Characterizing low contrast Galactic open clusters with GAIA DR2

Abstract

In this study, we characterized 16 objects previously classified as faint or low contrast Galactic open clusters (OCs). We employed parameters associated to the OCs dynamical evolution: core ($r_c$), tidal ($r_t$) and half-mass ($r_{hm}$) radii, age and crossing time ($t_{cr}$). Relations among these parameters were exploited to draw some evolutionary connections. We also included 11 OCs with previous characterizations to provide wider coverage of the parameters space. The investigated sample spans a considerable range in age (log (t/yr) ~7.0 - 9.7) and Galactocentric distance ($R_G$ ~ 6 - 11 kpc). Most of them present solar metallicity. We employed GAIA DR2 astrometry and photometry and selected member stars through a decontamination algorithm which explores the 3D astrometric space (${\mu }_{\alpha }$, ${\mu }_{\delta }$, $\varpi$) to assign membership likelihoods. Previous studies of most of these objects were based mostly on photometric information. All investigated OCs were proved to be real stellar concentrations and relations among their parameters indicate a general disruption scenario in which OCs tend to be more concentrated as they evolve. Internal interactions sucessively drive OCs to develop more dynamically relaxed structures and make them less subject to mass loss due to tidal effects. Tidal radius tends to increase with $R_G$ in accordance with the strength of the Galactic tidal field. Besides, the correlation between the $r_c$ and the dynamical ratio ${\tau }_{\text{dyn}}$ = age/$t_{cr}$ suggests two distinct evolutionary sequences, which may be consequence of different initial formation conditions.

19 pages, 5 figures. Accept for publication in MNRAS

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