Insights into the properties of the Local (Orion) spiral arm. NGC 2302: First results and description of the program

Abstract

The spiral structure of the Milky Way is highly uncertain and is the subject of much discussion nowadays. We present the first result from a program that determines the properties of the Local spiral arm (LOA), together with a full description of the program. In this context we have made a comprehensive study of the young LOA open cluster NGC 2302, which includes a UBVRI photometric analysis and determination of its kinematic properties - proper motion and radial velocity - and of its orbital parameters. We determined the mean PM of NGC 2302 relative to the local field of disk stars, and, through a comparison with the UCAC4 catalog, we transformed this relative PM into an absolute one. Using medium-resolution spectroscopy of 26 stars in the field of NGC 2302, we derived its mean RV. Isochrone fits to the photometric diagrams allowed us to determine the fundamental parameters of NGC 2302, including reddening, distance, and age. The kinematic data and derived distance allowed us to determine the space motion of NGC 2302. This was done by adopting a time-independent, axisymmetric, and fully analytic gravitational potential for the MW. We obtained an absolute PM for NGC 2302 of ($\mu_{\alpha} \cos\delta,\mu_{\delta}) = (-2.09,-2.11)$ mas/yr, with standard errors of 0.410 and 0.400 mas/yr. The mean RV of NGC 2302 turned out to be 31.2 km/sec with a standard error of 0.7 km/sec. Isochrone fits displaced for this reddening and for a distance modulus of (m-M)o = 10.69 indicate an age of log(t) = 7.90-8.00 with a slight tendency toward the younger age. Inspection of the shape of the orbit of NGC 2302 and the resulting orbital parameters indicate that it is a typical population I object.