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

Abstract

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.