CLUSTERING OF PRE-MAIN-SEQUENCE STARS IN THE ORION, OPHIUCHUS, CHAMELEON, VELA, AND LUPUS STAR-FORMING REGIONS

We study clustering of pre-main-sequence stars in the Orion, Ophiuchus , Chamaeleon, Vela, and Lupus star-forming regions. We calculate the a verage surface density of companions, Sigma(theta), as a function of a ngular distance, theta, from each star. We employ the method developed by Larson in a 1995 study for the calculation. In most of the regions studied, the function can be fitted by two power laws (Sigma proporti onal to theta(gamma)) with a break as found by Larson for the Taurus s tar-forming region. The power index, gamma, is smaller at small separa tions than at large separations. The power index at large separations shows significant variation from region to region (-0.8 < gamma < -0.1 ), while the power index at small separations does not (gamma similar to-2). The power index at large separations relates to the distributio n of the nearest-neighbor distance. When the latter can be fitted by t he Poisson distribution, the power index is close to 0. When the latte r is broader than the Poisson distribution, the power index is negativ ely large. This correlation can be interpreted as the result of the va riation in the surface density within the region. At large separations , the power-law fit may indicate star formation history in the region and not the spatial structure like the self-similar hierarchical, or f ractal, one. Because of the velocity dispersion, stars move from their birthplaces, and the surface density of coeval stars decreases with t heir age. When a star-forming region contains several groups of stars with different ages, a power law may fit the average surface density o f companions for it. The break of the power law is located around 0.01 -0.1 pc. There is a clear correlation between the break position and t he mean nearest-neighbor distance. The break position may reflect disp ersal of newly formed stars.