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

Abstract

The tendency of some diffuse interstellar band (DIB) carriers to concentrate in the outer, UV-illuminated layers of molecular clouds (MCs)--the ``skin effect''--makes their spatial distribution a powerful probe of their physical nature. We leverage Gaia DR3 measurements of the DIB at 862.1 nm to investigate its behavior across 12 nearby MCs, spanning diffuse to translucent regimes ($A_{\rm V}\,{\sim}\,0.2{-}3.5$ 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 (PLM) to fit the average slope ($α$) between the normalized DIB strength, ${\rm log_{10}}(W_{8621}/A_{\rm V})$, and dust extinction, ${\rm log_{10}}(A_{\rm V})$. In general, ${\rm log_{10}}(W_{8621}/A_{\rm V})$ declines with ${\rm log_{10}}(A_{\rm V})$ with $α$ between 0 and --1, becoming progressively steeper at higher $A_{\rm V}$. 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, clumpiness). Particularly, the Taurus cloud region uniquely displays an initial increase in ${\rm log_{10}}(W_{8621}/A_{\rm V})$ at low extinction, a signature predicted for a cationic carrier. By fitting the slope of this rising trend, we estimate an ionization potential of $E_{\rm IP}\,{=}\,12.40^{+1.90}_{-2.29}$ eV for the DIB$λ$8621 carrier, which aligns well with the secondary ionization energies of large carbonaceous molecules like polycyclic aromatic hydrocarbons (PAHs) or fullerenes.

Figures (10)

• Figure 1: Location of target cloud regions (colored rectangles with names marked below), overplotted on $^{12}\rm CO$ ($J\,{=}\,1{-}0$) intensity map from Dame2001.
• Figure 2: Variation of ${\rm log_{10}}(W_{8621}/A_{\rm V})$ as a function of ${\rm log_{10}}(A_{\rm V})$ for each target cloud. The corresponding $A_{\rm V}$ scale is shown on the top of each panel. Points are color-coded by their number density, estimated via a Gaussian KDE. Blue squares represent the median ${\rm log_{10}}(W_{8621}/A_{\rm V})$ in bins of ${\rm log_{10}}(A_{\rm V})$ (from --0.8 to 0.6 with a step of 0.1). The solid red line and shaded region show the selected Piecewise Linear Model (PLM) and its 99.7% credible interval, respectively. Vertical dashed orange lines indicate the knot locations (see Sect. \ref{['subsect:PLM']}). The full PLM fitting results with $n_p\,{=}\,1{-}4$ for each cloud can be found in Fig. \ref{['fig:mc-plm-all']}. For Cepheus South, points marked with black crosses were clipped as outliers and excluded from the fit (see Sect. \ref{['sect:results']} for details). Each panel is labeled with the cloud name, distance, and the number of sightlines used.
• Figure 3: Comparison of the slopes and knots of the selected PLM between target clouds. The slopes are arranged with increasing ${\rm log_{10}}(A_{\rm V})$ and marked with different colors.
• Figure 4: Linear fit to the relationship between ${\rm log_{10}}(W_{8621}/E_{\rm B{-}V})$ and $E_{\rm B{-}V}$ for sightlines in the Taurus cloud with $E_{\rm B{-}V}\,{<}\,0.2$ mag. The data points included in the fit are shown as red squares. The best-fit line is shown in red, and the resulting slope ($\alpha_T$) and the Pearson correlation coefficient ($r_p$) are indicated.
• Figure 5: The theoretical relationship between the ionization potential ($E_{\rm IP}$) of a DIB carrier and the expected observational slope ($\alpha_T$) from the PIE model of Sonnentrucker1997. The relationship is plotted for three different $R_{\rm V} = 3.1, 4.0,$ and $5.5$. The vertical orange line and shaded region represent our measured value of $\alpha_T$ for DIB $\lambda$8621 (from Fig. \ref{['fig:Taurus']}) and its 1$\sigma$ uncertainty, respectively. The intersection of our measurement with the $R_{\rm V}\,{=}\,3.1$ curve implies $E_{\rm IP}\,{\approx}\,12$ eV.