Research Article | Open Access
Haifeng Dai, Huiyu Yuan, Yuangui Yang, "Mass Transfer and Intrinsic Light Variability in the Contact Binary MT Cas", Advances in Astronomy, vol. 2019, Article ID 4593092, 9 pages, 2019. https://doi.org/10.1155/2019/4593092
Mass Transfer and Intrinsic Light Variability in the Contact Binary MT Cas
First CCD photometry for the contact binary MT Cas is performed in 2013 in December. The spectral type of F8V is determined from the low-precision spectrum observed on 2018 Oct 22. With Wilson-Devinney code, the photometric solutions are deduced from light curves (LCs) and AAVSO’s and ASAS-SN’s data, respectively. The results imply that MT Cas is a W-type weak-contact binary with a mass ratio of and a fill-out factor of , respectively. The asymmetric LCs in 2013 are modeled by a dark spot on the more massive component. By analyzing the curve, it is discovered that the orbital period may be undergoing a secular increase at a rate of , which may result from mass transfer from the less massive component to the more massive one. With mass transferring, MT Cas may evolve into a broken-contact configuration as predicted by TRO theory.
W Ursae Majoris binary contains two components, which are embedded in a common envelope [1, 2]. Models for contact binary have been recently constructed by several investigators (e.g., see [3–5]). However, their evolutionary status still remains unclear because the spectra cannot be analyzed for abundances due to the extreme broadening and blending of spectral lines. Therefore, it is crucially important to observe contact binaries, which may provide some special phenomena and processes, such as magnetic activity , third body , angular momentum evolution , flare , and stellar coalescence . It is helpful for us to understand their formation, structure, and evolution of contact binaries.
MT Cas (=SV SON 4671) was found by Götz & Wenzel  as a W UMa-type eclipsing binary. Its visual magnitude is mag, and the depths of both eclipses are mag and mag, respectively . Hoffmann  photoelectrically observed this binary. Unfortunately, the light curves did not cover the complete period. Pribulla et al.  derived a linear ephemeris with a period of days, which was updated to be days . Except for some photometric data performed by several amateur observers of AAVSO (https://www.aavso.org/data-download) and ASAS-SN database  (https://asas-sn.osu.edu/database/light_curves/335960), no additional observations for this binary have been presented up to now.
In this paper, the neglected binary MT Cas was studied photometrically and spectroscopically in Section 2. The orbital period variation is analyzed in Section 3, and three sets of light curves (i.e., LCs in 2013, AAVSO’s LC, and ASAS-SN LC) are modeled in Section 4. In Section 5, we estimated the absolute parameters and discuss mass transfer between two components and its evolutionary status.
2. New Observations
CCD photometry for MT Cas was first carried out at six nights of 2013, with the 85-cm telescope  at the Xinglong station (XLs) of National Astronomical Observatories of China (NAOC). The standard Johnson-Cousins systems were mounted onto this telescope. During the observation, files are used in order to provide the enough high time resolution. The image reductions are done by using the Image Reduction (IMRED) and Aperture Photometry (APPHOT) packages in the Image Reduction and Analysis Facility (IRAF) in a standard mode. Differential magnitudes were then determined by aperture photometry.
In the observing process, we chose TYC 3657-1637-1 ( and ) and TYC 3657-1245-1 ( and ) as the comparison and check stars, respectively. Typical exposure times are adopted to be in band and in band, respectively. In total, we obtained 383 and 373 images in and bands. The standard uncertainties are mag in band and mag in band. All individual observations (i.e., and ) are listed in Table 1. The differential magnitudes versus orbital phases are displayed in Figure 1, where phases are computed by a period of (Kreiner et al. 2004). The LCs in 2013 imply that MT Cas is an UMa-type eclipsing binary, whose amplitudes of variable light are mag and mag in and bands, respectively. There exists an unequal height between both maxima, i.e., O’Connell effect [17, 18]. Max.II at phase is brighter than Max.I at phase up to mag and mag in and bands, respectively. This kind of stellar activity occurs on other W UMa-type binaries, such as VW Cep , DV CVn , V532 Mon , and BB Peg (Kalomeni et al. 2007), and DZ Psc .
Note. The entire table is available only on the online journal.
2.2. Low-Precision Spectrum
The low-precision spectrum for MT Cas was obtained by using the Yunnan Faint Object Spectrograph and Camera (YFOSC), which is attached to the 2.4-m telescope at Lijiang station (LJs) of Yunnan Astronomical Observatory of China (YNAO) at UT 15:50:13 of 2018 October 22. During the observing process, we chose a 140-mm-length slit and a Grism-3 with a wavelength range from to . The exposure time is 10 minutes. The phase of 0.98 almost corresponds to the observed middle time HJD 245414.1634. Reduction of the spectra was performed by using IRAF packages, including bias subtraction, flat-fielding, and cosmic-ray removal. Finally, the one-dimensional spectrum was extracted. With the winmk software (http://www.appstate.edu/~grayro/MK/winmk.htm), we obtained a normalized spectrum, which is displayed in Figure 2. By comparing the spectra of standard stars , the spectral type is determined to be F8V for the primary (i.e., more massive component) of this binary because the secondary is eclipsed by the primary around phase 0.0 for the W-type contact binary (see Section 4).
3. Eclipse Times and Period Analysis
From our new observations and AAVSO’s data, several times of primary and secondary minima are generally determined by using the method of Kwee & van Woerden . From ASAS-SN database, we downloaded 134 data types in V band for this binary. With the Period04 package , we obtained the power spectrum, which is shown in Figure 3. The searched frequency is , corresponding to 0.156937 days (i.e., half of an orbital period). The derived epoch HJD 2457010.9922 (i.e., the secondary eclipse time) a bit differs from the given rough epoch HJD 2457011.76746 from the ASAS-SN database. The individual single-color minimum timings with their errors are listed in Table 2.
In order to construct the curve (i.e., observed values minus calculated ones), we collected all available light minimum times. From the gateway (http://var2.astro.cz/ocgate) and TIDAK  (http://www.as.up.krakow.pl/minicalc/CASMT.HTM), we accumulated 10 “pg” (i.e., photographic), 21 “pe” (i.e., photoelectric), and 34 CCD measurements. Table 3 lists all those eclipsing times, whose errors are not given for 10 pg, 4 pe, and 1 CCD from literature. The standard derivations for all pe and CCD data are averaged to be 0.00137 days. For light minimum times without errors, we adopted the errors of 0.0137 for 10 pg data and 0.0014 days for 4 pe and 2 CCD ones. Therefore, the used weights depend on their errors while fitting the curve.