Concluding remarks

We have reviewed the literature on the direct determination of metallicities of massive stars, concentrating on establishing the metallicity scale of OB stars and its relationship with the solar metallcity and H ii-region results, and investigated

Figure 6.3. Oxygen abundances for supergiants in M31 as a function of galacto-centric distance. The solid circles refer to NLTE results including line-blanketing and winds, while the open circles show the LTE results for the same stars. Also shown are LTE results for other B- and A-type supergiants in M31, while the nebular abundance gradients derived with various calibrations of R23 are shown as lines (see the key). Note the lack of supersolar stellar abundances in contrast with the nebular results at low galactocentric distances.

Figure 6.3. Oxygen abundances for supergiants in M31 as a function of galacto-centric distance. The solid circles refer to NLTE results including line-blanketing and winds, while the open circles show the LTE results for the same stars. Also shown are LTE results for other B- and A-type supergiants in M31, while the nebular abundance gradients derived with various calibrations of R23 are shown as lines (see the key). Note the lack of supersolar stellar abundances in contrast with the nebular results at low galactocentric distances.

the oxygen abundance of massive stars in supposedly metal-rich environments of the inner Milky Way and M31. We draw the following conclusions.

• OB stars within approximately 500 pc of the Sun have similar metallicities with [O/H] ~ 8.60 ± 0.1, in good agreement with the latest estimates of the Solar oxygen abundance derived using three-dimensional models of 8.66 dex.

• This value agrees well with the oxygen abundance of B-type stars in the Orion OB1 association and with the H ii-region oxygen abundance of this region.

• Little evidence is found for superSolar oxygen abundance in the Galactic disc, even at small Galactocentric distances, the abundance gradient for this element being rather flat within the Solar circle.

• Massive stars in the inner regions of M31 are at best only mildly metal-rich, again displaying a rather flat gradient in [O/H], in disagreement with H ii-region results.

• Estimates of [O/H] using the R23 index depend very sensitively on the calibration used, especially in the metal-rich regime. In addition, most calibrations predict very high [O/H] compared with that of the stars in the inner regions of M31.

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