MNRAS 000,1–8 (2018) Preprint 24 February 2021 Compiled using MNRAS LATEX style file v3.0 An Extreme Amplitude, Massive Heartbeat System in the LMC Characterized Using ASAS-SN and TESS T. Jayasinghe1,2¢, K. Z. Stanek1,2, C. S. Kochanek1,2, Todd A. Thompson1,2,3, B. J. Shappee4, M. Fausnaugh5 1Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA 2Center for Cosmology and Astroparticle Physics, The Ohio State University, 191 W. Woodruff Avenue, Columbus, OH 43210, USA 3Institute for Advanced Study, Princeton, NJ, 08540, USA 4Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 5MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, 37-241, Cambridge, MA 02139, USA Accepted XXX. Received YYY; in original form ZZZ ABSTRACT Using ASAS-SN data, we find that the bright (+∼13.5 mag) variable star MACHO 80.7443.1718 (ASASSN-V J052624.38-684705.6) is the most extreme heartbeat star yet dis- covered. This massive binary, consisting of at least one early B-type star, has an orbital period of %ASAS−SN = 32.83627 ± 0.00846 d, and is located towards the LH58 OB complex in the LMC. Both the ASAS-SN and TESS light curves show extreme brightness variations of ∼40% at periastron and variations of ∼10% due to tidally excited oscillations outside periastron. We fit an analytical model of the variability caused by the tidal distortions at pericenter to find orbital parameters of l = −61.4°, 8 = 44.8° and 4 = 0.566. We also present a frequency analysis to identify the pulsation frequencies corresponding to the tidally excited oscillations. Key words: stars: early-type – stars: oscillations – stars: massive –stars: variables: general – (stars:) binaries: general 1 INTRODUCTION multiples of the orbital frequency (Fuller 2017). TEOs were first discovered in the eccentric binary system HD 174884 (Maceroni et Heartbeat stars are short period (% 1 yr), eccentric (4 0.3) . & al. 2009) and later confirmed in KOI 54 and several other systems binaries where oscillations are excited by the tidal forcing at each (Shporer et al. 2016). The largest amplitude TEOs are driven by periastron passage. Heartbeat stars were first discovered in data resonances between harmonics of the orbital frequency and the from the Kepler space telescope (Thompson et al. 2012). The pro- normal mode frequencies of the star. Both the amplitudes and phases totypical heartbeat star, KOI-54, has been extensively studied and of TEOs can be predicted from linear theory (Fuller 2017). characterized (see for e.g., Welsh et al. 2011; Fuller, & Lai 2012; Burkart et al. 2012 and references therein), and Kepler (Kirk et al. The vast majority of the heartbeat stars that have been dis- 2016) has now identified over 170 heartbeat stars. covered are relatively low-mass A and F type stars. However, the heartbeat phenomenon extends to more massive OB stars as well. arXiv:1901.00005v2 [astro-ph.SR] 23 Feb 2021 The light curves of heartbeat stars are defined by oscillations outside of periastron combined with a brief, high amplitude ellip- ] Ori is the most massive heartbeat star system yet discovered and soidal variation at periastron that gives rise to a unique “heartbeat” it consists of a O9 III primary and a B1 III-IV companion (Pablo signature resembling the normal sinus rhythm of an electrocardio- et al. 2017). The dearth of massive heartbeat stars is likely an ob- gram. The light curves of these systems are dominated by the effects servational bias because massive stars are rare and Kepler observed of tidal distortion, reflection and Doppler beaming close to perias- only a small fraction of the sky, mostly lying off the Galactic plane. tron (Fuller 2017). Heartbeat stars continue to oscillate throughout Ground-based surveys cover most or all of the sky but find it chal- their orbit due to tidally excited stellar oscillations. The variability lenging to detect the low variability amplitudes of typical heartbeat −3 amplitude of most heartbeat stars is very small (. 1 mmag; Kirk et stars (Δ!/! ∼ 10 ; Fuller 2017). al. 2016; Hambleton et al. 2018). The All-Sky Automated Survey for SuperNovae (ASAS-SN, The tidally excited oscillations (TEOs) occur at exact integer Shappee et al. 2014; Kochanek et al. 2017) has been monitoring the entire visible sky for several years to a depth of + . 17 mag with a cadence of 2−3 days using two units in Chile and Hawaii, each with ¢ E-mail: [email protected] 4 telescopes. As of the end of 2018, ASAS-SN uses 20 telescopes to © 2018 The Authors 2 T. Jayasinghe et al. observe the entire sky daily, but all current observations are taken To evaluate the proper motions, we examined 56 Gaia DR2 with a 6-band filter. We have written a series of papers studying stars with 퐺 < 15 mag within 50 of MACHO 88.7443.1718. These variable stars using ASAS-SN data. In Paper I (Jayasinghe et al. 56 stars had medians (1f range) of −0.024 mas (−0.126 < c < −1 −1 2018a), we reported ∼66, 000 new variables that were discovered 0.020), 1.670 mas yr (0.540 < `U < 1.814) and 0.697 mas yr during the search for supernovae. In Paper II (Jayasinghe et al. (0.125 < ` X < 0.852) for their parallax and proper motions. Hence 2019a), we homogeneously analyzed ∼412, 000 known variables the parallax and proper motions of MACHO 88.7443.1718 are typ- from the VSX catalog, and developed a robust variability classifier ical of the local population of luminous stars. If we use the median utilizing the ASAS-SN V-band light curves and data from external proper motions to define a local standard of rest, the relative motion catalogues. In Paper III (Jayasinghe et al. 2019b), we conducted of MACHO 88.7443.1718 is 0.088 mas yr−1, or roughly 21 km s−1 a variability search towards the Southern Ecliptic pole in order to at a distance of 50 kpc. The median motion of the nearby stars rela- overlap with the The Transiting Exoplanet Survey Satellite (TESS; tive to this standard of rest is 0.215 mas yr−1 or roughly 50 km s−1. Ricker et al. 2015) continuous viewing zone (CVZ). We identified ASAS-SN V-band observations were made by the “Cassius" ∼11, 700 variables, of which ∼7, 000 were new discoveries. (CTIO, Chile) quadruple telescope between 2013 and 2018. Each TESS is currently conducting science operations by monitoring camera has a field of view of 4.5 deg2, the pixel scale is 800. 0 (eventually) most of the sky with a baseline of at least 27 days. and the FWHM is ∼2 pixels. The light curves for this source was Sources closer to the TESS CVZ will be observed for a substantially extracted as described in Kochanek et al.(2017) using aperture pho- longer period, approaching one year at the ecliptic poles. TESS tometry with a 2 pixel radius aperture. The AAVSO Photometric full-frame images (FFIs), sampled at a cadence of 30 min, are All-Sky Survey (APASS; Henden et al. 2015) DR9 catalog was made publicly available, allowing for the study of short time scale used for absolute photometric calibration. The ASAS-SN V-band variability across most of the sky. light curve has a time baseline of ∼1669 d. We derived possi- Here we discuss the identification of the most extreme ble periods for this source following the procedure described in amplitude heartbeat star yet discovered, MACHO 80.7443.1718 Jayasinghe et al.(2018a, 2019a). The astrobase implementation (ASASSN-V J052624.38-684705.6, TIC 373840312), using both (Bhatti et al. 2018) of the Generalized Lomb-Scargle (GLS, Zech- ASAS-SN and TESS photometry. The MACHO survey reported meister & Kürster 2009; Scargle 1982), the Multi-Harmonic Anal- that the source was a variable, but classified it as an eclipsing binary ysis Of Variance (MHAOV, Schwarzenberg-Czerny 1996), and the (Alcock et al. 1997). MACHO 80.7443.1718 was first identified as Box Least Squares (BLS, Kovács et al. 2002) periodograms were a likely heartbeat star during our ASAS-SN variability search. We used to search for periodicity in these light curves. We calculated discuss archival data and the ASAS-SN and TESS observations in the Lafler-Kinmann (Lafler & Kinman 1965; Clarke 2002) string section 2. In section 3, we fit an analytical model for the tidal dis- length statistic ) ¹qj%º on the phased light curve for each period tortions to estimate several orbital parameters of the binary system. using the definition In section 4, we discuss our SED fits to this source and the physical ÍN 2 ¹<8¸1 − <8º ¹# − 1º implications of these fits. In section 5, we identify tidally excited ) ¹qj%º = 8=1 × (1) ÍN ¹ − º2 2# oscillations through a periodogram analysis and present a summary 8=1 <8 < of our results in section 6. from Clarke(2002), where the <8 are the magnitudes sorted by phase and < is the mean magnitude. The best period is the period with the smallest ) ¹qj%º, which in this case is the best BLS period. 2 ARCHIVAL, ASAS-SN AND TESS DATA FOR MACHO % − = 32.83627 ± 0.00846 d, 80.7443.1718 ASAS SN which agrees with the MACHO periods to within ∼0.02%. The The source MACHO 80.7443.1718 was identified as a variable by ASAS-SN light curve hence contains ∼50 orbits of this system. the MACHO survey (Alcock et al. 1997), who classified it as a The error in the period was estimated using the spacing in the BLS generic eclipsing binary.