Alpha Cygni variable

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Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time other parts expand. They are supergiant stars of spectral types B or A. Variations in brightness on the order of 0.1 magnitudes are associated with the pulsations, which often seem irregular, due to beating of multiple pulsation periods. The pulsations typically have periods of several days to several weeks.

Contents

The prototype of these stars, Deneb (α Cygni), exhibits fluctuations in brightness between magnitudes +1.21 and +1.29. Small amplitude rapid variations have been known in many early supergiant stars, but they were not formally grouped into a class until the 4th edition of the General Catalogue of Variable Stars was published in 1985. It used the acronym ACYG for Alpha Cygni variable stars. [1] Many luminous blue variables (LBVs) show Alpha Cygni-type variability during their quiescent (hot) phases, but the LBV classification is generally used in these cases.

A large number (32) were discovered by Christoffel Waelkens and colleagues analysing Hipparcos data in a 1998 study. [2]

Pulsations

The pulsations of Alpha Cygni Variable stars are not fully understood. They are not confined to a narrow range of temperatures and luminosities in the way that most pulsating stars are. Instead, most luminous A and B supergiants, and possibly also O and F stars, show some type of unpredictable small-scale pulsations. Nonadiabatic strange mode radial pulsations are predicted but only for the most luminous supergiants. Pulsations have also been modelled for less luminous supergiants by assuming they are low mass post-red supergiant stars, but most Alpha Cygni variables do not appear to have passed through the red supergiant stage. [3] [4]

The pulsations are likely induced by kappa mechanism, caused by iron opacity variations, with strange modes producing the observed short periods for both radial and non-radial pulsations. Non-adiabatic g-modes may produce longer period variations, but these have not been observed in Alpha Cygni variables. [5]

List

Galactic

Designation (name)ConstellationDiscoveryMaximum Apparent magnitude (mV) [6] Minimum Apparent magnitude (mV) [6] Period (days)Spectral classLuminosityComment
CE Cam (HD 21389) Camelopardalis Percy & Welsh (1983) [7] 4m.54A0Iab63,000 [8]  
CS Cam Camelopardalis Rufener (1982) [9] 4m.259B9Ia75,900 [10]  
η CMa (Aludra) Canis Major Kazarovets et al. (1999) [11] 2m.382m.484.70433B5Ia105,000 [8]  
ο2 CMa Canis Major Waelkens et al. (1998) [2] 2m.983m.0424.44B3Ia219,000 [12]  
κ Cas Cassiopeia Percy & Welsh (1983) [7] 4m.124m.212.64690B1Ia331,000 [12]  
6 Cas Cassiopeia Abt (1957) [13] 5m.345m.4530B2 Ia+200,000 [14] Hypergiant
ο2 Cen Centaurus 5m.125m.2246.3A2Ia136,000 [15]  
ν Cep Cepheus Percy & Welsh (1983) [7] 4m.254m.35A2 Iab254,000 [8]  
DL Cru Crux Waelkens et al. (1998) [2] 6m.246m.282.8778B1.5Ia242,000 [16]  
Deneb [4] Cygnus Lee (1910) [17] 1m.211m.29A2 Ia196,000 [18] Prototype
σ Cyg Cygnus Abt (1957) [13] 4m.194m.26120.2B9 Iab39,000 [19]  
55 Cyg Cygnus Hill et al. (1976) [20] 4m.814m.87B2.5 Ia~400,000 [21]  
3 Gem Gemini Waelkens et al. (1998) [2] 5m.7513.70B3 Ia200,000 [22]  
ρ Leo Leo Olsen (1974) [23] 3m.833m.93.4271B1Iab295,000 [12]  
β Ori (Rigel) [4] Orion Waelkens et al. (1998) [2] 0m.170m.22B8Ia279,000 [24]

218,000 [25]

Brightest member
ε Ori (Alnilam) Orion Cousins (1960) [26] 1m.641m.74B0.5Iabea275,000 [12]

537,000 [27]

 
χ2 Ori Orion Waelkens et al. (1998) [2] 4m.684m.722.8682B2Ia446,000 [12]  
9 Per Perseus Abt (1957) [13] 5m.155m.25A2 Ia141,000 [28]  

Extragalactic

Designation (name)GalaxyDiscoveryMaximum Apparent magnitude (mV)Minimum Apparent magnitude (mV)Period (days)Spectral classLuminosityComment
LHA 115-S 18 [29] SMC 13m.3ComplexB[e]sg Possible LBV
HDE 268835 [29] LMC 10m.6010m.68>100B8p  
HD 37974 [29] LMC 10m.9211m.00400B0.5Ia+  
HD 37836 [30] LMC 10m.55B0IaeLBV candidate

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References

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  2. 1 2 3 4 5 6 Waelkens, C.; Aerts, C.; Kestens, E.; Grenon, M.; Eyer, L. (1998). "Study of an unbiased sample of B stars observed with Hipparcos: the discovery of a large amount of new slowly pulsating B star". Astronomy and Astrophysics. 330: 215–21. Bibcode:1998A&A...330..215W.
  3. ↑ Saio, H.; Georgy, C.; Meynet, G. (2013). "Strange-Mode Instability for Micro-Variations in Luminous Blue Variables". Progress in Physics of the Sun and Stars: A New Era in Helio- and Asteroseismology. Proceedings of a Fujihara Seminar held 25–29 November. Astronomical Society of the Pacific Conference Series. Vol. 479. p. 47. arXiv: 1305.4728 . Bibcode:2013ASPC..479...47S.
  4. 1 2 3 Saio, H.; Georgy, C.; Meynet, G. (2013). "Evolution of blue supergiants and Cygni variables: puzzling CNO surface abundances". Monthly Notices of the Royal Astronomical Society. 433 (2): 1246–1257. arXiv: 1305.2474 . Bibcode:2013MNRAS.433.1246S. doi: 10.1093/mnras/stt796 .
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  10. ↑ McErlean, N. D.; Lennon, D. J.; Dufton, P. L. (1999). "Galactic B-supergiants: A non-LTE model atmosphere analysis to estimate atmospheric parameters and chemical compositions". Astronomy and Astrophysics. 349: 553. Bibcode:1999A&A...349..553M.
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  12. 1 2 3 4 5 Crowther, P. A.; Lennon, D. J.; Walborn, N. R. (2006). "Physical parameters and wind properties of galactic early B supergiants". Astronomy and Astrophysics. 446 (1): 279–293. arXiv: astro-ph/0509436 . Bibcode:2006A&A...446..279C. doi:10.1051/0004-6361:20053685. S2CID   18815761.
  13. 1 2 3 Abt, Helmut A. (1957). "The Variability of Supergiants". The Astrophysical Journal. 126: 138. Bibcode:1957ApJ...126..138A. doi:10.1086/146379. ISSN   0004-637X.
  14. ↑ Achmad, L.; Lamers, H. J. G. L. M.; Pasquini, L. (1997). "Radiation driven wind models for A, F and G supergiants". Astronomy and Astrophysics. 320: 196. Bibcode:1997A&A...320..196A.
  15. ↑ Kaufer, A.; Stahl, O.; Wolf, B.; Gaeng, T.; Gummersbach, C. A.; Kovacs, J.; Mandel, H.; Szeifert, T. (1996). "Long-term spectroscopic monitoring of BA-type supergiants. I. Halpha_ line-profile variability". Astronomy and Astrophysics. 305: 887–901. Bibcode:1996A&A...305..887K.
  16. ↑ Lamers, Henny J. G. L. M.; Snow, Theodore P.; Lindholm, Douglas M. (1995). "Terminal Velocities and the Bistability of Stellar Winds". The Astrophysical Journal. 455: 269. Bibcode:1995ApJ...455..269L. doi:10.1086/176575. ISSN   0004-637X.
  17. ↑ Lee, O. J. (1910). "Four stars having variable radial velocities". The Astrophysical Journal. 31: 176. Bibcode:1910ApJ....31..176L. doi:10.1086/141741. ISSN   0004-637X.
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  19. ↑ Markova, N.; Puls, J. (2008). "Bright OB stars in the Galaxy". Astronomy and Astrophysics. 478 (3): 823–842. arXiv: 0711.1110 . Bibcode:2008A&A...478..823M. doi:10.1051/0004-6361:20077919. ISSN   0004-6361. S2CID   14510634.
  20. ↑ Kholopov, P. N.; Kukarkina, N. P.; Perova, N. B. (1979). "64th Name-List of Variable Stars". Information Bulletin on Variable Stars. 1581: 1. Bibcode:1979IBVS.1581....1K.
  21. ↑ Kraus, M.; Haucke, M.; Cidale, L. S.; Venero, R. O. J.; Nickeler, D. H.; Németh, P.; Niemczura, E.; Tomić, S.; Aret, A.; Kubát, J.; Kubátová, B.; Oksala, M. E.; Curé, M.; Kamiński, K.; Dimitrov, W.; Fagas, M.; Polińska, M. (2015). "Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198 478". Astronomy & Astrophysics. 581: A75. arXiv: 1507.01846 . Bibcode:2015A&A...581A..75K. doi:10.1051/0004-6361/201425383. S2CID   55392478.
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  23. ↑ Olsen, E. H. (1974). "Variable Stars among the Four-Colour (uvby) Standard Stars". Information Bulletin on Variable Stars. 925: 1. Bibcode:1974IBVS..925....1O.
  24. ↑ Chesneau, O.; Kaufer, A.; Stahl, O.; Colvinter, C.; Spang, A.; Dessart, L.; Prinja, R.; Chini, R. (2014). "The variable stellar wind of Rigel probed at high spatial and spectral resolution". Astronomy & Astrophysics. 566: 18. arXiv: 1405.0907 . Bibcode:2014A&A...566A.125C. doi:10.1051/0004-6361/201322894. S2CID   118404460. A125.
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  27. ↑ Searle, S. C.; Prinja, R. K.; Massa, D.; Ryans, R. (2008). "Quantitative studies of the optical and UV spectra of Galactic early B supergiants. I. Fundamental parameters". Astronomy and Astrophysics. 481 (3): 777–97. arXiv: 0801.4289 . Bibcode:2008A&A...481..777S. doi:10.1051/0004-6361:20077125. S2CID   1552752.
  28. ↑ Verdugo, E.; Talavera, A.; Gómez De Castro, A. I. (1999). "Understanding A-type supergiants. II. Atmospheric parameters and rotational velocities of Galactic A-type supergiants". Astronomy and Astrophysics. 346: 819. Bibcode:1999A&A...346..819V.
  29. 1 2 3 van Genderen, A. M.; Sterken, C. (2002). "Light variations of massive stars (α Cyg variables)". Astronomy and Astrophysics. 386 (3): 926–935. Bibcode:2002A&A...386..926V. doi: 10.1051/0004-6361:20020360 .
  30. ↑ Balona, L A; Handler, G; Chowdhury, S; Ozuyar, D; Engelbrecht, C A; Mirouh, G M; Wade, G A; David-Uraz, A; Cantiello, M (2019-02-27). "Rotational modulation in TESS B stars". Monthly Notices of the Royal Astronomical Society. 485 (3): 3457–3469. arXiv: 1902.09470 . doi: 10.1093/mnras/stz586 . ISSN   0035-8711.

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