217 research outputs found
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A survey of routing techniques in store-and-forward and wormhole interconnects.
This paper presents an overview of algorithms for directing messages through networks of varying topology. These are commonly referred to as routing algorithms in the literature that is presented. In addition to providing background on networking terminology and router basics, the paper explains the issues of deadlock and livelock as they apply to routing. After this, there is a discussion of routing algorithms for both store-and-forward and wormhole-switched networks. The paper covers both algorithms that do and do not adapt to conditions in the network. Techniques targeting structured as well as irregular topologies are discussed. Following this, strategies for routing in the presence of faulty nodes and links in the network are described
A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations
We discovered evidence for a possible additional 0.75 R_Earth transiting
planet in the NASA EPOXI observations of the known M dwarf exoplanetary system
GJ 436. Based on an ephemeris determined from the EPOXI data, we predicted a
transit event in an extant Spitzer Space Telescope 8 micron data set of this
star. Our subsequent analysis of those Spitzer data confirmed the signal of the
predicted depth and at the predicted time, but we found that the transit depth
was dependent on the aperture used to perform the photometry. Based on these
suggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5
microns spanning a time of transit predicted from the EPOXI and Spitzer 8
micron candidate events. The 4.5 micron data permit us to rule out a transit at
high confidence, and we conclude that the earlier candidate transit signals
resulted from correlated noise in the EPOXI and Spitzer 8 micron observations.
In the course of this investigation, we developed a novel method for correcting
the intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of the
Infrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of Warm
Spitzer observations of M dwarfs to confirm sub-Earth sized planets. Our
analysis will inform similar work that will be undertaken to use Warm Spitzer
observations to confirm rocky planets discovered by the Kepler mission.Comment: 22 pages, 8 figures, accepted for publication in PAS
The Transit Light Curve Project. VII. The Not-So-Bloated Exoplanet HAT-P-1b
We present photometry of the G0 star HAT-P-1 during six transits of its
close-in giant planet, and we refine the estimates of the system parameters.
Relative to Jupiter's properties, HAT-P-1b is 1.20 +/- 0.05 times larger and
its surface gravity is 2.7 +/- 0.2 times weaker. Although it remains the case
that HAT-P-1b is among the least dense of the known sample of transiting
exoplanets, its properties are in accord with previously published models of
strongly irradiated, coreless, solar-composition giant planets. The times of
the transits have a typical accuracy of 1 min and do not depart significantly
from a constant period.Comment: To appear in AJ [19pg, 3 figures]. New co-author added. Minor
revisions to match published versio
Studying the atmosphere of the exoplanet HAT-P-7b via secondary eclipse measurements with EPOXI, Spitzer and Kepler
The highly irradiated transiting exoplanet, HAT-P-7b, currently provides one
of the best opportunities for studying planetary emission in the optical and
infrared wavelengths. We observe six near-consecutive secondary eclipses of
HAT-P-7b at optical wavelengths with the EPOXI spacecraft. We place an upper
limit on the relative eclipse depth of 0.055% (95% confidence). We also analyze
Spitzer observations of the same target in the infrared, obtaining secondary
eclipse depths of 0.098+/-0.017%, 0.159+/-0.022%, 0.245+/-0.031% and
0.225+/-0.052% in the 3.6, 4.5, 5.8 and 8.0 micron IRAC bands respectively. We
combine these measurements with the recently published Kepler secondary eclipse
measurement, and generate atmospheric models for the day-side of the planet
that are consistent with both the optical and infrared measurements. The data
are best fit by models with a temperature inversion, as expected from the high
incident flux. The models predict a low optical albedo of ~< 0.13, with
subsolar abundances of Na, K, TiO and VO. We also find that the best fitting
models predict that 10% of the absorbed stellar flux is redistributed to the
night side of the planet, which is qualitatively consistent with the
inefficient day-night redistribution apparent in the Kepler phase curve. Models
without thermal inversions fit the data only at the 1.25 sigma level, and also
require an overabundance of methane, which is not expected in the very hot
atmosphere of HAT-P-7b. We also analyze the eight transits of HAT-P-7b present
in the EPOXI dataset and improve the constraints on the system parameters,
finding a period of P = 2.2047308+/-0.0000025 days, a stellar radius of R* =
1.824+/-0.089Rsun, a planetary radius of Rp = 1.342+/-0.068RJup and an
inclination of i = 85.7+3.5-2.2 deg.Comment: 21 pages, 8 figures, accepted by the Astrophysical Journa
How do Most Planets Form? -- Constraints on Disk Instability from Direct Imaging
Core accretion and disk instability have traditionally been regarded as the
two competing possible paths of planet formation. In recent years, evidence
have accumulated in favor of core accretion as the dominant mode, at least for
close-in planets. However, it might be hypothesized that a significant
population of wide planets formed by disk instabilities could exist at large
separations, forming an invisible majority. In previous work, we addressed this
issue through a direct imaging survey of B2--A0-type stars, and concluded that
<30% of such stars form and retain planets and brown dwarfs through disk
instability, leaving core accretion as the likely dominant mechanism. In this
paper, we extend this analysis to FGKM-type stars by applying a similar
analysis to the Gemini Deep Planet Survey (GDPS) sample. The results strengthen
the conclusion that substellar companions formed and retained around their
parent stars by disk instabilities are rare. Specifically, we find that the
frequency of such companions is <8% for FGKM-type stars under our most
conservative assumptions, for an outer disk radius of 300 AU, at 99%
confidence. Furthermore, we find that the frequency is always <10% at 99%
confidence independently of outer disk radius, for any radius from 5 to 500 AU.
We also simulate migration at a wide range of rates, and find that the
conclusions hold even if the companions move substantially after formation.
Hence, core accretion remains the likely dominant formation mechanism for the
total planet population, for every type of star from M-type through B-type.Comment: 10 pages, 4 figures, accepted for publication in Ap
Preliminary Results on HAT-P-4, TrES-3, XO-2, and GJ 436 from the NASA EPOXI Mission
EPOXI (EPOCh + DIXI) is a NASA Discovery Program Mission of Opportunity using
the Deep Impact flyby spacecraft. The EPOCh (Extrasolar Planet Observation and
Characterization) Science Investigation will gather photometric time series of
known transiting exoplanet systems from January through August 2008. Here we
describe the steps in the photometric extraction of the time series and present
preliminary results of the first four EPOCh targets.Comment: 4 pages, 2 figures. To appear in the Proceedings of the 253rd IAU
Symposium: "Transiting Planets", May 2008, Cambridge, M
Microporosity of a guanidinium organodisulfonate hydrogen-bonded framework
CITATION: Brekalo, I. et al. 2020. Microporosity of a Guanidinium Organodisulfonate Hydrogen-Bonded Framework. Angewandte Chemie, 59(5):1997-2002. doi:10.1002/anie.201911861The original publication is available at https://onlinelibrary.wiley.com/journal/15213757Guanidinium organosulfonates (GSs) are a large and well-explored archetypal family of hydrogen-bonded organic host frameworks that have, over the past 25 years, been regarded as nonporous. Reported here is the only example to date of a conventionally microporous GS host phase, namely guanidinium 1,4-benzenedisulfonate (p-G2BDS). p-G2BDS is obtained from its acetone solvate, AcMe@G2BDS, by single-crystal-to-single-crystal (SC-SC) desolvation, and exhibits a Type I low-temperature/pressure N2 sorption isotherm (SABET=408.7(2) m2 g−1, 77 K). SC-SC sorption of N2, CO2, Xe, and AcMe by p-G2BDS is explored under various conditions and X-ray diffraction provides a measurement of the high-pressure, room temperature Xe and CO2 sorption isotherms. Though p-G2BDS is formally metastable relative to the “collapsed”, nonporous polymorph, np-G2BDS, a sample of p-G2BDS survived for almost two decades under ambient conditions. np-G2BDS reverts to zCO2@p-G2BDS or yXe@p-G2BDS (y,z=variable) when pressure of CO2 or Xe, respectively, is applied.https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201911861Publishers versio
Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations
We present a method to confirm the planetary nature of objects in systems
with multiple transiting exoplanet candidates. This method involves a
Fourier-Domain analysis of the deviations in the transit times from a constant
period that result from dynamical interactions within the system. The
combination of observed anti-correlations in the transit times and mass
constraints from dynamical stability allow us to claim the discovery of four
planetary systems Kepler-25, Kepler-26, Kepler-27, and Kepler-28, containing
eight planets and one additional planet candidate.Comment: Accepted to MNRA
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