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Ap Stars with Resolved Magnetically Split Lines: Stars for Which New Measurements of the Mean Magnetic field Modulus Are Presented in This Paper

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Astronomy & Astrophysics manuscript no. apres c ESO 2018 March 12, 2018 Ap stars with resolved magnetically split lines: Magnetic field determinations from Stokes I and V spectra⋆ G. Mathys Joint ALMA Observatory & European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile e-mail: [email protected] Received ... / Accepted ... ABSTRACT Aims. We present the results of a systematic study of the magnetic fields and other properties of the Ap stars with resolved magnetically split lines. Methods. This study is based on new measurements of the mean magnetic field modulus, the mean longitudinal magnetic field, the crossover, the mean quadratic magnetic field, and the radial velocity of 43 stars, complemented by magnetic data from the literature for 41 additional stars. Results. Stars with resolved magnetically split lines represent a significant fraction, of the order of several percent, of the whole population of Ap stars. Most of them are genuine slow rotators, whose consideration provides new insight into the long-period tail of the distribution of the periods of the Ap stars. Emerging correlations between rotation periods and magnetic properties provide important clues for the understanding of the braking mechanisms that have been at play in the early stages of stellar evolution. The geometrical structures of the magnetic fields of Ap stars with magnetically resolved lines appear in general to depart slightly, but not extremely, from centred dipoles. However, there are a few remarkable exceptions, which deserve further consideration. We suggest that pulsational crossover can be observed in some stars; if confirmed, this would open the door to the study of non-radial pulsation modes of degree ℓ too high for photometric or spectroscopic observations. How the lack of short orbital periods among binaries containing an Ap component with magnetically resolved lines is related with their (extremely) slow rotation remains to be fully understood, but the very existence of a correlation between the two periods lends support to the merger scenario for the origin of Ap stars. Key words. Stars: chemically peculiar – Stars: magnetic field – Stars: rotation – Binaries: general – Stars: oscillations 1. Introduction servations of sharp, resolved line components implies that the star has a low projected equatorial velocity v sin i, and that its Babcock (1960) was the first to report the observationof spectral magnetic field is fairly uniform.From consideration of the wave- lines resolved into their Zeeman-split components by a magnetic length separation of the resolved line components, Babcock in- field in a star other than the Sun: the B9p star HD 215441 (since ferred that the average over the visible stellar disk of the modu- 1 then known as Babcock’s star). As Babcock realised, the ob- lus of its magnetic field (the mean magnetic field modulus) is of the order of 34 kG. Quite remarkably, more than half a century ⋆ Based on observations collected at the European Southern later, this still stands as the highest mean magnetic field modulus Observatory, Chile (ESO Programmes 56.E-0688, 56.E-0690, 57.E- measured in a non-degenerate star. 0557, 57.E-0637, 58.E-0155, 58.E-0159, 59.E-0372, 59.E-0373, 60.E- By the end of 1970, Preston had identified eight more Ap 0564, 60.E-0565, 61.E-0711, and Period 56 Director Discretionary stars with magnetically resolved lines (see Preston 1971 and ref- Time); at Observatoire de Haute Provence (CNRS), France; at Kitt erences therein). Preston recognised that the mean magnetic field Peak National Observatory, National Optical Astronomy Observatory modulus is fairly insensitive to the geometry of the observation, (NOAO Prop. ID: KP2442; PI: T. Lanz), which is operated by the Association of Universities for Research in Astronomy (AURA) under contrary to the mean longitudinal magnetic field (i.e. the aver- age over the visible stellar hemisphere of the line-of-sight com- arXiv:1612.03632v1 [astro-ph.SR] 12 Dec 2016 cooperative agreement with the National Science Foundation; and at the Canada-France-Hawaii Telescope (CFHT), which is operated from the ponent of the magnetic vector). This field moment, which is de- summit of Mauna Kea by the National Research Council of Canada, the termined from the analysis of the circular polarisation of spec- Institut National des Sciences de l’Univers of the Centre National de tral lines, is the main diagnostic of Ap star magnetic fields, of la Recherche Scientifique of France, and the University of Hawaii. The which the largest number of measurements have been published. observations at the Canada-France-Hawaii Telescope were performed The interest of combining measurements of the mean longitu- with care and respect from the summit of Mauna Kea, which is a sig- dinal field and mean field modulus to derive constraints on the nificant cultural and historic site. structure of stellar magnetic fields was also recognised early on. 1 However, the Babcock (1960) observations of HD 215441, on However, for the next two decades, Ap stars with magnetically which his discovery of magnetically resolved lines was based, were resolved lines, which represented only a very small subset of all obtained in October 1959. HD 126515 (also known as Preston’s star) actually appears to be the first star for which spectra showing mag- Ap stars, remained hardly more than a curiosity, to which very netically resolved lines were recorded, in February 1957— as Preston little attention was paid. (1970) found later, motivated by a remark made by Babcock (1958) on Yet these stars are especially worth studying for various rea- the unusual appearance of the line profiles. sons: 1 G. Mathys: Magnetic fields of Ap stars with magnetically resolved lines Table 1. Ap stars with resolved magnetically split lines: stars for which new measurements of the mean magnetic field modulus are presented in this paper HD/HDE Otherid. V Sp. type Period Ref. HJD0 Phase origin Ref. 965 BD 0 21 8.624 A8p SrEuCr > 13 y 1 2453 BD +−31 59 6.893 A1p SrEuCr 521 d 2 2442213.000 B min. h zi 9996 HR 465 6.376 B9p CrEuSi 7936.5 d 3 2433301.360 Bz min. 3 12288 BD +68 144 7.750 A2p CrSi 34d.9 4 2448499.870 hB imax. 4 d h i 14437 BD +42 502 7.261 B9p CrEuSi 26.87 4 2448473.846 Bz max. 4 18078 BD +55 726 8.265 A0p SrCr 1358 d 5 2449930.000 hB imax. 5 29578 CPD 54 685 8.495 A4p SrEuCr 5 y h i 47103 BD +−20 1508 9.148 Ap SrEu ≫ 50169 BD 1 1414 8.994 A3p SrCrEu 8 y 51684 CoD− 40 2796 7.950 F0p SrEuCr 371≫ d 2449947.000 B max. 55719 HR 2727− 5.302 A3p SrCrEu 10 y h i 59435 BD 8 1937 7.972 A4p SrCrEu 1360≫ d 6 2450580.000 B max. 6 61468 CoD− 27 4341 9.839 A3p EuCr 322 d 2450058.500 hBi max. 65339 53 Cam− 6.032 A3p SrEuCr 8d.02681 7 2448498.186 positiveh i crossover 7 70331 CoD 47 3803 8.898 B8p Si 1d.9989 or 1d.9909 2446987.100 arbitrary 75445 CoD −38 4907 7.142 A3p SrEu 6d.291? 2449450.000 arbitrary 81009 HR 3724− 7.200 A3p CrSrSi 33d.984 8 2444483.420 max. brightness in v 8 93507 CPD 67 1494 8.448 A0p SiCr 556 d 2 2449800.000 B min. 2 94660 HR 4263− 6.112 A0p EuCrSi 2800 d 2447000.000 hBi min. 110066 HR 4816 6.410 A1p SrCrEu 4900: d 9 h i 116114 BD 17 3829 7.026 F0p SrCrEu 27d.61 2447539.000 B min. − d h i 116458 HR 5049 5.672 A0p EuCr 148.39 2448107.000 Bz max. 119027 CoD 28 10204 10.027 A3p SrEu h i 126515 BD +−1 2927 7.094 A2p CrSrEu 129d.95 2 2437015.000 B max. 10 134214 BD 13 4081 7.467 F2p SrEuCr h i 137909 β CrB− 3.900 A9p SrEuCr 18d.4868 11 2434204.700 B pos. extr. 11 h zi 137949 33 Lib 6.659 F0p SrEuCr 5195: d 2453818.000 Bz max. d h i 142070 BD 0 3026 7.966 A0p SrCrEu 3.3718 2449878.200 Bz max. 144897 CoD− 40 10236 8.600 B8p EuCr 48d.57 2449133.700 hB imin. 150562 CoD −48 11127 9.848 A5p EuSi? > 4.5 y h i 318107 CoD −32 13074 9.355 B8p 9∼d.7088 12 2448800.000 B max. 165474 HR 6758B− 7.449 A7p SrCrEu 9 y h i 166473 CoD 37 12303 7.953 A5p SrEuCr >≫10 y 13 177765 CoD −26 13816 9.155 A5p SrEuCr ∼ 5 y − ≫ 187474 HR 7552 5.321 A0p EuCrSi 2345 d 14 2446766.000 Bz pos. extr. 15 d h i 188041 HR 7575 5.634 A6p SrCrEu 223.78 2432323.000 Bz min. 16 192678 BD +53 2368 7.362 A2p Cr 6d.4193 17 2449113.240 hB imax. 18 335238 BD +29 4202 9.242 A1p CrEu 48d.70 2447000.000 arbitraryh i d 200311 BD +43 3786 7.708 B9p SiCrHg 52.0084 19 2445407.513 Bz pos. extr. 19 201601 γ Equ 4.700 A9p SrEu > 97 y 20 h i d 208217 CPD 62 6281 7.196 A0p SrEuCr 8∼.44475 21 2447028.000 Bz pos. extr. 213637 BD −20 6447 9.611 F1p EuSr > 115 d? h i 216018 BD −12 6357 7.623 A7p SrEuCr 6 y? − ≫ References. (1) Romanyuk et al. (2014); (2) Mathys et al. (1997); (3) Metlova et al. (2014); (4) Wade et al. (2000c); (5) Mathys et al. (2016); (6) Wade et al. (1999); (7) Hill et al.
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