Документ взят из кэша поисковой машины. Адрес оригинального документа : http://star.arm.ac.uk/~csj/nuffield/posters/2012_MURPHY.pdf Дата изменения: Mon Sep 10 16:57:54 2012 Дата индексирования: Fri Feb 28 04:57:24 2014 Кодировка: Поисковые слова: ngc 6559

Helium-Rich Subdwarfs
Helium rich subdwarfs, in general, are very evolved stars. The surfaces of these stars have little or no hydrogen left in them. They are usually about half the mass of the Sun and between a tenth and a half its radius. Their surface temperatures range from about 30,000 Kelvin to 45,000 Kelvin. They are therefore much hotter than the sun which has a surface temperature of 5,778 Kelvin. They are also much smaller and less massive than the sun.

Binary Stars: Why are they important?
A binary star system is a star system consisting of two stars orbiting their common centre of mass. The brighter star is called the primary star and the other is known as the secondary or the companion star. Binary stars are extremely important in astrophysics because calculations of the orbit of a binary star system allows the mass of the two component stars to be directly determined, which indirectly allows the density and radii of the component stars to be calculated. This also determines an empirical Mass-Luminosity relationship which can be used to estimate the masses of stars which are not part of a binary system.

How can the spectra of a star tell us whether or not it's moving?
The spectra of stars are not continuous. They exhibit absorption lines at well defined frequencies that correlate with the frequencies needed to excite electrons from one level to another in various elements. The doppler effect is noticeable in these spectra because the exhibited absorption lines do not always appear at the frequency/wavelength that you would come to expect. They are instead, shifted slightly. If the source is approaching us, then the wavelength will decrease and the frequency will increase therefore the absorption line will shift toward the blue end of the electromagnetic spectrum. It will be blue shifted. Reciprocally, If the wavelength increases and the frequency decreases it will be Redshifted. Shown Left.

Above: Artist's impression of a binary star system

Methods:
The images on the left show the observed wavelength of a He I absorption line being measured. To get an accurate reading for the radial velocity, I used the program ELF to fit a parabola to a He I absorption line. The parabola could only be fitted to emission lines so the spectra ha to flipped to fit one. ELF gave the exact wavelength of the peak of the parabola, which gave a more accurate reading for the observed wavelength and, therefore, the radial velocity. Using SIMBAD I got the right ascension an declination of each star an the date and time of each observation an input them into the ESO airmass and Heliocentric correction online calculator. This gave me the value of the heliocentric correction and allowed me to see if they had a variation of radial velocity between observation large enough to constitute being in a binary system.

James Murphy, St Malacy's College Conclusions & Evaluations:

This project successfully identified at least one star that was part of a binary star system, with the possibility of one or two more. The star HE0111-1526 exhibited definite signs of being in a binary system. It showed a difference of 52.5km/s in radial velocity between the two observations, as shown in the results table. The error for the radial velocity was calculated to be was ±5.4km/s for the first spectra and ±5.2km/s for the second. The image on the left shows the results table for all ten stars analysed.