Stability of g modes in rotating B-type stars

Abstract

We have studied the stability of low-degree g modes in uniformly rotating B-type stars, taking into account the effects of the Coriolis force and the rotational deformation. From an analysis treating rotation frequency as a small parameter, it is found that slow rotation tends to destabilize high-radial-order retrograde g modes, although the effect is very small or absent for relatively low-order modes. Calculating eigenfrequencies at selected rotation rates, we find, on the other hand, that rapid rotation tends to stabilize retrograde g modes. The stabilizing effect appears stronger for less-massive B-type stars having low effective temperatures. If we change the rotation rate continuously, the frequency of a g mode belonging to ( , m) crosses frequencies of other g modes belonging to ( , m). If the parity of the two encountering modes is the same, then they interact with each other and the stability (i.e. imaginary part of eigenfrequency) of each mode is modified. Using an asymptotic method, we discuss the property of such mode crossings and couplings. For rapidly rotating stars, mode couplings are important for the stability of low-degree g modes. In particular, we find that the stabilization of retrograde g modes in rapidly rotating stars is due to many strong mode couplings, while prograde sectoral modes are exceptionally immune to the damping effects from the mode couplings.

Amplitudes of low-frequency modes in rotating B-type stars

“…The problem of differential rotation in a rotating star is quite difficult to answer and is beyond the scope of this paper. Since we used the traditional approximation to compute low‐frequency modes in a rotating star, we could not correctly take account of the effects of linear coupling between the modes associated with different λ km s. As discussed by Aprilia, Lee & Saio (2011), the linear coupling between low‐frequency modes tends to preferentially stabilize retrograde g ‐modes for rapidly rotating B stars, particularly for those having lower effective temperatures. It is therefore desirable to use the expansion method (e.g.…”

Domains of pulsational instability of low-frequency modes in rotating upper main sequence stars

“…Nevertheless, the computed mode frequencies and instabilities have to be treated with some caution, especially for the prograde sectoral modes which are mostly affected by the centrifugal force. It should also be kept in mind that fast rotation can stabilize retrograde g modes if a truncated expansion of the Legendre functions for the eigenfunctions is used (Lee 2008;Aprilia et al 2011). The ranges of Ω/Ω crit , corresponding to fixed values of equatorial velocities for models in a given instability strip are listed in the last columns of Tables 1, 3 and 4. Linear nonadiabatic pulsational calculations were done with the Dziembowski code in its version which includes the effects of rotation in the traditional approximation ).…”

Period–luminosity relations of fast-rotating B-type stars in the young open cluster NGC 3766

“…In rapidly rotating stars, retrograde g modes and tesseral g modes tend to be damped (Lee 2008;Aprilia et al 2011), while sectoral prograde g modes are less affected by rotation. In our models for rapidly rotating B-type mainsequence stars of intermediate mass stars, all excited retrograde modes are found to be r modes.…”