Sunday, January 23, 2011

It Is Never a Dull Moment With Cataclysmic Variables

I am in the middle of getting a presentation together for Cataclysmic Variables (CV) and thought you might like a crash course in it as well. Cataclysmic variables are binary systems that consist of an normal star and a white dwarf. They are typically small. The  typical binary system is roughly the size of the Earth-Moon system - with an orbital period in the range 1-10 hrs. The companion star, a more or less normal star like our Sun, loses material onto the white dwarf by accretion. This part is kind of interesting so I will give you some background on the setup. The white dwarf  was at one time a regular schlep of a star. For few billion years or so it cooked hydrogen into helium by fusion. Sadly the star eventually ran out of hydrogen so what do you do? you start fusing helium together until you wind up with a mostly carbon rich star that has ceased fusion and is just hanging out cooling down for the next billion years. While all of this was happening the star went through the red giant phase etc. So there it sits a sun like mass squished down into the size of the earth.This makes for a very dense and more to the point, a huge gravity well. Ah but what if there was a close by companion star? A regular star in most ways except for the fact that it is in an orbit with the earlier mentioned white dwarf? This scenario has the makings for a CV

There are several types of CVs out there and all of them have different ways of doing their business. Let's break CVs down into some bite sized chunks. For starters there are two major types of CVs. One has a fusion dominated  phase. The second has an accretion disk dominated phase.

Fusion Dominated  includes 
1.Classical Novae
2.Super Soft Sources (SSS)

 Classical Novae
A classical nova occurs when material (drawn from the companion star) accretes onto the surface of a white dwarf star's surface and thus begins an unstable thermonuclear fusion reaction. Material transfer will gradually occur between novae explosions  The mass will be accreted onto the white dwarf's surface at a low rate, generating only weak X-ray emission. The nova outburst will increase the apparent brightness of the binary star system by ten thousand to a million times (an increase in stellar magnitude of 10 to 15). Can I get a Wow! there? The reason is  fairly simple  in that the slow build up of material causes the bottom layer to compress and finally is set off on the fusion path. At this point all of the material is fused at the same moment releasing a massive explosion of light /energy  They are thought to recur, though with recurrence time scales of 10,000 years, We will just have to wait and see if it happens again. No known  classical novae recurrence have been observed.

Super Soft Sources
Super Soft Sources (or SSSs) are the new kids on the block in the family of cataclysmic variables. This CV was first categorized by ROSAT observations.  SSSs are objects with temperatures of between 200,000 and 800,000 K and luminosities around 1038 ergs/s. More than 90% of their observed X-ray emission is below 0.5 keV. The leading theory is  SSSs are white dwarfs with classic hydrogen fusion occurring from material accreting onto their surfaces. This could make SSSs the progenitor for Type Ia supernovae.
In the white dwarf scenario, the observed black body radii is simply the size of the white dwarf star itself with nuclear fusion occurring on its surface. If accretion occurs onto the white dwarf surface at low rates, fusion will be sporadic and violent, just like classical novae type explosions. If accretion is at a high rate, the white dwarf will acquire a red-giant-like atmosphere. Continuous nuclear fusion on the dwarf star surface would be possible only for a narrow range of accretion rates of the order of 10-7 solar masses per year. For relatively massive white dwarfs 0.7 - 1.2 solar masses, this raises the distinct possibility that such objects could eventually exceed the Chandrasekhar limit,  making them the candidates for Type Ia supernovae. White dwarfs have a limit to how big they can be  If they get larger than 1.4 solar masses they will inevitably supernova and collapse into a neutron star. That is what the Chandrasekhar limit is. That takes care of  the fusion dominated side of CV's. Now let's take a look at accretion dominated CVs.

Accretion dominated CVs can be chopped up into three main categories. all of which are fascinating in their workings. They are:

1.Dwarf Novae
3.Intermediate Polars

Dwarf Novae

Dwarf novae (DN) outbursts are dimmer events than classical novae outbursts. Their peak absolute magnitudes are weaker by at least 100 times. DN  are known to recur with times as short as a few weeks. DN also have short durations, lasting just a few days. Interestingly, dwarf novae also can exhibit a variety of unusual behaviors. Some occasionally exhibit long outbursts known as superoutbursts


In a polar system  matter will overflow the Roche lobe of the companion star. However, the white dwarf possesses a strong magnetic field, which prevents the formation of a accretion disk. Instead, the overflowing material is directed by the magnetic field structure until it impacts on the surface of the white dwarf at its magnetic pole. Until impact, the material essentially free falls, thus reaching substantial velocities which are seen in the optical spectra. The collision generates a shock wave which is the source of hard (energetic) X-rays. Hard X-rays emitted in the direction of the white dwarf from the shock wave heat the local area around the pole enough for the pole to become a source of intense soft (less energetic) X-rays.  Polars are generally much stronger sources of soft X-rays than hard X-rays. This is probably due to uneven matter streaming. knots in the accretion flows would most likely cause energy to also be let go deep within the atmosphere of the white dwarf. This results in more soft X-ray emission. The strong magnetic field will also tidally lock the orientation of the white dwarf relative to the companion, so that orbital and rotational periods are identical.
X-ray emission from polar systems is entirely due to the accretion column and its impact, so in quiescent times when matter is not accreting onto the system, the entire system is much dimmer.

Intermediate Polars

When a Polar has a wide separation between companions or does not have an outstanding magnetic field  but still a very good one  an accretion disk can and will form. When this action is found  The moniker of Intermediate Polar is bestowed on the pair As material migrates inwards in the disk, it may eventually encounter a magnetic field strong enough to control the flow of material, at which point matter would instead stream from the inner edge of the disk along magnetic field lines onto the pole of the white dwarf. Such a system would therefore be expected to be a source of hard X-rays from the shock at the magnetic poles. Intermediate polars, either due to weaker magnetic fields or wider star separations, will not necessarily have orbital and spin rates locked. Observed systems have longer orbital periods than polars, which given that the systems have comparable masses verifies their wider separation.

Now if that wet your whistle for the wild world of variable stars then hop on over to the American Association of Variable Star Observers here and get a handle on variable stars  You could even discover one! They happen all the time

Until next time
Keep looking up!

Steve T

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