661 research outputs found
Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)-Mo(VI) Dyad
Nature uses molybdenum-containing enzymes to catalyze oxygen atom transfer (OAT) from water to organic substrates. In these enzymes, the two electrons that are released during the reaction are rapidly removed, one at a time, by spatially separated electron transfer units. Inspired by this design, a Ru(II)-Mo(VI) dyad was synthesized and characterized, with the aim of accelerating the rate-determining step in the cis-dioxo molybdenum-catalyzed OAT cycle, the transfer of an oxo ligand to triphenyl phosphine, via a photo-oxidation process. The dyad consists of a photoactive bis(bipyridyl)-phenanthroline ruthenium moiety that is covalently linked to a bioinspired cis-dioxo molybdenum thiosemicarbazone complex. The quantum yield and luminescence lifetimes of the dyad [Ru(bpy)2(L(2))MoO2(solv)](2+) were determined. The major component of the luminescence decay in MeCN solution (τ = 1149 ± 2 ns, 67%) corresponds closely to the lifetime of excited [Ru(bpy)2(phen-NH2)](2+), while the minor component (τ = 320 ± 1 ns, 31%) matches that of [Ru(bpy)2(H2-L(2))](2+). In addition, the (spectro)electrochemical properties of the system were investigated. Catalytic tests showed that the dyad-catalyzed OAT from dimethyl sulfoxide to triphenyl phosphine proceeds significantly faster upon irradiation with visible light than in the dark. Methylviologen acts as a mediator in the photoredox cycle, but it is regenerated and hence only required in stoichiometric amounts with respect to the catalyst rather than sacrificial amounts. It is proposed that oxidative quenching of the photoexcited Ru unit, followed by intramolecular electron transfer, leads to the production of a reactive one-electron oxidized catalyst, which is not accessible by electrochemical methods. A significant, but less pronounced, rate enhancement was observed when an analogous bimolecular system was tested, indicating that intramolecular electron transfer between the photosensitizer and the catalytic center is more efficient than intermolecular electron transfer between the separate components
Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared
The TRAPPIST-1 planetary system is a favorable target for the atmospheric
characterization of temperate earth-sized exoplanets by means of transmission
spectroscopy with the forthcoming James Webb Space Telescope (JWST). A possible
obstacle to this technique could come from the photospheric heterogeneity of
the host star that could affect planetary signatures in the transit
transmission spectra. To constrain further this possibility, we gathered an
extensive photometric data set of 25 TRAPPIST-1 transits observed in the
near-IR J band (1.2 m) with the UKIRT and the AAT, and in the NB2090 band
(2.1 m) with the VLT during the period 2015-2018. In our analysis of these
data, we used a special strategy aiming to ensure uniformity in our
measurements and robustness in our conclusions. We reach a photometric
precision of (RMS of the residuals), and we detect no significant
temporal variations of transit depths of TRAPPIST-1 b, c, e, and g over the
period of three years. The few transit depths measured for planets d and f hint
towards some level of variability, but more measurements will be required for
confirmation. Our depth measurements for planets b and c disagree with the
stellar contamination spectra originating from the possible existence of bright
spots of temperature 4500 K. We report updated transmission spectra for the six
inner planets of the system which are globally flat for planets b and g and
some structures are seen for planets c, d, e, and f.Comment: accepted for publication in MNRA
Global analysis of the TRAPPIST Ultra-Cool Dwarf Transit Survey
ABSTRACT
We conducted a global analysis of the TRAPPIST Ultra-Cool Dwarf Transit Survey – a prototype of the SPECULOOS transit search conducted with the TRAPPIST-South robotic telescope in Chile from 2011 to 2017 – to estimate the occurrence rate of close-in planets such as TRAPPIST-1b orbiting ultra-cool dwarfs. For this purpose, the photometric data of 40 nearby ultra-cool dwarfs were reanalysed in a self-consistent and fully automated manner starting from the raw images. The pipeline developed specifically for this task generates differential light curves, removes non-planetary photometric features and stellar variability, and searches for transits. It identifies the transits of TRAPPIST-1b and TRAPPIST-1c without any human intervention. To test the pipeline and the potential output of similar surveys, we injected planetary transits into the light curves on a star-by-star basis and tested whether the pipeline is able to detect them. The achieved photometric precision enables us to identify Earth-sized planets orbiting ultra-cool dwarfs as validated by the injection tests. Our planet-injection simulation further suggests a lower limit of 10 per cent on the occurrence rate of planets similar to TRAPPIST-1b with a radius between 1 and 1.3 R⊕ and the orbital period between 1.4 and 1.8 d.</jats:p
In search of lost hybridity: the French Daniel Deronda
Starting from a set of examples of borrowings from French in George Eliot’s Daniel Deronda, I explore the various ways in which the characters’ and narrator’s use of mixed English–French utterances generates inferences which make the transcending of their mono-cultural self possible. I go on to argue that in Jumeau’s recent French translation of the novel, the reader is not given access to those inferences, resulting in the erasing of an Anglo-European, cosmopolitan identity
Do differences between metal body residues reflect the differences between effects for Chironomus riparius exposed to different sediments?
TRAPPIST-1: Global results of the Spitzer Exploration Science Program Red Worlds
With more than 1000 hours of observation from Feb 2016 to Oct 2019, the
Spitzer Exploration Program Red Worlds (ID: 13067, 13175 and 14223) exclusively
targeted TRAPPIST-1, a nearby (12pc) ultracool dwarf star orbited by seven
transiting Earth-sized planets, all well-suited for a detailed atmospheric
characterization with the upcoming JWST. In this paper, we present the global
results of the project. We analyzed 88 new transits and combined them with 100
previously analyzed transits, for a total of 188 transits observed at 3.6 or
4.5 m. We also analyzed 29 occultations (secondary eclipses) of planet b
and eight occultations of planet c observed at 4.5 m to constrain the
brightness temperatures of their daysides. We identify several orphan
transit-like structures in our Spitzer photometry, but all of them are of low
significance. We do not confirm any new transiting planets. We estimate for
TRAPPIST-1 transit depth measurements mean noise floors of 35 and 25 ppm
in channels 1 and 2 of Spitzer/IRAC, respectively. most of this noise floor is
of instrumental origins and due to the large inter-pixel inhomogeneity of IRAC
InSb arrays, and that the much better interpixel homogeneity of JWST
instruments should result in noise floors as low as 10ppm, which is low enough
to enable the atmospheric characterization of the planets by transit
transmission spectroscopy. We construct updated broadband transmission spectra
for all seven planets which show consistent transit depths between the two
Spitzer channels. We identify and model five distinct high energy flares in the
whole dataset, and discuss our results in the context of habitability. Finally,
we fail to detect occultation signals of planets b and c at 4.5 m, and can
only set 3 upper limits on their dayside brightness temperatures (611K
for b 586K for c)
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