RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.
This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.
RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.
RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun.[citation needed]) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.
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Abundances and distribution
RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.
Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way[1].
Discovery and recognition
In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid 1890s (especially by E. C. Pickering).
The first star of definitely RR Lyrae type outside a cluster was probably Mu Leporis discovered by J. Kapteyn in 1890.
The prototype star RR Lyrae was discovered prior to 1899 by Williamina Fleming, and reported by Pickering in 1900 as "indistinguishable from cluster-type variables".
From 1915 to the 1930s, the RR Lyraes became more accepted as a distinct class of star from classical Cepheids, on account of their shorter periods, different location within the galaxy, and finally, they are chemically different from classical Cepheids, being mostly metal-poor, Population II stars[1].
RR Lyraes have proven difficult to observe in external galaxies, because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, and to re-consider the calibration of Cepheid variables and to propose stellar populations[1].) They were finally found in the 1980s by Pritchet & van den Bergh[2] in the halo of the Andromeda galaxy, and more recently in its globular clusters by the Hubble Space Telescope.
Subtypes
The RR Lyrae stars are conventionally divided into three main types[1], following a classification of S.I. Bailey based on the shape of the stars' brighness curve:
- RRab — the majority, with steep rise in brightness (about 91%)
- RRc — having shorter periods, more sinusoidal variation (about 9%).
- RRd — rare double-mode pulsators.
New and upcoming developments
The Hubble Space Telescope has identified several RR Lyrae "candidates" in globular clusters of the Andromeda galaxy[3] and has measured the distance to the prototype star RR Lyrae.
The Gaia mission is expected to greatly improve knowledge of RR Lyraes, by providing a homogeneous spectrographic information of a large population of such stars[4].
See also
References
- ^ a b c d Smith, Horace A., RR Lyrae Stars, Cambridge (2004)
- ^ Pritchet, C. J., & van den Bergh, S., "Observations of RR Lyrae stars in the halo of M31", Astrophysical Journal, 316, 517 (1987)
- ^ G. Clementini, L. Federici, C. Corsi, C. Cacciari, M. Bellazzini, and H. A. Smith, "RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates", The Astrophysical Journal, 559:L109-L112 (2001)
- ^ Bono, G., "The Cepheid and RR Lyrae instability strip with GAIA", GAIA Spectroscopy: Science and Technology, ASP Conference Proceedings, Vol. 298 (2002)
External links
- APOD M3: Inconstant Star Cluster four-frame animation of RR Lyrae variables in globular cluster M3
- Animation of RR Lyrae-Variables in globular cluster M15
- Animation with the variable stars RR Lyrae in the center area of the globular cluster M15
- RR Lyrae stars
- M3 variable stars 2 night animation RR Lyrae at the globular cluster M3, 10min intervals.
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