Investigating the Possible Anomaly between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM

We obtained new optical spectra of 13 H II regions in WLM with EFOSC2; oxygen abundances are derived for nine H II regions. The temperature-sensitive [O III] l4363 emission line was measured in two bright H II regions, HM 7 and HM 9. The direct oxygen abundances for HM 7 and HM 9 are 12 + log(O/H) = 7.72 +- 0.04 and 7.91 +- 0.04, respectively. We adopt a mean oxygen abundance of 12 + log(O/H) = 7.83 +- 0.06. This corresponds to [O/H] = -0.83 dex, or 15% of the solar value. In H II regions where [O III] l4363 was not measured, oxygen abundances derived with bright-line methods are in general agreement with direct values of the oxygen abundance to an accuracy of about 0.2 dex. In general, the present measurements show that the H II region oxygen abundances agree with previous values in the literature. The nebular oxygen abundances are marginally consistent with the mean stellar magnesium abundance ([Mg/H] = -0.62). However, there is still a 0.62 dex discrepancy in oxygen abundance between the nebular result and the A-type supergiant star WLM 15 ([O/H] = -0.21). Nonzero reddening values derived from Balmer line ratios were found in H II regions near a second H I peak. There may be a connection between the location of the second H I peak, regions of higher extinction, and the position of WLM 15 on the eastern side of the galaxy.