Impact of Cosmic Ray Acceleration on the Early Evolution of Bow Shocks around Massive Runaway Stars

Watanabe, K., Rathjen, T.-E. et al., arXiv:2510.11988, 2025

Massive runaway stars create bow shocks as their stellar winds collide with interstellar gas, accelerating particles that produce detectable gamma-ray and radio emission. We present 3D cosmic ray magnetohydrodynamic simulations of these bow shocks, tracking particle acceleration over 180,000 years across different cosmic ray diffusion rates and stellar velocities. Our simulations incorporate dynamic particle injection at shock fronts and calculate predicted emission spectra. Results show that cosmic ray diffusion significantly influences bow shock morphology and non-thermal emission luminosity, with efficient forward shock acceleration producing gamma-ray and synchrotron signals comparable to observations. This demonstrates that cosmic ray physics fundamentally shapes bow shock structure and detectability.

SILCC – IX. The multi-phase interstellar medium at low metallicity

Brugaletta, V., Rathjen, T.-E. et al., MNRAS, 543, 4286, 2025

The gas-phase metallicity strongly influences heating, cooling, and star formation in the galactic interstellar medium (ISM). Using magnetohydrodynamic simulations from the SILCC project with the FLASH code, we study metallicities from one-fiftieth to solar. The models include non-equilibrium thermo-chemistry, stellar radiation, winds, supernovae, and cosmic rays. With decreasing metallicity, the star formation rate drops by more than a factor of ten, the cold gas fraction declines, and the warm gas fills most of the ISM. Dense gas forms fewer structures, and molecular hydrogen becomes scarce, showing that metallicity strongly regulates star formation and ISM structure.

SILCC -- VIII: The impact of far-ultraviolet radiation on star formation and the interstellar medium

Rathjen, T.-E. et al., MNRAS, 540, 1462, 2025

Our latest study in the SILCC Project investigates the role of far-ultraviolet (FUV) radiation in star formation within the multiphase interstellar medium using magnetohydrodynamic simulations. While FUV radiation can significantly heat dust and affect the gas phases near star clusters, we find its overall impact on star formation rates is minimal compared to other feedback mechanisms like ionising radiation, stellar winds, and supernovae. FUV radiation mainly reduces star formation burstiness and promotes a diffuse molecular gas phase, but it does not strongly influence the integrated star formation rate. The research highlights the complex interplay between radiation and the interstellar environment, refining our understanding of how stars shape their surroundings.

The impact of cosmic-ray heating on the cooling of the low-metallicity interstellar medium

Brugaletta, V., Rathjen, T.-E. et al., MNRAS, 537, 482, 2025

Low-metallicity environments experience inefficient cooling and reduced photoelectric (PE) heating due to low dust-to-gas ratios. Using SILCC project simulations with 0.02 Z_⊙, we include non-equilibrium chemistry, stellar feedback (FUV/EUV radiation, winds, supernovae), and cosmic-ray (CR) heating. CR heating, variable in space and time, often dominates over PE heating. Models with uniform CR ionization suppress star formation, while variable rates allow localized star formation in cool, pristine gas. However, low metallicity limits stellar feedback, preventing large-scale outflows from the mid-plane.

The masses, structure and lifetimes of cold clouds in a high-resolution simulation of a low metallicity starburst

Fotopoulou, C.M., Rathjen, T.-E. et al., MNRAS, 534, 215, 2024

We analyse the cold gas phase in a low metallicity starburst from a high-resolution hydrodynamical simulation of a gas-rich dwarf galaxy merger, part of the GRIFFIN project. The simulation resolves the multi-phase interstellar medium with non-equilibrium chemistry and includes interactions from individually sampled massive stars through HII regions and supernovae. During the starburst, the ISM is dominated by cold (T < 300 K) filamentary clouds with a mass range of 10^2.6 to 10^5.6 M_⊙, following a power law distribution (dN/dM ∝ M^-1.78). These clouds obey Larson's relations and exhibit an exponential lifetime distribution with an e-folding time of ~3.5 Myr. Clouds below 10⁴ M_⊙ follow a power law for their lifetimes, which flattens for larger masses at <10 Myr.

CO and [C II] line emission of molecular clouds: the impact of stellar feedback and non-equilibrium chemistry

Ebagezio, S., Rathjen, T.-E. et al., MNRAS, 525, 5631 2023

We analyze SILCC-Zoom project simulations' synthetic emission maps (12CO, 13CO, [C II]) of molecular clouds. Our zoom-in simulations track H2, CO, and C+ evolution in the interstellar medium with and without radiative stellar feedback. Post-processing with CLOUDY addresses higher carbon ionization due to stellar radiation in H II regions. Synthetic [C II] emission maps reveal depletion around feedback bubbles, attributed to C+ ionization into C2+. The cloud-averaged luminosity ratio for 12CO and 13CO is unreliable for H2 mass fraction or cloud evolutionary stage determination. Chemical equilibrium assumption introduces intrinsic errors in chemical abundances, luminosities, and ratios.

How do supernova remnants cool? – I. Morphology, optical emission lines, and shocks

Makarenko, E., Rathjen, T.-E. et al., MNRAS, 523, 1421 2023

We introduce a FLASH code post-processing module for simulating Supernova (SN) remnants' cooling radiation, using collisional excitation data from MAPPINGS V. Applying it to an SNR simulation, we find dominant EUV emissions, while optical lines ([O III], [N II], [S II], Hα, Hβ) are typically more observable. Our shock detection scheme reveals [S II] and [N II] emissions from the thin shell, and [O III], H⁠α, and H⁠β from the hot gas within the SNR bubble. Optical lines are influenced by the SNR's structure and projection, with 10–80% reduction in line luminosity due to line-of-sight absorption. Contaminating background radiation subtraction is essential for accurate SNR classification on oxygen or sulfur BPT diagrams. Synthetic observations match well with simulation results, but electron temperature and density sensitivity to assumed metallicity is noted.

SILCC – VII. Gas kinematics and multiphase outflows of the simulated ISM at high gas surface densities

Rathjen, T.-E. et al., MNRAS, 522, 1843 2023

We present MHD simulations of the star-forming interstellar medium in stratified galactic patches (Σgas = 10, 30, 50, 100 M_☉ / pc^2). The SILCC project includes non-equilibrium thermal and chemical processes. The sink-based model incorporates stellar winds, UV radiation, core-collapse supernovae, and cosmic rays (CR). Simulations align with the observed Σgas-ΣSFR relation. CRs impact outflows; without them, a two-phase to single-phase transition occurs. With CRs, three phases persist, dominated by the warm phase. CR impact on mass loading decreases with higher Σgas, maintaining factors around unity, independent of ΣSFR. Vertical velocity dispersions of the warm ionized medium (WIM) and cold neutral medium (CNM) correlate with ΣSFR^0.20, consistent with observations. In the absence of stellar feedback, no correlation is observed. WIM's velocity dispersion is ~2.2 times higher than CNM, matching local observations.

SILCC VI - Multiphase ISM structure, stellar clustering, and outflows with supernovae, stellar winds, ionizing radiation, and cosmic rays

Rathjen, T.-E. et al., MNRAS, 504, 1039, 2021

We present simulations of the multiphase interstellar medium (ISM) under solar neighbourhood conditions, incorporating thermal and non-thermal processes, star cluster formation, and feedback from massive stars: stellar winds, ionising radiation (using the TREERAY method), supernovae, and cosmic rays (CR). We investigate the impact of feedback mechanisms on the ISM, finding that radiation and winds from massive stars regulate star formation, aligning with observed star formation rates and outflows. While supernova-only feedback leads to highly clustered star formation, the inclusion of radiation reduces star cluster masses and enhances consistency with local conditions. CRs have moderate effects on star formation.