J/A+A/618/A113 Spectropolarimetric survey of white dwarfs (Bagnulo+, 2018)
Searching for the weakest detectable magnetic fields in white dwarfs.
Highly-sensitive measurements from first VLT and WHT surveys.
Bagnulo S., Landstree J.D.
<Astron. Astrophys. 618, A113 (2018)>
=2018A&A...618A.113B 2018A&A...618A.113B (SIMBAD/NED BibCode)
ADC_Keywords: Stars, white dwarf ; Spectroscopy
Keywords: magnetic fields - polarisation - white dwarfs
Abstract:
Our knowledge of the magnetism in white dwarfs is based on an
observational dataset that is biased in favour of stars with very
strong magnetic fields. Most of the field measurements available in
the literature have a relatively low sensitivity, while current
instruments allow us to detect magnetic fields of white dwarfs with
sub-kG precision. With the aim of obtaining a more complete view of
the incidence of magnetic fields in degenerate stars, we have started
a long-term campaign of high-precision spectropolarimetric
observations of white dwarfs. Here we report the results obtained so
far with the low-resolution FORS2 instrument of the ESO VLT and the
medium-resolution ISIS instrument of the WHT. We have considered a
sample of 48 stars, of which five are known magnetic or suspected
magnetic stars, and obtained new longitudinal magnetic field
measurements with a mean uncertainty of about 0.6kG. Overall, in the
course of our survey (the results of which have been partially
published in papers devoted to individual stars) we have discovered
one new weak-field magnetic white dwarf, confirmed the magnetic nature
of another, found that a suspected magnetic star is not magnetic, and
suggested two new candidate magnetic white dwarfs. Even combined with
data previously obtained in the literature, our sample is not
sufficient yet to reach any final conclusions about the actual
incidence of very weak magnetic fields in white dwarfs, but we have
set the basis to achieve a homogeneous survey of an unbiased sample of
white dwarfs. As a by-product, our survey has also enabled us to carry
out a detailed characterisation of the ISIS and the FORS2 instruments
for the detection of extremely weak magnetic fields in white dwarfs,
and in particular to relate the signal-to-noise ratio to measurement
uncertainty for white dwarfs of different spectral types. This study
will help the optimisation of future observations.
Description:
Stokes I, V/I and Null spectra obtained with the FORS2 instrument of
the ESO VLT and of the ISIS instrument of the WHT are made available
in form of ASCII tables. Table MWDs.dat gives the name of the star,
the epoch of the observation, exposure time, setting and provide a
link to the ASCII file with the Stokes I and V/I spectra, their
uncertainties, and the null spectra. See paper for more details. For
the definition of the null spectra see Bagnulo et al.
(2009PASP..121..993B 2009PASP..121..993B). Dataset includes two spectra of WD0447+176
which is not a white dwarf.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
mwds.dat 98 109 Table with star names, information about
spectra, and link to 109 ASCII tables
sp/* . 109 Individual spectrum files
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See also:
J/A+A/583/A115 : FORS1 catalogue of stellar magnetic fields (Bagnulo+, 2015)
Byte-by-byte Description of file: mwds.dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Name Name of the star
12- 22 A11 --- OName Alternative name of the star
24- 33 A10 "YYYY-MM-DD" Obs.date Observation date
35- 39 A5 "h:m" Obs.time Time of mid observation (UT)
41- 44 I4 s Texp Total exposure time
46- 54 A9 --- Inst Instrument and telescope
56- 61 A6 --- Setting Grims or grating
63- 65 F3.1 arcsec Slit Slit width
67- 98 A32 ---- FileName Filename of the ASCII table with spectra
in subdirectory sp
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Byte-by-byte Description of file (#): sp/*
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Bytes Format Units Label Explanations
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2- 15 E14.7 0.1nm lambda Wavelength in Angstroem
17- 30 E14.7 --- StokesI Stokes I intensity in arbitrary units
32- 45 E14.7 --- e_StokesI Error on Stokes I (same units as Stokes I)
47- 60 E14.7 --- StokesV/I Reduced Stokes V/I parameter
62- 75 E14.7 --- e_StokesV/I Error on reduced Stokes V/I parameter
77- 90 E14.7 --- Null Null parameter
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Acknowledgements:
Stefano Bagnulo, stefano.bagnulo(at)gmail.com
(End) S. Bagnulo [AOP, Northern Ireland], P. Vannier [CDS] 31-Jul-2018