An optical and x-ray study of the contact binary, bh cassiopeiae

Abstract

We present the revised and high-quality CCD time series observations in 2017 and 2018 of the W UMa-type contact binary BHCas. The combination of our new determinations of minimum times and those collected from the literatures reveal a long-term period increase with a rate of dP/dt=+3.27×10−7 day yr−1, which can be accounted for by angular momentum transfer as the less-massive component loses its mass to the more massive one. One cyclical oscillation (A′=0.00300 day and P′=20.09 yr) superimposed on the long-term period increase tendency is attributed to one additional object that orbits BH Cas. The low-resolution spectra of this source are observed at phase ∼0.0 and ∼0.5 in 2018 and 2019, and the spectral types are identified as K3V±1 and G8V±2. According to the luminosity contributions and spectra at these phases, the spectral type of the primary star is close to K3V±1. We infer a cool spot on the hotter and less-massive component to fit the asymmetry of light curves. In addition, changes in the location, temperature, and area of spots on the secondary star in different observations may indicate that the magnetic field activity of the secondary is more active than that of the primary. Such strong spot activities and optical flare are both detected at the same time from a W UMa-type star for the first time. With the XMM-Newton observations taken in 2003 and 2012, the X-ray light curve is obtained and the X-ray spectra can be well described with a blackbody or a bremsstrahlung model. The soft X-ray luminosity of BH Cas is estimated as (8.8–9.7)×1030 erg s−1. situation is reverse. These two subtypes of W UMa-type are generally considered to have an evolutionary relationship with each other, but the direction of evolution between them remains controversial. For example, Li et al. (2004), based on the energy transfer in binary systems, infers that the A-subtype is the later evolution stage of the W-subtype. Gazeas & Niarchos (2006), on the other hand, propose the opposite evolutionary sequence by utilizing statistics of the mass and angular momentum of binary systems. Many W UMa-type binaries are X-ray emitters. Some of their fundamental features, including high chromospheric and coronal activity (Huenemoerder et al. 2006; Kandulapati et al. 2015; Hu et al. 2016), and synchronous fast-rotation common envelopes (Gondoin 2004; Chenetal. 2006), are believed to be the origin of the X-ray emission. Their X-ray intensities are related to their orbital periods and spectral types (Stȩpień et al. 2001;Chen et al. 2006). The studies of 2MASS J11201034-2201340.