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A Stellar Split Personality:
Vibrating Two Ways Around an Invisible Twin
David Laney
South African Astronomical Observatory
Double Springs
A team of South African and international astronomers have discovered the first known star which pulsates in two completely
different ways at once. There are 15 classes of pulsating stars; this is the first to be a member of two classes at the same time.
Basic Data
 HD 209295 is not a newly discovered star, though its unusual properties were not known before. If the Sun were at the same distance (about 400 light years), HD 209295 would appear 14 times brighter. With a temperature of 7480 +/- 100 degrees C, it's considerably hotter than the Sun (5490 degrees C) and about twice the Sun's diameter. The star has no name --- it is number 209295 in the Henry Draper (HD) star catalogue.
Multirhythmic
Unlike the Sun, HD 209295 varies significantly in light output and temperature. The star pulsates in at least 11 different frequencies, nine of them relatively slow vibrations (1 to 3 cycles per day) and two much faster vibrations (14 to 26 cycles per day). What is unusual is not the number of different frequencies (like different pulses in a rhythm section), but that they include both slow cycles and much faster ones.
A Matter of Gravity: It's a Gamma Doradus Star
The nine relatively `slow' vibrational pulses (frequencies) are `gravity modes', because the basic force involved in the pulsations is gravity. If a layer of the hot gases in the star is forced from its normal position, it `floats' back to its normal position like a rubber duck pushed underwater. The presence of these `slow' vibrations makes HD 209295 what astronomers call a `Gamma Doradus' star. The variations in Gamma Doradus were so unexpected and baffling that the South African astronomer who discovered them (the late Alan Cousins) used to buttonhole fellow astronomers in the early 1990s, waving the data at them and asking them if they had any idea
what it all meant. There are now two dozen variables known to be members of this class, and more are being discovered all the time.
Under Pressure: It's a Delta Scuti Star
The two relatively `fast' vibrations show that HD 209295 is also a `Delta Scuti' star. In these stars, if a layer of gas is displaced, the pressure of the surrounding gas is the primary force `pushing back'. Such vibrations (p-modes) are at higher frequencies than g-modes because the `spring' in pressure modes is inherently `stiffer'. Until the discovery of Gamma Dor stars, all the known classes of pulsating variables oscillated in p-modes, so the surprise at the discovery of significant g-modes in some stars was considerable. Delta Scuti stars, on the other hand, have been known since the early 20th century. Until now, nobody has ever seen a star
vibrating by both mechanisms at once.
Leaving the Main Sequence: HD 209295 as an Emerging Senior Citizen
 Stars like our Sun are on the `main sequence'. If we plot the brightness and temperatures of stars on a graph, with
brightness increasing toward the top, and stellar surface temperature increasing toward the left, most stars in the solar neighbourhood lie on a diagonal strip running from top left to lower right. (Since it dominated the diagram, it was the `main' location for local stars, and the continuous strip suggested some kind of `sequence'
- this led to all kinds of wrong conclusions, but that's another story.) For stars on this `main sequence', the dominant source of energy is thermonuclear `burning' of hydrogen into helium in the core. Normal stars spend most of their lifespans in this stage of their evolution. Delta Scuti and Gamma Dor stars have typically begun to evolve to higher luminosity (and lower surface temperature) because hydrogen in their cores has mostly fused to helium. HD 209295 has gotten only a little brighter (and cooler) than it was when it began fusing hydrogen atoms into helium. As you can see from the diagram, it hasn't really left the `crowd' quite yet.
Gamma Dor stars are somewhat cooler than Delta Scuti stars, but both types are hotter than the Sun. Both typically pulsate
`nonradially' -at any given time some parts of the star's surface are moving outward while others are moving inward. Some Delta Scuti stars have dozens of nonradial pulsation patterns going simultaneously, each with its characteristic frequency (i.e. a certain number of cycles per day).
The image below shows one of the ways in which the shape of a star might change
during `nonradial pulsation'. The star doesn't expand or contract, but its shape is different at different phases in the
cycle.
Split Personalities and Abnormal Companions
Why the double personality? HD 209295 and an invisible companion star orbit each other every 3 days. The orbits are not circular, so the tidal forces between the two stars are much stronger at some times than others. This probably allows the invisible companion to `excite' (energize) the `slow' g-modes, in spite of a temperature which would otherwise be too high (normal for a Delta Scuti star, hotter than normal Gamma Dor stars).
The invisible companion is probably not a normal star - a normal companion as old as HD 209295 would move in a nearly circular orbit, because the tides in such a close binary (acting most strongly, of course, at the time of when the two stars were closest) would rapidly decrease the orbital eccentricity. HD 209295 has evolved far enough from its original `main sequence' brightness and temperature that we know it has had time to exhaust most of the hydrogen in the core
- and time to circularize its orbit.
Invisible, Compact and Strange?
One way to explain the highly elliptical orbits would be to suppose that the companion star could have been quite massive originally, exploding as a `supernova' and leaving a `neutron star' (an object with about the mass of the Sun, but as dense as an atomic nucleus) behind in an eccentric orbit. Such an orbit would not circularize in the lifetime of HD 209295. But a neutron star in a close binary would be expected to radiate X-rays, and none are seen.
A normal companion star as massive as the one orbiting with HD 209295 (at least 98% of the mass of the Sun), on the other hand, would be emitting enough visible and infrared (longer wavelengths than seen by the eye) light to be detected ---unless by a statistical fluke HD 209295 has a companion at almost exactly the minimum possible mass. No such light was observed.
In what is undoubtedly a clue (even if astronomers don't understand it yet), HD 209295 does emit more ultraviolet light than expected for a star of its temperature and brightness. The trouble is that no known type of companion star could produce the right amount of ultraviolet at the right wavelengths, especially if it must also be undetectable in visible and infrared. New theories should apply at the street entrance, during office hours.
For More Information
Contact Persons: Gerald Handler (gerald@saao.ac.za)
David Laney (cdl@saao.ac.za)
Telephone: 021 4470025
Fax: 021 4473639
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