NGC 2180: A disrupting open cluster?

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 NGC 3680 (ℓ=286.76^o and b=16.92^o), which has been reported as being in an advanced state of dissolution, and NGC 2180 (ℓ=203.85^o, b=-7.01^o), a possible dynamically evolved open cluster. At the 1-σ level, NGC 2180 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), NGC 3680 does not present strong signs of dissolution, having luminosity and mass functions very similar to those of the τ_age≈3.2 Gyr open cluster M 67. On the other hand, NGC 2180 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, NGC 2180 may be the missing link between evolved open clusters and remnants. For NGC 2180 we infer a distance to the Sun d_⊙=0.91 ± 0.15 kpc, an age τ_age≈710 Myr and an observed stellar mass of m_obs∼47 M_&odot. For NGC 3680 we derive an age τ_age≈1.6 Gyr, E(B-V)=0.0 and d_⊙=1.00 ± 0.09 kpc, confirming previous estimates. The observed stellar mass m_obs≈130 M_⊙ 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, NGC 2180 is closer to open cluster remnants than NGC 3680.