Photoionization Heating of Nova Ejecta by the Post-Outburst Supersoft Source

TL;DR

This paper investigates how photoionization heating by a post-outburst supersoft X-ray source maintains nova ejecta temperatures around 10,000 to 40,000 K for up to a year, using models and spectral synthesis.

Contribution

It introduces a combined modeling approach using a one-zone model and Cloudy to study the impact of WD temperature on nova ejecta heating and temperature evolution.

Findings

Ejecta temperature remains nearly constant for up to a year due to photoionization heating.

The duration of the isothermal phase depends on ejecta velocity and mass.

Ejecta temperature correlates with the WD's effective temperature and mass.

Abstract

The expanding ejecta from a classical nova remains hot enough ($\sim10^{4}\, {\rm K}$) to be detected in thermal radio emission for up to years after the cessation of mass loss triggered by a thermonuclear instability on the underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the hot temperatures observed in radio observations. During this same period, the unstable thermonuclear burning transitions to a prolonged period of stable burning of the remnant hydrogen-rich envelope, causing the WD to become, temporarily, a super-soft X-ray source. We show that photoionization heating of the expanding ejecta by the hot WD maintains the observed nearly constant temperature of $(1-4)\times10^4\mathrm{~K}$ for up to a year before an eventual decline in temperature due to either the cessation of the supersoft phase or the onset of a predominantly adiabatic expansion. We simulate the expanding ejecta using a one-zone model as well as the Cloudy spectral synthesis code, both incorporating the time-dependent WD effective temperatures for a range of masses from $0.60\ M_{\odot}$ to $1.10\ M_{\odot}$. We show that the duration of the nearly isothermal phase depends most strongly on the velocity and mass of the ejecta and that the ejecta temperature depends on the WD's effective temperature, and hence its mass.