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GD 356
GD 356 is a white dwarf in the constellation of Draco showing an unusual emission of circular polarised light. The star is 65 light years from earth.
Other catalog names for this are LP 137-43, EGGR 329 and WD 1639+537.
The class of this white dwarf is DAe meaning that it has a cool helium rich atmosphere. This star exhibits emission lines showing the Zeeman effect in the hydrogen Balmer spectrum. GD 356 belongs to a class of high field magnetic white dwarfs (HFMWD), but it is unique in that the split lines are purely emission lines with no absorption. The emission region appears to be due to a heated upper layer in the photosphere in which the magnetic field is uniform to within 10%. The emission can be produced by an atmosphere at 7500K in a gravity field of 106 ms−2 and a magnetic field of 13 megaGauss. The magnetically split emission lines, Hα and Hβ, are circularly polarised. One explanation is that it is caused by a large electric current flowing between the poles of the star and a highly conducting planet. This planet was not detected in a later, more detailed analysis with new data. Rejecting the idea of an orbiting planet. Other explanations such as being due to Bondi-Hoyle accretion or due to a corona are ruled out by the lack of radio and X-ray emissions. Accretion of gas at a low rate over a broad area of the star, only results in heating at levels high in the atmosphere and not down to the opacity depth of 1.0 as observed with these lines.
The spectrum does not vary over periods of hours or days. This indicates that the rotation axis must match the magnetic dipole axis. The power radiated by the emission lines is 1027 erg s−1. Overall light from the white dwarf varies by 0.2% smoothly over a period of 117 minutes. Explanations given for the variation are a dark spot rotating with the star. This could be near the rotation pole when viewed nearly edge on, or could be on the equator with the pole pointing roughly towards Earth.
The mass of GD 356 is 0.67 M☉ whereas when it was a main sequence star it would have had a mass of 3.25 M☉. In order to reach a temperature of 7510 K it would have become a white dwarf about 1.6 Gya. Prior to this the main sequence lifetime would have been 500 million years giving it a total age of 2.1 billion years. The current magnitude is 15.
The absolute visual magnitude is +13.43±0.16. Proper motion is 0.24" pa, in direction 212°. The trigonometric parallax is 21.1 parsecs. Tangential motion is 25 km−1.
The Hα line splitting is 44.5 nm. In similar white dwarfs an absorption line is expected to be seen instead, so that means the emission has sufficient energy to overpower any absorption. The emission was originally discovered by Jesse L. Greenstein. The original Hα line has a wavelength at 655.2 nm and is called the π component. The blue shifted component σ− has wavelength 633.4 nm and red shifted component line σ+ is at 678.2 nm.
The unipolar-inductor theory says that a high-conduction companion orbits. As it moves through the star's magnetic field, a high voltage is produced between the star facing side of the planet and the dark side. A current then flows along field lines to the point on the star where the field lines meet the star's photosphere, the current is completed through the photosphere heating it up.
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GD 356
GD 356 is a white dwarf in the constellation of Draco showing an unusual emission of circular polarised light. The star is 65 light years from earth.
Other catalog names for this are LP 137-43, EGGR 329 and WD 1639+537.
The class of this white dwarf is DAe meaning that it has a cool helium rich atmosphere. This star exhibits emission lines showing the Zeeman effect in the hydrogen Balmer spectrum. GD 356 belongs to a class of high field magnetic white dwarfs (HFMWD), but it is unique in that the split lines are purely emission lines with no absorption. The emission region appears to be due to a heated upper layer in the photosphere in which the magnetic field is uniform to within 10%. The emission can be produced by an atmosphere at 7500K in a gravity field of 106 ms−2 and a magnetic field of 13 megaGauss. The magnetically split emission lines, Hα and Hβ, are circularly polarised. One explanation is that it is caused by a large electric current flowing between the poles of the star and a highly conducting planet. This planet was not detected in a later, more detailed analysis with new data. Rejecting the idea of an orbiting planet. Other explanations such as being due to Bondi-Hoyle accretion or due to a corona are ruled out by the lack of radio and X-ray emissions. Accretion of gas at a low rate over a broad area of the star, only results in heating at levels high in the atmosphere and not down to the opacity depth of 1.0 as observed with these lines.
The spectrum does not vary over periods of hours or days. This indicates that the rotation axis must match the magnetic dipole axis. The power radiated by the emission lines is 1027 erg s−1. Overall light from the white dwarf varies by 0.2% smoothly over a period of 117 minutes. Explanations given for the variation are a dark spot rotating with the star. This could be near the rotation pole when viewed nearly edge on, or could be on the equator with the pole pointing roughly towards Earth.
The mass of GD 356 is 0.67 M☉ whereas when it was a main sequence star it would have had a mass of 3.25 M☉. In order to reach a temperature of 7510 K it would have become a white dwarf about 1.6 Gya. Prior to this the main sequence lifetime would have been 500 million years giving it a total age of 2.1 billion years. The current magnitude is 15.
The absolute visual magnitude is +13.43±0.16. Proper motion is 0.24" pa, in direction 212°. The trigonometric parallax is 21.1 parsecs. Tangential motion is 25 km−1.
The Hα line splitting is 44.5 nm. In similar white dwarfs an absorption line is expected to be seen instead, so that means the emission has sufficient energy to overpower any absorption. The emission was originally discovered by Jesse L. Greenstein. The original Hα line has a wavelength at 655.2 nm and is called the π component. The blue shifted component σ− has wavelength 633.4 nm and red shifted component line σ+ is at 678.2 nm.
The unipolar-inductor theory says that a high-conduction companion orbits. As it moves through the star's magnetic field, a high voltage is produced between the star facing side of the planet and the dark side. A current then flows along field lines to the point on the star where the field lines meet the star's photosphere, the current is completed through the photosphere heating it up.