3,499 research outputs found
Enhancing digital soil mapping in southeastern Brazil: incorporating stream density and soil reflectance from multiple depths.
This study proposes a novel and simple method to incorporate laboratory soil spectral data in the production of digital soil maps
Uranyl complexes formed with apara-t-butylcalix[4]arene bearing phosphinoyl pendant arms on the lower rim. Solid and solution studies
The current interest in functionalized calixarenes with phosphorylated pendant arms resides in their coordination ability towards f elements and capability towards actinide/rare earth separation. Uranyl cation forms 1:1 and 1:2 (M:L) complexes with atetra-phosphinoylated p-tert-butylcalix[4]arene, B4bL4: UO2(NO3)2(B4bL4)n· xH2O (n = 1, x = 2, 1; n = 2, x = 6, 2). Spectroscopic data point to the inner coordination sphere of 1 containing one monodentate nitrate anion, one water molecule and the four phosphinoylated arms bound to UO22+ while in 2, uranyl is only coordinated to calixarene ligands. In both cases the U(VI) ion is 8-coordinate. Uranyl complexes display enhanced metal-centred luminescence due to energy transfer from the calixarene ligands; the luminescence decays are bi-exponential with associated lifetimes in the ranges 220μs <τs <250μs and 630μs <τL < 640μs, pointing to the presence of two species with differently coordinated calixarene, as substantiated by aXPS study of U(4f5/2,7/2), O(1s) and P(2p) levels on solid state samples. The extraction study of UO22+ cation and trivalent rare-earth (Y, La, Eu) ions from acidic nitrate media by B4bL4 in chloroform shows the uranyl cation being much more extracted than rare earth
Massive pre-main-sequence stars in M17: and overtone CO bandhead emission and the thermal infrared
Recently much progress has been made in probing the embedded stages of
massive star formation, pointing to formation scenarios akin to a scaled up
version of low-mass star formation. However, the latest stages of massive star
formation have rarely been observed. Using 1st and 2nd overtone CO bandhead
emission and near- to mid-infrared photometry we aim to characterize the
remnant formation disks around 5 unique pre-main-sequence (PMS) stars with
masses , that have constrained stellar parameters thanks to
their detectable photospheres. We seek to understand this emission and the
disks it originates from in the context of the evolutionary stage of the
studied sources. We use an analytic LTE disk model to fit the CO bandhead and
the dust emission, found to originate in different disk regions. For the first
time we modeled the 2nd overtone emission. Furthermore, we fit continuum
normalized bandheads and show the importance of this in constraining the
emission region. We also include in our models as an additional
probe of the young nature of the studied objects. We find that the CO emission
originates in a narrow region close to the star (<1 AU) and under very similar
disk conditions (temperatures and densities) for the different objects. This is
consistent with previous modeling of this emission in a diverse range of young
stellar objects. We discuss these results in the context of the positions of
these PMS stars in the Hertzsprung-Russel diagram and the CO emission's
association with early age and high accretion rates in (massive) young stellar
objects. We conclude that, considering their mass range and for the fact that
their photospheres are detected, the M17 PMS stars are observed in a relatively
early formation stage. They are therefore excellent candidates for longer
wavelength studies to further constrain the end stages of massive star
formation.Comment: 21 pages, 12 figure
Muonium as a hydrogen analogue in silicon and germanium; quantum effects and hyperfine parameters
We report a first-principles theoretical study of hyperfine interactions,
zero-point effects and defect energetics of muonium and hydrogen impurities in
silicon and germanium. The spin-polarized density functional method is used,
with the crystalline orbitals expanded in all-electron Gaussian basis sets. The
behaviour of hydrogen and muonium impurities at both the tetrahedral and
bond-centred sites is investigated within a supercell approximation. To
describe the zero-point motion of the impurities, a double adiabatic
approximation is employed in which the electron, muon/proton and host lattice
degrees of freedom are decoupled. Within this approximation the relaxation of
the atoms of the host lattice may differ for the muon and proton, although in
practice the difference is found to be slight. With the inclusion of zero-point
motion the tetrahedral site is energetically preferred over the bond-centred
site in both silicon and germanium. The hyperfine and superhyperfine
parameters, calculated as averages over the motion of the muon, agree
reasonably well with the available data from muon spin resonance experiments.Comment: 20 pages, including 9 figures. To appear in Phys. Rev.
The optical counterpart of the bright X-ray transient Swift J1745-26
We present a 30-day monitoring campaign of the optical counterpart of the
bright X-ray transient Swift J1745-26, starting only 19 minutes after the
discovery of the source. We observe the system peaking at i' ~17.6 on day 6
(MJD 56192) to then decay at a rate of ~0.04 mag/day. We show that the optical
peak occurs at least 3 days later than the hard X-ray (15-50 keV) flux peak.
Our measurements result in an outburst amplitude greater than 4.3 magnitudes,
which favours an orbital period < 21 h and a companion star with a spectral
type later than ~ A0. Spectroscopic observations taken with the GTC-10.4 m
telescope reveal a broad (FWHM ~ 1100 km/s), double-peaked H_alpha emission
line from which we constrain the radial velocity semi-amplitude of the donor to
be K_2 > 250 km/s. The breadth of the line and the observed optical and X-ray
fluxes suggest that Swift J1745-26 is a new black hole candidate located closer
than ~7 kpc.Comment: 5 pages, 4 figures, accepted for publication in MNRA
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