The Single-mode Complex Amplitude Refinement (SCAR) coronagraph

We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8–2.4λ∕D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of 2–3 × 10−4, and the 180 SCAR reaches a null of 1 × 10−4. Their monochromatic contrast is maintained within a range of ±0.16λ∕D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood.

The War in the Donbas in Historical Perspective

Does the Donbas represent the stronghold of Russian separatism? Since Russia’s military intervention in the Donbas (following its occupation of the Crimea), this view of the Donbas as un-Ukrainian or anti-Ukrainian has gained wide circulation in and outside Ukraine. Yet it is patently wrong. In the Donbas, there have never been ethnic, linguistic, or religious (sectarian) conflicts to speak of, nor did its population consistently manifest strong pro-Russian or pro-Union sentiments. True, such sentiments existed in the Donbas, like elsewhere in much of Dnieper Ukraine, but they never dominated the political scene of the Donbas. Instead, until the twenty-first century this region always tended to be anti-imperialist and anti-metropolitan. What is remarkable is that in 1991 the Donbas overwhelmingly supported the independence of Ukraine. What followed in the wake of Ukraine’s independence was an attempt by the Donbas power holders, in particular Viktor Yanukovych, to take over all of Ukraine. Moscow helped this attempt, which failed ultimately. The “free steppe” of the Donbas undeniably attracted, among others, radical Russian nationalists from outside and provided them with space for action. It is this historical characteristic of the Donbas as the “free steppe” that has colored the popular view of this region as a stronghold of Russian separatism. In the rest of Ukraine, a strong prejudice against the Donbas as a culturally dark region has only helped to boost this popular misconception.

The Brightest Point in Accretion Disk and Black Hole Spin: Implication to the Image of Black Hole M87*

We propose the simple new method for extracting the value of the black hole spin from the direct high-resolution image of black hole by using a thin accretion disk model. In this model, the observed dark region on the first image of the supermassive black hole in the galaxy M87, obtained by the Event Horizon Telescope, is a silhouette of the black hole event horizon. The outline of this silhouette is the equator of the event horizon sphere. The dark silhouette of the black hole event horizon is placed within the expected position of the black hole shadow, which is not revealed on the first image. We calculated numerically the relation between the observed position of the black hole silhouette and the brightest point in the thin accretion disk, depending on the black hole spin. From this relation, we derive the spin of the supermassive black hole M87*, a = 0.75 ± 0.15 .

Attribution of Greenland's ablating ice surfaces on ice sheet albedo using unmanned aerial systems

Surface albedo, a primary control on the amount of energy available for melt, has considerable spatial heterogeneity across the Greenland ice sheet ablation area. However, the relative importance of distinct surface types on albedo remains unclear. In this study, the causes of mesoscale (10² to 10³ m) albedo variability are assessed using high resolution (decimetre-scale) digital imagery and broadband albedo data acquired by a fixed-wing unmanned aerial system. We characterize the reflectance properties and terrain roughness associated with six distinct surface types identified from a 25 km longitudinal transect across the ablating dark region of the Kangerlussuaq sector. Principal component analysis of the fractional area of each surface type versus coincident Moderate Resolution Imaging Spectroradiometer (MODIS) albedo data reveals the relative importance of each surface type. The highest correlation with mesoscale albedo was the fractional area of distributed impurities. Although not the darkest surface type, their extensive coverage meant that they could explain 65 % of the albedo variability across the survey transect including the presence of the dark region. In contrast, the 2 % mean surface water coverage across our survey transect could only explain 12 % of albedo variation and crevasses, only 17 %. Localised cryoconite patches have the lowest albedo signature but comprise less than 1 % of the survey area and do not appear to reduce mesoscale albedo. We anticipate further reduction in ablation area albedo under future warming as localized areas of distributed impurities, supraglacial water and crevassing increase in extent and conclude that current bare ice area albedo models may advance significantly by representing the evolution of the surface types identified in this study.