Effect of optically thin cooling curves on condensation formation: Case study using thermal instability

TL;DR

This study investigates how different optically thin cooling curves influence the formation, morphology, and stability of condensations due to thermal instability in astrophysical plasmas, using 2D MHD simulations.

Contribution

It provides a comparative analysis of various cooling curves' effects on condensation formation and introduces a bootstrap method for non-linear regime analysis.

Findings

Cooling curves significantly affect condensation morphology.

Condensations with low-temperature cutoff below 20,000 K are more stable.

Non-linear and fragmentation behaviors differ between hydrodynamic and MHD cases.

Abstract

Non-gravitationally induced condensations are observed in many astrophysical environments. Such structures are formed due to energy loss by optically thin radiative emission. Instead of solving the full radiative transfer equations, precomputed cooling curves are typically used in numerical simulations. In the literature, there exists a wide variety of cooling curves and they are quite often used as unquestionable ingredients. We determine the effect of the optically thin cooling curves on the formation and evolution of condensations. We perform a case study using thermal instability as a mechanism to form in situ condensations. We compare 2D numerical simulations with different cooling curves using interacting slow magnetohydrodynamic (MHD) waves as trigger for the thermal instability. Furthermore, we discuss a bootstrap measure to investigate the far non-linear regime of thermal instability. In the appendix, we include the details of all cooling curves implemented in MPI-AMRVAC and briefly discuss a hydrodynamic variant of the slow MHD waves setup for thermal instability. For all tested cooling curves, condensations are formed. However, the growth rates of the thermal instability are different. Also, the morphology of the formed condensation widely varies. We find fragmentation that is affected by the low-temperature treatment of the cooling curves. Condensations formed using cooling curves that vanish for temperatures lower than 20 000 K seem to be more stable against dynamical instabilities. The non-linear regime and fragmentation in the hydrodynamic case differ greatly from the MHD case. We advocate the use of modern cooling curves, based on accurate computations and up-to-date atomic parameters and solar abundances. Our bootstrap procedure can be used in future multi-dimensional simulations, to study fine-structure dynamics in solar prominences.