We describe the 2023 release of the spectral synthesis code Cloudy. Since the previous major release, migrations of our online services motivated us to adopt git as our version control system. This change alone led us to adopt an annual release scheme, accompanied by a short release paper, the present being the inaugural. Significant changes to our atomic and molecular data have improved the accuracy of Cloudy predictions: we have upgraded our instance of the Chianti database from version 7 to 10; our H- and He-like collisional rates to improved theoretical values; our molecular data to the most recent LAMDA database, and several chemical reaction rates to their most recent UDfA and KiDA values. Finally, we describe our progress on upgrading Cloudy's capabilities to meet the requirements of the X-ray microcalorimeters aboard the upcoming XRISM and Athena missions, and outline future developments that will make Cloudy of use to the X-ray community.

We announce the C23.01 update of cloudy. This corrects a simple coding error, present since ∼1990, in one routine that required a conversion from the line-center to the mean normalization of the Lyα optical depth. This affects the destruction of H i Lyα by background opacities. Its largest effect is upon the Lyα intensity in high-ionization dusty clouds, where the predicted intensity is now up to three times stronger. Other properties that depend on Lyα destruction, such as grain infrared emission, change in response.

We present the 2025 release of the spectral synthesis code CLOUDY, highlighting the significant enhancements in the scope and accuracy of the physics that have been made since the previous release. This key development resolves the Lyman-alpha line into j-resolved fine-structure doublets, thereby making CLOUDY useful to the X-ray community. We also updated the inner-shell ionization line energies and incorporated the 1 keV feature that is commonly observed in X-ray binaries. In addition, we updated the Stout database for the carbon isoelectronic sequence, improving the CLOUDY microphysical calculations for all wavelengths. The molecular network has also been extended with new silicon-bearing species, titanium-related reactions, and phosphorus-containing molecules, enhancing CLOUDY’s ability to model the complex chemistry relevant to exoplanet atmospheres. Finally, we outline future developments aimed at maximizing scientific returns from current and upcoming generations of observatories.

The depletion of elements onto dust grains is characterized using a generalized depletion strength F_* for any sightline, and trend-line parameters A_X, B_X and z_X. The parameters A_X, B_X and z_X define the relative depletion pattern, for which values are published in previous works. The present study uses these parameters to calculate post-depleted gas-phase abundances of 15 different elements while varying F_* from 0 to 1. An analysis of emergent strong spectral line intensities, obtained by inputting the calculated abundances into a cloudy model, shows that the depletion strength has a non-trivial effect on predicted emission lines and the thermal balance of the ionized cloud. The amount by which elements deplete also affects the coolant abundances in the gas. Furthermore, it was found that each of the parameters - metallicity, ionization parameter U and depletion strength F_* have degenerate effects on the emission-line strengths, and thermal balance of the interstellar medium (ISM). Finally, comparing our results to a sample of H ii regions using data obtained from the Mapping Nearby Galaxies at Apache Point Observatory survey (MaNGA) revealed that the best-fit F_* was approximately 0.5. However, this best-fit value does not work well for all metallicities. Removing the sulfur depletion and changing the nitrogen abundance pattern can improve the fit. As a result, extra observational evidence is required to verify the choices of parameters and better constrain the typical depletion strength in galaxies.

Atomic species in the interstellar medium transition out of their gas phase mainly by depletion on to dust. In this study, we examine if there is any change to the spectral-line ratio predictions from a photoionization model of the Orion H ii region when the degree of dust depletions is altered according to the most recently published model. We use equations and parameters published by previous works, in order to streamline the calculation of depleted abundances within cloudy. Our aim is for cloudy users to be able to vary the level of depletion using a single parameter in the input file. This makes it possible to explore predictions for a large range of depletions more efficiently. Finally, we discuss the results obtained for a model of the Orion Nebula when the degree of depletions are manipulated in this way. We found that the intensity of line ratios are significantly affected by depletions on to dust grains. Further, we found that adjusting dust abundances along with depletion affects the structure and the overall temperature of the H⁺ layer across the H ii region.

Aims. We present a simple yet powerful column density diagnostic for plasmas enabled by X-ray microcalorimeter observations. Methods. With the recent developments of the spectral simulation code CLOUDY, inspired by the high spectral resolution of the X-Ray Imaging and Spectroscopy Mission (XRISM) and the Advanced Telescope for High Energy Astrophysics (Athena), we make predictions for the intensity ratio of the resolved fine-structure lines Lyα₁ and Lyα₂ of H-like ions. Results. We show that this ratio can be observationally constrained and used as a plasma column density indicator. We demonstrate this with an XRISM observation of the high-mass X-ray binary Centaurus X-3. Conclusions. This diagnostic is useful for a wide range of X-ray emitting plasmas that are either collisionally or radiatively ionized.