7. NGC2323 and the CMa OB1 association

NGC2323 lies near the edge of CMa OB1 at 940 pc from the sun, i.e., scarcely nearer than the association, which is located at 1150 pc (Clariá 1974). However, the age derived for NGC2323 (100 Myr) is much greater than that derived by Clariá (1974) for CMa OB1 (3 Myr). Therefore, it seems very improbable that NGC2323 and CMa OB1 originated from the same protostellar material. The actual location of NGC2323 could be explained taking into account the motion of the cluster along the galactic disk. In fact, it is possible to estimate the full period P for the expected oscillation of NGC2323 around the galactic plane, using the expression: , where K_z is the acceleration in the gravitational field perpendicular to the galactic plane, and z is the distance from the galactic plane (see, e.g., Trumpler & Weaver, 1953, paragraph 6.37). Adopting the acceleration K_z at z=-21 pc from the computed values for the solar neighbourhood (Bahcall 1984), then Myr. Consequently, NGC2323 could have been formed many years before the association in a region probably different from the one it occupies now, having crossed the galactic plane four times during its lifetime. Since its formation till the present NGC2323 could have shifted to the region of the CMA OB1 association.

   

V - M_v : apparent distance modulus = 10.62

E(B-V) : selective absorption

A_v : visual absorption (variable) = 0.76

V₀ - M_v : true distance modulus = 9.86

d : distance from the Sun = 940 pc

z : distance from the galactic plane = -21 pc Age

log t : log age 8.0

t : age Dimension

D : angular diameter = 20'

: linear diameter = 5.46 pc Luminosity function Integrated parameters

M_v : visual absolute mag. = -3.93

(B-V)₀ : intrinsic (B-V) colour = 0.16

(U-B)₀ : intrinsic (U-B) colour = -0.10

V₀ : visual magnitude = 5.94

: total mass > 285

: stellar density = 1.3 star pc-3 : mean space density

Table 6 (click here) summarizes the main results derived in this study.

Acknowledgements

We wish to thank J.C. Mermilliod for very generously providing unpublished radial velocity information. We are also happy to thank the Director, staff and technicians of Cerro Tololo Inter-American Observatory and Las Campanas Observatory for the allocation of observing time

and for their kind assistance and hospitality. We gratefully acknowledge financial support from the Argentinian institutions CONICET, CONICOR and Fundación Antorchas.