Abstract
ABSTRACT The late B-type star V772 Cas (HD 10260) was previously suspected to be a rare example of a magnetic chemically peculiar star in an eclipsing binary system. Photometric observations of this star obtained by the TESS satellite show clear eclipses with a period of 5.0137 d accompanied by a significant out-of-eclipse variation with the same period. High-resolution spectroscopy reveals V772 Cas to be an SB1 system, with the primary component rotating about a factor two slower than the orbital period and showing chemical peculiarities typical of non-magnetic HgMn chemically peculiar stars. This is only the third eclipsing HgMn star known and, owing to its brightness, is one of the very few eclipsing binaries with chemically peculiar components accessible to detailed follow-up studies. Taking advantage of the photometric and spectroscopic observations available for V772 Cas, we performed modelling of this system with the phoebe code. This analysis provided fundamental parameters of the components and demonstrated that the out-of-eclipse brightness variation is explained by the ellipsoidal shape of the evolved, asynchronously rotating primary. This is the first HgMn star for which such variability has been definitively identified.
Highlights
About 10 per cent of A and B main sequence stars possess stable, globally organized magnetic fields with a strength of at least 300 G (Auriere et al 2007; Sikora et al 2019)
Full Frame Images (FFIs) from Transiting Exoplanet Survey Satellite (TESS) are available at a 30-min cadence for the entire field of view, allowing light curves to be extracted for all objects that fall on the detector
We investigated the nature of the bright but poorly studied eclipsing binary system V772 Cas
Summary
About 10 per cent of A and B main sequence stars possess stable, globally organized magnetic fields with a strength of at least 300 G (Auriere et al 2007; Sikora et al 2019). The overall incidence rate of magnetic upper main sequence stars in close binaries is less than 2 per cent (Alecian et al 2015), this fraction is significantly higher if one includes wide long-period systems (Mathys 2017) This low incidence of magnetic ApBp stars in close binaries is frequently considered as an argument in favour of the stellar merger origin of fossil fields (de Mink et al 2014; Schneider et al 2016). He confirmed the presence of primary eclipses, derived an orbital period of 5.0138 d and demonstrated that archival photographic radial velocity measurements (Hube 1970) show coherent variation with the same period These results, along with the B8IIIpSi spectral classification and an evidence of the outof-eclipse variability, led Gandet (2008) to suggest V772 Cas as an α2 CVn variable in a short-period eclipsing binary – an exceptionally rare and interesting object akin to the recently discovered magnetic eclipsing binaries HD 66051 and HD 62658.
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