J/A+A/690/A10 Magnetic fields in featureless white dwarfs (Berdyugin+, 2024)
Searching for magnetic fields in featureless white dwarfs with the DIPOL-UF
polarimeter at the Nordic Optical Telescope.
Berdyugin A., Landstreet J.D., Bagnulo S., Piirola V., Berdyugin S.V.
<Astron. Astrophys. 690, A10 (2024)>
=2024A&A...690A..10B 2024A&A...690A..10B (SIMBAD/NED BibCode)
ADC_Keywords: Stars, white dwarf ; Magnetic fields
Keywords: techniques: polarimetric - stars: magnetic field - white dwarfs
Abstract:
About 20% of white dwarfs possess a magnetic field that may be
detected by the splitting and/or polarisation of their spectral lines.
As they cool, white dwarfs evolve to such a low effective temperature
that no spectral lines can be seen in the visible wavelength range. If
their atmosphere is not polluted by the debris of a planetary system,
such cool white dwarfs have featureless optical spectra. Until quite
recently, very little was known about the incidence of magnetic fields
in such objects. However, when observed with polarimetric techniques,
a significant number of featureless white dwarfs reveal the presence
of a strong magnetic field in their optical continuum spectra.
Measuring the occurrence rate and strength of magnetic fields in old
white dwarfs may help to understand how these fields are generated and
evolve. Here we report the results of an ongoing survey of cool white
dwarfs with the high precision broad-band polarimeter DIPOL-UF
deployed at the Nordic Optical Telescope, La Palma, Spain. This survey
has so far led to the firm discovery of 13 cool magnetic white dwarfs
in the solar neighborhood, including six new detections reported in
this paper.
Description:
During our observing run, we obtained measurements of various bright
nonpolarized nearby stars to calibrate the instrumental polarization.
In addition, we observed the well-known MWD WD 1900+705, which appears
to show a signal of circular polarization that is nearly constant with
time.
The measurements of unpolarized stars yield an instrumental
polarization to a precision better than 10-5 (a few parts per
million). In the B'V'R' bands, the values of the reduced Stokes
parameter V/I are 0.0121±0.0004%, 0.0109±0.0005%, and
0.0084±0.0004%, respectively. This instrumental polarization was
subtracted from the measured polarization of all targets.
Table 1, "Basic parameters of stars observed., gives the list of DC
WDs that we observed during four observing runs on the NOT in 2022
November, 2023 March and November, and 2024 February. We also included
some observations that were obtained in a previous run in July 2022,
which were not published in our previous papers because of an
oversight.
The resulting measurements of the broad-band circular polarization in
each of the three filter bands, together with timing information,
including JD, date, and time at the middle of each observation, and
the total exposure time of each target, are listed in Table 2,
"Observing log of WDs"
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 78 84 Basic parameters of stars observed
table2.dat 83 111 Observing log of WDs
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See also:
B/wd : Spectroscopically identified white dwarfs (McCook+, 2014)
J/ApJ/901/93 : Model atmosphere analysis of hot WDs from SDSS DR12
(Bedard+, 2020)
J/MNRAS/508/3877 : Catalogue of white dwarfs in Gaia EDR3 (Gentile+, 2021)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Star name according to McCook & Sion,
WDHHMM+DDd (1999ApJS..121....1M 1999ApJS..121....1M, Cat. B/wd)
12 A1 --- f_Name [*] A "*" in this column means that the star
was newly discovered magnetic
14- 41 A28 --- SName Simabd main identified
43- 46 F4.1 mag Gmag G magnitude
48- 52 F5.2 pc d distance
54- 58 I5 K Teff Effective temperature from Gentile Fusillo et
al. (2021MNRAS.508.3877G 2021MNRAS.508.3877G,
Cat. J/MNRAS/508/3877)
60- 63 F4.2 Msun Mass Mass from Gentile Fusillo et al.
(2021MNRAS.508.3877G 2021MNRAS.508.3877G, Cat. J/MNRAS/508/3877
65- 69 F5.3 Gyr Age Age interpolated from the cooling tables by
Bedard et al. (2020ApJ...901...93B 2020ApJ...901...93B,
Cat. J/ApJ/901/93)
71- 78 A8 --- Comp Main atmospheric composition
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 --- Name Star name according to McCook & Sioni,
WDHHMM+DDd (1999ApJS..121....1M 1999ApJS..121....1M, Cat. B/wd)
12 A1 --- f_Name [*] A "*" in this column means that there is
a definite detection
14- 23 A10 "date" Obs.date Date of the observation
25- 29 A5 "h:m" Obs.time UT of the mid observation
31- 39 F9.3 d JD Julian date of the observation (JD-2400000)
41- 44 I4 s ExpTime Exposure time of the observation
46- 51 F6.3 % CP(B) Circular polarisation in % in the B filter
53- 57 F5.3 % e_CP(B) Uncertainty of the circular polarisation
in % in the B' filter
59- 64 F6.3 % CP(V) Circular polarisation in % in the B filter
66- 70 F5.3 % e_CP(V) Uncertainty of the circular polarisation
in % in the V' filter
72- 77 F6.3 % CP(R) Circular polarisation in % in the B filter
79- 83 F5.3 % e_CP(R) Uncertainty of the circular polarisation
in % in the R' filter
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Acknowledgements:
Stefano Bagnulo, Stefano.Bagnulo(at)Armagh.ac.uk
(End) Patricia Vannier [CDS] 15-Jul-2024