Delta Scuti stars
Delta Scuti variables belong to the same family of pulsating stars as Cepheids and RR Lyrae stars - all these objects reside in the same instability strip in the Hertzsprung-Russell diagram. In fact, there is a continuity between delta Scuti stars and classical Cepheids, at least the overtone ones. Delta Scuti variables are fainter, have shorter pulsation periods, and smaller masses than classical Cepheids, but both groups obey the same period-luminosity relation and it is simply a matter of convention which period defines a boundary between them.
Delta Scuti stars pulsate in radial and non-radial modes with periods in the range 0.02-0.3 days (0.5-7 hours) and amplitudes up to 0.9 mag in the V band. Their spectral types are between early A and late F (spanning the instability strip). This class of pulsators contains stars of different ages and therefore at different evolutionary stages: young stellar objects during their contraction toward the main sequence, stars with core hydrogen burning on the main sequence, and subgiants and giants evolving off the main sequence. Old delta Scuti variables belong to a subgroup which is known as SX Phoenicis stars. The OGLE project published catalogs of over 27 000 delta Scuti stars in the Milky Way (Pietrukowicz et al. 2020, Soszyński et al. 2021) and Large Magellanic Cloud (Poleski et al. 2010). Delta Scuti stars shown below were found by OGLE in the Galactic bulge and in the Galactic disk.
Single-mode pulsators
Delta Scuti stars with V-band amplitudes larger than 0.3 mag are called high-amplitude delta Scuti stars (HADS). In most cases these are single-mode, fundamental-mode radial pulsators with asymmetric, sawtooth light curves. Such light curves resemble those of classical Cepheids, therefore HADS were sometimes called "dwarf Cepheids". Below we plot six I-band light curves of delta Scuti stars pulsating in the fundamental mode. In each case, a steep rise of the light is followed by a slower decline, but note the differences in the light curve shapes between individual objects. All light curves in this sample but one are HADS. Hover the cursor over the images to see the designations and coordinates of the stars.
Single-mode delta Scuti stars pulsating in the overtone modes have much more symmetric light curves, and therefore they are difficult to distinguish from other types of variable stars. Here we present two light curves of delta Scuti stars oscillating probably in the first overtone.
As you may have noticed, the light curve in the right panel (BLG501.06.104263) is almost indistinguishable from the fundamental-mode light curve shown previously in the top right panel (BLG500.15.25378). So, how did we identify their pulsation modes? We used the fact that both light curves have additional, very weak pulsation modes (so these two objects are not strictly single-mode delta Scuti stars, but the secondary modes have very small amplitudes). Based on the characteristic period ratios we were able to identify their modes of pulsation.
Multi-mode pulsators
As shown above, when a pulsating star has at least two modes excited, it is much easier to identify these modes. Fortunately, multi-mode oscillations are very common among delta Scuti stars, although most often for variables with smaller amplitudes. Radial modes can be identified on the basis of the ratio of their periods. For example, the ratios of the first-overtone and the fundamental-mode periods are in the range 0.75-0.79. Below we present three examples of double-mode delta Scuti stars pulsating simultaneously in the fundamental and first-overtone modes. The left panels show the original photometry folded with one of the periods, the middle and right panels display separated fundamental-mode and first-overtone components, respectively. Note that the overtone modes usually produce nearly sinusoidal light curves, although there are exceptions (like in BLG611.22.36003).
For delta Scuti stars with the first and the second overtones excited, the characteristic period ratios are around 0.80:
This is an example of a double-mode delta Scuti star oscillating in the second and third overtones (period ratios about 0.84):
There are known delta Scuti variables with three, four, or even more modes excited. Here we present light curves of two triple-mode delta Scuti stars. Each panel displays a different mode, separated from the two others. On this page, we present only the radial modes, but delta Scuti stars often oscillate also in the non-radial modes, however their identification is much more complicated.
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