Expanding atmosphere models for SSS spectra of novae

TL;DR

This paper introduces spherically symmetric, expanding atmosphere models for Super Soft Source spectra of novae, showing that wind-driven expansion significantly affects spectral features and inferred parameters.

Contribution

The paper develops and compares expanding, dynamic atmosphere models with static ones, highlighting their impact on spectral interpretation and elemental abundance estimates.

Findings

Expanding models produce less pronounced absorption edges.

Harder X-ray spectra are generated by dynamic models.

Lower effective temperatures are needed to fit observed spectra.

Abstract

Super Soft Source (SSS) spectra are powered by nuclear burning on the surface of a white dwarf. The released energy causes a radiatively-driven wind that leads to a radially extended atmosphere around the white dwarf. Significant blue shifts in photospheric absorption lines are found in the spectra of novae during their SSS phase, being an evidence of continued mass loss in this phase. We present spherically symmetric PHOENIX models that account for the expansion of the ejecta. A comparison to a plane parallel, hydrostatic atmosphere model demonstrates that the mass loss can have a significant impact on the model spectra. The dynamic model yields less pronounced absorption edges, and harder X-ray spectra are the result. Therefore, lower effective temperatures are needed to explain the observed spectra. Although both types of models are yet to be fine-tuned in order to accurately determine best fit parameters, the implications on the chemical abundances are going in opposite directions. With the expanding models the requirement for strong depletion of the crucial elements that cause these edges is now avoidable.