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

¹ Chinese Academy of Sciences South America Center for Astronomy (CASSACA), Camino El Observatorio 1515, Las Condes, Santiago, Chile
² Departamento de Fisica y Astronomia, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, 8320000 Santiago, Chile
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 October 2025
Accepted: 5 March 2026

Abstract

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, log₁₀(W₈₆₂₁/A_V), and dust extinction, log₁₀(A_V). In general, log₁₀(W₈₆₂₁/A_V) declines with log₁₀(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 log₁₀(W₈₆₂₁/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 log₁₀(W₈₆₂₁/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.