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Physics of Magnetic Stars, 2007, pp. 81­88

New stars with strong magnetic fields
Kudryavtsev D. O., Romanyuk I. I.
Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia

Abstract. During realization of the program of search for new magnetic stars at the 6 m telescop e we managed to discover 6 stars with very strong magnetic fields with the longitudinal comp onent stronger than 4 kG. Three of them, HD 45583, HD 178892 and HD 221936 have b een studied in details. 1) The star HD 45583 has a complex field structure, the longitudinal magnetic field curve shows double­wave variations from -2 kG to +4 kG. Mo deling of the curve predicts the surface magnetic field value up to 20­30 kG. 2) HD 178892 shows one of the strongest dip olar magnetic fields among co ol CP stars with the longitudinal comp onent up to +7 kG and the surface magnetic field value Bs = 17.5 kG. The magnetic field configuration can b e represented as a dip ole with the dip olar strength Bd = 23 kG. 3) Clearly observed Zeeman splitting of some lines in the sp ectrum of HD 221936 indicates a surface magnetic field with the value of 25­30 kG. At the same time the longitudinal comp onent varies from -2.5 kG to +2.5 kG. There are some reasons to b elieve that the field of this star may have a complex structure.

1

Intro duction

Since 2000, a program of searching for magnetic chemically p eculiar stars has b een carried out with the 6 m telescop e of the Sp ecial Astrophysical Observatory. The basic results of this research are presented in the pap er by Kudryavtsev et al. (2006). The urgency of this work is conditioned by a relatively small numb er of known CP stars and by the emerged p ossibility of measuring magnetic fields by direct metho ds in noticeably fainter ob jects than it was p ossible earlier. Further investigations of detected ob jects are capable to provide new data useful for the development of such the active directions of mo dern astrophysics as the researches of chemically p eculiar stars and physics of stellar atmospheres. In this pap er, we would like to consider in more details a few stars with extremely strong magnetic fields discovered in the course of realization of our program. These ob jects are certainly most outstanding of all the list of new magnetic stars. For this reason, their further, more careful investigation is necessary. We have started additional observations of stars with extremely large magnetic fields with sp ectrographs of mo derate (R = 15000) and high (R = 50000) resolution. The first results of these observations will b e presented in this pap er.

2

Observations

The observations were p erformed with two sp ectrographs: the Main stellar sp ectrograph (MSS) with mo derate resolution and the high-resolution Nasmyth echelle sp ectrograph (NES) (Panchuk et al. 2002) of the 6 m telescop e. 81


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Table 1: Stars with strong magnetic fields Star HD 45583 HD 178892 HD 221936 HD 258686 HD 293764 HD 343872 HD 349321 mV 8.0 8.9 9.2 9.7 9.5 9.9 9.3 Pec Si SrCrEu Si Si SrCrEu Si Si Be extrema -2000/ + 4000 +2000/ + 7000 -2500/ + 2500 +5000/ + 6500 +3000/ + 4200 -750/ + 4600 -4500/ + 2000 Cluster NGC 2232 Sto ck 12 Collinder 95

Using of mo derate resolution allows one to make sp ectrop olarimetric measurements for relatively faint ob jects with the least time losses. For this purp ose a circular p olarization analyzer with a double image slicer and a rotated /4 plate (Chountonov 2004) was mounted in the slit part of the sp ectrograph. The applied image slicer has a diaphragm of ab out 5 , cutting the incoming b eam into 7 slices for each p olarization, which makes it p ossible to p erform observations most effectively even with bad seeings. Two sp ectra were observed at a different orientation of the /4 plate for every measurement of the longitudinal magnetic field, which allows for comp ensation of the influence of instrumental effects during subsequent reduction. ° The observations with the MSS were p erformed in a wavelength region of 4400­4600 A, which is rich of metal lines, mostly Fe and Cr, which allows p erforming the longitudinal magnetic field measurements with the highest p ossible accuracy. The pro cedure of the measuring is describ ed in more details in the pap er by Kudryavtsev et al. (2006). The measurements were p erformed with the co de of Kudryavtsev (2000). High resolution observations are b eing carried out with the NES sp ectrograph of the 6 m telescop e. The sp ectrop olarimetric technique, however, is not applied here, b ecause most of new magnetic stars are to o faint for sp ectrop olarimetry with high S/N ratio. In observations with the NES sp ectrograph we use a standard configuration of a sp ectrograph with an image slicer, cutting the ° incoming b eam into three slices. The wavelength range is 4700­6100 A. Most of the high-resolution sp ectra observed with the NES still require reduction and analyzing. Here we present only some results concerning the star HD 178892.

3

Results

During realization of our program, we succeeded in detection of 6 stars showing the value of the longitudinal magnetic field of more than 4 kG. The general information ab out these ob jects is presented in Table 1. The star HD 221936 is also included in the Table. In spite of the normal strength of the longitudinal comp onent (ab out 2.5 kG), an analysis of sp ectra allows susp ecting that the star has a very strong surface magnetic field, that will b e discussed b elow. Table1 shows the stellar magnitude, p eculiarity typ e, extrema of the longitudinal magnetic field, and memb ership in op en clusters. It should b e noted that for ma jority of stars the numb er of magnetic field measurements is not sufficient and do es not cover the whole rotation p erio d of the star. For this reason, the real extrema of the longitudinal magnetic field may b e larger in some cases. For most of stars presented in the Table, the observational data have already b een published in the pap er by Kudryavtsev et al. (2006). In this pap er we discuss new data obtained for the stars HD 45583 and HD 178892. In addition, we have found the magnetic CP star HD 221936, the magnetic field measurements for which are presents for the first time.


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Table 2: Longitudinal magnetic field measurements for HD 45583 JD 2450000+ 3273.550 3274.581 3275.570 3363.401 3364.323 3365.493 3667.600 3717.560 3717.572 3718.332 3718.515 3719.394 3784.416 3786.285 3786.417 Be +3060 ± +3850 ± -1650 ± +3780 ± +2010 ± +2130 ± +1950 ± +2690 ± +3560 ± +3450 ± +1920 ± +1330 ± +2570 ± -2140 ± -80 ± ± 770 510 860 590 320 330 280 500 340 230 470 420 350 310 400 S/N ratio 240 190 150 300 270 370 350 260 250 570 520 270 320 410 290

3.1

HD 45583

The star is a memb er of the op en cluster NGC 2232 with an estimated age of 30 Myr. The longitudinal magnetic field Be reaches 4 kG, a partial Zeeman splitting of lines is observed in the sp ectrum. Kudryavtsev et al. (2006) have susp ected that the star has a non-sinusoidal curve of B e variations, that p oints to the presence of a complex non-dip ole magnetic field in the star. The numb er of measurements, however, was not sufficient for a definite conclusion. We continued observations of this star. Table 2 presents the magnetic field measurements obtained earlier and the new ones. The rotation p erio d (P = 1.177177 days) of the star was estimated by North (1987) from the photometric variability. The variability curve of B e that we constructed from our measurements with the p erio d rep orted by North (1987), is shown in Fig. 1. The curve is explicitly double-wave. Thus, we can assert that we have found a star with essentially non-dip ole structure of the global magnetic field. Using the curve of Be measurements, we made an attempt to simulate the magnetic field configuration at the surface with the program FLDCURV, written by J. D. Landstreet. In the simplest case the curve can b e drown under the assumption that the field structure is a combination of a strongly non-central dip ole and an o ctup ole comp onent coaxial with the dip ole. We have derived two resembling mo dels which describ e the observed curve in an approximately similar manner: 1. i = 50 , = 130 , Bd = -17500 G, a = 0.59, B 2. i = 40 , = 120 , Bd = -17000 G, a = 0.61, B = -35000 G = -45000 G line of sight, is the tilt angle of the magnetic dip olar comp onent, a is the shift of the dip ole the strength of the o ctup ole comp onent. mo dels must vary within 20­30 kG dep ending sp ectrum of the star in left and right circular

oct oct

Here i is the tilt angle of the rotational axis to the axis to the axis of rotation, Bd is the magnitude of the expressed in the fractions of the star's radius, B oct is The surface magnetic field of the star predicted by on the rotation phase. The figure 2 shows a part of a


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HD45583 JD2453272.490 + 1.177177
4000 3000 2000

Be (G)

1000 0 -1000 -2000 -3000 0 0,5 1 1,5

Phase

Figure 1: The longitudinal magnetic field variations of HD 45583 with rotation p erio d, calculated by North (1987). p olarizations obtained simultaneously. It can b e clearly seen that sp ectra in different p olarizations are considerably different, there are signs of the partial Zeeman splitting. Such effects may only b e caused by the presence of a strong magnetic field, sufficient to b ecome apparent against rather rapid rotation of the star, v sin i = 75 k m/s (Uesugi & Fukuda 1981), which considerably broadens sp ectral line profiles. Using the derived mo del of the magnetic field, we have made an attempt to calculate a synthetic sp ectrum using the programm ZEEMAN2, written by J. D. Landstreet. Fig. 3 shows the result of mo deling of one region in the sp ectra of Stokes I and V parameters. The mo del was calculated for the temp erature Te = 13000 K. The sp ectral line profiles (Stokes I) are fitted rather well. A preliminary very rough estimate of the chemical comp osition shows the abundances Fe: -3.7, Ti: -5.8, Cr: -4.8, Si: -2.6. At the same time it is evident that the Stokes parameter V is not describ ed by the suggested mo del. Summing up, it can b e noted that we have found a star with a strong magnetic field of complex structure. However, the configuration of the magnetic field is still op en to question. Most likely, an accurate description of the magnetic field structure can only b e made by the metho d of DopplerZeeman imaging.

3.2

HD 178892

Results of our observation of HD 178892 were presented in pap er by Ryab chikova et al. (2006), therefore now we only rep eat shortly previously published data. HD 178892 shows the largest value of the longitudinal magnetic field, which reaches the value of 7 kG. It is very interesting that the star has SrCrEu p eculiarity typ e while such the extremely strong magnetic fields were found previously only in hotter Si typ e stars, for exception of co ol SrCrEu star HD 154708 (Hubrig et al. 2005) with T e = 6800 K and longitudinal field ab out 7.5 kG. Thus HD 178892 and HD 154708 are very unusual ob jects and more detailed study of them is needed. We have made a series of sp ectrop olarimetric observations of HD 178892 allowing the rotation p erio d of the star to b e determined and a curve of the longitudinal magnetic field B e variations to b e plotted. Afterwards, in the pap er by Ryab chikova et al. (2006) the p erio d was refined by using photometric data. The curve of the B e variations with this p erio d, P = 8d 2478, is presented in Fig. 4. . We have also obtained several sp ectra with the NES sp ectrograph of the 6 m telescop e


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Figure 2: A sp ectrum of HD 45583. Red line -- right circular p olarization, blue line -- left circular p olarization.

1 0,95

I

0,9 0,85 0,8 0,03 0,02 0,01 4510 4520 4530 4540 4550 4560

V
0 -0,01 -0,02 4510 4520 4530 4540 Wavelength, A 4550 4560

Figure 3: Stokes I and V sp ectra for HD 45583. Dashed line -- observed sp ectra. Solid line -- synthetic sp ectra calculated with the dip ole+o ctup ole magnetic field mo del, derived from the longitudinal magnetic field curve.


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HD178892 HJD2452452.935 + 8.2478 days
8000

6000

Be (G)

4000

2000

0

0,5

1
Phase

1,5

Figure 4: The longitudinal magnetic field variations of HD 178892 with rotation p erio d.

Fe II 5018.45 1

Ca II 5019.97

Residual intensity

0.8

0.6

0.4 5016

5017

5018 5019 Wavelength, A

5020

5021

Figure 5: The Zeeman splitting of Fe I I 5018.45 ° line in the NES sp ectra. A


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HD 221936
-2490 +- 230

RCP & LCP

+2550 +- 200 +170+- 610 1 0,8 4510 4520 +1510 +- 440 4530 4540 4550 4560

V
0 -0.025 4510 4520 4530 4540 Wavelength, A 4550 4560

Figure 6: Top: the Zeeman splitting in p olarized sp ectra of HD 221936, solid line -- left circular p olarization, dashed line -- right circular p olarization; b ottom: Stokes V parameter for the same sp ectra. (Panchuk et al. 2002) with a sp ectral resolution R = 40000. These sp ectra clearly show the Zeeman splitting of sp ectral lines (Fig 5). By the results of mo deling of sp ectra presented in pap er by Ryab chikova et al. (2006), the surface magnetic field has b een estimated as B s = 17.5 kG. Estimates of the chemical comp osition and parameters of the star have also b een obtained: T e = 7700 K, lg g = 4.0, v sin i = 9 km/s. The mo del of the star's magnetic field has b een obtained using b oth the curve of longitudinal magnetic field variations and measured B s values: i = 36 , = 37 , Bd = 23000 G.

3.3

HD 221936 (V 629 Cas)

For HD 221936 we have obtained the first data giving an evidence that the star has an extremely strong surface magnetic field. The star is rather faint (m V = 9.29) and p o orly studied. The typ e of p eculiarity is Si. The star is a memb er of the op en cluster Sto ck 12. According to the database WEBDA, the age of the cluster is ab out 280 million years. We have obtained four sp ectra of the star showing usual values of the longitudinal magnetic field varying from -2.5 kG to +2.5 kG. However, in Zeeman sp ectra one can clearly see a Zeeman splitting whose value corresp onds to a strong surface magnetic field of ab out 25­30 kG. The upp er part of Fig. 6 shows the Zeeman sp ectra in the right (dashed line) and left (solid line) circular p olarizations. The presented sp ectra have rather high quality, the S/N ratio is over 300. In the left circularly p olarized sp ectra at two phases the Zeeman splitting of lines is clearly observable.


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The lower part of the figure shows the corresp onding sp ectra of the Stokes V parameter reaching the value of ab out 2.5%. Thus, one can state with confidence that a strong surface magnetic field is present in the star. However, it should b e noted that the effect of splitting with the given sp ectral resolution (R 15000) will b e noticeable only in p olarized sp ectra, a usual sp ectrum will show no splitting. v sin i of the star do es not exceed 60 km/s. Panov & Scho eneich (1976) determined the rotation p erio d of the star from light variations as P = 0.63195. If the p erio d is correct, then with a typical for CP stars radius of 2 - 3R and the observed v sin i, the angle i must b e not larger than 20­25 . At the same time, the longitudinal magnetic field of the star shows a variation of p olarity, the variation b eing rather fast (no more than 24 hours), which confirms the fast rotation of the star. A relatively small longitudinal magnetic field combined with a strong surface field and, at the same time, changing p olarity allow one to make an assumption that the star has a magnetic field of complex structure. Observation of HD 221936 will b e continued.
Acknowledgements. When preparing the paper, we made use of NASA's Astrophysical Data System, SIMBAD database, operated at CDS, Strasbourg, France and WEBDA database, operated at the Institute for Astronomy of the University of Vienna. This work was partially supported by Russian Foundation for Basic Research (RFBR grant no. 06-02-16110a).

References
Chountonov G. A., 2004, in: Proc. of Intern. Conf. "Magnetic stars", Glagolevskij Yu. V., Kudryavtsev D. O., Romanyuk I. I. (eds.), SAO RAS Press, Nizhnij Arkhyz, 286 Hubrig S., Nesvacil N., Scholler M., North P., Mathys G., Kurtz D. W., Wolff B., Szeifert T., Cunha M. S., Elkin V. G., 2005, Astron. Astrophys., 440, L37 Kudryavtsev D. O., 2000, Baltic Astron., 9, 649 Kudryavtsev D. O., Romanyuk I.I., Elkin V. G., Paunzen E., Mon. Not. R. Astron. Soc., 2006, 372, 1804 North P., 1987, Astron. Astrophys. Suppl. Ser., 69, 371 Panchuk V. E., Piskunov N. E., Klochkova V. G., Yushkin M. V., Ermakov S. V., 2002, SAO Preprint no. 169 Panov K., Schoeneich W., 1976, Astronomische Nachrichten, 297, 177 Ryabchikova T., Kochukhov O., Kudryavtsev D., Romanyuk I., Semenko E., Bagnulo S., Lo Curto G., North P., Sachkov M., 2006, Astron. Astrophys., 445, L47 Uesugi A., Fukuda I., 1981, Revised Catalogue of Stellar Rotational Velocities, C.D.S., Strasbourg, France