Simulations and interpretation of the 6-cm MERLIN images of the classical nova V723 Cas

TL;DR

This paper compares simple spherical and ellipsoidal models of radio emission from nova V723 Cas with MERLIN observations, demonstrating that complex observed structures can be explained by basic models and emphasizing the importance of modeling in interferometric imaging.

Contribution

It introduces a modeling approach that reproduces observed nova radio structures using simple models, highlighting the impact of uv coverage on image interpretation.

Findings

Simple spherical models can reproduce complex observed structures.

24-hour tracking improves the accuracy of synthetic radio maps.

Sparse uv coverage increases susceptibility to instrumental effects.

Abstract

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the uv coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas. It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations showthat a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse uv coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations.