Semiregular variable star

Semi- regular variable stars are giants or supergiants with mid to late spectral type. They show partial or periodic light constantly changing, which is accompanied or interrupted by irregularities in the light curve. The periods are in the range from 20 to more than 2000 days, while the shape of the light curve may be variable in each cycle. The amplitudes can range from a few hundredths to several magnitudes. In the General Catalogue of Variable Stars, the semi- regular variable stars tend to have an amplitude of 1-2 mag in V filter. However, this is due to selection effects, since the light curves for classification are often based on brightness estimates and only under appropriate amplitude periodicity can be detected.

Subgroups

Semi- regular variable stars are divided into the following groups ( SR stands for semi- regular variable star, in addition to the lower-case letters there is also the variant with only capital letters, so for example, instead of SRa SRA ):

• SRa: Late giant stars in the spectral type M, C, S or Me, Ce, Se, always show the periods of usually small amplitudes (less than 2.5 mag in the visual ). Z Aquarii is an example of this class. The amplitudes and shapes of the light curves are variable, and the periods are in the range from 35 to 1200 days. Many of these stars differ from Mira variables only in that they show smaller amplitudes of the brightness.

• SRb: Late giant stars in the spectral type M, C, S or Me, Ce, Se with poorly defined periodicity ( mean cycles are in the range 20-2300 days ) or with alternating intervals of her period and slow irregular changes. Occasionally, some of them may interrupt their permutations for a given time completely. RR Coronae Borealis and AF Cygni are examples of this behavior. Each of the stars of this type can usually be assigned to a specific period. In a number of cases, a simultaneous presence of two or more periods are observed.

• SRc: Late supergiants in the spectral types M, C, S or Me, Ce, Se, with amplitudes of about 1 m and periods of 30 days to several thousand days. The Garnet Star μ Cephei is a bright star of this class.

• SRD: giants and supergiants of spectral classes F, G, or K, sometimes with emission lines in their spectra. The amplitude of its brightness changes are in the range of 0.1 m to 4 m, and their periods are 30-1100 days. SX Herculis and SV Ursae Majoris are members of this class.

Criticism

The above division into classes based on the appearance of the light curve, which often covers only a short period of time, is present exclusively in the optical range and due to estimation errors have a large scatter. The classification does not support the separation according to astrophysical parameters and the separation from Mira- stars and the slow irregular variable stars is inaccurate. A physical difference between the slow irregular and semi- regular variable stars does not seem to exist.

Causes of light variation

Over the last decade the search for microlensing effects ( EROS, MACHO, OGLE ) incurred thousands of high-precision light curves of long -period variables in the frame. While the Mirasterne always resonate with the fundamental frequency of the pulsating semi-regular as well as the slow irregular variable stars in one or more harmonics.

In addition to the dominant radial oscillations in the atmospheres of two periodic processes have been found in the long -period variables, which can lead to a classification with semi-regular change of light:

• Ellipsoidal light variation due to the distortion of the shape of the red giant's by a companion in a binary star system. This can be detected by the phase difference between the radial speed and the brightness curve. The amplitude may be up to 0.3 may be for periods between 50 and 1000 days.

The elliptical light change occurs in red giants and stars on the asymptotic giant. Based on the radius of a red giant's is a close binary star system in which it is used in most cases to a mass exchange in the context of further development. The result will be a merging of the two nuclei in a rapidly rotating FK Comae - - Berenices star or a binary star system as the core of a planetary nebula depending on the mass and distance of the two stars.

• Long secondary periods are detected in approximately 30 % of the stars on the asymptotic giant branch and with a lower average amplitude in red supergiant

It involves superimposed broad minima that occur with a period in the light curves, which is approximately nine times longer than the dominant pulsation period. The amplitude may be up to a magnitude. So far, oscillations in the atmosphere of the red giant's and the influence of a companion could be excluded in a binary star system as the cause. All the stars with a long secular period show a strong infrared excess and therefore unbalanced distributed circumstellar dust with the phenomena is associated. The exact mechanism of the formation of the minima is not yet understood. Also for the yellow supergiant Rho Cassiopeiae and HR 8752 long secondary periods have been reported. These periods should be around 1000 days and are considered the Umwälzdauer giant convection cells interpreted in the atmospheres of stars. This hypothesis is supported by the observation of the variable radial velocity at the yellow supergiants and also represented for red supergiants.

The excitation mechanism of the oscillations differs in semi-regular variable stars of the classical Pulsationsveränderlicher as the Mira stars, the Cepheids and RR Lyrae stars from. In these classes, the star shrinkage caused by the mechanism in the kappa ionization of helium. The excitation mechanism of the semi- regular on the other hand corresponds to the 5-minute oscillation of the sun. These are excited by random convection near the stellar atmosphere, reflected at the photosphere due to the density jump and run into the star. Most vibrations cancel each other except for those who just meet in the fundamental or harmonics thereof.

Flares

In the literature, there are notes on short-term brightness changes on the order of hours to days in long -period variables. These events usually have the shape of a flare with a steep slope, the one often slower descent follows as at Y Scopii in studies using the STEREO spacecraft. A systematic study of light curves of long-period variables by flares could only show that if the events are real, they occur very rarely with less than 0.15 events per star per year., Deviations from a smooth light curve is only on a time scale of 10 days.