Galactic H II Regions in the LAMOST Medium-Resolution Spectroscopic Survey of Nebulae

Abstract

Based on LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) data and the Wide-field Infrared Survey Explorer Galactic H II region catalog, we construct a sample of 280 Galactic H II regions and candidates in the outer Galaxy (80° ≲ l ≲ 220°). Using MRS-N optical spectra, we measure four emission lines (Hα, [N II] λ6584, [S II] λλ6717, 6731) and use line ratios to spectroscopically confirm 255 H II regions, including 90 previously “Known” H II regions and 165 newly classified ones. We measure their Te, ne, and oxygen abundance and determine distances via associated OB stars and the kinematic method. The sample spans Rgal from 8.16 to 15.36 kpc, enabling investigation of radial gradients in physical properties. We find that [N II]/Hα and [S II]/Hα decrease with increasing Rgal, while [S II]/[N II] remains nearly flat; these trends are quite different from diffuse ionized gas (DIG). We derive the Te gradient of 344.530 ± 78.083 K kpc−1 and the logne gradient of −0.143 ± 0.041 cm−3 kpc−1. Oxygen abundance shows a steep slope of −0.044 ± 0.010 dex kpc−1 in the inner disk and a shallow slope of −0.016 ± 0.005 dex kpc−1 in the outer disk, with a global slope of −0.014 ± 0.005 dex kpc−1. We also examine the two-dimensional distributions of Te, ne, and oxygen abundance and find that the gradients vary with azimuth. There is no obvious difference between spiral arm and interarm regions, and no trend appears along individual arms. From the [N II]/Hα[S II] λ6717/Hα diagram, H II regions have a S+/S ratio (0.32) lower than that of DIG (0.43); however, heavy overlap prevents clear separation from this diagram alone.