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The Webfooted Astronomer - November 2000


Minutes: Cataclysmic Variables

by Dave Irizarry

GERALD West announced the annual awards banquet will be held on Jan. 13, 2001, at the Yankee Grill and Roaster. The 2001 Table Mountain Star Party will be held from July 19–July 21. Shelly Rosen announced that Jack Newton has opened a bed and breakfast at Lake Osius. See Project ASTRO is looking for volunteers for 2001. SAS elections are coming up next month. Members are encouraged to apply for all positions, in particular the Publicity VP. You can order calendars and observers handbooks for 2001 from Brian Allen.

Cataclysmic Variables

Dr. Paula Szkody gave a presentation on cataclysmic variables (CV), which are binary star systems that exhibit the following criteria: There must be mass transfer from the secondary to the primary star; The primary star must be a white dwarf; The primary and secondary stars have a low mean orbital distance.

The orbital periods of such systems, due to their proximity to one another, is about 1–4 hours. The activity of a cataclysmic variable is strongly dependent on the mass transfer rate from the secondary to the primary star. When the system is quiet, it is in quiescence, whereas high activity periods are called outbursts. To observe such systems requires both ground-based telescopes (to observe the visible spectrum of light) and spaced-based observatories for the UV and X-Ray bands.

CVs are characterized by a white dwarf orbited by a M-Class (cool) secondary star from which material flows onto the dwarf. As the material flows towards the dwarf star, it forms an accretion disk. Observations of the systems combined spectrum can lead to hints as to how thick this disk is. If the white dwarf spectrum is seen through the disk, it is considered thin, whereas the total absence of the dwarf's spectrum may indicate the dwarf is eclipsed by its accretion disk. Some dwarf stars exhibit extremely strong magnetic fields (200 million gauss), and systems containing such stars may lack an accretion disk, transferring mass along magnetic field lines directly onto the white dwarf. If this occurs, all the mass is dumped onto the white dwarf at it's poles, and this concentration of matter will cause the white dwarf to emit significantly more radiation in the high energy UV spectrum. Systems with moderate strength magnetic fields will exhibit both an accretion disk and a magnetic funnel that extracts material from the inner portion of the disk. A feedback reaction can also occur. This is when the extra energy produced by the outburst may increase the mass transfer rate from the secondary star, which will in turn affect the energy output of the primary over time. Some mass transfer may occur in "clumps" leading to an extremely "choppy" light curve for the system. Observations show that these white dwarfs can get extremely hot, the spectrum of one dwarf suggesting surface temperatures near 46,000 K. There is also a relationship between the frequency of CV outbursts and the rate of mass transfer. Lower mass transfer rates lead to longer periods between outbursts (20 –30 years), but the outbursts are much stronger.

The puzzle for astronomers is to deduce the structure of a given CV based on its observed behavior. About 60% of all star systems in our galaxy are binary. Of the ones containing 1–8 solar mass primaries, 10% will undergo close binary evolution in which the secondary star will find an orbit within the gas envelope of the primary. About 10% of these systems will become cataclysmic variables. There are an estimated 1 to 10 million such systems in our galaxy. The systems start off with longer orbital periods, but the shorter the period, the more evolved the system is. In some cases, the mass loss of the secondary can be so extreme, that it becomes a brown dwarf, as its mass drops to less than 0.06 solar masses.

Dr. Szkody is using the Sloan Digital Sky Survey to attain spectra of suspected CV systems. In the past year she has discovered 12 new cataclysmic variables, and 50 M-Star – White dwarf systems.

Amateur astronomers equipped with telescopes and CCD cameras can take magnitude and color measurements of CV systems. When groups of amateurs compile the data into a database, light curves can be derived, which are very useful to professional astronomers. Resources for further study are listed below:

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