Probing disk properties with open clusters

We use the open clusters (OCs) with known parameters available in the WEBDA database and in recently published papers to derive properties related to the disk structure such as the thin-disk scale height, displacement of the Sun above the Galactic plane, scale length and the OC age-distribution function. The sample totals 654 OCs, consisting basically of Trumpler types I to III clusters whose spatial distribution traces out the local geometry of the Galaxy. We find that the population of OCs with ages younger than 200 Myr distributes in the disk following an exponential-decay profile with a scale height of zh pc. For the clusters with ages in the range 200 Myr to 1 Gyr we derive zh pc. Clusters older than 1 Gyr distribute nearly uniformly in height from the plane so that no scale height can be derived from exponential fits. Considering clusters of all ages we obtain an average scale height of zh pc. We confirm previous results that zh increases with Galactocentric distance. The scale height implied by the OCs younger than 1 Gyr outside the Solar circle is a factor ${\sim}1.4{-}2$ larger than zh of those interior to the Solar circle. We derive the displacement of the Sun above the Galactic plane as $\rm\mbox{$z_\odot$}=14.8\pm2.4$ pc, which agrees with previous determinations using stars. As a consequence of the completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile, instead it falls off both for regions external to the Solar circle and more sharply towards the Galactic center. We simulate the effects of completeness assuming that the observed distribution of the number of OCs with a given number of stars above the background, measured in a restricted zone outside the Solar circle, is representative of the intrinsic distribution of OCs throughout the Galaxy. Two simulation models are considered in which the intrinsic number of observable stars are distributed: (i) assuming the actual positions of the OCs in the sample, and (ii) random selection of OC positions. As a result we derive completeness-corrected radial distributions which agree with exponential disks throughout the observed Galactocentric distance range 5–14 kpc, with scale lengths in the range RD, smaller than those inferred by means of stars. In particular we retrieve the expected exponential-disk radial profile for the highly depleted regions internal to the Solar circle. The smaller values of RD may