Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Magnetic field model for slowly rotating CP-stars. γEqu= HD201601
A magnetic field model is constructed for the extremely slow rotatorγEqu based on measurements of its magnetic field over many yearsand using the “magnetic charge” method. An analysis ofγEqu and of all the data accumulated up to the present on themagnetic field parameters of chemically peculiar stars leads to someinteresting conclusions, of which the main ones are: the fact that theaxis of rotation and the dipole axis are not parallel in γEqu andthe other slowly rotating magnetic stars which we have studiedpreviously is one of the signs that the braking of CP stars does notinvolve the participation of the magnetic field as they evolve “tothe main sequence.” The axes of the magnetic field dipole in slowrotators are oriented arbitrarily with respect to their axes ofrotation. The substantial photometric activity of these CP stars alsoargues against these axes being close. The well-known absence ofsufficiently strong magnetic fields in the Ae/Be Herbig stars alsopresents difficulties for the hypothesis of “magneticbraking” in the “pre-main sequence” stages ofevolution. The inverse relation between the average surface magneticfield Bs and the rotation period P is yet another fact in conflict withthe idea that the magnetic field is involved in the braking of CP stars.We believe that angular momentum loss involving the magnetic field canhardly have taken place during evolution immediately prior “to themain sequence,” rather the slow rotation of CP stars most likelyoriginates from protostellar clouds with low angular momentum. Some ofthe slowly rotating stars have a central dipole magnetic fieldconfiguration, while others have a displaced dipole configuration, wherethe displacement can be toward the positive or the negative magneticpole.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| 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 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.
| The influence of the magnetic field orientation on the angular momentum loss in the pre-main sequence phase: The case of very slowly rotating magnetic Ap stars
Landstreet & Mathys (\cite{lan00}) have discovered that almost allthe magnetic Ap stars having rotation periods longer than about onemonth have their magnetic and rotation axes fairly closely aligned, incontrast to the more common magnetic Ap stars of shorter period, inwhich the two axes are usually inclined to one another at a large angle.Furthermore, as shown earlier by Mathys et al. (\cite{mathys}) and byHubrig et al. (\cite{hnm00}), these most slowly rotating magnetic Apstars have also magnetic fields several times larger than are typicalfor shorter period magnetic Ap stars, and generally have masses below 3Msun. In this paper, we present calculations to show that theobserved axial alignment in the slowest rotators may have arisen duringthe pre-main sequence (PMS) phase, when stars of a few solar masses areobserved as Herbig Ae/Be stars. During the PMS phase, a star is expectedto exchange angular momentum with a disk and with a stellar wind. Asshown by Stȩpień (\cite{ste00}), a disk around a PMSmagnetic Ap star tends to lock the rotation rate at a few days, whilemass loss in the absence of a disk is able to slow the star to quitelong periods. We argue that is it plausible to assume that the fractionof the PMS phase during which a disk persists depends on the orientationof the field axis to the plane of the disk, and that disk dissipationmay have occurred more quickly in magnetic stars with aligned magneticand rotation axes than in stars in which the angle between these axes islarge. In this case, the magnetic Ap stars with aligned rotation andmagnetic axes, especially those with large fields, could lose much moreangular momentum than the stars of large obliquity. However, it is alsoshown that loss of nearly all the angular momentum is only possible forstars with masses below about 3 Msun (and with relativelylarge fields); for higher masses the available time is too short. Ourmodel thus provides a reasonable explanation for all of the principleobservational facts about the very slowly rotating magnetic Ap stars.
| 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.
| 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 HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.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.
| 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.
| A list of candidates for high-velocity AP stars
'Reduced proper motion' is used to select from a catalog of over fifteenhundred Ap stars those that probably have a high velocity. A list of 164candidates is presented, 22 of them having very probably high velocity.Some of them could be the first specimens of late Ap stars with highvelocity. The possible relation to horizontal branch stars is alsodiscussed.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Carina |
| Ascensió Recta: | 10h45m50.63s |
| Declinació: | -68°07'49.6" |
| Magnitud Aparent: | 8.439 |
| Distancia: | 341.297 parsecs |
| Moviment propi RA: | -12.3 |
| Moviment propi Dec: | -0.9 |
| B-T magnitude: | 8.485 |
| V-T magnitude: | 8.443 |
Catàlegs i designacions:
|
