Starship Asterisk*

A schematic diagram of the surface of TW Hydrae, illustrating where strong X-ray emission might arise. Accreting material can produce winds and shocks at the stellar photosphere; some parameters are specified. [i](Credit: N. Brickhouse, et al)[/i]

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The star TW Hydrae is located about 150 light-years from earth in the direction of the constellation of Hydrae, the Water Snake. This star is relatively young -- at about 10 million years old it has passed out of its infancy but is not yet mature. Astronomers are trying to understand the processes at work around stars at this stage in their lives, when planets might be developing from disks around them, and TW Hydrae is a valuable example for two reasons: it is relatively close by and therefore bright, and it is rotating with its pole pointed nearly directly towards earth, enabling scientists to view the star's surrounding disk of material nearly face on.

Like other young stars of its size and age, TW Hydrae emits strong X-rays. The question is why, and how might they effect the star's proto-planetary disk? Several mechanisms have been proposed, including coronal magnetic field activity similar to that on the sun, accretion onto the stellar surface that might also contribute to winds and flares, and shocks from jets that develop. Each mechanism has associated with it hot gas with characteristic temperatures and densities.