Feedback by massive stars and the emergence of superbubbles

Abstract

Context. In a previous paper we investigated the energy transfer of massive stars to the interstellar medium (ISM) as a function of time and the geometrical configuration of three massive stars via 3D-mesh-refining hydrodynamics simulations, following the complete evolution of the massive stars and their supernovae with the exception of non-thermal processes. Aims. To compare our results against observations we derive thermal X-ray properties of the ISM from our simulations and compare them to observations of superbubbles in general, to the well-studied nearby Orion-Eridanus superbubble and to the diffuse soft X-ray emission of nearby galaxies. Methods. We analysed our ISM simulation results with the help of spectra for plasma temperatures between 0.1 and 10 keV and computed the spectral evolution and the spatio-temporal distribution of the hot gas. Results. Despite significant input of high-temperature gas from supernovae and fast stellar winds, the resulting thermal X-ray spectra are generally very soft, with most of the emission well below 1 keV. We show that this is due to mixing triggered by resolved hydrodynamic instabilities. Supernovae enhance the X-ray luminosity of a superbubble by 1-2 orders of magnitude for a time span of about 0.1 Myr; which is longer if a supernova occurs in a larger superbubble and shorter in higher energy bands. Peak superbubble luminosities of the order of 10 36 erg s −1 are reproduced well. The strong decay of the X-ray luminosity is due to bubble expansion, hydrodynamic instabilities related to the acceleration of the superbubble's shell thanks to the sudden energy input, and subsequent mixing. We also find global oscillations of our simulated superbubbles, which produce spatial variations of the X-ray spectrum, similar to what we see in the Orion-Eridanus cavity. We calculated the fraction of energy emitted in X-rays and find that with a value of a few times 10 −4 , it is about a factor of ten below the measurements for nearby galaxies. Conclusions. Our models explain the observed soft spectra and peak X-ray luminosities of individual superbubbles. Each supernova event inside a superbubble produces a fairly similar heating-entrainment-cooling sequence, and the energy content of superbubbles is always determined by a specific fraction of the energy released by one supernova. For a given superbubble, soft X-rays trace the internal energy content well with moderate scatter. Some mechanism seems to delay the energy loss in real superbubbles compared to our simulations. Alternatively, some mechanism other than thermal emission of superbubbles may contribute to the soft X-ray luminosity of star-forming galaxies.

Wind bubbles within H ii regions around slowly moving stars

“…Similarly, the winds from main sequence stars in three-dimensional superbubble simulations by Krause et al (2014) are significantly stronger than in our models, and the X-ray emission they predict is consequently larger (L X ∼ 3 × 10 33 erg s −1 ). They found that only a few times 10…”

The26Al Gamma-ray Line from Massive-Star Regions

“…A large interstellar cavity, called Eridanus, extends away from these molecular clouds towards the Sun, and is constrained though X-ray [19] and HI data [20]. Our recent re-analysis of X-ray emission from this region and its comparison with 3D hydrodynamical simulations showed that energy injection and cooling of the superbubble interiors are not in a steady state [21].…”

Nonthermal particles and photons in starburst regions and superbubbles

“…The rate at which the gas is converted into stars is controlled locally by the radiation of young stars, turbulence, magnetic fields, and feedback by winds of massive stars and supernovae with the emergence of superbubbles though largescale galactic structures such as the spiral arms and the central bar (e.g., Price and Bate 2009, Elmegreen 2012, Krause et al 2014. The relative role and the links between the processes listed above are still not very clear, but the pressure is an important factor.…”

“…Evolution and X-ray spectra of three massive star system of 25, 32, and 60 M in a background matrix of 10 cm −3 density was studied by Krause et al (2013Krause et al ( , 2014 in 3D hydrodynamics with an account for optically thin radiative cooling and photo-electric gas heating. They found that the energy injected by supernovae is entirely dissipated in a superbubble on the timescale of about 1 Myr, and that the deposition from the stars to the superbubble did not vary substantially if the massive stars were localized within a distance of < ∼ 30 pc.…”