2024A&A...689A.278K

The Simeis 147 nebula (S147) is particularly well known for a spectacular net of H_α-emitting filaments. It is often considered one of the largest and oldest (∼10⁵ yr) cataloged supernova remnants in the Milky Way, although the kinematics of the pulsar PSR J0538+2817 suggests that this supernova remnant might be a factor of three younger. The former case is considered in a companion paper, while here we pursue the latter. Both studies are based on the data of SRG/eROSITA All-Sky Survey observations. Here, we confront the inferred properties of the X-ray emitting gas data with the scenario of a supernova explosion in a low-density cavity, such as a wind-blown-bubble. This scenario assumes that a ∼20 M_⊙ progenitor star has had a low velocity with respect to the ambient interstellar medium, and so stayed close to the center of a dense shell created during its main-sequence evolution till the moment of the core-collapse explosion. The ejecta first propagate through the low-density cavity until they collide with the dense shell, and only then does the reverse shock go deeper into the ejecta and power the observed X-ray emission of the nebula. The part of the remnant inside the dense shell remains non-radiative till this point, plausibly in a state with T_e < T_i and nonequilibrium ionization. On the contrary, the forward shock becomes radiative immediately after entering the dense shell, and, being subject to instabilities, gives the nebula its characteristic “foamy” appearance in H_α and radio emission.