1,084 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
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
Prey-predator dynamics with periodic solar input. Part II
We study a two-component model ecosystem driven by a sinusoidal solar radiation. The governing dynamical system is expressed by two nonlinear differential equations, where the driving term appears factorized to one of the two
unknown functions. We show that the solution is asymptotically periodic, with the period of the driving term. Moreover, we find that the asymptotic solution, with the variation of the frequency of the input, shows a resonant-like behaviour. We discuss the interesting similarity between the response of the ecosystem to the external driving term and the response of a genuine resonant system
Constraints in the coupling Star-Life
If life is sustained by a process of photosynthesis, not necessarily the same
existing on Earth, the surface temperature of the star and the orbit of the
host planet cannot be whatsoever. In fact the global life cycle, no matter how
complicated, must contain in general an upper photochemical branch and a lower
dark branch, characterized by a higher and a lower temperature. These two
temperatures are star-orbit related. The velocity along the cycle or, in other
words, the power of the life machine, depends in general on several other
parameters. First of all the Gibbs photon availability, which is a star-orbit
parameter and is the input for the upper branch. Then follows the energy
cascade that develops along the organic web with a large number of interactions
and typical times that must match the typical times generated by the
combination of spin value and orientation, eccentricity and precession.
Finally, the capacity of the web to keep the global life cycle running along
the life span of the star, comes from some inner form of self-endurance and
self-balance. The property of not being transient could be the right way of
introducing the concept of intelligent life.Comment: 56 pages, 21 figures. In order to reduce size, the version on the
archive has low-resolution figures. A version of the paper with full
resolution may be requested to [email protected]
Repeatability of Spitzer/IRAC exoplanetary eclipses with Independent Component Analysis
The research of effective and reliable detrending methods for Spitzer data is of paramount importance for the characterization of exoplanetary atmospheres. To date, the totality of exoplanetary observations in the mid- and far-infrared, at wavelengths 3 m, have been taken with Spitzer. In some cases, in the past years, repeated observations and multiple reanalyses of the same datasets led to discrepant results, raising questions about the accuracy and reproducibility of such measurements. Morello et al. 2014, 2015 proposed a blind-source separation method based on the Independent Component Analysis of pixel time series (pixel-ICA) to analyze IRAC data, obtaining coherent results when applied to repeated transit observations previously debated in the literature. Here we introduce a variant to pixel-ICA through the use of wavelet transform, wavelet pixel-ICA, which extends its applicability to low-S/N cases. We describe the method and discuss the results obtained over twelve eclipses of the exoplanet XO3b observed during the "Warm Spitzer" era in the 4.5 m band. The final results will be reported also in Ingalls et al. (in prep.), together with results obtained with other detrending methods, and over ten synthetic eclipses that were analyzed for the "IRAC Data Challenge 2015". Our results are consistent within 1 with the ones reported in Wong et al. 2014. The self-consistency of individual measurements of eclipse depth and phase curve slope over a span of more than three years proves the stability of Warm Spitzer/IRAC photometry within the error bars, at the level of 1 part in 10 in stellar flux
Prey-predator dynamics driven by the solar radiation. Part I
We study a model ecosystem represented by two components: prey and predator. The predator feeds only on the prey, the prey, in turn, feeds on the solar radiation. In this scheme the two-species dynamics is no longer independent of the external physical conditions. Such independence was instead postulated in the Lotka-Volterra scheme. In this paper we consider the growth of the prey not unbounded (exponential), but logistic, where the saturation factor is governed by the available solar flux, more precisely by the percent of the solar flux that contains the photon frequencies which can drive the photosynthesis. In this way the solar flux represents the driving term of the dynamics, as we expect in general for a realistic ecosystem. The system is asymptotically stable. The equilibrium values of the prey and predator numbers depend on several parameters. The system contains two nonlinear coupling terms and two coupling parameters. The dependence of the equilibrium point on the coupling parameters is studied in detail. According to this model, we can define a predator efficiency and a global solar efficiency. We discuss the relationship between these two functions of the coupling parameters and the maximum value that the predator population can reach
A new approach to analysing HST spatial scans: the transmission spectrum of HD 209458 b
The Wide Field Camera 3 (WFC3) on Hubble Space Telescope (HST) is currently
one of the most widely used instruments for observing exoplanetary atmospheres,
especially with the use of the spatial scanning technique. An increasing number
of exoplanets have been studied using this technique as it enables the
observation of bright targets without saturating the sensitive detectors. In
this work we present a new pipeline for analyzing the data obtained with the
spatial scanning technique, starting from the raw data provided by the
instrument. In addition to commonly used correction techniques, we take into
account the geometric distortions of the instrument, whose impact may become
important when combined to the scanning process. Our approach can improve the
photometric precision for existing data and also push further the limits of the
spatial scanning technique, as it allows the analysis of even longer spatial
scans. As an application of our method and pipeline, we present the results
from a reanalysis of the spatially scanned transit spectrum of HD 209458 b. We
calculate the transit depth per wavelength channel with an average relative
uncertainty of 40 ppm. We interpret the final spectrum with T-Rex, our fully
Bayesian spectral retrieval code, which confirms the presence of water vapor
and clouds in the atmosphere of HD 209458 b. The narrow wavelength range limits
our ability to disentangle the degeneracies between the fitted atmospheric
parameters. Additional data over a broader spectral range are needed to address
this issue.Comment: 13 pages, 15 figures, 7 tables, Accepted for publication in Ap
A Principal Component Analysis-based method to analyse high-resolution spectroscopic data
High-Resolution Spectroscopy (HRS) has been used to study the composition and
dynamics of exoplanetary atmospheres. In particular, the spectrometer CRIRES
installed on the ESO-VLT has been used to record high-resolution spectra in the
Near-IR of gaseous exoplanets. Here we present a new automatic pipeline to
analyze CRIRES data-sets. Said pipeline is based on a novel use of Principal
Component Analysis (PCA) and Cross-Correlation Function (CCF). The exoplanetary
atmosphere is modeled with the -REx code using opacities at high
temperature from the ExoMol project. In this work, we tested our analysis tools
on the detection of CO and HO in the atmospheres of the hot-Jupiters
HD209458b and HD189733b. The results of our pipeline are in agreement with
previous results in the literature and other techniques.Comment: 14 pages, 12 figures, 2 tables, published in Ap
Molecular Signatures in the Near Infrared Dayside Spectrum of HD 189733b
We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5
microns using the NICMOS instrument on the Hubble Space Telescope. The emergent
spectrum contains significant modulation, which we attribute to the presence of
molecular bands seen in absorption. We find that water (H2O), carbon monoxide
(CO), and carbon dioxide (CO2) are needed to explain the observations, and we
are able to estimate the mixing ratios for these molecules. We also find
temperature decreases with altitude in the ~0.01 < P < ~1 bar region of the
dayside near-infrared photosphere and set an upper limit to the dayside
abundance of methane (CH4) at these pressures.Comment: 13 pages, 3 figures. accepted in Astrophysical Journal Letter
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