610 research outputs found
Tidal stripping as a mechanism for placing globular clusters on wide orbits: the case of MGC1 in M31
The globular clusters of large spiral galaxies can be divided into two
populations: one which formed in-situ and one which comprises clusters tidally
stripped away from other galaxies. In this paper we investigate the
contribution to the outer globular cluster population in the M31 galaxy through
donation of clusters from dwarf galaxies. We test this numerically by comparing
the contribution of globular clusters from simulated encounters to the observed
M31 globular cluster population. To constrain our simulations, we specifically
investigate the outermost globular cluster in the M31 system, MGC1. The remote
location of MGC1 favours the idea of it being captured, however, the cluster is
devoid of features associated with tidal interactions. Hence we separate
simulations where tidal features are present and where they are hidden. We find
that our simulated encounters can place clusters on MGC1-like orbits. In
addition, we find that tidal stripping of clusters from dwarf galaxies leaves
them on orbits having a range of separations, broadly matching those observed
in M31. We find that the specific energies of globular clusters captured by M31
closely matches those of the incoming host dwarf galaxies. Furthermore, in our
simulations we find an equal number of accreted clusters on co-rotating and
counter-rotating orbits within M31 and use this to infer the fraction of
clusters that has been accreted. We find that even close in roughly 50% of the
clusters are accreted, whilst this figure increases to over 80% further out.Comment: 17 pages, 12 figures. Accepted for publication in MNRA
Long-term stability of the HR 8799 planetary system without resonant lock
HR 8799 is a star accompanied by four massive planets on wide orbits. The
observed planetary configuration has been shown to be unstable on a timescale
much shorter than the estimated age of the system (~ 30 Myr) unless the planets
are locked into mean motion resonances. This condition is characterised by
small-amplitude libration of one or more resonant angles that stabilise the
system by preventing close encounters. We simulate planetary systems similar to
the HR 8799 planetary system, exploring the parameter space in separation
between the orbits, planetary masses and distance from the Sun to the star. We
find systems that look like HR 8799 and remain stable for longer than the
estimated age of HR 8799. None of our systems are forced into resonances. We
find, with nominal masses and in a narrow range of orbit separations, that 5 of
100 systems match the observations and lifetime. Considering a broad range of
orbit separations, we find 12 of 900 similar systems. The systems survive
significantly longer because of their slightly increased initial orbit
separations compared to assuming circular orbits from the observed positions. A
small increase in separation leads to a significant increase in survival time.
The low eccentricity the orbits develop from gravitational interaction is
enough for the planets to match the observations. With lower masses, but still
comfortably within the estimated planet mass uncertainty, we find 18 of 100
matching and long-lived systems in a narrow orbital separation range. In the
broad separation range, we find 82 of 900 matching systems. Our results imply
that the planets in the HR 8799 system do not have to be in strong mean motion
resonances.Comment: Accepted for publication in A&
Supernova enrichment and dynamical histories of solar-type stars in clusters
We use N-body simulations of star cluster evolution to explore the hypothesis
that short-lived radioactive isotopes found in meteorites, such as 26-Al, were
delivered to the Sun's protoplanetary disc from a supernova at the epoch of
Solar System formation. We cover a range of star cluster formation parameter
space and model both clusters with primordial substructure, and those with
smooth profiles. We also adopt different initial virial ratios - from cool,
collapsing clusters to warm, expanding associations. In each cluster we place
the same stellar population; the clusters each have 2100 stars, and contain one
massive 25M_Sun star which is expected to explode as a supernova at about
6.6Myr. We determine the number of Solar (G)-type stars that are within 0.1 -
0.3pc of the 25M_Sun star at the time of the supernova, which is the distance
required to enrich the protoplanetary disc with the 26-Al abundances found in
meteorites. We then determine how many of these G-dwarfs are unperturbed
`singletons'; stars which are never in close binaries, nor suffer sub-100au
encounters, and which also do not suffer strong dynamical perturbations.
The evolution of a suite of twenty initially identical clusters is highly
stochastic, with the supernova enriching over 10 G-dwarfs in some clusters, and
none at all in others. Typically only ~25 per cent of clusters contain
enriched, unperturbed singletons, and usually only 1 - 2 per cluster (from a
total of 96 G-dwarfs in each cluster). The initial conditions for star
formation do not strongly affect the results, although a higher fraction of
supervirial (expanding) clusters would contain enriched G-dwarfs if the
supernova occurred earlier than 6.6Myr. If we sum together simulations with
identical initial conditions, then ~1 per cent of all G-dwarfs in our
simulations are enriched, unperturbed singletons.Comment: 14 pages, 5 figures, accepted for publication in MNRA
[Introduction to] In Uncertain Times: American Foreign Policy after the Berlin Wall and 9/11
In Uncertain Times considers how policymakers react to dramatic developments on the world stage. Few expected the Berlin Wall to come down in November 1989; no one anticipated the devastating attacks on the World Trade Center and the Pentagon in September 2001. American foreign policy had to adjust quickly to an international arena that was completely transformed.
Melvyn P. Leffler and Jeffrey W. Legro have assembled an illustrious roster of officials from the George H. W. Bush, Clinton, and George W. Bush administrations—Robert B. Zoellick, Paul Wolfowitz, Eric S. Edelman, Walter B. Slocombe, and Philip Zelikow. These policymakers describe how they went about making strategy for a world fraught with possibility and peril. They offer provocative reinterpretations of the economic strategy advanced by the George H. W. Bush administration, the bureaucratic clashes over policy toward the breakup of the USSR, the creation of the Defense Policy Guidance of 1992, the expansion of NATO, the writing of the National Security Strategy Statement of 2002, and the invasion of Iraq in 2003.
A group of eminent scholars address these same topics. Bruce Cumings, John Mueller, Mary Elise Sarotte, Odd Arne Westad, and William C. Wohlforth probe the unstated assumptions, the cultural values, and the psychological makeup of the policymakers. They examine whether opportunities were seized and whether threats were magnified and distorted. They assess whether academicians and independent experts would have done a better job than the policymakers did. Together, policymakers and scholars impel us to rethink how our world has changed and how policy can be improved in the future.https://scholarship.richmond.edu/bookshelf/1264/thumbnail.jp
Close encounters involving free-floating planets in star clusters
Instabilities in planetary systems can result in the ejection of planets from
their host system, resulting in free-floating planets (FFPs). If this occurs in
a star cluster, the FFP may remain bound to the star cluster for some time and
interact with the other cluster members until it is ejected. Here, we use
-body simulations to characterise close star-planet and planet-planet
encounters and the dynamical fate of the FFP population in star clusters
containing single or binary star members. We find that FFPs ejected
from their planetary system at low velocities typically leave the star cluster
40% earlier than their host stars, and experience tens of close ( AU)
encounters with other stars and planets before they escape. The fraction of
FFPs that experiences a close encounter depends on both the stellar density and
the initial velocity distribution of the FFPs. Approximately half of the close
encounters occur within the first 30 Myr, and only 10% occur after 100 Myr. The
periastron velocity distribution for all encounters is well-described by a
modified Maxwell-Bolzmann distribution, and the periastron distance
distribution is linear over almost the entire range of distances considered,
and flattens off for very close encounters due to strong gravitational
focusing. Close encounters with FFPs can perturb existing planetary systems and
their debris structures, and they can result in re-capture of FFPs. In
addition, these FFP populations may be observed in young star clusters in
imaging surveys; a comparison between observations and dynamical predictions
may provide clues to the early phases of stellar and planetary dynamics in star
clusters.Comment: Accepted for publication in MNRAS; 18 pages, 12 figure
Response to Book Review (To Lead the World: American Strategy after the Bush Doctrine
Response to Book Review (To Lead the World: American Strategy after the Bush Doctrine)
We want to thank the commentators for their thoughtful and constructive remarks on our book. We think they highlight some of the key attributes of the volume and raise key issues for further reflection.
In order for readers of H-Diplo to understand the comments, we want to reiterate here what we stated in the introduction to the book. We tried to bring together some of the nation’s most renowned scholars and public intellectuals from all sides of the political spectrum to focus on what should be done after the Bush administration left office. Although many of the contributors shared a view that recent foreign policy had been either disappointing or a disaster, their task was not to dwell on the past, but to focus on the future. We asked each of them to author a basic national security paper in which they identified key threats, defined overriding goals, assigned priorities to objectives, examined the tradeoffs between “interests” and values, and addressed the challenges of mobilizing domestic support for preferred policies, designing effective tactics, and re-configuring multinational institutions
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