VV Cephei

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VV Cephei
Cepheus constellation crop VV Cephei location.png

Location of VV Cephei in Cepheus constellation
Observation data
Epoch J2000       Equinox J2000
Constellation Cepheus
Right ascension 21h 56m 39.14385s [1]
Declination +63° 37′ 32.0174″ [1]
Apparent magnitude  (V)4.91 [2] (4.80 - 5.36 [3] )
Characteristics
U−B color index +0.43 [4]
B−V color index +1.73 [4]
Variable type EA + SRc [3]
A
Evolutionary stage Red supergiant
Spectral type M2 Iab [2] (M1p 0) [5]
U−B color index +2.07 [4]
B−V color index +1.82 [4]
B
Evolutionary stage B-type main-sequence star
Spectral type B0-2 V [2]
U−B color index −0.52 [4]
B−V color index +0.36 [4]
Astrometry
Proper motion (μ)RA: −1.066 [6]   mas/yr
Dec.: −0.425 [6]   mas/yr
Parallax (π)1.0033 ± 0.1073  mas [6]
Distance 3,319 –4900  ly
(1,018+142
−99 [7]  –1500±400 [8]   pc)
Absolute magnitude  (MV)−6.93 [9]
Orbit
Period (P)7,430.5 days [10]
Semi-major axis (a)16.2 ± 3.7 [2] "
(24.8 [8] AU)
Eccentricity (e)0.346 ± 0.01 [10]
Inclination (i)84 [11] °
Semi-amplitude (K1)
(primary)		19.43 ± 0.33 [10] km/s
Semi-amplitude (K2)
(secondary)		19.14 ± 0.68 [10] km/s
Details
A
Mass 2.5 [12] or 18.2 [8]   M☉
Radius 660, [13] 1,050 [11]   R☉
Luminosity 72,880±16,300 [14]   L☉
Surface gravity (log g)–0.26 [15]   cgs
Temperature 3,660 [13]  –3,826 [2]   K
Metallicity [Fe/H]−0.06 [16]   dex
Age 25±0.1 [17]   Myr
B
Mass 8 [12] or 18.6 [8]   M☉
Radius 13 [10] or 25 [18]   R☉
Metallicity [Fe/H]−0.14 [19]   dex
Age 25±0.1 [17]   Myr
Other designations
VV Cep, HR  8383, HIP 108317, HD 208816, BD+62°2007, WDS J21567+6338, 2MASS J21563917+6337319, IRAS 21552+6323, AAVSO 2153+63, Gaia DR3 2216536246703152256, Gaia DR2 2216536246703152256
Database references
SIMBAD data

VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus. It is both a B[e] star and shell star.

Contents

VV Cephei is an eclipsing binary with the third longest known period. A red supergiant fills its Roche lobe when closest to a companion blue star, the latter appearing to be on the main sequence. Matter flows from the red supergiant onto the blue companion for at least part of the orbit and the hot star is obscured by a large disk of material. The supergiant primary, known as VV Cephei A, is currently recognised as one of the largest stars in the galaxy although its size is not certain. Estimates range from 660 R☉ to over 1,000 R☉.

Variability

A visual band light curve for one of the eclipses of VV Cephei, adapted from Hopkins et al. (2015) VVCepLightCurve.png
A visual band light curve for one of the eclipses of VV Cephei, adapted from Hopkins et al. (2015)

The fact that VV Cephei is an eclipsing binary system was discovered by American astronomer Dean McLaughlin in 1936. VV Cephei experiences both primary and secondary eclipses during a 20.3 year orbit. [20] The primary eclipses totally obscure the hot secondary star and last for nearly 18 months. Secondary eclipses are so shallow that they have not been detected photometrically since the secondary obscures such a small proportion of the large cool primary star. [11] The timing and duration of the eclipses is variable, although the exact onset is difficult to measure because it is gradual. Only ε Aurigae (period = 27.08 years), and AS Leonis Minoris (period = 69.1 years) have longer periods. [21] [22]

VV Cephei also shows semiregular variations of a few tenths of a magnitude. Visual and infrared variations appear unrelated to variations at ultraviolet wavelengths. A period of 58 days has been reported in UV, [23] while the dominant period for longer wavelengths is 118.5 days. [24] The short wavelength variations are thought to be caused by the disc around the hot secondary, while pulsation of the red supergiant primary caused the other variations. It has been predicted that the disc surrounding the secondary would produce such brightness variability. [25]

Spectrum

The spectrum of VV Cep can be resolved into two main components, originating from a cool supergiant and a hot small star surrounded by a disk. The material surrounding the hot secondary produces emission lines, including [FeII] forbidden lines, the B[e] phenomenon known from other stars surrounded by circumstellar disks. The hydrogen emission lines are double-peaked, caused by a narrow central absorption component. This is caused by seeing the disk almost edge on where it intercepts continuum radiation from the star. This is characteristic of shell stars. [20]

Forbidden lines, mainly of FeII but also of CuII and NiII, are mostly constant in radial velocity and during eclipses, so they are thought to originate in distant circumbinary material. [26]

The spectrum varies dramatically during the primary eclipses, particularly at the ultraviolet wavelengths produced most strongly by the hot companion and its disc. The typical B spectrum with some emission is replaced by a spectrum dominated by thousands of emission lines as portions of the disc are seen with the continuum from the star blocked. During ingress and egress, the emission line profiles change as one side or the other of the disc close to the star becomes visible while the other is still eclipsed. [11] The colour of the system as a whole is also changed during eclipse, with much of the blue light from the companion blocked. [2]

Out of eclipses, certain spectral lines vary strongly and erratically in both strength and shape, as well as the continuum. Rapid random variations in the short wavelength (i.e. hot) continuum appear to arise from the disc around the B component. Shell absorption lines show variable radial velocities, possibly due to variations in accretion from the disk. Emission from FeII and MgII strengthens around periastron or secondary eclipses, which occur at about the same time, but the emission lines also vary randomly throughout the orbit. [20]

In the optical spectrum, the Hα is the only clear emission feature. Its strength varies randomly and rapidly out of eclipse, but it becomes much weaker and relatively constant during the primary eclipses. [27]

Distance

The distance to VV Cephei is uncertain. The Hipparcos and Gaia Data Release 2 parallax measurements imply distances of 0.752 and 0.6 kiloparsecs respectively, [1] [28] while an analysis by Bailer-Jones et al. using the Gaia Data Release 3 parallax provide a photogeometric distance of 1.02 kpc. [7] These parallaxes are subject to inaccuracies, as changness in VV Cephei A's brightness can cause a measurable astrometric shift and therefore affect the parallax measurement. [29] Another study give a higher distance of 1.5±0.4 kpc based on comparisons of linear and angular orbits. [11] An older analysis from 1977 applied the Barnes-Evans relation to give a distance of 1.28 kpc. [30]

Properties

Mass

It should be possible to calculate the masses of eclipsing binary stars with some accuracy, but in this case mass loss, changes in the orbital parameters, a disk obscuring the hot secondary, and doubt about the distance of the system have led to wildly varying estimates. The traditional model, from the spectroscopically derived orbit, has the masses of both stars around 20  M☉, which is typical for a luminous red supergiant and an early B main sequence star. [10] However, another model has been proposed based on the unexpected timing of the 1997 eclipse. Assuming that the change is due to mass transfer altering the orbit, dramatically lower mass values are required. In this model, the primary is a 2.5 M☉ AGB star and the secondary is an 8 M☉ B star. The spectroscopic radial velocities showing the secondary with equal mass to the primary is explained as being of a portion of the disc rather than the star itself. [12]

Radius

Radius estimates of VV Cephei vary widely. Early estimates of the radius of the primary range from 1,200 to 1,600 solar radii, with an upper limit of 1,900 R☉. [31] The roche lobe is calculated to be about 1,800 R☉, thus the radius cannot be larger than this. [20] Infrared and near-infrared photometry, together with the spectral energy distribution of an M2 supergiant give an angular diameter of 6.38 milliarcseconds. This give a physical radius of either 694 R☉ [lower-alpha 1] or 1,050 R☉, [11] adopting distances of 1,018 and 1,500 parsecs respectively. The angular diameter has later been measured in 2021 at 7.251 mas, giving a radius of 779 R☉ at the adopted Gaia DR3 distance of 1 kpc. However, this measurement might have been affected by the binarity of VV Cephei. [14] A 2024 study give a radius of 660 R☉ adopting the Gaia DR3 distance. [13]

The size of the secondary is even more uncertain, since it is physically and photometrically obscured by a much larger disc several hundred R☉ across. The secondary is certainly much smaller than either the primary or the disc, and has been calculated at 13 R☉ to 25 R☉ from the orbital solution. [10] [18]

Effective temperature

The temperature of the VV Cephei stars is again uncertain, partly because there simply isn't a single temperature that can be assigned to a significantly non-spherical diffuse star orbiting a hot companion. The effective temperature generally quoted for stars is the temperature of a spherical blackbody that approximates the electromagnetic radiation output of the actual star, accounting for emission and absorption in the spectrum. VV Cephei A is fairly clearly identified as an M2 supergiant, and as such, it is given a temperature around 3,800 K. The secondary star is heavily obscured by a disk of material from the primary, and its spectrum is almost undetectable against the disc emission. Detection of some ultraviolet absorption lines narrow down the spectral type to early B and it is apparently a main-sequence star, but likely to be abnormal in several respects due to mass transfer from the supergiant. [32]

VV Cephei A has some emission lines, but these are produced from the accretion disc around the hot secondary. [33]

See also

Notes

  1. ↑ Calculated using R = 107.5 •𝜃 •D, where 𝜃 is the angular diameter in arcseconds, and D the distance in parsecs.

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References

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  2. 1 2 3 4 5 6 7 Hopkins, Jeffrey L.; Bennett, Philip D.; Pollmann, Ernst (2015). "VV Cephei Eclipse Campaign 2017/19". The Society for Astronomical Sciences 34th Annual Symposium on Telescope Science. Published by Society for Astronomical Sciences. 34: 83. Bibcode:2015SASS...34...83H.
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  5. ↑ Keenan, Philip C. (1942-05-01). "Luminosities of the M-Type Variables of Small Range". The Astrophysical Journal. 95: 461. Bibcode:1942ApJ....95..461K. doi:10.1086/144418. ISSN   0004-637X.
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  7. 1 2 Bailer-Jones, C. A. L.; Rybizki, J.; Fouesneau, M.; Demleitner, M.; Andrae, R. (2021-03-01). "Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3". The Astronomical Journal. 161 (3): 147. arXiv: 2012.05220 . Bibcode:2021AJ....161..147B. doi: 10.3847/1538-3881/abd806 . ISSN   0004-6256. Data about this star can be seen here.
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  9. ↑ Ginestet, N.; Carquillat, J. M. (2002). "Spectral Classification of the Hot Components of a Large Sample of Stars with Composite Spectra, and Implication for the Absolute Magnitudes of the Cool Supergiant Components". The Astrophysical Journal Supplement Series. 143 (2): 513. Bibcode:2002ApJS..143..513G. doi: 10.1086/342942 .
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  11. 1 2 3 4 5 6 Bauer, W. H.; Gull, T. R.; Bennett, P. D. (2008). "Spatial Extension in the Ultraviolet Spectrum of Vv Cephei". The Astronomical Journal. 136 (3): 1312. Bibcode:2008AJ....136.1312H. doi: 10.1088/0004-6256/136/3/1312 . S2CID   119404901.
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  13. 1 2 3 Healy, Sarah; Horiuchi, Shunsaku; Molla, Marta Colomer; Milisavljevic, Dan; Tseng, Jeff; Bergin, Faith; Weil, Kathryn; Tanaka, Masaomi (2024-03-23). "Red Supergiant Candidates for Multimessenger Monitoring of the Next Galactic Supernova". Monthly Notices of the Royal Astronomical Society. 529 (4): 3630–3650. arXiv: 2307.08785 . Bibcode:2024MNRAS.529.3630H. doi: 10.1093/mnras/stae738 . ISSN   0035-8711.
  14. 1 2 Baines, Ellyn K.; Thomas Armstrong, J.; Clark, James H.; Gorney, Jim; Hutter, Donald J.; Jorgensen, Anders M.; Kyte, Casey; Mozurkewich, David; Nisley, Ishara; Sanborn, Jason; Schmitt, Henrique R. (November 2021). "Angular Diameters and Fundamental Parameters of Forty-four Stars from the Navy Precision Optical Interferometer". The Astronomical Journal. 162 (5): 198. arXiv: 2211.09030 . Bibcode:2021AJ....162..198B. doi: 10.3847/1538-3881/ac2431 . ISSN   0004-6256. S2CID   238998021.
  15. ↑ Ayres, Thomas (2023-05-01). "In the Trenches of the Solar-Stellar Connection. VII. Wilson-Bappu 2022". The Astrophysical Journal Supplement Series. 266 (1): 6. Bibcode:2023ApJS..266....6A. doi: 10.3847/1538-4365/acb535 . ISSN   0067-0049. VV Cephei's databse entry at VizieR.
  16. ↑ Carr, John S.; Sellgren, K.; Balachandran, Suchitra C. (2000). "The First Stellar Abundance Measurements in the Galactic Center: The M Supergiant IRS 7". The Astrophysical Journal. 530 (1): 307–322. arXiv: astro-ph/9909037 . Bibcode:2000ApJ...530..307C. doi:10.1086/308340. S2CID   12036617.
  17. 1 2 Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190–200. arXiv: 1007.4883 . Bibcode:2011MNRAS.410..190T. doi: 10.1111/j.1365-2966.2010.17434.x . S2CID   118629873. VV Cephei's database entry at VizieR.
  18. 1 2 Hack, M.; Engin, S.; Yilmaz, N.; Sedmak, G.; Rusconi, L.; Boehm, C. (1992). "Spectroscopic study of the atmospheric eclipsing binary VV Cephei". Astronomy and Astrophysics Supplement Series. 95: 589. Bibcode:1992A&AS...95..589H.
  19. ↑ Ramírez, Solange V.; Sellgren, K.; Carr, John S.; Balachandran, Suchitra C.; Blum, Robert; Terndrup, Donald M.; Steed, Adam (2000). "Stellar Iron Abundances at the Galactic Center". The Astrophysical Journal. 537 (1): 205–220. arXiv: astro-ph/0002062 . Bibcode:2000ApJ...537..205R. doi:10.1086/309022. S2CID   14713550.
  20. 1 2 3 4 Bauer, Wendy Hagen; Bennett, Philip D. (2000). "The Ultraviolet Spectrum of VV Cephei Out of Eclipse". The Publications of the Astronomical Society of the Pacific. 112 (767): 31. Bibcode:2000PASP..112...31B. doi: 10.1086/316479 .
  21. ↑ "eps Aur". The International Variable Star Index. AAVSO. Retrieved 6 December 2021.
  22. ↑ Rodriguez, Joseph E.; Stassun, Keivan G.; Lund, Michael B.; Siverd, Robert J.; Pepper, Joshua; Tang, Sumin; Kafka, Stella; Gaudi, B. Scott; Conroy, Kyle E.; Beatty, Thomas G.; Stevens, Daniel J.; Shappee, Benjamin J. (May 2016). "An extreme analogue of ε Aurigae: an M-giant eclipsed every 69 years by a large opaque disk surrounding a small hot source". The Astronomical Journal. 151 (5): 123. arXiv: 1601.00135 . Bibcode:2016AJ....151..123R. doi: 10.3847/0004-6256/151/5/123 . S2CID   24349954.
  23. ↑ Baldinelli, L.; Ghedini, S.; Marmi, S. (1979). "Semiregular 58 Days Variation in VV Cep". Information Bulletin on Variable Stars. 1675: 1. Bibcode:1979IBVS.1675....1B.
  24. ↑ McCook, G. P; Guinan, E. F (1978). "118 Day Optical Variations in VV Cep". Information Bulletin on Variable Stars. 1385: 1. Bibcode:1978IBVS.1385....1M.
  25. ↑ Hutchings, J. B.; Wright, K. O. (1971). "Rotationally extended stellar envelopes - III. The Be component of VV Cephei". Monthly Notices of the Royal Astronomical Society. 155 (2): 203. Bibcode:1971MNRAS.155..203H. doi: 10.1093/mnras/155.2.203 .
  26. ↑ Kawabata, Shusaku; Saito, Mamoru (1997). "Expanding Atmosphere of the M-Type Supergiant in VV Cephei". Publications of the Astronomical Society of Japan. 49: 101–107. Bibcode:1997PASJ...49..101K. doi: 10.1093/pasj/49.1.101 .
  27. ↑ Pollmann, E.; Bennett, P. D.; Hopkins, J. L. (2016). "The Long-term Binary System VV Cep". Information Bulletin on Variable Stars. 6156: 1. Bibcode:2016IBVS.6156....1P.
  28. ↑ 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 .
  29. ↑ McDonald, I.; Zijlstra, A. A.; Boyer, M. L. (2012-11-01). "Fundamental parameters and infrared excesses of Hipparcos stars". Monthly Notices of the Royal Astronomical Society. 427 (1): 343–357. arXiv: 1208.2037 . Bibcode:2012MNRAS.427..343M. doi: 10.1111/j.1365-2966.2012.21873.x . ISSN   0035-8711.
  30. ↑ Lacy, C. H. (1977-04-01). "Distances to eclipsing binaries: an application of the Barnes-Evans relation". The Astrophysical Journal. 213: 458–463. Bibcode:1977ApJ...213..458L. doi:10.1086/155176. ISSN   0004-637X.
  31. ↑ Table 4 in Levesque, Emily M.; Massey, Philip; Olsen, K. A. G.; Plez, Bertrand; Josselin, Eric; Maeder, Andre; Meynet, Georges (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal. 628 (2): 973–985. arXiv: astro-ph/0504337 . Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID   15109583.
  32. ↑ Bauer, W. H.; Stencel, R. E.; Neff, D. H. (1991). "Twelve years of IUE spectra of the interacting binary VV Cephei". Astronomy and Astrophysics Supplement Series. 90: 175. Bibcode:1991A&AS...90..175B.
  33. ↑ Habets, G. M. H. J.; Heintz, J. R. W. (1981). "Empirical bolometric corrections for the main-sequence". Astronomy and Astrophysics Supplement Series . 46: 193–237. Bibcode:1981A&AS...46..193H.
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