Search

Ground-based radar observations show that, over the eastern North Atlantic, 50 % of warm marine boundary layer (WMBL) hydrometeors occur below 1.2 km and have reflectivities of −17 dBZ, thus making their detection from space susceptible to the extent of surface clutter and radar sensitivity. Surface clutter limits the ability of the CloudSat cloud profiling radar (CPR) to observe the true cloud base in ∼52 % of the cloudy columns it detects and true virga base in ∼80 %, meaning the CloudSat CPR often provides an incomplete view of even the clouds it does detect. Using forward simulations, we determine that a 250 m resolution radar would most accurately capture the boundaries of WMBL clouds and precipitation; that being said, because of sensitivity limitations, such a radar would suffer from cloud cover biases similar to those of the CloudSat CPR. Observations and forward simulations indicate that the CloudSat CPR fails to detect 29 %–43 % of the cloudy columns detected by ground-based sensors. Out of all configurations tested, the 7 dB more sensitive EarthCARE CPR performs best (only missing 9.0 % of cloudy columns) indicating that improving radar sensitivity is more important than decreasing the vertical extent of surface clutter for measuring cloud cover. However, because 50 % of WMBL systems are thinner than 400 m, they tend to be artificially stretched by long sensitive radar pulses, hence the EarthCARE CPR overestimation of cloud top height and hydrometeor fraction. Thus, it is recommended that the next generation of space-borne radars targeting WMBL science should operate interlaced pulse modes including both a highly sensitive long-pulse mode and a less sensitive but clutter-limiting short-pulse mode.

White dwarfs are useful objects with which to study the local interstellar medium (ISM). High ionisation state absorption features that cannot be attributed to the photosphere or the ISM have been observed along the line-of-sight to a number of white dwarf stars. Suggested origins of these lines include ionisation from past supernovae, stellar winds, circumstellar disks, photoionisation from nearby hot stars or also from the white dwarf itself. In this study we consider the origin of these non-photospheric highly ionised lines in two stars towards a rarefied region of the galaxy known as the extended β CMa Tunnel. We present preliminary results from our analysis of the first of these two stars.

Using newly obtained high-resolution data ($R\sim{1\times{10}^{5}}$) from the \textit{Hubble Space Telescope}, and archival UV data from the \textit{Far Ultraviolet Spectroscopic Explorer} we have conducted a detailed UV survey of the three hot, metal-polluted white dwarfs WD0455-282, WD0621-376, and WD2211-495. Using bespoke model atmospheres we measured $T_{\mathrm{eff}}$, log $g$, and photospheric abundances for these stars. In conjunction with data from Gaia we measured masses, radii, and gravitational redshift velocities for our sample of objects. We compared the measured photospheric abundances with those predicted by radiative levitation theory, and found that the observed Si abundances in all three white dwarfs, and the observed Fe abundances in WD0621-376 and WD2211-495, were larger than those predicted by an order of magnitude. These findings imply not only an external origin for the metals, but also ongoing accretion, as the metals not supported by radiative levitation would sink on extremely short timescales. We measured the radial velocities of several absorption features along the line of sight to the three objects in our sample, allowing us to determine the velocities of the photospheric and interstellar components along the line of sight for each star. Interestingly, we made detections of circumstellar absorption along the line of sight to WD0455-282 with three velocity components. To our knowledge, this is the first such detection of multi-component circumstellar absorption along the line of sight to a white dwarf., 19 pages, 23 figures, 8 tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

White dwarf atmospheres are subjected to gravitational potentials around $10^5$ times larger than occur on Earth. They provide a unique environment in which to search for any possible variation in fundamental physics in the presence of strong gravitational fields. However, a sufficiently strong magnetic field will alter absorption line profiles and introduce additional uncertainties in measurements of the fine structure constant. Estimating the magnetic field strength is thus essential in this context. Here we model the absorption profiles of a large number of atomic transitions in the white dwarf photosphere, including first-order Zeeman effects in the line profiles, varying the magnetic field as a free parameter. We apply the method to a high signal-to-noise, high-resolution, far-ultraviolet HST/STIS spectrum of the white dwarf G191-B2B. The method yields a sensitive upper limit on its magnetic field of $B < 2300$ Gauss at the $3\sigma$ level. Using this upper limit we find that the potential impact of quadratic Zeeman shifts on measurements of the fine structure constant in G191-B2B is 4 orders of magnitude below laboratory wavelength uncertainties., Comment: 10 pages, 5 figures

The experimental thermonuclear reactor, ITER, is currently being constructed in Cadarache, France. The reactor vessel will be constructred with a beryllium coated wall, and a tungsten coated divertor. As a plasma-facing component, the divertor will be under conditions of extreme temperature, resulting in the sputtering of tungsten impurities into the main body plasma. Modelling and understanding the potential cooling effects of these impurities requires detailed collisional-radiative modelling. These models require a wealth of atomic data for the various atomic species in the plasma. In particular, partial, final-state resolved dielectronic/radiative recombination (DR/RR) rate coefficients for tungsten are required. In this manuscript, we present our calculations of detailed DR/RR rate coefficients for the lanthanide-like, and low ionization stages of tungsten, spanning charge states W$^{13+}$ to W$^{1+}$. The calculations presented here constitutes the first detailed exploration of such low ionization state tungsten ions. We are able to reproduce the general trend of calculations performed by other authors, but find significant differences between ours and their DR rate coefficients, especially at the lowest temperatures considered., 23 pages, 23 figures, 3 tables. Accepted for publication in Journal of Physics B: Atomic, Molecular, and Optical Physics

We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992-1996 revealed a 1.15d periodicity with a 25 percent amplitude, hypothesised to be due to metals in a surface accretion spot. We obtained phase-resolved HST/Space Telescope Imaging Spectrograph (STIS) high-resolution far-ultraviolet (FUV) spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate and radial velocity of GD 394 constant over the observed timescales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high-excitation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf., Accepted for publication in MNRAS

Electron–ion recombination of Be-like 40Ar14+ has been measured by employing the electron–ion merged-beams method at the cooler storage ring CSRm. The measured absolute recombination rate coefficients for collision energies from 0 to 60 eV are presented, covering all dielectronic recombination (DR) resonances associated with 2s 2 → 2s2p core transitions. In addition, strong trielectronic recombination (TR) resonances associated with 2s 2 → 2p 2 core transitions were observed. Both DR and TR processes lead to series of peaks in the measured recombination spectrum, which have been identified by the Rydberg formula. Theoretical calculations of recombination rate coefficients were performed using the state-of-the-art multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE to compare with the experimental results. The plasma rate coefficients for DR+TR of Ar14+ were deduced from the measured electron–ion recombination rate coefficients in the temperature range from 103 to 107 K, and compared with calculated data from the literature. The experimentally derived plasma rate coefficients are 60% larger and 30% lower than the previously recommended atomic data for the temperature ranges of photoionized plasmas and collisionally ionized plasmas, respectively. However, good agreement was found between experimental results and the calculations by Gu and Colgan et al. The plasma rate coefficients deduced from experiment and calculated by the current AUTOSTRUCTURE code show agreement that is better than 30% from 104 to 107 K. The present results constitute a set of benchmark data for use in astrophysical modeling.

Tungsten is an important element for magnetically confined fusion plasmas but has the potential to cool, or even quench the plasma due to it being an efficient radiator. Total and level-resolved dielectronic recombination (DR) rate coefficients, for all ionization stages, are essential to model tungsten. We describe a set calculations performed using the distorted wave code AUTOSTRUCTURE for the tungsten ions W$^{37+}$ to W$^{28+}$. We demonstrate the importance of relativistic configuration mixing in such calculations. In particular, we show that the partial DR rate coefficients calculated in level and configuration resolution can differ by as little as 5%, and up to as much as 75%. Using the new data, we calculate a revised steady-state ionization fraction for tungsten. We find that, relative to the ionization fraction calculated using the recombination rate coefficients of Putterich et al. (Plasma Phys. Control. Fusion, 50, 085016), the peak temperatures of W$^{37+}$ to W$^{28+}$ ionization states are shifted to lower temperatures spanning 0.9-1.6keV. This temperature range is important for understanding the performance of large tokamaks, such as ITER, because the temperatures in the pedestal, edge, scrape-off-layer and divertor region fall in this range., 15 pages, 16 figures, 3 tables. Accepted for publication in Journal of Physics B: Atomic, Molecular, and Optical Physics. Updated with DOI and bibliographic information

Under the auspices of the IAEA Atomic and Molecular Data Center and the Korean Atomic Energy Research Institute, our assembled group of authors has reviewed the current state of dielectronic recombination (DR) rate coefficients for various ion stages of tungsten (W). Subsequent recommendations were based upon available experimental data, first-principle calculations carried out in support of this paper and from available recombination data within existing atomic databases. If a recommendation was possible, data were compiled, evaluated and fitted to a functional form with associated uncertainty information retained, where available. This paper also considers the variation of the W fractional abundance due to the underlying atomic data when employing different data sets.

Dielectronic recombination (DR) is the dominant mode of recombination in magnetically confined fusion plasmas for intermediate to low-charged ions of W. Complete, final-state resolved partial isonuclear W DR rate coefficient data is required for detailed collisional-radiative modelling for such plasmas in preparation for the upcoming fusion experiment ITER. To realize this requirement, we continue {\it The Tungsten Project} by presenting our calculations for tungsten ions W$^{55+}$ to W$^{38+}$. As per our prior calculations for W$^{73+}$ to W$^{56+}$, we use the collision package {\sc autostructure} to calculate partial and total DR rate coefficients for all relevant core-excitations in intermediate coupling (IC) and configuration average (CA) using $\kappa$-averaged relativistic wavefunctions. Radiative recombination (RR) rate coefficients are also calculated for the purpose of evaluating ionization fractions. Comparison of our DR rate coefficients for W$^{46+}$ with other authors yields agreement to within 7-19\% at peak abundance verifying the reliability of our method. Comparison of partial DR rate coefficients calculated in IC and CA yield differences of a factor $\sim{2}$ at peak abundance temperature, highlighting the importance of relativistic configuration mixing. Large differences are observed between ionization fractions calculated using our recombination rate coefficient data and that of P\"{u}tterich~\etal [Plasma Phys. and Control. Fusion 50 085016, (2008)]. These differences are attributed to deficiencies in the average-atom method used by the former to calculate their data., Comment: Accepted for publication in Journal of Physics B. Manuscript contains 21 pages, 30 figures, and 7 tables

Tungsten is an important metal in nuclear fusion reactors. It will be used in the divertor component of ITER (Latin for 'the way'). The Tungsten Project aims to calculate partial and total DR rate coefficients for the isonuclear sequence of Tungsten. The calculated data will be made available as and when they are produced via the open access database OPEN- ADAS in the standard adf09 and adf48 file formats. We present our progress thus far, detailing calculational methods, and showing comparisons with other available data. We conclude with plans for the future., Seven pages, four figures. To appear in the proceedings of the 19th International Conference on Atomic Processes in Plasmas, Paris 2016

Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This variation manifests as shifts in the observed wavelengths of absorption lines, such as quadruply ionized iron (FeV) and quadruply ionized nickel (NiV), when compared to laboratory wavelengths. Berengut et al. (Phys. Rev. Lett. 2013, 111, 010801) demonstrated the validity of such an analysis using high-resolution Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) spectra of G191-B2B. We have made three important improvements by: (a) using three new independent sets of laboratory wavelengths, (b) analysing a sample of objects, and (c) improving the methodology by incorporating robust techniques from previous studies towards quasars (the Many Multiplet method). A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. Here we describe our approach and present preliminary results from nine objects using both FeV and NiV., Comment: 6 pages, 1 figure, published on 30th March 2017 in Universe as part of VARCOSMOFUN'16 proceedings

Dielectronic recombination (DR) is a key atomic process which affects the spectroscopic diagnostic modelling of tungsten, most of whose ionization stages will be found somewhere in the ITER fusion reactor: in the edge, divertor, or core plasma. Accurate DR data is sparse while complete DR coverage is unsophisticated (e.g. average-atom or Burgess General Formula) as illustrated by the large uncertainties which currently exist in the tungsten ionization balance. To this end, we present a series of partial final-state-resolved and total DR rate coefficients for W$^{73+}$ to W$^{56+}$ Tungsten ions. This is part of a wider effort within {\it The Tungsten Project} to calculate accurate dielectronic recombination rate coefficients for the tungsten isonuclear sequence for use in collisional-radiative modelling of finite-density tokamak plasmas. The recombination rate coefficients have been calculated with {\sc autostructure} using kappa-averaged relativistic wavefunctions in level resolution (intermediate coupling) and configuration resolution (configuration average). The results are available from OPEN-ADAS according to the {\it adf09} and {\it adf48} standard formats. Comparison with previous calculations of total DR rate coefficients for W$^{63+}$ and W$^{56+}$ yield agreement to within 20\% and 10\%, respectively, at peak temperature. It is also seen that the J\"{u}ttner correction to the Maxwell distribution has a significant effect on the ionization balance of tungsten at the highest charge states, changing both the peak abundance temperatures and the ionization fractions of several ions.

Electron–ion recombination rate coefficients for beryllium-like calcium ions in the center of mass energy from 0 to51.88 eV have been measured by means of the electron–ion merged-beam technique at the main cooler storage ringat the Institute of Modern Physics in Lanzhou, China. The measurement energy range covers the dielectronicrecombination (DR) resonances associated with the 2s2 1S0 -> 2s2p 3P0,1,2, 1P1 core excitations and the trielectronicrecombination (TR) resonances associated with the 2s2 1S0 -> 2p2 3P0,1,2, 1D2, 1S0 core excitations. In addition,the AUTOSTRUCTURE code was used to calculate the recombination rate coefficients for comparison with theexperimental results. Resonant recombination originating from parent ions in the long-lived metastable state 2s2p 3P0 ions has been identified in the recombination spectrum below 1.25 eV. A good agreement is achievedbetween the experimental recombination spectrum and the result of the AUTOSTRUCTURE calculations whenfractions of 95% ground-state ions and 5% metastable ions are assumed in the calculation. It is found thatthe calculated TR resonance positions agree with the experimental peaks, while the resonance strengths areunderestimated by the theoretical calculation. Temperature dependent plasma rate coefficients for DR and TR inthe temperature range of 103–108 K were derived from the measured electron–ion recombination rate coefficientsand compared with the available theoretical results from the literature. In the temperature range of photoionizedplasmas, the presently calculated rate coefficients and the recent results of Gu & Colgan et al. are up to 30% lowerthan the experimentally derived ones, and the older atomic data are even up to 50% lower than the presentexperimental result. This is because strong resonances situated below electron–ion collision energies of 50 meVwere underestimated by the theoretical calculation, which also has a severe influence on the rate coefficients inlow-temperature plasmas. In the temperature range of collisionally ionized plasmas, agreement within 25% wasfound between the experimental result and the present calculation as well as the calculation by Colgan et al. Thepresent result constitutes a set of benchmark data for use in astrophysical modeling.

To calculate realistic models of objects with Ni in their atmospheres, accurate atomic data for the relevant ionization stages needs to be included in model atmosphere calculations. In the context of white dwarf stars, we investigate the effect of changing the Ni {\sc iv}-{\sc vi} bound-bound and bound-free atomic data has on model atmosphere calculations. Models including PICS calculated with {\sc autostructure} show significant flux attenuation of up to $\sim 80$\% shortward of 180\AA\, in the EUV region compared to a model using hydrogenic PICS. Comparatively, models including a larger set of Ni transitions left the EUV, UV, and optical continua unaffected. We use models calculated with permutations of this atomic data to test for potential changes to measured metal abundances of the hot DA white dwarf G191-B2B. Models including {\sc autostructure} PICS were found to change the abundances of N and O by as much as $\sim 22$\% compared to models using hydrogenic PICS, but heavier species were relatively unaffected. Models including {\sc autostructure} PICS caused the abundances of N/O {\sc iv} and {\sc v} to diverge. This is because the increased opacity in the {\sc autostructure} PICS model causes these charge states to form higher in the atmosphere, moreso for N/O {\sc v}. Models using an extended line list caused significant changes to the Ni {\sc iv}-{\sc v} abundances. While both PICS and an extended line list cause changes in both synthetic spectra and measured abundances, the biggest changes are caused by using {\sc autostructure} PICS for Ni., Comment: 14 pages, 9 figures, 7 tables. Accepted for publication in MNRAS

The plasma facing components in ITER will be constructed using Tungsten. Sputtering of plasma against these walls introduces Tungsten impurities into the plasma, possibly causing quenching. As data for Tungsten in the relevant charge states is historically poor, this project aims to calculate this data, providing partial and total Dielectronic/Radiative Recombination rates for use in collisional-radiative modelling of the ITER plasma.

Synopsis The Opacity Project is a highly successful venture in which energy levels, oscillator strengths, and photoionization cross sections were calculated for astrophysically important elements up to iron. However, metals heavier than iron are being discovered in stellar atmospheres. The photoionization cross sections for elements and ions not covered by the Opacity Project are usually approximated with hydrogenic formulae. We present a series of valence and inner-shell distorted wave photoioniza-tion cross section calculations for use in model atmosphere calculations covering Ni-Ni10+. We discuss current progress, and future work.

Spectra of molecular hydrogen (${\mathrm{H}}_{2}$) are employed to search for a possible proton-to-electron mass ratio ($\ensuremath{\mu}$) dependence on gravity. The Lyman transitions of ${\mathrm{H}}_{2}$, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions ($T\ensuremath{\sim}13\text{ }000\text{ }\text{ }\mathrm{K}$) of their photospheres. We derive sensitivity coefficients ${K}_{i}$ which define how the individual ${\mathrm{H}}_{2}$ transitions shift due to $\ensuremath{\mu}$ dependence. The spectrum of white dwarf star GD133 yields a $\mathrm{\ensuremath{\Delta}}\ensuremath{\mu}/\ensuremath{\mu}$ constraint of $(\ensuremath{-}2.7\ifmmode\pm\else\textpm\fi{}4.{7}_{\text{stat}}\ifmmode\pm\else\textpm\fi{}0.{2}_{\text{syst}})\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}5}$ for a local environment of a gravitational potential $\ensuremath{\phi}\ensuremath{\sim}1{0}^{4}$ ${\ensuremath{\phi}}_{\text{Earth}}$, while that of $\mathrm{G}29\ensuremath{-}38$ yields $\mathrm{\ensuremath{\Delta}}\ensuremath{\mu}/\ensuremath{\mu}=(\ensuremath{-}5.8\ifmmode\pm\else\textpm\fi{}3.{8}_{\text{stat}}\ifmmode\pm\else\textpm\fi{}0.{3}_{\text{syst}})\ifmmode\times\else\texttimes\fi{}1{0}^{\ensuremath{-}5}$ for a potential of $2\ifmmode\times\else\texttimes\fi{}1{0}^{4}$ ${\ensuremath{\phi}}_{\text{Earth}}$.

Spectra of molecular hydrogen (H2) are employed to search for a possible proton-to-electron mass ratio (μ) dependence on gravity. The Lyman transitions of H2, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions (T∼13 000 K) of their photospheres. We derive sensitivity coefficients Ki which define how the individual H2 transitions shift due to μ dependence. The spectrum of white dwarf star GD133 yields a Δμ/μ constraint of (-2.7±4.7stat±0.2syst)×10(-5) for a local environment of a gravitational potential ϕ∼10(4) ϕEarth, while that of G29-38 yields Δμ/μ=(-5.8±3.8stat±0.3syst)×10(-5) for a potential of 2×10(4) ϕEarth.

We propose a new probe of the dependence of the fine structure constant, alpha, on a strong gravitational field using metal lines in the spectra of white dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, respectively, at a dimensionless gravitational potential relative to Earth of (Delta phi) ~ 5x10^(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to two orders of magnitude., Comment: 8 pages, 2 figures. v2: minor corrections