On the structure of the open cluster NGC 2420.

The structure of the old galactic cluster NGC 2420 is investigated by counting 2071 stars in a field with a diameter of 2 deg 24 min around the cluster. Of these stars, 143 belong to the cluster core, 1290 are located in its corona, and 637 are field stars. The counted stars are divided into two groups: those brighter than 14.5 magnitudes (Group A) and those with magnitudes ranging from 14.5 to 16.20 (Group B). Apparent isopycnic lines and density distribution curves are constructed and evaluated for the two groups. It is concluded that: (1) the cluster has a well-defined core and an extended corona; (2) the cluster core radius for Group A is 8.9 arcmin, and the coronal radius is 56.7 arcmin; (3) the linear cluster diameter is 112 pc if a distance of 2400 pc is adopted; and (4) for Group B, the cluster core radius is 10.9 arcmin, the coronal radius is 44.3 arcmin, and the linear cluster diameter is 88 pc.