The delta Scuti stars are a class of pulsating
variable star named for the class prototype -- the fourth-brightest star
in the constellation Scutum. They are related to the Cepheid variables,
though the delta Scuti stars lie closer to the main sequence, have lower
masses, and higher effective temperatures.
Like most other pulsating stars, the delta Scuti stars pulsate because of
what is called the opacity- or κ-mechanism. Radiation
gets blocked in
a narrow ionization layer found inside all stars cooler than the ionization
temperatures of hydrogen and helium. When this occurs, the temperature
within the layer increases, causing an increase in pressure. This pressure
increase can do mechanical work on the outer layers of the star. Delta
Scuti stars pulsate because this layer is a just the right height so that
- the energy doesn't dissipate into the photosphere, and
- it isn't trapped deep inside, or used for convection.
The former case occurs in hotter, more massive stars, while the latter occurs
in cooler, less massive stars. Specifically, what happens is that any
compression of the surface of a delta Scuti star yields positive work
(at the expense of the radiative luminosity). Thus, once you start the star
pulsating, it will keep going and the amplitude will increase. No one yet
understands exactly how these stars start pulsating, but we at least
know what keeps them going. Like the Cepheids, the RR Lyrae, and the
pulsating white dwarf stars, they lie on the instability strip of the
Hertzsprung-Russell diagram.
Delta Scuti stars are interesting because unlike their larger Cepheid cousins,
they often pulsate in what are called nonradial modes. If you've ever
had a course on spherical harmonics, I just mean that the azimuthal quantum
number is greater than 0. If you haven't, imagine a balloon: inflating it
and deflating would be a radial mode, since the expansion is spherically
symmetric. Squishing it flat in the middle and letting it go again would be
a nonradial mode
(in this case, an "l=2" mode). The reason this is interesting is because
the pulsations of a star can tell you something about the interior conditions
that we can't normally see, using the principles of asteroseismology. Thus
we study these objects to learn more about the physical properties of stars
and stellar evolution.
As a group, the delta Scuti stars have spectral types between
roughly A5V on the hot side, and F5III on the cool side, and masses about
twice that of the Sun. If a delta Scuti star has a chemical composition
more deficient in metals than the Sun, it is known as an
SX Phoenicis star, again named for the subclass prototype. The SX
Phoenicis stars have lower masses, are more evolved, and tend to pulsate
only in radial modes. As such, they have been called "dwarf Cepheids" in
the past. Like the Cepheids, these stars also have a period-luminosity
relation, which means that if you know the period, you can measure
how far away it is by measuring its
brightness.
Delta Scuti itself is a very ordinary-looking star
(α 18h
42m 16.43s, δ -09° 03'
09.2'', mV = 4.7) of
spectral type F2III. Its variability was first discovered spectroscopically
around the year 1900, and the periodic nature of its pulsations confirmed in
the 1930's. Several hundred of these stars are now known, though many of
these are recently discovered
SX Phoenicis blue straggler stars found in globular clusters and the
galactic center.
Delta Scuti was also the subject of the first professional paper I ever wrote:
"A new pulsation spectrum and asteroseismology of delta Scuti," Astronomical
Journal vol 114, p1592 (1997).