A new photometric and dynamical study of the eclipsing binary star HW Virginis

Abstract

A growing number of eclipsing binary systems of the “HW Vir” kind (i.e. composed by a subdwarf-B/O primary star and an M dwarf secondary) show variations in their orbital period, also called Eclipse Time Variations (ETVs). Their physical origin is not yet known with certainty: while some ETVs have been claimed to arise from dynamical perturbations due to the presence of circumbinary planetary companions, other authors suggest that the Applegate effect or other unknown stellar mechanisms could be responsible for them. In this work, we present twenty-eight unpublished high-precision light curves of one of the most controversial of these systems, the prototype HW Virginis. We homogeneously analysed the new eclipse timings together with historical data obtained between 1983 and 2012, demonstrating that the planetary models previously claimed do not fit the new photometric data, besides being dynamically unstable. In an effort to find a new model able to fit all the available data, we developed a new approach based on a global-search genetic algorithm and eventually found two new distinct families of solutions that fit the observed timings very well, yet dynamically unstable at the 105-year time scale. This serves as a cautionary tale on the existence of formal solutions that apparently explain ETVs but are not physically meaningful, and on the need of carefully testing their stability. On the other hand, our data confirm the presence of an ETV on HW Vir that known stellar mechanisms are unable to explain, pushing towards further observing and modelling efforts.

Eclipse Timing Modeling of Three Post-Common Envelope Binaries: Hybrid Solutions

“…For HW Vir, we found no stable solutions that could fit the ETV time signature, although it was possible to find adequate 3 and 4 component fits, albeit with highly unstable orbits. This has also been the unfortunate conclusion of several previous studies of HW Vir (e.g., Horner et al 2012;Brown-Sevilla et al 2021) and although not definitive, this strongly suggests that substellar companions cannot account for all of the ETV time history observed in these (and likely other) PCEB binaries.…”

“…This provides a much better fit (𝜒 2 ≈ 7) but is highly unstable over short time internals (several hundred years). This unfortunate scenario (acceptable O-C model fits are invariably unstable) was also found by (Brown-Sevilla et al 2021) who analyzed a series of best-fit six multi-component models for HW Vir. In the section we discuss possible alternatives to pure substellar component models.…”

“…Variations in the orbital period were first discovered by Kilkenny et al (1994), who reported a period decrease over nine years. Subsequent ETV studies of HW Vir have been carried out by several authors (Çakirli & Devlen 1999;Kiss et al 2000;Kilkenny et al 2000Kilkenny et al , 2003İbanoǧlu et al 2004;Lee et al 2009;Qian et al 2010;Beuermann et al 2012b;Skelton 2012;Lohr et al 2014;Baran et al 2018;Esmer et al 2021;Brown-Sevilla et al 2021). These papers invoked one or more substellar companions as the primary cause of the observed ETVs.…”

Eclipse timing variations in post-common envelope binaries: Are they a reliable indicator of circumbinary companions?

“…More recently the stability of circumbinary objects around HW Vir has also been investigated by Esmer et al (2021), Brown-Sevilla et al (2021 and Mai & Mutel (2022). Esmer included both new data from Baran et al (2018) and their data through to 2019 March.…”

Orbital stability of proposed NY virginis exoplanets

“…Third, several published multi-component models have been subsequently shown to be highly unstable on timescales that are short compared with the expected lifetimes of the post-common envelope phase, vitiating the viability of the respective sub-component models. (e.g., Tinney et al 2012;Wittenmyer et al 2012Wittenmyer et al , 2013Brown-Sevilla et al 2021;Esmer et al 2021).…”