On the nature of small amplitude peaks in $\delta$ Scuti oscillation spectra

Abstract

The standard assumption in interpretation of stellar oscillation spectra from photometry is that the excited mode have low angular degrees, typically $\ell< 3$. Considering the case of FG Vir, the $\delta$ Scuti star with the richest known oscillation spectrum, we show that this assumption is not justified for low amplitude peaks. The $\ell<3$ identifications have been found for 12 dominant peaks from pulsation amplitudes and phases. However, we show that for the rest of the peaks (55), whose amplitudes are typically below 1 mmag, much higher $\ell$'s are most likely. We argue that improving amplitude resolution to the micromagnitude level, as expected from space observations, is not likely to be rewarded with a credible mode identifications because the spectra will be dominated by high-$\ell$ modes of unknown azimuthal order, $m$.