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The Webfooted Astronomer - August 1999

 

Minutes: Catacysmic Variables

by Leslie Irizarry

The July SAS meeting was held on July 21, 1999, at 7:30 p.m. in the UW’s Physics-Astronomy Building.

Announcements

Table Mountain registration has to be postmarked Friday. (At the entrance you pay $10.00 extra.)

Telescope Makers will take place July 31. Greenlake Star Party will be next Saturday. Astrophotograher’s meeting is Sunday at 2 p.m. at Keith Allred’s house. Viva La Lune will meet this Friday.

There will be a partial lunar eclipse next full moon (July 28), but we will only see the last phases here on the West Coast. George Best reminded those who go to see the solar eclipse to bring their slides to the September meeting where Guillermo Gonzales will talk about eclipses.

Cataclysmic Variables
Paula Scoti presented a talk on cataclysmic variables. There are several types of cataclysmic events in space: X-ray bursts, which are likely to be associated with neutron stars; supernovae, which are connected with objects that became neutron stars or black holes; novae and dwarf novae. Some novae and dwarf novae are cataclysmic variables.

There are three pathways to stellar death, depending on the mass of the star. In the case of a low mass star such as the sun, it will end up being a white dwarf about the size of Earth whose rotation can be measured in minutes.

A higher mass star greater than 6 times the mass of the Sun becomes a neutron star with a size of about 15 km and rotation measured in seconds. The highest mass stars become black holes. The vast majority of stars are of lower mass, which will end up as white dwarf stars.

The process of active mass transfer refers to a white dwarf accreting the outer gas layers of a late M cool star. As the gas falls in toward the white dwarf, it forms an accretion disc around the dwarf star. At the place where the gas falls onto the white dwarf there is a hot spot with a temperature of 10,000 degrees. The in-falling gas gets concentrated at the twp magnetic poles of the white dwarf, emitting x-rays and heating the surface of the recipient star up to 100,000 degrees.

You can explore such stellar systems in a variety of wavelengths. There must be a multiwavelength approach to understanding cataclysmic variables because specific wavelengths provide insights to the physical processes within the system. By isolating a portion of the spectrum, you isolate objects to study.

Gamma rays elucidate bursters; X rays elucidate X-ray binaries with degenerate stars, supernova remnants, gas in galaxy clusters, ultraviolet elucidates O and B stars; infrared elucidates cool stars, brown dwarfs, and planets, and radio waves elucidate molecular clouds.

There are currently many satellites studying the heavens at various wavelengths: HST (UV/optical); Solar A—hard X-ray (ASCA); EUVE (ultraviolet); XTE (hard X-ray); FUSE (ultraviolet); AXAF (soft and hard X-rays); SIRTF; XXM; and ASTRO E.

A nova is a new star—a star that has brightened. The spectrum of a nova appears different at visual wavelengths as compared with the ultraviolet wavelengths. In the visual, a brightness vs. time graph appears as a gentle descending slope.

In the ultraviolet, however, there is initially a downward spike, followed by a steep ascent, softening back into a gentle downward slope. To gain a better understanding of the process, astronomers have to combine observations made at different wavelengths.

The following is a summary of the process:

  • A white dwarf accretes mass until the thick degenerate gas envelope forms around it and leads to a thermonuclear runaway reaction.
  • A quantity of 10-6 to 10-4 solar masses of material is ejected. The envelope then returns to quasi-static equilibrium and steady nuclear burning phase.
  • Star's luminosity is now near Eddington limit
  • The gas envelope is gradually ejected by radiation driven winds or a fast magnetic field rotation
  • Photosphere recedes, temperature increases, and peak of radiation is now in the SOFT X-ray. Constant bolometric phase lasts until envelope is exhausted. (1-100 years)

The interval between such cycles can be a few thousand to 10,000 years. Between major explosive events, one can have dwarf novae outburster events. There is 3-5 magnitude increase in the visible spectrum and matter is not ejected. There is a difference in had X-ray (constant source) and softer X-ray emissions during such events. There is also a dip in X-ray emissions at the point at which visible wavelengths are getting brighter. This occurs because the hot spot is blocking out the soft x-rays by eclipsing it.

Cataclysmic Variable Systems
VW Hydrae has an outburst around one time per month. A super outburst can last 15 days. There have been 4 systems found that have extremely short cycles: 43-day cycles, 19-day cycles, and 44.5-day cycles.

Probably the accretion disc is going through dramatic changes in brightness. During an outburst, there is high accretion; during quiescence, there is low accretion. There was a recent study in which 4 satellites were looking at the same source for 40 days.

Amateur astronomers with CCD cameras as well as professionals participated in the study which observed eight separate outbursts. On Day 141 it was high in X-ray and low in ultraviolet and optical; on day 142 the X-ray decreased and the ultraviolet and optical increased. Thus, X-ray is inversely correlated with optical.

See the following Web sites for more information:

http://www.astro.washington.edu/szkody/
http://www.kusastro.kyoto-u.ac.jp/vsnet/
http://www.aavso.org/
http://cba.phys.columbia.edu/

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