Compact Massive Clusters Research Articles

Compact Star Clusters as Sources of Cosmic Rays and Neutrinos

The origin of cosmic rays is one of the most important questions in modern astrophysics. Leading world observatories are engaged in their search in our Galaxy and beyond. Current observations of bright sources of hard X-rays and gamma radiation require precise models of cosmic-ray accelerators and emission generation. Recent detection of astrophysical neutrinos with the energy exceeding 1015 eV achieved by the IceCube Neutrino Observatory and registration by the LOFAR observatory of the light cosmic-ray component at 1016–1017 eV have given rise to a wide discussion of whether its sources are galactic or extragalactic. H.E.S.S. observations of high-energy gamma-ray sources in compact massive clusters Westerlund 1 and Cl*1806-20 triggered a new search for the models of effective accelerators of high-energy cosmic rays, socalled galactic PeVatrons. In this paper, the properties of compact massive young stellar clusters as possible sources of cosmic rays, neutrinos, and gamma emission are discussed. We suppose that an expanding supernova remnant in a compact massive cluster is an effective cosmic-ray accelerator up to the energies of 1015 eV and beyond. Energy spectra of particles being accelerated in the system of colliding shocks generated by a supernova remnant interaction with nearby stellar winds have a broken power-law shape with a strong upturn around several teraelectronvolts and a cutoff energy at tens of petaelectronvolts. These sources provide the maximal emission flux at the maximal energies of the spectrum. Teraelectronvolt sources detected by H.E.S.S. could be related to supernova remnants in Westerlund 1 and Cl*1806-20 and be responsible for the 10% of the IceCube neutrino events. We have also shown that PeVatrons in the systems of colliding shocks can provide up to 80% of the light cosmic-ray component at the energies of 1016–1017 eV detected at the Earth.

The expansion of massive young star clusters – observation meets theory

Most stars form as part of a star cluster. The most massive clusters in the Milky Way exist in two groups - loose and compact clusters - with significantly different sizes at the end of the star formation process. After their formation both types of clusters expand up to a factor 10-20 within the first 20 Myr. Gas expulsion at the end of the star formation process is usually regarded as only possible process that can lead to such an expansion.We investigate the effect of gas expulsion by a direct comparison between numerical models and observed clusters concentrating on clusters with masses >10^3 M(sun). For these clusters the initial conditions before gas expulsion, the characteristic cluster development, its dependence on cluster mass, and the star formation efficiency (SFE) are investigated. We perform N-body simulations of the cluster expansion process after gas expulsion and compare the results with observations. We find that the expansion processes of the observed loose and compact massive clusters are driven by completely different physical processes. As expected the expansion of loose massive clusters is largely driven by the gas loss due to the low SFE of ~30%. One new revelation is that all the observed massive clusters of this group seem to have a very similar size of 1-3 pc at the onset of expansion. It is demonstrated that compact clusters have a much higher effective SFE of 60-70% and are as a result much less affected by gas expulsion. Their expansion is mainly driven by stellar ejections caused by interactions between the cluster members. The reason why ejections are so efficient in driving cluster expansion is that they occur dominantly from the cluster centre and over an extended period of time. Thus during the first 10 Myr the internal dynamics of loose and compact clusters differ fundamentally.

Reaction of massive clusters to gas expulsion – The cluster density dependence

The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group as long as stellar interactions can be neglected. We investigate under which circumstances stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different SFEs. Concentrating on the most massive clusters observed in the Milky Way, we test to what extend the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size.We find that stellar interactions are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way, making ejections the prime mass loss process in such systems. Even in the loosely bound OB associations stellar interactions are responsible for at least ~5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density-dependence of the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.

The massive star population in the giant H II region Tol 89 in NGC 5398

We present new high spectral resolution Very Large Telescope (VLT)/UV–Visual Echelle Spectrograph (UVES) spectroscopy and archival Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) imaging and spectroscopy of the giant H ii region Tol 89 in NGC 5398. From optical and ultraviolet (UV) HST images, we find that the star-forming complex as a whole contains at least seven young compact massive clusters. We resolve the two brightest optical knots, A and B, into five individual young massive clusters along our slit, A1–4 and B1, respectively. From UV spectral modelling using the starburst99 code of Leitherer et al., and nebular Hβ equivalent widths in the optical, we derive ages that are consistent with the formation of two separate burst events, of ∼4 ± 1 Myr and <3 Myr for knots A (A1–4) and B (B1), respectively. A Large Magellanic Cloud (LMC) metallicity is measured for both knots from a nebular line analysis, while nebular He ii 4686 is observed in knot B and perhaps in knot A. We detect underlying broad wings on the strongest nebular emission lines indicating velocities up to 600 km s−1. From UV and optical spectroscopy, we estimate that there are ∼95 early WN stars and ∼35 early WC stars in Tol 89-A, using empirical template spectra of LMC Wolf–Rayet (WR) stars from Crowther & Hadfield, with the WC population confined to cluster A2. Remarkably, we also detect a small number of approximately three mid WNs in the smallest (mass) cluster in Tol 89-A, A4, whose spectral energy output in the UV is entirely dominated by the WN stars. From the strength of nebular Hβ, we obtain N(O) ∼ 690 and 2800 for knots A and B, respectively, which implies N(WR)/N(O) ∼ 0.2 for knot A. We also employ a complementary approach using starburst99 models, in which the O star content is inferred from the stellar continuum, and the WR population is obtained from spectral synthesis of optical WR features using the grids from Smith et al. We find reasonable agreement between the two methods for the O star content and the N(WR)/N(O) ratio but find that the WR subtype distribution is in error in the starburst99 models, with far too few WN stars being predicted. We attribute this failure to the neglect of rotational mixing in evolutionary models. Our various modelling approaches allow us to measure the cluster masses. We identify A1 as a super star cluster (SSC) candidate with a mass of ∼1–2 × 105 M⊙. A total mass of ∼6 × 105 M⊙ is inferred for the ionizing sources within Tol 89-B.