On the Survival of High-Altitude Open Clusters Within the Milky Way Galaxy Tides

Abstract

It is a common assumption that high-altitude open clusters live longer compared with clusters moving close to the Galactic plane. This is because at high altitudes, open clusters are far from the disruptive effects of in-plane substructures, such as spiral arms, molecular clouds and the bar. However, an important aspect to consider in this scenario is that orbits of high-altitude open clusters will eventually cross the Galactic plane, where the vertical tidal field of the disk is strong. In this work we simulate the interaction of open clusters with the tidal field of a detailed Milky Way Galactic model at different average altitudes and galactocentric radii. We find that the life expectancy of clusters decreases as the maximum orbital altitude increases and reaches a minimum at altitudes of approximately 600 pc. Clusters near the Galactic plane live longer because they do not experience strong vertical tidal shocks from the Galactic disk; then, for orbital altitudes higher than 600 pc, clusters start again to live longer due to the decrease in the number of encounters with the disk. With our study, we find that the compressive nature of the tides in the arms region and the bar have an important role on the survival of small clusters by protecting them from disruption: clusters inside the arms can live up to twice as long as those outside the arms at similar galactocentric distance.

Star clusters in evolving galaxies

“…This quickly depletes this family of orbits such that Rossi and Hurley (2015a) suggested that such clusters should be rare. Martinez-Medina et al (2017) showed that tidal compression generated by local cores in the overlapping potentials of the bar and spiral arms (i.e. mainly at the tips of the bar) play a protective role on clusters (see also , for considerations on cluster formation in these areas).…”

“…By varying the orbital inclination, Martinez-Medina et al (2017) showed, on the one hand, that clusters with a low maximum orbital latitude (i.e. staying near the disc) cross the disc at a lower speed than those on a more inclined orbit.…”

“…On the other hand, clusters with a high orbital inclination cross the disc less frequently, which also results in a longer lifetime. By combining these two effects, Martinez-Medina et al (2017) found that clusters with a maximum altitude of 600 pc above the disc plane yield the shortest lifetime. At large galactic distances (i.e.…”

The GALAH survey and Gaia DR2: (non-)existence of five sparse high-latitude open clusters

“…The clusters addressed in this work are generally believed to be old and far from the Galactic plane. At first glance, this is surprising because the survival time of such clusters is lower than their thin disk counterparts (Martinez-Medina et al 2017). A simple model where most star formation happens in the thin disk has to be updated to explain open clusters at high galactic latitudes, assuming these are real 1 .…”

Structure and mass segregation in Galactic stellar clusters

“…As clusters age, the expulsion of gas by stellar feedback as well as dynamical interactions between stars and binary systems in the cluster soften its gravitational potential, leading to their expansion and to their partial or total dissolution into the field of their host galaxy (e.g., Spitzer & Harm 1958;Parker & Meyer 2012;Parker & Dale 2013;Dib et al 2011a,b;Dib 2011;Dib et al 2013;Pfalzner & Kaczmarek 2013;Brinkmann et al 2017). Clusters can also be ⋆ E-mail, SD: sami.dib@gmail.com † Royal Society Dorothy Hodgkin Fellow disrupted by close encounters with giant molecular clouds as they orbit the Galactic centre (e.g., Gieles et al 2006) or by being subjected to strong tidal fields (e.g., Dalessandro et al 2015;Zhai et al 2017;Martinez-Medina et al 2017). …”