Competition between gravity waves excited by convection and tides in stars that host a companion

Abstract

Asteroseismology has become a powerful tool in stellar astrophysics, offering unprecedented insights into the internal structures and dynamics of stars. It enables precise characterization of stellar interiors across a wide range of stellar masses and of evolutionary phases, from the main sequence to the white dwarf phase. At the same time, the number of detected close stellar and planetary companions throughout the entire stellar evolutionary phases has increased significantly, prompting key questions about the interplay between stellar evolution and binarity. We investigate the competition between gravity waves (IGW) excited by internal convection and those excited by tides in stars that host a companion. By modelling the energy and angular momentum luminosities transported by IGWs stochastically excited by convection and by tides, we seek to quantify their relative contributions and identify the key parameters that govern their efficiency. We compute the energy and angular momentum luminosities transported by both types of waves for a range of stellar masses and evolutionary stages, with a particular focus on understanding how the presence of a companion influences the angular momentum transport of the radiative layers of the host star. The competition between the two excitation mechanisms is sensitive to the mass and orbital properties of the companion, as well as the internal structure of the host star. We find that for a Jupiter-mass companion, the stochastic excitation dominates over tidal excitation during all evolutionary phases. Only for close-in stellar companions around late-type stars does the tidal excitation become more efficient. The presence of a companion is unlikely to significantly alter the internal angular momentum transport in the radiative layers of the host star, simplifying the modelling of IGW-driven angular momentum transport in stars that host a companion.

Figures (16)

• Figure 1: Schematic view of the radiative and convective shells in the three-layer model used in this work. Radii are not to scale (see Kippenhahn diagrams, e.g. Fig. \ref{['fig:Kippenhahn']}).
• Figure 2: Kippenhahn diagram throughout the evolutionary stages of a $M_\mathrm{ZAMS} = 1$ M$_\odot$ star. All stellar evolutionary phases are indicated at the top. The yellow dotted region illustrates the radiative regions while the brown hatched region illustrates the convective regions. Stellar evolutionary phases (PMS to WD) are indicated.
• Figure 3: Angular momentum luminosities carried by stochastically (blue) and tidally (red and purple) excited gravity waves (left axis) and stellar radius (brown; right axis) as a function of stellar age for a $M_\mathrm{ZAMS} = 1$ M$_\odot$ star with a 1 M$_\mathrm{Jup}$ (red) and a 1 M$_\odot$ (purple) companion orbiting at 1 AU. Stellar evolutionary phases (PMS to WD) are indicated.
• Figure 4: Different parameters influencing the angular momentum luminosity carried by stochastically excited gravity waves (Eq. \ref{['eq:LLS']}) as a function of stellar age for a $M_\mathrm{ZAMS} = 1$ M$_\odot$ star. The change in radius of the radiative-convective boundary, density, convective timescale, convective velocity, and change in Brunt-Väisälä frequency and Brunt-Väisälä frequency squared at this radius are represented in blue (solid) orange (solid), green (dashed), red (dashed), purple (dash-dotted) and brown (dash-dotted), respectively. Stellar evolutionary phases (PMS to WD) are indicated.
• Figure 5: Different parameters influencing the angular momentum luminosity carried by tidally excited gravity waves (Eq. \ref{['eq:LLT']}) as a function of stellar age for a $M_\mathrm{ZAMS} = 1$ M$_\odot$ star with a 1 M$_\mathrm{Jup}$ companion orbiting at 1 AU. The change in radius of the radiative-convective boundary is represented in blue (solid), the change in density at this radius is represented in orange (solid), the change in tidal forcing is represented in green (dashed) and the change in Brunt-Väisälä frequency squared is represented in red (dash-dotted). Stellar evolutionary phases (PMS to WD) are indicated.