Inverse tides in pulsating binary stars

Abstract

In close binary stars, the tidal excitation of pulsations typically dissipates energy, causing the system to evolve towards a circular orbit with aligned and synchronized stellar spins. However, for stars with self-excited pulsations, we demonstrate that tidal interaction with unstable pulsation modes can transfer energy in the opposite direction, forcing the spins of the stars away from synchronicity, and potentially pumping the eccentricity and spin-orbit misalignment angle. This “inverse” tidal process only occurs when the tidally forced mode amplitude is comparable to the mode’s saturation amplitude, and it is thus most likely to occur in main sequence gravity mode pulsators with orbital periods of a few days. We examine the long-term evolution of inverse tidal action, finding the stellar rotation rate can potentially be driven to a very large or very small value, while maintaining a large spin-orbit misalignment angle. Several recent asteroseismic analyses of pulsating stars in close binaries have revealed extremely slow core rotation periods, which we attribute to the action of inverse tides.

Asteroseismology of Close Binary Stars: Tides and Mass Transfer

“…In addition to the tidally excited oscillation, self-excited oscillations may also affect the orbital evolution. In the "inversetide" scenario (Fuller, 2021), angular momentum can be transferred from the self-excited modes to the orbit, and this may explain some of the very-slowly rotating convective cores discovered in γ Dor binaries Li et al, 2020a).…”

NGC 2004 #115: a black hole imposter containing three luminous stars

“…Some systems in the literature that are similar to NGC 2004 #115 include the triple HD 201433, which contains a B star with surface 𝑣 sin 𝑖 ≈ 8 km s −1 , and even slower rotation in the interior, in a 3.3 day orbit (Kallinger et al 2017); KIC 8429450, another triple with a 2.7 day inner period and a ≈38 day core rotation period (Li et al 2020), and KIC 9850387, a binary with a 2.7 day period, surface 𝑣 sin 𝑖 10 km s −1 , and core rotation period of ≈190 days (Li et al 2020;Sekaran et al 2021). Additional examples with slightly longer orbital periods can be found in Fuller (2021).…”

“…An unusually strong magnetic field can also lead to slow rotation via magnetic braking, but there is no evidence of an unusual magnetic field in the spectra. Another possibility, recently explored by Fuller (2021), is that in stars with self-excited pulsations, tidal interactions with unstable modes can drive stars away from synchronicity rather than toward it, leading to very low (or in other cases, very high) rotation rates. This process is predicted to be most efficient in binaries with periods of a few days and is thus a potentially promising avenue for explaining the slow rotation of the B star in NGC 2004 #115.…”

Tidally excited oscillations in MACHO 80.7443.1718: Changing amplitudes and frequencies, high-frequency tidally excited mode, and a decrease in the orbital period

“…Some address the role of TEOs in the dissipation of the orbital energy and dynamic evolution of a binary, especially when resonance locking occurs (Fuller et al 2017;Fuller 2017;Zanazzi & Wu 2021). A possibility of transferring energy from pulsations to orbit which may increase the non-synchronicity of the components or eccentricity of the system, the so-called inverse tides, was also found (Li et al 2020;Fuller 2021). A review of both the theory and observations of TEOs in massive binaries has been recently presented by Guo (2021).…”