86°. Because of the displaced dipole the magnitudeof the magnetic field differs at the poles: Bp = +6300 and 11600 G. AMercator map of the distribution of the magnetic field over the surfaceis obtained. The 7 slowly rotating CP stars studied thus far have anaverage angle β = 62°, which equals the average value for arandom orientation of dipoles.
| 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
| Probable detection of radial magnetic field gradients in the atmospheres of Ap stars
For the first time the possible presence of radial gradients of magneticfields in the atmospheres of three magnetic Ap stars has been criticallyexamined by measurements of the mean magnetic field modulus fromspectral lines resolved into magnetically split components lying on thedifferent sides of the Balmer jump. A number of useful diagnostic linesbelow and above the Balmer discontinuity, only slightly affected byblends, with simple doublet and triplet Zeeman pattern have beenidentified from the comparison between synthetic spectra computed withthe SYNTHMAG code and the high resolution and S/N spectra obtained inunpolarized light with the ESO VLT UVES spectrograph. For all threestars of our sample, HD 965, HD 116114 and 33 Lib, an increase of themagnetic field strength of the order of a few hundred Gauss has beendetected bluewards of the Balmer discontinuity. These results should betaken into account in future modelling of the geometric structure of Apstar magnetic fields and the determination of the chemical abundances inAp stars with strong magnetic fields.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO program No. 70.D-0470).
| Stellar activity and magnetism studied by optical interferometry
By means of numerical simulations, we investigate the ability of opticalinterferometry, via the fringe phase observable, to address stellaractivity and magnetism. To derive abundance maps and stellar rotationaxes, we use color differential interferometry which couples highangular resolution to high spectral resolution. To constrain magneticfield topologies, we add to this spectro-interferometer a polarimetricmode. Two cases of well-known Chemically Peculiar (CP) stars (βCrBand α2CVn) are simulated to derive instrumentalrequirements to obtain 2D-maps of abundance inhomogeneities and magneticfields. We conclude that the near-infrared instrument AMBER of the VLTIwill allow us to locate abundance inhomogeneities of CP stars largerthan a fraction of milliarcsecond whereas the polarimetric mode of theFrench GI2T/REGAIN interferometer would permit one to disentanglevarious magnetic field topologies on CP stars. We emphasize the crucialneed for developing and validating inversion algorithms so that futureinstruments on optical aperture synthesis arrays can be optimally used.
| No evidence of a strong magnetic field in the Blazhko star RR Lyrae
In this paper we report a new series of high-precision (medianσB 80 G) longitudinal magnetic field measurementsof RR Lyrae, the brightest Blazhko star, obtained with the MuSiCoSspectropolarimeter over a period of almost 4 years from 1999-2002. Thesedata provide no evidence whatsoever for a strong magnetic field in thephotosphere of RR Lyrae, a result consistent with Preston's(\cite{Preston67}) results, but inconsistent with apparent magneticfield detections by Babcock (\cite{Babcock58}) and Romanov et al.(\cite{Romanov87}, \cite{Romanov94}). Following discussion of thesedisparate results, we conclude that RR Lyrae is a {bona fide}non-magnetic star, a conclusion which leads to the general falsificationof models of the Blazkho effect requiring strong photospheric magneticfields. Furthermore, due to the 4 year baseline of our observations, wecan also dismiss the hypothesis that RR Lyrae undergoes a 4-year``magnetic cycle''. Therefore the origin of the observed 4-yearmodulation of RR Lyrae remains unexplained, and we stress the need foradditional theoretical efforts to come to a better understanding of thisphenomenon.Based on observations obtained with the MuSiCoS spectropolarimeterattached to the 2 m Bernard-Lyot Telescope of the Pic du MidiObservatory, France.
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| Abundance Analysis of the Silicon Star HR 6958
The elemental composition of the chemically peculiar star HR6958 hasbeen studied with emphasis on doubly ionized rare earths. A visualregion spectrum taken with the ELODIE spectrograph at the Haute-ProvenceObservatory was analyzed. A total of 40 elements including Pr III , Nd III , Tb III , Dy III , Ho III , and Er III were identified andtheir abundances computed. He is deficient by over 1dex with respect tothe Sun; the light elements (C-Ca), except for Si, have solarabundances; the iron group elements (Sc-Fe) are overabundant by 1 dex to2dex, with Ti and Cr highly overabundant; and the lanthanide rare earthsare overabundant by 3dex to 4dex. This abundance pattern with Hedeficient, Si, Ti, Cr, Sr, and Pr overabundant indicates that HR6958 isa member of Si stars.
| SCUBA observations of dust around Lindroos stars: evidence for a substantial submillimetre disc population
We have observed 22 young stars from the Lindroos sample at 850 μmwith SCUBA on the James Clerk Maxwell Telescope to search for evidenceof dust discs. Stars in this sample are the less massive companions ofB-type primaries and have well-defined ages that are 10- 170 Myr; i.e.they are about to, or have recently arrived on the main sequence. Dustwas detected around three of these stars (HD 112412, 74067 and 99803B).The emission around HD 74067 is centrally peaked and is approximatelysymmetrically distributed out to ~70 arcsec from the star. This emissionarises from either a two-component disc, one circumstellar and the othercircumbinary with dust masses of 0.3 and <27 M⊕,respectively, or an unrelated background object. The other twodetections we attribute to circumsecondary discs with masses of 0.04 and0.3 M⊕. We were also able to show that a circumprimarydisc is present around HD 112413 with a similar mass to that around thecompanion HD 112412. Cross-correlation of our sample with the IRAScatalogues only showed evidence for dust emission at 25 and 60 μmtoward one star (HD 1438); none of the submillimetre detections wereevident in the far-infrared data implying that these discs are cold(>40 K assuming β= 1). Our submillimetre detections are some ofthe first of dust discs surrounding evolved stars that were not detectedby IRAS or ISO and imply that 9-14 per cent of stars could harbourpreviously undetected dust discs that await discovery in unbiased sub-mmsurveys. If these discs are protoplanetary remnants, rather thansecondary debris discs, dust lifetime arguments show that they must bedevoid of small <0.1 mm grains. Thus it may be possible to determinethe origin of these discs from their spectral energy distributions. Thelow inferred dust masses for this sample support the picture thatprotoplanetary dust discs are depleted to the levels of the brightestdebris discs (~1 M⊕) within 10 Myr, although if theextended emission of HD 74067 is associated with the star, this wouldindicate that >10 M⊕ of circumbinary material canpersist until ~60 Myr and would also support the theory that T Tauridiscs in binary systems are replenished by circumbinary envelopes.
| Implementing TOPbase/Iron Project: continuous absorption from FeII
We discuss implementation of TOPbase and Iron Project opacities forstellar spectral codes. We use a technique employed by Peach, where aBoltzmann-averaged cross-section is calculated for selectedtemperatures, and the opacity obtained from double interpolation intemperature and wavelength. It is straightforward to include all levelsfor which cross-sections have been calculated. Boltzmann-averagedcross-sections for FeII show a local maximum between 1700 and 2000Å. We suggest this feature arises from 3d54snl->3d54pnl transitions within FeII. IUE spectra of iron-rich CPstars show local minima in this region. Theoretical calculations of arepresentative stellar continuum demonstrate that FeII photoionizationcontributes significantly to the observed minima.
| Radiative lifetime, oscillator strength and Landé factor calculations in doubly ionized europium (Eu III)
A set of transition probabilities has been calculated for Eu IIItransitions of astrophysical interest particularly for the study ofchemically peculiar stars. They were obtained taking configurationinteraction and core-polarization effects into account. The accuracy ofthe new scale of oscillator strengths, which differs substantially fromprevious results, has been assessed through comparisons with recenttime-resolved laser-induced fluorescence measurements of radiativelifetimes.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| 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.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. V. NGC 2169
We present results of a search for variable stars in the field of theyoung open cluster NGC 2169. The General Catalogue of Variable Stars(GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variablestars in the field we observed, viz., two beta Cep stars, V 916 and V917Ori, an alpha^2CVn variable, V 1356Ori, and an RRc variable, V1154Ori. We find V 916 and V 1154Ori to be constant in light. We confirmthe variability of V 917Ori, but not the period given in GCVS. For thechemically peculiar A0 V Si star V 1356Ori we definitely establish theperiod of 1.565 d, thus settling the uncertainty persisting in theliterature since the star was discovered to be variable. In addition, wefind two other stars to be variable in light. Both show irregularvariations.For V 917Ori, one of the two GCVS beta Cep variables, we determine aperiod of 0.267 d (frequency 3.7477d^-1). However, prewhitening withthis period leaves a significant amount of the star's light-variationunaccounted for. Since the star shows emission at Hα, wehypothesize that the unaccounted for variation is caused by an erratic,Be-type activity. As to the periodic term, we consider three hypotheses:(1) beta Cep-type pulsation, (2) rotational modulation of the lambda Eritype, and (3) ellipsoidal variation due to distorted primary componentin a close binary system. After deriving the star's effectivetemperature from Stromgren indices and the luminosity from the distancemodulus of the cluster, we show that while the third hypothesis isuntenable, the first two should be retained. However, neither isentirely satisfactory.For a number of stars we provide the V magnitudes. For 14 brighteststars in our field we also obtain the photometric alpha-index, a measureof the Hα equivalent width. From the alpha index, we detect mildemission at Hα in two stars, V 917Ori and NGC 2169-8.
| 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
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| Experimental and theoretical studies of DyIII: radiative lifetimes and oscillator strengths of astrophysical interest
The lifetimes of three short-lived levels belonging to the4f96p configuration and of two long-lived levels of the4f95d configuration of DyIII have been measured for the firsttime using time-resolved laser-induced fluorescence techniques. They arein good agreement with theoretical values calculated within theframework of a pseudo-relativistic configuration interactionapproximation. Using the experimental lifetimes and the theoreticalbranching fractions, a first set of transition probabilities ofastrophysical interest has been obtained for DyIII.
| 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.
| New Magnetic Chemically Peculiar Stars
Not Available
| Eu III Oscillator Strengths and Europium Abundances in Ap Stars
Not Available
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| Doppler imaging of stellar surfaces - techniques and issues
The development of Doppler imaging has allowed us to observe stellaractivity on the surface of stars other than the Sun for over a decade.We are now in a position to compare activity on rapidly rotating TTauri, RS CVn and young main-sequence stars and to compare the activityon those, in turn, with that of the Sun. The images produced show somestartling differences between stellar and solar activity. The strengthsand weakness of the Doppler imaging technique must be reviewed regularlyto remind us of what observed features are reliable and of when weshould have doubts. This review is a general survey of the techniqueemphasizing the issue of testing and of potential artifacts withoutattempting excessive detail on variations in application or results. Thedifficulties faced as the technique is extended from images of surfacetemperature or abundance to images that include magnetic information arebriefly surveyed.
| Magnetic survey of bright northern main sequence stars
The first results of a systematic search for magnetic fields in thebrightest upper main sequence (MS) stars are presented. The main goal isto survey the stars with about the same detection limit and to improveexisting statistics of their magnetism. The target list contains 57upper MS stars and represents well B0.5-F9 stars. High-resolution Zeemanspectra were obtained for 30 stars of the list. The accuracy of themagnetic field measurements ranges from 20 to 300 G depending mainly onspectral class. In the majority of studied stars we did not detectmagnetic fields. In some stars we suspect the presence of a weakmagnetic field. These are the best candidates for more extensivestudies. A particular case is the star chi Dra where we probablydetected the global magnetic field. The longitudinal field strength isB_l= -54+/-12 G. Further observations of this star are needed to confirmthe detection and to ascertain if the magnetic field is variable withthe period of rotation. Based on observations collected at the 1 mtelescope of the Special Astrophysical Observatory (Nizhnij Arkhyz,Russia).
| A statistical analysis of the magnetic structure of CP stars
We present the results of a statistical study of the magnetic structureof upper main sequence chemically peculiar stars. We have modelled asample of 34 stars, assuming that the magnetic morphology is describedby the superposition of a dipole and a quadrupole field, arbitrarilyoriented. In order to interpret the modelling results, we haveintroduced a novel set of angles that provides one with a convenient wayto represent the mutual orientation of the quadrupolar component, thedipolar component, and the rotation axis. Some of our results aresimilar to what has already been found in previous studies, e.g., thatthe inclination of the dipole axis to the rotation axis is usually largefor short-period stars and small for long-period ones - see Landstreet& Mathys (\cite{Landstreet2000}). We also found that forshort-period stars (approximately P<10 days) the plane containing thetwo unit vectors that characterise the quadrupole is almost coincidentwith the plane containing the stellar rotation axis and the dipole axis.Long-period stars seem to be preferentially characterised by aquadrupole orientation such that the planes just mentioned areperpendicular. There is also some loose indication of a continuoustransition between the two classes of stars with increasing rotationalperiod.
| Transition probabilities in Gd III
Theoretical lifetimes, calculated with inclusion of core-polarizationeffects, have been determined for five 4f76p levels of doublyionized gadolinium. They agree quite well with recent experimentalvalues measured by time-resolved laser-induced fluorescencespectroscopy. From this agreement, the accuracy of a first set of Gd IIItransition probabilities, calculated for4f75d-4f76p and 4f76s-4f76ptransitions of astrophysical interest, has been assessed.
| No magnetic field variations with pulsation phase in the classical Cepheid star eta Aquilae
We report new high resolution Stokes V spectropolarimetry and ultra-highprecision longitudinal magnetic field measurements of the pulsatingclassical Cepheid eta Aql which fail to corroborate the report byPlachinda (\cite{Plachinda}) of a ~ 100 G magnetic field in this star.Based on observations obtained using the MuSiCoS spectropolarimeter atPic du Midi observatory.
| Doppler Imaging of stellar magnetic fields. III. Abundance distribution and magnetic field geometry of alpha 2 CVn
We used the new magnetic Doppler Imaging code to reconstruct themagnetic field geometry and surface abundance distributions for theclassical magnetic CP star alpha 2 CVn. High-resolutionspectropolarimetric observations in the Stokes I and V parameters werecollected with the SOFIN échelle spectrograph at the NordicOptical Telescope. This superb observational material in combinationwith the advanced modelling technique allowed to achieve the firstsimultaneous self-consistent mapping of the vector magnetic field andthe abundance distributions of six chemical elements. In recovering thestellar magnetic distribution no prior assumptions about the fieldgeometry or strength were made. Instead, we restricted possiblesolutions of the inverse problem by means of the multipolarregularization method which searches for the field map close to ageneral non-axisymmetric multipolar configuration but allows departuresfrom this geometry if that is required by the observational data. Wefound that the magnetic field of alpha 2 CVn/ is dominated bya dipolar component and has a minor quadrupole contribution. The surfacedistributions of the chemical species form symmetric patterns whichclosely follow the magnetic geometry. This discovery constitutes one ofthe first direct observational constraints on the horizontal diffusionprocesses acting in the upper envelope of a strongly magnetized stellaratmosphere. In addition to the extensive magnetic Doppler Imaginganalysis, we derived new accurate estimates of the atmosphericparameters and basic physical properties of alpha 2 CVn/using the energy distribution, hydrogen line profiles and the recentHipparchos parallax. Based on observations obtained with the NordicOptical Telescope (NOT) and Isaac Newton Telescope (INT), La Palma,Spain.
| The laboratory analysis of Bi II and its application to the Bi-rich HgMn star HR 7775
The bismuth spectrum emitted from a hollow cathode discharge has beenrecorded with a Fourier Transform Spectrometer (FTS). Accuratewavelengths have been determined for 104 Bi Ii lines, and several newenergy levels have been found, while the accuracy of previously known BiIi level energies have been improved. The hyperfine structure of allobserved Bi Ii lines has been analyzed, yielding hyperfine constants Aand B for 56 Bi Ii levels. With the aid of the laboratory measurementsthe optical region spectrum of the HgMn star HR 7775 has been studiedfor all observable Bi Ii lines. The wavelengths and hfs constantsestablished from the laboratory work have been combined with theoreticalgf values to identify spectral lines and make an abundance estimation ofbismuth. It has been established that bismuth is present in HR 7775 atan enhancement level of approximately 5 orders of magnitude relative tothe meteoritic abundance, consistent with previous observations in theultraviolet region of this star. Astrophysical gf values are presentedfor a number of Bi Ii lines.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | りょうけん座 |
| Right ascension: | 12h56m01.70s |
| Declination: | +38°19'06.0" |
| Apparent magnitude: | 2.9 |
| Distance: | 33.784 parsecs |
| Proper motion RA: | 0 |
| Proper motion Dec: | 0 |
| B-T magnitude: | 2.788 |
| V-T magnitude: | 2.845 |
Catalogs and designations:
|
