2021A&A...650A..86G

Context. The Arches Cluster - Arched Filaments (AF) system is our Galaxy's prime example of the complexity involved in the interaction between the strong radiation field of numerous OB stars and their surrounding ISM in extremely harsh environments such as the Galactic center (GC) of the Milky Way. It offers a unique opportunity to study the close relationship between photon-dominated regions (PDRs) and HII regions and their relative contributions to the observed [CII] emission.
Aims. We aim to investigate the I([CII]) versus I([NII]) integrated intensity behavior in the AF region in order to assess the [CII] emission contribution from the HII region, which is traced by [NII] line observations, and PDR components in the high-metallicity environment of the GC.
Methods. We used [CII] 158µm and [NII] 205µm fine structure line observations of the AF in the literature to compare their observational integrated intensity distribution to semi-theoretical predictions for the contribution of HII regions and adjacent PDRs to the observed [CII] emission. We explored variations in the [C/N] elemental abundance ratio to explain the overall behavior of the observed relationship. Based on our models, the HII region and PDR contributions to the observed [CII] emission is calculated for a few positions within and near to the AF. Estimates for the [C/N] abundance ratio and [N/H] nitrogen elemental abundance in the AF can then be derived.
Results. The behavior of the I([CII]) versus I([NII]) relationship in the AF can be explained by model results satisfying 0.84<[C/N]_AF<1.41, with model metallicities ranging from 1Z_☉ to 2Z_☉, hydrogen volume density logn(H)=3.5, and ionization parameters logU from -1 to -2. A least-squares fit to the model data points yields log I([CII])=1.068xlogI([NII])+0.645 to predict the [CII] emission arising from the HII regions in the AF. The fraction of the total observed [CII] emission arising from within PDRs varies between ∼0.20 and ∼0.75. Our results yield average values for the carbon-to-nitrogen ratio and nitrogen elemental abundances of [C/N]_AF=1.13±0.09 and [N/H]_AF=6.21x10^–4 for the AF, respectively. They are a factor of ∼0.4 smaller and ∼7.5 larger than their corresponding Galactic disk values.
Conclusions. The large spatial variation of the fraction of [CII] emission arising either from HII regions or PDRs suggests that both contributions must be disentangled before any modeling attempt is made to explain the observed [CII] emission in the AF. We suggest thatsecondary production of nitrogen from low- to intermediate-mass stars in the Galactic bulge is a plausible mechanism to explain the large abundance differences between the GC and the Galactic disk. The mass loss of such stars would enrich the GC ISM with nitrogen as the gas falls into the inner GC orbits where the AF are located. Overall, our results show that tight constraints are needed on the [C/N] abundance ratio for the GC, significantly tighter than previous abundance measurements have discerned.