Star Stories
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How are White Dwarfs Violent?
By Kieran Mynott, your guide to the dark skies of Tenerife.

Today we continue with our violent universe series.  We are going to look at how white dwarfs we spoke about last time are violent.  If you remember we were talking about absorption and Emission lines.  To explain the change in their spectra, we need to think about how matter and radiation interact. If a cloud of gas is tenuous, then any radiation generated by it will easily escape. In this case, you tend to get emission lines in a spectrum. This is called an “optically thin” gas cloud. If, on the other hand, some object out in space is so dense that any photon emitted deep in its interior will be absorbed before it reaches the surface, then the object is called “optically thick”. In this case, it will show a “black body” spectrum, given by the Planck law.

If you have some cool gas in front of a black body, then it will cause absorption lines. The cool gas can just be a cooler part of the same object that is causing the black body radiation – as is the case in stellar atmospheres.

So – we have optically thin gas when the dwarf nova is quiet, which becomes optically thick (but surrounded by cooler gas) during an eruption. MORE CLUES If you look at the spectrum, you can also see the spectrum of a red star combined with that of the gas. The red star and gas show oscillating wavelength shifts, indicating that they are both orbiting around their common centre of mass. The gas cloud must be as heavy as a star to make the star wobble like this. The speeds are around 120 km/s, which allows you to estimate the separation of the star and gas cloud by balancing centrifugal force and gravity.  They are typically only a couple of million kilometres apart – so very close, almost touching! If we repeatedly measure the brightness of these objects, we see that in many cases there are eclipses as the gas cloud moves behind the red star.

If we were to look at a graph showing some features of a typical light curve we could explain some more. The transit is when the gas cloud moves behind the star. The steps in the side of the transit indicate that there are at least two bright point-like objects within the gas cloud. The hump occurs because one of these point-like objects only shines in one direction – acting like a glowing paint on an opaque surface. The flicking indicates that the brightness of this “paint” varies very rapidly, which means it must come from a tiny area. A final clue is that the gas spectra show emission lines with double peaks. During the transit, first one peak weakens then the other. This indicates that the gas is rotating in a disk.

We try to come up with a physical model that can explain all these puzzling observations. The basic idea is that we have a red star and a white dwarf in orbit around their common centre of mass. The red star has come to the end of its life and is swelling up, but its gas is spilling over onto the white dwarf. We can calculate the gravitational potential of a small object moving through a system dominated by two massive objects – this is called the “Roche surface”. Gas flows from the red star through a saddle point, then forms a disk around the white dwarf.

The spinning disk of gas around the white dwarf gets its power from the gravitational potential energy as the gas moves closer to the white dwarf. The power radiated can be calculated from the energy lost.  In practice, however, the inner parts of the disk will be much hotter and will dominate the total emission, producing a spectrum smoothly rising towards shorter wavelengths.

A small fraction of white-dwarfs are magnetic, and in these cases, the accretion disk cannot reach the white dwarf surface. Instead, the infalling gas is threaded onto the magnetic field lines and slides along them down to the surface. The charged particles doing spirals around the magnetic field lines can produce cyclotron radiation.

These dwarf novae are very common. Most stars are binaries, and the more massive star will die first and swell up. At this point, tidal drag often brings the two stars close together. The more massive star then turns into a white dwarf, and at some later stage the less massive star begins to swell up. At that point, you get a dwarf nova as gas from the second star spills over onto the white dwarf. We don’t really know, however, what causes the big outbursts. Most likely it is some form of instability in the accretion disk – matter builds up but cannot move inwards, until some threshold is reached, at which point a huge amount of matter is dumped onto the white dwarf.

Classical novae are explosions much brighter than the dwarf nova explosions we’ve been talking about so far. They too repeat, but often on much longer timescales (technically we have different names for those that have and have not been seen to repeat, but they are really the same thing). Unlike dwarf novae, these classical novae actually blow material out into space. rD Curiously, at least one of the classical novae (Nova Persei 1901) has subsequently turned into a dwarf nova! So classical novae seem to involve binary systems where a red star is transferring mass to a white dwarf companion – just like dwarf novae. In this case, the energy source is fusion, not gravity. As gas falls to the surface of the white dwarf, a thin shell of hydrogen builds up (on top of the carbonoxygen core). Because the gravity of the white dwarf is so immense, the pressure at the base of this thin hydrogen level becomes enormous – and can get up to the same pressure as the middle of our Sun. When this happens, nuclear fusion will begin. Normally, when some gas gets hotter, its pressure rises and so it expands and cools down. This keep the fusion in the middle of our Sun nice and steady – if the fusion rate increases for any reason, the temperature and pressure will increase, which will drop the density of the core of the sun and reduce the fusion rate to balance things out. But the base of the hydrogen layer on the white dwarf is degenerate – supported by quantum mechanics, not heat. So the pressure does not increase as it gets hotter. But as it gets hotter, and the pressure remains the same, the fusion rate will get bigger and bigger, causing an explosion. All sorts of highly radioactive elements are produced, which get convected to the surface where they radiate away their prodigious energy.

Next time we will explore more types of novae and look deeper at the classical novae.  I hope there wasn’t too much maths involved here and I haven’t sent too many of you to sleep.

Until next time this is Kieran signing off.

Any questions please leave a comment or email me at reservations@darkskiestenerifeguide.com

Kieran.

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