Remarkable Non-Dipolar Magnetic Field of the Bp Star HD 137509

arXiv:astro-ph/0603831v1 30 Mar 2006 eevd3 aur 2006 January 31 Received Univers Uppsala Physics, Space and Astronomy of Department EODTPormI 266.D-5655). ID Program DDT (ESO e words. Key ch magnetic among field ext second-largest the an with of object presence the the as confirms This predictions. model polar ieo ih edcmoet–lniuia field longitudinal – component disk-integr field the sight of of modulation line rotational establis the and of measurements form the the ing on focused observational studies early field The debate. netic intense of matter a is stars geometry. field o magnetic modulation stellar rotational the the of of intrin projection result not visible a are rather changes but star observed the its to and plasma stellar the in eovdZea pi opnnsi oeo h pcrllin spectral the of hav some I in 137509. components HD split of Zeeman topology resolved field surface the dominate framewo to rot the nent in complex variability exhibits magnetic 137509 this HD of star Interpretation Bp magnetic southern The ioa ed r fe insu purely often that found are was it fields 2002), al. dipolar et Bagnulo 2000; (Landstr Mathys procedure & modelling field the in observa Landstre incorporated dipo- magnetic were & additional dominant Borra several (e.g. the after topology However, for 1980). field evidence the to an contribution as lar considered rotation stellar was the This during single-wa field smooth, longitudinal exhibit of stars variation CP magnetic the of majority tbeadcvrtewoeselrsrae h edstrengt field to up The G surface. hundred few stellar a from whole ranges the cover fiel and magnetic magneti CP-star stable stars, spotted late-type complex active of evolving, topologies rapidly the Unlike field. these of Many region. ve culiar forming and line surface the stellar in the cally over horizontally both distribute uniformly elements s with solar, these in from layers significantly outer departs the sequence prop of surface composition main unusual chemical shows The upper ties. classes spectral and B–F middle the of the stars of fraction Significant Introduction 1. ec oecmlxqarplradotplrfilshdto had fields octupolar and quadrupolar complex more hence eakbenndplrmgei edo h psa D13750 HD star Bp the of field magnetic non-dipolar Remarkable ue1,2018 19, June ⋆ Astrophysics & Astronomy ae ntedt rmteUE aaa bevtr Project Observatory Paranal UVES the from data the on Based h tutr n rgno h lblmgei ed fCP of fields magnetic global the of origin and structure The C)sasas oss tog elodrdmagnetic well-ordered strong, a possess also stars (CP) ie rfie tr:mgei ed tr:ceial pe chemically stars: – fields magnetic stars: – profiles line: / cetd2 ac 2006 March 26 Accepted aucitn.4932 no. manuscript ffi in oepanobservations, explain to cient ≈ 5k.Tefil sfrozen is field The kG. 35 hmclype- chemically h B ko h o-re utplrfil oessget eyst very a suggests models field multipolar low-order the of rk z i h vast The . s h nerdma ufc edmodulus, field surface mean inferred The es. mclypcla stars. peculiar emically eeysrn o-ioa antcfil nH 359adest and 137509 HD in field magnetic non-dipolar strong remely .Kochukhov O. non- d sare ds cycle. t,S-5 0 psl,Sweden Uppsala, 20, SE-751 ity, xmndtehg-ult VLT high-quality the examined e mag- the f ABSTRACT ated toa ouaino h ogtdnlfil n te magne other and field longitudinal the of modulation ational bles tars rti- eet sic er- ve h- et h c uir–sas niiul D137509 HD individual: stars: – culiar l 00 n D170 Mty az19) h third distorte The strongly 1997). a shows Lanz single-wave 1990) & (Landstreet (Mathys 133880 et HD 137509 Khokhlova star, HD 1985; and Landstreet 2000) & al. sta (Thompson These 37776 found. HD been are have fields, non-dipolar clearly cu hence field longitudinal double-wave extraordinary with the jects with stars the of of nant investigation understanding ca detailed better stars through magnetic a early-type gained evolu- that in and fields suggest non-dipolar generation I of nature field stars. the CP of in in tion models constr great theoretical important of additional the certainly provide for is will problem it this because terest of Solution smal field. unresolved repr scale the or of approximation reality mathematical physical a th the merely from reflect structure deviations dipolar octupolar basic and wheth quadrupolar unclear suggested remains it the models, field multipolar low-order mtismygv iet eetvl ipe erysinu- nearly simple, deceptively a to soidal rise ge give magnetic complex may 2004) exceedingly ometries al. stars et some (Kochukhov in fo stars that the revealed CP of of spectra analysis parameter detailed Stokes recent Moreover, introduced. be h epo h ikitgae ufc edstrength field surface disk-integrated with the topology of magnetic multipol help spot-like, low-order complex, the a more a distinguish from and field objects these in fields fiel non-dipolar strong unusually an with star the of example neomu tegh o ntne Gpa-opa mod- peak-to-peak o kG 4 field a octupolar instance, or For strength. quadrupolar enormous a fi an by longitudinal produced is non-sinusoidal variability low-amplitude relatively predicts a scenario field multipolar low-order The surement. hs ept eti ucs ftephenomenological the of success certain a despite Thus, n ol oet osri h tutr fnon-dipolar of structure the constrain to hope could One o-ioa edcmoet.U onwol w ob- two only now to Up components. field non-dipolar h B z i variation. h B z i aito n sotnrfre oa another as to referred often is and variation / VSsetao D170 n discovered and 137509 HD of spectra UVES h B i = 9k,are ihtemulti- the with agrees kG, 29 ogqarplrcompo- quadrupolar rong bihsti star this ablishes i observables. tic

c h S 2018 ESO B i domi- 9 esent mea- rves, aints be n that eld ⋆ d. ur er ar rs l- d e - f - 2 O. Kochukhov: Remarkable non-dipolar magnetic field of the Bp star HD 137509 ulation of hBzi in HD37776 (Thompson & Landstreet 1985) should be interpreted as a signature of > 50 kG surface field (Bohlender 1994). In contrast, discarding a low-order multipolar field parameterization, one can successfully reproduce the observed non-sinusoidal hBzi phase curves with a high-contrast surface field of a modest strength (only a factor of 1.5–2 larger than the hBzi extremum). In this paper I try to constraindifferent possible non-dipolar field structures in the Bp star HD137509. Of the three early- type stars with distinctively non-dipolar magnetic field configu- rations, HD137509 has the longest rotation period, lowest projected rotational velocity and, therefore, presents the best op- portunity for the direct diagnostic of the surface field strength through the high-resolution spectroscopic observations and line profile analysis. The rest of the paper is organized as follows. In Sect. 2 I summarize the physical properties of HD137509 and refine the value of the stellar rotation period. Sect. 3 overviews previous magnetic field measurements and presents interpretation of the magnetic field variability in terms of the low-order multipolar field topologies. Detection of the resolved Zeeman split lines in the spectrum of HD137509 and the first field strength measurement are reported in Sect. 4. The study concludes with a summary and discussion in Sect. 5. Fig. 1. Time-series analysis of the Hipparcos epoch photom- 2. Atmospheric parameters and rotation period etry of HD137509. Upper panel: amplitude spectrum of the photometric variation. The inset shows the window function HD137509 (HIP 76011, NN Aps) was assigned a B9p SiCrFe of the Hipparcos observations. Lower panel: photometric mea- spectral classification by Cowley & Houk (1975). These au- surements folded with the period P = 4.49257 d. The solid     thors mention strong lines of Si , Cr , Fe , Ti , and the ab- curve illustrates the least-squares fit by a sinusoid and its first  sence of He features. Atmospheric parameters of HD 137509, harmonic. Teff = 12600 ± 200K and log g = 4.06 ± 0.11, were determined by Hunger & Groote (1999) using the IR flux method and comparison with the stellar evolutionary tracks. On the other hand, of these data is illustrated in Fig. 1. The only statistically sig- Hubrig et al. (2000) suggested Teff = 11900 ± 600 K from nificant peak is detected at the frequency ν = 0.223 d−1. The the published spectral type and deduced log g = 3.87 ± 0.16 least-squares fit by a cosine function and its first harmonic from the Hipparcos parallax and theoretical stellar models.I find that the Hβ and Hγ hydrogen lines in the UVES spectrum V = V + V cos[2π(T − T )/P + ϕ ] H 0 1 0 1 (1) of HD137509 discussed below (Sect. 4) are best fitted with + V2 cos[4π(T − T0)/P + ϕ2] Teff = 12750 ± 500 K and log g = 3.8 ± 0.1 if the enhanced metallicity [M/H] = +1.0 9 (Kurucz 1993) stellar model yields P = 4.49257±0.00029d, V0 = 6.846 mag, V1 = 0.019± atmosphere is employed. 0.001 mag, V2 = 0.012±0.001 mag. Here the epoch of positive Photometric variability of HD137509 was detected by magnetic extremum JD = 2448344.575(Lanz & Mathys 1991) Lodén & Sundman (1989). Mathys (1991) discovered a strong was adopted as the reference time T0. reversing longitudinal magnetic field and pointed to spectac- The new estimate of the rotation period differs by < 3σ ular variations of the Si  and iron-peak element line intensi- from the value derived by Mathys & Lanz (1997) and has ties, confirming earlier tentative spectrum variability reports by similar formal precision. Moreover, the present analysis of the Cowley & Houk (1975) and Pedersen (1979). Mathys & Lanz Hipparcos photometric time-series overcomes aliasing problem (1997) have combined all available magnetic measurements typical of the previous photometric observations (see fig. 1 in and obtained new time-series photometry in the Geneva sys- Mathys & Lanz 1997) and hence gives more confidence in the tem. Simultaneous consideration of these observables allowed derived period. the authors to obtain a refined value P = 4.4916 ± 0.0002 d for The brightness variation of HD137509 as measured by the rotation period of HD137509. Hipparcos correlates remarkably well with the magnetic field In addition to the previous reports of periodic brightness modulation. Comparison of the photometric curve (Fig. 1) and modulation, HD137509 also shows pronounced variability in hBzi variability (Fig. 2) indicates that the star shows maximum the Hipparcos epoch photometry (ESA 1997). The Hipparcos brightness at the phase of positive magnetic extremum (ϕ = satellite observed the star over the period of 3.2 years, obtain- 0.0). The secondary longitudinal field maximum at ϕ = 0.5 ing 192 individual photometric measurements. Fourier analysis also has its counterpart in the photometric curve. O. Kochukhov: Remarkable non-dipolar magnetic field of the Bp star HD 137509 3

3. Magnetic field topology Table 1. Parameters of the best-fit multipolar magnetic field models of HD137509. The first longitudinal field measurements were obtained for HD137509 by Mathys (1991) using metal line spectropo- Parameter Model A Model B Model C ◦ larimetry. Additional hBzi observations based on the photopo- i ( ) 81 ± 15 78 ± 7 64 ± 21 larimetric measurements in the Hβ line were acquired by Bd (kG) 3.1 ± 1.8 4.9 ± 1.3 5.5 ± 2.2 Bohlender et al. (1993). Variation of other integral magnetic β (◦) 64 ± 12 123 ± 26 84 ± 16 γ (◦) 372 ± 3 367 ± 16 372 ± 11 observables, the crossover ve sin ihxBzi and the mean quadratic field hB2 + B2i1/2, was studied by Mathys (1995a, 1995b) and Bq (kG) 41.9 ± 1.3 −46.8 ± 1.3 51.6 ± 2.9 z ◦ β1 ( ) 43 ± 6 47 ± 18 Mathys & Hubrig (1997). ◦ β2 ( ) = β1 114 ± 18 All longitudinal field measurements available for ◦ γ1 ( ) 81 ± 4 125 ± 9 ◦ HD137509 are illustrated in Fig. 2. In total there are 16 γ2 ( ) = γ1 216 ± 10 hBzi determinations, but only four of them correspond to a Bo (kG) −0.5 ± 2.1 2 > 3σ field detection. Nevertheless, the longitudinal field vari- χν 1.36 1.34 1.16 ation of HD137509 is reasonably well defined and exhibits a remarkable double-wave shape, with a peak-to-peak amplitude of ≈ 3.5 kG and unusually narrow positive field extremum at rotation phases ϕ = 0.8–1.2. An even more pronounced double- wave variation is evident in the crossover measurements (see Fig. 2). The mean quadratic field reaches up to 36 kG, which 2 2 1/2 is the largest hBz + B i found in magnetic CP stars (Mathys 1995b). These measurements provide a hint for the presence of a very strong surface field in HD137509. However, the quadratic field diagnostic by itself is rather inconclusive because it relies on the interpretation of subtle magnetic line broadening effects and neglects horizontal chemical inhomogeneities. For several spotted magnetic CP stars very large quadratic fields inferred by Mathys (1995b) could not be confirmed by the subsequent detailed line profile analyses of high-resolution spectra (Kupka et al. 1996; Kochukhov et al. 2004). Uncertainties of the magnetic observables notwithstanding, I have attempted to probe the field structure of HD137509 in the framework of the low-order multipolar magnetic geome- tries and using all available measurements of hBzi, ve sin ihxBzi, 2 2 1/2 and hBz + B i . The purpose of this exercise is not to study Fig. 2. Observed variation of the magnetic field moments in details of the stellar magnetic field topology, but to assess HD137509 and predictions of the best-fit multipolar mod- the range of the disk-averaged surface field strength hBi ex- els of the magnetic field topology. The longitudinal field, pected for such simple models. The multipolar field modelling quadratic field, and crossover observations are taken from procedure applied to HD137509 is similar to the analysis by Mathys (1991, 1995a, 1995b), Mathys & Hubrig (1997) (filled Landstreet & Mathys (2000) and Bagnulo et al. (2002). Having circles), and Bohlender et al. (1993) (open triangles). A single no a priori reason to prefer one or another form of the multi- data point shown in the lower right panel represents the field polar field parameterization suggested in the literature, I have modulus measurement reported in the present paper. Smooth tested the three most widely used types of magnetic models. curves show the fit by multipolar models discussed in the text: The first one (model A) is identical to the field parameteriza- model A (solid line), model B (dashed line), and model C tion used by Landstreet & Mathys (2000). The field is rep- (dash-dotted line). resented by an aligned superposition of dipole, axisymmetric quadrupole and octupole. The 6 free model parameters include the respective field strengths Bd, Bq, Bo, two angles, β and γ, model B, but includes non-linear quadrupole, whose geometri- giving orientation of the magnetic field axis, and inclination i cal properties are now defined with the 4 angles. Since the field of the stellar rotation axis. The second model (B) disposes of in HD137509 is dominated by a higher-order magnetic com- the field symmetry assumption and approximates the magnetic ponent, it is convenient to reckon the azimuth angles γ1 and γ2 structure by a sum of dipole and linear quadrupole, not nec- from the same zero-phase plane as the dipolar azimuth angle γ. essarily aligned with respect to each other (Khokhlova et al. The projected rotational velocity, ve sin i, influences the ampli- 2000). This model has 7 free parameters: the field intensities tude of the ve sin ihxBzi variation and is adjusted for each of the Bd and Bq, four angles specifying orientation of the dipolar and three magnetic models to fit the crossover observations. quadrupolar components, and stellar inclination. Finally, model Parameters of all magnetic models were optimized using C is the one employed by Bagnulo et al. (2002). It is similar to the least-squares procedure. Resulting best-fit multipolar field 4 O. Kochukhov: Remarkable non-dipolar magnetic field of the Bp star HD 137509

Fig. 3. Comparison of the observed (thin line) and computed (thick line) spectrum of HD137509. Synthetic spectrum is computed −1 for ve sin i = 20 kms and hBi = 29 kG using the  code. Zeeman patterns are schematically illustrated for the Si  6347.11, 6371.37 Å and Fe  6357.16 Å lines. The length of vertical bars is proportional to the relative strength of the respective Zeeman components; π components are shown above the horizontal dotted line, and σ± components below this line. characteristics are listed in Table 1. Theoretical phase curves one can see that the mean surface field variation expected for of the four magnetic observables are compared with available the alternative magnetic models of HD137509 is substantially observations in Fig. 2. The three magnetic models yield re- different, especially for the rotation phase interval ϕ =0.6–1.0. duced χ2 of 1.16–1.36, indicating acceptable fitting of the data. Predicted hBi ranges from 22 to 31 kG. CorrespondingZeeman The general superposition of dipole and non-linear quadrupole splitting of the magnetically sensitive spectral lines should be (model C) provides a marginally better description of the mag- comparable or larger than the rotational Doppler broadening −1 netic measurements. It is encouraging to find consistency in of the HD137509 spectrum (ve sin i = 28 ± 5kms according some of the geometrical parameters recovered in the fit with to the estimate by Mathys (1995b)). Thus, if one believes that different multipolar parameterizations. All models favour large the low-order multipolar field models provide an adequate de- inclination angle, i = 64◦–81◦, and suggest similar strength and scription of the non-dipolar field in HD137509, there are good orientation for the dipolar component.The strength of the dom- prospects for detecting Zeeman split lines in high-resolution inant quadrupolarcomponentrangesfrom42 to 52 kG – at least optical spectra of this star. 9 times larger than the dipolar contribution. The projected rotational velocity deduced from the crossover observations is in the interval 9–13 kms−1, which is smaller than the spectroscop- 4. Zeeman split lines in the spectrum of HD 137509 ically determined v sin i (see Sect. 4). The stellar radius esti- e High-resolution spectra of HD137509 were obtained with the mated using the aforementioned range of possible stellar incli- UVES instrument of the ESO VLT on July 12, 2001 (JD = nation and the rotation period of HD137509 is also unrealisti- 2452102.60, phase 0.67 for the 4.4916 d rotation period). cally small (R ≈ R ) for v sin i ≈ 10 kms−1. On the other hand, ⊙ e Observations were carried out in the context of the UVES one finds R ≈ 1.8–3.0 R if v sin i = 20–30 kms−1 is adopted. ⊙ e Paranal Observatory Project1 (ESO DDT program 266.D- The three magnetic models predict nearly identical rota- 5655, Bagnulo et al. 2003). The spectra were recorded in ′′ tional modulation of hBzi and ve sin ihxBzi. Expected variation dichroic modes, using the slit width of 0.5 . A spectral resolu- 2 2 1/2 of hBz + B i is marginally different, but corresponding the- tion of about 80000 and almost complete spectral coverage of oretical curves cannot be distinguished based on observations the wavelength interval from 3070 to 10400 Å were achieved. because of the large errors of the quadratic field measurements. The signal-to-noise ratio of the HD137509 spectrum is above Thus, no unique multipolar field model can be derived for 300 for the optical region. Reduction of the UVESpop obser- HD 137509 with the available magnetic observations. This situ- vations is described by Bagnulo et al. (2003). ation is often encountered in the analysis of the disk-integrated Inspection of the UVES spectra of HD137509 revealed a magnetic observables of magnetic CP stars when using some significant distortion in the profiles of many spectral lines. In specific parameterization of magnetic geometry (Bagnulo et al. addition to the overall asymmetry of many features, some lines 2002) or when comparing results obtained with different multipolar parameterizations (Kochukhov 2004). At the same time, 1 http://www.eso.org/uvespop/ O. Kochukhov: Remarkable non-dipolar magnetic field of the Bp star HD 137509 5 exhibit resolved absorption components which are consider- ably narrower than expected for the normal late B stellar spec- −1 trum with a ve sin i ≈30 kms Doppler broadening. This may be a signatureof the Zeeman splitting in a very strong magnetic field or a manifestation of an inhomogeneous surface chemical distribution. To separate these effects, I have investigated several groups of spectral lines, belonging to the same ions, but with different sensitivity to magnetic field. Remarkable proof of the presence of a very strong magnetic field in HD137509comes fromthe analysis of the red Si  doublet at λ 6347 and 6371 Å. These lines originate from the same lower atomic level and have similar strength. Nevertheless, as evident from Fig. 3, profile shape of the two Si  lines is very different. The 6371 Å line shows an asymmetric double structure, whereas the Si  6347 Å lines exhibits no apparent anomaly. Obviously, horizontal abundance spots cannot be re- sponsible for such a drastically different shape of the two similar lines. Consideration of the Zeeman effect allows one to un- derstand the puzzling behaviour of the Si  doublet. The Si  6371 Å line has a relatively simple, nearly doublet, Zeeman Fig. 4. Zeeman resolved Nd  lines in the spectrum of splitting pattern with two π and two σ components. On the HD137509. The four panels show lines with different Zeeman other hand, the 6347 Å line splits into 6 close Zeeman com- splitting patterns. The strength and position of the π and σ ponents, which are blended together and do not produce re- Zeeman components for hBi = 29 kG are illustrated as in solved structure in HD137509. In Fig. 3 I compare observa- Fig. 3. The classical triplet pattern with z = 0.9 is adopted for tions and theoretical spectrum calculated with the magnetic the unclassified line Nd  6273.67 Å. spectrum synthesis code  (Piskunov 1999). I have −1 adopted Teff = 12750 K, log g = 3.8, ve sin i = 20 kms , and hBi = 29 kG. The Zeeman splitting of Si  6371 Å line and obtained from individual lines). The aforementioned spectrum  broadening of the Si  6347 Å and Fe  6357 Å lines are re- synthesis modelling of the Si doublet is in agreement with produced fairly well, although a homogeneous magnetic field this estimate of the surface field strength. adopted in the  calculations is insufficient to model The first field modulus measurement obtained for relative strengths of the Si  6371 Å Zeeman components. HD137509 in the present paper is compared with the multi- A number of doubly ionized Nd lines can be identified in polar field model predictions in Fig. 2. The maximum attained the spectrum of HD137509. Some of these features show con- field strength is consistent with observations for all three mod- spicuous narrow components in the triplet-like splitting pat- els. The non-axisymmetric superposition of the dipole and lin- terns (Fig. 4). Detailed investigation of several lines confirms ear quadrupole (model B, see Sect. 3) shows the best agreement this to be a magnetic field effect. The Nd  lines with sig- with the observational estimate of hBi, whereas the axisymmet- nificant Landéfactors and quasi-triplet Zeeman splitting (e.g. ric model A is probably ruled out.  5845 and 6145 Å) exhibit resolved components.A well-defined Magnetic spectrum synthesis with the code was triplet structure is also observed for an unidentified line at used to assess chemical abundancepattern of HD137509. Even 6273.67 Å. According to the Nd laboratory line list compiled when the very strong magnetic field is taken into account, one by H. Crosswhite, this line belongs to the Nd  spectrum (T. finds a large enhancement of the Si and iron-peak element Ryabchikova, private communication). On the other hand, as concentration with respect to the solar chemical composition: expected from its small Landéfactor and complex Zeeman pat- [Si/H] =+0.8, [Fe/H] =+1.4, [Cr/H] =+1.9, and [Ti/H] =+2.6. tern, the resonance Nd  6550 Å line shows no magnetic split- In contrast, several light elements are strongly underabundant: ting. Resolved Zeeman components of the Nd  lines suggest [He/H] < −3.5, [Ca/H] = −2.2, [Mg/H] = −1.2. −1 ve sin i ≈ 10 kms , which is substantially smaller than the pro-  jected rotational velocity inferred from the Si and other spec- 5. Discussion tral lines. This anomaly may be attributed to a high-contrast inhomogeneous horizontal distribution of Nd. In such a situa- In this paper I have presented analysis of the magnetic field tion Nd lines will sample a fraction of the visible stellar surface in the unique peculiar B-type star HD137509. This object is and, therefore, will be less broadened by rotation. one of the very few early-type stars with distinctively non- To improvethe accuracy of the magnetic field strength mea- dipolar magnetic field geometry. Interpretation of the previ- surement, I have determined hBi from several lines showing ous magnetic measurements of HD137509 with the help of Zeeman components: Si  6371, Fe  5018, Ba  6141, Nd  the low-order multipolar field models suggests that the dom- 5845 and 6145 Å. Fitting superposition of Gaussian profiles to inant contribution to the surface field in this star comes from these five lines yields hBi = 28.7 ± 0.8 kG (the quoted uncer- the > 40 kG quadrupolar component. Mutipolar models predict tainty is the standard error of the mean for the measurements the disk-averaged surface field strength in the range from 22 6 O. Kochukhov: Remarkable non-dipolar magnetic field of the Bp star HD 137509 to 31 kG. My investigation of the high-resolution UVES spec- References tra of HD 137509 revealed resolved Zeeman splitting in several Bagnulo, S., Landi Degl’Innocenti, M., Landolfi, M., & Mathys, G. spectral lines. The inferredfield strength of 28.7kG is in accord 2002, A&A, 394, 1023 with the multipolar model predictions. Bagnulo, S., Jenin, E., Ledoux, C., et al. 2003, Messenger, 114, 10 Unlike several other cases when a large quadratic field de- Bohlender, D. A. 1994, in IAU Symp. 162, Pulsation, rotation and duced for spotted magnetic CP stars contradicts high-resolution mass loss in early-type stars, eds. L.A. Balona, H.F. Henrichs, line profile studies (see Kupka et al. (1996) for α Cir and J.M. Le Contel, Kluwer Academic Publishers, 155 Kochukhov et al. (2004) for HR3831), the quadratic field mea- Bohlender, D. A., Landstreet, J. D., & Thompson, I. B. 1993, A&A, surements of HD137509 appear to be more reliable. This is 269, 355 Borra, E. F., & Landstreet, J. D. 1980, ApJS, 42, 421 likely to be a result of a comparatively strong influence of the Cowley, C. R., & Houk, N. 1975, PASP, 87, 527 magnetic field on the line profile shapes in HD137509. In stars ESA 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200. ESA with weaker fields effects of the horizontal chemical inhomo- Publications Division, Noordwijk geneities often dominate the line profiles, which distorts the Hubrig, S., North, P., & Mathys, G. 2000, ApJ, 539, 352 quadratic field diagnostic (Kochukhov 2004) and sometimes Hunger, K., & Groote, D. 1999, A&A, 351, 554 leads to a spurious detection of strong magnetic fields. Khokhlova, V. L., Vasilchenko, D. V., Stepanov, V. V., & Romanyuk, The present study establishes HD137509 as an outstand- I. I. 2000, Astron. Letters, 26, 177 Kochukhov, O. 2004, in Magnetic Stars, eds. Yu. V. Glagolevskij, D. ing object with the second-largest field among CP and main O. Kudryavtsev and I.I. Romanyuk, Nizhnij Arkhyz, 64 sequence stars in general. The only star with a stronger field is Kochukhov, O., Bagnulo, S., Wade, G. A., et al. 2004, A&A, 414, 613 the well-known Babcock’s star (HD215441). The latter shows Kochukhov, O., Drake, N. A., Piskunov, N., & de la Reza, R. A&A, varying field modulus between 32 and 34 kG (Preston 1969). 424, 935 However, orientation of the rotation axis and magnetic field Kochukhov, O., Khan, S., & Shulyak, D. 2005, A&A, 433, 671 geometry of HD215441 are not favourable for detailed anal- Kupka, F., Ryabchikova, T. A., Weiss, W. W., et al. 1996, A&A, 308, ysis of the field topology (Landstreet et al. 1989). In con- 886 trast, large inclination angle and unusual, very strong field of Kurucz, R. L. 1993, Kurucz CD-ROM 13, Cambridge, SAO HD137509 open remarkable possibilities for the detailed mag- Landstreet, J. D. 1990, ApJ, 352, L5 netic field mapping (Piskunov & Kochukhov 2002) and for the Landstreet, J. D., & Mathys, G. 2000, A&A, 359, 213 analysis of relation between chemical spots and magnetic struc- Landstreet, J. D., Barker, P. K., Bohlender, D. A., & Jewison, M. S. 1989, ApJ, 344, 876 tures. Outstanding chemical enrichment of the HD137509 at- Lanz, T., & Mathys, G. 1991, IBVS, 3655 mosphere allows to use this star as a test ground for the appli- Lodén, L. O., & Sundman, A. 1989, JA&A, 10, 183 cation of the new generation model atmospheres of early-type Mathys, G. 1991, A&AS, 89, 121 stars, incorporating effects of anomalous chemical composition Mathys, G. 1995a, A&A, 293, 733 and strong magnetic field (Shulyak et al. 2004; Valyavin et al. Mathys, G. 1995b, A&A, 293, 746 2004; Kochukhov et al. 2005). Mathys, G., & Hubrig, S. 1997, A&AS, 124, 475 The measurement of the field strength in HD137509 is the Mathys, G., & Lanz, T. 1997, A&A, 323, 881 Pedersen, H. 1979, A&AS, 35, 313 first case when we are able to impose a strong constraint on Piskunov, N. E. 1999, in 2nd International Workshop on Solar the fundamental field characteristic of an early-type star with Polarization, eds. K. Nagendra and J. Stenflo, Kluwer Acad. Publ. a non-dipolar field topology. Good agreement between hBi de- ASSL, 243, 515 termination and the outcome of the multipolar field analysis Piskunov, N., & Kochukhov, O. 2002, A&A, 381, 736 supports the common (through not as yet thoroughly verified) Preston, G. W. 1969, ApJ, 156, 967 assumption that deviations from the basic dipolar field structure Shulyak, D., Tsymbal, V., Ryabchikova, T., Stütz, Ch., & Weiss, W. in the early-type magnetic stars has the form of quadrupole or W. 2004, A&A, 428, 993 octupole. Justification of the mutipolar field modelling frame- Thompson, I. B., & Landstreet, J. D. 1985, ApJ, 289, L9 work hints that another star with the complex, double-wave Valyavin, G., Kochukhov, O., & Piskunov, N. 2004, A&A, 420, 993 variability of the longitudinal field – HD37776 (Thompson & Landstreet 1985) – also hosts a similarly large magnetic field, or may even possess the strongest field of all known non- degenerate stars. Unfortunately, the rapid rotation of HD 37776 precludes us from detection of the Zeeman splitting in spectral lines and hence leaves no chance of direct confirmation of the presence of an extraordinarily large field in this star.

Acknowledgements. The author is grateful to Dr. T. Ryabchikova for her valuable remarks and help with the Nd  line identification. Resources provided by the electronic databases (VALD, SIMBAD, NASA’s ADS) are acknowledged. This work was supported by the grant from the Kungliga Fysiografiska Sällskapet i Lund.