Spectroscopic Study of M33 with the LAMOST Survey. I. Chemical Gradients from Nebulae

Morphological and chemical structures of M33 are investigated with the LAMOST DR7 survey. Physical parameters; extinction; chemical composition of He, N, O, Ne, S, Cl, and Ar (where available); and radial velocities were determined for 110 nebulae (95 H II regions and 15 planetary nebulae) in M33. Among them, 8 planetary nebulae and 55 H II regions in M33 are newly discovered. We obtained the following O abundance gradients: - ${0.199}_{-0.030}^{+0.030}$ dex ${R}_{25}^{-1}$ (based on 95 H II regions), - ${0.124}_{-0.036}^{+0.036}$ dex ${R}_{25}^{-1}$ (based on 93 H II regions), and - ${0.207}_{-0.174}^{+0.160}$ dex ${R}_{25}^{-1}$ (based on 21 H II regions), utilizing abundances from N2 at O3N2 diagnostics and the T _e -sensitive method, respectively. The He, N, Ne, S, and Ar gradients resulted in slopes of - ${0.179}_{-0.146}^{+0.145}$ , - ${0.431}_{-0.281}^{+0.282}$ , - ${0.171}_{-0.239}^{+0.234}$ , - ${0.417}_{-0.182}^{+0.174}$ , and - ${0.340}_{-0.157}^{+0.156}$ , respectively, utilizing abundances from the T _e -sensitive method. Our results confirm the existence of the negative axisymmetric global metallicity distribution that is assumed in the literature. We noticed one new WC star candidate and one transition W-R WN/C candidate. The grand-design pattern of the spiral structure of M33 is presented.