Star Formation and Feedback in Molecular Clouds

A short overview

Project C5 was established in the second funding period of CRC 956. In C5 we study the conditions and the impact of star formation in state-of-the-art, three-dimensional, high-resolution, magneto-hydrodynamical simulations including e.g. (self-) gravity, sink particles, radiative transfer from point sources, and a chemical network to model the formation and dissociation of molecular hydrogen and CO as well as non-equilibrium heating and cooling effects.

We have carried out a comprehensive study of molecular cloud formation, where we zoom-in onto forming molecular clouds. The initial molecular clouds are taken from larger-scale (500 pc x 500 pc x 10,000 pc) simulations of vertically stratified pieces of disk galaxies, that have already been carried out (see www.astro.uni-koeln.de/silcc). The resolution of these large-scale models is too coarse to follow the star formation process in detail, but high enough to model the assembly of the molecular clouds on galactic scales. However, these simulations have shown that molecular clouds are not isolated objects, but accrete significant amounts of mass, merge, or get partly sheared apart during their lifetime. Moreover, they are frequently affected by nearby Supernova explosions. This shows that, quite possibly, the environment within which molecular clouds form and live cannot be neglected during the star formation process. In the “galaxy zoom simulations“ carried out during the funding period (2015 - 2018) we self-consistently follow the collapse, formation of filaments, fragmentation, and star formation process within different molecular clouds down to scales of ~0.1 pc. At the same time, we continue the large-scale simulation at low resolution as the molecular clouds accrete significant amounts of mass while collapsing and forming stars, which alters the dynamics of clouds and drives turbulence.

Further, we have investigated the relative impact of stellar wind and ionizing radiation feedback from different massive stars in different environments, ranging from the parental, dense and cold molecular cloud where new stars are born, to more evolved environments as present in a young star cluster which hosts multiple massive stars. The dispersal of the dense molecular clouds is found to be driven by ionizing radiation as long as the stars are quite well embedded in pristine cloud material. Once the HII regions break out of the cloud, stellar winds gain importance. The time scale of molecular cloud dispersal depends on the local energy and momentum input of the feedback, which in turn depends on the detailed substructure of the clouds. We could show that the local visual extinction, set by the 3D column density distribution within the clouds, determines the vulnerability of the clouds. Deeply shielded parts of the clouds are more resistent to radiative feedback and can continue to form stars on their own without being disturbed by nearby stellar feedback, while regions of low extinction are easily blown apart. Overall, the stellar feedback significantly reduces the star formation efficiency of the clouds down to a few per cent, while at the same time star formation is triggered (new stars are born in density structures that are swept up by the ionizing radiation).

In the upcoming funding period, we would branch out in two directions: (1) to larger scales, modeling molecular cloud formation in full galactic environments to investigate the effects of shear and different surface densities, gravitational potentials, and insterstellar radiation fields; and (2) to smaller scales, modeling the star formation content of individual hubs and networks of filaments using a novel "deep-zoom" technique.

Project C5 will fundamentally further our understanding of star formation and feedback in different molecular clouds, and even in different galactic environments (e.g. local or high surface density conditions, see A2). The models will provide the basis for detailed comparisons with observations (projects A2, A3, A4, A6), PDR (C1), and chemical modelling (C3). Furthermore, we will provide valuable information about the complex physics of stellar feedback, which cannot be resolved in galaxy-scale simulations (project C4), but which has been shown to regulate the gas cycle and star formation process in galaxies.

Due to our active collaboration with the observational groups within CRC 956, it has become apparent that the theoretical work done in project C5 is significantly strengthened by detailed comparsions between simulations and observations by means of realistic mock observations. This would be realized in the newly proposed project C6, which would be concerned with synthetic observations and improved astro-chemical modeling.

Selected Publications

• 2022

• Bisbas, Thomas G.; Walch, Stefanie; Naab, Thorsten; Lahén, Natalia; Herrera-Camus, Rodrigo; Steinwandel, Ulrich P.; Fotopoulou, Constantina M.; Hu, Chia-Yu; Johansson, Peter H. 2022. "The Origin of the [C II] Deficit in a Simulated Dwarf Galaxy Mergerdriven Starburst." The Astrophysical Journal 934. doi:10.3847/1538-4357/ac7960
• Gaches, B. A. L., Bialy, S., Bisbas, T. G., Padovani, M., Seifried, D., Walch, S. 2022."Cosmic-ray-induced H₂ line emission. Astrochemical modeling and implications for JWST observations." Astronomy and Astrophysics 664. doi:10.1051/0004-6361/202244090
• Markert, J., Walch, S., Gassner, G. 2022. "A discontinuous Galerkin solver in the FLASH multiphysics framework." Monthly Notices of the Royal Astronomical Society 511, 4179–4200. doi:10.1093/mnras/stac298
• Rohde, P. F., Walch, S., Seifried, D., Whitworth, A. P., Clarke, S. D. 2022. "Protostellar outflows: a window to the past." Monthly Notices of the Royal Astronomical Society 510, 2552–2571. doi:10.1093/mnras/stab3572
• Borchert, E. M. A., Walch, S., Seifried, D., Clarke, S. D., Franeck, A., Nürnberger, P. C. 2022. "Synthetic CO emission and the X_CO factor of young molecular clouds: a convergence study." Monthly Notices of the Royal Astronomical Society 510, 753–773. doi:10.1093/mnras/stab3354
• Rathjen, T.-E., Naab, T., Walch, S., Seifried, D., Girichidis, P., Wünsch, R. 2022. "SILCC VII – Gas kinematics and multiphase outflows of the simulated ISM at high gas surface densities." arXiv e-prints 2211.15419.
• Weis, M., Walch, S., Seifried, D., Ganguly, S. 2022. "The Virial Balance of Molecular Clumps and Cores in Colliding Magnetized Flows." arXiv e-prints 2208.11705.
• Klepitko, A., Walch, S., Wünsch, R., Seifried, D., Dinnbier, F., Haid, S. 2022. "Tree-based solvers for adaptive mesh refinement code FLASH – III: a novel scheme for radiation pressure on dust and gas and radiative transfer from diffuse sources." arXiv e-prints 2204.09072.
• Ganguly, S., Walch, S., Clarke, S. D., Seifried, D. 2022. "SILCC-Zoom: the dynamic balance in molecular cloud substructures." arXiv e-prints 2204.02511.

• 2021

• Wünsch, Richard; Walch, Stefanie; Dinnbier, František; Seifried, Daniel; Haid, Sebastian; Klepitko, Andre; Whitworth, Anthony P.; Palouš, Jan 2021. "Tree-based solvers for adaptive mesh refinement code FLASH - II: radiation transport module TreeRay." Monthly Notices of the Royal Astronomical Society 505, 3730–3754. doi:10.1093/mnras/stab1482
• Girichidis, P., Naab, T., Walch, S., Berlok, T. 2021. "The in situ formation of molecular and warm ionized gas triggered by hot galactic outflows." Monthly Notices of the Royal Astronomical Society 505, 1083–1104. doi:10.1093/mnras/stab1203
• Rathjen, Tim-Eric, Naab, Thorsten, Girichidis, Philipp, Walch, Stefanie, Wünsch, Richard, Dinnbier, Frantis̆ek, Seifried, Daniel , Klessen, Ralf S. , Glover, Simon C. O., 2021. "SILCC VI - Multiphase ISM structure, stellar clustering, and outflows with supernovae, stellar winds, ionizing radiation, and cosmic rays." Monthly Notices of the Royal Astronomical Society 504, 1039–1061. doi:10.1093/mnras/stab900
• Rohde, P. F., Walch, S., Clarke, S. D., Seifried, D., Whitworth, A. P., Klepitko, A. 2021. "The impact of episodic outflow feedback on stellar multiplicity and the star formation efficiency." Monthly Notices of the Royal Astronomical Society 500, 3594–3612. doi:10.1093/mnras/staa2926
• Markert, J., Gassner, G., Walch, S. 2020. "A Sub-Element Adaptive Shock Capturing Approach for Discontinuous Galerkin Methods. 2021, Communications on Applied Mathematics and Computation," arXiv e-prints 2011.03338, https://doi.org/10.1007/s42967-021-00120-x

• 2020

• Clarke, S. D., Williams, G. M., Walch, S. 2020. "The hierarchical fragmentation of filaments and the role of sub-filaments." Monthly Notices of the Royal Astronomical Society 497, 4390–4406. doi:10.1093/mnras/staa2298
• Gaches, B. A. L., Walch, S., Offner, S. S. R., Münker, C. 2020. "Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays." The Astrophysical Journal 898. doi:10.3847/15384357/ab9a38

• 2019

• Mackey, J., Walch, S., Seifried, D., Glover, S. C. O., Wünsch, R., Aharonian, F. 2019. "Non-equilibrium chemistry and destruction of CO by X-ray flares." Monthly Notices of the Royal Astronomical Society 486, 1094–1122. doi:10.1093/mnras/stz902
• Ibáñez-Mejía, J. C., Walch, S., Ivlev, A. V., Clarke, S., Caselli, P., Joshi, P. R. 2019. "Dust charge distribution in the interstellar medium." Monthly Notices of the Royal Astronomical Society 485, 1220–1247. doi:10.1093/mnras/stz207
• Clarke, S. D., Williams, G. M., Ibáñez-Mejía, J. C., Walch, S. 2019. "Determining the presence of characteristic fragmentation length-scales in filaments." Monthly Notices of the Royal Astronomical Society 484, 4024–4045. doi:10.1093/mnras/stz248
• Joshi, P. R., Walch, S., Seifried, D., Glover, S. C. O., Clarke, S. D., Weis, M. 2019. "On the resolution requirements for modelling molecular gas formation in solar neighbourhood conditions." Monthly Notices of the Royal Astronomical Society 484, 1735–1755. doi:10.1093/mnras/stz052
• Rohde, P. F., Walch, S., Seifried, D., Whitworth, A. P., Clarke, S. D., Hubber, D. A. 2019. "Evolution of Hubble wedges in episodic protostellar outflows." Monthly Notices of the Royal Astronomical Society 483, 2563–2580. doi:10.1093/mnras/sty3302
• Haid, S., Walch, S., Seifried, D., Wünsch, R., Dinnbier, F., Naab, T. 2019. "SILCC-Zoom: The early impact of ionizing radiation on forming molecular clouds." Monthly Notices of the Royal Astronomical Society 482, 4062–4083. doi:10.1093/mnras/sty2938

• 2018

• Franeck, A.; Walch, S.; Seifried, D.; Clarke, S. D.; Ossenkopf-Okada, V.;Glover, S. C. O.; Klessen, R. S.; Girichidis, P.; Naab, T.; Wünsch, R.;Clark, P. C.; Pellegrini, E.; Peters, T. 2018. "Synthetic [C II] emission maps of a simulated molecular cloud in formation." Monthly Notices of the Royal Astronomical Society 481, 4277–4299. doi:10.1093/mnras/sty2507
• S D Clarke, A P Whitworth, R L Spowage, A Duarte-Cabral, S T Suri, S E Jaffa, S Walch, P C Clark, 2018. "Synthetic C^₁₈O observations of fibrous filaments: the problems of mapping from PPV to PPP." Monthly Notices of the Royal Astronomical Society 479, 1722–1746. doi:10.1093/mnras/sty1675
• Derigs, Dominik; Winters, Andrew R.; Gassner, Gregor J.; Walch, Stefanie; Bohm, Marvin: "Ideal GLM-MHD: About the entropy consistent nine-wave magnetic field divergence diminishing ideal magnetohydrodynamics equations", JCoPh, 364, 420
• Gronke, Max; Girichidis, Philipp; Naab, Thorsten; Walch, Stefanie: "The imprint of cosmic ray driven outflow on Lyman-alpha spectra", ApJL, submitted, arxiv:180512251
• Haid, S.; Walch, S.; Seifried, D.; Wünsch, R.; Dinnbier, F.; Naab, T.: "The relative impact of photoionizing radiation and stellar winds on different environments", MNRAS accepted, arxiv:1804.10218
• Seifried, D.; Walch, S.; Reissl, S.; Ibáñez-Mejía, J. C.: "SILCC-Zoom: Polarisation and depolarisation in molecular clouds", MNRAS, submitted, arxiv:1804.10157
• Seifried, D.; Walch, S.; Haid, S.; Girichidis, P.; Naab, T.: "Is Molecular Cloud Turbulence Driven by External Supernova Explosions?", MNRAS, 855, 81
• Wünsch, R.; Walch, S.; Dinnbier, F.; Whitworth, A.: "Tree-based solvers for adaptive mesh refinement code FLASH - I: gravity and optical depths", MNRAS, 475, 3393

• 2017

• Derigs, Dominik; Winters, Andrew R.; Gassner, Gregor J.; Walch, Stefanie: "A novel averaging technique for discrete entropy-stable dissipation operators for ideal MHD", JCoPh, 230, 624
• Gatto, A.; Walch, S.; Naab, T.; Girichidis, P.; Wünsch, R.; Glover, S. C. O.; Klessen, R. S.; Clark, P. C.; Peters, T.; Derigs, D.; Baczynski, C.; Puls, J.: "The SILCC project - III. Regulation of star formation and outflows by stellar winds and supernovae", MNRAS, 466, 1903
• Hu, Chia-Yu; Naab, Thorsten; Glover, Simon C. O.; Walch, Stefanie; Clark, Paul C.: "Variable interstellar radiation fields in simulated dwarf galaxies: supernovae versus photoelectric heating", MNRAS, 471, 2151
• Peters, T.; Naab, T.; Walch, S.; Glover, S. C. O.; Girichidis, P.; Pellegrini, E.; Klessen, R. S.; Wünsch, R.; Gatto, A.; Baczynski, C.: "The SILCC project - IV. Impact of dissociating and ionizing radiation on the interstellar medium and Hα emission as a tracer of the star formation rate", MNRAS, 466, 3293
• Seifried, D.; Walch, S.; Girichidis, P.; Naab, T.; Wünsch, R.; Klessen, R. S.; Glover, S. C. O.; Peters, T.; Clark, P.: "SILCC-Zoom: the dynamic and chemical evolution of molecular clouds", MNRAS, 472, 4797

• 2016

• Haid, S.; Walch, S.; Naab, T.; Seifried, D.; Mackey, J.; Gatto, A.: "Supernova blast waves in wind-blown bubbles, turbulent, and power-law ambient media", MNRAS, 460, 2962
• Girichidis, P.; Walch, S.;  Naab, T.; Gatto, A.; Glover, S. C. O.; Wünsch, R.; Klessen R. S.; Clark, P. C.; Peters, T.; Derigs D.; Baczynski, C.: "The SILCC (SImulating the LifeCycle of molecular Clouds) project - II. Dynamical evolution of the supernova-driven ISM and the launching of outflows", MNRAS, 456, 3432
• Seifried, D.; Walch, S.: "Modelling the chemistry of star-forming filaments - I. H2 and CO chemistry", MNRAS Letters, 459, L11

• 2015

• Walch, S.; Girichidis, P.; Naab, T.; Gatto, A.; Glover, S. C. O.; Wünsch, R.; Klessen R. S.; Clark, P. C.; Peters, T.; Derigs D.; Baczynski, C.; "The SILCC (SImulating the LifeCycle of molecular Clouds) project - I. Chemical evolution of the supernova-driven ISM", MNRAS, 454, 238.

• 2013

• Walch, S.; Whitworth, A. P.; Bisbas, T.G.; Wünsch, R.; Hubber, D.; “Clumps and triggered star formation in ionized molecular clouds”, MNRAS, 435, 917

• 2012

• Walch, S.; Whitworth, A. P.; Bisbas, T.G.; Wünsch, R.; Hubber, D.; “Dispersal of molecular clouds by ionizing radiation”, MNRAS, 427, 625.
• Walch, S.; Whitworth, A. P.; Girichidis, P.; “The influence of the turbulent perturbation scale on pre-stellar core fragmentation and disc formation”, MNRAS, 419, 760.

• 2011

• Walch, S.; Wünsch, R.; Burkert, A.; Glover, S.C.O.; Whitworth, A. P.; “The turbulent fragmentation of the interstellar medium: the impact of metallicity on global star formation ”, ApJ, 733, 47.

• 2010

• Walch, S.; Naab, T.; Whitworth, A. P.; Burkert, A.; Gritschneder, M., “Protostellar discs formed from turbulent cores”, MNRAS, 402, 2253ff.

• 2009

• Walch, S.; Burkert, A.; Whitworth, A. P.; Naab, T.; Gritschneder, M., “Protostellar discs formed from rigidly rotating cores”, MNRAS, 400, 13ff.
• Gritschneder, M.; Naab, T.; Walch, S.; Burkert, A.; Heitsch, F.; “Driving turbulence and triggering star formation by ionizing radiation”, ApJ, 694, 26.