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Register a new userWeak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
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We have investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields, with the aim of exploring the weak part of the magnetic field distribution of Ap/Bp stars. Using the MuSiCoS and NARVAL spectropolarimeters we have obtained 282 LSD Stokes V signatures of our 28 sample stars. All stars were detected, showing clearly that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. To better characterise the surface magnetic field intensities and geometries of the sample, we have inferred the dipolar field intensity and the magnetic obliquity. The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this magnetic threshold as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.
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Various observational properties of Ap/Bp stars have been well-established such as the often-cited 10% incidence rate of strong, organized magnetic fields amongst all A- and B-type stars. However, these inferences have generally been drawn fromsurveys biased towards the strongest most easily detectable fields. A volume-limited spectropolarimetric survey of all intermediate-mass stars within 100 pc was initiated in 2007 in order to avoid the biases inherent in previous studies. This workyielded the magnetic properties of a large number of Ap/Bp stars in the sample; however, nearly half of the sample remained either unobserved or had relatively poor constraints on their field strengths and geometries. We have recently completed thissurvey using measurements obtained by ESPaDOnS and NARVAL. We discuss here some of the recent findings of this survey.
Ap/Bp stars are magnetic chemically peculiar early A and late B type stars of the main sequence. They exhibit peculiar surface abundance anomalies that are thought to be the result of gravitational settling and radiative levitation. The physics of diffusion in these stars are reviewed briefly and some model predictions are discussed. While models reproduce some observations reasonably well, more work is needed before the behavior of diffusing elements in a complex magnetic field is fully understood.
The brightest magnetic chemically peculiar stars theta Aur and eps UMa were targeted by numerous studies of their photometric and spectroscopic variability. Detailed maps of chemical abundance spots were repeatedly derived for both stars. However, very little information on the magnetic field geometries of these stars is available. In this study we aim to determine detailed magnetic field topologies of theta Aur and eps UMa based on modern, high-resolution spectropolarimetric observations. Both targets were observed in all four Stokes parameters using the Narval and ESPaDOnS spectropolarimeters. A multi-line technique of least-squares deconvolution was employed to detect polarisation signatures in spectral lines. These signatures were modelled with a Zeeman-Doppler imaging code. We succeeded in detecting variable circular and linear polarisation signatures for theta Aur. Only circular polarisation was detected for eps UMa. We obtained new sets of high-precision longitudinal magnetic field measurements using mean circular polarisation metal line profiles as well as hydrogen line cores, which are consistent with historical data. Magnetic
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. Using the new spectropolarimeter ESPaDOnS at CFHT, we have obtained 44 measurements of the mean longitudinal fields 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. The ESPaDOnS data contain a large amount of useful information not readily obtained from lower resolution spectropolarimetry. With the new observations we are able to expand the available data on fields of 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 mo 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 solar mass to 15 Myr for stars of 4 - 5 solar mass. Our data are consistent either with emergent flux conservation for most (but not all) Ap stars, or with modest decline in flux with age.
High energy photons from blazars can initiate electromagnetic pair cascades interacting with the extragalactic photon background. The charged component of such cascades is deflected and delayed by extragalactic magnetic fields (EGMF), reducing thereby the observed point-like flux and leading potentially to multi degree images in the GeV energy range. We calculate the fluence of 1ES 0229+200 as seen by Fermi-LAT for different EGMF profiles using a Monte Carlo simulation for the cascade development. The non-observation of 1ES 0229+200 by Fermi-LAT suggests that the EGMF fills at least 60% of space with fields stronger than {cal O}(10^{-16}-10^{-15})G for life times of TeV activity of {cal O}(10^2-10^4)yr. Thus the (non-) observation of GeV extensions around TeV blazars probes the EGMF in voids and puts strong constraints on the origin of EGMFs: Either EGMFs were generated in a space filling manner (e.g. primordially) or EGMFs produced locally (e.g. by galaxies) have to be efficiently transported to fill a significant volume fraction, as e.g. by galactic outflows.
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