R Doradus

R Doradus (HD 29712 or P Doradus) is a red giant variable star in the far-southern constellation Dorado. Its distance from Earth is 178 light-years (55 parsecs). Having a uniform disk diameter of 57±5 mas, it is thought to be the extrasolar star with the largest apparent size as viewed from Earth.

R Doradus

Infrared interferometric image of the star
Credit: ESO
Observation data
Epoch J2000      Equinox J2000
Constellation Dorado
Right ascension 04h 36m 45.59127s[1]
Declination −62° 04′ 37.7974″[1]
Apparent magnitude (V) 4.8–6.6[2]
5.40[3]
Characteristics
Evolutionary stage AGB
Spectral type M8III:e[4]
Apparent magnitude (J) −2.6[5]
Apparent magnitude (K) −4.2[5]
U−B color index +0.86[1]
B−V color index +1.58[1]
Variable type SRb[2]
Astrometry
Radial velocity (Rv)+26.1[6] km/s
Proper motion (μ) RA: −69.36[1] mas/yr
Dec.: −75.78[1] mas/yr
Parallax (π)18.31 ± 0.99 mas[1]
Distance178 ± 10 ly
(55 ± 3 pc)
Details
Mass0.7–1.0[7] M☉
Radius298±21[7] R☉
Luminosity4,350±520[7] L☉
Surface gravity (log g)−0.6±0.1[7] cgs
Temperature2,710±70[7] K
Metallicity0.0[7]
Rotation57.5 years[8]
Rotational velocity (v sin i)1±0.1[8] km/s
Age6–14[7] Gyr
Other designations
P Dor, R Dor, AAVSO 0435-62, CCDM J04368-6205A CD−62°175, CPC 20.1 1002, CPD−62°372, GCRV 2726, GSC 08880-01071, HD 29712, HIC 21479, HIP 21479, HR 1492, 2MASS J04364544-6204379, PPM 354226, SAO 249066, WDS J04368-6205A[9]
Database references
SIMBADdata

Variability
A visual band light curve for R Doradus, adapted from Bedding et al. (1998)[10]

The visible magnitude of R Doradus varies between 4.8 and 6.6, which means it is usually visible to the naked eye, but in the infrared it is one of the brightest stars in the sky. With a near-infrared J band magnitude of −2.6,[5] only Betelgeuse at −2.9 is brighter. In the infrared K band, it is sometimes the brightest star in the sky, although usually Betelgeuse is brighter.[11]

It is classified as a semiregular variable star of type SRb, indicating giants with slow poorly-defined variations, often alternating between periodic and irregular brightness changes.[2] Some studies show it alternating between periods of about 175 and 332 days, and a period of 117.3 days has also been identified.[12] It has been likened to a Mira variable when its variations are relatively regular, although its amplitude of only 1.5 magnitudes is smaller than Mira variables.[13]

Angular diameter

The angular diameter of R Doradus is easily measured using interferometry. Its uniform disc diameter, the diameter when interpreted as a disc of uniform brightness, when viewed at 1.25 μm is 57±5 mas.[14] When viewed at 2.3 μm and interpreted as a limb-darkened disc, the diameter is 51.18±2.24 mas.[12]

The angular diameter of R Doradus is larger than any other measured star other than the Sun. The angular diameter of the next-largest star, Betelgeuse, is around 45 mas.[14]

Properties

The Hipparcos parallax of R Doradus is 18.31±0.99 mas, corresponding to a distance of 55±3 pc.[1]

The bolometric luminosity of R Doradus, derived from its bolometric flux at a distance of 55 pc, is 4,350±520 L☉. The measured angular diameter, again assuming a distance of 55 pc gives a radius of 298±21 R☉. The effective temperature corresponding to this luminosity and radius is 2,710±170 K.[12]

Comparison of its properties with theoretical evolutionary tracks gives an age of between 6 and 14 billion years, with a current mass of between 0.7 and 1.0 M☉. Its initial mass would have been between 1 and 1.25 M☉. It is on the asymptotic giant branch having exhausted its core helium.[12]

The radius of 298 R☉ means that the diameter of R Doradus is 415 million km (2.77 AU). If placed at the centre of the Solar System, the perihelion of Mars would be within the star.

R Doradus has a projected equatorial rotation velocity of 1±0.1 km/s. It is calculated to take 57.5 years to rotate once on its axis.[8]

R Doradus figures almost at the western limit of Dorado, next to Reticulum (map to be held, as standard, upwards, with south horizon in front of viewer). Like Alpha Reticulum, which outshines it, it is a little south of a line between the bright stars Canopus and Achernar, as is the narrow kite-shaped asterism of Reticulum.

References
  1. van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. "GCVS Query=R Dor". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2012-08-22.
  3. Nicolet, B. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
  4. Finzi, A.; Wolf, R. A. (1967). "Type I Supernovae". The Astrophysical Journal. 150: 115. Bibcode:1967ApJ...150..115F. doi:10.1086/149317.
  5. Skrutskie, Michael F.; Cutri, Roc M.; Stiening, Rae; Weinberg, Martin D.; Schneider, Stephen E.; Carpenter, John M.; Beichman, Charles A.; Capps, Richard W.; Chester, Thomas; Elias, Jonathan H.; Huchra, John P.; Liebert, James W.; Lonsdale, Carol J.; Monet, David G.; Price, Stephan; Seitzer, Patrick; Jarrett, Thomas H.; Kirkpatrick, J. Davy; Gizis, John E.; Howard, Elizabeth V.; Evans, Tracey E.; Fowler, John W.; Fullmer, Linda; Hurt, Robert L.; Light, Robert M.; Kopan, Eugene L.; Marsh, Kenneth A.; McCallon, Howard L.; Tam, Robert; Van Dyk, Schuyler D.; Wheelock, Sherry L. (1 February 2006). "The Two Micron All Sky Survey (2MASS)". The Astronomical Journal. 131 (2): 1163–1183. Bibcode:2006AJ....131.1163S. doi:10.1086/498708. ISSN 0004-6256. S2CID 18913331.
  6. Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Washington: Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
  7. Ohnaka, Keiichi; Weigelt, Gerd; Hofmann, Karl-Heinz (2019). "Infrared Interferometric Three-dimensional Diagnosis of the Atmospheric Dynamics of the AGB Star R Dor with VLTI/AMBER". The Astrophysical Journal. 883 (1): 89. arXiv:1908.06997. Bibcode:2019ApJ...883...89O. doi:10.3847/1538-4357/ab3d2a. S2CID 201103617.
  8. Vlemmings, W. H. T.; Khouri, T.; De Beck, E.; Olofsson, H.; García-Segura, G.; Villaver, E.; Baudry, A.; Humphreys, E. M. L.; Maercker, M.; Ramstedt, S. (2018). "Rotation of the asymptotic giant branch star R Doradus". Astronomy and Astrophysics. 613: L4. arXiv:1804.04709. Bibcode:2018A&A...613L...4V. doi:10.1051/0004-6361/201832929. S2CID 52088412.
  9. "* P Dor". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2015-03-05.
  10. Bedding, T. R.; Zijlstra, Albert A.; Jones, A.; Foster, G. (December 1998). "Mode switching in the nearby Mira-like variable R Doradus". Monthly Notices of the Royal Astronomical Society. 301 (4): 1073–1082. arXiv:astro-ph/9808322. Bibcode:1998MNRAS.301.1073B. doi:10.1046/j.1365-8711.1998.02069.x. S2CID 15358667. Retrieved 2 January 2022.
  11. Wing, Robert F. (1971). "The Spectral Type and Infrared Brightness of R Doradus". Publications of the Astronomical Society of the Pacific. 83 (493): 301. Bibcode:1971PASP...83..301W. doi:10.1086/129124.
  12. Tabur, V.; Bedding, T. R.; Kiss, L. L.; Moon, T. T.; Szeidl, B.; Kjeldsen, H. (2009). "Long-term photometry and periods for 261 nearby pulsating M giants". Monthly Notices of the Royal Astronomical Society. 400 (4): 1945. arXiv:0908.3228. Bibcode:2009MNRAS.400.1945T. doi:10.1111/j.1365-2966.2009.15588.x. S2CID 15358380.
  13. Bedding, T. R.; Zijlstra, Albert A.; Jones, A.; Foster, G. (1998). "Mode switching in the nearby Mira-like variable R Doradus". Monthly Notices of the Royal Astronomical Society. 301 (4): 1073–1082. arXiv:astro-ph/9808322. Bibcode:1998MNRAS.301.1073B. doi:10.1046/j.1365-8711.1998.02069.x. S2CID 15358667.
  14. Bedding, T. R.; et al. (April 1997). "The angular diameter of R Doradus: a nearby Mira-like star". Monthly Notices of the Royal Astronomical Society. 286 (4): 957–962. arXiv:astro-ph/9701021. Bibcode:1997MNRAS.286..957B. doi:10.1093/mnras/286.4.957. S2CID 15438522.
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