Polytropic models of filamentary interstellar clouds - I. Structure and stability

TL;DR

This paper models filamentary interstellar clouds using polytropic models, showing that non-thermal pressure support explains observed density profiles and stability, with implications for understanding their structure and support mechanisms.

Contribution

It introduces polytropic models with specific exponents to accurately reproduce observed filament profiles and discusses their stability under non-isentropic support mechanisms.

Findings

Negative-index polytropes fit observed density profiles.

Non-thermal pressure support is more realistic than external heating.

Polytropic filaments can be stable with high density contrasts.

Abstract

The properties of filamentary interstellar clouds observed at sub-millimetre wavelengths, especially by the Herschel Space Observatory, are analysed with polytropic models in cylindrical symmetry. The observed radial density profiles are well reproduced by negative-index cylindrical polytropes with polytropic exponent $1/3\lesssim \gamma_{\rm p} \lesssim 2/3$ (polytropic index $-3\lesssim n \lesssim -3/2$), indicating either external heating or non-thermal pressure components. However, the former possibility requires unrealistically high gas temperatures at the filament's surface and is therefore very unlikely. Non-thermal support, perhaps resulting from a superposition of small-amplitude Alfv\'en waves (corresponding to $\gamma_{\rm p}=1/2$), is a more realistic possibility, at least for the most massive filaments. If the velocity dispersion scales as the square root of the density (or column density) on the filament's axis, as suggested by observations, then polytropic models are characterised by a uniform width. The mass per unit length of pressure-bounded cylindrical polytropes depends on the conditions at the boundary and is not limited as in the isothermal case. However, polytropic filaments can remain stable to radial collapse for values of the axis-to-surface density contrast as large as the values observed only if they are supported by a non-isentropic pressure component.