TIC 5724661: A Long-Period Binary with a Pulsating sdB Star and $\delta$ Scuti Variable

TL;DR

This paper reports the discovery of a rare long-period binary system containing a pulsating subdwarf B star and a delta Scuti star, using TESS data, and models its possible formation history.

Contribution

It identifies and characterizes a unique long-period sdB binary with pulsating components, expanding the known population of such systems and providing insights into their formation.

Findings

The binary includes a delta Scuti star with pulsations 32-41 d$^{-1}$.

The sdB star shows a large-amplitude pulsation at 524 d$^{-1}$.

The orbital period of the sdB companion exceeds a few tens of days.

Abstract

Using TESS 20-sec cadence data, we have discovered an unusual combination of pulsating stars in what we infer to be a binary system. The binary consists of a standard $\delta$ Scuti star with pulsations over the range 32-41 d$^{-1}$; this is in a likely wide orbit with a hot subdwarf B (sdB) star, which itself has a large-amplitude p-mode pulsation at 524 d$^{-1}$. We establish constraints on the period of the putative binary by using radial velocity measurements of the $\delta$~Scuti star and show that any sdB companion star must orbit with a period greater than a few tens of days. Our identification of this sdB binary serves as an important addition to the relatively small number of sdB binaries known to have orbital periods longer than a few days. We model such a binary using {\tt MESA} and find that this system could be formed through stable, nonconservative mass transfer from either a low- or intermediate-mass progenitor, without undergoing a common envelope phase.