5,412 research outputs found
Available Energy for Life on a Planet, with or without Stellar Radiation
The quest for life in the Universe is often affected by the free use of
extrapolations of our phenomenological geocentric knowledge. We point out that
the existence of a living organism, and a population of organisms, requires the
existence of available energy or, more precisely, available power per unit
volume (Sect. 1). This is not a geocentric concept, but a principle that
belongs to the foundations of thermodynamics. A quest about availability in the
Universe is justified. We discuss the case in which power comes from mining
(Sect. 2), and from thermal disequilibrium (Sect. 3). Thermal disequilibrium
may show up in two ways: on planets without a star (Sect. 4), and on planets
where the surface thermal disequilibrium is dominated by the incoming photon
flux from the nearest star (Sect. 6). In the first case we study the
availability by simulating the structure of the planet with a simple model that
contains the general features of the problem. For the first case we show that
the availability is in general very small (Sect. 5). In the second case we show
that the availability is in general large; the order of magnitude depends first
of all on the star's temperature and the planet's orbit, but is also controlled
by the greenhouse gases present on the planet.Comment: 30 pages, 10 figures, to be published in Nuovo Cimento
Molecular Detectability in Exoplanetary Emission Spectra
Of the many recently discovered worlds orbiting distant stars, very little is
yet known of their chemical composition. With the arrival of new transit
spectroscopy and direct imaging facilities, the question of molecular
detectability as a function of signal-to-noise (SNR), spectral resolving power
and type of planets has become critical. In this paper, we study the
detectability of key molecules in the atmospheres of a range of planet types,
and report on the minimum detectable abundances at fixed spectral resolving
power and SNR. The planet types considered - hot Jupiters, hot super-Earths,
warm Neptunes, temperate Jupiters and temperate super-Earths - cover most of
the exoplanets characterisable today or in the near future. We focus on key
atmospheric molecules, such as CH4, CO, CO2, NH3, H2O, C2H2, C2H6, HCN, H2S and
PH3. We use two methods to assess the detectability of these molecules: a
simple measurement of the deviation of the signal from the continuum, and an
estimate of the level of confidence of a detection through the use of the
likelihood ratio test over the whole spectrum (from 1 to 16). We find
that for most planetary cases, SNR=5 at resolution R=300 ()
and R=30 () is enough to detect the very strongest spectral
features for the most abundant molecules, whereas an SNR comprised between 10
and 20 can reveal most molecules with abundances 10^-6 or lower, often at
multiple wavelengths. We test the robustness of our results by exploring
sensitivity to parameters such as vertical thermal profile, mean molecular
weight of the atmosphere and relative water abundances. We find that our main
conclusions remain valid except for the most extreme cases. Our analysis shows
that the detectability of key molecules in the atmospheres of a variety of
exoplanet cases is within realistic reach, even with low SNR and spectral
resolving power.Comment: ICARUS Accepte
Primary transit of the planet HD189733b at 3.6 and 5.8 microns
The hot Jupiter HD 189733b was observed during its primary transit using the
Infrared Array Camera on the Spitzer Space Telescope. The transit depths were
measured simultaneously at 3.6 and 5.8 microns. Our analysis yields values of
2.356 +- 0.019 % and 2.436 +- 0.020$ % at 3.6 and 5.8 microns respectively, for
a uniform source. We estimated the contribution of the limb-darkening and
star-spot effects on the final results. We concluded that although the limb
darkening increases by ~0.02-0.03 % the transit depths, and the differential
effects between the two IRAC bands is even smaller, 0.01 %. Furthermore, the
host star is known to be an active spotted K star with observed photometric
modulation. If we adopt an extreme model of 20 % coverage with spots 1000K
cooler of the star surface, it will make the observed transits shallower by
0.19 and 0.18 %. The difference between the two bands will be only of 0.01 %,
in the opposite direction to the limb darkening correction. If the transit
depth is affected by limb darkening and spots, the differential effects between
the 3.6 and 5.8 microns bands are very small. The differential transit depths
at 3.6 and 5.8 microns and the recent one published by Knutson et al.(2007) at
8 microns are in agreement with the presence of water vapour in the upper
atmosphere of the planet. This is the companion paper to Tinetti et al.
(2007b), where the detailed atmosphere models are presented.Comment: 6 pages, 4 figures, Astrophysical Journal 675. Accepted Nov 21,
20007, to appear on March 10, 200
Nightside Pollution of Exoplanet Transit Depths
Out of the known transiting extrasolar planets, the majority are gas giants
orbiting their host star at close proximity. Both theoretical and observational
studies support the hypothesis that such bodies emit significant amounts of
flux relative to the host star, increasing towards infrared wavelengths. For
the dayside of the exoplanet, this phenomenon typically permits detectable
secondary eclipses at such wavelengths, which may be used to infer atmospheric
composition. In this paper, we explore the effects of emission from the
nightside of the exoplanet on the primary transit lightcurve, which is
essentially a self-blend. Allowing for nightside emission, an exoplanet's
transit depth is no longer exclusively a function of the ratio-of-radii. The
nightside of an exoplanet is emitting flux and the contrast to the star's
emission is of the order of ~10^(-3) for hot-Jupiters. Consequently, we show
that the transit depth in the mid-infrared will be attenuated due to flux
contribution from the nightside emission by ~10^(-4). We show how this effect
can be compensated for in the case where exoplanet phase curves have been
measured, in particular for HD 189733b. For other systems, it may be possible
to make a first-order correction by using temperature estimates of the planet.
Unless the effect is accounted for, transmission spectra will also be polluted
by nightside emission and we estimate that a Spitzer broadband spectrum on a
bright target is altered at the 1-sigma level. Using archived Spitzer
measurements, we show that the effect respectively increases the 8.0um and
24.0um transit depths by 1-sigma and 0.5-sigma per transit for HD 189733b.
Consequently, we estimate that this would be 5-10 sigma effect for near-future
JWST observations.Comment: Accepted in MNRA
The extrasolar planet atmosphere and exosphere: Emission and transmission spectroscopy
We have entered the phase of extrasolar planets characterization, probing
their atmospheres for molecules, constraining their horizontal and vertical
temperature profiles and estimating the contribution of clouds and hazes. We
report here a short review of the current situation using ground based and
space based observations, and present the transmission spectra of HD189733b in
the spectral range 0.5-24 microns.Comment: 8 pages, 3 figures, invited talk at IAU Symposium 253, Transiting
planet, Boston May 2008. Pont F., Queloz D., Sasselov., Torres M. and Holman
M. editor
An Updated Study of Potential Targets for Ariel
Ariel has been selected as ESA's M4 mission for launch in 2028 and is
designed for the characterisation of a large and diverse population of
exoplanetary atmospheres to provide insights into planetary formation and
evolution within our Galaxy. Here we present a study of Ariel's capability to
observe currently-known exoplanets and predicted TESS discoveries. We use the
Ariel Radiometric model (ArielRad) to simulate the instrument performance and
find that ~2000 of these planets have atmospheric signals which could be
characterised by Ariel. This list of potential planets contains a diverse range
of planetary and stellar parameters. From these we select an example Mission
Reference Sample (MRS), comprised of 1000 diverse planets to be completed
within the primary mission life, which is consistent with previous studies. We
also explore the mission capability to perform an in-depth survey into the
atmospheres of smaller planets, which may be enriched or secondary. Earth-sized
planets and Super-Earths with atmospheres heavier than H/He will be more
challenging to observe spectroscopically. However, by studying the time
required to observe ~110 Earth-sized/Super-Earths, we find that Ariel could
have substantial capability for providing in-depth observations of smaller
planets. Trade-offs between the number and type of planets observed will form a
key part of the selection process and this list of planets will continually
evolve with new exoplanet discoveries replacing predicted detections. The Ariel
target list will be constantly updated and the MRS re-selected to ensure
maximum diversity in the population of planets studied during the primary
mission life
Principles of distributed database systems, third edition
We could consider this book as a classical one from many points of view. To start with, the book is the third in a series of editions since the first one more than 20 years ago, thus containing the evolution in the field of distributed databases as well as the basic concepts which are not strongly affected by technology evolution. The thorough review of relational databases as well as a discussion of several aspects of distributed systems (as computer networks) in Chapter 2 makes the book highly self-contained, enhancing reading and understanding underlying principles. The book covers all the problems expected to be encountered in distributed databases.\n\n(Párrafo extraído del texto a modo de resumen)</i
Performance of scientific processing in networks of workstations: matrix multiplication example
Parallel computing on networks of workstations are intensively used in some application areas such as linear algebra operations. Topics such as processing as well as communication hardware heterogeneity are considered solved by the use of parallel processing libraries, but experimentation about performance under these circumstances seems to be necessary. Also, installed networks of workstations are specially attractive due to its extremely low cost for parallel processing as well as its great availability given the number of installed local area networks. The performance of such networks of workstations is fully analyzed by means of a simple application: matrix multiplication. A parallel algorithm is proposed for matrix multiplication derived from two main sources: a) previous proposed algorithms for this task in traditional parallel computers, and b) the bus based interconnection network of workstations. This parallel algorithm is analyzed experimentally in terms of workstations workload and data communication, two main factors in overall parallel computing performance
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