J/A+A/583/A115 FORS1 catalogue of stellar magnetic fields (Bagnulo+, 2015)
The FORS1 catalogue of stellar magnetic field measurements.
Bagnulo S., Fossati L., Landstreet J.D., Izzo C.
<Astron. Astrophys. 583, A115 (2015)>
=2015A&A...583A.115B 2015A&A...583A.115B (SIMBAD/NED BibCode)
ADC_Keywords: Magnetic fields ; Polarization ; Spectroscopy
Keywords: polarization - catalogs - stars: magnetic fields
Abstract:
The FORS1 instrument of the ESO Very Large Telescope was used to
obtain low resolution circular polarized spectra of nearly a thousand
of different stars, with the aim of measuring their mean longitudinal
magnetic fields. Magnetic fields were measured by different authors,
and using different methods and software tools.
A catalogue of FORS1 magnetic measurements would provide a valuable
resource with which to better understand the strengths and limitations
of this instrument and of similar low-dispersion, Cassegrain
spectropolarimeters. However, FORS1 data reduction has been carried
out by a number of different groups using a variety of reduction and
analysis techniques. Both our understanding of the instrument and our
data reduction techniques have improved over time. A full re-analysis
of FORS1 archive data using a consistent and fully documented
algorithm would optimise the accuracy and usefulness of a catalogue of
field measurements.
Based on ESO FORS pipeline, we have developed a semi-automatic
procedure for magnetic field determinations, which includes
self-consistent checks for field detection reliability. We have
applied our procedure to the full content of circular
spectropolarimetric measurements of the FORS1 archive.
We have produced a catalogue of spectro-polarimetric observations and
magnetic field measurements for 1400 observations of 850 different
objects. The spectral type of each object has been accurately
classified. We have also been able to test different methods for data
reduction is a systematic way. The resulting catalogue has been used
to produce an estimator for an upper limit to the uncertainty in a
field strength measurement of an early type star as a function of the
signal-to-noise ratio of the observation.
While FORS1 is not necessarily an optimal instrument for the discovery
of weak magnetic fields, it is very useful for the systematic study of
larger fields, such as those found in Ap/Bp stars and in white dwarfs.
Description:
Magnetic fields measurements obtained with the FORS1 instrument of the
ESO VLT from 2002 to 2009.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 312 1391 Magnetic field measurements
sp/* . 1391 Individual tarball files with the spectra
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See also:
II/134 : Catalog of Magnetic Field Measurements (Didelon 1983)
J/A+A/407/631 : Stellar effective magnetic fields catalogue (Bychkov+, 2003)
J/A+A/430/1143 : Stellar magnetic rotational phase curves (Bychkov+, 2005)
J/MNRAS/394/1338 : Stellar effective magnetic fields. II. (Bychkov+, 2009)
J/PASP/126/469 : PolarBase catalogue of stellar spectra (Petit+, 2014)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension from the FORS1
fits-headers (J2000) (1)
4- 5 I2 min RAm Right ascension from the FORS1
fits-headers (J2000) (1)
7- 10 F4.1 s RAs [0/60] Right ascension from the FORS1
fits-headers (J2000) (1)
12 A1 --- DE- Declination sign from the FORS1
fits-headers (J2000) (1)
13- 14 I2 deg DEd Declination from the FORS1
fits-headers (J2000) (1)
16- 17 I2 arcmin DEm Declination from the FORS1
fits-headers (J2000) (1)
19- 20 I2 arcsec DEs [0/60] Declination from the FORS1
fits-headers (J2000) (1)
22- 36 A15 --- Name Star identifier
38- 52 A15 --- Class Classification
54- 63 A10 --- PID Programme ID
65- 73 F9.3 d MJD Modified Julian Date at mid observing series
75- 84 A10 "date" Obs.date Date of the observation (mid exposure)
86- 93 A8 "h:m:s" Obs.time Time of the observation (mid exposure)
95- 99 I5 s Texp Exposure time
101-103 I3 --- N Number of exposures
105-109 A5 --- grism Grism name
111-114 F4.2 arcsec width Slit width
116-119 I4 --- R Spectral resolution
121-125 I5 0.1nm blambda Lower limit of the observed spectral range
127-130 I4 0.1nm Blambda Upper limit of the observed spectral range
132-136 I5 --- SNR Signal-to-noise ratio per Angstrom
138-141 I4 0.1nm lsnr Central wavelength of the 100Å interval
range in which the S/N ratio reaches the
maximum
143-148 I6 gauss <Bz>(H) Longitudinal field measured from H Balmer
lines
150-153 I4 gauss e_<Bz>(H) Error bar of <Bz>(H)
155-160 F6.2 --- chi<Bz>(H) Reduced minimum chi-square from the fit
to measure <Bz>(H)
163-168 I6 gauss <Nz>(H) Null field measured from H Balmer lines
170-173 I4 gauss e_<Nz>(H) Error bar of the Null field measured
from <Nz>(H)
175-180 F6.2 --- chi<Nz>(H) Reduced minimum chi-square from the fit to
measure the <Nz>(H)
183-188 I6 gauss <Bz>(m) Longitudinal field measured from metal lines
190-193 I4 gauss e_<Bz>(m) Error bar of <Bz>(m)
195-200 F6.2 --- chi<Bz>(m) Reduced minimum chi-square from the fit
to measure <Bz>(m)
203-208 I6 gauss <Nz>(m) Null field measured from metal lines
210-213 I4 gauss e_<Nz>(m) Error bar of the Null field measured
from <Nz>(m)
215-220 F6.2 --- chi<Nz>(m) Reduced minimum chi-square from the fit
to measure the <Nz>(m)
223-228 I6 gauss <Bz>(T) Longitudinal field measured from H Balmer
and metal lines
230-233 I4 gauss e_<Bz>(T) Error bar of <Bz>(T)
235-240 F6.2 --- chi<Bz>(T) Reduced minimum chi-square from the fit
to measure <Bz>(T)
243-248 I6 gauss <Nz>(T) Null field measured from H Balmer and
metal lines
250-253 I4 gauss e_<Nz>(T) Error bar of the Null field measured
from <Nz>(T)
255-260 F6.2 --- chi<Nz>(T) Reduced minimum chi-square from the fit
to measure the <Nz>(T)
262-312 A51 --- FileName Name of the tarball file with the intensity
spectra in subdirectory sp (2)
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Note (1): Coordinates in the fits-headers are close but not identical to
Simbad coordinates of the object named in record "Name"
(the star identifier).
Note (2): The full description of the CDS material (as well of its purpose) is
given in the paper, in particular the tarball file is described in Sect. 6.4,
titled: "The archive of intensity spectra".
Each tarball includes an ASCII file named as the tarball itself
(less the extension .tar) with the list of original frames used for the
field determination. In this file, each filename is followed by:
the exposure time in seconds of each individual frame (fits-header keyword
EXPTIME); the position angle of the retarder waveplate with respect to the
parallel beam of the Wollaston prism (fits-header keyword INS.RETA4.ROT);
the exposure number, and the total number of exposures in each OB template,
and the OB number, given by fits-keywords NEXP EXP and OB, respectively.
The file name list is followed by two lines with the grism name,
the slit width, the spectral resolution, the detector name and the readout
mode.
The tarball includes an ASCII file for each frame of the observing series
used to determine the longitudinal magnetic field. This file is identified
by the same name as the original FORS frame archive name, having replaced
the extension .fits with .prof. In case of observations obtained in
multi-object mode, the name of the .prof file refers also to the slitlet
number where the star was centred, e.g.,
FORS1.YYYY-MM-DDThh-mm-ss.xxx_Syy.fits, where yy may be 02, 04,..., 18.
The spectrum files (FORS1.YYYY-DD-MMThh:mm:ss.sss.prof) are in
the following format :
----------------------------------------------------------------------------
Bytes Format Units Label Explanations
----------------------------------------------------------------------------
1- 9 F9.3 0.1nm lambda Wavelength
13- 23 E11.6 --- Flux Flux
26- 36 E11.6 --- e_Flux rms uncertainty on Flux (NaN when no value)
----------------------------------------------------------------------------
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Acknowledgements:
Stefano Bagnulo, sba(at)arm.ac.uk
Luca Fossati, luca.fossati(at)oeaw.ac.at
Landstreet, jlandstr(at)uwo.ca
(End) Patricia Vannier [CDS] 04-Aug-2015