Delta Scuti oscillations - How low can they go?

Delta Scutis are pulsating stars located in the instability strip just above the main sequence. Their typical pulsation amplitudes generally lie between one hundred and a few thousand ppm. We are interested to test whether there is any lower limit to their amplitudes. Previous searches found pulsations down to about 20 ppm, although those samples only considered very faint (14-16 magnitude) long-cadence Kepler stars. We present evidence for an amplitude threshold in δ Scuti pulsations based on TESS and Gaia observations of stars on the main sequence. Analyzing amplitude as a function of colour, we find a distinct difference among stars within the instability strip: they either show clear δ Scuti pulsators (above about 80 ppm) or show no detectable pulsations (below about 20 ppm), with very few intermediate cases. The gap between these groups points to an intrinsic lower limit on δ Scuti mode amplitudes, offering fresh constraints on the excitation and damping mechanisms operating in these stars. Our results highlight a clear observational boundary for δ Scuti variability that future models must explain.