Magnetic Doppler imaging of 53 Camelopardalis in all four Stokes parameters

Abstract

Abstract. 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 Fe  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 loworder 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.

Bi-dimensional element stratifications computed for magnetic Ap star atmospheres

“…In that case, one might obtain a sequence of largish spots (where the field angle stays close enough to 90 • ), instead of a belt. This may possibly help in explaining some of the features observed in stars such as 53 Cam (Kochukhov et al 2004) where, clearly, no equatorial belt can be seen.…”

“…In addition to these rather qualitative/schematic predictions from theoretical models, observational evidence for such abundance inhomogeneities has are only available in electronic form at http://www.aanda.org accumulated over recent years (see Ryabchikova et al 2002;Ryabchikova 2005;Shulyak et al 2009). Zeeman Doppler mapping (Kochukhov et al 2004) provides simultaneous magnetic and abundance maps for several elements and indicates the richness of physical processes acting in these stars.…”

Doppler imaging of the helium-variable star a Centauri

“…In the particular case of the star 53 Cam with its relatively slow rotation and strong magnetic field it was seen (Kochukhov et al 2004) that the field was quite complex. A future attempt at producing a Zeeman Doppler image of a Cen will be a challenge, but would be of considerable interest since the abundance distribution of the lighter elements examined here suggests that the magnetic field may deviate considerably from a simple structure.…”

HD 24355 observed by theKepler K2mission: a rapidly oscillating Ap star pulsating in a distorted quadrupole mode

“…This is similar to the Ap star 53 Cam (Kochukhov et al 2004). In their analysis, they used different methods to determine the T eff and log g of the star, and arrive at different values.…”