A comprehensive analysis toward the Fermi-LAT source 4FGL J1846.9-0227 -- Jets of a proto-planetary nebula producing gamma-rays?

Most of the gamma-ray sources in the Fermi-LAT 14-year Source Catalog are associated with pulsars and blazars. However, unveiling the nature of the still unassociated gamma-ray sources is important for the understanding of high energy emission mechanisms in astrophysical objects. This work presents a comprehensive study toward the region covered by the Fermi source 4FGL J1846.9$-$0227 previously suggested to be a blazar or a massive protostar. Using multiwavelength observations, we analyzed several astrophysical objects in the region as possible counterparts of the Fermi-LAT source. Having discarded most of them after a comprehensive analysis, we suggest that the most likely candidate to be such a counterpart is IRAS 18443$-$0231. We discovered that this source, previously cataloged as a planetary nebula candidate, actually is a proto-planetary nebula. The radio continuum image at 3 GHz associated with such a nebula allowed us to identify a jet-like structure. Additionally, we identified an associated red-shifted CO molecular outflow and a dense molecular clump in which the source is embedded. We obtained a radio spectral index of -0.47 ± 0.08 for the source, indicating syncrothron emission due to accelerated particles. Thus, we suggest that processes such as proton-proton collisions and relativistic Bremsstrahlung are likely to occur. IRAS 18443$-$0231, lying almost at the center of the Fermi confidence ellipse and related to the hard X-ray source 4XMM J184700.4$-$022752, would be the first association between a proto-planeatry nebula and gamma emission.