Accurate age determinations of several nearby open clusters containing magnetic Ap stars

Aims: Our aim is to obtain ages that are as accurate as possible for the seven nearby open clusters alpha Per, Coma Ber, IC 2602, NGC 2232, NGC 2451A, NGC 2516, and NGC 6475, each of which contains at least one magnetic Ap or Bp star. Simultaneously, we test the current calibrations of Te and luminosity for the Ap/Bp star members, and identify clearly blue stragglers in the clusters studied. Methods: We explore the possibility that isochrone fitting in the theoretical Hertzsprung-Russell diagram (i.e. log (L/L&sun;) vs. log Te), rather than in the conventional colour-magnitude diagram, can provide more precise and accurate cluster ages, with well-defined uncertainties. Results: Well-defined ages are found for all the clusters studied. For the nearby clusters studied, the derived ages are not very sensitive to the small uncertainties in distance, reddening, membership, metallicity, or choice of isochrones. Our age determinations are all within the range of previously determined values, but the associated uncertainties are considerably smaller than the spread in recent age determinations from the literature. Furthermore, examination of proper motions and HR diagrams confirms that the Ap stars identified in these clusters are members, and that the presently accepted temperature scale and bolometric corrections for Ap stars are approximately correct. We show that in these theoretical HR diagrams blue stragglers are particularly easy to identify. Conclusions: Constructing the theoretical HR diagram of a nearby open cluster makes possible an accurate age determination, with well defined uncertainty. This diagnostic of a cluster also provides a useful tool for studying unusual stars such as Ap stars and blue stragglers

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

The stars of the middle main sequence have relatively quiescent outer layers, and unusual chemical abundance patterns may develop in their atmospheres. The presence of chemical peculiarities reveal the action of such subsurface phenomena as gravitational settling and radiatively driven levitation of trace elements, and their competition with mixing processes such as turbulent diffusion. We want to establish whether abundance peculiarities change as stars evolve on the main sequence, and provide observational constraints to diffusion theory. We have performed spectral analysis of 15 magnetic Bp stars that are members of open clusters (and thus have well-known ages), with masses between about 3 and 4 M_sun. For each star, we measured the abundances of He, O, Mg, Si, Ti, Cr, Fe, Pr and Nd. We have discovered the systematic time evolution of trace elements through the main-sequence lifetime of magnetic chemically peculiar stars as their atmospheres cool and evolve toward lower gravity. During the main sequence lifetime, we observe clear and systematic variations in the atmospheric abundances of He, Ti, Cr, Fe, Pr and Nd. For all these elements, except He, the atmospheric abundances decrease with age. The abundances of Fe-peak elements converge toward solar values, while the rare-earth elements converge toward values at least 100 times more abundant than in the Sun. Helium is always underabundant compared to the Sun, evolving from about 1% up to 10% of the solar He abundance. We have attempted to interpret the observed abundance variations in the context of radiatively driven diffusion theory, which appears to provide a framework to understand some, but not all, of the observed anomalous abundance levels and variations.Comment: 13 pages, 5 figures, 5 tables, accepted for publication in A&

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

CONTEXT. The ESO archive of FORS1 spectropolarimetric observations may be used to create a homogeneous database of magnetic field measurements. However, no systematic comparison of FORS field measurements to those obtained with other instruments has been undertaken so far. AIMS. We exploit the FORS archive of circular spectropolarimetric data to examine in a general way how reliable and accurate field detections obtained with FORS are. METHODS. We examine the observations of Ap and Bp stars, on the grounds that almost all of the unambiguous detections of magnetic fields in the FORS1 archive are in these kinds of stars. We assess the overall quality of the FORS1 magnetic data by examining the consistency of field detections with what is known from previous measurements obtained with other instruments, and we look at patterns of internal consistency. RESULTS. FORS1 magnetic measurements are fully consistent with those made with other instruments, and the internal consistency of the data is excellent. However, it is important to recognise that each choice of grism and wavelength window constitutes a distinct instrumental measuring system, and that simultaneous field measurements in different instrumental systems may produce field strength values that differ up to 20 %, or more. Furthermore, we found that field measurements using hydrogen lines only yield results that meaningfully reflect the field strength as sampled specifically by lines of hydrogen for stars with effective temperatures above about 9000 K. CONCLUSIONS. In general the magnetic field measurements of Ap and Bp stars obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1 provides an extremely useful example that offers valuable lessons for field measurements with other low- resolution Cassegrain spectropolarimeters.Comment: 14 pages, 8 figures. Accepted for publication in section 13 of Astronomy & Astrophysics on 13 October 201

An Analysis of the Rapidly Rotating Bp Star HD 133880

HD 133880 is a rapidly rotating Bp star and host to one of the strongest magnetic fields of any Bp star. A member of the Upper Centaurus Lupus association, it is a star with a well-determined age of 16 Myr. Twelve new spectra obtained from the FEROS, ESPaDOnS, and HARPS instruments, provide sufficient material from which to re-evaluate the magnetic field and obtain a first approximation to the atmospheric abundance distributions of various elements. Using the new magnetic field measurements and optical photometry together with previously published data, we refine the period of HD 133880 to P = 0.877476 \pm 0.000009 days. The magnetic field structure was characterised by a colinear multipole expansion from the observed variations of the longitudinal and surface fields with rotational phase. This simple axisymmetric magnetic field model is based on a predominantly quadrupolar component that roughly describes the field variations. Using spectrum synthesis, we derived mean abundances for O, Mg, Si, Ti, Cr, Fe, Nd and Pr. All elements, except Mg (which has a uniform distribution), are overabundant compared to the Sun and are more abundant in the negative than in the positive magnetic hemisphere. In contrast to most Bp stars the abundance of O in HD 133880 is overabundant compared to the solar abundance ratio. In studying the Halpha and Paschen lines in the optical spectra we could not unambiguously detect information about the magnetosphere of HD 133880. However, radio emission data at both 3 and 6 cm suggests that the magnetospheric plasma is held in rigid rotation with the star by the magnetic field and further supported against collapse by the rapid rotation. Subtle differences in the shapes of the optically thick radio light curves at 3 and 6 cm suggest that the large-scale magnetic field is not fully axisymmetric at large distances from the star.Comment: Accepted for publication in MNRAS. 17 pages, 10 figure

Discovery of an extremely weak magnetic field in the white dwarf LTT 16093 = WD2047+372

Magnetic fields have been detected in several hundred white dwarfs, with strengths ranging from a few kG to several hundred MG. Only a few of the known fields have a mean magnetic field modulus below about 1 MG. We are searching for new examples of magnetic white dwarfs with very weak fields, and trying to model the few known examples. Our search is intended to be sensitive enough to detect fields at the few kG level. We have been surveying bright white dwarfs for very weak fields using spectropolarimeters at the Canada-France-Hawaii telescope, the William Herschel telescope, the European Southern Observatory, and the Russian Special Astrophysical Observatory. We discuss in some detail tests of the WHT spectropolarimeter ISIS using the known magnetic strong-field Ap star HD 215441 (Babcock's star) and the long-period Ap star HD 201601 (gamma Equ). We report the discovery of a field with a mean field modulus of about 57 kG in the white dwarf LTT 16093 = WD2047+372. The field is clearly detected through the Zeeman splitting of Halpha seen in two separate circularly polarised spectra from two different spectropolarimeters. Zeeman circular polarisation is also detected, but only barely above the 3 sigma level. The discovery of this field is significant because it is the third weakest field ever unambiguously discovered in a white dwarf, while still being large enough that we should be able to model the field structure in some detail with future observations

The FORS1 catalogue of stellar magnetic field measurements

The FORS1 instrument on the ESO Very Large Telescope was used to obtain low-resolution circular polarised spectra of nearly a thousand different stars, with the aim of measuring their mean longitudinal magnetic fields. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Our understanding of the instrument and our data reduction techniques have both improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements. Based on the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for about 1400 observations of about 850 different objects. The spectral type of each object has been accurately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.Comment: Accepted for publication by A&

Monitoring and modelling of white dwarfs with extremely weak magnetic fields

Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus >= 1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (<= 200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered "super-weak field" magnetic white dwarfs, WD2047+372 and WD2359-434. WD2047+372 has a mean longitudinal field that reverses between about -12 and +15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be intepreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD2359-434 always shows a weak positive longitudinal field with values between about 0 and +12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the Halpha line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD2359-434 is a light variable with an amplitude of only 0.005 mag, our own photometry shows that if WD2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD2359-434 has a much more complex surface field than the much younger WD2047+372.Comment: Accepted for publication in Astronomy & Astrophysic