T. R. Bedding — Page 11

arXiv1994

An Optical Interferometer with Wavelength Dispersion

MAPPIT is an optical interferometer installed at the coude focus of the 3.9-m Anglo-Australian Telescope. The instrument combines non-redundant masking with wavelength dispersion and is able to record fringes simultaneously over a wide bandwidth. For typical observations centred near 600 nm, the bandwidth is 55 nm and the spectral resolution is 3 nm. This paper describes the instrument and the data processing methods and presents some results. We find the star sigma Sgr to be a close binary; the system is only partially resolved, with a separation of (11.5 +/- 2) milliarcsec (assuming the components to have equal magnitudes). We also give angular diameter measurements of two red giant stars, alpha Sco and beta Gru. The observations of beta Gru (spectral type M5 III) resolve the star for the first time and give an equivalent uniform-disk diameter of (27 +/- 3) milliarcsec.

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arXiv1994

Amplitudes of Stellar Oscillations: The Implications for Asteroseismology

There are no good predictions for the amplitudes expected from solar-like oscillations in other stars. In the absence of a definitive model for convection, which is thought to be the mechanism that excites these oscillations, the amplitudes for both velocity and luminosity measurements must be estimated by scaling from the Sun. In the case of luminosity measurements, even this is difficult because of disagreement over the solar amplitude. This last point has lead us to investigate whether the luminosity amplitude of oscillations (dL/L) can be derived from the velocity amplitude v_osc. Using linear theory and observational data, we show that p-mode oscillations in a large sample of pulsating stars satisfy (dL/L)_bol proportional to v_osc/T_eff. Using this relationship, together with the best estimate of v_osc(Sun) = (23.4 +/- 1.4) cm/s, we estimate the luminosity amplitude of solar oscillations at 550 nm to be dL/L = (4.7 +/- 0.3) ppm. Next we discuss how to scale the amplitude of solar-like (i.e., convectively-powered) oscillations from the Sun to other stars. The only predictions come from model calculations by (Christensen-Dalsgaard & Frandsen, Sol. Phys. 82, 469). However, their grid of stellar models is not dense enough to allow amplitude predictions for an arbitrary star. Nevertheless, although convective theory is complicated, we might expect that the general properties of convection -- including oscillation amplitudes -- should change smoothly through the colour-magnitude diagram. Indeed, we find that the velocity amplitudes predicted by the model

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