237 research outputs found
Detection of a Stellar Stream Behind Open Cluster NGC 188: Another Part of the Monoceros Stream
We present results from a WIYN/OPTIC photometric and astrometric survey of
the field of the open cluster NGC 188 ((l,b) = (122.8\arcdeg, 22.5\arcdeg)). We
combine these results with the proper-motion and photometry catalog of Platais
et al. and demonstrate the existence of a stellar overdensity in the background
of NGC 188. The theoretical isochrone fits to the color-magnitude diagram of
the overdensity are consistent with an age between 6 and 10 Gyr and an
intermediately metal poor population ([Fe/H] = -0.5 to -1.0). The distance to
the overdensity is estimated to be between 10.0 and 12.6 kpc. The
proper-motions indicate that the stellar population of the overdensity is
kinematically cold.
The distance estimate and the absolute proper motion of the overdensity agree
reasonably well with the predictions of the Pe\~{n}arrubia et al. model of the
formation of the Monoceros stream. Orbits for this material constructed with
plausible radial-velocity values, indicate that dynamically, this material is
unlikely to belong to the thick disk. Taken together, this evidence suggests
that the newly-found overdensity is part of the Monoceros stream.Comment: accepted by A
Geometric Approach to Lyapunov Analysis in Hamiltonian Dynamics
As is widely recognized in Lyapunov analysis, linearized Hamilton's equations
of motion have two marginal directions for which the Lyapunov exponents vanish.
Those directions are the tangent one to a Hamiltonian flow and the gradient one
of the Hamiltonian function. To separate out these two directions and to apply
Lyapunov analysis effectively in directions for which Lyapunov exponents are
not trivial, a geometric method is proposed for natural Hamiltonian systems, in
particular. In this geometric method, Hamiltonian flows of a natural
Hamiltonian system are regarded as geodesic flows on the cotangent bundle of a
Riemannian manifold with a suitable metric. Stability/instability of the
geodesic flows is then analyzed by linearized equations of motion which are
related to the Jacobi equations on the Riemannian manifold. On some geometric
setting on the cotangent bundle, it is shown that along a geodesic flow in
question, there exist Lyapunov vectors such that two of them are in the two
marginal directions and the others orthogonal to the marginal directions. It is
also pointed out that Lyapunov vectors with such properties can not be obtained
in general by the usual method which uses linearized Hamilton's equations of
motion. Furthermore, it is observed from numerical calculation for a model
system that Lyapunov exponents calculated in both methods, geometric and usual,
coincide with each other, independently of the choice of the methods.Comment: 22 pages, 14 figures, REVTeX
Kinematics in Kapteyn's Selected Area 76: Orbital Motions Within the Highly Substructured Anticenter Stream
We have measured the mean three-dimensional kinematics of stars in Kapteyn's
Selected Area (SA) 76 (l=209.3, b=26.4 degrees) that were selected to be
Anticenter Stream (ACS) members on the basis of their radial velocities, proper
motions, and location in the color-magnitude diagram. From a total of 31 stars
ascertained to be ACS members primarily from its main sequence turnoff, a mean
ACS radial velocity (derived from spectra obtained with the Hydra multi-object
spectrograph on the WIYN 3.5m telescope) of V_helio = 97.0 +/- 2.8 km/s was
determined, with an intrinsic velocity dispersion sigma_0 = 12.8 \pm 2.1 km/s.
The mean absolute proper motions of these 31 ACS members are mu_alpha cos
(delta) = -1.20 +/- 0.34 mas/yr and mu_delta = -0.78 \pm 0.36 mas/yr. At a
distance to the ACS of 10 \pm 3 kpc, these measured kinematical quantities
produce an orbit that deviates by ~30 degrees from the well-defined swath of
stellar overdensity constituting the Anticenter Stream in the western portion
of the Sloan Digital Sky Survey footprint. We explore possible explanations for
this, and suggest that our data in SA 76 are measuring the motion of a
kinematically cold sub-stream among the ACS debris that was likely a fragment
of the same infalling structure that created the larger ACS system. The ACS is
clearly separated spatially from the majority of claimed Monoceros ring
detections in this region of the sky; however, with the data in hand, we are
unable to either confirm or rule out an association between the ACS and the
poorly-understood Monoceros structure.Comment: Accepted to ApJ. 48 pages, 20 figures, preprint forma
Energy landscape and phase transitions in the self-gravitating ring model
We apply a recently proposed criterion for the existence of phase
transitions, which is based on the properties of the saddles of the energy
landscape, to a simplified model of a system with gravitational interactions,
referred to as the self-gravitating ring model. We show analytically that the
criterion correctly singles out the phase transition between a homogeneous and
a clustered phase and also suggests the presence of another phase transition,
not previously known. On the basis of the properties of the energy landscape we
conjecture on the nature of the latter transition
Geometry of dynamics, Lyapunov exponents and phase transitions
The Hamiltonian dynamics of classical planar Heisenberg model is numerically
investigated in two and three dimensions. By considering the dynamics as a
geodesic flow on a suitable Riemannian manifold, it is possible to analytically
estimate the largest Lyapunov exponent in terms of some curvature fluctuations.
The agreement between numerical and analytical values for Lyapunov exponents is
very good in a wide range of temperatures. Moreover, in the three dimensional
case, in correspondence with the second order phase transition, the curvature
fluctuations exibit a singular behaviour which is reproduced in an abstract
geometric model suggesting that the phase transition might correspond to a
change in the topology of the manifold whose geodesics are the motions of the
system.Comment: REVTeX, 10 pages, 5 PostScript figures, published versio
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