Alpha Cygni variable explained

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.

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.

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.^[2]

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.^[3]

List

Galactic

Designation (name)	Constellation	Discovery	Maximum Apparent magnitude (mV)	Minimum Apparent magnitude (mV)^[4]	Period (days)	Spectral class	Luminosity	Comment
CE Cam (HD 21389)	Camelopardalis	Percy & Welsh (1983)	4^m.54			A0Iab	63,000
CS Cam	Camelopardalis	Rufener (1982)^[5]	4^m.259			B9Ia	75,900^[6]
η CMa (Aludra)	Canis Major	Kazarovets et al. (1999)^[7]	2^m.38	2^m.48	4.70433	B5Ia	105,000
ο^2 CMa	Canis Major	Waelkens et al. (1998)	2^m.98	3^m.04	24.44	B3Ia	219,000
κ Cas	Cassiopeia	Percy & Welsh (1983)	4^m.12	4^m.21	2.64690	B1Ia	331,000
6 Cas	Cassiopeia	Abt (1957)^[8]	5^m.34	5^m.45	30	B2 Ia^+	200,000^[9]	Hypergiant
ο^2 Cen	Centaurus		5^m.12	5^m.22	46.3	A2Ia	136,000^[10]
ν Cep	Cepheus	Percy & Welsh (1983)^[11]	4^m.25	4^m.35		A2 Iab	254,000
DL Cru	Crux	Waelkens et al. (1998)	6^m.24	6^m.28	2.8778	B1.5Ia	242,000^[12]
Deneb^[13]	Cygnus	Lee (1910)^[14]	1^m.21	1^m.29		A2 Ia	196,000^[15]	Prototype
σ Cyg	Cygnus	Abt (1957)	4^m.19	4^m.26	120.2	B9 Iab	39,000^[16]
55 Cyg	Cygnus	Hill et al. (1976)^[17]	4^m.81	4^m.87		B2.5 Ia	~400,000^[18]
3 Gem	Gemini	Waelkens et al. (1998)	5^m.75		13.70	B3 Ia	200,000^[19]
ρ Leo	Leo	Olsen (1974)^[20]	3^m.83	3^m.9	3.4271	B1Iab	295,000
β Ori (Rigel)	Orion	Waelkens et al. (1998)^[21]	0^m.17	0^m.22		B8Ia	279,000 218,000	Brightest member
ε Ori (Alnilam)	Orion	Cousins (1960)^[22]	1^m.64	1^m.74		B0.5Iabea	275,000 537,000^[23]
χ^2 Ori	Orion	Waelkens et al. (1998)	4^m.68	4^m.72	2.8682	B2Ia	446,000^[24]
9 Per	Perseus	Abt (1957)	5^m.15	5^m.25		A2 Ia	141,000^[25]

Extragalactic

Designation (name)	Galaxy	Discovery	Maximum Apparent magnitude (mV)	Minimum Apparent magnitude (mV)	Period (days)	Spectral class	Luminosity	Comment
LHA 115-S 18	SMC		13^m.3		Complex	B[e]sg		Possible LBV
HDE 268835	LMC		10^m.60	10^m.68	>100	B8p
HD 37974	LMC		10^m.92	11^m.00	400	B0.5Ia^+
HD 37836^[26]	LMC		10^m.55			B0Iae		LBV candidate

Sources

• Samus N.N., Durlevich O.V., et al. Combined General Catalog of Variable Stars (GCVS4.2, 2004 Ed.)

Notes and References

1. 1996yCat.2139....0K. VizieR Online Data Catalog: General Catalog of Variable Stars, 4th Ed. (GCVS4) (Kholopov+ 1988). VizieR On-Line Data Catalog: II/139B. Originally Published in: Moscow: Nauka Publishing House (1985–1988). 2139. 0. Kholopov. P. N.. Samus'. N. N.. Frolov. M. S.. Goranskij. V. P.. Gorynya. N. A.. Kireeva. N. N.. Kukarkina. N. P.. Kurochkin. N. E.. Medvedeva. G. I.. Perova. N. B.. 1996.
2. Book: 2013ASPC..479...47S. 1305.4728 . 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. 479 . 47 . Saio . H. . Georgy . C. . Meynet . G. . 2013 .
3. Saio. Hideyuki. Linear analyses for the stability of radial and non-radial oscillations of massive stars. Monthly Notices of the Royal Astronomical Society. 412. 3. 2011. 1814–1822. 0035-8711. 10.1111/j.1365-2966.2010.18019.x. 1011.4729 . 2011MNRAS.412.1814S . 118552910.
4. 2009yCat....102025S. 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. 02025. Samus. N. N.. Durlevich. O. V.. 2009. etal.
5. 1982A&AS...48..503R. List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry. Astronomy and Astrophysics Supplement Series. 48. 503. Rufener. F.. Bartholdi. P.. 1982.
6. 1999A&A...349..553M. Galactic B-supergiants: A non-LTE model atmosphere analysis to estimate atmospheric parameters and chemical compositions. Astronomy and Astrophysics. 349. 553. McErlean. N. D.. Lennon. D. J.. Dufton. P. L.. 1999.
7. 1999IBVS.4659....1K. The 74th Special Name-list of Variable Stars. Information Bulletin on Variable Stars. 4659. 1. Kazarovets. E. V.. Samus. N. N.. Durlevich. O. V.. Frolov. M. S.. Antipin. S. V.. Kireeva. N. N.. Pastukhova. E. N.. 1999.
8. Abt. Helmut A.. The Variability of Supergiants.. The Astrophysical Journal. 126. 1957. 138. 0004-637X. 10.1086/146379. 1957ApJ...126..138A .
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11. Percy. J. R.. Welch. D. L.. Photometric variability of B- and A-type supergiants. Publications of the Astronomical Society of the Pacific. 95. 1983. 491. 0004-6280. 10.1086/131198. 1983PASP...95..491P . free.
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17. 1979IBVS.1581....1K. 64th Name-List of Variable Stars. Information Bulletin on Variable Stars. 1581. 1. Kholopov. P. N.. Kukarkina. N. P.. Perova. N. B.. 1979.
18. 2015A&A...581A..75K. 1507.01846. Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198 478. Astronomy & Astrophysics. 581. A75. 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. 10.1051/0004-6361/201425383. 55392478.
19. 1999A&A...350..970K. astro-ph/9910449 . The wind momentum-luminosity relationship of galactic A- and B-supergiants . Astronomy and Astrophysics . 350 . 970 . Kudritzki . R. P. . Puls . J. . Lennon . D. J. . Venn . K. A. . Reetz . J. . Najarro . F. . McCarthy . J. K. . Herrero . A. . 1999 .
20. 1974IBVS..925....1O. Variable Stars among the Four-Colour (uvby) Standard Stars. Information Bulletin on Variable Stars. 925. 1. Olsen. E. H.. 1974.
21. 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. 1998A&A...330..215W.
22. 1960MNSSA..19...56C. New Bright Variable Stars. Monthly Notes of the Astronomical Society of Southern Africa. 19. 56. Cousins. A. W. J.. 1960.
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24. Crowther . P. A. . Lennon . D. J. . Walborn . N. R. . Physical parameters and wind properties of galactic early B supergiants . 10.1051/0004-6361:20053685 . Astronomy and Astrophysics . 446 . 1 . 279–293 . 2006 . astro-ph/0509436 . 2006A&A...446..279C . 18815761 .
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