3,325 research outputs found
Interpretation of the evolution parameter of the Feynman parametrization of the Dirac equation
The Feynman parametrization of the Dirac equation is considered in order to
obtain an indefinite mass formulation of relativistic quantum mechanics. It is
shown that the parameter that labels the evolution is related to the proper
time. The Stueckelberg interpretation of antiparticles naturally arises from
the formalism.Comment: 6 pages, RevTex, no figures, submitted to Phys. Lett.
Gadolinium-vacancy clusters in the (111) surface of gadolinium-doped ceria: a density functional theory study
Solid-oxide fuel cells are promising devices for sustainable power generation. Electrolyte materials play an important role in connecting the anode and cathode, and they influence the performance of the device. In this context, gadolinium-doped ceria (GDC) has proven to be an efficient electrolyte material, although the presence of dopant clusters can lower its efficiency. After usage, dopant clusters start appearing at dislocations, translocations, grain boundaries, or surfaces. Hence, the study of dopant clustering at the atomic level near these regions becomes of vital importance, as it allows us to understand the reasons for the occurrence of this phenomenon and its impact on the oxygen conduction. In this context, the present paper studies the impact of dopant clustering near the (111) GDC surface. We have studied two different gadolinium concentrations in the material, of approximately 7% and 14%, which are close to the optimum concentration of 10%. Our results indicate that surface relaxation is a key factor in determining the preference of defect clusters to be found in the surface. We have also calculated the relative abundance of different defect clusters at different temperatures, including the configurational entropy term. It was revealed that working temperatures (650–1100 K) show the relative abundance of different cluster structures, displaying that, at high concentrations, preferred dopant clusters resemble the structure of Gd2O3, showing the formation of gadolinia domains. Finally, we show that oxygen diffusion will be affected by the formation of these domains. After evaluating the oxygen mobility, we conclude that oxygen vacancies will be trapped by the gadolinium clusters at the surface. These vacancy traps prevent oxygen diffusion, thereby affecting negatively the performance of the material and the fuel cell in general
Accretion and photodesorption of CO ice as a function of the incident angle of deposition
Non-thermal desorption of inter- and circum-stellar ice mantles on dust
grains, in particular ultraviolet photon-induced desorption, has gained
importance in recent years. These processes may account for the observed gas
phase abundances of molecules like CO toward cold interstellar clouds. Ice
mantle growth results from gas molecules impinging on the dust from all
directions and incidence angles. Nevertheless, the effect of the incident angle
for deposition on ice photo-desorption rate has not been studied. This work
explores the impact on the accretion and photodesorption rates of the incidence
angle of CO gas molecules with the cold surface during deposition of a CO ice
layer. Infrared spectroscopy monitored CO ice upon deposition at different
angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet
spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp.
Molecules ejected from the ice to the gas during irradiation or warm-up were
characterized by a quadrupole mass spectrometer. The photodesorption rate of CO
ice deposited at 11 K and different incident angles was rather stable between 0
and 45. A maximum in the CO photodesorption rate appeared around
70-incidence deposition angle. The same deposition angle leads to the
maximum surface area of water ice. Although this study of the surface area
could not be performed for CO ice, the similar angle dependence in the
photodesorption and the ice surface area suggests that they are closely
related. Further evidence for a dependence of CO ice morphology on deposition
angle is provided by thermal desorption of CO ice experiments
Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
Recent studies have shown that the extended main-sequence turn off (eMSTO) is
a common feature of intermediate-age star clusters in the Magellanic Clouds
(MCs). The most simple explanation is that these stellar systems harbor
multiple generations of stars with an age difference of a few hundred Myrs.
However, while an eMSTO has been detected in a large number of clusters with
ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in
nearby galaxies do not find any evidence for a prolonged star-formation
history, i.e. for multiple stellar generations. These results have suggested
alternative interpretation of the eMSTOs observed in intermediate-age star
clusters. The eMSTO could be due to stellar rotation mimicking an age spread or
to interacting binaries. In these scenarios, intermediate-age MC clusters would
be simple stellar populations, in close analogy with younger clusters.
Here we provide the first evidence for an eMSTO in a young stellar cluster.
We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr
old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened
MSTO that is consistent with a prolonged star-formation which had a duration of
about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and
blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars,
respectively. We discuss these findings in the context of
multiple-stellar-generation, stellar-rotation, and interacting-binary
hypotheses.Comment: 14 pages, 10 figures, accepted for publication in MNRA
Recent Star Formation in Sextans A
We investigate the relationship between the spatial distributions of stellar
populations and of neutral and ionized gas in the Local Group dwarf irregular
galaxy Sextans A. This galaxy is currently experiencing a burst of localized
star formation, the trigger of which is unknown. We have resolved various
populations of stars via deep UBV(RI)_C imaging over an area with diameter \sim
5.'3. We have compared our photometry with theoretical isochrones appropriate
for Sextans A, in order to determine the ages of these populations. We have
mapped out the history of star formation, most accurately for times \lesssim
100 Myr. We find that star formation in Sextans A is correlated both in time
and space, especially for the most recent (\lesssim 12 Myr) times. The youngest
stars in the galaxy are forming primarily along the inner edge of the large H I
shell. Somewhat older populations, \lesssim 50 Myr, are found inward of the
youngest stars. Progressively older star formation, from \sim 50--100 Myr,
appears to have some spatially coherent structure and is more centrally
concentrated. The oldest stars we can accurately sample appear to have
approximately a uniform spatial distribution, which extends beyond a surface
brightness of \mu_B \simeq 25.9 mag arcsec^{-2} (or, a radius r \simeq 2.'3$).
Although other processes are also possible, our data provides support for a
mechanism of supernova-driven expansion of the neutral gas, resulting in cold
gas pileup and compression along the H I shell and sequential star formation in
recent times.Comment: 64 pages, 22 figures, to appear in A
Global and non-global parameters of horizontal branch morphology of globular clusters
The horizontal branch (HB) morphology of globular clusters (GCs) is mainly
determined by metallicity. However, the fact that GCs with almost the same
metallicity exhibit different HB morphologies demonstrates that at least one
more parameter is needed to explain the HB morphology. It has been suggested
that one of these should be a global parameter that varies from GC to GC, and
the other a non-global parameter that varies within the GC. In this study we
provide empirical evidence corroborating this idea. We used the photometric
catalogs obtained with the Advanced Camera for Surveys (ACS) of the Hubble
Space Telescope (HST) and analyse the CMDs of 74 GCs. The HB morphology of our
sample of GCs has been investigated on the basis of the two new parameters L1
and L2 that measure the distance between the RGB and the coolest part of the
HB, and the color extension of the HB, respectively. We find that L1 correlates
with both metallicity and age, whereas L2 most strongly correlates with the
mass of the hosting GC. The range of helium abundance among the stars in a GC,
characterised by Delta Y and associated with the presence of multiple stellar
populations, has been estimated in a few GCs to date. In these GCs we find a
close relationship among Delta Y, GC mass, and L2. We conclude that age and
metallicity are the main global parameters while the range of helium abundance
within a GC is the main non-global parameter defining the HB morphology of
Galactic GCs.Comment: 34 pages, 13 figures, accepted for publication in Ap
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