RR Lyrae star

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RR Lyrae stars are pulsation-variable stars with a regular light change and a period of 0.2 to 1.2 days. The brightness amplitudes are up to 2 magnitudes and the spectral type A to F. They are also called cluster variables due to their frequent occurrence in globular clusters .

history

The RR Lyrae stars were discovered in 1895 by Solon Irving Bailey while analyzing several images of globular clusters from Boyden Station at Harvard College Observatory in Arequipa , Peru. The variables found showed strong similarities in their light curves with the Cepheids , but their periods were much shorter in comparison with the Cepheids with between 80 minutes and 20 hours. The first variable of this type found in the galactic field was probably U Leporis , but it was only the prototype RR Lyrae in the constellation Lyra that was described by Pickering as indistinguishable from the cluster variables.

Subgroups

RR Lyrae stars are divided into three subgroups based on their light curve :

  • RRab: This subgroup represents the majority of the discovered RR Lyrae stars with a steep rise and a large amplitude . Due to the higher probability of detection, the apparently high frequency is a selection effect. The stars pulsate in the fundamental oscillation with a period between 0.3 and 0.9 days. They are also known as RR0 stars.
  • RRc: The change in light is sinusoidal and the amplitude does not exceed 0.6 magnitudes. These stars usually pulsate in the first harmonic with a period of 0.2 to 0.5 days. An alternative designation is RR1. A very small group of RRc stars probably only pulsates in the second harmonic and is known as RR2 stars.
  • RRd: In this subgroup, the variable pulsates with two or more periods of comparable amplitude. In the case of an oscillation with the fundamental frequency and the first harmonic , these stars would be called RR01. The proportion of RRd stars in a star system or population is a few percent, whereby the value can fluctuate between 0.5 and 30%. The ratio of P 0 to P 1 is between 0.742 and 0.748, the values depending on the metallicity . For RR Lyrae stars that pulsate in the fundamental and the second harmonic, the period ratio is between 0.585 and 0.595.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists around 8,500 stars with the abbreviation RR , which means that almost 20% of all stars in this catalog belong to the class of RR Lyrae stars.

classification

HR diagram of the globular cluster M5 . The position of the RR Lyrae stars on the horizontal branch is marked in green.

RR Lyrae stars have about half the solar mass, about five times the solar diameter and the giant stars change their surface temperature between 6000 and 7500 ° C over the course of the period  . The cause of the changeability is the kappa mechanism as with the Cepheids . They are developed stars on the horizontal branch in the Hertzsprung-Russell diagram . Coming from the red giant branch, they migrate to the left and back again, crossing the instability strip . RR Lyrae stars are found in globular clusters , the galactic halo , the bulge of the Milky Way and, more recently, in extragalactic systems. The proportion of heavy elements in their atmosphere is low, between 0.00001 and 0.01 that of the sun .

Change of period

Since the RR Lyrae stars show a strictly periodic change in light, small changes due to summation over time should lead to a shift in the point in time of maximum brightness. This makes it possible to measure the direction and speed when passing through the instability strip, whereby the value of the period change expected from the model calculations should be 0.01 days per million years. In contrast, the observations show a result that is difficult to interpret. While the mean period change is as expected, only 40% of all RRab stars show a uniform period change over a period of one century. 15% could be interpreted as abrupt changes, superimposed with a regular change in period, while the other stars show only irregular and abrupt change in period. Most stars with irregular period changes also show a Blazhko effect.

Blazhko effect

A long-term modulation of the light curve between 10 and 500 days can be superimposed on the regular light change , whereby the amplitude of the fundamental oscillation can vary by up to 50 percent. In addition to the amplitude, the phase of the changes in brightness is also modulated. About 40 to 50 percent of all RR-Lyrae-type RRab and RRc stars show the Blazhko effect. Several hypotheses have been developed to explain the Blazhko effect:

  • A superimposed (non-radial) higher order pulsation
  • Modification of the pulsation by a stellar magnetic field and rotation
  • A 9: 2 resonance between the fundamental frequency and the 9th harmonic
  • A variable turbulent convection caused by a quasi-periodic change in the stellar magnetic field
  • A non-linear interaction between the fundamental and the first harmonic. These hypotheses are not supported by new observations from the COROT and Kepler satellite missions, as strong changes in the Blazhko period have already been observed from cycle to cycle. In addition to the RR Lyrae stars, the Blazhko effect has also been demonstrated in the Cepheids and the Delta Scuti stars . Whether long-period modulations in the light curves of sdB stars and white dwarfs are also based on the Blazhko effect is the subject of current scientific discussions.

The case of V445 Lyrae: Complex behavior

Current observations indicate that this class of stars can show a considerably more complex, possibly chaotic behavior and the previous assumption that RR Lyrae stars are viewed as variables pulsating radially with a period is only a simplification in order to understand the basic properties.

The Kepler satellite, used to search for exoplanets, observed star fields intensively at high frequency photometrically, which also includes long-term observations of variable stars.

The RR Lyrae star V445 Lyrae showed the following properties, which were previously observed on the RR Lyrae star CoRoT 105288363 :

  • Radial pulsations not only in the basic oscillation, but also with low amplitudes in the first and second harmonics .
  • At least one non-radial pulsation.
  • Further probably non-radial oscillations in the frequency band between the fundamental and first harmonic
  • The Blazhko modulation is periodically variable with at least two cycle lengths.
  • The radial oscillations show signs of a doubling of the period , this means the transition from a stable oscillation to a chaotic state .

RR Lyrae stars as probes

The pulsation masses of RR Lyrae stars are 0.7 solar masses and these developed low-mass stars are at least 10 billion years old. Therefore, RR Lyrae stars can only occur in star populations of type II and are an easily determinable indicator of the evolutionary history of a star system. In addition, with the help of these variable stars, both the metal frequency and the distance within the local group can be derived.

The absolute magnitude is between +0 M .5 and +1 M . It depends on the period, the mass and the mean surface temperature . These factors are combined into a period-luminosity relationship . Due to this relationship, the extinction within a star system can also be investigated with RR Lyrae stars due to the dependence on the surface temperature , as this leads to a reddening of the starlight.

The high frequency and brightness of the RR Lyrae stars make it possible to analyze structures in the halo of the Milky Way and in other galaxies of the local group . In the halo of the Milky Way, numerous star currents have been discovered with the help of the pulsating variables , and these are likely to be the remains of dwarf galaxies cannibalized by the Milky Way .

The metallicity , the proportion of the atmosphere with elements heavier than helium, can be derived from the light curve of RRab stars. There is a connection between the amplitude and the sandage metallicity as well as with a parameter derived from a Fourier transformation , the metallicity according to Jurcsik and Kovacs, between the period and the metal content. Using the light curve, it is therefore possible to determine both the distance and the content of heavy elements and to analyze the historical development of the star system under study with little effort.

The light curves of the RR Lyrae stars are also mimicked by a rare group of stars that are not in the nuclear helium burn stage. RR Lyrae stars are evolved stars with low mass of less than one solar mass, which have already passed through the red giant stage and, after igniting the helium burn in their core, migrated to the horizontal branch of the Hertzsprung-Russell diagram . OGLE-BLG-RRLYR-02792 shows the light curve of an RR Lyrae star in terms of both shape and amplitude. Since it is also an eclipse variable , it was possible to calculate the mass of only 0.26 solar masses with the help of radial velocity measurements instead of the approximately 0.7 solar masses for RR Lyrae stars. This pulsation variable is also two magnitudes less light than RR Lyrae stars. Probably these unusual RRLyr evolve with double mass exchange in some binary star systems . The determination of the distance to an RR Lyrae star is therefore potentially inaccurate, as it could be an unusual RRLyr that is considerably fainter.

Preston's Spectral Index

The investigation of the spectra of RR Lyrae stars revealed a lower metal abundance, the abundance of elements heavier than lithium, than in the solar atmosphere. This low frequency is quantified with the Preston spectral index ΔS. It is defined as ten times the difference between the spectral type derived from the hydrogen lines and the spectral type derived from the calcium lines. RR Lyrae stars with ΔS <3 are in the Milky Way plane and have periods less than 0.4 days. In contrast, the metal-poor RR Lyrae variables with ΔS> 5 are stars of the galactic halo and their periods are generally greater than 0.4 days. When applying the period-luminosity relationship with RR Lyrae stars, the metal frequency must always be taken into account.

Oosterhoff's dichotomy

In 1939 the Dutch astronomer P. Oosterhoff discovered while working on RR Lyrae stars in globular clusters of the Milky Way that there is no continuous distribution of the periods. The mean period of RRab stars is either 0.55 days or 0.65 days. There is also a corresponding dichotomy in the case of RRc stars, which are now referred to as Oosterhoff groups I and II. The dichotomy is surprising in that there is no parameter for globular clusters that occurs in two distinguishable forms, such as age or chemical composition. In the dwarf galaxies of the Milky Way, in contrast to the globular clusters, a continuous distribution of the mean period length of RR Lyrae stars was observed.

The most commonly used explanation for the Oosterhoff dichotomy assumes a hysteresis effect in the development of the RR Lyrae stars on the horizontal branch. The hysteresis prevents the pulsation from changing between the fundamental (RRab) and the harmonic (RRc). In Group II, which has a lower metal content, the RR Lyrae phase begins at higher temperatures and the development leads to the right in the Hertzsprung-Russell diagram . On the other hand, the group I stars develop from low temperatures to the left and later switch from the RRab to the RRc phase.

The Oosterhoff dichotomy does not occur in dwarf galaxies of the local group and their globular clusters. The distribution of the periods of the RR Lyrae stars in these star systems is continuous. In some globular clusters outside the Milky Way, the variable stars, sometimes called "Oosterhoff's intermediate objects", represent the largest proportion of all RR Lyrae stars. This is incompatible with the assumption that the Milky Way captured dwarf galaxies in the past and that their globular clusters are now part of the Milky Way.

Examples

  • RRab with Blazhko effect: RR Lyrae
  • RRab: U Leporis
  • RRc: SX Ursae Majoris
  • RRd or RR (B): AQ Leonis

See also

Web links

Commons : RR Lyrae variables  - collection of images, videos and audio files

literature

  1. ↑ A. Umsold, B. Baschek: The new cosmos . 5th edition. Springer, Berlin 1991, ISBN 3-540-53757-0 .
  2. ↑ B.-Q. Chen, B.-W. Jiang, M. Yang: Analysis of a selected sample of RR Lyrae stars in LMC from OGLE III . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.4711 .
  3. ↑ P. Moskalik: Multi-Periodic Oscillations in Cepheids and RR Lyrae-Type Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.4246 .
  4. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved February 2, 2019 .
  5. ↑ C. Hoffmeister, G. Richter, W. Wenzel: Veränderliche Sterne . 3. Edition. Springer, Berlin 1990, ISBN 3-335-00224-5 .
  6. ↑ JR Percy: Understanding Variable Stars . Cambridge University Press, Cambridge 2007, ISBN 978-0-521-23253-1 .
  7. ↑ J. Jurcsik, G. Hajdu, B. Szeidl, K. Olah, J. Kelemen, A. Sodor, A. Saha, P. Mallick, J. Claver: Long-term photometric monitoring of RR Lyr stars in M3 . In: Monthly Notice of the Royal Astronomical Society . tape 419 , 2011, pp. 2173-2194 , doi : 10.1111 / j.1365-2966.2011.19868.x .
  8. ^ Robert Szabo: Blazhko effect in Cepheids and RR Lyrae stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1309.3969v1 .
  9. ^ D. Gillet: Atmospheric dynamics in RR Lyrae stars - The Blazhko effect . In: Astronomy & Astrophysics . tape 554 , 2013, p. A46 , doi : 10.1051 / 0004-6361 / 201220840 .
  10. ^ J. Robert Buchler, Zoltan Kollath: On the Blazhko Effect in RR Lyrae Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1101.1502 .
  11. ↑ R. Smolec, P. Moskalik, K. Kolenberg, S. Bryson, MT Cote, RL Morris: Variable turbulent convection as the cause of the Blazhko effect - testing the model Stothers . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1102.4845 .
  12. ^ Michel Breger: The Blazhko Effect in Delta Scuti and Other Groups of Pulsating Stars . In: VARIABLE STARS, THE GALACTIC HALO AND GALAXY FORMATION . 2010, p. 111-116 .
  13. ↑ E. Guggenberger et al.: The complex case of V445 Lyr observed with Kepler: Two Blazhko modulations, a non-radial mode, possible triple mode RR Lyrae pulsation, and more . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.1344v1 .
  14. ↑ Ata Sarajedini: RR Lyrae variable in M31 and M33 . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1105.5116v1 .
  15. ↑ Laura Watkins: Galactic substructure traced by RR Lyraes in SDSS Stripe 82 . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1111.4390v1 .
  16. ↑ Cecilia Mateu, A. Katherina Vivas, Juan José Downes, César Briceño, Robert Zinn, Gustavo Cruz-Diaz: The QUEST RR Lyrae Survey: III. The Low Galactic Latitude Catalog . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.4599 .
  17. ↑ G. Pietrzynski, IB Thompson, W. yaw, D. Graczyk, K. Stępień, G. Bono, PG Prada Moroni, B. Pilecki, A. Udalski, I. Soszynski, GW Preston, N. Nardetto, A. McWilliam , IU Roederer, M. Górski, P. Konorski & J. Storm: RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system . In: Nature . tape 484 , 2012, p. 75-77 , doi : 10.1038 / nature10966 .
  18. ^ HA Smith: RR Lyrae Stars . Cambridge University Press, Cambridge 2003, ISBN 0-521-54817-9 .
  19. ^ Charles A. Kuehn, Horace A. Smith, Marcio Catelan, Barton J. Pritzl, Nathan De Lee, Jura Borissova: VARIABLE STARS IN LARGE MAGELLANIC CLOUD GLOBULAR CLUSTERS I: NGC 1466 . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1107.5515v1 .