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 so-called BPT diagnostic diagrams, we show that accreting WD nebulae possess characteristics distinct from those of H ii-like regions, while they have line ratios similar to those in galactic low-ionization emission-line regions. Finally, we compare our results with 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 that 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 relevant observations.
We present optical ccd images of the large supernova remnant (SNR) G132.7+1.3 (HB3) covering its full extent for the first time, in the emission lines of Hα +[N ii], [S ii] and [O iii], where new and known filamentary and diffuse structures are detected. These observations are supplemented by new low-resolution long-slit spectra and higher-resolution images in the same emission lines. Both the flux-calibrated images and spectra confirm that the optical emission originates from shock-heated gas since the [S ii]/Hα > 0.4. Our findings are also consistent with the recently developed emission line ratio diagnostics for distinguishing SNRs from H ii regions. A multi-wavelength comparison among our optical data and relevant observations in radio, X-rays, γ-rays and CO bands, provided additional evidence on the interaction of HB3 with the surrounding clouds and clarified the borders of the SNR and the adjacent cloud. We discuss the supernova (SN) properties and evolution that led to the current observables of HB3 and we show that the remnant has most likely passed at the pressure driven snowplow phase. The estimated SN energy was found to be (3.7 ± 1.5) × 1051 erg and the current SNR age (5.1 ± 2.1) × 104 yrs. We present an alternative scenario according to which the SNR evolved in the wind bubble cavity excavated by the progenitor star and currently is interacting with its density walls. We show that the overall mixed morphology properties of HB3 can be explained if the SN resulted by a Wolf-Rayet progenitor star with mass $\sim 34 \rm ~M_{\rm \odot }$.
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