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$^{\circ}$. A maximum in the CO photodesorption rate appeared around 70$^{\circ}$-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

Science Star over Asia

The founding director of Singapore's Institute of Molecular and Cell Biology illustrates the rise of science in Asia