Pavel Kroupa: Moovies

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For research on simulating the formation and evolution of galaxies in Milgromian dynamics, we first needed to develop the code Phantom of Ramses ( Lueghausen et al. 2015 ). This MOVIE shows the dynamical evolution of a Milky-Way-type galaxy which is composed only of stellar particles. Note the buckling and the development of the bar instability and an X-feature in the inner region with radius near 2 kpc in the edge-on view.

Star clusters

For research on the formation and evolution of star clusters in Newtonian dynamics, we mostly use the Aarseth suite of Nbody5-7 programmes ( Aarseth Nody codes ).

Ejections of massive stars from young star clusters:
These two MOVIES show the dynamical evolution of a 30000 Msun (left) and a 3000 Msun (right) very young cluster. The detailed realistic initial conditions are described in Oh et al. (2015) and Oh & Kroupa (2016) . The clusters are initially mass-segregated, and every star is in a binary. The stars and binaries follow realistic distribution functions. Note how incredibly dynamically active the clusters are, shooting out their massive stars at a high rate and with different velocities, and also with high-order multiplicities. Note also the nice example of the two-step ejection process ( Pflamm-Altenburg & Kroupa 2010 ) which occurs at 5.6 Myr in the right movie: a massive star binary is expelled and propagates to the right of the cluster, the stars evolve and when the final supernova occurs the two neutron stars are left moving on their near-tangential orbital motion such that the neutron star moving to the left passes the cluster on a trajectory which an observer would not trace back to the cluster.

Multiple populations of young stars in the Orion Nebula Cluster (ONC):
The big surprise was the report by Beccari et al. (2017) that the ONC has three populations of very young stars within it, each separated in age by about 0.5-1Myr. This seems to shatter the notion that such clusters form monolithically within a few hundred thousand years followed by the expulsion of the majority of mass in the form of ionised gas ( Kroupa, Aarseth & Hurley 2001 ). But if the massive (ionising) stars form in compact multiple systems near the centre of a freshly formed population then they can eject each other dynamically from the cluster, allowing the inflowing gas to recombine and form the next population of stars. This scenario implies a narrow mass range for clusters where this process of "repeated stellar-dynamical termination of feedback-halted filament-accretion model" may operate and has been published by Kroupa, Jerabkova, Dinbier et al. (2018). Our N-body simulations confirm that complete ejections of all ionising stars is likely if these form at the centre of their embedded cluster and in a very compact non-hierarchical multiple system ( Wang, Kroupa & Jerabkova 2019 ). This MOVIE visualises the process of repeated ejections.

The initial dynamical evolution of a small cluster of binary stars (Heidelberg, March 2000):

Aarseth's Nbody6 with modifications ( Kroupa, Aarseth & Hurley 2001).

Standard local Galactic tidal field; Plummer density profile, Rhalf=0.2pc, virial equilibrium; Stellar masses: 0.01-50Msol from KTG93 IMF; 40 binaries: Companion masses paired randomly; Taurus-Auriga-like period distribution (Kroupa'95); Latest stellar evolution (Hurley et al. 2000).

Short description:
The movie exemplifies dynamical processes that are important for young clusters. Stellar masses: Magenta: BDs (0.01-0.08Msol); Red: M dwarfs (0.08-0.5Msol); Green: "K" dwarfs (0.5-1Msol); Blue: massive stars (>1Msol).

Cluster evolution begins with the two most massive stars sinking to the cluster core within one crossing time, and forming a binary, after ejecting it's original companions as well as other low-mass members. It recoils after a very fast (barely visible) ejection of an original Mdwarf companion to the upper left. This hardens the massive binary. The cluster gains momentum and moves slowly towards the lower right. Further ejection events, mostly from the tight central binary, remove further stars from the cluster.

Observe the fast development of mass segregation, taking note of the distribution of BDs, and note how the dynamical evolution time-scale slows as the cluster expands and looses stars.

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