Delta Scuti Star Newsletter

Issue 5, October 1992

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Tau Peg and Theta2 Tau: Mode identification from line profile variation

by: E.J.Kenelly

Time series observations at high-resolution and high-S/N were obtained at CFHT for the rapidly rotating Delta Scuti stars Epsilon Cep, Theta 2 Tau, and Tau Peg. Each of these stars demonstrated variations in their line profiles believed to be due to high-degree oscillations.

The time series of Epsilon Cep was interrupted by fog but 5 and 7 hour data strings of Theta 2 Tau and Tau Peg were obtained. Considered to be multiperiodic based on photometric data, the detection of additional high-degree modes is of particular interest in these stars. Additional observations were gathered on 97 Tau, which proved to be variable, and 14 Ari, a suspected (photometrically) constant star in the Delta Scuti instability strip.

Analysis of the data was performed by constructing a two-dimensional Fourier transform in both time and "Doppler space" from the variable component of a chosen line profile. In this representation, the apparent frequency of oscillation n and the apparent azimuthal order m are determined directly from the peaks in the 2-D amplitude spectrum.

Numerical simulations have demonstrated that the apparent azimuthal order provides a good indication of the modes actually present in the star (though it systematically overestimates low-degree modes). The apparent frequency of oscillation includes the effects of rotation.

A detailed investigation of Tau Peg identified a single mode of oscillation at a frequency of 18.4052 cycles per day (Breger, Astron. Astrophys. 250, 107, 1991). However, changes in the amplitude of the photometric variations suggest that additional modes of oscillation might be present. From only seven hours of observation, four distinct peaks were identified in our 2-D Fourier spectrum of Tau Peg. Apparent mode identifications are:

m =   2.6, v = 18.5 cycles per day 
m =   6.6, v = 22.3 cycles per day 
m =  11.1, v = 21.0 cycles per day 
m =  14.9, v = 29.7 cycles per day

The results were found to be in good agreement with the theoretical limits imposed on the frequencies of oscillation by the models of Dziembowski (in Progress of Seismology of the Sun and Stars, Lecture Notes in Physics 367, 1990).

The results obtained for t Peg have currently been submitted for publication (Kennelly, Walker & Merryfield, ApJ Letters). An intensive photometric campaign detected several low-degree modes in Theta 2 Tau at frequencies near 13.5 cycles per day (Breger et al., Astron. Astrophys. 214, 209, 1989). Though these individual frequencies are not resolved in our data, a peak located at m = 4.3, n = 13.8 cycles per day is identified in the 2-D Fourier spectrum of this star. Additional power is identified with a peak at m = 11.1, v = 15.9 cycles per day. The time series observations of 97 Tau and e Cep were not complete enough to provide reliable mode identifications, but multimode oscillations are also expected for these stars. Ultimately, multisite, continuous, spectroscopic observations of these and other stars could provide both the frequency resolution and spatial information required to undertake detailed investigations of Delta Scuti oscillations.