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The diagnosis of the mean quadratic magnetic field of Ap stars
Aims.We assess the validity of the method of determination of the meanquadratic field modulus and we explore its limits. Methods.Weanalyse high spectral resolution, high signal-to-noise spectra of a fewAp stars, and of a superficially normal main-sequence A star, recordedover a broad wavelength range with EMMI at the NTT.Results.Weintroduce a revised form of the regression equation describing thedependence of the second-order moment of the line profiles about theircentre, in natural light, on various parameters of the correspondingtransitions. We show that interpretation of the observed dependencesallows one to determine the mean quadratic magnetic field modulus of thestudied stars, and their v sin i. We explain why the contributions tothe quadratic field of the mean square magnetic field modulus and of themean square longitudinal field cannot in general be disentangled. Forthose stars of the sample that have resolved magnetically split lines,we show that the derived values of the quadratic field are mostlyconsistent with the values of the mean longitudinal magnetic field andof the mean magnetic field modulus at the observed phase. However thereare some hints that they may occasionally slightly underestimate theactual field. This suggests that the method is unlikely to yieldspurious field detections. In addition, we illustrate the importance forthis type of analyses of using, as far as possible, samples of lines ofa single ion, and to specify in the presentation of the results whichion was used.Conclusions.The results presented in this paper lendstrong support to the validity of the quadratic field diagnostic methodto obtain a realistic quantitative characterisation of the magneticfields Ap and related stars.
| Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Measurements of magnetic fields over the pulsation cycle in six roAp stars with FORS 1 at the VLT
With FORS 1 at the VLT we have tried for the first time to measure themagnetic field variation over the pulsation cycle in six roAp stars tobegin the study of how the magnetic field and pulsation interact. Forthe star HD 101065, which has one of the highest photometric pulsationamplitudes of any roAp star, we found a signal at the known photometricpulsation frequency at the 3σ level in one data set; however thiscould not be confirmed by later observations. A preliminary simplecalculation of the expected magnetic variations over the pulsation cyclesuggests that they are of the same order as our current noise levels,leading us to expect that further observations with increased S/N have agood chance of achieving an unequivocal detection.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes Nos. 69.D-0210 and 270.D-5023).
| 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 14.5 kG magnetic field in the NGC 2516 star HD 66318
We have been searching for magnetic Ap stars in open clusters, in orderto clarify the time evolution of magnetic fields in middle main sequencestars from the ZAMS to the TAMS. We have discovered that the star HD66318 in the open cluster NGC 2516 has an extraordinarily large magneticfield: the measured mean longitudinal component Bl ~ 4.5 kG,and the mean field modulus Bs ~ 14.5 kG. This star thus hasone of the largest fields so far discovered in a non-degenerate star,and the largest field known in a current Ap star cluster member.We estimate that HD 66318 has completed about 16 +/- 5% of its mainsequence life. It thus appears to contradict the hypothesis of Hubrig etal. that magnetic fields are only found in stars that have completed atleast 30% of their main sequence lifetimes.There is no indication that the spectrum or brightness of the star isvariable, and the spectral lines are very sharp. The star probably has avery long rotation period (years).We have modelled some parts of the observed spectrum, assuming that thechemical composition is uniform both horizontally and vertically, andusing a simple multipolar expansion for the field structure; althoughour model does not reproduce exactly the observed spectrum, it is clearthat the atmospheric chemical composition of the star is very peculiar,with Ti, Cr and Fe overabundant by between 1.5 and 2.5 dex. Both La IIand III are apparently about 4 dex overabundant. In contrast, evidencefor the presence of other rare earths is difficult to find in thespectrum. It appears that Ce III, Pr III, Nd II and III, and Eu II aredetected with inferred overabundances ranging between 1.5 and 5 dex, butfor most of these elements, the abundance of the non-detected ionizationstate is significantly lower than that inferred for the detected state.HD 66318 thus seems to exhibit strong discrepancies between abundancesdeduced for different ionization states of rare earths, a phenomenon sofar found only in somewhat cooler stars.Even within one ionization state it has not been found possible to fitmost observed lines with a single value of abundance. For example, whenwe fit medium strength lines of Fe II, the calculated strong lines ofthis ion are deeper than observed, and the calculated weak lines areless deep than observed. This situation is probably due to strongvertical abundance stratification of most of the elements studied.Finally, HD 66318 also shows a new form of core-wing anomaly in Hαin which the observed line profile falls below the computed one in thecore. These characteristics clearly suggest that the atmosphericstructure of HD 66318 is not closely similar to that of a normal mainsequence A star of similar parameters.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 68.D-0403 and DDT-269.D-05022.
| 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.
| Measuring magnetic fields of early-type stars with FORS1 at the VLT
This paper describes an experiment aimed at evaluating the capability ofthe FORS1 instrument at the VLT for measuring weak ( =~ 0.1%) Zeemancircular polarization signatures in stellar H Balmer lines. We haveobtained low-resolution polarized spectra at 3500-5800 Å of twobright A-type stars, HD 94660, and HD 96441. The former is a well knownmagnetic star, the latter an apparently non-magnetic, unpolarized starthat was observed for comparison purposes. In order to test thepossibility of performing multi-object spectropolarimetric measurements(e.g., for cluster studies), observations were taken both on-axis (i.e.,at the CCD field center) and off-axis (i.e., at the edges of the CCD).In HD 94660 (the magnetic star), we detected a clear signal of circularpolarization in all observed hydrogen Balmer lines (i.e., from Hβto the Balmer limit), with a typical peak-to-peak amplitude of about0.8%. From the analysis of Stokes V under the weak-field approximation,we have estimated a mean longitudinal field of -2085 +/- 85 G, a valuefully consistent with previous studies of this star. Notably, we foundthat at wavelengths shorter than about 4100 Å, the polarizationsignal detected off-axis is about 20-30% smaller than that detectedon-axis. No polarization due to the Zeeman effect was detected in thecomparison star HD 96441, and we estimate that the contribution due toinstrumental circular polarization is limited to the order of 0.01%.This experiment demonstrates the effectiveness of FORS1 at the VLT as atool for high-sensitivity diagnosis of the magnetic field in upper mainsequence stars, providing the potential for measuring fields in fainterand more rapidly rotating stars than has previously been possible. Basedon observations obtained at the VLT within the context of DDT266.D-5649.
| Detecting magnetic fields of upper-main-sequence stars with FORS1 at ANTU.
Not Available
| Magnetic AP Stars in the Hertzsprung-Russell Diagram
The evolutionary state of magnetic Ap stars is rediscussed using therecently released Hipparcos data. The distribution of the magnetic Apstars of mass below 3 Msolar in the H-R diagram differs fromthat of the normal stars in the same temperature range at a high levelof significance. Magnetic stars are concentrated toward the center ofthe main-sequence band. This is shown in two forms of the H-R diagram:one where logL is plotted against logTeff and a version moredirectly tied to the observed quantities, showing the astrometry-basedluminosity (Arenou & Luri) against the (B2-G)0 index ofGeneva photometry. In particular, it is found that magnetic fieldsappear only in stars that have already completed at least approximately30% of their main-sequence lifetime. No clear picture emerges as to thepossible evolution of the magnetic field across the main sequence. Hintsof some (loose) relations between magnetic field strength and otherstellar parameters are found: stars with shorter periods tend to havestronger fields, as do higher temperature and higher mass stars. Amarginal trend of the magnetic flux to be lower in more slowly rotatingstars may possibly be seen as suggesting a dynamo origin for the field.No correlation between the rotation period and the fraction of themain-sequence lifetime completed is observed, indicating that the slowrotation in these stars must already have been achieved before theybecame observably magnetic. Based on data from the ESA Hipparcossatellite and on observations collected at the European SouthernObservatory (La Silla, Chile; ESO programs Nos. 43.7-004, 44.7-012,49.7-030, 50.7-067, 51.7-041, 52.7-063, 53.7-028, 54.E-0416, and55.E-0751), at the Observatoire de Haute-Provence (Saint-Michell'Observatoire, France), at Kitt Peak National Observatory, and at theCanada-France-Hawaii Telescope.
| Magnetic models of slowly rotating magnetic Ap stars: aligned magnetic and rotation axes
As a result of major surveys carried out during the past decade byMathys and collaborators, we now have measurements with full phasecoverage of several magnetic field moments, including the meanlongitudinal field B_l, the mean field modulus B_s, and in most casesthe mean quadratic field B_mq and mean crossover field B_xover, for asample of 24 chemically peculiar magnetic (Ap) stars. This represents anincrease of a factor of order five in the stellar sample with data ofthis quality, compared to the situation a decade ago. We exploit thisdataset to derive general and statistical properties of the stars in thesample, as follows. First, we fit the available field momentobservations assuming a simple, axisymmetric multipole magnetic fieldexpansion (with dipole, quadrupole, and octupole components) over eachstellar surface. We show that this representation, though not exact,gives an adequate description of the available data for all the stars inthis sample, although the fit parameters are in many cases not unique.We find that many of the stars require an important quadrupole and/oroctupole field component to satisfy the observations, and that some(usually small) deviations from our assumed axisymmetric fielddistributions are certainly present. We examine the inclination i (0<= i <= 90o) of the rotation axis to the line of sightand the obliquity beta (0 <= beta <= 90o) of themagnetic field with respect to the rotation axis, and show that thestars with periods of the order of a month or longer have systematicallysmall values of beta : slowly rotating magnetic stars generally havetheir magnetic and rotation axes aligned to within about 20o,unlike the short period magnetic Ap stars, in which beta is usuallylarge. This is a qualitatively new result, and one which is veryimportant for efforts to understand the evolution of magnetic fields andangular momentum in the magnetic Ap stars.
| Loss of angular momentum of magnetic Ap stars in the pre-main sequence phase
A model for rotation evolution of an intermediate mass star with theprimordial magnetic field in the pre-main sequence (PMS) phase wasdeveloped. It takes into account the accretion of matter along themagnetic field lines, the stellar field-disk interaction and amagnetized wind. Variations of stellar moment of inertia were includedbased on evolutionary models of PMS evolution of such stars. Stellarmass and magnetic moment were assumed constant during the PMS evolution.Values of the parameters describing the strength of the magnetic field,accretion rate and mass loss rate were taken from observations. Inaddition, the life time of the disk was varied. An equation describingthe evolution of the rotation rate of a magnetic PMS star was derivedand solved for different stellar masses. The results indicate that theinteraction of the stellar ymagnetic field with circumstellarenvironment wipes out quickly a memory of the initial rotation period.The ZAMS period depends solely on the details of this interaction.Accretion spins up a star early in its PMS life and if the diskdisappears right after that the star may keep its faster rotation untilZAMS and appear there as a Be star. A wide variety of parametersdescribing the evolution of stellar AM results in typical ZAMS rotationperiods of magnetic stars several times longer than of normal stars.This agrees well with the observations. Under special circumstances astar can reach an exceptionally long rotation period of several years(up to 100 years). This requires a long PMS life time, an existence of adisk for only a part of the PMS phase and the wind in the strongmagnetic field existing for the rest of the PMS life. The observationsconfirm indeed that extremely slowly rotating Ap stars are lower massstars with strong magnetic fields.
| Why are magnetic AP stars slowly rotating?
Observational data on rotation of Ap stars suggest that the bulk oftheir rotation rates orm a separate Maxwellian distribution with anaverage value 3-4 times lower than the normal star distribution. Noevidences for a significant angular momentum (AM) loss on the mainsequence (MS) have been found. It is thus concluded that Ap stars mustlose a large fraction of their initial angular momentum (AM) in thepre-MS phase of evolution, most probably as a result of the interactionof their primordial magnetic fields with accretion disks and stellarwinds. The observationally most acceptable values of accretion rate fromthe disk, 10^(-8) M_(Sun)/year, of mass loss rate via a magnetized wind,10^(-8) M_(Sun)/year, and of the surface magnetic field, 1 kG on theZAMS, result in the AM loss in full agreement with observations. Thereexists a separate group of extremely slowly rotating Ap stars, withperiods of the order of 10-100 years. They are too numerous to come fromthe distribution describing the bulk of Ap stars. It is conjectured thattheir extremely low rotation rates are the result of additional AM losson the MS.
| The ROSAT all-sky survey catalogue of optically bright main-sequence stars and subgiant stars
We present X-ray data for all main-sequence and subgiant stars ofspectral types A, F, G, and K and luminosity classes IV and V listed inthe Bright Star Catalogue that have been detected as X-ray sources inthe ROSAT all-sky survey; several stars without luminosity class arealso included. The catalogue contains 980 entries yielding an averagedetection rate of 32 percent. In addition to count rates, sourcedetection parameters, hardness ratios, and X-ray fluxes we also listX-ray luminosities derived from Hipparcos parallaxes. The catalogue isalso available in electronic form via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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
| 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.
| 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
| Spectropolarimetric observations of active stars
This paper reports the results of five years (five runs, 23 nights) ofspectropolarimetric observations of active stars with the UCL EchelleSpectrograph of the Anglo-Australian Telescope. A total of 225circularly (and four linearly) polarized spectra were recorded on 28objects (21 active stars and seven calibration standards) using the newtechnique of Zeeman-Doppler imaging. To extract polarization echellespectra from raw frames, we developed a new dedicated automatic softwarepackage (called ESPRIT, which utilizes optimal extraction techniques)whose detailed description is given. For each recorded spectrum, weextract 'mean' polarized and unpolarized profiles using 'least-squaresdeconvolution', a technique similar to cross-correlation, which canenhance enormously the sensitivity of Zeeman-Doppler imaging, by up to7.5 mag in flux with respect to a single average line analysis. Magneticfield is detected unambiguously on 14 objects. Marginal field detectionsare also obtained for the weak-line T Tauri star HD 283572 and theHerbig Ae star HD 104237.
| Spectropolarimetry of magnetic stars. VI. Longitudinal field, crossover and quadratic field: New measurements
New determinations of the mean longitudinal magnetic field, of thecrossover, and of the mean quadratic magnetic field of Ap stars arepresented. They are based on spectra recorded simultaneously in bothcircular polarizations at ESO with the CASPEC spectrograph fed by the3.6 m telescope. This paper discusses 95 observations of 44 stars. Amajor result of this study is the discovery that HD 137509 has apredominantly quadrupolar magnetic field, a strucuture previously foundin only a couple of stars. Improvement or revision of the determinationof the rotation period has been achieved for 3 stars. The stars studiedin this work include 14 rapidly oscillating Ap stars (for 6 of which noprevious attempt to detect a magnetic field had ever been made) and 21Ap stars with spectral lines resolved into their magnetically splitcomponents when observed at high enough dispersion in unpolarized light(for 9 of these stars, no determination of the longitudinal field hadbeen performed before). The observations discussed in this paper havebeen performed between 1989 and 1994, a period during which CASPEC andits Zeeman analyzer have progressively undergone various configurationchanges. The results reported here demonstrate that the polarimetricperformance of the instrument has remained unaltered through thesemodifications. Thanks to the latter, the achieved resolving power wasincreased, which resulted in improved magnetic measurement accuracies.Based on observations collected at the European Southern Observatory (LaSilla, Chile; ESO programmes Nos. 47.7-045 and 49.7-029).
| The mean magnetic field modulus of AP stars
We present new measurements of the mean magnetic field modulus of asample of Ap stars with spectral lines resolved into magnetically splitcomponents. We report the discovery of 16 new stars having thisproperty. This brings the total number of such stars known to 42. Wehave performed more than 750 measurements of the mean field modulus of40 of these 42 stars, between May 1988 and August 1995. The best of themhave an estimated accuracy of 25 - 30 G. The availability of such alarge number of measurements allows us to discuss for the first time thedistribution of the field modulus intensities. A most intriguing resultis the apparent existence of a sharp cutoff at the low end of thisdistribution, since no star with a field modulus (averaged over therotation period) smaller than 2.8 kG has been found in this study. Formore than one third of the studied stars, enough field determinationswell distributed throughout the stellar rotation cycle have beenachieved to allow us to characterize at least to some extent thevariations of the field modulus. These variations are oftensignificantly anharmonic, and it is not unusual for their extrema not tocoincide in phase with the extrema of the longitudinal field (for thefew stars for which enough data exist about the latter). This, togetherwith considerations on the distribution of the relative amplitude ofvariation of the studied stars, supports the recently emerging evidencefor markedly non-dipolar geometry and fine structure of the magneticfields of most Ap stars. New or improved determinations of the rotationperiods of 9 Ap stars have been achieved from the analysis of thevariations of their mean magnetic field modulus. Tentative values of theperiod have been derived for 5 additional stars, and lower limits havebeen established for 10 stars. The shortest definite rotation period ofan Ap star with magnetically resolved lines is 3.4 deg, while thosestars that rotate slowest appear to have periods in excess of 70 or 75years. As a result of this study, the number of known Ap stars withrotation periods longer than 30 days is almost doubled. We brieflyrediscuss the slow-rotation tail of the period distribution of Ap stars.This study also yielded the discovery of radial velocity variations in 8stars. There seems to be a deficiency of binaries with short orbitalperiods among Ap stars with magnetically resolved lines. Based onobservations collected at the European Southern Observatory (La Silla,Chile; ESO programmes Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067,51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at theObservatoire de Haute-Provence (Saint-Michel-l'Observatoire, France), atKitt Peak National Observatory, and at the Canada-France-HawaiiTelescope. Tables 2, 3, and 4 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.
| Radial velocities and axial rotation for a sample of chemically peculiar stars.
As part of a systematic project we have determined radial velocities andprojected rotational velocities for a sample of 186 chemically peculiarstars which have been observed by the Hipparcos' satellite. The purposeis to provide necessary data to study the space velocities of peculiarstars.
| Spectropolarimetry of magnetic stars. V. The mean quadratic magnetic field.
Systematic determinations of the mean quadratic magnetic field of Apstars have been performed for the first time. The mean quadraticmagnetic field (or, in short, the quadratic field) is the square root ofthe sum of the mean square magnetic field modulus and of the mean squarelongitudinal magnetic field. The latter are averages over the visiblestellar disk of the square of, respectively, the modulus of the magneticfield and its component along the line of sight. These averages areweighted by the local emergent line intensities. The quadratic field isdiagnosed from the study of the magnetic broadening of the stellarspectral lines as observed in unpolarized light, through thecharacterization of the widths of the lines by the second-order momentsof their profiles (in the Stokes parameter I) about their centre. Thetheoretical basis of the interpretation of these moments in terms ofmagnetic field and the strategy followed in the analysis are presented.It is shown that this analysis yields, as a by-product, the projectedequatorial velocity v_e_sini of the studied stars. Observations of asample of 29 stars are analyzed. For 22 of them, meaningful values orupper limits of the quadratic field can be determined. The lower limitof detection of the quadratic fields, set by the spectral resolution ofthe observations, is of the order of 5 kG. The observed quadratic fieldsrange from this value up to 37 kG, in the star HD 137509. The magneticfield of this star is likely the second strongest known field in Apstars. Quadratic field values derived for stars where resolvedmagnetically split lines are observed in higher-dispersion spectra areconsistent with the values of the mean field modulus measured in thosestars from the line splitting. For the stars of the sample repeatedlyobserved through their rotation cycle, the variations of the quadraticfield are well represented by a cosine with the rotation frequency ofthe star, or by the superposition of such a cosine and of a cosine withtwice that frequency. However, it appears that it is essential to have alarge number of observations distributed sufficiently uniformly andsufficiently densely over the rotation phases to determine unambiguouslythe shape of the variations. The extrema of the quadratic field tend tooccur at phases close to those of the extrema of the longitudinal field,but in some stars, the two quantities definitely vary out of phase. Theratio between the maximum and the minimum of the quadratic field isalways smaller than 1.7.
| Spectropolarimetry of magnetic stars. IV. The crossover effect.
This paper is devoted to the study of the crossover effect in magneticAp stars. It is shown that this effect can be measured by the secondorder moment about their centre of the profiles of spectral linesrecorded in the Stokes parameter V. The interpretation of thesemeasurements in terms of magnetic field is developed. It is shown thatone can derive from them a quantity called the mean asymmetry of thelongitudinal magnetic field, which is the first moment of the componentof the magnetic field along the line of sight, about the plane definedby the line of sight and the stellar rotation axis. The consistency ofthe determination of this quantity with that of the mean longitudinalmagnetic field from measurements of wavelength shifts of lines betweenright and left circular polarization is demonstrated. This technique ofanalysis is applied to observations of a sample of 29 stars, among which10 have a detectable crossover effect. For 8 of them, the availableobservational data allow the study of the variations of the asymmetry ofthe longitudinal field with rotation phase. In most cases, thisvariation is sinusoidal and essentially symmetric about 0, and it occursin quadrature with the variation of the mean longitudinal field. A morecomplex behaviour is definitely observed in HD 147010 and HD 175362,where the variation of the asymmetry of the longitudinal field is betterrepresented by the superposition of two sinusoids, one with the rotationfrequency of the star, and the other with twice that frequency.
| The Cape rapidly oscillating AP star survey - III. Null results of searches for high-overtone pulsation.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.271..129M&db_key=AST
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| Some problems of non-reversive CP stars. I. Spatial distribution.
Not Available
| Spectropolarimetry of magnetic stars. III. Measurement uncertainties.
The estimation of the uncertainties affecting the determination ofvarious parameters characterizing spectral line profiles recorded inStokes I and V that are used in the diagnosis of stellar magnetic fieldsthrough the moment technique is described. Simple statisticalconsiderations are presented, to demonstrate that the errors areproperly assessed and to illustrate how their evaluation can beexploited to derive information about the physical properties of thestudied stars. It is shown that taking these uncertainties properly intoaccount allows one to achieve better determination of the meanlongitudinal magnetic field. Improved values of the latter are derivedfor observations already analysed in earlier work. The approach sketchedin this paper is intended as a basis for future studies of magneticfields of Ap stars.
| Longterm Photometry of Variables at ESO - Part Two - the Second Data Catalogue 1986-1990
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102...79S&db_key=AST
| A study of magnetic fields in AP SI and He weak stars
A survey of 44 Ap and Bp Si and He-weak stars for the presence ofdetectable longitudinal magnetic fields (Be) is reported. Fields havebeen detected in about half of the sample, with typical standard errorsin the range of 100-400 G. In 11 stars, fields have been detected forthe first time. For most of the stars with detected fields, thevariation of Be with rotational phase has been determined at leastapproximately. In a number of cases the phase relationship withphotometric or spectroscopic variations have also been established.Several of the newly detected fields are relatively large, with rmsvalues between 1 and 4 kG, and thus may be especially suitable fordetailed mapping and modelling. A particularly interesting result ofthese observations is the identification of at least four stars in whichextrema of the photometric or spectroscopic variations do not coincidewith the magnetic extrema, which implies that the distribution ofchemical elements over the stellar surface that leads to photometricvariations is definitely not symmetric about the magnetic axis of thestar.
| AP stars with resolved magnetically split lines
New high-resolution spectroscopic observations of Ap stars with resolvedmagnetically split spectral lines are presented. Six new such stars havebeen discovered: HD 2453, HD 9996, HD 18078, HD 50169, HD 137949, and HD192678. This raises the total number of known Ap stars with resolvedmagnetically split lines to 21. The spectral line profiles observed intwo more stars, HD 111133 and HD 147010, are strongly affected by themagnetic field, but they are not fully resolved, due to nonnegligiblerotational Doppler effect. Observations of fairly sharp-lined Ap starswhose lines are unresolved are also presented. The relative differencebetween the equivalent widths of the Fe II lines 6147.7 A and 6149.2 Ais shown to increase with the mean magnetic field modulus. The existenceof a simple, unambiguous correlation between the mean magnetic fieldmodulus and the index Z of Geneva photometry, which had previously beenadvocated, is not confirmed. At most, stars with large negative valuesof Z may be more prone to have strong magnetic fields, but the evidencefor this is not statistically strong.
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| Deklináció: | -42°15'04.0" |
| Vizuális fényesség: | 6.11 |
| Távolság: | 151.976 parszek |
| RA sajátmozgás: | -40.9 |
| Dec sajátmozgás: | -2 |
| B-T magnitude: | 6.007 |
| V-T magnitude: | 6.072 |
Katalógusok és elnevezések:
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