TICs 167692429 and 220397947: the first compact hierarchical triple stars discovered with TESS

Abstract

We report the discovery and complex analyses of the first two compact hierarchical triple star systems discovered with TESS in or near its southern continuous viewing zone during Year 1. Both TICs 167692429 and 220397947 were previously unknown eclipsing binaries, and the presence of a third companion star was inferred from eclipse timing variations exhibiting signatures of strong third-body perturbations and, in the first system, also from eclipse depth variations. We carried out comprehensive analyses, including the simultaneous photodynamical modelling of TESS and archival ground-based WASP light curves, as well as eclipse timing variation curves. Also, for the first time, we included in the simultaneous fits multiple star spectral energy distribution data and theoretical PARSEC stellar isochrones, taking into account Gaia DR2 parallaxes and catalogued metallicities. We find that both systems have twin F-star binaries and a lower mass tertiary star. In the TIC 167692429 system, the inner binary is moderately inclined (imut = 27°) with respect to the outer orbit, and the binary versus outer (triple) orbital periods are 10.3 versus 331 d, respectively. The mutually inclined orbits cause a driven precession of the binary orbital plane that leads to the disappearance of binary eclipses for long intervals. In the case of TIC 220397947, the two orbital planes are more nearly aligned and the inner versus outer orbital periods are 3.5 and 77 d, respectively. In the absence of radial velocity observations, we were unable to calculate highly accurate masses and ages for the two systems. According to stellar isochrones TIC 167692429 might be either a pre-main sequence (MS) or an older post-MS system. In the case of TIC 220397947, our solution prefers a young pre-MS scenario.

The compact triply eclipsing triple star TIC 209409435 discovered with TESS

“…The reason is that the Gaia DR2 data might contain systematics for compact triple systems (see, e.g. the discussion of this problem in Borkovits et al 2020). However, as we will discuss above, our results are in perfect agreement with the Gaia DR2 catalog data.…”

“…Turning back to the most prominent characteristic of our triple, i.e. its flatness, other similarly very flat (im < 5 • ) and compact (P2 < 200 d) triple systems with accurately known parameters that were reported previously are HD 181068 (Borkovits et al 2013), HD 144548 (Alonso et al 2015), ξ Tau (Nemravová et al 2016), EPIC 249432662 (Borkovits et al 2019a), HIP 41431 (Borkovits et al 2019b) and TIC 220397947 (Borkovits et al 2020). 6 All of these systems show remarkable similarities, as one can see in Table 5.…”

“…We carried out complex photodynamical modelling of TIC 209409435 with the use of the software package Lightcurvefactory (see Borkovits et al 2019aBorkovits et al , 2020, and further references therein). In order to obtain a comprehen-sive, as well as physically and dynamically consistent model for this triple we simultaneously analysed: (i) four sets of photometric lightcurves (the two sectors of TESS measurements, the historical WASP observations, and T. G. Tan's Cousins RC-band and F.-J.…”

“…Compact Triply Eclipsing Triple TIC 209409435 9 namical masses for each component. Therefore, in the second stage of the analysis, in order to obtain physical quantities within the framework of a self-consistent model, similar to the method followed in Borkovits et al (2020), we added the observed composite SED of the triple system to the fit, and made efforts to find consistent, coeval PARSEC evolutionary tracks (Bressan et al 2012) for the three stars. For this latter purpose we used machine readable PAR-SEC isochrone tables generated via the web based tool CMD 3.3 3 .…”

“…These tables contain theoretically computed fundamental stellar parameters and absolute passband magnitudes in several different photometric systems, for a large three-dimensional grid of ages, metallicities and initial stellar masses (see also Chen et al 2019, and further references therein). The preselection of the most appropriate subsets of the tabulated isochrones for the current analysis, as well as the interpolation of fundamental stellar parameters and absolute passband magnitudes from the grid of values for an actual trial run is described in Section 3 of Borkovits et al (2020).…”