Dr
Richard Wunsch
(Astronomical Institute, Czech Academy of Sciences)
Globular clusters (GCs) are very massive spherically symmetric objects consisting of $10^5$ - $10^6$ stars concentrated within a radius of several parsecs. There are approximately 150 GCs in our Galaxy, distributed in a spherical galactic halo. Until recently, it has been believed that they are simple systems consisting of a single generation of stars with the same age and chemical composition. However, approximately 15 years ago it has been found that most if not all GCs consist of two or more stellar populations differing photometrically. Moreover, recent spectroscopic observations show that some populations of stars include products of high temperature hydrogen burning, which takes place in certain types of stars. This chemical feature is universal and unique for GCs.
Several hypotheses have been suggested, most of them interpreting the individual stellar populations as subsequent generations with later ones formed out of winds or outflows of the previous generation stars, enriched by the H-burning products. However, all the existing hypotheses include important unsolved problems. For instance, it is not clear how the stellar winds with velocities several thousand km/s (much more than the escape velocity from the cluster) can be captured inside the star cluster and participate on the secondary star formation. Another (so called "mass budget") problem is that the observed enriched generations of stars are substantially more massive than the total amount of mass released by the first generation stars.
We develop a model that tries to overcome the two aforementioned problems. The subsequent stellar populations in GCs are formed out of winds of massive stars of the first generation. In dense and massive young star clusters, the hot gas originating from wind-wind collisions becomes thermally unstable and forms dense warm gaseous structures that sink into the cluster centre where they accumulate, cool further due to self shielding of the ionising radiation of stars, and form new stars. This can solve the first problem of the wind capturing, and help with the mass budget: if the second generation is more centrally concentrated, a fraction of the first generation can be lost by galactic tidal forces.
We study this model using radiation-hydrodynamic simulations describing the interacting winds of massive stars. The numerical implementation is based on MPI-parallel adaptive mesh refinement code FLASH4 (Fryxell+2000) widely used in many astrophysical applications. Effects of ionising radiation are calculated with the algorithm TreeRay combining the method of reverse ray-tracing with tree (Wunsch+2018). This module is developed in our research group as a part of the wide international collaboration SILCC (Walch+2015) aiming to simulate the life cycle of molecular clouds in galaxies with unprecedented physical complexity.
The results confirm that the fast stellar winds can be captured by the suggested mechanism and that the secondary star formation occurs in clusters with parameters expected for young GCs. Additionally, we predict shapes of the spectral line H$\alpha$30 that could be detected in young massive star clusters using the ALMA radiotelescope to test the model observationally.
Summary
Globular star clusters (GCs) are fossils remaining from the epoch when star formation in the Universe was at its peak and when galaxies were assembling. Current theories of star formation are unable to explain the observed peculiar chemical composition of stars in GCs. We develop the radiation-hydrodynamic model of a young massive star cluster describing the flow of the gas inserted into the cluster by massive stars in the form of stellar winds. We use it to test the hypothesis that several generations of stars were formed in GCs, the latter ones out of the gas enriched by products of nuclear burning in previous generations. The model predicts that if the cluster is very dense and massive, as the young GCs were, the shocked winds rapidly cool by emitting soft X-ray radiation forming dense clumps which fall into the cluster centre and form new stars.
References
Fryxell, B.; Olson, K.; Ricker, P. et al. (2000), The Astrophysical Journal Supplement Series, 131, 273
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. (2015), Monthly Notices of the Royal Astronomical Society, 454, 238
Wünsch, R.; Palouš, J.; Tenorio-Tagle, G.; Ehlerová, S. (2017), The Astrophysical Journal, 835, 60
Wünsch, R.; Walch, S.; Dinnbier, F.; Whitworth, A. (2018), Monthly Notices of the Royal Astronomical Society, 475, 3393
Dr
Richard Wunsch
(Astronomical Institute, Czech Academy of Sciences)
