DSCT [S Scuti stars

- These are pulsating variables of spectral types A0-F5 1IIV displaying light amplitudes from 0".'003 to 0".'9 in V (usually several hundredths of a magnitude) and periods from ff.'Ol to 0?2. The shapes of the light curves, periods, and amplitudes usually vary greatly. Radial as well as non-radial pulsations are observed. The variability of some members of this type appears sporadically and sometimes completely ceases, this being a consequence of strong amplitude modulation with the lower value of the amplitude not exceeding 0".'001 in some cases. The maximum of the surface layer expansion does not lag behind the maximum light for more than 0.1 periods. DSCT stars are representatives of the galactic disk (flat component) and are phenomenologically close to the SX Phe variables. DSCTC (subtype) - Low amplitude group of S Set variables (light amplitude < 0".'l in V). The majority of this type's representatives are stars of luminosity class V; objects of this subtype generally are representative of the S Set variables in open clusters. GCVS

Observation Key

"^T Bright stars fXoj Small amplitudes

^^ Short periods

8 Scuti7 stars are spectral types A to early F pulsating variable stars, luminosity classes V to III. They pulsate in radial and non-radial pulsation modes, and possibly also gravitation modes, displaying periods between approximately 30 minutes and 8 hours. Photometric amplitudes are usually less than 1 magnitude. After the white dwarf ZZ Ceti variables, 8 Scuti variables are the second most abundant type of pulsating variable star within our Galaxy.

The first mention of variability found within the star 8 Scuti was made in 1900. At this time, a period was determined and <5 Scuti was placed in the ft Canis Majoris variable star group. Later investigations found that S Scuti resembled the Cepheid variable stars rather than the hotter ft Canis Majoris variables. In 1956, astrophysical developments pointed out the existence of a separate type of variable star.

Not surprisingly, the first S Scuti stars discovered turned out to be unusual for their class because of their large photometric amplitudes. Only after 1965 could numerous discoveries of 8 Scuti stars be made when photoelectric measurements with milli-magnitude precision became possible. As a result of the new precision available to astronomers, several systematic searches for S Scuti variables were made in the late 1960s and early 1970s.

In 1970, after examining nine small-amplitude 5 Scuti stars within the Hyades, astronomers proposed that all these variables with P < 0?2 should be called ultrashort-period Cepheids.

The very extensive photometric campaigns carried out 10-20 years later for specific 8 Scuti stars and the discovery of dozens of stable frequencies show that the variability of 8 Scuti stars is multiperiodic and regular in frequency.

A range of about 0™2 is typical for the 8 Scuti stars. The stars form a group which lies in an instability strip in the HR diagram which includes the classical Cepheids at its bright end and the pulsating white dwarfs at its faintest limit.

Today, the former dwarf Cepheids of the disk population are mostly called high-amplitude 8 Scuti stars. Their pulsational behavior differs substantially from that of the stars with low amplitudes. They appear

'Scutum, or Sobicski's Shield, was a creation of Johannes Hevelius of Gdansk, who commemorated King John Sobeski III, who led the army which saved Vienna from the Turkish invasion of 1683.

to have only one or two radial modes excited with the remarkable exception of AI Velorum. In most cases, they oscillate in the fundamental mode or first overtone and closely resemble classical pulsational variables, like Cepheids or RR Lyrae stars. However, it is not clear whether or not non-radial pulsations are excited in a number of high-amplitude <5 Scuti stars.

Since most known 8 Scuti stars are brighter than 8th magnitude, small telescopes equipped with photometers are still used extensively for their study.

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