Date: September 7, 2009
Title: Cataclysmic Variables
Podcasters: Mike Simonsen, Virginia Renehan, and Rebecca Turner of AAVSO
Organization: American Association of Variable Star Observers www.aavso.org
Description: Today we are going to talk about cataclysmic variable stars, CVs for short. Because hundreds of CVs get bright enough to study with even modest telescopes, and due to their unpredictability, they have become the favorite class of stars for many amateur variable star observers to follow.
Bio: The AAVSO is an international non-profit organization whose mission is: to observe and analyze variable stars; to collect and archive observations for worldwide access; to forge strong collaborations between amateur and professional astronomers; and to promote scientific research and education using variable star data.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Mark Duwe.
Transcript:
Cataclysmic Variables
Mike: Hi again, and welcome to the Restless Universe, the podcast of the American Association of Variable Star Observers. You can find us on the web at www.aavso.org.
I’m Mike Simonsen, and with me today are Virginia Renehan and Rebecca Turner.
Today we are going to talk about cataclysmic variable stars, CVs for short. Because hundreds of CVs get bright enough to study with even modest telescopes, and due to their unpredictability, they have become the favorite class of stars for many amateur variable star observers to follow. I have to confess I have been waiting for this episode, because I am a hopeless “CV junkie.” So let’s dive right in and tell everyone exactly what cataclysmic variables are.
Rebecca: Of the roughly four billion stars in our Galaxy, more than half occur in binary or multiple systems. Binary star systems come in many flavors: red stars orbiting blue stars, huge stars orbiting tiny stars, black holes orbiting blue giants, and so on. In a CV binary, one star is a white dwarf; a collapsed star with the mass of the Sun squeezed into the volume of the Earth. The other star is a red dwarf, about the size of our Sun, but redder and less massive. This pair of stars is so close together that they orbit each other in just a few hours.
Virginia: These are semidetached binary stars, where one of the components fills its Roche lobe and the other does not. We talked about this in the last episode about eclipsing binaries.
The red star in a CV is so close to the white dwarf that it becomes tidally distorted, stretched out of shape. Gas is stripped off the red star and falls towards the white dwarf. Because of the conservation of angular momentum, the infalling gas can’t stream directly onto the surface of the white dwarf. Instead it forms a disc, an accretion disc, with the white dwarf at its centre. The accretion disc usually outshines both the red star and the white dwarf in visible light.
Rebecca: If the white dwarf is strongly magnetic, other interesting things happen. We’ll talk about that later.
Mike: CVs with accretion discs come in several flavors. The first to be discovered were the novae. Long ago these stars were thought to be literally new stars. Novae Stella means new stars, and that is why we call them novae today. They drew attention to themselves by their stupendous amplitude of variation — 6 to 19 magnitudes. You can imagine the ancients’ surprise when suddenly a bright star shone where there was none just the night before. The novae with the largest outburst amplitudes fade the fastest, so these are not long-lived visual events. But astronomers can follow the fading back down to pre-outburst levels for years.
Rebecca: Nova outbursts are due to thermonuclear runaways of the hydrogen-rich material that has accreted onto the white dwarf. Most novae known have only been observed to undergo one nova outburst, but several are recurrent novae.
Virginia: Another group of CVs is the dwarf novae. Their outbursts are not quite as spectacular as those of the novae, but the outbursts occur more often. In general, the more frequent the outburst, the smaller the amplitude of outburst. For example, V1159 Ori, has outbursts once every four days with an amplitude of about two magnitudes. On the other end of the scale, WZ Sge goes into outburst only once every thirty years, and the outburst amplitude is 7 or 8 magnitudes.
Rebecca: An interesting subset of the dwarf novae is the SU Ursa Majoris stars. They show two distinct kinds of outbursts, normal outbursts and super-outbursts. Super-outbursts can last 5-10 times longer and are slightly brighter than normal outbursts. Most dwarf novae exhibiting super-outbursts have short orbital periods, less than 2 hours. Some of the SU UMa stars with long outburst intervals, like WZ Sge, show interesting re-brightenings on the way back to quiescence after a super-outburst.
During a super-outburst, a SU UMa star shows an additional subtle modulation in the light curve called a super-hump. This is caused by a wobbling, or precession, of the accretion disc. Super-humps show up in the light curve as a modulation with a period a few percent longer than the orbital period.
Mike: WZ Sge is now considered the prototype for a subset of the SU UMa class. A variety of SU UMa stars in which the interval between super-outbursts is measured in decades, while normal outbursts are few and far between. Other WZ Sge stars include AL Com and EG Cnc, which have super-outburst intervals of approximately 20 years.
WZ Sge stars are the most inactive group of the SU UMa type stars. The factor determining the different timescales between outbursts in dwarf novae appears to be mass-transfer rate. WZ Sge stars have a very low mass-transfer rate, relative to other dwarf novae. Given the slow rate of mass-transfer, it takes decades to accumulate enough material for a super-outburst.
One of the puzzling characteristics of these stars is why they show few or no normal outbursts during this time. Even with a low mass-transfer rate, material should accumulate, drifting into the inner disc, triggering an outburst. But that doesn’t happen, and we don’t know why.
Virginia: Z Cam type stars also show cyclic outbursts, but they differ from normal dwarf novae by the fact that sometimes after an outburst they do not return to their normal quiescent magnitude. Instead, they get stuck about half way down and may stay at that intermediate brightness for days or weeks before eventually falling off to minimum. The outburst amplitudes are generally from 2 to 5 magnitudes in V.
Rebecca: Another group of CVs with accretion discs are the nova-like variables. The difference between the nova-like variables and dwarf novae is that nova-like variables don’t undergo dwarf nova outbursts. This is because the rate of transfer of matter in their discs is, by-and-large, stable. The overall brightness varies only slightly. In addition, the rate of mass-transfer in the discs of nova-like variables is much higher than that in quiescent dwarf novae, so the accretion discs are very bright.
Some nova-like variables show super-humps, like those seen in the SU UMa stars. Unlike the super-outbursting systems, however, these nova-likes have super-humps in their light curves all the time. So they are referred to as permanent super-humpers.
Mike: Magnetic CVs are systems where the white dwarf has an appreciable magnetic field, several tens of millions of Gauss. Because the matter in the accretion stream is partially ionized, it can’t form a disc, because charged particles can’t cross field lines. They can only spiral around them. Instead, the gas is threaded onto the field lines and plunges straight down onto the magnetic poles of the white dwarf. Copious X-ray and extreme ultra-violet emission is liberated at the poles.
In some systems the magnetic field of the white dwarf is strong enough to synchronize the rotation of the white dwarf with that of the red dwarf companion, much like the Moon is locked to the Earth, with the same face towards earth at all times. These are the polars, or AM Herculis stars. The magnetic field in these stars is so strong (it’s about fifty million times the strength of the Earth’s magnetic field,) that no accretion disc can form.
Rebecca: In the intermediate polars, which have lower magnetic fields than the polars, the spin period of the white dwarf is shorter than the orbital period. The accretion process in the intermediate polars is through a disc with a disrupted inner radius, or an accretion stream as in the polars, or both.
Virginia: Well I can see why you love these stars, Mike. White dwarfs and red dwarfs whipping around each other with orbital periods of a few hours to 90 minutes. Gases streaming from the secondary into a disk around the white dwarf, and of course the unpredictable outbursts. Dwarf novae are a lot of fun to watch aren’t they?
Mike: Absolutely! Visual observers can still contribute to science by watching and monitoring these stars, especially the ones that rarely go into outburst, so they can notify astronomers with CCDs and big telescopes when something unusual is happening, sort of like fire spotters in the forest service.
Amateurs with CCDs can take time series data and determine the orbital and super-hump periods of newly discovered CVs and monitor faint CVs for flickering and strange behavior between outbursts.
These are some pretty wild stars in our Restless Universe, and they are in no hurry to give up all their secrets, so there is more to learn. Unfortunately, that is all we have time for today. From Rebecca, Virginia, and me, thank you for tuning in the Restless Universe, and we’ll see you again next month. Good-bye.
End of podcast:
365 Days of Astronomy
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