scholarly journals Gradient of the stellar magnetic field in measurements of hydrogen line cores

2008 ◽  
Vol 4 (S259) ◽  
pp. 411-412
Author(s):  
Dimitry O. Kudryavtsev ◽  
Iosif I. Romanyuk
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Longitudinal Magnetic Field ◽  
Stellar Atmospheres ◽  
Overwhelming Majority ◽  
Hydrogen Line ◽  
The Core ◽  
Hydrogen Lines ◽  
Ap Stars ◽  
The Magnetic Field

AbstractWe report the observed systematic differences in longitudinal magnetic field values, obtained from measurements of metal lines and the core of the Hβ line for a number of Ap stars, having strong global magnetic fields. In overwhelming majority of cases the magnetic field values, obtained from measurements of hydrogen lines cores, is smaller then the ones obtained from metal lines. We discuss some possible explanations of this effect, the most probable of which is the existence of the gradient of the magnetic field in stellar atmospheres.

scholarly journals Modeling surface magnetic fields in stars with radiative envelopes

2013 ◽  
Vol 9 (S302) ◽  
pp. 290-299
Author(s):  
Oleg Kochukhov
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Doppler Imaging ◽  
Magnetic Field Mapping ◽  
Ap Stars ◽  
Surface Fields ◽  
The Magnetic Field

AbstractStars with radiative envelopes, specifically the upper main sequence chemically peculiar (Ap) stars, were among the first objects outside our solar system for which surface magnetic fields have been detected. Currently magnetic Ap stars remains the only class of stars for which high-resolution measurements of both linear and circular polarization in individual spectral lines are feasible. Consequently, these stars provide unique opportunities to study the physics of polarized radiative transfer in stellar atmospheres, to analyze in detail stellar magnetic field topologies and their relation to starspots, and to test different methodologies of stellar magnetic field mapping. Here I present an overview of different approaches to modeling the surface fields in magnetic A- and B-type stars. In particular, I summarize the ongoing efforts to interpret high-resolution full Stokes vector spectra of these stars using magnetic Doppler imaging. These studies reveal an unexpected complexity of the magnetic field geometries in some Ap stars.

scholarly journals Magnetic fields of rapidly oscillating Ap stars

1993 ◽  
Vol 139 ◽  
pp. 132-132
Author(s):  
G. Mathys
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Zeeman Effect ◽  
Rotation Frequency ◽  
Spectral Lines ◽  
Driving Mechanism ◽  
Line Splitting ◽  
Ap Stars ◽  
The Magnetic Field

Magnetic field appears to play a major role in the pulsations of rapidly oscillating Ap (roAp) stars. Understanding of the behaviour of these objects thus requires knowledge of their magnetic field. Such knowledge is in particular essential to interpret the modulation of the amplitude of the photometric variations (with a frequency very close to the rotation frequency of the star) and to understand the driving mechanism of the pulsation. Therefore, a systematic programme of study of the magnetic field of roAp stars has been started, of which preliminary (and still very partial) results are presented here.Magnetic fields of Ap stars can be diagnosed from the Zeeman effect that they induced in spectral lines either from the observation of line-splitting in high-resolution unpolarized spectra (which only occurs in favourable circumstances) or from the observation of circular polarization of the lines in medium- to high-resolution spectra.

scholarly journals Pulsation of Rotating Magnetic Stars

1993 ◽  
Vol 139 ◽  
pp. 134-134
Author(s):  
H. Shibahashi ◽  
M. Takata
Keyword(s):  
Magnetic Field ◽  
Fine Structure ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Dipole Field ◽  
Dipole Mode ◽  
Magnetic Stars ◽  
Ap Stars ◽  
The Magnetic Field ◽  
Split Frequency

Recently, one of the rapidly oscillating Ap stars, HR 3831, has been found to have an equally split frequency septuplet, though its oscillation seems to be essentially an axisymmetric dipole mode with respect to the magnetic axis which is oblique to the rotation axis (Kurtz et al. 1992; Kurtz 1992). In order to explain this fine structure, we investigate oscillations of obliquely rotating magnetic stars by taking account of the perturbations due to the magnetic fields and the rotation. We suppose that the star is rigidly rotating and that the magnetic field is a dipole field and its axis is oblique to the rotation axis. We treat the effects of the rotation and of the magnetic field as perturbations. In doing so, we suppose that the rotation of the star is slow enough so that the effect of the rotation on oscillations is smaller than that of the magnetic field.

scholarly journals The magnetic field of the nearest cool star A Paradigm for Stellar Activity

1996 ◽  
Vol 176 ◽  
pp. 1-16
Author(s):  
Carolus J. Schrijver
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Differential Rotation ◽  
Stellar Atmospheres ◽  
The Sun ◽  
Flux Tubes ◽  
Cool Star ◽  
Temperature Regimes ◽  
The Magnetic Field ◽  
Selection Of

Looking at the Sun forges the framework within which we try to interpret stellar observations. The stellar counterparts of spots, plages, flux tubes, chromospheres, coronae, etc., are readily invoked when attempting to interpret stellar data. This review discusses a selection of solar phenomena that are crucial to understand stellar atmospheric activity. Topics include the interaction of magnetic fields and flows, the relationships between fluxes from different temperature regimes in stellar atmospheres, the photospheric flux budget and its impact on the measurement of the dynamo strength, and the measurement of stellar differential rotation.

scholarly journals Magnetic Field Diagnosis Through Spectropolarimetry)

1993 ◽  
Vol 138 ◽  
pp. 232-246 ◽  
Author(s):  
Gautier Mathys
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Longitudinal Magnetic Field ◽  
Quadratic Field ◽  
Spectral Lines ◽  
Stellar Rotation ◽  
The Asymmetry ◽  
The Moment ◽  
Ap Stars ◽  
Original Approach

AbstractAn original approach, the moment technique, is applied to analyze the shapes of spectral lines of Ap stars recorded in both circular polarizations. The longitudinal magnetic field, the asymmetry of the longitudinal magnetic field, and the quadratic field of the studied stars are derived. From the consideration of these quantities and of their variations through the stellar rotation cycle, constraints on the spatially unresolved structure of the magnetic fields are obtained.

scholarly journals Pulsation of magnetic stars

2013 ◽  
Vol 9 (S301) ◽  
pp. 197-204 ◽  
Author(s):  
Hideyuki Saio
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Amplitude Distribution ◽  
High Order ◽  
Frequency Separation ◽  
Strong Magnetic Fields ◽  
Magnetic Stars ◽  
Ap Stars ◽  
The Magnetic Field ◽  
Large Frequency

AbstractSome Ap stars with strong magnetic fields pulsate in high-order p modes; they are called roAp (rapidly oscillating Ap) stars. The p-mode frequencies are modified by the magnetic fields. Although the large frequency separation is hardly affected, small separations are modified considerably. The magnetic field also affects the latitudinal amplitude distribution on the surface. We discuss the properties of axisymmetric p-mode oscillations in roAp stars.

scholarly journals The Role of Magnetic Fields in the Heating of Stellar Atmospheres — Theory

1983 ◽  
Vol 102 ◽  
pp. 375-386 ◽  
Author(s):  
C. Chiuderi
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Theoretical Approach ◽  
General Framework ◽  
Stellar Atmospheres ◽  
Point Of View ◽  
The Subject ◽  
Stellar Envelopes ◽  
The Magnetic Field

The last ten years of observations have shown beyond doubt the fundamental role played by the magnetic field in the heating of stellar atmospheres. After the recognition of the extreme inhomogeneity of the solar corona, two basic new trends have appeared in the theoretical literature on the coronal heating problem. One is the adoption of a global point of view that stresses the connection of the properties of the upper layers to those of the underlying ones. In this way a general framework is provided, capable of accomodating many possible heating mechanisms that need not to be specified at this stage. The second novelty is the explicit inclusion in the theory of the inhomogeneous nature of the stellar envelopes, as a result of the presence of magnetic fields. The present status of knowledge on the subject as determined by the above evolution of the theoretical approach will be reviewed.

scholarly journals Magnetic Properties of Rapidly Oscillating Ap Stars

1998 ◽  
Vol 185 ◽  
pp. 313-314
Author(s):  
G. Mathys ◽  
S. Hubrig
Keyword(s):  
Magnetic Field ◽  
Magnetic Properties ◽  
Longitudinal Magnetic Field ◽  
Rotation Frequency ◽  
Pulsation Amplitude ◽  
Field Geometry ◽  
The Mean ◽  
Ap Stars ◽  
The Magnetic Field ◽  
Maximum Pulsation

Rapidly oscillating Ap stars generally pulsate in multiple modes, characterized by different frequencies. The amplitudes of these modes may furthermore be modulated with the rotation frequency of the star. For the two roAp stars whose magnetic fields have been sufficiently studied, the maximum pulsation amplitude coincides in phase with one of the extrema of the mean longitudinal magnetic field. Two interpretations of this property have been proposed: the oblique pulsator model, according to which the pulsation modes are aligned with the magnetic axis of the star, and the spotted pulsator model, which assumes that the pulsation modes are symmetric about the rotation axis of the star, and that pulsation amplitude modulation is due to the inhomogeneity of the stellar surface (which, itself, is related to the magnetic field geometry). At present, no definite choice between these two models can be made, though the oblique pulsator model is often preferred.

scholarly journals Hanle and Zeeman effects: from solar to stellar diagnostics

2013 ◽  
Vol 9 (S302) ◽  
pp. 130-133
Author(s):  
A. López Ariste
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Strong Field ◽  
Zeeman Effect ◽  
Stellar Atmospheres ◽  
Line Of Sight ◽  
Magnetic Topology ◽  
Hanle Effect ◽  
The Magnetic Field

RésuméWe suggest the use of the area asymmetries of the Stokes V profile of a line sensitive to the Zeeman effect to diagnose variatios of the magnetic field along the line of sight in stellar atmospheres. This tool could allow to disentangle the magnetic topology of the observed stellar features in analogy to the solar case: a fibril topology as in plage and netwrok magnetic fields vs. a homogeneous and strong field as in sunspots. We also suggest the use of the Hanle effect as a means to observe weak global dipoles.

Quick Interpretation of Unresolved Hyperfine and/or Zeeman Structures in Stellar Spectra

1988 ◽  
Vol 102 ◽  
pp. 263-265
Author(s):  
J. Bauche ◽  
J. Oreg
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Hyperfine Structure ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Experimental Resolution ◽  
Average Wavelength ◽  
Local Magnetic Fields ◽  
The Magnetic Field ◽  
Landé Factors

AbstractSome atomic or ionic lines emitted by the stellar atmospheres are very useful clues for estimating the local magnetic fields. However, their use is often complicated by insufficient experimental resolution and by the occurence of hyperfine structure. But the average wavelength and the width of the component set corresponding to each type of polarisation are given by compact formulae, in terms of the magnetic field, and of the Landé factors and hyperfine magnetic and electric constants of both involved levels.

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