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Photometric variability in helium-rich subdwarfs
Sara McCarney - Bloomfield Collegiate School
Abstract My Nuffield Bursary took place over four weeks at the Armagh Observatory during Summer 2012. I studied a variable star with small light variations found on the Kepler satellite. Spectroscopy showed this star to be a hot, helium rich subdwarf. I calculated the surface properties and created a hypothesis of a reflecting binary star system to be the cause of this variation in luminosity. I formulated mathematical proofs to test this theory.
What is a variable star? ·A Variable star is one whose physical properties vary with time. This varying property is usually brightness. ·Reflecting binary stars are variable stars. The variation in light is due to the motion of the stars ·The variable star I am studying was found on the Kepler satellite
The atomic spectrum of the Kepler star

What is the Kepler Mission? ·The Kepler Mission is run by NASA and aims to study a portion of the Milky Way in the hope of finding earth sized planets in the habitable zone. ·The star I am observing was picked up by this satellite

Methods and Materials ·I first needed to find out the surface properties of my star. I can do this by studying the atomic spectra of the star. ·I used a program called DIPSO, which is a spectral analysis program. I measured the wavelengths of the lines and compared to an atomic spectra database, NIST. ·I found the star to be rich in Helium. ·I then compared the spectrum against model spectra of Helium rich stars to work out the surface gravity, surface temperature and Helium abundance of the star. ·I used a mathematical model known as The Method of Least Squares to give me a reliable answer over a wide range of possibilities.

Observed spectrum and model spectrum of temperature 38kK, log g 4.5 cms-2 and helium 99% . This model spectrum fitted my observed spectrum best.

Light curve of Kepler star: shows small variation in brightness of about 0.02%

Results & Conclusion ·After finding the surface properties of the star, I was able to work out the radius of the star, the separation of the hypothetical binary stars and the orbital radius using Keplers Laws. ·I then created a mathematical model of the reflection effect to see if the light variation I observed in the light curves could be attributed to this model. ·I believe my hypothesis was proved ­ my hypothetical value for light variation corresponded well to my actual value. ·My results taken were reliable ­ I compared a number of model spectra to my observed spectrum. ·There were many errors in my results, however; I only had model spectra for particular surface properties ­ there could have been a better fit in between these. I also made many estimations when working with my raw data, such as the albedo of the light reflected from the secondary star. ·A better way to estimate surface properties would have been to use a program that uses the Chi Square Method. ·I have also attempted to account for any errors in my experiment. ·To conclude, the reflecting binary hypothesis appears to be a viable theory to explain the light variation from this star.