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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - RadFeedback (The radiative interstellar medium)

Teaser

Summary

The pressure, radiation, and ionization from the warm (UV emitting) and hot (X-ray emitting) gas has a significant impact on the cold, star-forming interstellar medium. In the RADFEEDBACK project we carry out a comprehensive 3D study of the turbulent, multi-phase ISM in different environments that includes, for the first time, a proper treatment of UV and X-ray emission from stellar (primary) sources and extended (secondary) sources like cooling shock fronts and evaporating clouds. We do this by means of massively parallel, high-resolution 3D simulations that capture the complex interplay of gravity, magnetic fields, feedback from massive stars (ionizing radiation, radiation pressure, stellar winds, supernovae), heating and cooling including X-rays and cosmic rays, and chemistry. We are developing a novel, original and highly efficient method to accurately treat the transfer of radiation from multiple point and extended sources in the 3D simulations. Radiation and chemistry will be coupled to achieve self-consistent heating, cooling, and ionization rates. Moreover, accurate synthetic observations covering the large dynamic range from X-rays down to radio emission will be generated to set the results in the proper observational context. This will enable us to address the key science questions: How efficient is stellar feedback in different environments and which feedback process is dominant? What is the precise role of UV radiation and X-rays, also from secondary sources? Are the observations following the key dynamical players? How do we best interpret ISM observations from ALMA, SKA, or ATHENA? How do we assist in designing future observations? Within RADFEEDBACK we perform the most self-consistent theoretical study of the multi-phase ISM so far, thus building up a leading group for ISM research in Europe. To stimulate worldwide scientific activities and interactions we will make all data available to the community through an open-access web interface.

Work performed

The first two main objectives of RADFEEDBACK were the questions â€œHow is the joint energy and momentum output from evolving massive stars processed by the interstellar medium?â€ and â€œWhat is the relative contribution of stellar winds, (ionizing) radiation, radiation pressure, and blast waves to shaping the multi-phase, ISM, driving supersonic turbulence and dispersing molecular clouds?â€. We have so far solved these questions for low- to intermediate mass molecular clouds using the best 3D simulations to date. Further, we have implemented a new method to treat the radiative transfer and absorption of soft X-rays and their effects on the chemistry of the gas. This has led to new, exciting results, that CO is more vulnerable to a high X-ray irradiation than the H_2 molecule. We have further made significant progress in comparing simulations and observations in a quantitative manner (O6 and O7) and put several software packages online, which are useful to compare simulations and observations. The synthetic observation pipeline is operating well and we have implemented several molecules, dust, C+ and atomic hydrogen. We have also carried out polarization studies on our high-resolution simulations of molecular cloud formation with the POLARIS code. We are currently working on applying all these well-working tools to the wealth of simulation data, which we have obtained in the SILCC-Zoom Gauss project, which ended in fall 2017.

Final results

The radiative transfer scheme TreeRay is working very well and scales perfectly with the number of radiative sources, i.e. the method is independent of the number of sources. The method paper is currently in progress. We have coupled the radiative transfer scheme to the chemical network used in our simulations. In the process, the network has also been updated and now includes state-of-the art rates. Further, it has been expanded to be able to treat the impact of a time- and energy-dependent X-ray radation field.
The new numerical scheme has been applied in several papers. For instance, we have studied the evolution of molecular species in clouds which are subject to an X-ray flare, which is prototypical for molecular clouds near the Galactic Center. We found that CO is much more vulnerable than molecular hydrogen and therefore the CO/H2 ratio varies significantly in such environments. The main channel to destroy CO is not He+ as predicted by equilibrium models, but rather by the local far-UV field produced by collisions of non-thermal electrons with H2.
In another study, where we exploit the power of TreeRay to treat many ionizing sources, we study the impact of UV radiation from massive stars that are born in molecular clouds, which are formed from the warm interstellar medium in the SILCC-Zoom project. We find that HII regions flicker on scales of >10 parsec before they are fully developed. This leads to relatively large photo-dissociation regions (defined as regions of intermediate ionization degree) with respect to what is predicted by static or 1D models. We will now use these simulations to look at observable star formation tracers like Halpha.
The developments achieved in the first half of this ERC project will allow us to answer the following objectives as a next step: (i) We will demonstrate the importance of secondary emission in the form of infrared radiation, UV, and X-rays produced in shocks, which are ubiquitous in the turbulent interstellar medium. In particular, we will show if these effects have the power to boost stellar feedback in massive giant molecular clouds, where feedback is subject to strong cooling and therefore possibly too weak to regulate the star formation efficiency of these clouds; (ii) We will investigate the impact of X-rays on the heating and ionization of the dense gas as well as on the chemical and therefore observational signatures of molecular clouds in different environments more violent than the quiescent solar neighbourhood. These results will contribute fundamentally to the understanding of the evolution of the interstellar medium in different galactic environments and will help us to interpret existing and upcoming observations.