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Fig. 5.6 Images of the environment around the star HL Tau, obtained with the VLA at a wavelength of 7 mm, and with a resolution of 40 mas. Left: the overall structure of the disk, determined from continuum emission; right (enlarged): image of the inner disk obtained from measurements made as a very high resolution, showing the collimated jets, perpendicular to the plane of the disk (After S.P. Wilmer et al., 2000, as cited by Najita, 2001)

The kinematics of disks is studied by high-resolution (R = 106) heterodyne spectroscopy in the millimetric interferometry domain - by IRAM, for example -based on the Doppler effect measured from molecular transitions (that of CO in particular). It is these observations that reveal the streams of material that are being accreted and ejected, and which play a part in star formation. Closer to the heart of the protostar, it is also possible to determine the kinematics of the hotter and denser, inner disk, by examining the vibration-rotation molecular transitions (for example the (2-0) band of CO at 2.35 |m; Fig. 5.7).

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