799 research outputs found
Magnetic Doppler imaging of alpha^2 Canum Venaticorum in all four Stokes parameters. Unveiling the hidden complexity of stellar magnetic fields
Strong organized magnetic fields have been studied in the upper main sequence
chemically peculiar stars for more than half a century. However, only recently
have observational methods and numerical techniques become sufficiently mature
to allow us to record and interpret high-resolution four Stokes parameter
spectra, leading to the first assumption-free magnetic field models of these
stars. Here we present a detailed magnetic Doppler imaging analysis of the
spectropolarimetric observations of the prototypical magnetic Ap star alpha^2
CVn. The surface abundance distributions of Fe and Cr and a full vector map of
the stellar magnetic field are reconstructed in a self-consistent inversion
using our state-of-the-art magnetic Doppler imaging code Invers10. We succeeded
in reproducing most of the details of the available spectropolarimetric
observations of alpha^2 CVn with a magnetic map which combines a global
dipolar-like field topology with localized spots of higher field intensity. We
demonstrate that these small-scale magnetic structures are inevitably required
to fit the linear polarization spectra; however, their presence cannot be
inferred from the Stokes I and V observations alone. Our magnetic Doppler
imaging analysis of alpha^2 CVn and previous results for 53 Cam support the
view that the upper main sequence stars can harbour fairly complex surface
magnetic fields which resemble oblique dipoles only at the largest spatial
scales. Spectra in all four Stokes parameters are absolutely essential to
unveil and meaningfully characterize this field complexity in Ap stars. We
therefore suggest that understanding magnetism of stars in other parts of the
H-R diagram is similarly incomplete without investigation of their linear
polarization spectra.Comment: 16 pages, 12 figures; Accepted for publication by Astronomy &
Astrophysic
Remarkable non-dipolar magnetic field of the Bp star HD 137509
The southern magnetic Bp star HD 137509 exhibits complex rotational
modulation of the longitudinal field and other magnetic observables.
Interpretation of this magnetic variability in the framework of the low-order
multipolar field models suggests a very strong quadrupolar component to
dominate the surface field topology of HD 137509. I have examined the
high-quality VLT/UVES spectra of HD 137509 and discovered resolved Zeeman split
components in some of the spectral lines. The inferred mean surface field
modulus, =29 kG, agrees with the multipolar model predictions. This confirms
the presence of an extremely strong non-dipolar magnetic field in HD 137509 and
establishes this star as the object with the second-largest field among
magnetic chemically peculiar stars.Comment: accepted by A&A; 6 pages, 4 figure
Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles
Information about stellar magnetic field topologies is obtained primarily
from high-resolution circular polarisation (Stokes ) observations. Due to
their generally complex morphologies, the stellar Stokes profiles are
usually interpreted with elaborate inversion techniques such as Zeeman Doppler
imaging (ZDI). Here we further develop a new method of interpretation of
circular polarisation signatures in spectral lines using cumulative Stokes
profiles (anti-derivative of Stokes ). This method is complimentary to ZDI
and can be applied for validation of the inversion results or when the
available observational data are insufficient for an inversion. Based on the
rigorous treatment of polarised line formation in the weak-field regime, we
show that, for rapidly rotating stars, the cumulative Stokes profiles
contain information about the spatially resolved longitudinal magnetic field
density. Rotational modulation of these profiles can be employed for a simple,
qualitative characterisation of the stellar magnetic field topologies. We apply
this diagnostic method to the archival observations of the weak-line T Tauri
star V410 Tau and Bp He-strong star HD 37776. We show that the magnetic field
in V410 Tau is dominated by an azimuthal component, in agreement with the ZDI
map that we recover from the same data set. For HD 37776 the cumulative Stokes
profile variation indicates the presence of multiple regions of positive
and negative field polarity. This behaviour agrees with the ZDI results but
contradicts the popular hypothesis that the magnetic field of this star is
dominated by an axisymmetric quadrupolar component.Comment: 11 pages, 9 figures; accepted for publication in A&
Magnetic field topology and chemical spot distributions of the Ap star HD119419
Analysis of high-resolution spectropolarimetric observations of early-type
magnetic stars is currently the most advanced method of obtaining detailed
information on their surface magnetic field topologies and horizontal spot
distributions. In this study we analyse a new set of high-quality four Stokes
parameter observations of the magnetic Ap star HD119419 - a member of the 14
Myr old Lower Cen-Cru association - for the purpose of studying the surface
field topology and mapping the chemical abundance spots. We made use of the
circular and linear polarisation data collected for HD119419 with the HARPSpol
instrument at the ESO 3.6-m telescope. These observations were analysed with a
multi-line magnetic diagnostic technique and modelled in detail with a Magnetic
Doppler imaging code. We present a new set of mean longitudinal magnetic field
measurements and derive a revised stellar rotational period. We also
redetermine the basic stellar atmospheric parameters. Our four Stokes parameter
magnetic inversions reveal a moderately complex surface field topology with a
mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal
dipolar component dominates the magnetic energy spectrum of the surface field
in HD119419. However, significant contributions of the higher-order spherical
harmonic components are also present. We show that the dipole plus quadrupole
part of the reconstructed field geometry is incapable of reproducing the
observed amplitudes and shapes of the Stokes Q and U profiles. The chemical
abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with
the magnetic field geometry, are characterised by large abundance gradients and
a lack of clear correlation with the magnetic field structure. This full Stokes
vector analysis of HD119419 extends the modern hot-star magnetic mapping
investigations to an open cluster Ap star with a well-determined age.Comment: 15 pages, 11 figures; accepted for publication in A&
Observations of magnetic fields in hot stars
The presence of magnetic fields at the surfaces of many massive stars has
been suspected for decades, to explain the observed properties and activity of
OB stars. However, very few genuine high-mass stars had been identified as
magnetic before the advent of a new generation of powerful spectropolarimeters
that has resulted in a rapid burst of precise information about the magnetic
properties of massive stars. During this talk, I will briefly review modern
methods used to diagnose magnetic fields of higher-mass stars, and summarize
our current understanding of the magnetic properties of OB stars.Comment: Proceedings of IAUS272: Active OB stars, review talk (11 pages
Pulsational line profile variation of the roAp star HR 3831
We report the first comprehensive investigation of the line profile variation
caused by non-radial pulsation in a magnetic oscillating chemically peculiar
star. Spectrum variation of the well-known roAp star HR 3831 is detected using
very high-resolution high signal-to-noise spectroscopic time-series
observations and are followed through the whole rotation cycle of the star. We
confirm outstanding diversity of pulsational behaviour of different lines in
the HR 3831 spectrum and attribute this phenomenon to an interplay between
extreme vertical chemical inhomogeneity of the HR 3831 atmosphere and a running
pulsation wave, propagating towards the upper photospheric layers with
increasing amplitude. Rapid profile variation of the NdIII 6145 A line is
characterized by measuring changes of its equivalent width and the first three
moments. We demonstrate that rotational modulation of the radial velocity
oscillations cannot be fully explained by an oblique axisymmetric dipole
(ell=1, m=0) mode, implied by the classical oblique pulsator model of roAp
stars. Pulsational variation of the higher moments reveal substantial
contribution of the high-order (ell=3) spherical harmonics which appear due to
distortion of pulsations in HR 3831 by the global magnetic field. We interpret
observations with a novel numerical model of the pulsational variation and
rotational modulation of the line profile moments in roAp stars. The comparison
between observed and computed amplitudes and phases of the radial velocity and
line width variation is used to establish parameters of the oblique pulsator
model of HR 3831. Furthermore, definite detection of pulsational variation in
lines of light and iron-peak elements enables the first 3-D mapping of
pulsations in non-radially oscillating star.Comment: 21 pages, 11 figures; accepted by A&
Pushing the limit of instrument capabilities
Chemically Peculiar (CP) stars have been subject of systematic research since
more than 50 years. With the discovery of pulsation of some of the cool CP
stars, the availability of advanced spectropolarimetric instrumentation and
high signal- to-noise, high resolution spectroscopy, a new era of CP star
research emerged about 20 years ago. Together with the success in ground-based
observations, new space projects are developed that will greatly benefit for
future investigations of these unique objects. In this contribution we will
give an overview of some interesting results obtained recently from
ground-based observations and discuss on future outstanding Gaia space mission
and its impact on CP star research.Comment: Joint Discussion 04, Secsion 1, To appear in Highlights of Astronomy,
Proc. of the XXVIIth IAU General Assembly, Rio de Janeiro, Brazil, August
2009, 9 page
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