The distance to the Orion Nebula Cluster

Abstract

The distance to the Orion Nebula Cluster (ONC) is estimated using the rotational properties of its low-mass pre main-sequence (PMS) stars. Rotation periods, projected equatorial velocities and distance-dependent radius estimates are used to form an observational sin i distribution (where i is the axial inclination), which is modelled to obtain the distance estimate. A distance of 440+/-34 pc is found from a sample of 74 PMS stars with spectral types between G6 and M2, but this falls to 392+/-32 pc when PMS stars with accretion discs are excluded on the basis of their near-infrared excess. Since the radii of accreting stars are more uncertain and probably systematically underestimated, then this closer distance is preferred. The quoted uncertainties include statistical errors and uncertainties due to a number of systematic effects including binarity and inclination bias. This method is geometric and independent of stellar evolution models, though does rely on the assumption of random axial orientations and the Cohen & Kuhi (1979) effective temperature scale for PMS stars. The new distance is consistent with, although lower and more precise, than most previous ONC distance estimates. A closer ONC distance implies smaller luminosities and an increased age based on the positions of PMS stars in the Hertzsprung-Russell diagram.Comment: Accepted for publication in MNRAS (12 pages) Table 1 available from the autho