SR Semiregular variable stars

- Giant or supergiants of intermediate and late spectral types showing noticeable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to >2000 days, while the shapes of the light curves are rather different and variable, and the amplitudes may be from several hundredths to several magnitudes (usually lm-2m in V). SRA (subtype) - Semiregular late-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small (< 2".'5 in V) light amplitudes (Z Aqr). Amplitudes and light-curve shapes generally vary and periods are in the range of 35-1200 days. Many of these stars differ from Miras only by showing smaller light amplitudes. SRB (subtype) -Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20-2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals (RR CrB, AF Cyg). Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalog. In a number of cases, the simultaneous presence of two or more periods of light variation is observed. SRC (subtype) - Semiregular late-type (M, C, S or Me, Ce, Se) supergiants (Mu Cep) with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. SRD (subgroup) - Semiregular variable giants and supergiants of F, G, or K spectral types, sometimes with emission lines in their spectra. Amplitudes of light variation are in the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days (SX Her, SV UMa). SRS (subgroup) - Pulsating red giants with short periods (several days to months), probably high-overtone pulsators (added to the GCVS 9

Observation

Key f Bright stars

| Mixed amplitude

^ Mixed periods

Figure 4.9. Light curve

ZeSRB4ype variable star, Z UMa. Annual dates are indicated olong Hte horizontal axis. Dota provided by the VSNET Used with permission

Figure 4.9. Light curve

ZeSRB4ype variable star, Z UMa. Annual dates are indicated olong Hte horizontal axis. Dota provided by the VSNET Used with permission

July 2001 [The 76"' Name List of Variable Stars - IBVS 5135]). GCVS

As you are probably beginning to notice, variability seems to be a fundamental characteristic of cool luminous stars. In fact, it is suggested that almost all late stars of spectral class M are variable at some level (Figure 4.9).

Semiregular variable stars possess some similarities to the Mira variables. The SRA and SRB stars are giants while the SRC are supergiants. The major difference between the SRA class and the Miras is that an SRA may have a visual light amplitude of less than 2™5. In principle, the light curves can also be less regular than those of the Miras, however, Mira light curves can also be less regular. The SRB is similar to the SRA class but with a less obvious demonstration of period.

Like the Miras, SRA and SRB stars include spectral class types M, S and C.

SRC stars are generally thought to be massive with progenitors in excess of about 8 solar masses.

SRD stars are semiregular giants and supergiants of spectral type F, G or K. Occasionally, the spectra of theses stars will possess emission lines. SRD variables are generally considered poorly studied. A subgroup, discussed later in this chapter, is sometimes called UU Her stars.

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