Open cluster Dolidze 25: Stellar parameters and the metallicity in the
  Galactic Anticentre
  The young open cluster Dolidze 25, in the direction of the Galactic
Anticentre, has been attributed a very low metallicity, with typical abundances
between $-0.5$ and $-0.7$ dex below solar. We intend to derive accurate cluster
parameters and accurate stellar abundances for some of its members. We have
obtained a large sample of intermediate- and high-resolution spectra for stars
in and around Dolidze 25. We used the FASTWIND code to generate stellar
atmosphere models to fit the observed spectra. We derive stellar parameters for
a large number of OB stars in the area, and abundances of oxygen and silicon
for a number of stars with spectral types around B0. We measure low abundances
in stars of Dolidze 25. For the three stars with spectral types around B0, we
find $0.3$ dex (Si) and $0.5$ dex (O) below the values typical in the solar
neighbourhood. These values, even though not as low as those given previously,
confirm Dolidze 25 and the surrounding H II region Sh2-284 as the most
metal-poor star-forming environment known in the Milky Way. We derive a
distance $4.5\pm0.3\:$kpc to the cluster ($r_{\textrm{G}}\approx12.3\:$kpc).
The cluster cannot be older than $\sim3\:$Myr, and likely is not much younger.
One star in its immediate vicinity, sharing the same distance, has Si and O
abundances at most $0.15\:$dex below solar. The low abundances measured in
Dolidze 25 are compatible with currently accepted values for the slope of the
Galactic metallicity gradient, if we take into account that variations of at
least $\pm0.15\:$dex are observed at a given radius. The area traditionally
identified as Dolidze 25 is only a small part of a much larger star-forming
region that comprises the whole dust shell associated with Sh2-284 and very
likely several other smaller H II regions in its vicinity.
