Excitation and charge transfer in low-energy hydrogen atom collisions with neutral oxygen

Barklem, P. S.

Excitation and charge transfer in low-energy O+H collisions is studied; it is a problem of importance for modelling stellar spectra and obtaining accurate oxygen abundances in late-type stars including the Sun. The collisions have been studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multichannel Landau-Zener model. The method has been extended to include configurations involving excited states of hydrogen using an estimate for the two-electron transition coupling, but this extension was found to not lead to any remarkably high rates. Rate coefficients are calculated for temperatures in the range 1000-20 000 K, and charge transfer and (de)excitation processes involving the first excited S-states, 4s.5So and 4s.3So, are found to have the highest rates.


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Charge exchange in galaxy clusters

Gu, Liyi; Mao, Junjie; de Plaa, Jelle; Raassen, A. J. J.; Shah, Chintan; Kaastra, Jelle S.

Context. Though theoretically expected, the charge exchange emission from galaxy clusters has never been confidently detected. Accumulating hints were reported recently, including a rather marginal detection with the Hitomi data of the Perseus cluster. As previously suggested, a detection of charge exchange line emission from galaxy clusters would not only impact the interpretation of the newly discovered 3.5 keV line, but also open up a new research topic on the interaction between hot and cold matter in clusters. Aim. We aim to perform the most systematic search for the O VIII charge exchange line in cluster spectra using the RGS on board XMM-Newton.
Methods: We introduce a sample of 21 clusters observed with the RGS. In order to search for O VIII charge exchange, the sample selection criterion is a >35σ detection of the O VIII Lyα line in the archival RGS spectra. The dominating thermal plasma emission is modeled and subtracted with a two-temperature thermal component, and the residuals are stacked for the line search. The systematic uncertainties in the fits are quantified by refitting the spectra with a varying continuum and line broadening.
Results: By the residual stacking, we do find a hint of a line-like feature at 14.82 Å, the characteristic wavelength expected for oxygen charge exchange. This feature has a marginal significance of 2.8σ, and the average equivalent width is 2.5 × 10-4 keV. We further demonstrate that the putative feature can be barely affected by the systematic errors from continuum modeling and instrumental effects, or the atomic uncertainties of the neighboring thermal lines.
Conclusions: Assuming a realistic temperature and abundance pattern, the physical model implied by the possible oxygen line agrees well with the theoretical model proposed previously to explain the reported 3.5 keV line. If the charge exchange source indeed exists, we expect that the oxygen abundance could have been overestimated by 8-22% in previous X-ray measurements that assumed pure thermal lines. These new RGS results bring us one step forward to understanding the charge exchange phenomenon in galaxy clusters.



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Special Session EWASS 2018: Atomic and molecular data needs for astronomy and astrophysics

The aim of this session is to encourage further dialogue and promote knowledge transfer between astronomers with strong atomic and molecular data needs and those who produce this data from laboratory measurements and calculations. We hope this will foster new collaborations for the future, which, through targeted measurements and calculations, will accelerate the rate at which astronomers’ needs can be met.

Talks and posters will focus on four main areas:

* Data requirements of astronomers
* Laboratory-measured atomic and molecular data
* Theoretical calculations
* Databases

We encourage all astronomers involved in the ongoing spectroscopic surveys such as Gaia-ESO, APOGEE or GALAH or in future ones such as WEAVE, 4MOST or DESI to attend in order to explain their data needs.

The emphasis throughout the special session will remain on knowledge transfer; with astronomers highlighting their data needs, and spectroscopists and theorists presenting new data of astronomical interest and the capabilities of their facilities for future collaborations.

Link: http://eas.unige.ch/EWASS/session.jsp?id=SS4

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Line Ratios for Solar Wind Charge Exchange with Comets

Mullen, P. D.; Cumbee, R. S.; Lyons, D.; Gu, L.; Kaastra, J.; Shelton, R. L.; Stancil, P. C.

Charge exchange (CX) has emerged in X-ray emission modeling as a significant process that must be considered in many astrophysical environments—particularly comets. Comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (SWCX). X-ray observatories provide astronomers and astrophysicists with data for many X-ray emitting comets that are impossible to accurately model without reliable CX data. Here, we utilize a streamlined set of computer programs that incorporate the multi-channel Landau-Zener theory and a cascade model for X-ray emission to generate cross sections and X-ray line ratios for a variety of bare and non-bare ion single electron capture (SEC) collisions. Namely, we consider collisions between the solar wind constituent bare and H-like ions of C, N, O, Ne, Na, Mg, Al, and Si and the cometary neutrals H2O, CO, CO2, OH, and O. To exemplify the application of this data, we model the X-ray emission of Comet C/2000 WM1 (linear) using the CX package in SPEX and find excellent agreement with observations made with the XMM-Newton RGS detector. Our analyses show that the X-ray intensity is dominated by SWCX with H, while H2O plays a secondary role. This is the first time, to our knowledge, that CX cross sections have been implemented into a X-ray spectral fitting package to determine the H to H2O ratio in cometary atmospheres. The CX data sets are incorporated into the modeling packages SPEX and Kronos.


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The impact of atomic data selection on nebular abundance determinations

Juan de Dios, Leticia; Rodríguez, Mónica

Atomic data are an important source of systematic uncertainty in our determinations of nebular chemical abundances. However, we do not have good estimates of these uncertainties since it is very difficult to assess the accuracy of the atomic data involved in the calculations. We explore here the size of these uncertainties by using 52 different sets of transition probabilities and collision strengths, and all their possible combinations, to calculate the physical conditions and the total abundances of O, N, S, Ne, Cl and Ar for a sample of planetary nebulae and H ii regions. We find that atomic data variations introduce differences in the derived abundance ratios as low as 0.1-0.2 dex at low density, but that reach or surpass 0.6-0.8 dex at densities above 104 cm-3 in several abundance ratios, like O/H and N/O. Removing from the 52 data sets the four data sets that introduce the largest differences, the total uncertainties are reduced, but high-density objects still reach uncertainty factors of four for their values of O/H and N/O. We identify the atomic data that introduce most of the uncertainty, which involves the ions used to determine density, namely, the transition probabilities of the S+, O+, Cl++ and Ar+3 density diagnostic lines, and the collision strengths of Ar+3. Improved calculations of these data will be needed in order to derive more reliable values of chemical abundances in high-density nebulae. In the meantime, our results can be used to estimate the uncertainties introduced by atomic data in nebular abundance determinations.



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PhD position in atomic astrophysics

Malmö University and Lund Observatory jointly announce a PhD position in Atomic Astrophysics.

The candidate will work within the project ‘Experimental and Computational Atomic Astrophysics’
For information and online application, please see the link below.


The deadline for application is August 15.

Please forward to potential candidates.

Contact: henrik.hartman@mah.se

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Testing atomic collision theory with the two-photon continuum of astrophysical nebulae

Guzmán, F.; Badnell, N. R.; Chatzikos, M.; van Hoof, P. A. M.; Williams, R. J. R.; Ferland, G. J.

Accurate rates for energy-degenerate l-changing collisions are needed to determine cosmological abundances and recombination. There are now several competing theories for the treatment of this process, and it is not possible to test these experimentally. We show that the H I two-photon continuum produced by astrophysical nebulae is strongly affected by l-changing collisions. We perform an analysis of the different underlying atomic processes and simulate the recombination and two-photon spectrum of a nebula containing H and He. We provide an extended set of effective recombination coefficients and updated l-changing 2s – 2p transition rates using several competing theories. In principle, accurate astronomical observations could determine which theory is correct.


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