A cationic carrier for the diffuse interstellar band at 862.1 nm: Evidence of the skin effect in nearby diffuse to translucent clouds

The tendency of some diffuse interstellar band (DIB) carriers to concentrate in the outer UV-illuminated layers of molecular clouds (MCs), the so-called skin effect, makes their spatial distribution a powerful probe of their physical nature. We leveraged Gaia DR3 measurements of the DIB at 862.1 nm to investigate its behavior across 12 nearby MCs, spanning diffuse to translucent regimes ( A V ~ 0.2-3.5 mag). We find significant diversity in the DIB behavior, both between different clouds and within individual clouds from their outer to inner regions. To quantify these trends, we employed a piecewise linear model to fit the average slope ( α ) between the normalized DIB strength, log 10 ( W 8621 / A V ), and dust extinction, log 10 ( A V ). In general, log 10 ( W 8621 / A V ) declines with log 10 ( A V ) with α between 0 and −1 and becomes progressively steeper at higher A V . These observed slopes and their variations are consistent with the photoionization equilibrium models, where the carrier abundance is governed by local conditions, particularly the UV radiation field and cloud structure (e.g., density profiles or clumpiness). Particularly, the Taurus cloud region uniquely displays an initial increase in log 10 ( W 8621 / A V ) at low extinction, a signature predicted for a cationic carrier. By fitting the slope of this rising trend, we estimate an ionization potential of E IP = 12.40 −2.29 +1.90 eV for the DIB λ8621 carrier, which agrees well with the secondary ionization energies of large carbonaceous molecules such as polycyclic aromatic hydrocarbons or fullerenes. Furthermore, by comparing the peak position of log 10 ( W 8621 / E B-V ) in Taurus with other prominent optical DIBs, we propose a spatial sequence within clouds, placing the layer of the λ8621 carrier between the layers of λ5780 and λ6614. Our results support a cationic carrier for DIB λ8621 and help situate it within the broader context of the DIB family.