Grain Physics and Rosseland Mean Opacities

Tables of mean opacities are often used to compute the transfer of radiation in a variety of astrophysical simulations from stellar evolution models to proto-planetary disks. Often tables, such as Ferguson et al. (2005), are computed with a predetermined set of physical assumptions that may or may not be valid for a specific application. This paper explores the effects of several assumptions of grain physics on the Rosseland mean opacity in an oxygen rich environment. We find that changing the distribution of grain sizes, either the power-law exponent or the shape of the distribution, has a marginal effect on the total mean opacity. We also explore the difference in the mean opacity between solid homogenous grains and grains that are porous or conglomorations of several species. Changing the amount of grain opacity included in the mean by assuming a grain-to-gas ratio significantly affects the mean opacity, but in a predictable way.Comment: 19 pages, 6 figures, accepted for publication in Ap

Shorter Lines Facilitate Reading in Those Who Struggle

People with dyslexia, who ordinarily struggle to read, sometimes remark that reading is easier when e-readers are used. Here, we used eye tracking to observe high school students with dyslexia as they read using these devices. Among the factors investigated, we found that reading using a small device resulted in substantial benefits, improving reading speeds by 27%, reducing the number of fixations by 11%, and importantly, reducing the number of regressive saccades by more than a factor of 2, with no cost to comprehension. Given that an expected trade-off between horizontal and vertical regression was not observed when line lengths were altered, we speculate that these effects occur because sluggish attention spreads perception to the left as the gaze shifts during reading. Short lines eliminate crowded text to the left, reducing regression. The effects of attention modulation by the hand, and of increased letter spacing to reduce crowding, were also found to modulate the oculomotor dynamics in reading, but whether these factors resulted in benefits or costs depended on characteristics, such as visual attention span, that varied within our sample

Low Temperature Opacities

Previous computations of low temperature Rosseland and Planck mean opacities from Alexander & Ferguson (1994) are updated and expanded. The new computations include a more complete equation of state with more grain species and updated optical constants. Grains are now explicitly included in thermal equilibrium in the equation of state calculation, which allows for a much wider range of grain compositions to be accurately included than was previously the case. The inclusion of high temperature condensates such as Al$_2$O$_3$ and CaTiO$_3$ significantly affects the total opacity over a narrow range of temperatures before the appearance of the first silicate grains. The new opacity tables are tabulated for temperatures ranging from 30000 K to 500 K with gas densities from 10$^{-4}$ g cm$^{-3}$ to 10$^{-19}$ g cm$^{-3}$. Comparisons with previous Rosseland mean opacity calculations are discussed. At high temperatures, the agreement with OPAL and Opacity Project is quite good. Comparisons at lower temperatures are more divergent as a result of differences in molecular and grain physics included in different calculations. The computation of Planck mean opacities performed with the opacity sampling method are shown to require a very large number of opacity sampling wavelength points; previously published results obtained with fewer wavelength points are shown to be significantly in error. Methods for requesting or obtaining the new tables are provided.Comment: 39 pages with 12 figures. To be published in ApJ, April 200

Summary of additional analyses: Influence of variation within the sample.

<p><b>Device:</b> (1 = POD, 0 = PAD); <b>Crowding:</b> (1 = SPACED, 0 = NORMAL); <b>Hand:</b> (1 = HAND, 0 = NO HAND).</p

Lapses in high level lexical processing rarely cause regression.

<p>There are twice as many horizontal regressions (LEFT) in PAD as in POD. If the mechanism for regression was entirely due to lapses in conceptual understanding, we would expect regressions made horizontally to trade off against regressions made upwards (UP), as margins are made narrow in the POD condition. And yet, this expected tradeoff is not observed. Narrowed columns halve the number of horizontal regressions, but hardly increase the number of regressions upward. This indicates that the mechanism for regression is restricted to the line, and is likely proximal to fixation. (Error bars are +/−1 s.e.)</p

Schematic illustration of effects of attention and crowding in effective and ineffective readers.

<p>In typical readers, attention (grey oval) is primarily directed to the direction of reading, to provide a parafoveal preview benefit, and reduce attention to words previously inspected. In dyslexia, we propose that sluggish attention shifting causes attention to be slow to disengage from previously fixated locations, effectively spreading attention to the left of the fixated word. Given that crowding (indicated with stippling) is more severe in dyslexia, text perceived to the left is likely to be misperceived, increasing the potential for confusion.</p

Fidelity as dependent variable.

<p>Fidelity as dependent variable.</p

Holding the device in the hand alters erratic reading patterns.

<p>The observed interaction of Hand and global report (VAS) is plotted for TOT, the number of fixations that depart from efficient reading. A significant interaction of Hand*VAS was observed when TOT was taken as the dependent variable. Here, the TOT is shown as a function of VAS, the number of letters correctly identified on a six-letter global report paradigm. The NO-HAND condition is indicated in red, and HAND is indicated in blue. The figure shows that those with high scores on the global report task make more TOT errors when the device is held in the hand than when they do not hold it, while the reverse is true for those with low scores. When the hand is placed in the lap, variation in VAS makes little difference. (The colored shading indicates a confidence interval for this interaction, defined by a +/−1-sigma within-subjects standard error of the mean <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071161#pone.0071161-Cousineau1" target="_blank">[72]</a>. The graph is based on the POD and SPACED conditions.)</p