Research Note

A Field-length based refinement criterion for adaptive mesh simulations of the interstellar medium

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: o.gressel@qmul.ac.uk
² Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK

Received: 1 September 2008
Accepted: 2 February 2009

Abstract

Context. Adequate modelling of the multiphase interstellar medium requires optically thin radiative cooling, containing an inherent thermal instability. The size of the resulting condensation and evaporation interfaces is determined by the so-called Field-length, which represents the dimension across which the instability is damped significantly by thermal conduction.

Aims. We study the relevance of conduction scale effects in the numerical modelling of a bistable medium and check the applicability of conventional and alternative adaptive mesh techniques.

Methods. The low value of the thermal conduction coefficient in the ISM defines a multiscale problem, promoting the use of adaptive meshes. We introduce a refinement strategy that applies the Field condition by Koyama & Inutsuka as a refinement criterion. The described method is similar to the Jeans criterion for gravitational instability by Truelove and allows us to trace efficiently the unstable gas situated at the thermal interfaces.

Results. We present test computations that demonstrate the higher accuracy of our proposed refinement criterion compared to refinement based on the local density gradient. Apart from its usefulness as a refinement trigger, we do not find evidence in favour of the Field criterion being a prerequisite for numerical stability.