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Projected distance from source (AU)

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Projected distance from source (AU)

Fig. 4. Variation of jet widths against projected distance from the central source. Jet widths are measured from jet [O l]/[S II] emission intrinsic FWHM (corrected from the PSF FWHM), derived from space HST and ground-based AO observations. Figure adapted from [39]

tion space and ground-based observations (HL Tau, HH 30: [40]; DG Tau, CW Tau, and RW Aur: [13, 46]; HN Tau, UZ Tau E: [21]). At the highest spatial resolution currently achievable (0".1 = 14 AU for the Taurus Auriga Star Formation Region) the jet innermost regions (below ~20 AU) are unresolved. Jet widths are then seen to increase up to 20-30 AU at projected distances of around 50 AU from the source. Further out, jet widths increase slowly with distance with full opening angles <5°. These observations show that jets achieve almost cylindrical collimation very early on, on scales ~50 au. A few constraints available from younger flows show similar collimation properties. These observations put strong constraints on theories of jet launching and appear to rule out pure hydrodynamic models for jet focusing (see the chapter by Sylvie Cabrit in the first volume of this series).

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