AG Pegasi

AG Pegasi is a symbiotic binary star in the constellation Pegasus. It is a close binary composed of a red giant and white dwarf, estimated to be around 2.5 and 0.6 times the mass of the Sun respectively. It is classified as a symbiotic nova; it has undergone one extremely slow nova outburst and a smaller outburst.

AG Pegasi

A visual band light curve for AG Pegasi. The inset plot shows the 2015 flare with an expanded time scale. Adapted Skopal from et al. (2012)[1] and Skopal et al. (2017)[2]
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension 21h 51m 01.97340s[3]
Declination +12° 37′ 32.1240″[3]
Apparent magnitude (V) 6.0 - 9.4[4]
Characteristics
Spectral type var + M3III[5]
B−V color index 1.158±0.031[6]
Variable type Symbiotic nova[2]
Astrometry
Radial velocity (Rv)−15.86±0.15[6] km/s
Proper motion (μ) RA: −0.890±0.084[3] mas/yr
Dec.: −1.461±0.091[3] mas/yr
Parallax (π)0.3803 ± 0.0820 mas[3]
Distanceapprox. 9,000 ly
(approx. 2,600 pc)
Absolute magnitude (MV)/-1.0[7]
Details
AG Peghot
Mass0.6[7] M☉
Radius0.08-16[8] R☉
Luminosity400-3,700 L☉
Surface gravity (log g)6.0[9] cgs
Temperature10,000-100,000[7] K
AG Peggiant
Mass2.5[7] M☉
Radius85[7] R☉
Luminosity1,150[7] L☉
Temperature3,650[7] K
Other designations
BD+11°4673, HD 207757, HIP 107848, SAO 107436[10]
Database references
SIMBADdata

Initially a magnitude 9 star, AG Pegasi brightened and peaked at an apparent magnitude of 6.0 around 1885 before gradually fading to magnitude 9 in the late 20th century. Its spectrum was noted by earlier observers to resemble P Cygni.[11] The spectrum of the hotter star has changed drastically over 160 years, leading investigators Scott Kenyon and colleagues to surmise that its hotter component, originally a white dwarf, accumulated enough material from the donor giant star to begin burning hydrogen and enlarge and brighten into an A-type white supergiant around 1850. It had this spectrum and an estimated surface temperature of around 10000 K in 1900,[7] with a likely radius 16 times that of the Sun,[8] before becoming a B-class star in 1920, then an O-class star in 1940, and finally a Wolf-Rayet star in 1970,[7] with a surface temperature of 95000 K since 1978. It has shrunk to star with a diameter 1.1 times that of the Sun in 1949, then 0.15 times in 1978 and 0.08 times that of the Sun in 1990.[8] AG Pegasi has been described as the slowest nova ever recorded,[7] with a constant bolometric luminosity of the hotter star over 130 years from 1850 to 1980. By the late 20th century, the hotter star has evolved into a hot subdwarf on its way to eventually returning to white dwarf status.[5]

Vogel and colleagues calculated the hotter star must have been accreting material from the red giant for around 5000 years before erupting. Both stars are ejecting material in stellar winds.[7] The resulting nebula contains material from both stars and is complex in nature.[9]

From 1997 until 2015, AG Pegasi entered a quiescent phase with no further change to its brightness. Then the hot component increased in temperature, which caused the nebulosity around the stars to become more ionised and increase in brightness. The combination of the extremely slow nova and smaller outburst means that AG Pegasi is classed as a symbiotic nova.[2]

References
  1. Skopal, A.; Shugarov, S.; Vanko, M.; Dubovsky, P.; Peneva, S. P.; Semkov, E.; Wolf, M. (April 2012). "Recent photometry of symbiotic stars". Astronomische Nachrichten. 333 (3): 242–255. arXiv:1203.4932. Bibcode:2012AN....333..242S. doi:10.1002/asna.201111655.
  2. Skopal, A; Shugarov, S. Yu; Sekeráš, M; Wolf, M; Tarasova, T. N; Teyssier, F; Fujii, M; Guarro, J; Garde, O; Graham, K; Lester, T; Bouttard, V; Lemoult, T; Sollecchia, U; Montier, J; Boyd, D (2017). "New outburst of the symbiotic nova AG Pegasi after 165 yr". Astronomy & Astrophysics. 604: A48. arXiv:1705.00076. Bibcode:2017A&A...604A..48S. doi:10.1051/0004-6361/201629593. S2CID 55311688.
  3. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  4. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
  5. Kenyon, S. J.; Proga, D.; Keyes, C. D. (2001). "The Continuing Slow Decline of AG Pegasi". The Astronomical Journal. 122 (1): 349–359. arXiv:astro-ph/0103426. Bibcode:2001AJ....122..349K. doi:10.1086/321107. S2CID 18188047.
  6. Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644.
  7. Kenyon, S. J.; Mikolajewska, J.; Mikolajewski, M.; Polidan, R. S.; Slovak, M. H. (1993). "Evolution of the symbiotic binary system AG Pegasi - the slowest classical nova eruption ever recorded". The Astronomical Journal. 106: 1573–98. Bibcode:1993AJ....106.1573K. doi:10.1086/116749.
  8. Vogel, M.; Nussbaumer, H. (1994). "The hot wind in the symbiotic nova AG Pegasi". Astronomy and Astrophysics. 282 (1): 145–55. Bibcode:1994A&A...284..145V.
  9. Lü, G.; Zhu, C.; Han, Z.; Wang, Z. (2008). "Chemical Abundances in Symbiotic Stars". The Astrophysical Journal. 683 (2): 990–1005. arXiv:0805.0832. Bibcode:2008ApJ...683..990L. doi:10.1086/589876. S2CID 1651798.
  10. "AG Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-12-02.
  11. Boyarchuk, A.A. (1967). "The Nature of AG Pegasi" (PDF). Soviet Astronomy. 11 (1): 8–15. Bibcode:1967SvA....11....8B.
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