Galactic HII regions in LAMOST Medium-Resolution Spectroscopic Survey of Nebulae

Abstract

Based on LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) data and WISE Galactic HII region catalog, we construct a sample of 280 Galactic HII regions and candidates in the Outer Galaxy (80${}^{\circ }$ $\lesssim$ l $\lesssim$ 220${}^{\circ }$). Using MRS-N optical spectra, we measure four emission lines (H$\hat{I}\pm$, [NII]$\hat{I}»$6584, [SII]$\hat{I}»\hat{I}»$6717,6731) and use line-ratios to spectroscopically confirm 255 HII regions, including 90 previously "Known" HII regions and 165 newly classified ones. We measure their $T_{\mathrm{e}}$, $n_{\mathrm{e}}$ and oxygen abundance, and determine distances via associated OB stars and the kinematic method. The sample spans $R_{\mathrm{gal}}$ from 8.16 to 15.36 kpc, enabling investigation of radial gradients in physical properties. We find [NII]/H$\hat{I}\pm$ and [SII]/H$\hat{I}\pm$ decrease with increasing $R_{\mathrm{gal}}$, while [SII]/[NII] remains nearly flat; these trends are quite different from diffuse ionized gas (DIG). We derive the $T_{\mathrm{e}}$ gradient of 344.530 $\pm$ 78.083 K kpc${}^{-1}$, and the $\log n_{\mathrm{e}}$ gradient of -0.143 $\pm$ 0.041 cm${}^{-3}$ kpc${}^{-1}$. Oxygen abundance shows a steep slope of -0.044 $\pm$ 0.010 dex kpc${}^{-1}$ in the inner disk and a shallow slope of -0.016 $\pm$ 0.005 dex kpc${}^{-1}$ in the outer disk, with a global slope of -0.014 $\pm$ 0.005 dex kpc${}^{-1}$. We also examine the two-dimensional distributions of $T_{\mathrm{e}}$, $n_{\mathrm{e}}$, and oxygen abundance, and find the gradients vary with azimuth. There is no obvious difference between spiral arm and interarm regions, and no trend appears along individual arms. From [NII]/H$\hat{I}\pm$-[SII]$\hat{I}»$6717/H$\hat{I}\pm$ diagram, HII regions have a S${}^{+}$/S ratio (0.32), lower than DIG (0.43); however, heavy overlap prevents clear separation from this diagram alone.

21 pages, 13 figures, accepted by AJ

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