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Measurements of stellar magnetic fields using synthetic spectrum fitting Nielsen, Krister; Wahlgren, Glenn Published in: Astronomy & Astrophysics DOI: 10.1051/0004-6361:20021276 2002 Link to publication Citation for published version (APA): Nielsen, K., & Wahlgren, G. (2002). Measurements of stellar magnetic fields using synthetic spectrum fitting. Astronomy & Astrophysics, 395(2), 549-556. https://doi.org/10.1051/0004-6361:20021276 Total number of authors: 2 General rights Unless other specific re-use rights are stated the following general rights apply: Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Read more about Creative commons licenses: https://creativecommons.org/licenses/ Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. LUND UNIVERSITY PO Box 117 221 00 Lund +46 46-222 00 00 A&A 395, 549–556 (2002) Astronomy DOI: 10.1051/0004-6361:20021276 & c ESO 2002 Astrophysics Measurements of stellar magnetic fields using synthetic spectrum fitting? K. Nielsen and G. M. Wahlgren Atomic Astrophysics, Lund Observatory, Box 43, 221 00 Lund, Sweden Received 26 June 2002 / Accepted 30 August 2002 Abstract. A sample of Ap and Bp stars have been investigated with respect to their surface magnetic field using synthetic spectrum fitting. The advantage of this technique is the simplicity in determining the surface field strength and the possibility to apply it to objects whose spectrum does not display resolved Zeeman components. We show the usefulness of the method by demonstrations on stellar spectra with resolved Zeeman components, where the results have been compared with values based on measurements of the Zeeman splitting for the magnetically sensitive Fe ii λ6149 line. The objects HD 192678 and HD 165474 have been observed over their rotational period to investigate magnetic field variations. The analysis is then extended to objects where no resolved structure is observed. The rapidly rotating stars HD 22316 and HD 10783 are used as examples where this method is useful to achieve a value of the surface field. The relative intensification of the Fe ii λλ6147, 6149 lines is investigated in an attempt to understand its relation to the surface magnetic field strength for the stars HD 22316 and HD 10783. Key words. magnetic fields – line: profiles – stars: chemically peculiar 1. Introduction structure (Mathys & Lanz 1992; Lanz & Mathys 1993; Hubrig et al. 1999). Stenflo & Lindegren (1977) developed a method Numerous investigations of stellar magnetic fields have been to derive the interdependence of the magnetic field and the performed since Babcock (1947) detected the Zeeman effect in equivalent width based on solar observations, a method later the spectrum of 78 Vir. Most of these studies have measured refined (Mathys & Lanz 1992) to applications in hotter stars the mean longitudinal field (MLF), which is the averaged mag- without resolved Zeeman structure. Takeda (1991) investigated netic field over the stellar surface in the line-of-sight. An anal- the sensitivity of the relative intensification to changes in iron ysis of this kind uses the fact that transitions between magnetic abundance, aspect angle and microturbulence. Unfortunately, sublevels are differently polarized. Therefore, stellar spectra of the method is highly sensitive to line blending and the method opposite circular polarity are used to calculate the Stokes V pa- in converting the equivalent width into a magnetic field strength rameter. is still not fully developed. I Investigations have also made use of the Stokes profile to We present an approach to measure magnetic fields, use- measure the field affecting the atoms in the stellar photosphere ful for spectra without resolved Zeeman structure, built on (Huchra 1972; Preston 1969a,b; Preston & Wolff 1970; Mathys synthetic spectrum fitting. Each spectral line shows a charac- 1990) and uses the Zeeman splitting of magnetically sensitive teristic Zeeman pattern when influenced by a magnetic field spectral lines to measure the surface field, also referred to as and through analysis of the line profile information regarding the mean field modulus (MFM). This method is restricted to the stellar magnetic field can be obtained. To investigate spec- objects with resolved Zeeman components and is not useful for tral line profiles, the observed spectrum is compared with a objects whose spectral lines are predominantly broadened by synthetic spectrum including complete Zeeman patterns. The stellar rotation. method requires calculated Zeeman patterns and knowledge of The equivalent width, Wλ, has been used as a magnetic field the other broadening effects influencing the analyzed spectral indicator for stars with weak field and no observable Zeeman line. The first is simplified by assuming pure LS-coupling and a weak magnetic field to avoid partial Paschen-Back effect, Send offprint requests to: K. Nielsen, e-mail: [email protected] which restricts the investigation to objects with surface fields ? Based on observations obtained with the Nordic Optical less than 7–8 kG. The measured magnetic field is integrated Telescope, operated on the island of La Palma jointly by Denmark, over the stellar surface and is treated as perpendicular to the Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio line-of-sight. Our approach is capable of investigating line pro- del Roque de los Muchachos of the Instituto de Astrofisica de files in the absence of resolved Zeeman components and can be Canarias. used for objects with a weak surface field even if the rotation 550 K. Nielsen and G. M. Wahlgren: Measurements of stellar magnetic fields using synthetic spectrum fitting is the dominating broadening mechanism. Our investigation of Table 2. Observed data. the stellar magnetic field using synthetic spectrum fitting does a not consider the polarization state of the radiation and the influ- Object Sp. type V HJD S=N ence on the radiative transfer by the polarized radiation is not (2 451 700+) included in our synthetic spectrum code. HD 2453 A2p 6.91 74.701 173 HD 10783 A2p 6.55 68.736 180 74.680 223 Table 1. Spectral lines used in the magnetic field analysis. 75.682 193 HD 22316 B9p 6.28 65.738 182 66.607 212 Ion λ (Å) g g log g f b air Jl Ju 67.732 158 Cr ii 6147.154 1.200 1.376 2.843 − HD 165474 A8p 7.50 64.472 193 Fe iia 6147.736 1.200 2.700 2.721 65.480 190 − 6149.248 0 2.700 2.724 66.466 176 − 67.464 175 a Wavelengths from S. Johansson, private communication. 68.493b 83c b Kurucz (1988). 68.519 177 69.413 170 70.413 178 70.499b 139c 2. Observations 71.426 192 72.419 178 All data used in this analysis were obtained with the 2.5 m 73.421 209 Nordic Optical Telescope (NOT) at La Palma, Canary Islands, HD 192678 A4p 7.40 64.437 162 using the SOviet FINish high resolution echelle spectrograph 65.453 169 λ (SOFIN) at a resolving power of R = ∆λ =75 000. The data 66.439 166 set consists of a sequence of observations for each of two 67.437 172 primary targets, HD 192678 and HD 165474 obtained on 10 68.440 114 b c consecutive nights. In addition, single-phase observations of 68.466 105 two stars (HD 2453 and HD 200311) with expected long rota- 69.466 132 67.464b 177c tional periods were made with the purpose of testing the mag- 70.461 139 netic field measurement technique. Two objects, HD 22316 and 71.451 164 HD 10783, do not show resolved Zeeman structure and were 72.444 151 each observed at three phases. The spectral coverage between 73.445 151 5600 7000 Å is not complete and is centered on a spectral re- HD 200311 B9p 7.68 67.490 162 gion− including the magnetically sensitive lines Fe ii λλ6147, a Estimated S=N for the spectral order including Fe ii λλ6147, 6149. 6149 and Cr ii λ6147 (Table 1). Additional observations for b Additional observation for less magnetically sensitive lines. some of the targets were made at shorter wavelengths to cover c Estimated S=N for the spectral order including Fe ii λλ4491, 4508. the less magnetic sensitive lines Fe ii λλ4508, 4491 Å. The complete data set is presented in Table 2. Zeeman components rely on less magnetically sensitive spec- 3. Atmospheric models tral lines, such as Fe ii λλ4491, 4508, with gJ < 0:5 for both The determination of the atmospheric parameters has been the upper and lower level, to place an upper limit on v sin i. accomplished through application of the uvbybeta photo- For the included objects with resolved Zeeman structure v sin i metric calibration program uvbybeta developed by Moon & was estimated using iron lines, especially the less magnetically Dworetsky (1985) and later improved by Napiwotzki et al. sensitive Fe ii λλ4491, 4508 lines, and fine tuned with individ- (1993) based on photometric indices from Hauck & Mermilliod ual Zeeman component of the Fe ii λ6149 line. The rotational (1980). Stellar atmospheric models have been computed using velocity for HD 10783 is based on spectrum fitting using a sam- the LTE approximation with the ATLAS9 code (Kurucz 1993) ple of Fe ii lines in the blue spectral region while for HD 22316 under the assumption of no turbulent velocity.
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