Sunday, October 13, 2013

An Extremely Cool White Dwarf Star

White dwarfs are dense stellar remnants that mark the final evolutionary state of intermediate-mass and low-mass stars. Right after formation, a white dwarf is very hot and will appear blue-white in colour. Since material in a white dwarf no longer undergoes fusion reactions like in a normal star, a white dwarf does not have a source of energy and will gradually cool as it radiates away its energy. As a white dwarf cools, the radiation emitted by it will shift from the hot blue-white colour of an O-type star (> 30,000 K) to the cool red colour of an M-type red dwarf star (< 4,000 K). Given sufficient time, a white dwarf will cool until it no longer emits significant heat or light, and become what is known as a black dwarf. Nonetheless, no black dwarfs are expected to exist in the present Universe because the length of time required for a white dwarf to cool to this state far exceeds the current age of the Universe.

Artist’s impression of a white dwarf shown to scale with the Earth.

In 1997, a white dwarf identified as WD 0346+246 was serendipitously discovered during re-examination of images taken for a survey to detect brown dwarfs in the Pleiades cluster. The images were acquired by the UK Schmidt Telescope at Siding Spring Observatory in New South Wales, Australia. WD 0346+246 is observed to have an estimated temperature of 3,900 K, making it one of coolest white dwarfs currently known. A subsequent estimate gives WD 0346+246 a slightly lower temperature of 3,750 K. At that temperature, it will have a colour temperature similar to a red dwarf star. Parallax measurements yield an estimated distance of approximately 90 light years for WD 0346+246. This places WD 0346+246 within the Sun’s neighbourhood.

WD 0346+246 also shows unusually large velocity components with respect to the disk of the Milky Way galaxy. This indicates WD 0346+246 is a member of the galactic halo and is currently passing through the Sun’s neighbourhood. Most stars in the Milky Way galaxy exist within a single plane known as the galactic plane. However, a population of stars inhabit a spherical region around the galaxy known as the galactic halo. Stars in the galactic halo tend to be much older and less enrich in elements heavier than hydrogen and helium than stars in the galactic disk. Assuming WD 0346+246 is a carbon white dwarf with a pure hydrogen atmosphere, the length of time required for it to cool to its current state is at least 10 billion years. The existence of WD 0346+246 suggests that a significant number of such cool white dwarfs reside in the solar neighbourhood and have yet to be identified.

- Hambly, N. C., Smartt, S. J., & Hodgkin, S. T., “WD 0346+246: A Very Low Luminosity, Cool Degenerate in Taurus”, ApJ 489: L157-L160, 1997 November 10
- N. C. Hambly et al., “On the parallax of WD 03461246: a halo white dwarf candidate”, MNRAS (1999) 309 (4): L33-L36.
- P. Bergeron (2001), “The Halo White Dwarf WD 0346+246 Revisited”, arXiv:astro-ph/0105333