Probing disk properties with open clusters

Abstract

We use the open clusters (OCs) with known parameters available in theWEBDA 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 = 48 ± 3 pc. For the clusters with ages in the range 200 Myr to 1 Gyr we derive zh = 150 ± 27 pc. Clusters older than 1Gyr 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 = 57 ± 3 pc. We confirm previous results that zh increases with Galactocentric distance. The scale height implied by the OCs younger than 1Gyr outside the Solar circle is a factor ∼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 z = 14.8 ± 2.4pc, 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 = 1.5−1.9 kpc, 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 reflect intrinsic differences in the spatial distributions of OCs and stars. We derive a number-density of Solar-neighbourhood (with distances from the Sun dʘ ≤ 1.3 kpc) OCs of ρʘ = 795 ± 70 kpc-³, which implies a total number of (Trumpler types I to III) OCs of ∼730 of which ∼47% would already have been observed. Extrapolation of the completeness-corrected radial distributions down to the Galactic center indicates a total number of OCs in the range (1.8−3.7) × 105. These estimates are upper-limits because they do not take into account depletion in the number of OCs by dynamical effects in the inner parts of the Galaxy. The observed and completeness-corrected age-distributions of the OCs can be fitted by a combination of two exponential-decay profiles which can be identified with the young and old OC populations, characterized by age scales of ∼100 Myr and ∼1.9Gyr, respectively. This rules out evolutionary scenarios based on constant star-formation and OC-disruption rates. Comparing the number of observed embedded clusters and candidates in the literature with the expected fraction of very young OCs, derived from the observed age-distribution function, we estimate that 3.4–8% of the embedded clusters do actually emerge from the parent molecular clouds as OCs.