140 research outputs found
Constraints on the Formation of the Planet Around HD188753A
The claimed discovery of a Jupiter-mass planet in the close triple star
system HD 188753 poses a problem for planet formation theory. A circumstellar
disk around the planet's parent star would be truncated close to the star,
leaving little material available for planet formation. In this paper, we
attempt to model a protoplanetary disk around HD 188753A using a fairly simple
alpha-disk model, exploring a range of parameters constrained by observations
of T Tauri-type stars. The disk is truncated to within 1.5 to 2.7 AU, depending
on model parameters. We find that the in situ formation of the planet around HD
188753A is implausible.Comment: Accepted version, to appear in ApJ. 23 pages, 5 figures (3 in color
Trailgazers: A Scoping Study of Footfall Sensors to Aid Tourist Trail Management in Ireland and Other Atlantic Areas of Europe
This paper examines the current state of the art of commercially available outdoor footfall sensor technologies and defines individually tailored solutions for the walking trails involved in an ongoing research project. Effective implementation of footfall sensors can facilitate quantitative analysis of user patterns, inform maintenance schedules and assist in achieving management objectives, such as identifying future user trends like cyclo-tourism. This paper is informed by primary research conducted for the EU funded project TrailGazersBid (hereafter referred to as TrailGazers), led by Donegal County Council, and has Sligo County Council and Causeway Coast and Glens Council (NI) among the 10 project partners. The project involves three trails in Ireland and five other trails from Europe for comparison. It incorporates the footfall capture and management experiences of trail management within the EU Atlantic area and desk-based research on current footfall technologies and data capture strategies. We have examined 6 individual types of sensor and discuss the advantages and disadvantages of each. We provide key learnings and insights that can help to inform trail managers on sensor options, along with a decision-making tool based on the key factors of the power source and mounting method. The research findings can also be applied to other outdoor footfall monitoring scenarios
First Structure Formation: A Simulation of Small Scale Structure at High Redshift
We describe the results of a simulation of collisionless cold dark matter in
a LambdaCDM universe to examine the properties of objects collapsing at high
redshift (z=10). We analyze the halos that form at these early times in this
simulation and find that the results are similar to those of simulations of
large scale structure formation at low redshift. In particular, we consider
halo properties such as the mass function, density profile, halo shape, spin
parameter, and angular momentum alignment with the minor axis. By understanding
the properties of small scale structure formation at high redshift, we can
better understand the nature of the first structures in the universe, such as
Population III stars.Comment: 31 pages, 14 figures; accepted for publication in ApJ. Figure 1 can
also be viewed at http://cfa-www.harvard.edu/~hjang/research
Type I planet migration in nearly laminar disks - long term behavior
We carry out 2-D high resolution numerical simulations of type I planet
migration with different disk viscosities. We find that the planet migration is
strongly dependent on disk viscosities. Two kinds of density wave damping
mechanisms are discussed. Accordingly, the angular momentum transport can be
either viscosity dominated or shock dominated, depending on the disk
viscosities. The long term migration behavior is different as well. Influences
of the Rossby vortex instability on planet migration are also discussed. In
addition, we investigate very weak shock generation in inviscid disks by small
mass planets and compare the results with prior analytic results.Comment: Accepted for publication in Ap
No evidence of a hot Jupiter around HD 188753 A
The discovery of a short-period giant planet (a hot Jupiter) around the
primary component of the triple star system HD 188753 has often been considered
as an important observational evidence and as a serious challenge to
planet-formation theories. Following this discovery, we monitored HD 188753
during one year to better characterize the planetary orbit and the feasibility
of planet searches in close binaries and multiple star systems. We obtained
Doppler measurements of HD 188753 with the ELODIE spectrograph at the
Observatoire de Haute-Provence. We then extracted radial velocities for the two
brightest components of the system using our multi-order, two-dimensional
correlation algorithm, TODCOR. Our observations and analysis do not confirm the
existence of the short-period giant planet previously reported around HD 188753
A. Monte Carlo simulations show that we had both the precision and the temporal
sampling required to detect a planetary signal like the one quoted. From our
failure to detect the presumed planet around HD 188753 A and from the available
data on HD 188753, we conclude that there is currently no convincing evidence
of a close-in giant planet around HD 188753 A.Comment: 8 pages, 3 figures, accepted for publication in A&A. Corrected typos
and minor mistake
Against all odds? Forming the planet of the HD196885 binary
HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose
orbit places it at the limit for orbital stability. The presence of a planet in
such a highly perturbed region poses a clear challenge to planet-formation
scenarios. We investigate this issue by focusing on the planet-formation stage
that is arguably the most sensitive to binary perturbations: the mutual
accretion of kilometre-sized planetesimals. To this effect we numerically
estimate the impact velocities amongst a population of circumprimary
planetesimals. We find that most of the circumprimary disc is strongly hostile
to planetesimal accretion, especially the region around 2.6AU (the planet's
location) where binary perturbations induce planetesimal-shattering of
more than 1km/s. Possible solutions to the paradox of having a planet in such
accretion-hostile regions are 1) that initial planetesimals were very big, at
least 250km, 2) that the binary had an initial orbit at least twice the present
one, and was later compacted due to early stellar encounters, 3) that
planetesimals did not grow by mutual impacts but by sweeping of dust (the
"snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab
was formed not by core-accretion but by the concurent disc instability
mechanism. All of these 4 scenarios remain however highly conjectural.Comment: accepted for publication by Celestial Mechanics and Dynamical
Astronomy (Special issue on EXOPLANETS
Halting Type I planet migration in non-isothermal disks
Aims: We investigate the effect of including a proper energy balance on the
interaction of a low-mass planet with a protoplanetary disk. Methods: We use a
three-dimensional version of the RODEO method to perform hydrodynamical
simulations including the energy equation. Radiation is included in the
flux-limited diffusion approach. Results: The sign of the torque depends
sensitively on the ability of the disk to radiate away the energy generated in
the immediate surroundings of the planet. In the case of high opacity,
corresponding to the dense inner regions of protoplanetary disks, migration is
directed \emph{outward}, instead of the usual inward migration that was found
in locally isothermal disks. For low values of the opacity we recover inward
migration, and we show that torques originating in the coorbital region are
responsible for the change in migration direction.Comment: 4 pages, 5 figures, accepted for A&A letter
Dark Matter Halo Environment for Primordial Star Formation
We study the statistical properties (such as shape and spin) of high-z halos
likely hosting the first (PopIII) stars with cosmological simulations including
detailed gas physics. In the redshift range considered () the
average sphericity is , and for more than 90% of halos the
triaxiality parameter is , showing a clear preference for
oblateness over prolateness. Larger halos in the simulation tend to be both
more spherical and prolate: we find and , with and at z = 11.
The spin distributions of dark matter and gas are considerably different at
, with the baryons rotating slower than the dark matter. At lower
redshift, instead, the spin distributions of dark matter and gas track each
other almost perfectly, as a consequence of a longer time interval available
for momentum redistribution between the two components. The spin of both the
gas and dark matter follows a lognormal distribution, with a mean value at z=16
of , virtually independent of halo mass. This is in good
agreement with previous studies. Using the results of two feedback models (MT1
and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a
PopIII IMF, we find that at high- the IMF closely tracks the spin lognormal
distribution. Depending on the feedback model, though, the distribution can be
centered at (MT1) or (MT2). At later
times, model MT1 evolves into a bimodal distribution with a second prominent
peak located at as a result of the non-linear relation between
rotation and halo mass. We conclude that the dark matter halo properties might
be a key factor shaping the IMF of the first stars.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Imaging the heart of astrophysical objects with optical long-baseline interferometry
The number of publications of aperture-synthesis images based on optical
long-baseline interferometry measurements has recently increased due to easier
access to visible and infrared interferometers. The interferometry technique
has now reached a technical maturity level that opens new avenues for numerous
astrophysical topics requiring milli-arcsecond model-independent imaging. In
writing this paper our motivation was twofold: 1) review and publicize
emblematic excerpts of the impressive corpus accumulated in the field of
optical interferometry image reconstruction; 2) discuss future prospects for
this technique by selecting four representative astrophysical science cases in
order to review the potential benefits of using optical long baseline
interferometers. For this second goal we have simulated interferometric data
from those selected astrophysical environments and used state-of-the-art codes
to provide the reconstructed images that are reachable with current or
soon-to-be facilities. The image reconstruction process was "blind" in the
sense that reconstructors had no knowledge of the input brightness
distributions. We discuss the impact of optical interferometry in those four
astrophysical fields. We show that image reconstruction software successfully
provides accurate morphological information on a variety of astrophysical
topics and review the current strengths and weaknesses of such reconstructions.
We investigate how to improve image reconstruction and the quality of the image
possibly by upgrading the current facilities. We finally argue that optical
interferometers and their corresponding instrumentation, existing or to come,
with 6 to 10 telescopes, should be well suited to provide images of complex
sceneries.Comment: Acccepted to Astronomy and Astrophysics Revie
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