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Cygnus OB2 is a rich OB association in the Galaxy which has experiencedintense star formation in the last 20-25 Myr. Its stellar population shows acorrelation between age and Galactic longitude. Exploring the chemicalcomposition of its stellar content we will be able to check the degree ofhomogeneity of the natal molecular cloud and possible effects ofself-enrichment processes. Our aim is to determine silicon and oxygenabundances for a sample of eight early-type slow rotators in Cygnus OB2 inorder to check possible inhomogeneities across the whole association andwhether there exists a correlation of chemical composition with Galacticlongitude. We have performed a spectroscopic analysis of a sample of late O andearly B stars with low rotational velocity, which have been chosen so as tocover the whole association area. We have carried out an analysis based onequivalent widths of metal lines, the wings of the H Balmer lines and FASTWINDstellar atmosphere models to determine their stellar fundamental parameters aswell as the silicon and oxygen surface abundances. We derive a ratherhomogeneous distribution of silicon and oxygen abundances across the region,with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition inCygnus OB2 with no clear evidence for significant chemical self-enrichment,despite indications of strong stellar winds and possible supernovae during thehistory of the region. Comparison with different scenarios of chemicalenrichment by stellar winds and supernovae point to star forming efficienciesnot significantly above 10%. The degree of homogeneity that we find isconsistent with the observed Milky Way oxygen gradient based on HII regions. Wealso find that the oxygen scatter within Cygnus OB2 is at least of the sameorder than among HII regions at similar Galactocentric distance.

Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators