RAA 2021 Vol. 21 No. 7, 180(7pp) doi: 10.1088/1674-4527/21/7/180 R c 2021 National Astronomical Observatories, CAS and IOP Publishing Ltd. esearch in Astronomy and http://www.raa-journal.org http://iopscience.iop.org/raa Astrophysics The deep and low-mass-ratio contact binary CSS J022914.4+044340 with a luminous additional companion Liang Liu and Xu-Zhi Li Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China; [email protected] Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China Center for Astronomical Mega-Science, Chinese Academy of Sciences, Beijing 100101, China University of Chinese Academy of Sciences, Beijing 100049, China Received 2021 January 27; accepted 2021 March 12 Abstract The first B-, V -, Rc- and Ic-band light curves of CSS J022914.4+044340 are presented and analyzed. It is found that CSS J022914.4+044340is a low mass ratio (0.198 ± 0.005) deep (63.7 ± 7.9 %) contact binary, indicating that it is already at the end evolutionary stage of tidally-locked evolution via magnetized wind. Because of the totally eclipsing character, the photometric solutions are reliable. The temperature and metallicity are determined from the spectroscopic data as T = 5855 ± 15 K and [Fe/H] = −0.842 ± 0.031, respectively. Based on the parallax of Gaia EDR3, the physical parameters +0.25 +0.05 of CSS J022914.4+044340 are estimated as M1 = 1.44−0.22 M⊙, M2 = 0.29−0.05 M⊙, R1 = +0.08 +0.03 +0.186 +0.039 1.26−0.06 R⊙, R2 = 0.65−0.04 R⊙, L1 = 1.718−0.191 L⊙ and L2 = 0.416−0.050 L⊙. Combining the fraction of light from the third body via the photometric solution (54%), the luminosity of the third body is estimated as 2.705 L⊙. The third body may be inferred to be a subgiant. Thus, why the primary component of CSS J022914.4+044340 has higher mass compared to similar systems is explained, as well as why its metallicity is so poor. Key words: binaries : eclipsing — binaries : close — stars: individuals (CSS J022914.4+044340)— stars: evolution 1 INTRODUCTION O’Connell effect (O’Connell 1951). For some of them, the dark spots were found to change on a short timescale (e.g., GN Boo, Wang et al. 2015; V789 Her, Li et al.2018). A contact binary is a close binary system whose More importantly, contact binaries were associated with components overflow their respective Roche lobes (Kopal some special celestial systems. The progenitor of the 1959) and share a common envelope (CE). The CE is luminous red nova (LRN) V1309 Sco (e.g., Tylenda et al. radiativeif the system consists of O, B or maybe A spectral 2011; Ste¸pie´n2011) would be a deep and low-mass-ratio type stars, or else it is convective if the system is composed contact binary (DLMRCB) (Zhu et al. 2016). The Am- of later spectral type stars. Lucy (1968a,b) put forward eclipsing binary V2787 Ori is also a shallow contact and a widely accepted convective common envelope (CCE) extreme mass ratio contact binary (Tian et al. 2019). V53, model for the W UMa-type contact binaries which contains a member of the globular cluster M4, could be a blue two unevolved low mass components. Because of the tight straggler (BS) (Li et al. 2017), and recently, Ferreira et al. orbit, contact binaries have the shortest periods, the lowest (2019) reported that the post-burst V1309 Sco, a merger angular momenta and the strongest interactions among from a contact binary, is located in the BS region. close binaries. They are very important for studying the processes of free mass and energy transfer, the interaction There is a period cutoff for contact binaries. This phe- of components and characteristics of the CE. Due to nomenon was discovered by Rucinski (1992). According the rapidly rotating later-type component, contact binaries to the data at that time, he found the value of the period usually display magnetic activities. In these cases, the light cutoff is about 0.22d and he suggested that the limitation curves would be distorted by dark spots, the so-called could be due to the fully convective structure of the 180–2 L. Liu & X.-Z. Li: DLMR Contact Binary CSS J022914.4+044340 Table 1 New Times of Light Minima for CSS J022914.4+044340 with the 85cm Telescope at Xinglong Station 1.15 1.10 HJD Error (d) Min Filter 2458053.18427 0.00092 p B 1.05 2458053.18284 0.00132 p V 1.00 2458053.18365 0.00064 p RC 2458053.18401 0.00115 p IC 0.95 2458055.17658 0.00048 s B Relativeflux 2458055.17673 0.00044 s V 0.90 2458055.17663 0.00044 s RC 2458055.17565 0.00044 s IC 0.85 0.80 4200 4500 4800 5100 5400 5700 6000 6300 6600 W avelength (Angstrom) B 0.3 Fig. 2 Spectrum of CSS J022914.4+044340. The black 0.4 solid line is the observed spectrum, while the red solid V line is the fitting. The spectrum was observed by the 2.4-m 0.5 R C telescope at Lijiang Gaomeigu Station, with Grism-14 and m (mag)m ′′ I the 2.5 long-slit. The exposure time was 3600s. C 0.6 0.7 -0.90 0.0075 -0.95 I C -1.00 R C -1.05 0.0070 V m (mag)m -1.10 B -1.15 0.00 0.25 0.50 0.75 1.00 1.25 0.0065 Phase 2 (O-C) 0.0060 Fig. 1 In the upper panel, the colored solid circles refer to the observed differential light curves for CSS J022914.4+044340, while the black solid lines 0.0055 signify the theoretical light curves. Different colors denote different filters. The Ic-band light curve is shifted by 0.0050 +0.05 mag. The light curves in the lower panel correspond 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 to the comparison star minus the check star (C-Ch). The Mass Ratio q symbols are the same as those in the upper panel. Fig. 3 The relation between q and the residual sum of M-type component. Later, Ste¸pie´n (2006) explained this squares for CSS J022914.4+044340. The optimally initial period limit as the shorter period detached binaries have q is about 0.18. not evolved into contact binaries within the age of the Universe, yet. Li et al. (2019) collected 55 such samples LAMOST, Qian et al. (2017, 2018) found that EA- and suggested that in some cases, the effect of the third type binaries have higher metallicities than EW-type body should be taken into account except for those two binaries and they suggest that long-period EWs should reasons. They also suggested a statistical value for the be formed by the EAs. During the contact evolutionary period cutoff as 0.1763835d. Very recently, Zhang & Qian phase, there are at least two very important evolutionary (2020) suggested this value as 0.15d. More information stages for contact binaries, namely the marginal contact about the short period cutoff of contact binaries can be phase and the deep and low-mass-ratio (DLMR) contact found in the review Qian et al. (2020, and the references phase (Qian et al. 2020). The marginal contact phase therein). was predicted by thermal relaxation oscillation (TRO) Contact binaries are usually formed from detached theory (Lucy 1976; Flannery 1976; Robertson & Eggleton binaries (e.g., Eggleton 2012) via angular momentum loss 1977). This theory assumes that matter can be transferred (AML) or via third bodies through the Kozai mechanism via the inner Lagrangian point while energy can be (Kozai 1962). According to the spectra provided by exchanged through the CCE. Mass transfer changes the L. Liu & X.-Z. Li: DLMR Contact Binary CSS J022914.4+044340 180–3 Table 2 Photometric Solutions for Table 3 Estimated Physical Parameters of CSS CSS J022914.4+044340 J022914.4+044340, Based on the Resolution and Parallax Parameters Photometric elements Errors from Gaia EDR3 g1 = g2 0.32 assumed Parameters Value Range A1 = A2 0.50 assumed T1 (K) 5855 5840 ∼ 5870 x1bol,x2bol 0.180,0.194 assumed T2 (K) 5747 5723 ∼ 5771 y1bol,y2bol 0.537,0.523 assumed M1 (M⊙) 1.44 1.22 ∼ 1.69 x1B ,x2B 0.491,0.532 assumed M2 (M⊙) 0.29 0.24 ∼ 0.34 y1B ,y2B 0.395,0.353 assumed R1 (R⊙) 1.26 1.20 ∼ 1.34 x1V ,x2V 0.229,0.257 assumed R2 (R⊙) 0.65 0.61 ∼ 0.68 y1V ,y2V 0.606,0.581 assumed L1 (L⊙) 1.718 1.527 ∼ 1.904 x1R ,x2R 0.119,0.143 assumed c c L2 (L⊙) 0.416 0.366 ∼ 0.455 y1R ,y2R 0.650,0.632 assumed c c A (R⊙) 2.289 2.166 ∼ 2.418 x1I ,x2I 0.046,0.066 assumed c c log g1 4.39 4.37 ∼ 4.42 y1I ,y2I 0.638,0.623 assumed c c log g2 4.27 4.25 ∼ 4.29 Phase shift −0.0092 ±0.0005 Mbol1 4.22 4.10 ∼ 4.34 T1 (K) 5855 ±15 Mbol2 5.75 5.63 ∼ 5.87 T2 (K) 5747 ±24 mV 14.255 14.204 ∼ 14.306 ± q = M2/M1 0.198 0.005 Distance (pc) 1600.0 1548.0 ∼ 1655.6 Ωin 2.2287 – (m − M)V 11.021 10.949 ∼ 11.095 Ωout 2.1021 – BCV −0.121 −0.171 ∼−0.071 Ω1 =Ω2 2.1480 ±0.0099 Mbol 3.982 3.863 ∼ 4.102 i(◦) 88.4 ±1.2 L1/(L1 + L2)(B) 0.8159 ±0.0351 The value of Mbol does not include the third light.