Work

SDSS-V LVM: A Spatially Resolved Study of the Physical Conditions and the Chemical Abundance Discrepancy in the Lagoon Nebula (M 8)

Year: 2026

Type: article

Abstract: The abundance discrepancy problem refers to the systematic differences observed between chemical abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) of heavy ions. It remains a major unsolved problem in the study of ionized nebulae and is quantified by the abundance discrepancy factor (ADF). In this work, we present a deep integral field spectroscopic data set of the entire Lagoon Nebula (M8), obtained by the Sloan Digital Sky Survey V Local Volume Mapper project, at a spatial resolution of 0.21 pc spaxel −1 . This unique data set allows us, for the first time, to investigate spatially resolved maps of oxygen RL intensities (O ii V1), together with maps of H i RLs, heavy-ion CELs, and dust attenuation across a whole H ii region. We map the electron temperature using CELs and RLs of O 2+ and CELs of N + , and we map the electron density using CELs of S + . We derive CEL-based ionic and elemental oxygen abundances and, for the first time, a spatially resolved map of the RL-based O 2+ abundance in an H ii region. These measurements enable construction of the first spatially resolved ADF(O 2+ ) map of an H ii region and yield a global mean ADF of ∼0.47 ± 0.02 dex. Focusing on the central region of M8, where ionization is dominated by the O-type star Her 36, we find radial variations in the ADF, ranging between ∼0.35 and 0.50 dex. Our findings provide novel constraints on the spatial behavior and origin of the abundance discrepancy in the H ii regions.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/ae563a