Astronomers reveal new insights into the origin and evolution of open cluster NGC 6791

This is a ground-based telescopic view of NGC 6791.
Credit: NASA, ESA, DSS, and L. Bedin (STScI)

(Phys.org)—By conducting an orbital analysis, a team of
astronomers led by Luis Martinez-Medina of the National
Autonomous University of Mexico has uncovered new details
about the origin and evolution of the old, metal-rich open
cluster NGC 6791. The findings were presented Sept. 4 in a
paper published on arXiv.org.

Located about 13,300 light years from the Earth in the Lyra
constellation, NGC 6791 is one of the most studied . It has an estimated age of
approximately 8 billion years and an iron to hydrogen abundance
ratio that is more than twice that of our sun. This makes it
one of the oldest and most metal-rich clusters in the Milky Way
galaxy. With a mass of approximately 5,000 solar masses, it is
also one of the most massive open clusters known to date.

Recently, Martinez-Medina’s team has carried out studies of NGC
6791, performing an orbital analysis of this cluster within a
more detailed model of our galaxy. The researchers focused on
investigating possible locations for the formation of the
studied cluster, hoping to learn more about its origin and
evolution.

“In this work, we explore possible locations for the formation
of NGC 6791, improving previous determinations by including,
among other things, a more detailed model of the Milky Way
galaxy, as well as a more rigorous criteria in the selection of
the orbits,” the astronomers wrote in the paper.

The team performed a comprehensive orbital study of NGC 6791
that included integrating a half-million orbits, representing
the galaxy’s stellar disc, for 8 billion years in order to find
those with similar position, proper motion, and radial velocity
to the current values of the studied open cluster.

Based on the conducted analysis, the authors hypothesize that
NGC 6791 may have formed in the inner thin disc or in the bulge
of the Milky Way galaxy. Afterwards, the galaxy may have been
displaced by radial migration to its current orbit – about
26,200 light years from the galactic center. According to the
study, the migration scenario would also explain the cluster’s
metal-rich composition, which is unusual for such old open
clusters.

“There is a probability that an orbit could have suffered an
outward radial migration that brought the cluster to its
current galactocentric position and its distance from the
plane. This scenario would explain its high metallicity, in
spite of its old age,” the paper reads.

The researchers emphasized that the birthplace and migration of
NGC 6791 could be derived from its chemical composition, its
mass loss, and its flat stellar mass function. When it comes to
NGC 6791’s initial mass, they assume that the should originally have been over 10 times
more massive than it is today.

“The birthplace and journeys of NGC 6791 are imprinted in its
chemical composition, in its mass loss, and in its flat stellar
function, further confirming its origin in
the inner thin disc or in the bulge,” the scientists concluded.

Explore further:

Researchers determine chemical composition of the stellar
cluster Gaia1

More information: New insights in the origin and
evolution of the old, metal-rich open cluster NGC 6791,
arXiv:1709.01079 [astro-ph.GA] arxiv.org/abs/1709.01079

Abstract
NGC 6791 is one of the most studied open clusters, it is
massive (∼5000M⊙), located at the solar circle, old
( 8Gyr) and yet the most metal-rich cluster ([Fe/H]≃0.4)
known in the Milky Way. By performing an orbital analysis
within a Galactic model including spiral arms and a bar, we
found that it is plausible that NGC 6791 formed in the inner
thin disc or in the bulge, and later displaced by radial
migration to its current orbit. We apply different tools to
simulate NGC 6791, including direct N-body summation in
time-varying potentials, to test its survivability when going
through different Galactic environments. In order to survive
the 8 Gyr journey moving on a migrating orbit, NGC 6791 must
have been more massive, M0≥5×104M⊙, when formed. We
find independent confirmation of this initial mass in the
stellar mass function, which is observed to be flat; this can
only be explained if the average tidal field strength
experienced by the cluster is stronger than what it is at its
current orbit. Therefore, the birth place and journeys of NGC
6791 are imprinted in its chemical composition, in its mass
loss, and in its flat stellar mass function, supporting its
origin in the inner thin disc or in the bulge.

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