1,108 research outputs found
Detectability of Free Floating Planets in Open Clusters with JWST
Recent observations have shown the presence of extra-solar planets in
Galactic open stellar clusters, as in the Praesepe (M44). These systems provide
a favorable environment for planetary formation due to the high heavy-element
content exhibited by the majority of their population. The large stellar
density, and corresponding high close-encounter event rate, may induce strong
perturbations of planetary orbits with large semimajor axes. Here we present a
set of N-body simulations implementing a novel scheme to treat the tidal
effects of external stellar perturbers on planetary orbit eccentricity and
inclination. By simulating five nearby open clusters we determine the rate of
occurrence of bodies extracted from their parent stellar system by
quasi-impulsive tidal interactions. We find that the specific free-floating
planet production rate (total number of free-floating planets per unit of time,
normalized by the total number of stars) is proportional to the stellar density
of the cluster, with a constant of proportionality equal to (23 +/- 5)10^-6
pc^3 Myr^-1. For the Pleiades (M45) we predict that about 26% of stars should
have lost their planets. This raises the exciting possibility of directly
observing these wandering planets with the James Webb Space Telescope in the
NIR band. Assuming a surface temperature of the planet of 500 K, a
free-floating planet of Jupiter size inside the Pleiades would have a specific
flux @4.4 micron of approximately 400 nJy, which would lead to a very clear
detection (S/N of order 100) in only one hour of integration.Comment: Accepted for publication in ApJ Letters on 4 November 201
Quasi-Resonant Theory of Tidal Interactions
When a spinning system experiences a transient gravitational encounter with
an external perturber, a quasi-resonance occurs if the spin frequency of the
victim matches the peak orbital frequency of the perturber. Such encounters are
responsible for the formation of long tails and bridges of stars during galaxy
collisions. For high-speed encounters, the resulting velocity perturbations can
be described within the impulse approximation. The traditional impulse
approximation, however, does not distinguish between prograde and retrograde
encounters, and therefore completely misses the resonant response. Here, using
perturbation theory, we compute the effects of quasi-resonant phenomena on
stars orbiting within a disk. Explicit expressions are derived for the velocity
and energy change to the stars induced by tidal forces from an external
gravitational perturber passing either on a straight line or parabolic orbit.
Comparisons with numerical restricted three-body calculations illustrate the
applicability of our analysis.Comment: 22 pages, 13 figures, ApJ submitted, numerical routines for
evaluation of special functions and analytical results are provided upon
reques
The Halo Density Profiles with Non-Standard N-body Simulations
We propose a new numerical procedure to simulate a single dark halo of any
size and mass in a hierarchical framework coupling the extended Press-Schechter
formalism (EPSF) to N-body simulations. The procedure consists of assigning
cosmological initial conditions to the particles of a single halo with a EPSF
technique and following only the dynamical evolution using a serial N-body
code. The computational box is fixed with a side of Mpc. This
allows to simulate galaxy cluster halos using appropriate scaling relations, to
ensure savings in computing time and code speed. The code can describe the
properties of halos composed of collisionless or collisional dark matter. For
collisionless Cold Dark Matter (CDM) particles the NFW profile is reproduced
for galactic halos as well as galaxy cluster halos. Using this numerical
technique we study some characteristics of halos assumed to be isolated or
placed in a cosmological context in presence of weak self-interacting dark
matter: the soft core formation and the core collapse. The self-interacting
dark matter cross section per unit mass is assumed to be inversely proportional
to the particle collision velocity: .Comment: Accepted for publication in MNRAS (2 figures added
How galaxies lose their angular momentum
The processes are investigated by which gas loses its angular momentum during
the protogalactic collapse phase, leading to disk galaxies that are too compact
with respect to the observations. High-resolution N-body/SPH simulations in a
cosmological context are presented including cold gas and dark matter. A halo
with quiet merging activity since z~3.8 and with a high spin parameter is
analysed that should be an ideal candidate for the formation of an extended
galactic disk. We show that the gas and the dark matter have similar specific
angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1.
All the gas involved in the merger loses a substantial fraction of its specific
angular momentum due to tidal torques and falls quickly into the center.
Dynamical friction plays a minor role,in contrast to previous claims. In fact,
after this event a new extended disk begins to form from gas that was not
involved in the 5:1 merger event and that falls in subsequently. We argue that
the angular momentum problem of disk galaxy formation is a merger problem: in
cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1
are expected to occur in almost all galaxies. We suggest that energetic
feedback processes could in principle solve this problem, however only if the
heating occurs at the time or shortly before the last substantial merger event.
Good candidates for such a coordinated feedback would be a merger-triggered
star burst or central black hole heating. If a large fraction of the low
angular momentum gas would be ejected as a result of these processes, late-type
galaxies could form with a dominant extended disk component, resulting from
late infall, a small bulge-to-disk ratio and a low baryon fraction, in
agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution
figures to the author
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