The Nonradial Oscillations of SPICA - Part Two - a Quasi-Toroidal Mode

Details The Nonradial Oscillations of SPICA - Part Two - a Quasi-Toroidal Mode The Nonradial Oscillations of SPICA - Part Two - a Quasi-Toroidal Mode

Oct 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985apj...297..224s&link_type=abstract

Astrophysical Journal, Vol.297, NO. 1/OCT01, P. 224, 1985

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36

Scientific paper

The Si III λλ 4552-4574 line profiles of Spica (α Vir; Spectral type B1 III-IV) show periodic variations of a qualitative type not yet reported in any other nonradially pulsating B star. The most prominent new feature is a quasi-stationary absorption "spike" near the red edge of the line, which grows and then recedes, only to reappear near the blue edge 4 hr later. The observations suggest a periodicity of about 8.03 hr, or about 1/12 the orbital period. The period seems to be stable over three observing runs separated by over a year. The amplitude of the spike features varies from cycle to cycle but may show a coherence over two cycles or more. The spikes seem to be strongest when the hemisphere of Spica "facing" its companion is also facing the Earth. Extensive line profile modeling tests indicate that the easiest way of producing the spikes and companion features is with periodic, horizontal-circulation motions on the star's surface, similar to the vortices imposed by toroidal pulsation modes. The geometrical requirements are that the region on the star most responsible for producing the spike be extended in latitude and confined in longitude. There are also indications that these flows are confined primarily to the star's upper atmosphere. We infer herein that these spike features are caused by a retrograde, tesseral, toroidal-like oscillation consisting of 12 pairs of vortices on the visible disk. Although the excitation mechanism for this "mode" is not yet understood, we explore the possibility that the rotational shear on Spica's surface provides that mechanism. In this picture, periodic influences (tides, irradiation) by the secondary star serve as a "trigger" for this shear instability.

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