Hubble Space Telescope WFPC2 study of the trapezium cluster:: The influence of circumstellar disks on the initial mass function

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 ( WFPC2) on board the Hubble Space Telescope. We obtained photometry for 91 stars in the U band ( F336W) and 71 stars in the B band ( F439W). The WFPC2 archive was also searched to obtain complementary V-band ( F547M) and I-band ( F791W) photometry. In this paper we focus our attention on a group of 40 stars with known spectral types and complete UBVI WFPC2 photometry. We locate each star on the H-R diagram, considering both the standard ISM reddening law with R-V = 3.1 and the anomalous'' reddening law with R-V = 5.5 more appropriate for the Orion Nebula. Then we derive the stellar masses and ages by comparing with the evolutionary tracks and isochrones calculated by D'Antona & Mazzitelli and Palla & Stahler. Approximately three-quarters of the sources show excess luminosity in the U band, which we attribute to mass accretion. The known correlation between the U-band excess and the total accretion luminosity, recalibrated for our photometric system, allows us to estimate the accretion rates, which are all found to be in the range 10(-8) to 10(-12) M-. yr(-1). For stars older than 1 Myr, there is some evidence of a relation between mass accretion rates and stellar age. 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 10(-5) M-. yr(-1) 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 ( IMF) may therefore be affected by the rapid evolution of the most massive cluster's stars, causing a surplus of 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.