Spectropolarimetric Mode Research Articles

Magnetic fields of massive stars

AbstractWe recently carried out a spectropolarimetric study of a sample of massive O-type stars and pulsating β Cephei stars using the SOFIN echelle spectrograph at the 2.56 m Nordic Optical Telescope and the low-resolution FORS 2 spectrograph at the VLT in spectropolarimetric mode. The sample consists of massive stars already detected as magnetic in the course of our previous low-resolution polarimetric observations with FORS 1 and a few O-type stars with magnetic field detections reported in the literature.

FABRY–PÉROT VERSUS SLIT SPECTROPOLARIMETRY OF PORES AND ACTIVE NETWORK: ANALYSIS OF IBIS ANDHINODEDATA

We discuss spectropolarimetric measurements of photospheric (Fe I 630.25 nm) and chromospheric (Ca II 854.21 nm) spectral lines. Our long-term goal is to diagnose properties of the magnetic field near the base of the corona. We compare ground-based two-dimensional spectropolarimetric measurements with (almost) simultaneous space-based slit spectropolarimetry. The ground-based observations were obtained May 20, 2008, with IBIS in spectropolarimetric mode, The space observations were obtained with the Spectro-Polarimeter aboard the HINODE satellite. The agreement between the near-simultaneous co-spatial IBIS and HINODE Stokes-V profiles at 630.25 nm is excellent, with V/I amplitudes compatible with to within 1 %. IBIS QU measurements are affected by residual crosstalk from V, arising from calibration inaccuracies, not from any inherent limitation of imaging spectroscopy. We use a PCA analysis to quantify the detected cross talk. Chromospheric magnetic fields are difficult to constrain by polarization of Ca II lines alone. However, we demonstrate that high cadence, high angular resolution monochromatic images of fibrils in Ca II and H-alpha, can be used to improve the magnetic field constraints, under conditions of high electrical conductivity. Such work is possible only with time series datasets from two-dimensional spectroscopic instruments under conditions of good seeing.

Magnetic field measurements of O stars with FORS 1 at the VLT

Context. The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of these fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Ha emission variations, chemical peculiarity, narrow X-ray emission lines, and non-thermal radio/X-ray emission. Aims. To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS 1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT. Methods. Spectropolarimetric observations were obtained at different phases for a sample of 13 O stars. Ten stars were observed in the spectral range 348-589 nm, HD36879 and HD148937 were observed in the spectral region 325-621 nm, and HD155806 was observed in both settings. To prove the feasibility of the FORS1 spectropolarimetric mode for the measurements of magnetic fields in hot stars, we present in addition 12 FORS 1 observations of the mean longitudinal magnetic field in theta(1) OriC and compare them with measurements obtained with the MuSiCoS, ESPaDOnS, and Narval spectropolarimeters. Results. Most stars in our sample, which were observed on different nights, show a change of the magnetic field polarity, but a field at a significance level of 3 sigma was detected in only four stars, HD36879, HD148937, HD152408, and HD164794. The largest longitudinal magnetic field, = -276 +/- 88 G, was detected in the Of?p star HD148937. We conclude that large-scale organized magnetic fields with polar field strengths larger than 1 kG are not widespread among O-type stars.

Impact Linear Polarization of the Hα Line Observed in a Proton Flare with the Irkutsk Large Solar Vacuum Telescope

The impulsive phase of the 23 July 2002 2B/X4.8 proton flare with a classical two-ribbon structure was observed with the Irkutsk Large Solar Vacuum Telescope (LSVT) in spectropolarimetric mode, with high spatial, spectral, and time resolution. On the basis of 49 spectrograms and 1200 spectral cuts across the flare ribbons, evidence for Hα line impact polarization has been obtained. A systematic change of the Stokes Q and U parameters has been detected across the ribbons for different flare regions measured with a scanning step of 0.85″. In the eastern side of the ribbons, the degree of polarization is 4 – 8%; its plane is oriented toward the solar disk center (radial direction). In the western side, the polarization degree runs up to 25%, and its plane is perpendicular to the disk center direction (tangential direction). A comparison of these results with hard X-ray data (RHESSI) allows us to conclude that high-energy electron beams reached the chromosphere during this flare. The observed changes of the direction of polarization and the vanishing polarization within the ribbons mean that, at the chromospheric level, the energy of electrons remaining in the beam is about 200 eV. A shift of the peak position of polarization relative to the intensity maximum in the ribbons may result from the inclination of the electron beam axis with respect to the solar surface.

A study of the β Cephei star γ Pegasi: binarity, magnetic field, rotation, and pulsations

Aims. We provide observational material to study the magnetic field variability of the classical β Cep-type star y Peg. Methods. The observations were carried out in the He I 6678 line in the course of 23 observing nights from 1997 to 2005 with using the Coude spectrograph in spectropolarimetric mode at the Crimean 2.6 m telescope. The behavior of stellar wind was studied in the UV region using data from the IUE satellite (the INES database). Results. It is shown that the UV stellar wind exhibits a variability. A variation of the wind due to stellar pulsations has been detected. In the He I 6678 line, the abnormally blueshifted radial velocities (y = -60.57 ± 0.29 km s -1 ) were detected during a single night in 2005. We do not confirm the 370.5-day orbital period. The most probable orbital period was estimated as P orb = 6.81608±0.00012 day. The ratio P orb /P puls = 44.92 appeared to be very close to integer. We have detected the presence of a weak magnetic field on the star. The longitudinal component of the field varies from -10 G to 30 G with the stellar rotation. The most probable rotational period is P rot = 6.6538 ± 0.0016 days. Both the orbital and the rotational periods are integral multiples of the difference between them: P orb /|P orb - P rot | = 42.002, and P rot /|P orb - P rot | = 41.002. Variation in the longitudinal magnetic field during the pulsation period with an amplitude about 7 G was detected.

Discovery of magnetic fields in three He variable Bp stars with He and Si spots

AbstractIt is essential for the understanding of stellar structure models of high mass stars to explain why constant stars, nonpulsating chemically peculiar hot Bp stars and pulsating stars co‐exist in the slowly pulsating B stars and β Cephei instability strips. We have conducted a search for magnetic fields in the four Bp stars HD55522, HD105382, HD131120, and HD138769 which previously have been wrongly identified as slowly pulsating B stars. A recent study of these stars using the Doppler Imaging technique revealed that the elements He and Si are inhomogeneously distributed on the stellar surface, causing the periodic variability. Using FORS 1 in spectropolarimetric mode at the VLT, we have acquired circular polarisation spectra to test the presence of a magnetic field in these stars. A variable magnetic field is clearly detected in HD55522 and HD105382, but no evidence for the existence of a magnetic field was found in HD131120. The presence of a magnetic field in HD138769 is suggested by one measurement at 3σ level. We discuss the occurrence of magnetic B stars among the confirmed pulsating B stars and find strong magnetic fields of order kG and oscillations to be mutually exclusive. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT

To properly understand the physics of Ap and Bp stars it is particularly important to identify the origin of their magnetic fields. For that, an accurate knowledge of the evolutionary state of stars that have a measured magnetic field is an important diagnostic. Previous results based on a small and possibly biased sample suggest that the distribution of magnetic stars with mass below 3M⊙ in the H-R diagram differs from that of normal stars in the same mass range (Hubrig et al. 2000). In contrast, higher mass magnetic Bp stars may well occupy the whole main-sequence width (Hubrig, Scholler & North 2005b). In order to rediscuss the evolutionary state of upper main sequence magnetic stars, we define a larger and bias-free sample of Ap and Bp stars with accurate Hipparcos parallaxes and reliably determined longitudinal magnetic fields. We used FORS 1 at the VLT in its spectropolarimetric mode to measure the magnetic field in chemically peculiar stars where it was unknown or poorly known as yet. In this first paper we present our results of the mean longitudinal magnetic field measurements in 136 stars. Our sample consists of 105 Ap and Bp stars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPB stars. A magnetic field was for the first time detected in 57 Ap and Bp stars, in four HgMn stars, one Pga star, one normal B-type star and four SPB stars. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiwavelength Analysis of the Impact Polarization of 2001 June 15 Solar Flare

We report here on the temporal and spatial evolution of the impact polarization of the Hα and Hβ lines during an M6.3 solar flare observed on 2001 June 15 with the THEMIS telescope in the multiwavelength spectropolarimetric mode. Typical spectral intensity and polarization profiles are presented. Both lines are linearly polarized. The Hαline degree of polarization exceeds 4% at line center and in the near line wings. The Hβ line is also linearly polarized, with a degree of polarization reaching 6%. The directions of polarization are either parallel or perpendicular to the local transverse magnetic field (i.e., either radial or tangential because the transverse magnetic field is directed almost in the flare-to-disk center direction). However, contrary to Hα, the Hβ polarization direction is radial only. The Hα and Hβ polarization islands are located at the edges of flare kernels. Only for radial polarization are these islands cospatial. No Hβ polarization is found at the places where tangential Hα polarization is present. The origin of the observed polarization is discussed. Bombardment by low-energy protons or high-energy electrons associated with return currents can explain the radial polarization observed in the lowest flare kernel. The tangential Hα polarization observed in the surge near the upper flare location is interpreted as due to the electric current at the origin of the electromagnetic force that lifts the surge.

Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

We have discovered an extraordinarily large mean longitudinal magnetic field of 7.5 kG in the ultra-cool low mass Ap star HD 154708 using FORS 1 in spectropolarimetric mode. From UVES spectra, we have measured a mean magnetic field modulus of 24.5 kG. This is the second-largest mean magnetic field modulus ever measured in an Ap star. Furthermore, it is very likely that this star is one of the coolest and least massive among the Ap stars and is located in the H-R diagram in the same region in which rapidly oscillating Ap stars have been detected. We note that all known roAp stars have much smaller magnetic fields, by at least a factor of three.

Fast photospheric flows and magnetic fields in a flaring active region

We present new results from the coordinated observations between the THEMIS telescope (in the multi-line spectropolarimetric mode) and Michelson Doppler Imager (MDI) on SOHO obtained in November 2000 for active re- gion NOAA 9236 which was the source of several X-class flares. The goal of these observations was twofold: to verify MDI measurements of the line-of-sight components of flow velocity and magnetic field, and to obtain more information about the photospheric flows and magnetic fields in flaring regions. Using the simultaneous observational data in several lines we have analyzed the structure and dynamics of this active region at the photospheric level before and after a X4.0 flare of November 26, the last major flare produced by this very active region. Vector magnetic field maps are computed from the THEMIS data by full inversion of the Stokes line profiles. In the Doppler velocity maps from THEMIS and MDI, we observe fast photospheric flows which appear to be supersonic in two regions located close to the region where the flare occurred. These flows seem to be long-lived (several hours at least). In one position, we observe a supersonic downflow strongly inclined with respect to the vertical (by 51), while in another position, a flow suggesting a strong shear with a supersonic component as well, although almost horizontal upflows and downflows cannot be ruled out in that case. These flows seem to be present at least 8 hours before the flare, and the amplitude in the second case appeared to be modified during the flare, especially, during the first minutes. In the MDI data, we observed strong permanent changes of the longitudinal magnetic flux, associated with the flare. The role of the strong flows and their interaction with the magnetic field in the development of the active region and the flare is not understood yet.

Search for polarimetric sensitivity in the first observations with THEMIS spectropolarimetric mode MTR (August 1998 campaign)

The present paper is devoted to the search for the polarimetric sensitivity level in observations of the $\ion{Fe}{i}$ 5576 A line performed with the THEMIS spectropolarimetric mode MTR on August 23 1998. This line is insensitive to the Zeeman effect and the present work is thus useful to calibration purposes. The upper level of the line is unpolarizable ($J=0$) and insensitive to the Hanle effect, and the observations have been performed at disk center to avoid any scattering polarization of lower level atomic polarization origin. In the present paper, we describe the steps of a method that is the basis of a data reduction code implemented on systems at the Meudon Observatory for the interpretation of observations where a large number (~150) of images are averaged, and where the signal is in addition averaged along the slit. First, we describe the numerical methods used to determine the line position in the images, and to perform operations on the profiles by FFT techniques (such as translation, dilation, defocusing, apodization). Then, the preprocessing steps are described: dark current subtraction, destretching and flat-field correction. The polarization analysis is then performed, based on the idea that, as the flat-field images are unpolarized, they can be used to correct spurious polarization occuring in the observations. As a result, the observed line is found to be unpolarized, and a sensitivity of 2–4$\times 10^{-4}$ is found for the polarization degree in the neighboring continuum.

Investigation of weak solar magnetic fields

Scattering polarization measurements were obtained with THEMIS in July 2000, close to the solar south Pole and to the east Equator and in a period of maximum solar activity. Using the THEMIS multi-lines spectro-polarimetric mode (MTR), we observed simultaneously four spectral domains containing the 460.7 nm Sr i line, several molecular lines around 515.9 nm and the Na i D 1 and Na i D 2 lines. This allows us to scan different altitudes in the solar atmosphere at the same time and provides us with a large set of constraints to study the behaviour of the magnetic field. This paper is devoted to the Sr i line which exhibits quite a strong linear polarization peak outside active regions. A detailed radiative transfer modeling is performed in order to interpret the observed center-to-limb variations of the line intensity and polarization. It was shown previously (Faurobert-Scholl [CITE]) that this line, which is sensitive to the Hanle effect, can be used as a diagnostic tool for the presence of weak turbulent magnetic fields in the solar photosphere outside active regions. The line polarization rates that we measured in July 2000 are 25% lower than what has been reported previously, for observations near the minimum, or in the increasing phase, of the activity cycle (Stenflo et al. [CITE]). They are in agreement with other observations performed with a different observational set-up in August 2000 (Bommier & Molodij [CITE]). We show that they are consistent with the presence of a weak turbulent magnetic field with an average strength between 20 G and 30 G in the upper solar photosphere. This is about twice the value which was derived from previous observations. This result raises the possiblity of a long-term variation of the turbulent photospheric magnetic field with the activity cycle.

Full-Stokes spectropolarimetry of solar prominences

We report on first spectropolarimetric observations of solar prominences made at THEMIS. Hereafter we shall emphasis on the preliminary analysis of He i D3 full-Stokes observations. Our measurements clearly show manifestations of both the Hanle and Zeeman effects. More generally, we demonstrate the very favourable performances of the THEMIS multi-line spectropolarimetric mode for further new investigations of the magnetic field in solar prominences.

First results from THEMIS spectropolarimetric mode

We present here the very first spectropolarimetric results obtained with the multiline spectroscopy mode (MTR) of THEMIS. The principal problems found during the data analysis are exposed. A first characterisation of the quality of this observing mode is given.