Polytropic models of filamentary interstellar clouds -II. Helical magnetic fields

TL;DR

This paper models magnetised filamentary interstellar clouds using polytropic cylinders with helical magnetic fields, analyzing how magnetic support or compression depends on the polytropic exponent and magnetic field geometry.

Contribution

It extends previous work by deriving scale-free solutions for magnetised filaments with helical fields, linking the polytropic exponent to density and magnetic field profiles and force directions.

Findings

Magnetic field decreases less steeply than density with radius.

Field lines form helices twisted over cylindrical surfaces.

Support or compression depends on the polytropic exponent and pitch angle.

Abstract

We study the properties of magnetised cylindrical polytropes as models for interstellar filamentary clouds, extending the analysis presented in a companion paper (Toci & Galli 2014a). We formulate the general problem of magnetostatic equilibrium in the presence of a helical magnetic field, with the aim of determining the degree of support or compression resulting from the magnetisation of the cloud. We derive scale-free solutions appropriate to describe the properties of the envelopes of filaments at radii larger than the flat-density region. In these solutions, the polytropic exponent determines the radial profiles of the density and the magnetic field. The latter decreases with radius less steeply than the density, and field lines are helices twisted over cylindrical surfaces. A soft equation of state supports magnetic configurations that preferentially compress and confine the filament, whereas in the isothermal limit the field provides support. For each value of the polytropic exponent, the Lorentz force is directed outward or inward depending on whether the pitch angle is below or above some critical value which is a function of the polytropic exponent only.