Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...287..893s&link_type=abstract
Astronomy and Astrophysics 287, 893-926 (1994)
Astronomy and Astrophysics
Astrophysics
117
Mhd, Solar Wind, Ism: Jets And Outflows, Stars: Mass-Loss, Stars: Pre-Main Sequence, Galaxies: Jets
Scientific paper
A new class of analytical solutions for rotating MHD outflows from the gravitational potential of a central object is discussed; the outflows are driven by thermal pressure gradients arising from a non polytropic heating, as well as by magnetic rotator forces. The solutions are obtained by a separation of the variables, of the spherical distance R and of the magnetic flux function α in several physical key quantities. Thus, the solutions are magnetic flux self-similar and treat fully the dynamics of the flow from its source to large distances. The topology and asymptotical behaviour of this broad class of solutions is examined in detail. At the Alfvenic and X-type critical points which the transAlfvenic solution crosses, the appropriate criticality conditions are applied. Attention is focused on the streamline shape which is calculated exactly and consistently throughout the outflow and allows us to formulate a criterion that distinguishes between collimated jets and non collimated winds. Thus, two classes of solutions with markedly different asymptotics are found. The first corresponds to outflows wherein the streamlines obtain asymptotically a conical shape on the poloidal plane, while the second is constituted of collimated jet-type outflows wherein the streamlines obtain asymptotically a cylindrical shape. Furthermore, it is shown that a basic feature of such collimated flows is an oscillatory jet width, without the help of an external confining pressure; an analytical formula is given for the wavelength of these oscillations. Hence, the jet does not need to focalize completely along the polar axis, but through successive contractions and expansions in its width, it naturally relaxes into a cylindrical pattern; it is argued that this is due to a consistent treatment of the current flowing along the axis of the jet. The study is applied to the relevant problem of jets from young stellar objects; this preliminary application suggests that several observational constraints are satisfied if we have a two-component outflow, one originating at the star while the other at the surrounding disc. A simple criterion is given for the transition from a magnetic rotator with collimated jets to a slow magnetic rotator with a noncollimated wind, in terms of the meridional anisotropy in the available energy in the streamlines of the outflow; it essentially amounts to saying that if the magnetic rotator results in a surplus of energy along any nonpolar streamline as compared to the available energy along the polar streamline, then the flow collimates into a jet. It is suggested therefore that the shape of the streamlines of the outflow from a rotating object switches over from cylindrical to conical asymptotics, as the central object looses angular momentum and passes successively from the stage of a pre-main sequence YSO to that of a main sequence star.
Sauty Christophe
Tsinganos Kanaris
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