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The icing on the snake

posted Nov 9, 2017, 7:37 AM by Jeremy Drake
Planets are born in the residual gas and dust from the star formation process that is left orbiting a nascent star in the form of a disk. The closest example of such a planet hatchery is in the constellation of Hydra, the water snake. The TW Hydrae system, whose Atacama Large Millimeter Array image famously shows a disk face-on to us carved out with dark lanes that could possibly be gaps swept out by newly-born planets.  The gas disk lasts for a few million years, eventually being dissipated by evaporation or loss to a disk wind, or by spiraling onto the parent star under gravity.

The inner edge of the disk is truncated at a few stellar radii by the star's strong  magnetic field.  Gas then follows the magnetic field lines, accreting onto the star in narrow columns down the magnetic poles. In free-fall, velocities reach up to 500 km per second and most of this energy of motion is converted into heating the gas to 2-3 million degreeshot enough to emit at X-ray wavelengthsin a shock as it hits the stellar surface. The shocked gas is slowed by a factor of about 4, so just before merging into the stellar surface it can still be moving at about 100 km per second.  Italian astrophysicist Costanza Argiroffi, from the Palermo Astronomical Observatory in Sicily, lead a study published in the November 2017 edition of Astronomy & Astrophysics to try and detect this motion of the shocked gas accreting onto TW Hydrae by its Doppler shift using the spectrometers onboard the Chandra X-ray Observatory. By combining many observations of the star and carefully comparing the spectrum with older stars long-bereft of accreting gas, the Doppler shift was detected. But the velocity was much smaller than expected - "only" 40 km per second.  This indicates that the accretion streams that provide the last 1% or so of the star's final massthe icing on the cakelie at an angle to our line-of-sight. The dominant magnetic pole of TW Hydrae is then not at the geographical stellar pole, but instead closer to the equator at a latitude of 10-30 degrees.
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