Delta Scuti Star Newsletter

Issue 5, October 1992

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A spectroscopic search for nonradial pulsations in the Delta Scuti Stars, Delta Delphini and Epsilon Cephei

by: D.Baade, S.Bardelli, J.Ph.Beaulieu, S.Vogel.

In high-resolution observations of the reported 40.5-day binary Delta Del, the spectra of the two almost identical components were found to be well separated. However, over 40% of the primary pulsation period of 0.135 days no significant line profile variations or asymmetries were detected in either component. The line widths of ~ 30 km per sec are too small for the detection of higher-order nonradial modes.

The latter goal was clearly achieved in Epsilon Cep, despite of exposure times of 20 minutes. The nonradial mode order, |m|, inferred from the line profiles is between six and eight. The timescale agrees with the dominating 61-minute photometric variability to within the errors whereas no evidence of radial or low-order nonradial pulsation was found. This is surprising because such a relatively high-order mode would not usually be expected to be very prominent in integrated-light observations such as photometry unless the limb darkening is substantial. On the other hand, similar coincidences have been seen in other Delta Scuti stars. An identity of these spectroscopic and photometric modes would provide a drastic renewal of warnings against the usage of period ratios for mode identification. If the two modes are different, the similarity of their periods in several stars still calls for an explanation. The existence of higher-order modes in Delta Scuti stars raises questions about the nature of the photometrically constant stars in the Delta Scuti strip and the true boundaries of the latter.

A manuscript by D. Baade (ESO), S. Bardelli (Bologna), J. Ph. Beaulieu (Paris), S.Vogel (Hamburg) was submitted to Astronomy and Astrophysics. Preprints are available from the first author.