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Register a new userMOBSTER -- III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic identical twin
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HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M$_odot$. The primarys longitudinal magnetic field $langle B_z rangle$ varies between about $+100$ and $-250$ G, suggesting a surface magnetic dipole strength $B_{rm d} = 850$~G. Bayesian analysis of the Stokes $V$ profiles indicates $B_{rm d} = 650$~G for the primary and $B_{rm d} < 110$ G for the secondary. The primarys line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that stars chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent with the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete.
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HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H$alpha$ emission. We have analysed the TESS light curve, and find that it is a $beta$ Cep pulsator with 11 detectable frequencies, 4 of which are independent $p$-modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of $sim$years; significant differences in the frequencies determined from TESS and RV datasets are consistent with a light-time effect from orbital motion. Close examination of the stars spectrum reveals the presence of a spectroscopic companion, however as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of LSD profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with $langle B_z rangle$ about $+1.5$ kG as compared to $langle B_z rangle sim -0.8$ kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H$alpha$ emission appears to originate around HD 156424B. Using H$alpha$, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.
Early-type magnetic stars are rarely found in close binary systems. No such objects were known in eclipsing binaries prior to this study. Here we investigated the eclipsing, spectroscopic double-lined binary HD66051, which exhibits out-of-eclipse photometric variations suggestive of surface brightness inhomogeneities typical of early-type magnetic stars. Using a new set of high-resolution spectropolarimetric observations, we discovered a weak magnetic field on the primary and found intrinsic, element-dependent variability in its spectral lines. The magnetic field structure of the primary is dominated by a nearly axisymmetric dipolar component with a polar field strength $B_{rm d}approx600$ G and an inclination with respect to the rotation axis of $beta_{rm d}=13^{rm o}$. A weaker quadrupolar component is also likely to be present. We combined the radial velocity measurements derived from our spectra with archival optical photometry to determine fundamental masses (3.16 and 1.75 $M_odot$) and radii (2.78 and 1.39 $R_odot$) with a 1-3% precision. We also obtained a refined estimate of the effective temperatures (13000 and 9000 K) and studied chemical abundances for both components with the help of disentangled spectra. We demonstrate that the primary component of HD66051 is a typical late-B magnetic chemically peculiar star with a non-uniform surface chemical abundance distribution. It is not an HgMn-type star as suggested by recent studies. The secondary is a metallic-line star showing neither a strong, global magnetic field nor intrinsic spectral variability. Fundamental parameters provided by our work for this interesting system open unique possibilities for probing interior structure, studying atomic diffusion, and constraining binary star evolution.
$tau^{9}$ Eri is a Bp star that was previously reported to be a single-lined spectroscopic binary. Using 17 ESPaDOnS spectropolarimetric (Stokes $V$) observations we identified the weak spectral lines of the secondary component and detected a strong magnetic field in the primary. We performed orbital analysis of the radial velocities of both components to find a slightly eccentric orbit ($e= 0.129$) with a period of $5.95382(2)$ days. The longitudinal magnetic field ($B_ell$) of the primary was measured from each of the Stokes $V$ profiles, with typical error bars smaller than 10 G. Equivalent widths (EWs) of LSD profiles corresponding to only the Fe lines were also measured. We performed frequency analysis of both the $B_ell$ and EW measurements, as well as of the Hipparcos, SMEI, and TESS photometric data. All sets of photometric observations produce two clear, strong candidates for the rotation period of the Bp star: 1.21 days and 3.82 days. The $B_ell$ and EW measurements are consistent with only the 3.82-day period. We conclude that HD 25267 consists of a late-type Bp star (M= $3.6_{-0.2}^{+0.1} M_odot$, T= $12580_{-120}^{+150}$ K) with a rotation period of 3.82262(4) days orbiting with a period of 5.95382(2) days with a late-A/early-F type secondary companion (M= $1.6pm 0.1 M_odot$, T= $7530_{-510}^{+580}$ K). The Bp stars magnetic field is approximately dipolar with $i= 41pm 2^{circ}$, $beta= 158pm 5^{circ}$ and $B_{rm d}= 1040pm 50$ G. All evidence points to the strong $1.209912(3)$ day period detected in photometry, along with several other weaker photometric signals, as arising from $g$-mode pulsations in the primary.
In the course of a project to study eclipsing binary stars in vinicity of the Sun, we found that the cooler component of LL Aqr is a solar twin candidate. This is the first known star with properties of a solar twin existing in a non-interacting eclipsing binary, offering an excellent opportunity to fully characterise its physical properties with very high precision. We used extensive multi-band, archival photometry and the Super-WASP project and high-resolution spectroscopy obtained from the HARPS and CORALIE spectrographs. The spectra of both components were decomposed and a detailed LTE abundance analysis was performed. The light and radial velocity curves were simultanously analysed with the Wilson-Devinney code. The resulting highly precise stellar parameters were used for a detailed comparison with PARSEC, MESA, and GARSTEC stellar evolution models. LL Aqr consists of two main-sequence stars (F9 V + G3 V) with masses of M1 = 1.1949$pm$0.0007 and M2=1.0337$pm$0.0007 $M_odot$, radii R1 = 1.321$pm$0.006 and R2 = 1.002$pm$0.005 $R_odot$, temperatures T1=6080$pm$45 K and T2=5703$pm$50 K and solar chemical composition [M/H]=0.02$pm$0.05 dex. The absolute dimensions, radiative and photometric properties, and atmospheric abundances of the secondary are all fully consistent with being a solar twin. Both stars are cooler by about 3.5 $sigma$ or less metal abundant by 5$sigma$ than predicted by standard sets of stellar evolution models. When advanced modelling was performed, we found that full agreement with observations can only be obtained for values of the mixing length and envelope overshooting parameters that are hard to accept. The most reasonable and physically justified model fits found with MESA and GARSTEC codes still have discrepancies with observations but only at the level of 1$sigma$.
Context. The existence of a significant population of Ap stars with very long rotation periods (up to several hundred years) has progressively emerged over the past two decades. However, only lower limits of the periods are known for most of them because their variations have not yet been observed over a sufficient timebase. Aims. We determine the rotation period of the slowly rotating Ap star HD 18078 and we derive constraints on the geometrical structure of its magnetic field. Methods. We combine measurements of the mean magnetic field modulus obtained from 1990 to 1997 with determinations of the mean longitudinal magnetic field spanning the 1999-2007 time interval to derive an unambiguous value of the rotation period. We show that this value is consistent with photometric variations recorded in the Stroemgren uvby photometric system between 1995 and 2004. We fit the variations of the two above-mentioned field moments with a simple model to constrain the magnetic structure. Results. The rotation period of HD 18078 is (1358 +/- 12) d. The geometrical structure of its magnetic field is consistent to first order with a colinear multipole model whose axis is offset from the centre of the star. Conclusions. HD 18078 is only the fifth Ap star with a rotation period longer than 1000 days for which the exact value of that period (as opposed to a lower limit) could be determined. The strong anharmonicity of the variations of its mean longitudinal magnetic field and the shift between their extrema and those of the mean magnetic field modulus are exceptional and indicative of a very unusual magnetic structure.
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