Title: Spectroscopic Observation and Analysis of HII regions in M33 with MMT: Temperatures and Oxygen Abundances,
Abstract: The spectra of 413 star-forming (or HII) regions in M33 (NGC 598) were
observed by using the multifiber spectrograph of Hectospec at the 6.5-m
Multiple Mirror Telescope (MMT). By using this homogeneous spectra sample, we
measured the intensities of emission lines and some physical parameters, such
as electron temperatures, electron densities, and metallicities. Oxygen
abundances were derived via the direct method (when available) and two
empirical strong-line methods, namely, O3N2 and N2. In the high-metallicity
end, oxygen abundances derived from O3N2 calibration were higher than those
derived from N2 index, indicating an inconsistency between O3N2 and N2
calibrations. We presented a detailed analysis of the spatial distribution of
gas-phase oxygen abundances in M33 and confirmed the existence of the
axisymmetric global metallicity distribution widely assumed in literature.
Local variations were also observed and subsequently associated with spiral
structures to provide evidence of radial migration driven by arms. Our O/H
gradient fitted out to 1.1 $R_{25}$ resulted in slopes of $-0.17\pm0.03$,
$-0.19\pm0.01$, and $-0.16\pm0.17$ dex $R_{25}^{-1}$ utilizing abundances from
O3N2, N2 diagnostics, and direct method, respectively.