Roche tomography of cataclysmic variables – III. Star-spots on AE Aqr

We present a Roche tomography reconstruction of the secondary star in the cataclysmic variable AE Aqr. The tomogram reveals several surface inhomogeneities that are due to the presence of large, cool star-spots. In addition to a number of lower latitude spots, the maps also show the presence of a large, high-latitude spot similar to that seen in Doppler images of rapidly rotating isolated stars, and a relative paucity of spots at a latitude of 40°. In total, we estimate that some 18 per cent of the Northern hemisphere of AE Aqr is spotted.
We have also applied the entropy landscape technique to determine accurate parameters for the binary system. We obtain optimal masses   M ₁= 0.74 M_⊙, M ₂= 0.50 M_⊙  , a systemic velocity  γ=−63 km s⁻¹  and an orbital inclination   i = 66°  .
Given that this is the first study to successfully image star-spots on the secondary star in a cataclysmic variable, we discuss the role that further studies of this kind may play in our understanding of these binaries.     

Roche tomography of cataclysmic variables – II. Images of the secondary stars in AM Her, QQ Vul, IP Peg and HU Aqr

We present a set of Roche tomography reconstructions of the secondary stars in the cataclysmic variables AM Her, QQ Vul, IP Peg and HU Aqr. The image reconstructions show distinct asymmetries in the irradiation pattern for all four systems that can be attributed to shielding of the secondary star by the accretion stream/column in AM Her, QQ Vul and HU Aqr, and increased irradiation by the bright-spot in IP Peg. We use the entropy landscape technique to derive accurate system parameters (   M ₁, M ₂  , i and γ) for the four binaries. In principle, this technique should provide the most reliable mass determinations available, since the intensity distribution across the secondary star is known. We also find that the intensity distribution can systematically affect the value of γ derived from circular orbit fits to radial velocity variations.     

Further variability of the compact radio nebula of P Cygni

Skinner et al. presented two high-resolution 6-cm (5 GHz) images of the B-supergiant star P Cygni. These show the observed morphology and flux densities to have changed over the intervening month. Following on from this, we present a series of seven high-resolution 6-cm images (including re-reductions of the two from Skinner et al.). These confirm that radio emission from the inner 400 mas of the wind is inhomogeneous, consisting usually of several separated bright spots, and that the total and peak flux densities and the observed morphology vary over all time-scales sampled. We suggest that recombination in cooling clumps of gas which will decrease the radio emission, followed by the appearance of other ionized clumps, could explain such rapid changes, but detailed models must await further observations.     

Insights into the Hadean Earth from experimental studies of zircon

Geologists investigate the evolution of the atmosphere, crust, and mantle through time by direct study of the rock record. However, the Hadean eon (>3.85 Ga) has been traditionally viewed as inaccessible due to the absence of preserved rocks. The discovery of >4.0 Ga detrital zircons from Western Australia in the 1980s — coupled with the development of new micro-analytical capabilities — made possible new avenues of early Earth research. The prevailing view that emerged is that the early Earth may have contained a stable hydrosphere, water-saturated or (near watersaturated) granitic magmas, and volcanic emanations dominated by neutral gas species (e.g., CO₂, H₂O, and SO₂). The Hadean Earth may have been capable of supporting life ～200 Ma after accretion and perhaps earlier. Many of these models are formulated — or have been subsequently supported — by laboratory experiments of zircon. Important petrological variables such as temperature, pressure, oxygen fugacity, and component activities (e.g., SiO₂/TiO₂-activities) can be controlled. These experiments are fundamental for extrapolation to ‘deep time’ because they provide a means to understand primary chemistry preserved in ancient zircons. This review paper specifically focuses on zircon experimental studies (oxygen isotope fractionations, Ti-thermometry, and redox sensitive element incorporation into zircon), which have influenced our view of the very early Earth.     

Shear instability of differential rotation in stars

Abstract

The two-dimensional (horizontal) shear instability of a differentially rotating star is examined. A solar-type rotation law is investigated. and it is found that for equatorial accelerations there is instability when there is a difference of 29% between the angular velocity of the equator and the poles.