Science Highlights

What is the structure and chemical composition of gas that may feed future star formation? Before interstellar gas turns dense enough to form new stars it is not fully molecular yet but in some so far unknown transitional state. A special case of such gas clouds are given by high-latitude clouds representing material that may fall onto the plane of the Milky Way.
In a recently accepted paper by Nicola Schneider and collaborators we reported results from SOFIA/upGREAT observations of a number of diffuse and high latitude clouds in the Milky Way, in particular the Draco, Spider, Polaris and Musca clouds. In only one of them, the Draco cloud, we detected emission of ionized carbon in spite of it being neither the densest cloud nor the one irradiated the strongest by known sources.
When trying to model the emission of all observations in terms of an astrochemical model of a photon-dominated region it turns out, that the model is not able to simultaneously explain the strength of the continuum emission from dust and the ionized carbon line. The ionized carbon detection and also the non-detections suggest a very low impinging UV field well below what is actually observed taking all the known stars in the environment. For the Draco cloud we can explain the difference by the additional energy from a shock that is produced when the cloud is hitting the interstellar gas of the Milky Way and some shielding of the cloud by interstellar dust in atomic gas. However, for the other clouds, we do not have a consistent explanation yet.
The paper combined the results from a fruitful collaboration between the CRC 1601 subprojects B2 and A6.

Figure caption: Schematic illustration of the illumination of a high-latitude cloud by UV radiation from stars in the plane of the Milky Way. The orange cylinder along the path between star and cloud indicates the dust column which attenuates the UV-field.

What roles do different stellar feedback processes play in governing star formation? From ionizing and non-ionizing radiation to stellar winds and supernovae, these forces interact with the surrounding stellar nurseries. However, understanding the precise significance of each process in shaping star formation remains an ongoing challenge.

We present new magneto-hydrodynamic (MHD) simulations conducted within the SILCC framework, exploring the multiphase interstellar medium (ISM) within a patch of a stratified galactic disk. Our study incorporates a self-consistent modeling of non-ionizing far-ultraviolet (FUV) radiation emitted by stellar clusters, aiming to understand its impact on star formation and the chemical composition of the nearby ISM. We observe locally intense interstellar radiation fields (ISRF) with values up to G₀ ≈ 10⁴ (in Habing units), contrasting with the canonical solar neighborhood value of G₀ = 1.7.

Our findings suggest that while FUV radiation influences star formation, its role in regulating the star formation rate (SFR) appears less significant compared to other stellar feedback mechanisms such as ionizing UV radiation, stellar winds, and supernovae. Additionally, our chemical analysis reveals enhancements in both the warm-ionized medium (WIM) and the cold-neutral medium (CNM) beyond the vicinity of stellar clusters, indicating a complex interplay influenced by the self-consistent and highly variable FUV radiation field, fostering the presence of a diffuse molecular hydrogen gas phase.

Further details will be available in Rathjen et al., currently in preparation.

Figure: Overview of the simulated ISM. Shown are the edge-on views of the total gas (Σ_gas, 1st panel), molecular hydrogen (Σ_H2, 4th panel), and ionized hydrogen (Σ_H⁺, 5th panel) column densities, as well as mass-weighted gas (T_gas, 2nd panel) and dust (T_dust, 3rd panel) temperatures and ionizing photon energy density (e_γ, 6th panel), the effective G₀ field (G_eff, 7th panel), and cosmic ray energy density (e_CR, 8th panel) in projection. The star-forming galactic ISM is concentrated around the midplane. White circles in the 1st panel indicate active star clusters.

To learn more about the formation and evolution of massive stars it is important to confront simulations and observations.
It is useful to interpret the observational data and to extract the cores’ physical parameters,. We can address e.g. the question how massive cores fragment and form (massive) stars, or how long the young, massive stellar objects are embedded in their parental core.

Doing so, we simulate a collapsing core scenario of a subvirial, 1000 M☉ core with an initial radius of 1 pc and a linear magnetic field of 100 μG, which is a birthplace of massive stars.

For the post-processing we use RADMC-3D, which is an open source radiative transfer code that is based on the Monte-Carlo method. Here, we present synthetic observations of the dust emission (top left panel). The advantage of our simulations is that we calculate the dust temperature self-consistently, hence taking into account radiative heating by all young stars as well as shock heating. Thus, RADMC-3D is directly working on the simulated dust temperature. These results are post-processed with CASA, where different, respectively a combination of, possible ALMA channels and predictable water vapour (pwv) in the atmosphere can be simulated.
We show the results for synthetic observations in different ALMA channels (labeled with AMLA TM1, TM2 and ACA; bottom panels), as well as their combination for two predictable water vapour settings (top middle and top right panel). In synthetic observations most of the structures in the less dense environment are not visible anymore; however, the emission of the main star forming regions remain.

This work was performed in collaboration with Dr. Álvaro Sánchez-Monge