Variability in X-ray line ratios in helium-like ions of massive stars: the wind-driven case

High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R=f/i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example. The f/i ratio can be significantly modulated owing to evolving wind properties. We evaluate how variable mass loss might bias measures of f/i.Comment: to appear in A&

Variability in X-ray Line Ratios in Helium-Like Ions of Massive Stars: The Wind-Driven Case

Context. High spectral resolution and long exposure times are providing unprecedented levels of data quality of massive stars at X-ray wavelengths. Aims. A key diagnostic of the X-ray emitting plasma are the fir lines for He-like triplets. In particular, owing to radiative pumping effects, the forbidden-to-intercombination line luminosity ratio, R = f∕i, can be used to determine the proximity of the hot plasma to the UV-bright photospheres of massive stars. Moreover, the era of large observing programs additionally allows for investigation of line variability. Methods. This contribution is the second to explore how variability in the line ratio can provide new diagnostic information about distributed X-rays in a massive star wind. We focus on wind integration for total line luminosities, taking account of radiative pumping and stellar occultation. While the case of a variable stellar radiation field was explored in the first paper, the effects of wind variability are emphasized in this work. Results. We formulate an expression for the ratio of line luminosities f∕i that closely resembles the classic expression for the on-the-spot result. While there are many ways to drive variability in the line ratio, we use variable mass loss as an illustrative example for wind integration, particularly since this produces no variability for the on-the-spot case. The f∕i ratio can be significantly modulated owing to evolving wind properties. The extent of the variation depends on how the timescale for the wind flow compares to the timescale over which the line emissivities change. Conclusions. While a variety of factors can ellicit variable line ratios, a time-varying mass-loss rate serves to demonstrate the range of amplitude and phased-dependent behavior in f∕i line ratios. Importantly, we evaluate how variable mass loss might bias measures of f∕i. For observational exposures that are less than the timescale of variable mass loss, biased measures (relative to the time-averaged wind) can result; if exposures are long, the f∕i ratio is reflective of the time-averaged spherical wind

Atomic-scale structure of gel materials by solid-state NMR

The underlying principles of solid-state NMR spectroscopy are outlined with an emphasis on the physical origins of the interactions that affect NMR spectra so that an understanding of the structural information they convey is clearly understood. The fundamental components of the experimental approach are described. How the experimental data can be analyzed to provide structural characterization of sol-gel materials is illustrated through a series of examples from the literature. The short-range structural sensitivity of NMR means that it is an ideal probe of sol-gel materials since they are structurally disordered. Given the importance of silicates in sol-gel science, 29Si magic-angle spinning (MAS) NMR is a widely used nucleus in solid-state NMR studies of sol-gel materials. However, it is emphasized that to derive maximum benefit fromNMR characterization, a multinuclear approach is used, although each nucleus will have its own particular considerations which are presented. In this second edition, key advances in the experimental methodology (e.g., much higher applied magnetic fields, faster MAS rates, more sophisticated excitation approaches) since 2005 are outlined. The use of first-principles computational approaches to calculate NMR interaction parameters and hence better constrain structure provides an important additional dimension to the NMR approach. Materials where there has been a substantial expansion of sol-gel approaches since 2005 are included, with, for example, novel sol-gel schemes opening up preparation of phosphates where 31P MAS NMR is a sensitive structural probe. Another area where there has been substantial sol-gel activity since 2005 is in the preparation of bioactive calcium silicate-based materials, where multinuclear NMR is an ideal probe, including the use of 43Ca, a quadrupolar nucleus with a small magnetic moment, which has only really become readily accessible in recent years. © Springer International Publishing AG, part of Springer Nature 2018