Semi-regular variable star

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Light curve of the semi-regularly variable Betelgeuse

Semi-regularly variable stars are giants or supergiants with a medium to late spectral type . They partially or constantly show periodic light changes, which are accompanied or interrupted by irregularities in the light curve .

The periods range between 20 and more than 2000 days, while the shape of the light curve can be variable in each cycle. The amplitudes of the brightness can range from a few hundredths to several magnitudes ; In the General Catalog of Variable Stars , the semi-regularly variable stars usually have an amplitude of 1 to 2 magnitudes in the V filter. However, this is based on selection effects, since the light curves for classification are often based on brightness estimates and a periodicity can only be detected with corresponding amplitudes.

Subgroups

Semi-regularly variable stars are divided into the following groups (SR stands for semiregular variable star ; in addition to the spelling with lowercase letters, e.g. SRa, there is also the variant with only uppercase letters, i.e. SRA):

Subgroup Star class Spectral type period Amplitude of the change in brightness Representative comment
SRa late giant stars M, C, S or Me, Ce, Se 35 to 1200 days usually small (<2.5 magnitudes in visual) Eta Geminorum , Z Aquarii The amplitudes and the shapes of the light curves are variable. Many of these stars differ from Mira variables only in that they show smaller amplitudes of brightness.
SRb late giant stars M, C, S or Me, Ce, Se poorly defined (an average of 20 to 2300 days) or with changing intervals of their periods and slow, irregular changes. Occasionally some of them may stop their variations altogether for a period of time. La Superba , Beta Gruis , Sigma Librae , R Sculptoris , RR Coronae Borealis, AF Cygni Each star of this type can usually be assigned to a specific period. In some cases, two or more periods can be observed at the same time.
SRc late supergiants M, C, S or Me, Ce, Se 30 to several thousand days approx. 1 magnitude Antares , garnet star μ Cephei , Betelgeuse
SRd Giants and supergiants F, G, K or Fe, Ge, Ke 30 to 1100 days 0.1 to 4 magnitudes RW Cephei , V509 Cassiopeiae , SX Herculis, SV Ursae Majoris

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists over 5000 stars with the abbreviations SR , SRA , SRB , SRC , SRD and SRS , which means that around 10% of all stars in this catalog belong to the class of semi-regularly variable stars.

criticism

The above-mentioned division into classes is based on the appearance of the light curve, which often only covers a short period of time, is only present in the optical range and has a large spread due to estimation errors. The classification does not support the separation according to astrophysical parameters, and the delimitation against Mira stars and slowly irregular variable stars is imprecise. A physical difference to the slowly irregular stars does not seem to exist at all.

Causes of the light change

Over the past decade, the search for micro lens effects ( EROS , MACHO , OGLE ) has resulted in thousands of high-precision light curves of long-period variables. While the mira stars always vibrate with the basic frequency , the semi-regular as well as the slowly irregularly changing stars pulsate in one or more harmonics .

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

The ellipsoidal light change occurs in red giants and stars on the asymptotic giant branch . In relation to the radius of the red giant, it is a close binary star system in which in most cases there is a mass exchange in the course of further development . Depending on the mass and distance between the two stars, the result will be a merging of the two nuclei to form a rapidly rotating FK-Comae-Berenices star or a binary star system as the nucleus of a planetary nebula .
  • Long secondary periods are detected in around 30% of the stars on the asymptotic giant branch and with a lower mean amplitude in red supergiants . These are superimposed broad minima that occur in the light curves with a period that is approx. Nine times longer than the dominant pulsation period. The amplitude can be up to one magnitude.

So far, vibrations in the atmosphere of the red giant and the influence of a companion in a binary star system could be excluded as the cause.

All stars with a long secondary period show a strong infrared excess , therefore asymmetrically distributed circumstellar dust is associated with the phenomenon. The exact mechanism of the formation of the minima is not yet understood.

Long secondary periods have also been reported for the yellow supergiants Rho Cassiopeiae and HR 8752 . These periods are said to be around 1000 days and are interpreted as the period of circulation of huge convection cells in the atmospheres of the stars. This hypothesis is supported by the observation of the variable radial velocity in the yellow supergiants. and also represented for red supergiants

The excitation mechanism of the vibrations in semi-regularly variable stars differs from that of classic pulsation variables , such as the Mira stars, the Cepheids and the RR Lyrae stars . In these classes of stars , the vibrations are generated by the kappa mechanism in the helium ionization zone. The excitation mechanism of the semi-regular, however, corresponds to the 5-minute oscillation of the sun . These are excited by random convection currents near the star's atmosphere , reflected on the photosphere due to the density jump and run back into the star. Most of the vibrations cancel each other out except for those that hit the fundamental or its harmonic.

Flares

In the literature there are indications of short-term changes in brightness in the order of magnitude of hours to days with long-period variables. These events usually take the form of a flare with a steep rise, which is often followed by a slower descent, as in the case of Y Scorpii during examinations with the STEREO space probes. A systematic investigation of light curves of long-period variables after flares could only show that, if the events are real, they only occur very rarely: with less than 0.15 events per star and year. Deviations from a smooth light curve only occur on a time scale of 10 days.

See also

supporting documents

  1. ↑ JR Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  2. ↑ C. Hoffmeister, G. Richter, W. Wenzel: Veränderliche Sterne . 3. Edition. Springer Verlag, Berlin 1990, ISBN 3-335-00224-5 .
  3. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved May 12, 2019 .
  4. ↑ HJ habing, H. Olofson: asymptotic giant branch stars . Springer Verlag, Berlin 2003, ISBN 0-387-00880-2 .
  5. ↑ T. Lebzelter, M. Obbrugger: How semiregular are irregular variables? In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0902.4096 .
  6. ^ Walter Nowotny, Bernhard Aringer, Susanne Höfner, Michael T. Lederer: Synthetic photometry for carbon-rich giants II. The effects of pulsation and circumstellar dust . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1103.5005 .
  7. ↑ CP Nicholls and PR Wood: Eccentric Ellipsoidal Red Giant Binaries in the LMC: Complete Orbital Solutions and Comments on Interaction at Periastron . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.1043v1 .
  8. ↑ CP Nicholls, PR Wood and M.-RL Cioni: Ellipsoidal Variability and the Difference between Sequence D and E Red Giants . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1002.3751v1 .
  9. ↑ JD Nie, PR Wood, CP Nicholls: Predicting the Fate of Binary Red Giants Using the Observed Sequence E Star Population: Binary Planetary Nebula Nuclei and Post-RGB Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.2648v1 .
  10. ^ Ming Yang and BW Jiang: Red Supergiant Stars in the Large Magellanic Cloud: I. The Period-Luminosity Relation . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1011.4998v1 .
  11. ^ PR Wood and CP Nicholls: Evidence for Mass Ejection Associated with Long Secondary Periods in Red Giants . In: Astrophysics. Solar and Stellar Astrophysics . 2009, arxiv : 0910.4418v1 .
  12. ^ JD Nie, XB Zhang and BW Jiang: Testing a pulsating binary model for long secondary periods in red variables . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1003.2553v2 .
  13. ↑ Christine Nicholls: Why variable AGB stars with Long Secondary Periods aren't binaries, but are dusty . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1201.1046v1 .
  14. ^ Stothers, Richard B .: Yellow Hypergiants Show Long Secondary Periods? In: The Astrophysical Journal . tape 751 , no. 2 , 2012, p. 151 .
  15. ^ Richard B. Stothers: Giant Convection Cell Turnover as an Explanation of the Long Secondary Periods in Semiregular Red Variable Stars. ApJ 725, 2010, p. 1170, doi: 10.1088 / 0004-637X / 725/1/1170 (free)
  16. ^ B. Mosser, WA Dziembowski, K. Belkacem, MJGoupil, E. Michel, R. Samadi, I. Soszynski, M. Vrard, E. Elsworth, S. Hekker, S. Mathur: Period-luminosity relations in evolved red giants explained by solar-like oscillations . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1310.0839v1 .
  17. ^ KT Wraight, D. Bewsher, Glenn J. White, W. Nowotny, AJ Norton and C. Paladini: STEREO observations of long period variables . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.1485 .
  18. ↑ Lebzelter, T .: Long-period variables in the CoRoT fields . In: Astronomy & Astrophysics . tape 530 , 2011, pp. A35 , doi : 10.1051 / 0004-6361 / 201116801 .