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Spectroscopic Study of M33 with LAMOST survey. I. Chemical gradients from nebulae

Morphological and chemical structures of M33 are investigated with LAMOST DR7 survey. Physical parameters, extinction, chemical composition of He, N, O, Ne, S, Cl, Ar (where available), and radial velocities were determined for 110 nebulae (95 H\ii\ regions and 15 PNe) in M33. Among them, 8 PNe 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\ region), utilizing abundances from N2 at O3N2 diagnostics and the $T_{\rm e}$-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}$, $-$0.340$_{-0.157}^{+0.156}$, respectively, utilizing abundances from the $T_{\rm e}$-sensitive method. Our results confirm the existence of negative axisymmetric global metallicity distribution that is assumed in the literature. We noticed one new WC star candidate and one transition WR WN/C candidate. The grand-design pattern of spiral structure of M33 is presented.