NGC 2180 : a disrupting open cluster?

Abstract

The spatial dependence of luminosity and mass functions of evolved open clusters is discussed in this work using J and H 2MASS photometry, which allows a wide spatial coverage and proper background determination. The target objects are the intermediate-age open cluster NGC3680 (l = 286.76° and b = 16.92°), which has been reported as being in an advanced state of dissolution, and NGC2180 (l = 203.85°, b = −7.01°), a possible dynamically evolved open cluster. At the 1 − σ level, NGC2180 presents a radial distribution of stars that can be represented by a King law. We conclude that, although in an advanced dynamical state (mass segregated), NGC3680 does not present strong signs of dissolution, having luminosity and mass functions very similar to those of the τage ≈ 3.2 Gyr open cluster M67. On the other hand, NGC2180 appears to have flat, eroded luminosity functions throughout its structure, suggesting that in addition to mass segregation, Galactic tidal stripping has been effective in depleting this object. Accordingly, NGC2180 may be the missing link between evolved open clusters and remnants. For NGC 2180 we infer a distance to the Sun dʘ = 0.91 ± 0.15 kpc, an age τage ≈ 710 Myr and an observed stellar mass of mobs ∼ 47 Mʘ. For NGC3680 we derive an age τage ≈ 1.6 Gyr, E(B − V) = 0.0 and dʘ = 1.00 ± 0.09 kpc, confirming previous estimates. The observed stellar mass mobs ≈ 130 Mʘ agrees with previous values. We study both clusters in the context of dynamical states estimated from diagnostic diagrams involving photometric and structural parameters. Both objects appear to be dynamically evolved systems. In particular, NGC2180 is closer to open cluster remnants than NGC3680.