2024A&A...690A..42L

Context. We present multi-wavelength observations of the Galactic supernova remnant (SNR) HB9 and the H II region G159.2+3.3 apparently projected nearby, in order to study their properties and potential physical connections.
Aims. Confirming the physical connections between SNRs and H II regions is crucial to understanding their origin and interactions with the environment. Optical emission lines are powerful tools with which to measure the physical, chemical, and dynamical properties of the ionised gas, so could further help us to confirm such physical connections.
Methods. We present new optical narrow-band images of G159.2+3.3, as well as long-slit medium-resolution optical spectroscopy of both G159.2+3.3 and the SNR HB9 projected nearby. We compared these new data to archival multi-wavelength data to study the properties of the multi-phase interstellar medium in and around these two objects.
Results. HB9 is bright in γ-rays, but the γ-ray morphology is centrally filled and most of it is not clearly associated with the surrounding molecular clouds. There is a weak apparent connection of HB9 to the infrared bright enclosing shell of G159.2+3.3 in the γ-ray. The diffuse Balmer line has an almost identical morphology to the radio emission in G159.2+3.3, indicating they are both thermal in origin. Using medium-band high-resolution optical spectra from selected regions of the southeast (SE) shell of HB9 and G159.2+3.3, we found the radial velocity dispersion of HB9 along the slit to be significantly higher than the full width at half maximum of the lines. In contrast, these two values are both smaller and comparable to each other in G159.2+3.3. This indicates that the gas in HB9 may have additional global motion triggered by the SNR shock. The [N II] λ6583 Å/Hα line ratio of both objects can be interpreted with photo-ionisation by hot stars or low-velocity shocks, except for the post-shock region in the SE shell of HB9, where the elevated [N II]/Hα line ratio suggests a contribution from shock ionisation. The measured electron density from the [S II] 6716/6730 line ratio is significantly lower in the brighter G159.2+3.3 compared to the SE shell of HB9.
Conclusions. Our density estimate suggests that G159.2+3.3, although appearing brighter and more compact, is likely located at a much larger distance than HB9, so the two objects have no physical connections, unless the shock compressed gas in HB9 has a significantly lower filling factor.