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Original URL: http://arstechnica.com/science/2016/07/white-dwarf-bombards-its-companion-with-relativistic-electrons/
White dwarf bombards its companion with relativistic electrons
When observing AR Scorpii, researchers noticed that its brightness varied over a 3.5 hour period. So they labelled it a periodic variable and paid it no further attention. Now, however, a large international team of astronomers has gone back and taken a more careful look at the star. The astronomers found that AR Scorpii is much more variable than first thought, with 400 percent changes in brightness occurring within only 30 seconds. The reason for this? AR Scorpii is actually two stars, and one of them is launching relativistic electrons at the other.
The paper describing these results was published this week in Nature .
The researchers were drawn to AR Scorpii because of seven years of archival images that revealed a lot of additional variability layered on top of its well-described 3.5 hour period. Rather than peaking at a similar level each time, the output could vary by as much as a factor of four.
This caused the astronomers to look at the M-class star's output more carefully. They found that the light alternated being red- and blue-shifted over the same 3.5 hour period that its brightness varies. This typically means that the star is being pulled around by something orbiting it, which causes it to accelerate towards and away from Earth, accounting for the Doppler shift. "The 3.56 hour period is therefore the orbital period of a close binary star," the authors conclude.
Based on the strength of the red- and blue-shifts, that companion must be roughly a third of the mass of the Sun, which places it squarely in white dwarf territory.
Since the changes in brightness line up so nicely with the orbit, the authors looked more carefully at those, too. And once again, something strange was up. We know the sorts of radiation that M dwarfs and white dwarfs produce, but AR Scorpii produces more than that. "In the infrared and radio in particular," the authors write, "is orders of magnitudes brighter than the thermal emission from its component stars." While the combined luminosities of the two stars should be about 1024 Watts, the maximum luminosity of the system is over 1025 Watts.
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(Score: 2) by stormwyrm on Tuesday August 02 2016, @03:08AM
No, white dwarf stars aren't usually formed by violent explosions, the way neutron stars are. A star's gravity is trying to make it collapse into itself, but as it collapses under gravity this also causes nuclear fusion to happen in its core, and that produces an outward radiation pressure that tries to make the star expand. Throughout its lifetime a star balances gravity and radiation pressure from fusion, until eventually the nuclear fuel runs out and the radiation pressure it produces stops. Then gravity causes what's left of the star to collapse in on itself. White dwarves are generally created from stars around the mass of our sun, where the gravity isn't strong enough to make fusion reactions that can fuse carbon, and when the star's core is mostly carbon, the core shrinks until it is stopped by electron degeneracy pressure, producing a white dwarf. This usually doesn't happen rapidly enough that an explosion occurs, as might happen to a more massive star, and all that happens is the core shrinks slowly and the outer layers of the dying star drift away relatively quietly to form a planetary nebula.
The star system in TFA probably was in the distant past two normal stars, one slightly heavier than the other. The heavier one transitioned into the white dwarf phase first.
Numquam ponenda est pluralitas sine necessitate.