4 research outputs found
Dissipative N - body code for galaxy evolution
The evolving galaxy is considered as a system of baryonic fragments embedded
into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to
gravitational and viscous interactions. Although the chemical evolution of each
separate fragment is treated in the frame of one -- zone close box model with
instantaneous recycling, its star formation (SF) activity is a function of mean
local gas density and, therefore, is strongly influenced by other interacting
fragments. In spite of its simplicity this model provides a realistic
description of the process of galaxy formation and evolution over the Hubble
timescale.Comment: 11 pages, LaTeX, 7 figures, using the article.sty, expected in
A&ApTr, 18, 83
Shape parameters of Galactic open clusters
(abridged) In this paper we derive observed and modelled shape parameters
(apparent ellipticity and orientation of the ellipse) of 650 Galactic open
clusters identified in the ASCC-2.5 catalogue. We provide the observed shape
parameters of Galactic open clusters, computed with the help of a
multi-component analysis. For the vast majority of clusters these parameters
are determined for the first time. High resolution ("star by star") N-body
simulations are carried out with the specially developed GRAPE code
providing models of clusters of different initial masses, Galactocentric
distances and rotation velocities. The comparison of models and observations of
about 150 clusters reveals ellipticities of observed clusters which are too low
(0.2 vs. 0.3), and offers the basis to find the main reason for this
discrepancy. The models predict that after Myr clusters reach an
oblate shape with an axes ratio of , and with the major axis
tilted by an angle of with respect to the
Galactocentric radius due to differential rotation of the Galaxy. Unbiased
estimates of cluster shape parameters require reliable membership determination
in large cluster areas up to 2-3 tidal radii where the density of cluster stars
is considerably lower than the background. Although dynamically bound stars
outside the tidal radius contribute insignificantly to the cluster mass, their
distribution is essential for a correct determination of cluster shape
parameters. In contrast, a restricted mass range of cluster stars does not play
such a dramatic role, though deep surveys allow to identify more cluster
members and, therefore, to increase the accuracy of the observed shape
parameters.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Quantitative analysis of clumps in the tidal tails of star clusters
Tidal tails of star clusters are not homogeneous but show well defined clumps
in observations as well as in numerical simulations. Recently an epicyclic
theory for the formation of these clumps was presented. A quantitative analysis
was still missing. We present a quantitative derivation of the angular momentum
and energy distribution of escaping stars from a star cluster in the tidal
field of the Milky Way and derive the connection to the position and width of
the clumps. For the numerical realization we use star-by-star -body
simulations. We find a very good agreement of theory and models. We show that
the radial offset of the tidal arms scales with the tidal radius, which is a
function of cluster mass and the rotation curve at the cluster orbit. The mean
radial offset is 2.77 times the tidal radius in the outer disc. Near the
Galactic centre the circumstances are more complicated, but to lowest order the
theory still applies. We have also measured the Jacobi energy distribution of
bound stars and showed that there is a large fraction of stars (about 35%)
above the critical Jacobi energy at all times, which can potentially leave the
cluster. This is a hint that the mass loss is dominated by a self-regulating
process of increasing Jacobi energy due to the weakening of the potential well
of the star cluster, which is induced by the mass loss itself.Comment: 14 pages, 17 figures; accepted by MNRA