Physical conditions in QSO absorbers from fine-structure absorption lines

We calculate theoretical population ratios of the ground fine-structure levels of some atoms/ions which typically exhibit UV lines in the spectra of QSO absorbers redward the Ly-alpha forest: C0, C+, O0, Si+ and Fe+. The most reliable atomic data available is employed and a variety of excitation mechanisms considered: collisions with several particles in the medium, direct excitation by photons from the cosmic microwave background radiation (CMBR) and fluorescence induced by a UV field present. The theoretical population ratios are confronted with the corresponding column density ratios of C I and C II lines observed in damped Ly-alpha (DLA) and Lyman Limit (LL) systems collected in the recent literature to infer their physical conditions. The volumetric density of neutral hydrogen in DLA systems is constrained to be lower than tens of cm^-3 (or a few cm^-3 in the best cases) and the UV radiation field intensity must be lower than two orders of magnitude the radiation field of the Galaxy (one order of magnitude in the best cases). Their characteristic sizes are higher than a few pc (tens of pc in the best cases) and lower limits for their total masses vary from 10^0 to 10^5 solar masses. For the only LL system in our sample, the electronic density is constrained to be n_e<0.15 cm^-3. We suggest that the fine-structure lines may be used to discriminate between the current accepted picture of the UV extragalatic background as the source of ionization in these systems against a local origin for the ionizing radiation as supported by some authors. We also investigate the validity of the temperature-redshift relation of the CMBR predicted by the standard model and study the case for alternative models.Comment: 16 pages, 12 figure

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

THE COMPARISON OF HELIUM-LIKE ION EMISSION LINE RATIOS WITH SOLAR X-RAY SPECTRAL DATA

Electron impact excitation rates recently calculated by Keenan, McCann and Kingston for transitions in the He-like ions Al XII , Si XIII and S XV are used t o derive the electron taperature sensitive emission line ratio G and the density sensitive ratio R in its low density limit (Ro). These ratios are compared with those calculated by other authors and with the values obtained for solar flares using instruments on board the P78-1 and Solar Maximum Mission satellites. In general it is found that our results resolve discrepancies which currently exist between observation and theory. which prwides experimental support for the methods and atanic data adopted in the present analysis

DIAGNOSTIC LINE RATIOS FOR HIGHLY IONIZED IONS OF THE BERYLLIUM ISOELECTRONIC SEQUENCE AND A COMPARISON WITH SOLAR OBSERVATIONAL DATA

Observations of the relative line strengths of Beryllium like ions in a high temperature plasma can be used to derive the electron temperature and density of the emitting region through diagnostic line ratios [1, 2]. However, to calculate such ratios reliably, accurate atomic data must be employed, especially for the electron impact excitation rates of the relevant transitions [3]. In this paper we compare the theoretical S XIII and Ar XV emission line ratios R1 = I (2s2p 1P - 2s2 1S) / I (2s2p 3P1 - 2s2 1S), and R2 = I (2s2p 1P - 2s2 1S) / I (2p2 3P2 - 2s2p 3P2) with observational data for several solar flares, obtained using the SO82A instrument on board Skylab. Good agreement is found between theory and observation for R1 for both ions, which provides support for the electron impact excitation rates adopted in the calculations. However, in the case of R2, all the observed values for S XIII and Ar XV are much smaller than the theoretical estimates, which is probably due to blending in the 2p2 3P2 - 2s2p 3P2) lines

PROTON EXCITATION OF THE 2s2 2p5 2 P3/2 -2s22p5 2P1/2 TRANSITION IN FLUORINE-LIKE Ti XIV and Ni XX

Rate coefficients for excitation of the 2s22p5 2P3/2 - 2s22p5 2P1/2 transition in fluorine-like Ti XIV and Ni XX by proton impact have been calculated using the close-coupled impact parameter method. These data are significantly different from earlier results with, for example, our proton rates for Ti XIV being approximately a factor of five smaller than those of Bely and Faucher (1970) at low temperatures. We show that the 2s22p5 2P3/2 - 2s22p5 2P1/2 transition in Ti XIV and Ni XX may be used to infer the electron density or ion temperature of a laboratory plasma through the diagnostic emission line ratios R1 = I(2s22p5 2P3/2 - 2 s22P52P1/2)/I(2s22P52P3/2 - 2s2p6 2S1/2) and R2 = I(2s22p5 2P3/2 - 2s22p5 2P1/2)/I(2s22p52P1/2 - 2s2p6 2S1/2), although the theoretical values of R1 and R2 are strongly dependant on the magnitude of the proton excitation rate for 2s22p5 2P3/2 - 2s22p5 2P1/2. The accurate calculation of this quantity for Ti XIV and Ni XX is therefore of great importance

THEORETICAL ELECTRON DENSITY AND TEMPERATURE SENSITIVE EMISSION LINE RATIOS FOR HELIUM-LIKE Si XIII COMPARED TO DITE TOKAMAK OBSERVATIONS

New calculations of the electron density sensitive emission line ratio R (= f/i) and temperature sensitive ratio G (= (f+i)/r) in helium-like Si XIII are presented, where f, i and r are the forbidden 1s2 1S - 1s2s 3S, intercombination 1s21S - 1s2p 3P1,2 and resonance ls21S - 1s2p 1P transitions, respectively. A comparison of these with R and G ratios measured from x-ray spectra of the DITE tokamak, for which the electron density and temperature have been well determined, reveals excellent agreement between theory and observation, with discrepancies of typically less than 10%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations. The theoretical results may therefore be applied with confidence to the analysis of remote sources for which no independant electron density and temperature estimates exist, such as solar flares