A&A 536, A45 (2011) Astronomy DOI: 10.1051/0004-6361/201117407 & c ESO 2011 Astrophysics Characterising the magnetic fields of the Herbig Ae/Be stars HD 97048, HD 150193, HD 176386, and MWC 480 S. Hubrig1, M. Schöller2, I. Ilyin1,C.R.Cowley3, Z. Mikulášek4,5, B. Stelzer6, M. A. Pogodin7,8, R. V. Yudin7,8,andM.Curé9 1 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [email protected] 2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany 3 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA 4 Department of Theoretical Physics and Astrophysics, Masaryk University, Brno, Czech Republic 5 Observatory and Planetarium of J. Palisa, VŠB - Technical University, Ostrava, Czech Republic 6 INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy 7 Pulkovo Observatory, 196140 Saint-Petersburg, Russia 8 Isaac Newton Institute of Chile, Saint-Petersburg Branch, Russia 9 Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Chile Received 4 June 2011 / Accepted 16 September 2011 ABSTRACT Context. Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. Aims. We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail. Methods. Our results for the three Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 are based on multi-epoch low- resolution spectra obtained in spectropolarimetric mode with FORS 2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC 480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS 2 low- resolution observations of the Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC 480 were measured using the moment tech- nique where wavelength shifts between right- and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field Bz. Results. Our search for periodicities resulted in P = 0.693 d for HD 97048, P = 1.317 d for HD 150193, and P = 0.899 d for HD 176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength Bd of several hundred Gauss. The longitudinal magnetic-field measurements of MWC 480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. A weak magnetic field was detected in the circumstellar components of the Ca ii H&K lines and the Na i D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment. Key words. stars: pre-main sequence – stars: atmospheres – stars: magnetic field – stars: variables: general – stars: general 1. Introduction the strongly magnetic Herbig Ae star HD 101412 (Hubrig et al. / 2010, 2011). The small amount of UVES spectra acquired for Spectropolarimetric observations of several Herbig Ae Be stars this star have been analysed uncovering variations in equiva- have indicated that magnetic fields are important ingredients of lent widths, radial velocities, line widths, line asymmetries, and the intermediate-mass star formation process. Models of mag- mean magnetic field modulus over the rotation period of 42.1 d. netically driven accretion and outflows successfully reproduce Magnetic field configurations are obviously of utmost impor- many observational properties of low-mass pre-main sequence tance to understanding the magnetospheres of Herbig Ae/Be stars (the classical T Tauri stars), but the picture is less clear stars. Since the accreting material has to be lifted from the for higher mass stars. It is important to understand how the / disk midplane to higher stellar latitudes near the stellar photo- magnetic fields in Herbig Ae Be stars are generated and how sphere, some coupling between the accreting plasma and a stellar these fields interact with the circumstellar environment, display- magnetic field is required. Progress in understanding the disk- ing a combination of disk, wind, accretion, and jets. To date, magnetosphere interaction can, however, only come from study- these fields have not been studied for a representative number of ffi / ing a su cient number of targets in detail to look for patterns Herbig Ae Be stars. Magnetically controlled behaviour has been encompassing this type of pre-main sequence stars. found in both the spectroscopic and photometric variability of In this work, we present series of mean longitudi- Based on observations obtained at the European Southern nal magnetic-field measurements for the Herbig Ae/Be stars Observatory (ESO programme 085.C-0137(A)) and SOFIN observa- HD 97048, HD 150193, and HD 176386 obtained at low reso- tions at the 2.56-m Nordic Optical Telescope on La Palma. lution with the multi-mode instrument FORS 2 at the VLT, and Article published by EDP Sciences A45, page 1 of 8 A&A 536, A45 (2011) Table 1. Target stars with multi-epoch spectropolarimetric observa- Table 2. Magnetic field measurements of Herbig Ae/Be stars with tions. FORS 1/2. Object Other V Spectral MJD Phase Bz all [G] Bz hyd [G] name identifier type HD 97048 MWC 480 HD 31648 7.7 A3Ve 54609.137 0.083 164 ± 42 188 ± 47 HD 97048 CU Cha 8.5 A0pshe 55279.131 0.349 −92 ± 41 −135 ± 58 HD 150193 V2307 Oph 8.9 A1Ve 55311.109 0.467 −163 ± 53 −195 ± 59 HD 176386 CD−37 13023 7.3 B9V 55312.035 0.802 −6 ± 74 12 ± 91 55320.094 0.426 −44 ± 54 −62 ± 62 Notes. Spectral types and visual magnitudes are taken from SIMBAD. 55322.119 0.346 −123 ± 42 −62 ± 48 55324.105 0.210 4 ± 59 9 ± 63 55326.987 0.367 −124 ± 34 −156 ± 44 describe their magnetic field geometries. Magnetic fields of the 55334.039 0.537 −154 ± 53 −193 ± 61 order of 120–250G were detected in these stars for the first 55335.119 0.094 127 ± 120 135 ± 49 time during our visitor run with FORS 1 in May 2008 (Hubrig 55345.987 0.768 54 ± 59 26 ± 63 et al. 2009). The follow-up program developed to search for 55347.180 0.488 −186 ± 61 −231 ± 64 55348.070 0.772 −36 ± 38 −12 ± 64 magnetic field variations over the rotation cycle was allocated − ± ± for observations with FORS 2 in 2010. The previous spectropo- 55349.102 0.260 7 74 65 78 HD 150193 larimetric observations of the star MWC 480 (=HD 31648) with 54609.092 0.118 −144 ± 32 −252 ± 48 FORS 1 revealed the presence of a weak longitudinal magnetic − ± − ± = ± 55321.364 0.523 124 60 184 62 field Bz 87 22 G and distinct circular polarisation signa- 55312.319 0.648 −97 ± 84 −239 ± 102 tures in spectral lines originating in the circumstellar (CS) envi- 55320.179 0.622 −148 ± 69 −147 ± 80 ronment (Hubrig et al. 2006, 2007). We report here on three new 55322.295 0.230 −109 ± 48 −111 ± 64 high-resolution polarimetric spectra obtained with the SOFIN 55323.311 0.003 −39 ± 34 −43 ± 46 spectrograph installed at the Nordic Optical Telescope, which 55324.285 0.744 −107 ± 43 −124 ± 64 confirm the presence of a magnetic field in this star in both pho- 55325.307 0.520 −189 ± 45 −228 ± 59 tospheric and CS lines. 55326.341 0.306 −142 ± 53 −176 ± 70 55327.340 0.066 −19 ± 36 −15 ± 48 55328.356 0.838 −62 ± 74 −46 ± 99 2. Magnetic field measurements and period 55334.314 0.366 −143 ± 32 −132 ± 74 55341.279 0.661 −217 ± 58 −235 ± 74 determination 55348.125 0.864 −120 ± 96 −306 ± 130 − ± − ± The targets for which we present new spectropolarimetric mea- 55354.296 0.555 201 55 229 82 surements are listed in Table 1. Multi-epoch polarimetric spec- HD 176386 / 54610.272 0.174 −119 ± 33 −121 ± 35 tra of the Herbig Ae Be stars HD 97048, HD 150193, and − ± − ± 1 55312.353 0.145 10 69 42 88 HD 176386 were obtained with FORS 2 on Antu (UT1) from 55322.313 0.224 −41 ± 65 −44 ± 89 2010 March 23 to 2010 June 7 in service mode. Using a slit − ± − ± 55323.247 0.263 62 62 126 64 width of 0. 4, the spectral resolving power of FORS 2 achieved 55324.308 0.444 −190 ± 62 −179 ± 65 with the GRISM 600B is about 2000. A detailed description of 55325.331 0.582 −201 ± 63 −225 ± 71 the assessment of the longitudinal magnetic-field measurements 55326.365 0.732 −115 ± 54 −119 ± 68 using FORS 2 was presented in our previous papers (e.g., Hubrig 55327.244 0.709 −6 ± 64 −49 ± 79 et al. 2004a,b, and references therein). The mean longitudinal 55328.375 0.967 99 ± 59 111 ± 79 55334.339 0.602 −65 ± 49 −50 ± 63 magnetic field, Bz, was derived using 55335.264 0.631 −102 ± 43 −144 ± 54 2 55341.317 0.364 −101 ± 65 −104 ± 71 V geffeλ 1 dI = − B , 55349.128 0.052 115 ± 50 121 ± 68 2 z (1) I 4πmec I dλ 55354.226 0.724 −130 ± 41 −162 ± 62 − ± − ± where V is the Stokes parameter that measures the circular po- 55355.134 0.733 25 38 30 61 larisation, I is the intensity in the unpolarised spectrum, geff is the effective Landé factor, e is the electron charge, λ is the wave- length, me the electron mass, c the speed of light, dI/dλ is the the entrance slit of the spectrograph and consists of a fixed cal- derivative of Stokes I,andBz is the mean longitudinal mag- cite beam splitter aligned along the slit and a rotating super- netic field.