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Semiregular variable star

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Light curveof semiregular variable starBetelgeuse

Inastronomy,asemiregular variable star,a type ofvariable star,is agiantorsupergiantof intermediate and late (cooler)spectral typeshowing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000days,while the shapes of thelight curvesmay be rather different and variable with each cycle. The amplitudes may be from several hundredths to severalmagnitudes(usually 1-2 magnitudes in the V filter).

Classification

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The semiregular variable stars have been sub-divided into four categories for many decades, with a fifth related group defined more recently. The original definitions of the four main groups were formalised in 1958 at the tenth general assembly of theInternational Astronomical Union(IAU). TheGeneral Catalogue of Variable Stars(GCVS) has updated the definitions with some additional information and provided newer reference stars where old examples such asS Vulhave been re-classified.

Semiregular variable subtypes
Subtype[1] IAU definition[1] GCVS code[2] GCVS definition[2] Standard
stars
SRa semi-regular variable giantsoflatespectral classes (M, C and S), which retain periodicity with comparative stability and possess, as a rule, small (less than 2m.5) light-variation amplitudes. Amplitudes and forms of light curves are usually liable to strong variations from period to period. Many of these stars differ from Mira Ceti type stars only owing to the smaller amplitude of light variation. SRA Semiregular late-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small (<2.5 mag in V) light amplitudes. 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 Z Aqr[1][2]
SRb semi-regular variable giantsoflatespectral classes (M, C and S) with a poorly expressed periodicity, i.e. with a different duration of individual cycles (which leads to the impossibility of predicting the epochs of maximum and minimum brightness), or with the replacement of periodical changes by slow irregular variations, or even by the constancy of brightness. Some of them are characterised by a certain mean value of the period, given in the catalogue. SRB Semiregular late-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes, and even with light constancy intervals. Every star of this type may usually be assigned a certain mean period (cycle), which is the value given in the Catalogue. In a number of cases, the simultaneous presence of two or more periods of light variation is observed AF Cyg[1][2]
RR CrB[1][2]
SRc semi-regular variable super-giantsoflatespectral classes SRC Spectral-type (M, C, S or Me, Ce, Se) supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. μ Cep[1][2]
RW Cyg[1]
SRd semi-regular variable giants and super-giantsbelonging to spectral classes F, G, K SRD 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 S Vul[1]
UU Her[1]
AG Aur[1]
SX Her[2]
SV UMa[2]
SRS Semiregular pulsating red giants with short period (several days to a month), probably high-overtone pulsators AU Ari[2]

Pulsation

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The semiregular variable stars, particularly the SRa and SRb sub-classes, are often grouped with theMira variablesunder thelong-period variableheading. In other situations, the term is expanded to cover almost all cool pulsating stars. The semi-regular giant stars are closely related to the Mira variables: Mira stars generally pulsate in thefundamental mode;semiregular giants pulsate in one or moreovertones.[3]

Photometric studies in theLarge Magellanic Cloudlooking forgravitational microlensingevents have shown that essentially all cool evolved stars are variable, with the coolest stars showing very large amplitudes and warmer stars showing only micro-variations. The semiregular variable stars fall on one of five mainperiod-luminosity relationshipsequences identified, differing from the Mira variables only in pulsating in an overtone mode. The closely related OSARG (OGLEsmall amplitude red giant) variables pulsate in an unknown mode.[4][5]

Many semiregular variables show long secondary periods around ten times the main pulsation period, with amplitudes of a few tenths of a magnitude at visual wavelengths. The cause of the pulsations is not known.[3]

Bright examples

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η Gemis the brightest SRa variable, and also an eclipsing binary.GZ Pegis an SRa variable andS-type starwith a maximum magnitude of 4.95.T Cenis listed as the next-brightest SRa example,[2]but it is suggested that it may actually be anRV Tauri variable,which would make it by far the brightest member of that class.[6]

There are numerous naked-eye SRb stars, with third-magnitudeL2Pupbeing the brightest listed in the GCVS.σ Libandρ Perare also third-magnitude SRb stars at maximum brightness.β Gruis a second magnitude star classified as aslow irregular variableby the GCVS, but reported to be of SRb type by later research.[7]These four are all class M giants, although some SRb variables arecarbon starssuch asUU Auror S-type stars such asPi1Gru.[2]

Catalogued SRc stars are less numerous, but include some of the brightest stars in the sky such asBetelgeuseandα Her.Although SRc stars are defined as being supergiants, a number of them have giant spectralluminosity classesand some such as α Her are known to beasymptotic giant branchstars.[2]

Many SRd stars are extremely luminoushypergiants,including the naked-eyeρ Cas,V509 Cas,andο1Cen.Others are classified as giant stars, but the brightest example is the seventh-magnitudeLU Aqr.[2]

Most SRS variables have been discovered in deep large-scale surveys, but the naked-eye starsV428 And,AV Ari,andEL Pscare also members.[2]

See also

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References

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  1. ^abcdefghijKukarkin, B. V. (2016)."27. Commission des Etoiles Variables".Transactions of the International Astronomical Union.10:398–431.doi:10.1017/S0251107X00020988.
  2. ^abcdefghijklmn"GCVS Variability Types".General Catalogue of Variable Stars@Sternberg Astronomical Institute, Moscow, Russia.12 Feb 2009.Retrieved2010-11-24.
  3. ^abNicholls, C. P.; Wood, P. R.; Cioni, M.-R. L.; Soszyński, I. (2009)."Long Secondary Periods in variable red giants".Monthly Notices of the Royal Astronomical Society.399(4): 2063.arXiv:0907.2975.Bibcode:2009MNRAS.399.2063N.doi:10.1111/j.1365-2966.2009.15401.x.S2CID19019968.
  4. ^Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (2009). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud".Acta Astronomica.59(3): 239.arXiv:0910.1354.Bibcode:2009AcA....59..239S.
  5. ^Soszynski, I.; Dziembowski, W. A.; Udalski, A.; Kubiak, M.; Szymanski, M. K.; Pietrzynski, G.; Wyrzykowski, L.; Szewczyk, O.; Ulaczyk, K. (2007). "The Optical Gravitational Lensing Experiment. Period--Luminosity Relations of Variable Red Giant Stars".Acta Astronomica.57:201.arXiv:0710.2780.Bibcode:2007AcA....57..201S.
  6. ^Watson, C. L. (2006). "The International Variable Star Index (VSX)".The Society for Astronomical Sciences 25th Annual Symposium on Telescope Science. Held May 23–25.25:47.Bibcode:2006SASS...25...47W.
  7. ^Otero, S. A.; Moon, T. (December 2006). "The Characteristic Period of Pulsation of β Gruis".The Journal of the American Association of Variable Star Observers.34(2): 156–164.Bibcode:2006JAVSO..34..156O.
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