N.Z. observations help star theorists

Press, 27 March 1980, Page 29

N.Z. observations help star theorists

New Zealand astronomers have observed certain classes of eruptive stars for decades. Their data,, published in this country. has attracted world-wide interest, so much so that in 1979 extensive use was made of these data to formulate theories of how these stars erupt. Astronomical literature for , 1979 contains well over 100 references to the New Zealand observations. A recent paper from the European Southern Observatory is based almost exclusively on these data. At present New Zealand astronomers are running a joint programme with the Royal Greenwich Observatory, the European Southern Observatory, and the Goddard Space Flight Centre to study these stars at all wave lengths. There is a wide variety of objects broadly termed cataclysmic variables. Such stars have two common features despite the wide diversity of objects in this group. First they all have eruptions, and, second; they are all binary systems. Within this broad class there is a group known as SU Ursae Majoris stars. A typical example consists of a white dwarf primary and a red secondary which is larger but less massive than the primary. This red star is unstable and ejects matter in a stream which because of its motion forms a disc around the white dwarf. At the point of impact of the infalling material on this disc a hot spot is formed, being heated bv the matter that fails upon it.

The New Zealand observations have proved that such systems can have only one of two types of eruptions. These can be either ' normal outbursts, during which the star increases bv from two to four magnitudes, or superoutbursts when the star becomes another magnitude brighter. Normal outbursts last for less than three days, while superoutbursts last for from ten to twenty-five days. The normal outbursts occur at random,' although for each particular star there is a mean cycle so that in any interval of 1000 days there are much the same outbursts . as in any other similar interval.

It was the New Zealand observations that shotved

that the super-outbursts recur in a semi-periodic manner. Once this period has been found for any star it becomes possible to predict future outbursts.

As these systems consist of two stars revolving around a common centre there are periodic eclipses. If the orbital inclination is such that these eclipses can be accurately timed then the period of revolution can be found. However most of the systems are not inclined at a sufficiently wide angle to allow for such timings. The orbital period can be determined by small amplitude humps superimposed on the variations in light. It has been found that all have periods under hours. This shows that the stars are close together and that rotation is rapid. When a super-outburst occurs the normal humps are invisible but superhumps are seen that have a -period slightly longer than that of the period of revolution.

During minimum light it has been found that the main light source comes from the hot spot on the disc. Flickering of the light in a rapid oscillation shows that matter is continually arriving at the hot spot but varies in intensity. This implies that the stream of matter is unsteady.

It was once thought that the eruptions were due to the steam of matter coming in the form of a series of

‘‘Hiccups’’ so that each. blob was the cause of an eruption. Between eruptions -the light was more or less steady because it was arriving in a thin stream. While this theory may still be valid for certain types of cataclysmic variables it no longer appears to be true for the SU Ursae Majoris stars, because there is no observational ' evidence that shows that the hot spot receives blobs of matter.

For these stars having super-outbursts it is now thought that the hot spot acts like a faucet dumping material on to the white dwarf as it arrives. The white dwarf acts like a reservoir which h? to be filled to a certain level before it erupts by varying amounts in accordance with the rate at which it has been filled. While this suggested model accounts

for the super-outbursts it does not account for their semi-periodic nature. There is still strong disagreement on the details of models to account for these stars and their eruptions. What has become clear is that the SU Ursae Majoris stars differ from, other cataclysmic Variables and their outbursts have different origins. Also this class is no longer confined to stars that have frequent normal outbursts but extends to inifude many stars that were formerly considered to be recurrent novae. Thus stars like WZ Sagittae which have superoutbursts every 32 years are considered to he of the same class as those that have such eruptions every 140 to 180 days. The aim of the present programmes is to obtain data at all wavelengths so that the behaviour of these stars can be compared in X-rays, infra-red, and ultra-violet with optical observations made at the same time. In this way it is hoped to determine which of the present models is correct. This in turn is likely to improve our knowledge of the evolution of stars. In April Mercury reaches its greatest elongation west of the Sun on April 3. For the first fortnight of the month it will be visible in the eastern morning sky rising before dawn. By watching it from one morning to the next its rapid motion amongst the stars will readily be apparent.

Venus continues to be a bright object in the evening sky after sunset. It is now getting low in the north-west and only visible for a short time after twilight ends. The three other bright planets have al! moved into a favourable viewing position for evening watching. Mars and Jupiter remain conspicuous in the constellation, Leo, Sa= turn is further to the east. Al) three set well after midnight. The Moon will pass through 'Leo on April 24. On that evening it will be close to Mars at 7 p.m. close to the bright star, Regulus, at 9 p.m., and close to Jupiter at 11 p.m. Watching the Moon on that evening one will notice how rapidly it moves against the background of stars. Two nights later, on April 26, the Moon will be close to Saturn.