The physical properties of the faint and extremely tenuous plasma in the far outskirts of galaxy clusters, the circumgalactic media of normal galaxies, and filaments of the cosmic web remain one of the biggest unknowns in our story of large-scale structure evolution. Modelling the spectral features due to emission and absorption from this very diffuse plasma poses a challenge, as both collisional and photoionization processes must be accounted for. In this paper, we study the ionization by photons emitted by the intracluster medium in addition to the photoionization by the cosmic ultraviolet/X-ray background on gas in the vicinity of galaxy clusters. For near-massive clusters such as A2029, the ionization parameter can no longer describe the ionization balance uniquely. The ionization fractions (in particular of C iv, C v, C vi, N vii, O vi, O vii, O viii, Ne viii, Ne ix, and Fe xvii) obtained by taking into account the photoionization by the cosmic background are either an upper or lower limit to the ionization fraction calculated as a function of distance from the emission from the cluster. Using a toy model of a cosmic web filament, we predict how the cluster illumination changes the column densities for two different orientations of the line of sight. For lines of sight passing close to the cluster outskirts, O vi can be suppressed by a factor of up to 4.5, O vii by a factor of 2.2, C v by a factor of 3, and Ne viii can be boosted by a factor of 2, for low-density gas.

The study of chemically peculiar (CP) stars in open clusters provides valuable information about their evolutionary status. Their detection can be performed using the ∆a photometric system, which maps a characteristic flux depression at λ ∼ 5200Å. This paper aims at studying the occurrence of CP stars in the earliest stages of evolution of a stellar population by applying this technique to Hogg 16, a very young Galactic open cluster (∼ 25 Myr). We identified several peculiar candidates: two B-type stars with a negative ∆a index (CD −60 4701, CPD −60 4706) are likely emissionline (Be) stars, even though spectral measurements are necessary for a proper classification of the second one; a third object (CD −60 4703), identified as a Be candidate in literature, appears to be a background B-type supergiant with no significant ∆a index, which does not rule out the possibility that it is indeed peculiar as the normality line of ∆a for supergiants has not been studied in detail yet. A fourth object (CD −60 4699) appears to be a magnetic CP star of 8 M , but obtained spectral data seem to rule out this hypothesis. Three more magnetic CP star candidates are found in the domain of early F-type stars. One is a probable nonmember and close to the border of significance, but the other two are probably pre-main sequence cluster objects. This is very promising, as it can lead to very strong constraints to the diffusion theory. Finally, we derived the fundamental parameters of Hogg 16 and provide for the first time an estimate of its metal content.

Understanding and modelling astrophysical plasmas on atomic levels while taking into account various assumptions (for example, collisional ionisation equilibrium or photoionisation equilibrium) became essential with the progress of high-resolution X-ray spectroscopy. In order to prepare for the upcoming X-ray spectroscopy missions such as XRISM or Athena, the plasma codes with their models and the atomic databases need to be up to date and accurate. One such update for the plasma code SPEX is presented in this paper where we focus on the radiative loss due to collisional excitation in the low-density, optically thin regime. We also update the atomic data for neutral hydrogen and include the contribution of the dielectronic recombination. With all these updates being implemented in SPEX we finally present the new cooling curve. We include the comparison to other plasma codes (MEKAL, APEC, Cloudy) and other atomic databases (CHIANTI, ADAS). We show how the updated cooling impacts the stability curve for photoionised plasmas and find a new stable branch.