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Johannesburg Centre,
Astronomical Society of Southern Africa
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Variable(s) of the Month
Danie’s choice of the Mira type variable R Cen as the variable of the month last month has coincided with some discussion on the variable star network about this star. It has been classed as a pulsating Mira star with a period around 420 days, with double humps in the light curve ie two peaks and troughs that are not the same. Astronomers taking a careful look at the light curve for the past 100 years have noticed that both its amplitude of variation and its period have been decreasing. Mira stars with periods in excess of 400 days seem to be unstable and are possibly settling down to a shorter, more regular pulsation pattern. This is one of the reasons why these stars should be observed by amateurs. Another 200 or 300 years of observations and astronomers may yet get to understand what is happening. The centre star in the line of three in the head of Scorpius underwent a remarkable brightening in July. An amateur astronomer in Argentina, Sebastian Otero, one day reported to the variable star network that this star had brightened by about a tenth of a magnitude. Now, if you have ever tried to do some variable star observing you will know that it is very difficult to get to one tenth of a magnitude accuracy - in fact it is hard enough guessing to about 0.3 magnitudes!! So everybody politely ignored him. Anyrate, a couple of days later Sebastian reported that it had brightened some more and then someone else had a good look and found that the star seemed to be different. Then an astronomer did some spectroscopy on delta and found that it WAS strange in that there were emission lines in the spectra (stars normally do not show emission lines). It turns out that the star brightened from 2.3 to about 1.8 and it is still very bright as I write this article. It has been identified as a gamma Cas type variable - a very hot young B type star that has experienced some unhappiness in its life cycle. ( Don’t we all! ). This one is dead easy to see. Why don’t you monitor it over the next couple of months? Just keep comparing it to the other two stars in the head of Scorpius and see what happens. Perhaps it will return to normal and perhaps it will get a lot brighter. What are Gamma Cas stars? GCAS stars are massive blue stars ( spectral type O8 to A1 ) that are evolving off the main sequence ( Luminosity classes V to III ). First of all they are Be stars, stars with emission lines in their spectra. This hydrogen Balmer emission lines are caused by the presence of a thin envelope of matter previously ejected by the star. In a normal star the light from the photosphere ( continuum spectrum ) passes through the star's atmosphere ( lower density ) resulting in an absorption spectrum. But in Be stars, we can see free emission from the light coming directly from the circumstellar matter which produces an emission spectrum. (See graphic) They may be non-variable Be stars (in the visual), they may be BE variable stars, Be stars with very small variations, or GCAS stars when they undergo important optical activity. Delta Scorpii was a normal B star ten years ago. It was discovered as a Be star in 1993 and now it has emerged as a new gamma Cassiopeiae variable. Why did these events take place? The common fact for all Be stars is that they are all very rapid rotators ( 100 - 500 km/ sec ). In some cases, the rotation speed overcomes gravity force and parts of the atmosphere are ejected by the star's equator forming a circumstellar envelope. The orientation and thickness of this ring are the main causes for GCAS variability. Rapid rotation may be an intrinsic property for Be stars and sometimes may be a consequence of mass transfer in a binary system. Tidal forces between a star and its companion may play a crucial role in cases such as delta Scorpii, since the proximity of the companion can increase the strength of the activity in the star's upper atmosphere. Delta Scorpii is a binary system ( the companion is another B star but considerably less luminous ) with a period of nearly 20 years, and the point of the orbit when both stars are closest together was reached in . . . . . . . . JULY 2000 !!!!!. So it's a kind of proof for the theory. This is the main cause for GCAS variations: rapid rotation. Brian Fraser Danie Overbeek. |
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