Linking the properties of accreting white dwarfs with the ionization state of their ambient medium

TL;DR

This study models the ionization effects of accreting white dwarfs on their surroundings, revealing how nebulae characteristics depend on WD properties and challenging their role as progenitors of certain supernovae.

Contribution

It provides the first detailed simulations linking white dwarf accretion properties with nebular emission spectra and ionization states, offering new insights into their observational signatures.

Findings

Nebular line fluxes peak for WD masses of 0.8-1.2 solar masses.

Accreting WD nebulae have distinct line ratios from H II regions.

Constraints on ambient medium density challenge the role of steady accretion in supernova progenitors.

Abstract

Steadily accreting white dwarfs (WDs) are efficient sources of ionization and thus, are able to create extended ionized nebulae in their vicinity. These nebulae represent ideal tools for the detection of accreting WDs, given that in most cases the source itself is faint. In this work, we combine radiation transfer simulations with known H and He accreting WD models, providing for the first time the ionization state and the emission line spectra of the formed nebulae as a function of the WD mass, the accretion rate and the chemical composition of the accreted material. We find that the nebular optical line fluxes and radial extent vary strongly with the WD's accretion properties, peaking in systems with WD masses of 0.8 - 1.2 $\rm~M_{\odot}$. Projecting our results on the 'BPT' diagnostic diagrams, we show that accreting WDs nebulae possess characteristics distinct from those of H II-like regions, while they share similar line ratios with the galactic low-ionization emission-line regions. Finally, we compare our results to the relevant constraints imposed by the lack of ionized nebulae in the vicinity of supersoft X-ray sources (SSSs) and Type Ia supernova remnants - sources which are related to steadily accreting WDs. The large discrepancies uncovered by our comparison rule out any steadily accreting WD as a potential progenitor of the studied remnants and additionally require the ambient medium around the SSSs to be less dense than 0.2 $\rm~cm^{-3}$. We discuss possible alternatives that could bridge the incompatibility between the theoretical expectations and the relevant observations.