The compact triply eclipsing triple star TIC 209409435 discovered with TESS

Abstract

We report the discovery in TESS Sectors 3 and 4 of a compact triply eclipsing triple star system. TIC 209409435 is a previously unknown eclipsing binary with a period of 5.717 d, and the presence of a third star in an outer eccentric orbit of 121.872-d period was found from two sets of third-body eclipses and from eclipse timing variations. The latter exhibits signatures of strong third-body perturbations. After the discovery, we obtained follow-up ground-based photometric observations of several binary eclipses as well as another of the third-body eclipses. We carried out comprehensive analyses, including the simultaneous photodynamical modelling of TESS and ground-based light curves (including both archival WASP data, and our own follow-up measurements), as well as eclipse timing variation curves. Also, we have included in the simultaneous fits multiple star spectral energy distribution data and theoretical PARSEC stellar isochrones. We find that the inner binary consists of near twin stars of mass 0.90 M⊙ and radius 0.88 R⊙. The third star is just 9 per cent more massive and 18 per cent larger in radius. The inner binary has a rather small eccentricity, while the outer orbit has e = 0.40. The inner binary and outer orbit have inclination angles within 0.1° and 0.2° of 90°, respectively. The mutual inclination angle is ≲1/4°. All of these results were obtained without radial velocity observations.

TIC 278825952: a triply eclipsing hierarchical triple system with the most intrinsically circular outer orbit

“…For example, Borkovits et al (2016) has reported 22 triple star candidates with outer period P 2 ≤ 240 d among the eclipsing binaries in the original Kepler-field, and only for five of them was an outer eccentricity of e 2 ≤ 0.1 found. More specifically, considering only the flat, compact hierarchical triple systems with accurately known parameters tabulated in Table 5 of Borkovits et al (2020b) only one of them has outer orbit less eccentric than e 2 ∼ 0.2. This only ex-ception, HD 181068 is similar to the present system not only in its flatness, but it also has two circular orbits (Borkovits et al 2013).…”

“…p 05 phase sections of the regular eclipses were kept, with the exception of the outer eclipses, where longer, 6-8-day-long sections of the light curves were also retained. Note that, in case of the WASP light curve, our treatment slightly departed from that which was followed in Borkovits et al (2020b), where the full WASP light curve was considered in the photodynamical analysis. The reason is that, in the present situation, the TESS light curve by itself made it possible to determine accurately the outer period and, therefore, we were able to pre-compute the expected locations of the outer eclipses in the intervals of the WASP observations.…”

“…Until now, less than two dozen triply eclipsing hierarchical triple stellar systems are known in the literature (see an informative E-mail: mtibor@titan.physx.u-szeged.hu collection of them in Borkovits et al 2020b). While, in order to observe both inner and outer eclipses in a hierarchical triple system, it is necessary to view both the inner and outer orbits almost edge-on, there are no restrictions (at least in theory) on the mutual inclination of the inner and outer orbital planes.…”

“…In the case of a flat system, however, especially, as far as none of its components have undergone a Roche-lobe filling stage, one can expect that the alignment of the orbital planes is a certain relic of the formation process of the given triple star (in this regard see, e. g. Tokovinin & Moe 2020, and references therein). The subset of compact, extremely flat systems with very accurately known mutual inclinations is very small, and listed in Table 5 of Borkovits et al (2020b). More than half of these systems (four of seven) come from the triply eclipsing triples, emphasizing the extremely fortuitous role of such systems in determining accurate mutual inclinations.…”

Detection of period variations of eclipsing binaries in the Catalina Sky Survey

“…Hajdu et al (2017) identified five relatively short period Algol systems observed in the 100-150 d long CoRoT ETV data series for which the computed periods of the outer bodies are between 82 and 272 d. Although their solution does not satisfy the criterion that observations should cover at least two outer orbital periods (Conroy et al 2014;Borkovits et al 2016) their solutions were found to be physically consistent. Later short triple candidates were revealed from K2 data (Borkovits et al 2019a, b) and most recently from TESS data (Borkovits et al 2020). Test on a synthetic EB: raw (top panel) and phased (middle panel) synthetic LCs of a detached EB with period variations, generated using PHOEBE 2.0, are shown, along with the ETV diagram constructed with ToM calculated by our pipeline (bottom).…”