Superbubble dynamics in globular cluster infancy

Abstract

The picture of the early evolution of globular clusters has been significantly revised in recent years. Current scenarios require at least two generations of stars of which the first generation (1G), and therefore also the protocluster cloud, has been much more massive than the currently predominating second generation (2G). Fast gas expulsion is thought to unbind the majority of the 1G stars. Gas expulsion is also mandatory to remove metal-enriched supernova ejecta, which are not found in the 2G stars. It has long been thought that the supernovae themselves are the agent of the gas expulsion, based on crude energetics arguments. Here, we assume that gas expulsion happens via the formation of a superbubble, and describe the kinematics by a thin-shell model. We find that supernovadriven shells are destroyed by the Rayleigh-Taylor instability before they reach escape speed for all but perhaps the least massive and most extended clusters. More power is required to expel the gas, which might plausibly be provided by a coherent onset of accretion onto the stellar remnants. The resulting kpc-sized bubbles might be observable in Faraday rotation maps with the planned Square Kilometre Array radio telescope against polarised background radio lobes if a globular cluster would happen to form in front of such a radio lobe.

Dynamical constraints on the origin of multiple stellar populations in globular clusters

“…The gas expulsion time-scales and the initial mass of the gas clouds the we obtain in this work, match results by Krause et al (2012Krause et al ( , 2013 very well. However it is not clear whether GCs can retain gas clouds with masses of order ∼ 10 6 M for ∼ 35 Myr.…”

“…In addition, Krause et al (2012) have shown that the Rayleigh-Taylor (Sharp 1984) instability destroys the huge gas shells (superbubbles) made by SN explosions before they build up enough speed to leave the cluster, thus such superbubbles are ineffective in expelling the gas even if they have enough energy. Instead Krause et al (2012) propose accretion onto dark remnants, such as neutron stars and black holes, as a promising mechanism which is capable to overcome the Rayleigh-Taylor instability and lead to rapid gas expulsion.…”

“…Instead Krause et al (2012) propose accretion onto dark remnants, such as neutron stars and black holes, as a promising mechanism which is capable to overcome the Rayleigh-Taylor instability and lead to rapid gas expulsion. According to their model, the dark remnants become active after the SN phase (t > 35 Myr) (Krause et al 2013) and are able to unbind the intra-cluster medium in 0.03 Myr to 0.06 Myr depending on if neutron stars also contribute to the gas expulsion (Krause et al 2012(Krause et al , 2013.…”

“…According to Krause et al (2012Krause et al ( , 2013, This model works for protoclusters whose masses are less than 2 × 10 7 M above which the gas cannot be ejected and will be retained in the cluster. This is intriguing, because firstly such a mass limit encompasses the majority of GCs except for very massive ones such as ω Cen, which is 10 times more massive, and secondly MSPs in such massive GCs have different iron abundances which could be a result of gas retention (Krause et al 2012).…”

“…This is intriguing, because firstly such a mass limit encompasses the majority of GCs except for very massive ones such as ω Cen, which is 10 times more massive, and secondly MSPs in such massive GCs have different iron abundances which could be a result of gas retention (Krause et al 2012). …”