Context : Elemental abundance gradients in galactic disks are important constraints for models of how spiral galaxies form and evolve .
However , the abundance structure of the outer disk region of the Milky Way is poorly known , which hampers our understanding of the spiral galaxy that is closest to us and that can be studied in greatest detail .
Young OB stars are good tracers of the present-day chemical abundance distribution of a stellar population and because of their high luminosities they can easily be observed at large distances , making them suitable to explore and map the abundance structure and gradients in the outer regions of the Galactic disk .

Aims : Using a sample of 31 main-sequence OB stars located between galactocentric distances 8.4 - 15.6 kpc , we aim to probe the present-day radial abundance gradients of the Galactic disk .

Methods : The analysis is based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m telescope on Las Campanas .
We used a non-NLTE analysis in a self-consistent semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric parameters and chemical abundances .

Results : Stellar parameters ( effective temperature , surface gravity , projected rotational velocity , microturbulence , and macroturbulence ) and silicon and oxygen abundances are presented for 28 stars located beyond 9 kpc from the Galactic centre plus three stars in the solar neighborhood .
The stars of our sample are mostly on the main-sequence , with effective temperatures between 20 800 - 31 300 K , and surface gravities between 3.23 - 4.45 dex .
The radial oxygen and silicon abundance gradients are negative and have slopes of -0.07 dex/kpc and -0.09 dex/kpc , respectively , in the region 8.4 \leq R _ { G } \leq 15.6 kpc .

Conclusions : The obtained gradients are compatible with the present-day oxygen and silicon abundances measured in the solar neighborhood and are consistent with radial metallicity gradients predicted by chemodynamical models of Galaxy Evolution for a subsample of young stars located close to the Galactic plane .