Magnetic Fields Of Ap Stars Research Articles

Ap stars with resolved magnetically split lines: Magnetic field determinations from StokesIandVspectra

We present the results of a systematic study of the magnetic fields and other properties of the Ap stars with resolved magnetically split lines. This study is based on new measurements of the mean magnetic field modulus, the mean longitudinal magnetic field, the crossover, the mean quadratic magnetic field, and the radial velocity of 43 stars, complemented by magnetic data from the literature for 41 additional stars. Stars with resolved magnetically split lines represent a significant fraction, of the order of several percent, of the whole population of Ap stars. Most of them are genuine slow rotators, whose consideration provides new insight into the long-period tail of the distribution of the periods of the Ap stars. Emerging correlations between rotation periods and magnetic properties provide important clues for the understanding of the braking mechanisms that have been at play in the early stages of stellar evolution. The geometrical structures of the magnetic fields of Ap stars with magnetically resolved lines appear in general to depart slightly, but not extremely, from centred dipoles. However, there are a few remarkable exceptions, which deserve further consideration. We suggest that pulsational crossover can be observed in some stars; if confirmed, this would open the door to the study of non-radial pulsation modes of degree $\ell$ too high for photometric or spectroscopic observations. How the lack of short orbital periods among binaries containing an Ap component with magnetically resolved lines is related with their (extremely) slow rotation remains to be fully understood, but the very existence of acorrelation between the two periods lends support to the merger scenario for the origin of Ap stars.

Measuring the surface magnetic fields of magnetic stars with unresolved Zeeman splitting

High-dispersion, archival spectra of magnetic Ap stars with resolved Zeeman components in Stokes I are used to derive a simple relation that can be utilised to estimate the mean surface field strengths of stars with v sin i > 10 km/s. For each star, the mean surface field, as measured from the observed splitting in Fe II at 6149 A, is compared to the differential broadening of spectral lines with large and small Lande factors in order to produce a relation to estimate the field strengths of magnetic stars with unresolved Zeeman patterns. The method is shown to be reliable for rotational velocities up to about 50 km/s for field strengths down to about 5 kG. These results should allow for better constraints to be placed on the mean surface magnetic fields of Ap stars where Zeeman patterns are unresolved.

Magnetic fields of Ap stars from full Stokes vector spectropolarimetric observations

Current knowledge about stellar magnetic fields relies almost entirely on circular polarization observations, with very few objects having been observed in all four Stokes parameters. We are investigating a sample of Ap stars in all four Stokes parameters using the HARPSpol instrument at the 3.6-m ESO telescope. In the context of this project we recently observed the magnetic Ap star HD 24712 (DO Eri, HR 1217). The resulting spectra have dense phase coverage, resolving power > 105, and S/N ratio of 300–600. These are the highest quality full Stokes observations obtained for any star other than the Sun. Furthermore, we have achieved good phase coverage for HD 125248 and HD 119419. Typical four Stokes parameters HARPSpol spectra are shown in Fig. 1. An analysis of the full Stokes vector spectropolarimetric data set of HD 24712 has been published in Rusomarov et al. (2013).

Magnetic fields of Ap stars as a result of the Tayler instability

AbstractAp star magnetism is often attributed to fossil magnetic fields which have not changed much since the pre‐main‐sequence epoch of the stars. Stable magnetic field configurations are known which could persist probably for the entire mainsequence life of the star, but they may not show the complexity and diversity exhibited by the Ap stars observed. We suggest that the Ap star magnetism is not a result of stable configurations, but is the result of an instability based on strong toroidal magnetic fields buried in the stars. The highly nonaxisymmetric remainders of the instability are reminiscent of the diversity of fields seen on Ap stars. The strengths of these remnant magnetic fields are actually between a few per cent up to considerable fractions of the internal toroidal field; this means field strengths of the order of kGauss being compatible with what is observed. The magnetic fields emerge at the surface rather quickly; rough estimates deliver time‐scales of the order of a few years. Since rotation stabilizes the instability, normal A stars may still host considerable, invisible toroidal magnetic fields (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Searching for links between magnetic fields and stellar evolution

Context. A small fraction of upper main sequence stars have strong, highly structured magnetic fields. The origin and evolution of these fields are not adequately understood. Aims. We are carrying out a survey of magnetic fields in Ap stars in open clusters in order to obtain the first sample of magnetic upper main sequence stars with precisely known ages. These data will constrain theories of field evolution in these stars. Methods. A survey of candidate open cluster magnetic Ap stars was carried out using the new ESPaDOnS spectropolarimeter at the CFHT. This instrument provides an alternative to the FORS1 spectropolarimeter used up to now for this survey. Results. We have obtained 44 measurements of the mean longitudinal fields Bz# of 23 B6 ‐ A2 stars that have been identified as possible Ap stars and that are possible members of open clusters, with a median uncertainty of about 45 G. Of these stars, 10 have definite field detections. Nine stars of our sample are found not to be magnetic Ap stars. These observations significantly increase the information available about low-mass stars near the TAMS compared to our previous sample. Conclusions. We find that ESPaDOnS provides field measurements comparable to those that we have previously obtained with FORS1, and that these data also contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new data we are able to expand the available data on low-mass, relatively evolved Ap stars, and identify more robustly which observed stars are actually magnetic Ap stars and cluster members. Re-analysis of the enlarged data set of cluster Ap stars indicates that such stars with masses in the range of 2 ‐ 5 M$ show RMS fields larger than about 1 kG only when they are near the ZAMS. The time scale on which these large fields disappear varies strongly with mass, ranging from about 250 Myr for stars of 2 ‐3 M$ to 15 Myr for stars of 4 ‐ 5 M$ . Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.

Searching for links between magnetic fields and stellar evolution

Context.The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars.

An evolutionary study of magnetic Ap stars in 4 northern open clusters

This poster describes the first results of a study of magnetic Ap stars in nearby northern open clusters. Using the MuSiCoS spectropolarimeter on the 2 m Bernard Lyot telescope at Pic du Midi observatory, high-resolution circular polarisation spectra have been obtained for 9 Ap stars proposed as members of 4 open clusters (M 39, IC 4756, the Pleiades and the Hyades). The aim of this study is to explore the evolution of chemical peculiarities and magnetic fields of Ap stars.

Magnetic Doppler imaging of 53 Camelopardalis in all four Stokes parameters

We present the first investigation of the structure of the stellar surface magnetic field using line profiles in all four Stokes parameters. We extract the information about the magnetic field geometry and abundance distributions of the chemically peculiar star 53 Cam by modelling time-series of high-resolution spectropolarimetric observations with the help of a new magnetic Doppler imaging code. This combination of the unique four Stokes parameter data and state-of-the-art magnetic imaging technique makes it possible to infer the stellar magnetic field topology directly from the rotational variability of the Stokes spectra. In the magnetic imaging of 53 Cam we discard the traditional multipolar assumptions about the structure of magnetic fields in Ap stars and explore the stellar magnetic topology without introducing any global a priori constraints on the field structure. The complex magnetic model of 53 Cam derived with our magnetic Doppler imaging method achieves a good fit to the observed intensity, circular and linear polarization profiles of strong magnetically sensitive spectral lines. Such an agreement between observations and model predictions was not possible with any earlier multipolar magnetic models, based on modelling Stokes I spectra and fitting surface averaged magnetic observables (e.g., longitudinal field, magnetic field modulus, etc.). Furthermore, we demonstrate that even the direct inversion of the four Stokes parameters of 53 Cam assuming a low-order multipolar magnetic geometry is incapable of achieving an adequate fit to our spectropolarimetric observations. Thus, as a main result of our investigation, we discover that the magnetic field topology of 53 Cam is considerably more complex than any low-order multipolar expansion, raising a general question about the validity of the multipolar assumption in the studies of magnetic field structures of Ap stars. In addition to the analysis of the magnetic field of 53 Cam, we reconstruct surface abundance distributions of Si, Ca, Ti, Fe and Nd. These abundance maps confirm results of the previous studies of 53 Cam, in particular dramatic antiphase variation of Ca and Ti abundances.

Observations of magnetic Ap stars at the Pic-du-Midi Observatory: From broadband linear polarization measurements with STERENN to spectropolarimetry with MuSiCoS

Observations of magnetic Ap stars taken at the 2m Telescope Bernard Lyot (TBL) of Pic-du-Midi Observatory are of the greatest importance in the study of the stellar magnetism. With the help of the polarimeter STERENN, in the '90s, Leroy and collaborators performed a vigorous campaign of observations of broadband linear polarization of Ap stars, virtually disclosing a new window on the transverse components of the stellar magnetic fields. Less than 10 years later, Donati, Landstreet, Wade and other collaborators are routinely using MuSiCoS in polarimetric mode to observe Stokes profiles of Ap and other kinds of magnetic stars. I will review the impact of TBL observations on the study of magnetic fields of Ap stars, and the progresses in the modelling that have been accomplished in the last ten years.

Looking for Twins of Przybylski’s Star

AbstractThe examination of high resolution spectra of two Ap stars, HD 965 and HD 217522, discloses a spectral appearance similar to Przybylski’s star. Here we present the preliminary results of a comparative study of all three stars. Further, we discuss the possibility of having a good spectroscopic indicator of magnetic fields in Ap stars. In all three studied stars the strong Lu II line at λ 5984 Å is present and very likely not affected by significant blends. Its hyperfine structure has been recently measured and we show the results of a model that accounts for the Paschen-Back effect on hyperfine structure.

Spectroscopic Diagnosis of Magnetic Fields of AP Stars

Diagnosis of Ap star magnetic fields through spectroscopic methods is a broad topic, which cannot be fully covered within the rather tight limits of this contribution. Accordingly, the latter is devoted to only one particular approach, namely the determination of the mean magnetic field modulus from the observation of resolved magnetically split lines in high-dispersion spectra taken in unpolarized light. This determination is straightforward, mostly approximation free and model independent. The derived quantity, the mean magnetic field modulus, is the line-intensity weighted average over the visible stellar hemisphere of the modulus of the magnetic vector.

Radio Astronomy Techniques of Observing Magnetic Fields: The Galaxy

Optical techniques were first used to detect magnetic fields in cosmic objects. Hale used the Zeeman effect to detect solar magnetic fields in 1908. Babcock extended the Zeeman technique to show the existence of magnetic fields in Ap stars in 1946. Optical polarization observations were made as early as 1920 by W.F. Meyer who observed the Hubble variable nebula NGC 2261. Polarization observations of the Andromeda nebula were made by Öhman in 1942. However at first the interpretation of these optical polarization observations was in terms of scattered light only. The theoretical work of Davis and Greenstein suggested that optical polarization could also be due to dust grains aligned in magnetic fields. Observations of Hiltner and Hall supported this interpretation. Extensive surveys of starlight polarization were made by many observers giving information about nearby magnetic fields in the Galaxy. Optical polarization observations of galaxies gave some information on the Magellanic Clouds and other nearby objects but due to lack of sensitivity progress was slow.

The Boundary between the Magnetic Fields of Ap Stars and the Fields of Solar-type Stars

AbstractThe boundary between Ap-type magnetic fields and the magnetic fields of solar-type stars occurs near Te ~ 7000K, about where deep envelope convection develops in main sequence stars. This seems natural for solar-type stars, in which the field is generated by the convection zone. However, among magnetic Ap stars the frequency of occurrence declines from about 10% of all A stars near A0 to about 1% near F0. It is not clear what produces this decline in frequency, but the convection zone is probably not responsible. In fact, it seems likely that if global fossil fields occur in main sequence F stars, such fields should be detectable even if the stars having them are not chemically peculiar.

The evolution of the magnetic fields of AP stars - Magnetic observations of stars in the Scorpius-Centaurus association

view Abstract Citations (58) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of the Magnetic Fields of AP Stars: Magnetic Observations of Stars in the Scorpius-Centaurus Association Thompson, Ian B. ; Brown, Douglas N. ; Landstreet, John D. Abstract Data concerning the positions, photometry, proper motions, and radial velocities have been compiled for a sample of Ap and Bp stars toward the Scorpius-Centaurus association. Probable members of the association are identified, and new magnetic observations of 13 of these member Ap stars in the mass range 2.5-4.5 solar masses are presented. These magnetic observations are compared with similar data for a sample of old field Ap stars. No evidence is found for a significant difference between the distributions of magnetic field strengths of the two samples, suggesting that little or no evolution of the magnetic fields in Ap stars occurs on a time scale of a few x 10 to the 8th yr. Publication: The Astrophysical Journal Supplement Series Pub Date: May 1987 DOI: 10.1086/191195 Bibcode: 1987ApJS...64..219T Keywords: Centaurus Constellation; Magnetic Stars; Peculiar Stars; Scorpius Constellation; Star Clusters; A Stars; Astronomical Photometry; Position (Location); Stellar Motions; Astrophysics; CLUSTERS: ASSOCIATIONS; STARS: EVOLUTION; STARS: PECULIAR A; STARS: MAGNETIC full text sources ADS | data products SIMBAD (79)

Photometrically determined mean surface magnetic fields of Ap stars

AbstractThe photometrically determined mean surface magnetic fields BS need a revision. None of the stars for which BS can be measured directly by Zeeman line splitting fulfils the relation between the photometric parameter Δ(V1 – G) and the mean surface field BS, which is used by NORTH, CRAMER and MAEDER to determine BS for other B2 – A3 stars. The ratio BeffMax/BS for stars, which define North's relation, shows unreasonable large values.

Magnetic Fields: Observational Aspects

A relatively short, time was available for the preparation of this paper. A number of excellent reviews of Ap observations are already in the Literature, notably that by Preston (1971) on surface fields. However, a recent review of developments in techniques for the measurement of magnetic fields in Ap stars was lacking.Upon arrival at this symposium, we found so many exciting new results and programs underway (and therefore unpublished) that we decided to throw out our original paper and open this session up for reports from people who are making magnetic observations themselves, or who are familiar with active programs in their home countries.