GENERAL MEETING REPORT 13 FEBRUARY
Speaker: Peter Nelson, Director Variable Star Section ASV
Topic: “Recent Variable Star Activity”
February must be Peter Nelson month; it was exactly two years ago when Peter last gave us a talk on Photometric Measuring of Variable Stars (Refer Vol.24, No 2; pp 16). It struck me then how diligent an astronomer Peter must be in the pursuit of his goals, to be not only well informed on such an (for me) obscure subject, but to actually perform cutting edge research on the subject. Astronomy is such a rapidly expanding field of activity that merely keeping up with developments seems to be all I can accomplish. I did look up GW Lib (Peter’s subject for tonight’s talk) on the Internet and, would you believe, Google brings up 6,000 references for this “obscure” 18th magnitude star. The cataclysmic variable behaviour of GW Lib was first noted in 1983, but no further outbursts have been observed until now. On April 12, 2007 at 11.50 UT Rod Stubbings (ASV) from Drouin, Victoria alerted the astronomical community to a rapid increase in brightness of the star. It rose by two magnitudes within six hours. Peter Nelson, Ellinbank, Victoria, started watching 15 minutes later, and has been following the outburst for its full duration, which lasted 70 days and spanned 6 magnitudes.
The data so collected can be analysed and interpreted to allow confirmation of hypotheses as to the source and the causes of these brightness changes, and to the category that would fit it. It is generally accepted that the mechanism responsible for such cataclysmic variations is a close binary star system where a “normal” White Dwarf accretes material by gravitationally ripping it from its companion. Accreting binaries are surprisingly common. Around 1600 are now known, and GW Lib seems to fit into the SU Uma (Ursae Majoris) subgroup, with outbursts that seem to be of the WZ Sge (Sagitae) type. The latter is an old and well known CV in which the process is reasonably well understood: The two stars orbit their common centre of mass every 82 minutes while gas is launched from the tip of the red donor star towards the white dwarf. The transferred gas forms a swirling disc around the white dwarf, known as the accretion disc. Through this disc matter slowly spirals in as potential energy and angular momentum is converted into heat, radiation and kinetic energy. Due to the large amount of energy released in the disc, it is generally the most luminous part of the binary and produces a large amount of radiation across a wide range of the electromagnetic spectrum. The cataclysmic part of the name refers to the periodic brightenings that many of the systems display. As matter builds up in the disk turbulent eddies lead to a Balbus-Hawley instability, the disk becomes opaque to radiation and heats up rapidly and can erupt in a dwarf nova. In most systems such outbursts reoccur every few months, last for a few weeks and typically result in the overall system brightening temporarily by factors of 30-100. WZ Sge is unusual in the sense that its outbursts are very rare with only 3 recorded outbursts, in 1913,1946 and 1978, suggesting a recurrence time of 33 years. Furthermore, the system brightens by a factor of 1500 compared to its usual brightness, and the whole outburst lasts several months.
Peter Nelson’s interpretation of the data collected on GW Lib is of a White Dwarf, mass 0.7 sol, with a Red Dwarf companion, of mass 0.13 sol, and an Orbital Period of 76.8 minutes. It is a CV of the SU UMa type with outbursts that match the WZ Sge pattern. His investigations of the rapid variations in the light curve, termed Superhumps, are ongoing, giving rise to theories of precessions on bulges in the accretion disk or the Lagrangian contact. If this is correct then Superhumps can be used to confirm the mass ratio of the system components. Cataclysmic variables (CVs) provide a rich laboratory to study the effects of mass transfer and accretion on the properties and evolution of white dwarfs. One interesting aspect is the above mentioned effect of accretion on the instability strip. UV spectra of the white dwarf in GW Lib obtained with the Hubble Space Telescope shows it to be the only known nonradially pulsating white dwarf in a cataclysmic variable, although the mean spectrum fits with an average white dwarf temperature of 14,700 K. Compared to all other known single white dwarf pulsators the current models for such stars may not apply.
Amateur astronomers play a vital role in monitoring stars such as GW Lib during its long dormant period. Very few professionals can afford to spend their precious allocations of large-telescope time on monitoring none events. Of course, once an outburst is reported, every observatory joins the hunt looking for clues with new equipment, new programs and new ideas to shed light on another aspect of the cosmic mystery.
I found a little research into the naming convention of Variable Stars a great help in making sense of their apparently random letter designation. There is in fact a system behind it. In any given constellation the first variable star discovered is designated with a letter starting with “R”. The terminology was developed by Friedrich W. Argelander, born on 22 March 1799 in Memel (at that time part of Prussia) who was the first astronomer to begin a careful study of variable stars. Every first previously unnamed variable star in a constellation he gave the capital letter “R” followed by the constellation’s name in the genitive. “R” was the first letter in the alphabet then not yet used in astronomical nomenclature and, as variable stars were then few and far between, Argelander reasoned there was sufficient scope in two letter permutations to the end of the alphabet: RR through RZ, SS through SZ, up to ZZ. With modern instruments it did not take long though for this range to be exhausted and recourse was made to double capital AA and so on, and that is how we get to GW Librae.
The vote of appreciation for another stimulating and educational evening was given by Barry Adcock to general acclaim.