RR Lyrae variable

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The RR Lyrae variable stars fall in a particular area on a Hertzsprung-Russell diagram of color versus brightness. HR-diag-instability-strip.svg
The RR Lyrae variable stars fall in a particular area on a Hertzsprung–Russell diagram of color versus brightness.

RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and brightest example, RR Lyrae.

Contents

They are pulsating horizontal branch stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have shed mass during the red-giant branch phase, and were once stars at around 0.8 solar masses.

In contemporary astronomy, a period-luminosity relation makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group. They are also frequent subjects in the studies of globular clusters and the chemistry (and quantum mechanics) of older stars.

Discovery and recognition

H-R diagram for globular cluster M5, with the horizontal branch marked in yellow and known RR Lyrae stars in green M5 colour magnitude diagram.png
H-R diagram for globular cluster M5, with the horizontal branch marked in yellow and known RR Lyrae stars in green

In surveys of globular clusters, these "cluster-type" variables were being rapidly identified in the mid-1890s, especially by E. C. Pickering. Probably the first star definitely of RR Lyrae type found outside a cluster was U 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". [1]

From 1915 to the 1930s, the RR Lyraes became increasingly accepted as a class of star distinct from the classical Cepheids, due to their shorter periods, differing locations within the galaxy, and chemical differences. RR Lyrae variables are 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, to reconsider the calibration of Cepheid variables, and to propose the concept of stellar populations. [1] ) Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & van den Bergh found RR Lyraes in Andromeda's galactic halo [2] and, more recently, in its globular clusters. [3]

Classification

The RR Lyrae stars are conventionally divided into three main types, [1] following classification by S.I. Bailey based on the shape of the stars' brightness curves:

Distribution

RR Lyrae-type variable stars close to the galactic center from the VVV ESO public survey Variable stars close to the Galactic Centre.jpg
RR Lyrae-type variable stars close to the galactic center from the VVV ESO public survey

RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, over 80% of all variables known in globular clusters are RR Lyraes. [5] RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheids, which are strongly associated with the galactic plane.

Because of their old age, RR Lyraes are commonly used to trace certain populations in the Milky Way, including the halo and thick disk. [6]

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 85,000 in the Milky Way. [1]

Though binary star systems are common for typical stars, RR Lyraes are very rarely observed in binaries. [7]

Properties

RR Lyrae stars pulse in a manner similar to Cepheid variables, but the nature and histories of these stars is thought to be rather different. Like all variables on the Cepheid instability strip, pulsations are caused by the κ-mechanism, when the opacity of ionised helium varies with its temperature.

RR Lyraes are old, relatively low mass, Population II stars, in common with W Virginis and BL Herculis variables, the type II Cepheids. Classical Cepheid variables are higher mass population I stars. RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae star is about +0.75, only 40 or 50 times brighter than the Sun. [8] Their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, including RR Lyrae itself, exhibit the Blazhko effect in which there is a conspicuous phase and amplitude modulation. [9]

Period-luminosity relationships

Typical RR Lyrae light curve Rr lyrae ltcrv en.svg
Typical RR Lyrae light curve

Unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infrared K band. [10] They are normally analysed using a period-colour-relationship, for example using a Wesenheit function. In this way, they can be used as standard candles for distance measurements although there are difficulties with the effects of metallicity, faintness, and blending. The effect of blending can impact RR Lyrae variables sampled near the cores of globular clusters, which are so dense that in low-resolution observations multiple (unresolved) stars may appear as a single target. Thus the brightness measured for that seemingly single star (e.g., an RR Lyrae variable) is erroneously too bright, given those unresolved stars contributed to the brightness determined. Consequently, the computed distance is wrong, and certain researchers have argued that the blending effect can introduce a systematic uncertainty into the cosmic distance ladder, and may bias the estimated age of the Universe and the Hubble constant. [11] [12] [13]

Recent 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. [14]

The Kepler space telescope provided accurate photometric coverage of a single field at regular intervals over an extended period. 37 known RR Lyrae variables lie within the Kepler field, including RR Lyrae itself, and new phenomena such as period-doubling have been detected. [15]

The Gaia mission mapped 140,784 RR Lyrae stars, of which 50,220 were not previously known to be variable, and for which 54,272 interstellar absorption estimates are available. [16]

Related Research Articles

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

<span class="mw-page-title-main">Lyra</span> Constellation in the northern celestial hemisphere

Lyra is a small constellation. It is one of the 48 listed by the 2nd century astronomer Ptolemy, and is one of the modern 88 constellations recognized by the International Astronomical Union. Lyra was often represented on star maps as a vulture or an eagle carrying a lyre, and hence is sometimes referred to as Vultur Cadens or Aquila Cadens, respectively. Beginning at the north, Lyra is bordered by Draco, Hercules, Vulpecula, and Cygnus. Lyra is nearly overhead in temperate northern latitudes shortly after midnight at the start of summer. From the equator to about the 40th parallel south it is visible low in the northern sky during the same months.

<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.

<span class="mw-page-title-main">Messier 5</span> Globular cluster in the constellation Serpens

Messier 5 or M5 is a globular cluster in the constellation Serpens. It was discovered by Gottfried Kirch in 1702.

<span class="mw-page-title-main">Messier 107</span> Globular cluster in Ophiuchus

Messier 107 or M107, also known as NGC 6171 or the Crucifix Cluster, is a very loose globular cluster in a very mildly southern part of the sky close to the equator in Ophiuchus, and is the last such object in the Messier Catalogue.

<span class="mw-page-title-main">Cosmic distance ladder</span> Succession of methods by which astronomers determine the distances to celestial objects

The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

<span class="mw-page-title-main">Messier 2</span> Globular cluster in the constellation Aquarius

Messier 2 or M2 is a globular cluster in the constellation Aquarius, five degrees north of the star Beta Aquarii. It was discovered by Jean-Dominique Maraldi in 1746, and is one of the largest known globular clusters.

<span class="mw-page-title-main">Messier 15</span> Globular cluster in the constellation Pegasus

Messier 15 or M15 is a globular cluster in the constellation Pegasus. It was discovered by Jean-Dominique Maraldi in 1746 and included in Charles Messier's catalogue of comet-like objects in 1764. At an estimated 12.5±1.3 billion years old, it is one of the oldest known globular clusters.

<span class="mw-page-title-main">Messier 19</span> Globular cluster in Ophiuchus

Messier 19 or M19 is a globular cluster in the constellation Ophiuchus. It was discovered by Charles Messier on June 5, 1764 and added to his catalogue of comet-like objects that same year. It was resolved into individual stars by William Herschel in 1784. His son, John Herschel, described it as "a superb cluster resolvable into countless stars". The cluster is located 4.5° WSW of Theta Ophiuchi and is just visible as a fuzzy point of light using 50 mm (2.0 in) binoculars. Using a telescope with a 25.4 cm (10.0 in) aperture, the cluster shows an oval appearance with a 3′ × 4′ core and a 5′ × 7′ halo.

<span class="mw-page-title-main">Messier 62</span> Globular cluster in the constellation Ophiuchus

Messier 62 or M62, also known as NGC 6266 or the Flickering Globular Cluster, is a globular cluster of stars in the south of the equatorial constellation of Ophiuchus. It was discovered in 1771 by Charles Messier, then added to his catalogue eight years later.

<span class="mw-page-title-main">NGC 5466</span> Class XII globular cluster in the constellation Boötes

NGC 5466 is a class XII globular cluster in the constellation Boötes. Located 51,800 light years from Earth and 52,800 light years from the Galactic Center, it was discovered by William Herschel on May 17, 1784, as H VI.9. This globular cluster is unusual insofar as it contains a certain blue horizontal branch of stars, as well as being unusually metal poor like ordinary globular clusters. It is thought to be the source of a stellar stream discovered in 2006, called the 45 Degree Tidal Stream. This star stream is an approximately 1.4° wide star lane extending from Boötes to Ursa Major.

<span class="mw-page-title-main">RR Lyrae</span> Star in the constellation Lyra

RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astronomers. RR Lyrae variables serve as important standard candles that are used to measure astronomical distances. The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the inverse-square law. Hence, understanding the period-luminosity relation for a local set of such stars allows the distance of more distant stars of this type to be determined.

<span class="mw-page-title-main">Terzan 5</span>

Terzan 5 is a heavily obscured globular cluster belonging to the bulge of the Milky Way galaxy. It was one of six globulars discovered by French astronomer Agop Terzan in 1968 and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC–20385. It is situated in the Sagittarius constellation in the direction of the Milky Way's center. Terzan 5 probably follows an unknown complicated orbit around the center of the galaxy, but currently it is moving towards the Sun with a speed of around 90 km/s.

<span class="mw-page-title-main">Type II Cepheid</span>

Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects.

<span class="mw-page-title-main">BL Boötis</span> Star in the constellation Boötes

BL Boötis is a pulsating star in the constellation Boötes. It is the prototype of a class of anomalous Cepheids which is intermediate in the H-R diagram between the type I classical Cepheids and the type II Cepheids.

<span class="mw-page-title-main">V473 Lyrae</span> Star in the constellation Lyra

V473 Lyrae is a variable star in the constellation Lyra. It is an unusual Classical Cepheid variable with a visual range of 5.99 to 6.35.

<span class="mw-page-title-main">NGC 6388</span> Globular cluster in the constellation Scorpius

NGC 6388 is a globular cluster of stars located in the southern constellation of Scorpius. The cluster was discovered by Scottish astronomer James Dunlop on May 13, 1826 using a 20 cm (9 in) reflector telescope. It was later determined to be a globular cluster by English astronomer John Herschel, who was able to resolve it into individual stars. NGC 6388 is located at a distance of approximately 35,600 light-years (10.90 kpc) from the Sun. Due to its apparent visual magnitude of +6.8, binoculars or a small telescope are required to view it.

<span class="mw-page-title-main">NGC 6441</span> Globular cluster in Scorpius

NGC 6441, sometimes also known as the Silver Nugget Cluster, is a globular cluster in the southern constellation of Scorpius. It was discovered by the Scottish astronomer James Dunlop on May 13, 1826, who described it as "a small, well-defined rather bright nebula, about 20″ in diameter". The cluster is located 5 arc minutes east-northeast of the star G Scorpii, and is some 43,000 light-years from the Sun.

<span class="mw-page-title-main">NGC 6426</span> Globular cluster in the constellation Ophiuchus

NGC 6426 is a globular cluster of stars located in the equatorial constellation of Ophiuchus. It was discovered by the German-English astronomer William Herschel on 3 June 1786. This cluster is at a distance of 67,000 light years from the Sun. It has an apparent visual magnitude of 10.9 and an angular diameter of 4.2′, making it difficult to observe with a small telescope.

<span class="mw-page-title-main">Period-luminosity relation</span> Astronomical principle

In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, sometimes called the Leavitt Law. Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances. The physical model explaining the Leavitt's law for classical cepheids is called kappa mechanism.

References

  1. 1 2 3 4 5 Smith, Horace A. (2004). RR Lyrae Stars. Cambridge University Press. ISBN   978-0-521-54817-5.
  2. ↑ Pritchet, Christopher J.; Van Den Bergh, Sidney (1987). "Observations of RR Lyrae stars in the halo of M31". Astrophysical Journal. 316: 517. Bibcode:1987ApJ...316..517P. doi:10.1086/165223.
  3. 1 2 Clementini, G.; Federici, L.; Corsi, C.; Cacciari, C.; Bellazzini, M.; Smith, H. A. (2001). "RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates". The Astrophysical Journal. 559 (2): L109. arXiv: astro-ph/0108418 . Bibcode:2001ApJ...559L.109C. doi:10.1086/323973. S2CID   48632444.
  4. ↑ Christensen-Dalsgaard, J.; Balona, L. A.; Garrido, R.; Suárez, J.C. (Oct 20, 2012). "Stellar Pulsations: Impact of New Instrumentation and New Insights". Astrophysics and Space Science Proceedings. ISBN   978-3-642-29630-7.
  5. ↑ Clement, Christine M.; Muzzin, Adam; Dufton, Quentin; Ponnampalam, Thivya; Wang, John; Burford, Jay; Richardson, Alan; Rosebery, Tara; Rowe, Jason; Hogg, Helen Sawyer (2001). "Variable Stars in Galactic Globular Clusters". The Astronomical Journal. 122 (5): 2587–2599. arXiv: astro-ph/0108024 . Bibcode:2001AJ....122.2587C. doi:10.1086/323719. S2CID   38359010.
  6. ↑ Vozyakova, O. V.; Sefako, R.; Rastorguev, A. S.; Kravtsov, V. V.; Kniazev, A. Y.; Berdnikov, L. N.; Dambis, A. K. (2013-11-11). "RR Lyrae variables: visual and infrared luminosities, intrinsic colours and kinematics". Monthly Notices of the Royal Astronomical Society. 435 (4): 3206–3220. arXiv: 1308.4727 . doi: 10.1093/mnras/stt1514 . ISSN   0035-8711.
  7. ↑ Hajdu, G.; Catelan, M.; Jurcsik, J.; Dékány, I.; Drake, A.J.; Marquette, B. (2015). "New RR Lyrae variables in binary systems". Monthly Notices of the Royal Astronomical Society. 449 (1): L113–L117. arXiv: 1502.01318 . Bibcode:2015MNRAS.449L.113H. doi: 10.1093/mnrasl/slv024 .
  8. ↑ Layden, A. C.; Hanson, Robert B.; Hawley, Suzanne L.; Klemola, Arnold R.; Hanley, Christopher J. (August 1996). "The Absolute Magnitude and Kinematics of RR Lyrae Stars via Statistical Parallax". Astron. J. 112: 2110–2131. arXiv: astro-ph/9608108 . Bibcode:1996AJ....112.2110L. doi:10.1086/118167. S2CID   8732647.
  9. ↑ Szabó, R.; Kolláth, Z.; Molnár, L.; Kolenberg, K.; Kurtz, D. W.; Bryson, S. T.; Benkő, J. M.; Christensen-Dalsgaard, J.; Kjeldsen, H.; Borucki, W. J.; Koch, D.; Twicken, J. D.; Chadid, M.; Di Criscienzo, M.; Jeon, Y.-B.; Moskalik, P.; Nemec, J. M.; Nuspl, J. (2010). "Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars". Monthly Notices of the Royal Astronomical Society. 409 (3): 1244. arXiv: 1007.3404 . Bibcode:2010MNRAS.409.1244S. doi: 10.1111/j.1365-2966.2010.17386.x . S2CID   119190883.
  10. ↑ Catelan, M.; Pritzl, Barton J.; Smith, Horace A. (2004). "The RR Lyrae Period-Luminosity Relation. I. Theoretical Calibration". The Astrophysical Journal Supplement Series. 154 (2): 633. arXiv: astro-ph/0406067 . Bibcode:2004ApJS..154..633C. doi:10.1086/422916. S2CID   119336592.
  11. ↑ Majaess, D.; Turner, D.; Gieren, W.; Lane, D. (2012). "The Impact of Contaminated RR Lyrae/Globular Cluster Photometry on the Distance Scale". The Astrophysical Journal Letters. 752 (1): L10. arXiv: 1205.0255 . Bibcode:2012ApJ...752L..10M. doi:10.1088/2041-8205/752/1/L10. S2CID   118528078.
  12. ↑ Lee, Jae-Woo; López-Morales, Mercedes; Hong, Kyeongsoo; Kang, Young-Woon; Pohl, Brian L.; Walker, Alistair (2014). "Toward a Better Understanding of the Distance Scale from RR Lyrae Variable Stars: A Case Study for the Inner Halo Globular Cluster NGC 6723". The Astrophysical Journal Supplement. 210 (1): 6. arXiv: 1311.2054 . Bibcode:2014ApJS..210....6L. doi:10.1088/0067-0049/210/1/6. S2CID   119280050.
  13. ↑ Neeley, J. R.; Marengo, M.; Bono, G.; Braga, V. F.; Dall'Ora, M.; Stetson, P. B.; Buonanno, R.; Ferraro, I.; Freedman, W. L.; Iannicola, G.; Madore, B. F.; Matsunaga, N.; Monson, A.; Persson, S. E.; Scowcroft, V.; Seibert, M. (2015). "On the Distance of the Globular Cluster M4 (NGC 6121) Using RR Lyrae Stars. II. Mid-infrared Period-luminosity Relations". The Astrophysical Journal. 808 (1): 11. arXiv: 1505.07858 . Bibcode:2015ApJ...808...11N. doi:10.1088/0004-637X/808/1/11. S2CID   117031686.
  14. ↑ Benedict, G. Fritz; et al. (January 2002). "Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae". The Astronomical Journal. 123 (1): 473–484. arXiv: astro-ph/0110271 . Bibcode:2002AJ....123..473B. doi:10.1086/338087. S2CID   59150013.
  15. ↑ Kinemuchi, Karen (2011). "RR Lyrae Research with the Kepler Mission". RR Lyrae Stars. 5: 74. arXiv: 1107.0297 . Bibcode:2011rrls.conf...74K.
  16. ↑ Riello, M.; Evans, D. W.; Szabados, L.; Sarro, L. M.; Regibo, S.; Ridder, J. De; Eyer, L.; Lecoeur-Taibi, I.; Mowlavi, N. (2019-02-01). "Gaia Data Release 2 - Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars". Astronomy & Astrophysics. 622: A60. arXiv: 1805.02079 . Bibcode:2019A&A...622A..60C. doi:10.1051/0004-6361/201833374. ISSN   0004-6361. S2CID   260496633.
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