Study of the Per OB2 star forming complex - II. Structure and kinematics

The study of a sky area with a 20 degree diameter centered on the nearby association Per OB2 has been carried out on the basis of the "Compiled Catalogue of Astronomical Data in the Per OB2 Field" presented in a previous paper. The catalogue contains about 30 000 stars with accurate proper motions in the Hipparcos system as well as magnitudes supplemented with spectral classes and other relevant data. We applied the reduced proper motion technique to estimate individual distances for almost all catalogue stars. The distributions of reddening and interstellar extinction were determined on the basis of the spectral classification of stars in the area. Proper motions were corrected for differential rotation of the Galactic disk and parallactic motion effects and used for the evaluation of association membership. The analysis of a uniform subset of main sequence (MS) stars earlier than A7 - which is complete within 500 pc - has clearly revealed the association as an area of enhanced density with an angular size of about 7degrees. In comparison to the previous results on Per OB2, we show that the presently determined area extends the association from its classical position towards the California nebula and the Auriga dark cloud. There are 1025 MS proper motion members residing in this region. The distance to the association centre derived from a kinematic calibration is found to be about 300 pc i.e., in perfect agreement with the Hipparcos data. The shape of the association is almost spherical, and its diameter found from stellar counts is about 40 pc. An examination of internal systematic motions from proper motion data has shown that there is no evidence of an expansion or a rotation of the association around the line of sight. The average tangential and spatial velocity vectors of Per OB2 with respect to the LSR are determined as (V-l; V-b) = (+7.2 +/- 0.4; +2.0 +/- 0.3) km s(-1) and (U; V; W) = (+12.7 +/- 1.6; 3.0 +/- 0.6; -0.9 +/- 0.8) km s(-1).