Nested bubbles in a supershell : new clues to the history of star formation in Canis Major OB1 and the origin of the Sh 2-296 (“Seagull”) nebula
Séminaire IPAG de Thierry Montmerle (IAP), mardi 4 décembre 2018 à 13h30, salle Manuel Forestini
The origin of the arc-shaped Sh 2-296 "Seagull" nebula is still mysterious. Some authors have proposed that it is a 0.5 Myr-old supernova remnant (SNR) shock, inducing star formation in the CMa OB1 association. We show instead that the nebula is part of a large, shell-like structure ( 60 pc), enclosing a bubble created by three successive supernova explosions having taken place 6, 2 and 1 Myr ago. We base this chronology on three runaway stars that appear to have been ejected from a compact, Trapezium-like progenitor cluster near its center.
A number of late-type O stars are known to be present in the region, but they cannot explain the ionization of the Sh 2-296 nebula. In contrast, SNRs are filled with an X-ray emitting plasma, and we show that the ionization of the nebula can be explained in terms of conductive heating at the contact surface between the hot SNR bubble and the adjacent, cold molecular clouds. Then the "Seagull" nebula may be a unique case where such a heating process may be directly observed.
In addition, we have discovered in Halpha-survey images a larger ( 140 pc), presumably older “superbubble”, which itself encloses the shell structure. We conclude that these nested SNRs probably testify to a succession of star formation episodes, each separated by a few Myr, and for which we find an "echo" in the distribution of stellar population ages. Then the "Seagull" nebula itself would represent only the final stages in the history of this star-forming region.