This new instrument is one of the only spectropolarimetric facilities worldwide and should be extremely well adapted for studying stellar magnetic fields and in particular their topologies through rotational modulation of linear (Stokes Q and U) and circular polarisation (Stokes V) Zeeman signatures in line profiles. It is also a very interesting tool for investigating geometries of non-axisymmetric circumstellar environments through depolarisation structures of spectral lines formed within the scattering envelope.
This page presents the basic characteristics of this instrument,
describes its operation and details the
first results obtained to date during the first three observing runs at TBL
(August 1996, February 1997) and CFH (November 1996). A more practical
users' guide to the MuSiCoS polarimeter is also available for new
observers.
The new MuSiCoS polarimeter is mounted at the TBL Cassegrain focus through an
interface module including all usual spectroscopic calibration facilities
(halogen and thorium/argon lamp). As shown below, stellar light is collected
in a 2" (500 micron) entrance aperture. Linear/circular sheet
polarisers
can be inserted in the beam for all tuning and checkout procedures. One quarter-wave
and one half-wave retarder can also be inserted in the beam
and rotated to achieve, in conjunction with a beamsplitter,
a circular or linear analysis of the stellar light respectively. A focal
reducer then converts the beam aperture from f/25 to f/2.5. Finally, two 50
micron core optical fibres collect the two beams emerging
from the Zeeman analyser (corresponding to both orthogonal states of the selected
polarisation form) and convey them to the MuSiCoS
spectrograph.
The two retarders were supposed to be
Halle
superachromatic
Quartz-MgF2 wave plates, i.e. the only commercial crystalline retarders with
close to nominal retardance and fast axis direction throughout the whole spectral
domain of the MuSiCoS spectrograph
(390 to 870 nm). However, we discovered that these wave plates
introduce large ripples both in polarisation and intensity
spectra (with respective peak-to-peak amplitudes of about 1% and 2% respectively),
most likely due to interference within the multiple cement layers of the retarders.
This is very likely a general problem of all
Halle
superachromatic
retarders (rather
than a specific defect of ours), since all similar plates that we know of (those of
the AAT and WHT polarimetric modules) also generate such ripples.
We therefore switched to a less achromatic Fichou quarter wave
plate for which such ripples are at least 20 times weaker.
The expense is of course that circular
spectropolarimetry is now restricted to a wavelength range of
400 to 700 nm only (till the
Halle waveplates are fixed and
if they can be fixed) and requires a special "intermediate" setup of the
MuSiCoS spectrograph
between the two nominal "blue" and "red" configurations. For the same
reason, we no longer use the Halle
half-wave plate (also replaced by a Fichou half-wave
plate) and rotate
instead the polarimeter as a whole, making linear spectropolarimetry
perfectly achromatic throughout the whole wavelength range of the
MuSiCoS spectrograph but slightly less convenient to
operate than circular spectropolarimetry (see below).
The beamsplitter is made of two crossed calcite blocks
(Savart plate). The
advantage of such a design is that both beams emerging from the analyser are
shifted by the same amount with respect to the incident beam and are therefore
more symmetric than with a conventional single calcite block.
The focal reducer was optimised for our purpose to
minimise longitudinal chromatic aberrations.
The optical fibre we selected to collect the two beams and
send them to the MuSiCoS spectrograph is the H-treated
low-OH Ceram-Optec fibre with 50/60 micron
core/cladding diameters, whose transmission
is optimised in the 350-1100 nm wavelength band. Transmission
measurements indicate that this fibre does not match our expectations in the
infrared domain, but is still acceptable up to 700 nm.
The total throughput of the MuSiCoS spectropolarimeter
(atmosphere, telescope and detector included) peaks at about 0.85%, i.e. the same as
that of the MuSiCoS spectrograph.
The polarimeter is controlled from a terminal in the
TBL
control room through a
serial link, a microcontroller card and attendant electronics. Rotating the
polarimeter wheel, the waveplate wheel, and the waveplate holders is achieved
with three stepper motors and three proximity detectors. Four microswitches
are used to check that the instrument has safely reached the requested positions.
Thirteen polarimeter configurations are available
altogether, each associated to a two-character command. A list of these
commands (and error messages) is given here.
For each polarisation exposure, one needs to obtain a sequence
of four individual subexposures at alternating waveplate or instrument azimuths:
A more extensive users' guide to this instrument (description of
set-up, operation and data reduction) is also available for potential observers.
We find that the MuSiCoS spectropolarimeter performs very well for detecting
polarisation/depolarisation structures in line profiles.
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Description of the instrument
Instrument control and data reduction
The last operation consists in feeding the dedicated reduction package
ESpRIT (installed on the DecAlpha workstation of the TBL
control room) with the sequence of four subexposures (plus flat fields, arc and bias
frames) to get the corresponding wavelength-calibrated optimally-extracted polarisation
spectrum (within typically 5 min). 
For Stokes V observations, one needs to set
the quarter-wave plate (once in the beam: command l1) at azimuth -45
degrees (command q1), take one exposure, then drive it to azimuth 45
degrees (q3), take 2 exposures, bring it back at azimuth -45
degrees (q1) and take a fourth exposure.
For Stokes Q observations, a first solution consists in
using the half-wave plate (command l3) and driving it
successively to azimuth 45 degrees (command d3, one
exposure), 0 degrees (d1, two exposures) and 45 degrees
(d3, one exposure). A second preferable solution
consists in removing the half-wave plate from the beam (l2)
and ask the telescope operator to rotate the instrument (not
the Cassegrain bonette) to azimuths 90 degrees (one exposure),
0 degrees (two exposures) and back to
90 degrees (one exposure). This second solution has the
advantage of being
perfectly achromatic (hence reducing possible crosstalk from circular to linear polarisation
to an absolute minimum), even if slightly less convenient to operate (one must check the
automatic guiding after each instrument rotation).
For Stokes U observations, the first solution consists in
driving the half-wave plate to azimuths 22.5 degrees (d2, one
exposure), 67.5 degrees (d4, two exposures) and 22.5 degrees
(d2, one exposure), while the second
(better) solution consists in
rotating the instrument to azimuths 45 degrees (one exposure),
-45 degrees (two exposures)
and back to 45 degrees (one exposure).
First results
As one can check on the graphs below, Stokes V Zeeman signatures
in line profiles (due to the presence of
stellar surface
magnetic fields)
are very easily detected on active stars like II Peg
(left panel) with cross-correlation techniques such as
Least-Squares
Deconvolution (LSD). Note in particular that the detected signal in the
average Stokes V spectrum (upper curve) is observed
in close association with the mean line profile in the Stokes
I spectrum (lower curve).
Very weakly active sharp-lined objects like the
integrated Sun (right panel) indicate that no spurious signatures are observed
down to a relative noise level of 0.003% rms (note the different expansion factors for
Stokes V). Monitoring such Zeeman signatures on a full stellar rotational cycle
allows one to
map
the parent surface magnetic field topology.
Our instrument can also be used to study the details of the field topologies of
magnetic chemically peculiar stars using circular
and linear Zeeman signatures such magnetic structures induce in spectral line
profiles. The integrated Stokes Q and U profiles we measure for standard stars
(e.g. 78 Vir) yield average linear polarisation angles and signs
that are in perfect agreement with results published by Leroy from broadband linear
spectropolarimetry. We even observe that linear polarisation profiles are
much more informative than broadband measurements. The two
graphs below present MuSiCoS Stokes Q, U, V and I
(from top to bottom, note the
different expansion factors for Stokes V and Stokes Q/U)
spectropolarimetric observations of 53 Cam at rotational phase
0.845, i.e. when broadband measurements indicate a very weak level of linear
polarisation.
LSD
Stokes Q and U profiles of 53 Cam at this phase, although
null in average, do possess a complex shape which should
reveal crucial clues on the details of the parent magnetic field topology and its
departures from simple large scale geometries.
The MuSiCoS spectropolarimeter should also be very useful for studying how
emission lines (and in particular forbidden lines) get depolarised with respect to
the surrounding continuum, and thus for determining where these
emission lines form with respect to the scattering environment.
Such depolarisation structures have been observed for instance in the case of the hot
star theta Ori (see graph below) whose continuum is circularly polarised at a rate of a
few %. The fact that the emission lines of nebular origin (the [N II] lines at 654.81 and
658.36 nm, the central peak of Halpha, but also the [O III] lines at 495.89 and 500.68 nm
not shown on graph) are depolarised in exact proportion to the line to
continuum flux ratio indicates that all these lines
are formed outside the scattering circumstellar environment and that the continuum
circular polarisation is therefore of stellar (rather than
interstellar) origin.
Another potential scientific applications of our instrument in this field is the
study of how strong winds of O stars depart from spherical
symmetry through the depolarisation of spectral lines formed in the wind.
Our polarimeter can in principle also be used to estimate continuum
polarisation in stellar spectra. However, we observe that our fibre setup introduces
spurious continuum polarisation through small motions of the
double image at fibre level (due to wave plate rotations, instrument flexures with telescope
movement or simply small random fluctuations in light injection) coupled to slight
differential mispositioning of the two images onto the two fibres (due to small
magnification errors in the focal reducer, to slight azimuthal misalignment of the
analyser with respect to the fibre or to the chromatism of the Zeeman analyser). The
accuracy to which continuum polarisation can be estimated with our instrument
is typically 0.8% rms for circular polarisation, and about
twice as much for linear polarisation. For such programs,
our instrument is thus
poorly competitive with photopolarimeters like Sterenn or Cassegrain low-resolution
spectropolarimeters, usually an order of magnitude more accurate at measuring continuum
polarisations of stellar objects. Note that this problem does not affect
measurements of line profile polarisation signatures as we usually remove a
posteriori any true or spurious
continuum polarisation from the observations in this case.
Any questions/comments/suggestions about either this page or the MuSiCoS polarimeter should be
sent to jean-francois.donatiATnospam.ast.obs-mip.fr
Related publications
Shorlin S.L.S, Wade G.A., Donati J.-F., Landstreet J.D.,
Petit P., Sigut T.A.A., Strasser S.,
``A sensitive search for magnetic fields in
B, A and F stars'' (2002) A&A 392, 637 Donati J.-F., Wade G.A., Babel J., Henrichs H.F.,
de Jong J.A., Harries T.J.,
``The magnetic field and wind confinement of
beta Cephei: new clues for interpreting the Be phenomenon?''
(2001) MNRAS 326, 1265 Bagnulo S., Wade G.A., Donati J.-F., Landstreet J.D.,
Leone F., Monin D.N., Stift M.J.,
``A study of polarised spectra of magnetic CP
stars: predicted vs. observed Stokes IQUV profiles for beta CrB and
53 Cam'' (2001) A&A 369, 889 Wade G.A., Donati J.-F., Landstreet J.D.,
``An atlas of Zeeman polarisation in the
Stokes IQUV spectrum of Beta Coraonae Borealis'' (2001) New
Astronomy 5, 455 Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S.,
``Spectropolarimetric measurements of
magnetic Ap and Bp stars in all four Stokes parameters'' (1999) MNRAS
313, 823 Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S.,
``High precision magnetic field
measurements of Ap and Bp stars'' (1999) MNRAS 313, 851 Donati J.-F., Wade G.A.,
``On
the magnetic field and circumstellar
environment of the young O7 star theta1 Orionis C''
(1999) A&A 341, 216 Donati J.-F., Catala C., Wade G.A., Gallou G., Delaigue G., Rabou P.,
``A
dedicated polarimeter for the MuSiCoS échelle spectrograph''
(1999) A&AS 134, 149 Donati J.-F., Semel M., Carter B.D., Rees D.E., Cameron A.C.,
``Spectropolarimetric observations of active stars''
(1997) MNRAS 291, 658
since 1999 Nov. 17.
© Jean-François Donati, last update on 2003 Jun. 16