Hubble Space Telescope WFPC2 Observations of the Binary Fraction among Pre-Main-Sequence Cluster Stars in Orion

Abstract

We present estimates of the binary frequency for optically visible (I_HST < 19 mag) members of the dense (mean star separation ~0.05 pc) embedded stellar clusters associated with NGC 2024, NGC 2068, and 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" < θ < 2.3"). The resulting multiplicity fraction is 15/99 or 0.15 +/- 0.04 in this separation range, while the comparable values observed for solar neighborhood G and K stars are 0.11 and 0.09, respectively. We also examined the archival F547M WFPC2 images of the Trapezium cluster obtained by O'Dell & Wen and found a binary frequency of 7/50 or 0.14 +/- 0.05 in the separation range 138-828 AU for stars with V_HST < 17. Our results compare well with the value of 0.16 +/- 0.03 determined recently by Reipurth & Zinnecker from observations of 238 pre-main-sequence (PMS) stars in nearby (d ~ 150 pc) low-density (mean star separation ~0.3 pc) star-forming regions over comparable separation ranges. Our results suggest an excess of PMS binaries over main-sequence binaries with the same separations. In addition, we find no evidence that the fraction of binaries formed in relatively dense clusters differs from that characterizing low-density star-forming regions.