4,687 research outputs found
Super-Earth Atmospheres: Self-Consistent Gas Accretion and Retention
Some recently discovered short-period Earth to Neptune sized exoplanets
(super Earths) have low observed mean densities which can only be explained by
voluminous gaseous atmospheres. Here, we study the conditions allowing the
accretion and retention of such atmospheres. We self-consistently couple the
nebular gas accretion onto rocky cores and the subsequent evolution of gas
envelopes following the dispersal of the protoplanetary disk. Specifically, we
address mass-loss due to both photo-evaporation and cooling of the planet. We
find that planets shed their outer layers (dozens of percents in mass)
following the disk's dispersal (even without photo-evaporation), and their
atmospheres shrink in a few Myr to a thickness comparable to the radius of the
underlying rocky core. At this stage, atmospheres containing less particles
than the core (equivalently, lighter than a few % of the planet's mass) can be
blown away by heat coming from the cooling core, while heavier atmospheres cool
and contract on a timescale of Gyr at most. By relating the mass-loss timescale
to the accretion time, we analytically identify a Goldilocks region in the
mass-temperature plane in which low-density super Earths can be found: planets
have to be massive and cold enough to accrete and retain their atmospheres,
while not too massive or cold, such that they do not enter runaway accretion
and become gas giants (Jupiters). We compare our results to the observed
super-Earth population and find that low-density planets are indeed
concentrated in the theoretically allowed region. Our analytical and intuitive
model can be used to investigate possible super-Earth formation scenarios.Comment: Updated (refereed) versio
Interplanetary magnetic fields, their fluctuations, and cosmic ray variations
The cause of Forbush decreases is examined using neutron monitor data and measurements of the interplanetary magnetic field. It is found that for the period examined (Dec. 15, 1965 to April 23, 1966) large enhancements of the interplanetary magnetic field correlate well with decreases in cosmic ray intensity, while various parameters connected with the fluctuations in the field do not display such good correlation. The inference is drawn that Forbush decreases are not related to the turbulence or random motions in the field but to the large scale features of the field
The Self-Similarity of Shear-Dominated Viscous Stirring
We examine the growth of eccentricities of a population of particles with
initially circular orbits around a central massive body. Successive encounters
between pairs of particles increase the eccentricities in the disk on average.
As long as the epicyclic motions of the particles are small compared to the
shearing motion between Keplerian orbits, there is no preferred scale for the
eccentricities. The simplification due to this self-similarity allows us to
find an analytic form for the distribution function; full numerical
integrations of a disk with 200 planetesimals verify our analytical
self-similar distribution. The shape of this non-equilibrium profile is
identical to the equilibrium profile of a shear-dominated population whose
mutual excitations are balanced by dynamical friction or Epstein gas drag.Comment: 8 pages, 2 figure
GeV Emission from Prompt and Afterglow Phases of Gamma-Ray Bursts
We investigate the GeV emission from gamma-ray bursts (GRBs), using the
results from the Energetic Gamma Ray Experimental Telescope (EGRET), and in
view of the Gamma-ray Large Area Space Telescope (GLAST). Assuming that the
conventional prompt and afterglow photons originate from synchrotron radiation,
we compare an accompanying inverse-Compton component with EGRET measurements
and upper limits on GeV fluence, taking Klein-Nishina feedback into account. We
find that EGRET constraints are consistent with the theoretical framework of
the synchrotron self-Compton model for both prompt and afterglow phases, and
discuss constraints on microphysical parameters in both phases. Based on the
inverse-Compton model and using EGRET results, we predict that GLAST would
detect GRBs with GeV photons at a rate >~20 yr^{-1} from each of the prompt and
afterglow phases. This rate applies to the high-energy tail of the prompt
synchrotron emission and to the inverse-Compton component of the afterglow.
Theory predicts that in a large fraction of the cases where synchrotron GeV
prompt emission would be detected by GLAST, inverse-Compton photons should be
detected as well at high energies >~10 GeV. Therefore GLAST will enable a more
precise test of the high-energy emission mechanism. Finally, we show that the
contribution of GRBs to the flux of the extragalactic gamma-ray background
measured with EGRET is at least 0.01% and likely around 0.1%.Comment: 11 pages, 7 figures; accepted by Ap
Formation of Kuiper Belt Binaries
The discovery that a substantial fraction of Kuiper Belt objects (KBOs)
exists in binaries with wide separations and roughly equal masses, has
motivated a variety of new theories explaining their formation. Goldreich et
al. (2002) proposed two formation scenarios: In the first, a transient binary
is formed, which becomes bound with the aid of dynamical friction from the sea
of small bodies (L^2s mechanism); in the second, a binary is formed by three
body gravitational deflection (L^3 mechanism). Here, we accurately calculate
the L^2s and L^3 formation rates for sub-Hill velocities. While the L^2s
formation rate is close to previous order of magnitude estimates, the L^3
formation rate is about a factor of 4 smaller. For sub-Hill KBO velocities (v
<< v_H) the ratio of the L^3 to the L^2s formation rate is 0.05 (v/v_H)
independent of the small bodies' velocity dispersion, their surface density or
their mutual collisions. For Super-Hill velocities (v >> v_H) the L^3 mechanism
dominates over the L^2s mechanism. Binary formation via the L^3 mechanism
competes with binary destruction by passing bodies. Given sufficient time, a
statistical equilibrium abundance of binaries forms. We show that the frequency
of long-lived transient binaries drops exponentially with the system's lifetime
and that such transient binaries are not important for binary formation via the
L^3 mechanism, contrary to Lee et al. (2007). For the L^2s mechanism we find
that the typical time, transient binaries must last, to form Kuiper Belt
binaries (KBBs) for a given strength of dynamical friction, D, increases only
logarithmically with D. Longevity of transient binaries only becomes important
for very weak dynamical friction (i.e. D \lesssim 0.002) and is most likely not
crucial for KBB formation.Comment: 20 pages, 3 figures, Accepted for publication in ApJ, correction of
minor typo
Predictions for The Very Early Afterglow and The Optical Flash
According to the internal-external shocks model for -ray bursts
(GRBs), the GRB is produced by internal shocks within a relativistic flow while
the afterglow is produced by external shocks with the ISM. We explore the early
afterglow emission. For short GRBs the peak of the afterglow will be delayed,
typically, by few dozens of seconds after the burst. For long GRBs the early
afterglow emission will overlap the GRB signal. We calculate the expected
spectrum and the light curves of the early afterglow in the optical, X-ray and
-ray bands. These characteristics provide a way to discriminate
between late internal shocks emission (part of the GRB) and the early afterglow
signal. If such a delayed emission, with the characteristics of the early
afterglow, will be detected it can be used both to prove the internal shock
scenario as producing the GRB, as well as to measure the initial Lorentz factor
of the relativistic flow. The reverse shock, at its peak, contains energy which
is comparable to that of the GRB itself, but has a much lower temperature than
that of the forward shock so it radiates at considerably lower frequencies. The
reverse shock dominates the early optical emission, and an optical flash
brighter than 15th magnitude, is expected together with the forward shock peak
at x-rays or -rays. If this optical flash is not observed, strong
limitations can be put on the baryonic contents of the relativistic shell
deriving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure
Broad-band Modeling of GRB Afterglows
Observations of GRB afterglows ranging from radio to X-ray frequencies
generate large data sets. Careful analysis of these broad-band data can give us
insight into the nature of the GRB progenitor population by yielding such
information like the total energy of the burst, the geometry of the fireball
and the type of environment into which the GRB explodes. We illustrate, by
example, how global, self-consistent fits are a robust approach for
characterizing the afterglow emission. This approach allows a relatively simple
comparison of different models and a way to determine the strengths and
weaknesses of these models, since all are treated self-consistently. Here we
quantify the main differences between the broad-band, self-consistent approach
and the traditional approach, using GRB000301C and GRB970508 as test cases.Comment: Appears in "Gamma-Ray Bursts in the Afterglow Era" proceedings of the
Roma 2000 GRB Workshop; 3 pages; 2 figure
Geographical Information System Mapping the Billboards in Samarinda
Advances in information technology on geography are increasingly needed by many people, for example information on distances between regions, locations, facilities and many other information. The information is needed by users for various purposes such as research, development, regional design and natural resource management. Because of this geographical presence can help the presentation of a more interactive information, where users can access complete geographical information using only a computer, web-browser and internet network. So to get that information all in need of a Geographical Information System (GIS). The purpose of this research is to create a web that contains information on the location of billboards in the city of Samarinda. While this research is expected to make it easier for users to obtain information on the location of billboards in Samarinda
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