δ Scuti variables¶
The δ Scuti variables are usually A- or early-F- type dwarfs or subgiants found where the classical instability strip intersects the main sequence. Like the classical Cepheids, their pulsations are driven by the instability of helium going back and forth between its first and second ionization states. The light curves of higher-amplitude δ Scuti variables even resemble classical Cepheids and are sometimes called dwarf Cepheids.
δ Scutis pulsate at a range of amplitudes, from most of a magnitude down to millimagnitudes. Many, especially the lower-amplitude pulsators, pulsate simultaneously at many different frequencies. The fundamental pulsation mode, when excited, usually has a period between about 1 and 6 hours (so frequencies roughly between 4 and 24 cycles per day).
There are several subclasses of δ Sct variable that I have chosen not to distinguish because the physical mechanism of the pulsations is presumably the same. Two notable subclasses are the high-amplitude δ Scutis (HADS), whose light curves look a bit like classical Cepheids, and SX Phoenicis variables, which are like HADS but distinguished for being metal-poor and often found in globular clusters.