Delta Scuti stars are pulsating variables of spectral types A
to early F with luminosity classes V to III. They pulsate in radial and nonradial
p (and possibly also g) modes with periods between about 30 minutes to 8 hours
and photometric amplitudes less than 1 magnitude. After white dwarfs, they are
the second most abundant pulsating variables in our Galaxy.
The variability of the prototype of these kind of stars was discovered by Fath (1935). Smith (1955) called these objects "Dwarf Cepheids" because they seemed to be different from RR Lyrae stars because of their (on the average) higher metallicity and different period-luminosity relation. Bessell (1969) introduced the designation "AI Velorum stars" for all objects with amplitudes higher than 0.3 mag. Since this difference is mainly phenomenological, Breger (1979, 1980) suggested to call all these objects "Delta Scuti stars". This designation is widely accepted today. The metal-poor objects are called "SX Phoenicis stars" (see Nemec & Mateo 1990).
Today, the former "dwarf cepheids" of the disk population are mostly called "high-amplitude Delta Scuti stars". Their pulsational behavior differs substantially from that of the objects with low amplitudes. They appear to have only one or two radial modes excited (with the remarkable exception of AI Velorum, Walraven et al. 1992). In most cases, they oscillate in the fundamental mode or first harmonic and resemble closely classical pulsational variables, like cepheids or RR Lyrae stars. However, it is not clear whether or not nonradial pulsations are excited in a number of high-amplitude Delta Scuti stars.
Location of the Delta Scuti and related stars in the H-R-Diagram. The positions of selected well studied Delta Scuti stars are marked (from Zima (1999, Master Thesis)).
Most Delta Scuti stars do, however, show very small amplitudes; the smaller the amplitudes become, the more variables are found (Breger 1979, Rodriguez et al. 1994). From searches for Delta Scuti-like variability among open cluster stars (Breger 1972, Slovak 1978) it could be deduced that about 30% of all stars with spectral types of A2 to F0 on the Main Sequence are pulsators. The above two points make it quite reasonable that all stars in the lower instability strip could be pulsators; their discovery just requires higher photometric precision.
Delta Scuti stars obey a period-luminosity-color relationship (PLCR). Since the pulsation "constant" Q does not change very much over the whole lower instability strip and since it is reasonable to assume that stars with similar temperature have similar overtones excited, the existence of a PLCR is not surprising. Breger (1979) determined the "observed" PLCR for Delta Scuti stars of Pop. I:
Mv = -3.05 log P + 8.46 (b-y)o - 3.12
They also noted that the average Q value of Delta Scuti stars decreases with increasing temperature, i.e. hotter stars oscillate in higher overtones. The "border" between the objects pulsating in the fundamental mode and those preferring the first or second overtone appeared to be located near an effective temperature of 7800 K. However, latest results show that such a separation might not be real (Breger 1995).
Finally, an interesting article by Fernie (1992) needs to be mentioned. He
attempted to determine a common PLCR for Delta Scuti stars and Cepheids. However,
since Cepheids and Delta Scuti stars are in a completely different evolutionary
state, and since Fernie used galactic Delta Scuti stars together with LMC Cepheids
to derive his PLCR, he could be mislead by hidden zeropoint differences (see
also Kurtz 1994).
The first clues, that Delta Scuti stars are also nonradial pulsators, came from the studies of the stars 14 Aur and 1 Mon. Fitch & Wisniewski (1979) concluded after a thorough photometric and radial velocity study that this star pulsates with k=5 and l=1. The star 1 Mon was first examined by Shobbrook & Stobie (1974). They determined that 1 Mon pulsates with 3 closely spaced frequencies, where the differences of consecutives frequency values are the same. Balona & Stobie (1980) acquired simultaneous radial velocity and photometric measurements of 1 Mon and were able to show that the three frequencies do not have exactly the same spacing. Because of the phase shifts of the color changes relatively to the variations in the star's brightness they were able to determine (using results of Balona & Stobie 1979) the l values of the pulsation modes. According to them, the modes with the highest and lowest frequency have l=1 with m=-1 and m=1, respectively, while the mode with the intermediate frequency is a radial one. This mode identification was confirmed by Smith (1982) by means of his analysis of line profile variations. Kurtz (1988) noted that further frequencies might be excited in 1 Mon, since the residuals between the light curve of Balona & Stobie and a three-frequency fit are larger than the uncertainties in their measurements. Interestingly, this was also suspected, and later confirmed, for the star AI Vel (Walraven et al. 1992).