Spica (B1 III-IV) is the primary of a detached 4.01 day binary in which a 4 hr pulsation mode died out a decade ago. Its spectral line profiles still exhibit a complicated traveling pattern of line profile "bumps", characteristic of nonradial pulsation (NRP). The author obtained high-resolution Reticon observations of the Si III λλ4552 - 4574 triplet of this star over 4 1/4 nights and has been able to ascribe these variations to four sectorial, equatorial nonradial modes: (1) an l = m = +2 "equilibrium" tidal mode with a period of 48.2 hr; (2) a tidally forced quasi-toroidal mode; (3) an l = m = -8 mode with an apparent period of 6.52±0.08 hr, and (4) an l = m = -16 mode with an apparent period of 3.20±0.016 hr. The ratio of the periods for the latter two modes is almost exactly equal to the ratio of their l-values, 2.0. This "commensurability" of modes is discussed within the context of a differential rotation model for Spica. The picture is that the two nonradial modes are in resonance with the interior rotation rate, the orbital rate, and one another.
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M. A. Smith (1985) studied this question.