  We have performed the first measures of mass accretion rates in the core of
the Orion Nebula Cluster. Four adjacent fields centered on the Trapezium stars
have been imaged in the U- and B-bands using the Wide Field Planetary Camera 2
on board the Hubble Space Telescope. In this paper we focus our attention on a
group of 40 stars with known spectral types and complete UBVI WFPC2 photometry.
Approximately three quarters of the sources show excess luminosity in the
U-band, that we attribute to mass accretion. The known correlation between the
U-band excess and the total accretion luminosity allows us to estimate the
accretion rates. Overall, mass accretion rates appear lower than those measured
by other authors in the Orion flanking fields or in Taurus-Auriga. Mass
accretion rates remain low even in the vicinity of the birth line of Palla &
Stahler, suggesting that in the core of the Trapezium cluster disk accretion
has been recently depressed by an external mechanism. We suggest that the UV
radiation generated by the Trapezium OB stars, responsible for the disk
evaporation, may also cause the drop of the mass accretion rate. In this
scenario, low-mass stars may terminate their pre-main sequence evolution with
masses lower than those they would have reached if disk accretion could have
proceeded undisturbed until the final disk consumption. In OB associations the
low-mass end of the Initial Mass Function may therefore be affected by the
rapid evolution of the most massive cluster's stars, causing a surplus
"accretion aborted" very low-mass stars and brown dwarfs, and a deficit of
intermediate mass stars. This trend is in agreement with recent observations of
the IMF in the Trapezium cluster.
