Planetary nebula NGC 6543 and white dwarf central star

WHITE DWARF EVOLUTION

 

 

White dwarfs are the dying remnant cores of stars similar to the Sun. During their lives stars generate energy from the process of fusing hydrogen into helium. As they begin to run out of fuel they become unstable and shed their outer layers before collapsing under their own gravity. At the same time the star becomes extremely hot, up to 200,000 degrees or so. Two groups of white dwarfs are known, those with mainly hydrogen atmospheres and some with helium atmospheres. Our work at Leicester is aimed at discovering the reason for this division and the subsequent compositional changes as the stars cool. Spectroscopic observations from the Extreme Ultraviolet Explorer (EUVE), ORFEUS and the Hubble Space Telescope are providing us with important new scientific results.

Diagram showing H-rich (DA) and He-rich (DO & DB) paths, noting possible mechanisms that may alter atmospheric composition as the stars cool. The circled numbers are the ratio of H and He-rich stars.

 

We have been able to match the observed far-UV and EUV spectra of heavy element-rich white dwarfs, such as the star G191-B2B, using state of the art theoretical models, to obtain accurate measurements of stellar temperature and composition. However, this good agreement is only possible if modifications are made to the basic model calculations. A component of ionized helium (HeII) must be included to suppress the predicted flux level below 228Å. If real, this may be interstellar/circumstellar material or could be in the atmosphere of the star. Proving whether or not the opacity really exists, and if so, finding its location is the goal of a new rocket-borne high resolution EUV spectrometer, on which we are working. Below 190Å, the spectral shape can only be reproduced if we assume that Iron present in the atmosphere has a depth dependent abundance, showing a relative depletion in the outer layers. This may be evidence for active mass-loss in the star.

Extreme Ultraviolet spectra of the H-rich white dwarf G191-B2B; Left hand panel - the best agreement that can be achieved with model spectrum containing a homogeneous Iron abundance (green and red histograms), right hand panel - improvement achieved when the outer layers of the atmosphere are assumed to be depleted of Iron.

 

EXPLOSIONS IN INTERSTELLAR SPACE

Our present picture of the local interstellar medium (the gas lying between the stars out to distances of about 300 light years), is that the Sun is embedded in and near the edge of a wispy diffuse cloud, known as the Local Cloud (or Local Fluff). This cloud, which is only 20-30 light years across, is itself in a larger much less dense region called the Local Bubble. Using the the shadowing effect of the interstellar medium on the EUV spectra of 13 nearby white dwarfs we have measured the density and level of ionization of gas in the vicinity of the Sun. Remarkably, while the gas density varies in different directions, the fraction of material ionized is highly uniform. This can be best explained if we assume that the Local Cloud was ionized by the shock wave from a nearby supernova explosion, since when the ions and electrons have been slowly recombining. The observed fractions of ionized hydrogen (27%) and helium (35%) indicate that the explosion occurred around 4 million years ago.

EUV spectrum of the pure H white dwarf REJ2156-546, showing the ionized and neutral helium features from which the helium ionization fraction is measured.

 

Observations of the helium ionization fraction, plotted as a function of the total column density of interstellar gas.The size of each cross indicates the measurement uncertainty .

 

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Web page created by Louise Twist.

Longslade Community College.

Last updated 26/5/00