HUBBLE-SPACE-TELESCOPE WFPC2 OBSERVATIONS OF THE BINARY FRACTION AMONG PRE-MAIN-SEQUENCE CLUSTER STARS IN ORION

We present estimates of the binary frequency for optically visible (I- HST < 19 mag) members of the dense (mean star separation similar to 0. 05 pc) embedded stellar clusters associated with NGC 2024, NGC 2068, a nd NGC 2071 (distance = 460 pc) based on Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations. Of 99 targets, 15 are found to be double stars with projected linear separations of 138- 1050 AU (0.3 '' < theta < 2.3 ''). The resulting multiplicity fraction is 15/99 or 0.15 +/- 0.04 in this separation range, while the compara ble values observed for solar neighborhood G and K stars are 0.11 and 0.09, respectively. We also examined the archival F547M WFPC2 images o f the Trapezium cluster obtained by O'Dell & Wen and found a binary fr equency of 7/50 or 0.14 +/- 0.05 in the separation range 138-828 AU fo r stars with V-HST < 17. Our results compare well with the value of 0. 16 +/- 0.03 determined recently by Reipurth & Zinnecker from observati ons of 238 pre-main-sequence (PMS) stars in nearby (d similar to 150 p c) low-density (mean star separation similar to 0.3 pc) star-forming r egions over comparable separation ranges. Our results suggest an exces s of PMS binaries over main-sequence binaries with the same separation s. In addition, we find no evidence that the fraction of binaries form ed in relatively dense clusters differs from that characterizing low-d ensity star-forming regions.