Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...419..117k&link_type=abstract
Astrophysical Journal v.419, p.117
Astronomy and Astrophysics
Astronomy
2
Galaxies: Individual Messier Number: M87, Galaxies: Jets, Magnetohydrodynamics: Mhd
Scientific paper
We apply the narrow-jet model developed by Koupelis & Van Horn (in which the MHD equations are projected onto the jet symmetry axis by defining radially averaged values for the parameters of interest) to the case of the M87 jet. The wind-type solutions for the motions in the jet, obtained by using the observed values of the speeds and locations of two of the features in the jet (N2 and A), lead to strict limits on the values of some of the parameters describing the outflow, both close to the nucleus and along the jet.
We find that the combined effects of rotation and magnetic fields play an active role in the physics of the M87 jet, not only in assisting in the confinement of the flow (as suggested by the observations), but also in accelerating the flow along the length of the jet. Our results show that the flow attains relativistic velocities close to the initial point (feature N2), and that it is sub-Alfvénic and supersonic along the entire length of the jet. The poloidal Mach number of the flow along the jet is at least of the order of Mp ˜ 2-10. The region of feature A is where the flow approaches its Alfvén point and the magnetic field configuration becomes Bω-dominated, suggesting that reconnection effects may become important. Since the true jet origin is much closer to the nucleus of M87 than the position of N2, our results suggest that the opening angle of the jet is not constant, especially in the region very close to the nucleus. The latter conclusion is also supported by the observations which suggest that some collimation occurs on scales between a few parsec and a few 100 pc.
By using the model to fit the observations we obtain limits on the values of the characteristic parameters for the M87jet. We find that the rate of mass outflow is 6.5 × 10-5 Msun yr-1 ≲ Mṡout ≲ 5.5 × 10-3 Msun yr-1, the number density at the initial point (N2) is 2 protons cm-3 n0 1≲ 145 protons cm-3, the magnetic field strength is 44 G ≲ B0 ≲ 404 G, the gas pressure is 3.2 × 10-5 dyn cm-2 ≲ P0 2.7 × 10-3 dyn cm-2, and the rotational velocity is Ω0 ≃ 2 × 10-10 rad s-1. Faraday rotation along the jet is negligible for all wavelengths up to a few centimeters.
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