1,423 research outputs found
Habitable planets around the star Gl 581?
Radial velocity surveys are now able to detect terrestrial planets at
habitable distance from M-type stars. Recently, two planets with minimum masses
below 10 Earth masses were reported in a triple system around the M-type star
Gliese 581. Using results from atmospheric models and constraints from the
evolution of Venus and Mars, we assess the habitability of planets Gl 581c and
Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ)
boundaries determination. We provide simplified formulae to estimate the HZ
limits that may be used to evaluate the astrobiological potential of
terrestrial exoplanets that will hopefully be discovered in the near future.
Planets Gl 581c and 'd' are near, but outside, what can be considered as the
conservative HZ. Planet 'c' receives 30% more energy from its star than Venus
from the Sun, with an increased radiative forcing caused by the spectral energy
distribution of Gl 581. Its habitability cannot however be positively ruled out
by theoretical models due to uncertainties affecting cloud properties.
Irradiation conditions of planet 'd' are comparable with those of early Mars.
Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better
candidate for the first exoplanet known to be potentially habitable. A mixture
of various greenhouse gases could also maintain habitable conditions on this
planet.Comment: Astronomy and Astrophysics (2007) accepted for publicatio
Un nou mètode sintètic per a les tiourees amb grups atraients d'electrons: les reaccions en fusió
Aplicacions analítiques dels equilibris de complexació del FE(III) amb els àcids 5-sulfosalicílic i cítric
Aluminosi, problema resolt o conflicte latent?
L’ensorrament del sostre d’un edifici del Turó de la Peira de Barcelona va desencadenar una gran alarma social a l’entorn de l’estat del parc edifica t. Una alarma que a poc a poc es va anar apaivagant amb mesures polítiques i tècniques que han permès assolir un bon coneixement dels edificis, del seu estat, de les seves mancances i de la manera de corregir- les. Avui, vint anys després, encara són molts els malentesos de caire tècnic i social que hi ha a l’entorn d’aquest tema i que cal clarificar per evitar les conseqüències perverses que comporten. L’aluminosi ha deixat de ser un problema tècnic per esdevenir un problema de caire socioeconòmic i judicial. El que va començar com un problema estructural dels edificis, la degradació de les biguetes fabricades amb ciment aluminós, –els mitjans de comunicació ho van popularitzar amb el nom d’aluminosi– ha passat a ser: un problema social, ja que els edificis on hi ha ciment aluminós són considerats edificis malalts i perillosos per als seus habitants; un problema econòmic, ja que les entitats financeres concedeixen un valor molt reduït a aquests edificis; i un problema judicial, perquè la simple presència de ciment aluminós en un edifici és considerada una patologia greu per part dels jutges. El concepte científic d’aluminosi el va definir per primer cop, l’any 1989, el químic Rafael Talero, com un fenomen que es podia produir en el formigó fabricat amb ciment aluminós i que necessitava tres processos patològics simultanis: la conversió, la carbonatació i la hidròlisi alcalina. El resultat és un formigó que, com deia Talero, es desfà dins de l’aigua com un terròs de sucre. Però també és cert que la hidròlisi alcalina que sí que existeix als laboratoris, fins ara no l’hem trobada en cap dels edificis ni en els milers de mostres analitzades.Peer ReviewedPostprint (published version
GU Boo: A New 0.6 Msun Detached Eclipsing Binary
We have found a new low-mass, double-lined, detached eclipsing binary, GU
Boo, among a sample of new variables from the ROTSE-I database. The binary has
an orbital period of 0.488728 +/- 0.000002 days, and estimated apparent
magnitudes Vrotse = 13.7 and I = 11.8. Our analysis of the light and radial
velocity curves of the system yields individual masses and radii of M1= 0.610
+/- 0.007 Msun, M2 = 0.599 +/- 0.006 Msun, R1= 0.623 +/- 0.016 Rsun, R2= 0.620
+/- 0.020 Rsun. The stars in GU Boo are therefore very similar to the
components of the eclipsing binary YY Gem. For this study we have adopted a
mean effective temperature for the binary of Teff = 3870 +/- 130 K. Based on
its space velocities we suggest that GU Boo is a main sequence binary, possibly
with an age of several Gyr. The metallicity of the binary is not well
constrained at this point but we speculate that it should not be very different
from solar. We have compared the physical parameters of GU Boo with current
low-mass stellar models, where we accounted for uncertainties in age and
metallicity by considering a wide range of values for those parameters. Our
comparisons reveal that all the models underestimate the radii of the
components of GU Boo by at least 10-15%. This result is in agreement with the
recent studies of YY Gem and CU Cnc.Comment: 41 pages, 10 figures, 11 tables; accepted by Ap
Masses and Radii of Low-Mass Stars: Theory versus Observations
Eclipsing binaries with M-type components are still rare objects. Strong
observational biases have made that today only a few eclipsing binaries with
component masses below 0.6 Msun and well-determined fundamental properties are
known. However, even in these small numbers the detailed comparison of the
observed masses and radii with theoretical predictions has revealed large
disagreements. Current models seem to predict radii of stars in the 0.4-0.8
Msun range to be some 5-15% smaller than observed. Given the high accuracy of
the empirical measurements (a few percent in both mass and radius), these
differences are highly significant. I review all the observational evidence on
the properties of M-type stars and discuss a possible scenario based on stellar
activity to explain the observed discrepancies.Comment: 8 pages, 1 figure, proceedings of the meeting "Close Binaries in the
21st Century
Design of Zn-, Cu-, and Fe-Coordination Complexes Confined in a Self-Assembled Nanocage
The encapsulation of coordination complexes in a tetragonal prismatic nanocage (1·(BArF)8) built from Zn-porphyrin and macrocyclic Pd-clip-based synthons is described. The functional duality of the guest ligand L1 allows for its encapsulation inside the cage 1·(BArF)8, along with the simultaneous coordination of ZnII, CuII, or FeIII metal ions. Remarkably, the coordination chemistry inside the host–guest adduct L1⊂1·(BArF)8 occurs in both solution solution and solid state. The resulting confined metallocomplexes have been characterized by means of UV-vis, ESI-HRMS, NMR, and EPR techniques. Furthermore, the emission of the Zn-porphyrin fluorophores of 1·(BArF)8 is strongly quenched by the encapsulation of paramagnetic complexes, representing a remarkable example of guest-dependent tuning of the host fluorescence
Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b
We present Spitzer Space Telescope infrared photometry of a primary transit
of the hot Neptune GJ 436b. The observations were obtained using the 8 microns
band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data
and the weak limb-darkening in the 8 microns IRAC band allow us to derive
(assuming M = 0.44 +- 0.04 Msun for the primary) a precise value for the
planetary radius (4.19 +0.21-0.16 Rearth), the stellar radius (0.463
+0.022-0.017 Rsun), the orbital inclination (85.90 +0.19-0.18 degrees) and
transit timing (2454280.78186 +0.00015-0.00008 HJD). Assuming current planet
models, an internal structure similar to that of Neptune with a small H/He
envelope is necessary to account for the measured radius of GJ 436b.Comment: Accepted for publication in A&A on 21/07/2007; 5 pages, 3 figure
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