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VV Cephei
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VV Cephei
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. As a 5th magnitude star, it is visible to the naked eye under good observing conditions.
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☉.
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. 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. 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.
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, while the dominant period for longer wavelengths is 118.5 days. 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.
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.
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.
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. The colour of the system as a whole is also changed during eclipse, with much of the blue light from the companion blocked.
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.
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VV Cephei
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. As a 5th magnitude star, it is visible to the naked eye under good observing conditions.
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☉.
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. 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. 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.
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, while the dominant period for longer wavelengths is 118.5 days. 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.
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.
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.
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. The colour of the system as a whole is also changed during eclipse, with much of the blue light from the companion blocked.
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.
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