Steven Parsons
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A star's mass determines its evolution and the final object formed. Once massive stars have used up their nuclear fuel they become black holes. Similarly, intermediate mass stars become incredibly dense neutron stars. For low mass stars (which account for over 90% of current main sequence stars), the consumption of nuclear fuel results in the formation of an electron-degenerate white dwarf. The importance of white dwarfs in the evolution of the Galaxy, as well as the universe as a whole, cannot be overstated. The planetary nebular phase of pre-white dwarf evolution injects large amounts of processed material back into the interstellar medium from which subsequent generations of stars form hence, study of white dwarfs can help trace the chemical evolution of the galaxy. On a larger scale, type I supernova, created by a thermonuclear explosion on the surface of an accreting white dwarf, can be used to determine the distance to faraway galaxies and indicate the acceleration of the universe. Precise measurements of masses and radii are of fundamental importance to the theory of stellar structure and evolution. Mass-radius relations are routinely used to estimate the masses and radii of stars and stellar remnants, such as white dwarfs. Additionally, the mass-radius relation for white dwarfs has played an important role in estimating the distance to globular clusters and the determination of the age of the galactic disk. However, the empirical basis for this relation is uncertain as there are very few circumstances where both the mass and radius of a white dwarf can be measured independently and with precision. One situation where this is possible is in close binary systems. In these cases, masses can be determined from the orbital parameters and radii from light-curve analysis. My research involve the study of a particular type of close binary system known as Post Common Envelope Binaries (PCEBs). In the past the more massive member of the binary has evolved off the main sequence and, as a result of this, has expanded to fill its Roche lobe. The unstable mass transfer to the secondary star results in the formation of a common envelope in which both members of the binary reside. Frictional forces within the envelope cause the two stars to spiral inwards and the envelope to be ejected. The result is a detached close binary containing a main sequence star and the core of the more massive star which will become a white dwarf. The unique set up of these systems allow us to potentially determine the masses and radii of both stars independently of any mass-radius relations allowing us to test them. My PhD has so far involved a detailed study of the eclipsing PCEB NN Serpentis. This system has the great advantage that it is eclipsing, which leads to far tighter constraints on the system parameters. Moreover, this system also displays a secondary eclipse allowing a complete solution of the systems parameters. A combined study using UVES spectroscopy and ULTRACAM photometry allowed us to measured the mass and radius of both components of the system independent of any mass radius relations. This is the first direct measurement of a white dwarfs mass and radius (all other studies have involved indirect methods of determining the radius). The secondary star also provided some of the best measurements for a low mass star ever.
The preprint of this paper is available here For a simple python script to convert UBVRI magnitudes to ugriz magnitudes and propagate the errors, use this program. |
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The figure on the left shows the mass and radius of the white dwarf in NN Ser in relation to other white dwarfs. Since the white dwarf is very hot it has a large radius placing it higher than the other white dwarfs (which are cooler). The measured mass and radius of the white dwarf are in excellent agreement with a 'thick' hydrogen envelope hot (~60,000K) white dwarf model. (Models are labelled by their temperature in units of 103 K and the exponent of the hydrogen layer fraction). Future work will aim to add additional point to this plot using similar systems to NN Ser (i.e. other eclipsing PCEBs).