J/A+A/545/A119 Stromgren v photometry of σ Lup (Henrichs+, 2012)
Discovery of a magnetic field in the early B-type star sigma Lupi.
Henrichs H.F., Kolenberg K., Plaggenborg B., Marsden S.C., Waite I.A.,
Landstreet J.D., Wade G.A., Grunhut J.H., Oksala M.E., MiMeS collaboration
<Astron. Astrophys. 545, A119 (2012)>
=2012A&A...545A.119H 2012A&A...545A.119H
ADC_Keywords: Stars, B-type ; Magnetic fields ; Photometry, uvby
Keywords: stars: early-type - stars: magnetic field - stars: winds, outflows -
stars: rotation - starspots - stars: abundances
Abstract:
Magnetic early B-type stars are rare. Indirect indicators are needed
to identify them before investing in time-intensive
spectropolarimetric observations.
We use the strongest indirect indicator of a magnetic field in B
stars, which is periodic variability of ultraviolet (UV) stellar wind
lines occurring symmetric about the approximate rest wavelength. Our
aim is to identify probable magnetic candidates which would become
targets for follow-up spectropolarimetry to search for a magnetic
field.
From the UV wind line variability the B1/B2V star sigma Lupi emerged
as a new magnetic candidate star. AAT spectropolarimetric measurements
with SEMPOL were obtained. The longitudinal component of the magnetic
field integrated over the visible surface of the star was determined
with the least- squares deconvolution method.
The UV line variations of sigma Lupi are similar to what is known in
magnetic B stars, but no periodicity could be determined. We detected
a varying longitudinal magnetic field with amplitude of about 100G
with error bars of typically 20G, which supports an oblique
magnetic-rotator configuration. The equivalent width variations of the
UV lines, the magnetic and the optical-line variations are consistent
with the photometric period of 3.02d, which we identify with the
rotation period of the star. Additional observations with ESPaDOnS
attached to the CFHT confirmed this discovery, and allowed the
determination of a precise magnetic period. Analysis revealed that
sigma Lupi is a helium-strong star, with an enhanced nitrogen
abundance and an underabundance of carbon, and has a chemically
spotted surface. Conclusions. sigma Lupi is a magnetic oblique
rotator, and is a He-strong star. Like in other magnetic B stars the
UV wind emission appears to originate close to the magnetic equatorial
plane, with maximum emission occurring when a magnetic pole points
towards the Earth. The 3.01972±0.00043d magnetic rotation period is
consistent with the photometric period, with maximum light
corresponding to maximum magnetic field.
Description:
Photometric data for sigma Lupi, obtained with the The Small and
Moderate Aperture Research Telescope System, see
http://www.astro.yale.edu/smarts/
Objects:
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RA (2000) DE Designation(s)
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14 32 37.06 -50 27 25.8 sigma Lupi = HD 127381
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
photom.dat 24 314 *Stroemgren v differential magnitudes
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Note on photom.dat: Photometry by M.E. Oksala.
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Byte-by-byte Description of file: photom.dat
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Bytes Format Units Label Explanations
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1- 11 F11.5 d MJD Modified Julian date
13- 18 F6.3 mag dvmag Differential Stroemgren v magnitude
20- 24 F5.3 mag e_dvmag 1 sigma error in dvmag
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Acknowledgements:
Huib Henrichs, H.F.Henrichs(at)uva.nl
(End) Huib Henrichs [Amsterdam], Patricia Vannier [CDS] 06-Sep-2012