Discovery of the magnetic field of the B1/B2V star σLupi
H.F. Henrichs, K. Kolenberg, B. Plaggenborg, S.C. Marsden, I.A. Waite, J. Landstreet, J. Grunhut, M. Oksala, G. Wade, MiMeS Collaboration
aa r X i v : . [ a s t r o - ph . S R ] A ug Discovery of the magnetic field of the B1/B2Vstar s Lupi
H.F. Henrichs ∗ , K. Kolenberg † , B. Plaggenborg ∗ , S.C. Marsden ∗∗ ,I.A. Waite ‡ , J. Landstreet § , J. Grunhut ¶ , M. Oksala k , G. Wade ¶ andthe MiMeS Collaboration †† ∗ University of Amsterdam, Amsterdam, Netherlands † Universität Wien, Vienna, Austria ∗∗ Anglo-Australian Observatory, Australia ‡ University of Southern Queensland, Toowoomba, Australia § University of Western Ontario, London, Canada ¶ Royal Military College of Canada, Kingston, Ontario, Canada k University of Delaware, Newark, DE, USA †† ∼ wade/mimes/MiMeS__Magnetism_in_Massive_Stars.html Abstract.
In our search for new magnetic massive stars we use the strongest indirect indicatorof a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurringin a velocity range symmetric around zero. Our aim is to obtain follow-up spectropolarimetry tosearch for a magnetic field in magnetic candidate stars. We quantify UV wind line variability, andanalyse its time behaviour. The B1/B2V star s Lupi emerged as a new magnetic candidate star. AATspectropolarimetric measurements with SEMPOL were obtained.The stellar wind line variations of s Lupi are similar to what is known in magnetic B stars,but no periodicity could be determined. We detected a longitudinal magnetic field with varyingstrength and amplitude of about 100 G with error bars of typically 20 G, which supports an obliquemagnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic andthe optical line variations are consistent with the well-known photometric period of 3.02 days, whichwe identify with the rotation period of the star. Additional observations with ESPaDOnS attached tothe CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period.Further analysis revealed that s Lupi is a helium-strong star, with an enhanced nitrogen abundanceand an underabundance of carbon, and has a spotted surface.We conclude that s Lupi is a magnetic oblique rotator, and is a He-strong star. It is the fourthB star for which a magnetic field is discovered from studying only its wind variability. Like inthe other magnetic B stars the wind emission originates in the magnetic equator, with maximumemission occurring when a magnetic pole points towards the Earth. The 3.01819 d magnetic rotationperiod is consistent with the photometric period, with maximum light corresponding to maximummagnetic field. A full paper will be submitted to
Astronomy & Astrophysics . Keywords:
Massive Stars; magnetic fields; star spots, abundances
PACS:
INTRODUCTION
High-resolution spectropolarimeters covering a wide wavelength range (Musicos, SEM-POL, ESPaDOnS, Narval, Sophin) allow to detect organized magnetic fields in B and Ostars. The MiMes Collaboration has as primary goal to search systematically for thesefields. It appears that in nearly all magnetic OB stars the dipole component is dominant.In general, these objects act as oblique rotators. The outflowing stellar wind is perturbedy the surface magnetic field, and is periodically modified. In fact, the discovery of anumber of magnetic early-type stars was preceded by the discovery of strictly periodicwind variability as observed in the UV, which appeared to be the strongest indirect in-dicator for the presence of a magnetic field. By this method three magnetic B stars havebeen found: b Cep, z Cas and V2052 Oph, with rotation periods of 12 d, 5.4 d and 3.6d, respectively. We describe here the discovery of the magnetic field, the period analysisand abundance determination of s Lup.The adopted stellar parameters (partly based on [1, 2]) are: Spectral Type B1/B2V, V = . d = + − pc, log( L / L ⊙ ) = . ± . T eff = ±
550 K, log g = . ± .
10 cms − , R / R ⊙ = . ± . M / M ⊙ = . ± .
5, log Age(y) = . ± . v sin i = ± − , P phot = . ± . P magn = . ± . v rad = . ± . − . STELLAR WIND VARIABILITY
We examineded the 4 usable archival IUE spectra, taken between 1992 and 1993, nearthe Si IV, C IV and N V lines, the prime stellar wind indicators. Fig. 2a shows an overplotalong with the significance of the variations. Significant variability was found, similarto what exclusively is found in magnetic stars. For EW variations see Fig. 2b (top).The three previously discovered magnetic B stars, mentioned above, showed a doublesine wave in the equivalent width of the UV wind lines, with maximum emission(minimum EW) coinciding with maximum field strength, i.e. when a magnetic poleis pointing towards the observer. The expected curve (with arbitrary scaling, see Fig.2b, top) suggests similar behavior for s Lup as well. This supports a model with theemitting material in the magnetic equator.
SPECTROPOLARIMETRY AND MAGNETIC RESULTS –300 –200 –100 0 100 200 3000.850.90.951–0.00100.001–0.00100.001 Velocity (km/s) I / I C N / I C V / I C AAT 1, 2007/12/28 B = 99 ± 16 GN = 0 ± 12 G B l ong ( G ) s Lup B1/B2V, magnetic data 28 Dec 2007 – 2 June 2010 AAT + SEMPOL (circles) CFHT + Espadons (squares)
FIGURE 1. (a) The discovery magnetic measurement of s Lupi at AAT. Displayed are LSD Stokesunpolarised I , (null) N profile (for integrity purposes) and circularly polarised V profiles of SEMPOLspectra. The integrated Zeeman signature in the V profile over the width of the line (within the outerdashed vertical lines) gives the value of the longitudinal component of the magnetic field, integrated overthe stellar surface: B l = ±
16 G. (b) Magnetic data of s Lup. he échelle spectropolarimeters SEMPOL attached to the 3.9 m AAT in Australiaand ESPaDOnS at the 3.6m CFHT at Hawaii were used. The reduction was provided bythe proprietary software of the observatory, called Libre ESpRIT version 2.06, see [3].Weak stellar magnetic fields can be detected through the Zeeman signatures generatedin the shape and polarisation state of spectral line profiles, applying a cross-correlationtechnique the Least-Squares Deconvolution (LSD). The LSD method combines the verysmall circularly polarised signatures, properly weighted, of all available line profiles inthe spectrum to increase the signal to noise ratio. Typical exposure times are 4 ×
200 to4 ×
400 sec. For s Lup we used 171 spectral lines to obtain a mean Stokes V profile.When a magnetic field is present, the Stokes V profile indicates a Zeeman signature. SeeFig. 1a for the discovery Stokes V spectra. –1000 0 1000 20001235 1240 12450240123 Velocity (km s –1 ) (stellar rest frame) s ob s / s e xp F l ux ( – e r g c m –2 s –1 Å –1 ) Wavelength (Å)HD 127381 s LupiIUE N V 4 spectra–1000 0 1000 2000 30001390 1395 1400 1405024012 Velocity (km s –1 ) (stellar rest frame) s ob s / s e xp F l ux ( – e r g c m –2 s –1 Å –1 ) Wavelength (Å) IUE Si IV 4 spectra–1000 0 10001545 1550 1555024012 Velocity (km s –1 ) (stellar rest frame) s ob s / s e xp F l ux ( – e r g c m –2 s –1 Å –1 ) Wavelength (Å) IUE C IV 4 spectra –1 –0.8 –0.6 –0.4 –0.2 0 0.2 0.4 0.6 0.8 1–100–50050100–100–500501001502000.911.11.20.9511.050.911.11.2–0.500.511.52 Magnetic Phase (3.01819 d) B l ong ( G ) ( O – C ) B l ong ( G ) E W ( Å ) E W ( Å ) E W ( Å ) C I V E W ( Å ) s Lup B1/B2V, Dec 2007 – Jun 2010 IUE data 1992 – 1993Si III 4575He I 4471N II 5696Magnetic data AAT + SEMPOL (circles)CFHT + Espadons(squares)Residuals
FIGURE 2. (a) Overplot of variable UV wind lines of s Lupi. Such variability is exclusively found inmagnetic B stars, which prompted a magnetic search. (b) Overplot of all magnetic and EW data foldedwith the adopted period of 3.0186 d. Lower two panels: magnetic data with their residuals with best-fitcosine curve. Middle panels EW of the Si III 4575, He I 4471 and N II 5696 lines with a best-fit sinusoidsuperposed. Note the opposite behavior of the different lines. Top: EW the CIV 1540 doublet 15 yearsearlier (Fig. 2a) with a suggested double sine wave, similar to what is observed in other magnetic B stars.
From the LSD spectra we computed the mean longitudinal field ( B l ), integrated overthe stellar surface. The results are plotted in Fig. 1b. The smallest error bars are about16 G. A best fit with reduced c = . B l ( t ) = B + B max cos ( p ( t − t ) / P ) gives: B = ( ± ) G, B max = ( ± ) G, P = . ± . t = JD2455103 . ± . i > ◦ . The magnetic tilt angle b isthen constrained by the observed ratio B max / B min = cos ( i + b ) / cos ( i − b ) = − . + . − . ,which implies that b is close to 90 ◦ . This gives a polar field of ∼
400 G.
PERIOD AND ABUNDANCE ANALYSIS
The photometric period is 3 . ± . I / I c s Lup B1/B2V, best fit
Fe IIIHe I O II C II
O II
Fe III N IIFe III
N IIHe IN IIO IIN IIO II
He I Mg II
FIGURE 3.
Portion of the optical spectrum of s Lup taken at phase 0.418, (with strongest He lines)with most lines identified, overplotted with the modelfit (thick line) to determine abundances.
CONCLUSIONS s Lup is the fourth magnetic B star found by its stellar wind variability. The polar fieldis ∼
400 G. With the 3.0 d period it is a relatively fast magnetic rotator. The star is a He-rich star, with N-enhancement. The photometric period being the same as the magneticperiod could serve as another indirect indicator for hosting a magnetic field. The EWvariations of the optical lines indicate significant spots on the stellar surface.
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