The Dipolar Solar Minimum Corona

The large-scale configuration of the UV solar corona at the minimum activity between solar cycles 22 and 23 is explored in this paper. Exploiting a large sample of spectroscopic observations acquired by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory in the two-year period of 1996–1997, this work provides the first-ever monochromatic O vi 1032 Å image of the extended corona, and the first-ever two-dimensional maps of the kinetic temperature of oxygen ions and the O vi 1037/1032 Å doublet intensity ratio (a proxy for the outflow velocity of the oxygen component of the solar wind), statistically representative of solar minimum conditions. A clear dipolar magnetic structure, both equator- and axis-symmetric, is distinctly shown to shape the solar minimum corona, both in UV emission and in temperature and expansion rate. This statistical approach allows for robust establishment of the key role played by the magnetic field divergence in modulating the speed and temperature of the coronal flows, and identification of the coronal sources of the fast and slow solar wind.

Enhancement of the near-band-edge electroluminescence from the active ZnO layer in the ZnO/GaN-based light emitting diodes using AlN-ZnO/ZnO/AlN-ZnO double heterojunction structure

In this work, an AlN-ZnO/ZnO/AlN-ZnO double heterojunction (DH) structure prepared using the cosputtering technology was deposited onto the p-type GaN epitaxial layer. The indiffusion of the oxygen atoms to the p-GaN epilayer was obstructed as the cosputtered AlN-ZnO film inset between n-ZnO/p-GaN interface. The near-ultraviolet (UV) emission from this ZnO/GaN-based light emitting diode (LED) was greatly improved as compared to an n-type ZnO film directly deposited onto the p-GaN epilayer. Meanwhile, the native defects in the n-ZnO layer associated with the green luminescence was less likely to form while it was sandwiched by the cosputtered AlN-ZnO film. As the thickness of the active n-ZnO layer in the DH structure reached 10 nm, the near-band-edge (NBE) emission became the predominated luminescence over the resulting LED spectrum.

AT 2018lqh: Black Hole Born from a Rotating Star?

Recently an intriguing transient, AT 2018lqh, with only a day-scale duration and a high luminosity of 7 × 1042 erg s−1, was discovered. While several possibilities are raised on its origin, the nature of this transient is yet to be unveiled. We propose that a black hole (BH) with ∼30 M ⊙ forming from a rotating blue supergiant can generate a transient like AT 2018lqh. We find that this scenario can consistently explain the optical/UV emission and the tentative late-time X-ray detection, as well as the radio upper limits. If super-Eddington accretion onto the nascent BH powers the X-ray emission, continued X-ray observations may be able to test the presence of an accretion disk around the BH.

Efficient photocatalytic hydrogen production over Ce/ZnO from aqueous methanol solution

Ce/ZnO crystallites along with bare ZnO were prepared by solution free mechanochemical method and characterized with powder XRD, SEM, EDX, XPS, UV–Visible and Photoluminescence (PL) spectra. The visible light photocatalytic performance of these materials was investigated for H2 evolution with the aqueous 10vol% methanol solution under one sun conditions using solar simulator. X-ray diffraction data suggests the hexagonal wurtzite structure for Ce/ZnO crystallites and the incorporation of Ce4+ ion in ZnO is supported by the shifting of XRD peaks to lower Bragg angles that indicate lattice expansion. With the increase of Ce content in ZnO, the crystallite size of Ce/ZnO decreases and the specific surface area increases. UV–Visible spectra propose the decrease in optical band gap of Ce incorporated ZnO with the increase of Ce content up to 3 mol. %. The XPS analysis supports the incorporation of Ce4+ in Ce/ZnO. The PL spectra propose that, with the insertion of Ce ions into ZnO, intensity of UV emission band decreases that reflects the low recombination rate of photogenerated charge carriers, which is responsible for higher photocatalytic H2 production. The extent of hydrogen production is affected by calcination temperature of Ce/ZnO. 2 mol. % Ce incorporated ZnO calcined at 600 °C produces43 μmolh−1 g−1 of hydrogen.

Moisture-assisted Near-UV Emission Enhancement of Lead-free Cs4CuIn2Cl12 Double Perovskite Nanocrystals

Lead-based halide perovskite nanocrystals (NCs) are recognized as emerging emissive materials with superior photoluminescence (PL) properties. However, the toxicity of lead and the swift chemical decomposition under atmospheric moisture severely hinder their commercialization process. Herein, we report the first colloidal synthesis of lead-free Cs4CuIn2Cl12 layered double perovskite NCs via a facile moisture-assisted hot-injection method stemming from relatively nontoxic precursors. While moisture is typically detrimental to NC synthesis, we demonstrate that the presence of water molecules in Cs4CuIn2Cl12 synthesis enhances the PL quantum yield (mainly in the near-UV range), induces a morphological transformation from 3D nanocubes to 2D nanoplatelets, and converts the dark transitions to radiative transitions for the observed self-trapped excitons relaxation. This work paves the way for further studies on the moisture-assisted synthesis of novel lead-free halide perovskite NCs for a wide range of applications.

A Study of the Extragalactic UV Radiation in Helix Nebula using GALEX

We have studied the ultraviolet sources using Galaxy Evolution Explorer medium imaging surveys in Helix Nebula and estimated UV fluxes by using aperture photometry in distant and near ultraviolet bands. The aperture photometric method produces reliable, accurate flux measurements and found inconsistent with the merged catalog of Galaxy Evolution Explorer. From the current results, the fluxes are consistent with brighter absolute magnitude up to 24.5 and the measurement error increases gradually to more than 50 % at the fainter magnitude side. Percentage of error in far UV is greater than near UV, due to the fact that brighter galaxies are more visible than the near UV sources. The diffuse UV contributors of zodiacal light, airglow contribution in the nebula were estimated. The total extragalactic UV radiation from the detected sources to the diffuse background in the nebula is of the order of 50 ± 14 photons cm-2sr-1s-1Å-1 in NUV band and 28 ±10 photons cm-2sr-1s-1Å-1 in FUV band. HIGHLIGHTS GALEX observations have the potential to find extragalactic UV sources Helix Nebula is first identified for distinct source detection Aperture photometric method can detect fainter sources up to the magnitude of 27 Extragalactic sources in the Helix nebula contribute to diffuse UV emission in the nebula GRAPHICAL ABSTRACT