| Abstract
| - Context. We recently discovered mid-infrared light scattering by micron-size grains deeply buried in dark clouds. We have named this coreshine. We also showed that this effect is widespread across the Galaxy except in the Gum/Vela region, the only region among those we explored without any trace of coreshine. Aims. We aim to check whether the Gum/Vela situation is a chance effect or if coreshine is really absent from the region. Methods. We explored the entire available Spitzer/InfraRed Red Array Camera (IRAC) archive centered on the Gum/Vela region in search of the coreshine effect. Results. Out of 24 validated objects (of a total of 32), we found three cases of coreshine and three possible other cases, while we detect nine cases of non-coreshine emission (bright rimmed clouds - BRC - or polycyclic aromatic hydrocarbon - PAH - emission). This is markedly different from our previous galactic-wide survey with a ratio of 7-8 coreshine cases per PAH case. In Gum/Vela, a majority of the clouds with protostars or young stellar objects do not show a coreshine effect, while in the galactic-wide survey, 75% of the protostellar clouds do. Conclusions. The rare occurence of coreshine, outnumbered by PAH and BRC cases, together with a large number of protostars, let us conclude that the Gum Nebula is a supernova remnant (SNR), and that the blast wave has both reset the grain size distribution and induced the formation of several protostars. The absence of coreshine in the vicinity of several of the Class I objects also implies that the growth time for grains to efficiently scatter mid-infrared radiation exceeds the Class I life duration, which is typically 2 × 10 5 years, and it also implies that the blast wave has reached these clouds only recently despite the age of the Gum region (over 1.5 My). This is consistent with their large distance from the center of the SNR.
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