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Physics of Magnetic Stars, 2007, pp. 262-270

Line profile variability and the possible magnetic field in the spectra of supergiant Leo
A.F. Kholtygin1 , G.A. Chountonov2 , S.N. Fabrika2 , T.E. Burlakova Kang Dong-il3 , G.A. Galazutdinov2,4 , M.V. Yushkin2
1 2

2,3

, G.G. Valyavin

2,3

,

Astronomical Institute of Saint-Petersburg State University Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia 3 Bohyunsan Optical Astronomy Observatory (BOAO), Jacheon P.O.B.1, YoungChun, KyungPook, 770-820, Korea 4 Korea Astronomy and Space Science Institute, Optical Astronomy Division, 61-1, Whaam-Dong, YuseongGu, Daejeon, 305-348, Korea

1

Intro duction

Line profiles in sp ectra of OB stars are usually strongly variable (Morel et al. 1998). One can detect b oth the sto chastic line profile variability (lpv) connected with formation of the small-scale structures in the stellar wind (Eversb erg et al. 1998, Kholtygin et al. 2003) and the regular lpv, induced by the large-scale structures in the wind (de Jong et al. 2001). The regular line profile variability are often connected with the co-rotation of the large-scale structures in the wind (Kap er et al. 1999). The latter might b e explained by accepting the hyp othesis that hot stars p ossess global magnetic fields (Neiner 2002, Donati et al. 2002). Magnetic field can also regularize the wind structures induced by stellar non-radial pulsations (Owo cki and Cranmer 1988). The recent measurements have shown that only two O stars and a small part of B stars p ossibly have magnetic fields (e.g. Donati et al. 2001, Donati et al. 2002, Henrichs et al. 2003). So, the problem of searching for the magnetic field of O and early B stars is still actual. Recently we have prop osed (Kholtygin et al. 2004) the program of searching for weak magnetic field of OB stars with the aim to know if the magnetic field is the common feature of all OB stars or not. In the present pap er we rep ort the results of searching for magnetic field of the B1I sup ergiant Leo.

2

Main information ab out Leo. Observations and data reduction

The sup ergiant Leo (HD 91316) is a slowly rotating (V sin i = 75 km/s) star of sp ectral class B1Ib. The effective temp erature of the star T eff is very uncertain. In the pap er by Morel et al. (2004) a value of Teff = 20260 K was given, but according to SteLib T eff = 24200 K, lg(g ) = 3.09. On the HR Diagram Leo is lo cated near the Cep star instability domain (see, for example, Pamyatnykh 1999). Parameters of the star are given in Table 1. In the table Teff -- effective temp erature of the star, M -- mass of the main comp onent of the system, M -- mass loss rate, L -- b olometric velo city, V -- terminal velo city of stellar wind, Vsin i -- rotation velo city of the star. The observations were made in 2004-2005. In 2004 the star was observed in the Sp ecial Astrophysical Observatory (SAO) with using the 6-m telescop e (Kholtygin et al. 2006c , sp ectrograph NES) and also in Bohyunsan Optical Astronomy Observatory (BOAO) at the 1.8-m telescop e with 262


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Table 1: Parameters of the star Leo
Parameter Teff , M /M M /M V M - lg M lg L/L Vsin i (km/s) Value 24200 22 32 1110 -6.20 5.18 75 References SteLib Morel et al. (2004) Morel et al. (2004) Howarth et al. (1997) Morel et al. (2004) Morel et al. (2004) Howarth et al. (1997)

the BOES sp ectrograph. In 2005 sp ectra of the program star were obtained at the 6-m telescop e of SAO with the NES and MSS sp ectrographs. The log of observations is given in Table 2. Table 2: Observation of Leo in 2004-2005
Number of spectra 30 7 11 15 8 2 2 Exposition (min) 6 9/10 4 4/7 4 20 20 Full time of observations Telescope Jan 10/11 2004 3.5 SAO1 , 6-m 0.6 BO, 1.8 m Jan 14/15 2004 2.5 BO, 1.8 m Feb 3/5 2004 3.0 BO, 1.8 m Jan 30/31 2005 0.5 SAO, 6-m Feb 22/23 2005 1.0 SAO, 6-m Feb 23/24 2005 1.0 SAO, 6-m Spectrograph, CCD NES2 , 2kx2k BOES, 2kx2k BOES, 2kx4k BOES, 2kx4k MSS3 , 2kx2k NES, 2kx2k NES, 2kx2k

Comments: 1 Special Astrophysical Observatory in the Northern Caucasus, Russia 2 Echelle spectrograph in the Nasmyth focus of the 6-meter telescope of SAO 3 Main Stellar spectrograph in the Nasmyth focus

Sp ectral observation in SAO on January 10/11 2004 were made in the region 4500 - 6000 А with using a quartz echelle sp ectrograph NES in Nasmyth fo cus (Panchuk 2002) with A 2048з2048 Uppsala CCD. The reduction of SAO sp ectra was made with using the MIDAS package (eg., Kholtygin et al. 2003). For finding the p ositions of the sp ectral order the metho d of Ballester (1994) was employed. For studying lpv, sp ectra was normalized to the individual continuum for each sp ectral order. The metho d of Shergin (1996) was used for determining the continuum level. Part of the sp ectra was obtained at BOAO 1.8-m telescop e on January 11, 14, 15 and February 3/5, 2004 with the fib er-fed echelle sp ectrograph BOES (BOES) with a large CCD (2048з4096 pixA A els, 15з15 ЕЕ p er pixel). All 17 sp ectra in BOAO were obtained in the 3782 А 9803 А region. Preliminary reduction of CCD frames was fulfilled with the help of IRAF. The next pro cessing steps were made with using the mo dified version (Dech20T) of the Dech package (Galazutdinov, 1992)). All BOAO sp ectra were normalized to the continuum level. The pro cedure of finding the continuum level has recently b een describ ed by us (Kholtygin et al. 2006a). Observations in SAO in 2005 were made b oth with the NES sp ectrograph and with the main


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stellar sp ectrograph (MSS). Observations were fulfilled with a new p olarization analyzer describ ed by Chountonov (2004) . Then analyzer consists of an achromatic waveplate, which can b e rotated and take 2 p ositions (0o and 45o ), a diaphragm of 5 arcsec, a dichroic p olarizer, a double slicer and a slit. The double slicers in the new analyzer is used to increase the efficiency of measurements. The numb er of strips is 14 (7 strips p er each p olarization). The light from a star passes through the liquid crystal mo dulator, which can b e in two states to create phase shifts of 0 o and 180o . The pro cedure of reduction for sp ectra obtained in 2005 is also describ ed by Chountonov (2004). All sp ectra was normalized to the continuum level. The quality of sp ectra app eared to b e very go o d with a signal-to-noise ratio of up to 2100.

3

Line profile variations

Night mean line profiles in sp ectra of OB stars show significant variations (de Jong et al. 2001). For А example in Fig. 1 we plot part of the sp ectra of Leo in the wavelengths interval 5650-5720 A in comparison with the mean over all the sp ectra of Leo that we obtained in 2004 (see Table 2).

Figure 1: Mean sp ectra of Leo obtained in 2004 and 2005. In the figure we can see that the amplitude of the lpv in sp ectra of Leo is ab out of 1-2% in the continuum units. Night mean profiles obtained for Leo in SAO and BOAO on January 11, 2004 practically coincide (Kholtygin et al. 2006c). That is evidence of the go o d internal quality of the pro cedures we used for drawing the continuum level.

3.1

The regular line profile variability in sp ectra of Leo

To illustrate the lpv in sp ectra of Leo, we plot different line profiles for 30 sp ectra obtained on January 10/11, 2004 in SAO together with the night mean sp ectrum in Fig. 2. One can see that the lpv o ccurs only in the limits of the line profiles and there is no lpv out of the lines. The Clean analysis of the lpv in sp ectra of Leo reveal 8 regular comp onents in the frequency region 0.14 6.2 d-1 with p erio ds from 6.2h to 7.3d (Kholtygin et al. 2006c). These regular comp onents are connected with non-radial pulsations and the rotational mo dulation of the line profiles. The slowest of the comp onents with a p erio d of 7.3 d is probably the rotational p erio d of the star.


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265

3. 5 3 2. 5 2 1. 5 1 0. 5 0

Figure 2: Top: density plot of the line profile variations in sp ectra of Leo in the wavelength region А 5665-5702 A. Bottom: mean line profiles in the same region averaged over all sp ectra obtained on Jan. 10/11 2004 .

3.2

Mo dels of cyclical comp onents of the line profile variability

Large time-scale line profile variations are often explained via formation of large-scale structures in the stellar wind. These structures are often connected with corotating interaction regions (CIR, Cranmer and Owo cki 1996) resulting from a lo calized "bright sp ot" on the stellar surface. These CIRs are thought to pro duce the cyclical mo dulation of the P-Cygni absorptions in optical and UV lines (e.g. Kap er et al. 1999, de Jong et al. 2001). An alternative explanation of the cyclical line profile variations can b e obtained in the framework of a "confined corotating wind" mo del. In this mo del a star is an oblique magnetic rotator (see, for example, Fig. 15 in Rauw et al. 2001). Such a mo del has b een prop osed to explain the lpv observed in the sp ectra of P up (Moffat and Michaud 1981) and 1 Ori C (Stahl 1996).

4
4.1

Magnetic fields of OB stars
Magnetically confined wind-sho ck mo del

Bab el and Montmerle (1997) have develop ed a magnetically confined wind-sho ck (MCWS) mo del. In this mo del the wind streams from b oth magnetic hemispheres, collide with each other and pro duce strong sho cks, an extended X-Ray-emitting p ost-sho ck region and a thin dense co oling disc in the magnetic equatorial plane. In this mo del it is p ossible to explain the stellar disks (Cassinelli et al. 2002) around Be stars (see also Brown et al. 2004). We can also remark that the material in this mo del may b e unstable against falling back (UdDoula and Owo cki 2002). The p ossibility of generation of magnetic fields on the surface of hot stars by a dynamo mechanism is often supp osed (eg., MacGregor and Cassinelli 2003). They found that fields in hot mainsequence stars are generated by a dynamo mechanism at the interface b etween the radiative core


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and convective envelop e of a star. The generated magnetic tub es could rise to the surface and reach the necessary level for the wind confining.

4.2

Magnetic field of Leo

Observations of Leo with the p olarized analyzer and data reduction pro cedures are descib ed in Section 2. The equipment for measuring stellar magnetic fields designed for the sp ectrograph MSS at the 6 m telescop e is describ ed by Chountonov (2004) . For example, we plot the part of sp ectra of Leo obtained with the p olarization analyzer in Fig. 3.

RELATIVE FLUX

NII5676 -

NII5667 -

V

rho Leo

V*1000

NII5667 -

5664

5669

5674

NII5676 -

5679

NII5680 -

- NII5680

5684

Figure 3: Top: left and right polarized components in spectra of Leo in the region 5664 - 5684 А Bottom: A. profile of the Stokes parameter V in the same spectral region. For determining the field value we used the cross-correlation metho d (see, for example, Semenko 2004). Results of our measurements of mean longitudinal magnetic fields B l are given in Table 3. Table 3: Field measurements of Leo
Date 2005-1-30 2005-1-10 2005-1-12 2005-1-13 2005-1-14 UT 19h 55m 12.00s 0h 43m 11.99s 0h 57m 36.00s 1h 55m 12.00s 0h 0m 0.00s Bl +33 -93 +31 -21 +69 ,G Б 19 Б 27 Б 24 Б 14 Б 17

As a first approximation, we can supp ose a dip ole geometry of the Leo's field. In this case the measured value B l is the line intensity weighted mean over the entire stellar disk (Eversb erg 1997): 1 Bl = W
2 /2

d
0 0

Bl cos( ) sin()d з

r ( , )d .

(1)

Here Bl = Bl ( , ) is the line of sight comp onent of the magnetic field at the p oint ( ,), where


LINE PROFILE VARIABILITY AND FIELD OF LEO

267

and are the co ordinates of the p oint at the stellar disk and r ( , ) is the residual intensity of the line at this p oint. For a tilted dip ole magnetic field geometry and a linear law of limb darkening r ( , ) 1 - u + u cos( ) (u is the parameter of limb darkening for the wavelength considered) the variation of B l with rotational phase can b e obtained by integration of Eq.1. Finally (see, e.g. Preston (1967) for details): 15 + u B l = Bp [cos cos i + sin sin i cos 2 ( - 0 )] , (2) 20(3 - u) where Bp is the p olar magnetic field strength, is the angle b etween the magnetic and rotational axes, i is the rotational axis inclination angle and 0 is the phase of the maximal longitudinal field. Using the standard least mean square approximation, we fit the obtained values of B l in the tilted magnetic dip ole mo del. For parameter u we use the standard value u = 0.350 for early B stars (Schrijvers 1997). Parameters of the fit for Leo are given in Table 4. The rotational phase for dates of observations were calculated using the p ossible rotational p erio d P = 7.267 d and the value of TO = 2453377.611 (Kholtygin et al. 2006c).

Figure 4: Fit of the longitude comp onents B l of the Leo magnetic field (triangles) in the mo del of Preston (1967), the tilted magnetic rotator -- dashed line. The rms error of values B l are also shown.

5

Discussion of results

In Table 4 we compile the results of the recent measurements of the magnetic field for OB stars basing on some recent investigations together with our data for Leo. In the table B p is the p olar field in the case where the tilted dip ole mo del is accessible and the mean field averaged over all measurements in other cases. We see that the parameters of the p ossible magnetic field of Leo are close to those of O and early B stars. That gives an additional argument in the favor of our supp osition ab out the dip ole geometry of the field of Leo. Other arguments go from the detection a weak regular lpv in sp ectra of Leo out of the V sin i zone (see Kholtygin et al. 2006c for details). Such a typ e of variability can o ccur if the matter just near the star (1-2 stellar radii) co-rotates with the stars itself. Connection b etween pulsations and regular structures in the wind was p ointed out by Owo cki and Cranmer (1988). Possibly the most intriguing problem of stellar physics is the origin of the magnetic field of the early-typ e stars. There exist two p ossibilities. The first is a hyp othesis that the field is b eing


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Table 4: Field measurements for OB stars
Spectral Class O6 Ipe 08 B0.2V B0.5III B1 II-III B1Ib B1III B1.5III B2III B2III B2III B2 B2IIIevar B2III B2IV B2IV B2 IV-V B2IV-V B2V B3IV B3IV B3IV B3V B3V B3 IIIe Vsini, km/s 45 102 5 16 11 95 20 97 21 8 20 18 17 16 60 95 9 13 41 264 104 172 P, d 15 538 41 Bp, G 1110 Б 100 -1500 Б 200 500 18 35 240 Б 50 232 73 33 -131 Б 42 -277 Б 108 67 360 Б 40 103 340 Б 90 -39 Б 21 250 Б 190 873 Б 66 -39 Б 21 200 146 37 -106 Б 46 133 530 Б 200

Star 1 Ori C HD 191612 Sco Cru CMa Leo 1 CMa Sco Eri HD 85953 HD 74195 V386 Cen Cep V335 Vel Cas Oph v 2052 Oph HR 2718 V539 Ara o Vel HY Vel V514 Car Cir 16 Peg Ori

45 Б 17 45o
o

i
o

45

o

7.3

59o Б 30

o

85o Б

o

85o Б 10 80o Б 4

o

60o Б 10 18o Б 4

o

5.4 3.6

o

o

35o Б 17

o

71o Б 10

o

1.3

50o Б 25

o

42o Б 7

o

References Donati et al. 2002 Donati et al. 2006b Donati et al. 2006a Hubrig et al. 2006b Hubrig et al. 2006b Present paper Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006a Hubrig et al. 2006a Hubrig et al. 2006b Henrichs et al. 2003 Hubrig et al. 2006b Henrichs et al. 2003 Hubrig et al. 2006b Henrichs et al. 2003 Hubrig et. al. 2006 Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006b Hubrig et al. 2006b Henrichs et al. 2003

Comments: - single measurement of longitudal magnetic field, all measurements



- average longitudal magnetic field over

generated by a contemp orary dynamo mechanism (MacGregor and Cassinelli 2003). The second p ostulates that the field is fossil. It means, that the field is a dynamically stable relic of the field in the molecular cloud where the star formed, or of a field built by a dynamo acting in a pre-main sequence phase of the star (Mestel 2003). In the case of dynamo action there have to b e a correlation b etween the field strengths and the rotation velo city. However, we have insp ected the data presented in Table 4 and have not found any correlation of such a kind (see Fig. 5). Moreover, Braithwaite and Nordlund 2006 have recently resolved the most serious problem of the fossil theory, the stability of the field during stellar evolution. It was found that stable magnetic field configurations exist under the conditions in the radiative interior of a star. Such configurations have roughly equal p oloidal and toroidal field strengths. We can conclude that the fossil nature of the OBA stellar field seems to b e more substantial.

6

Conclusion

We rep ort the results of a study of fast lpv in sp ectra of the bright B1 sup ergiant Leo and search for magnetic field. Regular long time-scale comp onents of lpv in sp ectra of the star have b een detected.


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Figure 5: The p olar magnetic field values B p (triangles) and the mean line of sight comp onent of the magnetic field Bl (asterisks) vs. pro jectional rotational velo cities V sini. The formation of such comp onents of lpv can b e explained in the framework of the MCWS mo del by Bab el and Montmerle (1997) .
Acknowledgements. The authors are grateful for the support provided by RFBR grants 5-02-16995 and 06-02-17096

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