Superbubble dynamics in globular cluster infancy

Abstract

Context. The self-enrichment scenario for globular clusters (GC) requires large amounts of residual gas after the initial formation of the first stellar generation. Recently, we found that supernovae may not be able to expel that gas, as required to explain their presentday gas-free state, and suggested that a sudden accretion onto the dark remnants at a stage when type II supernovae have ceased may plausibly lead to fast gas expulsion. Aims. Here, we explore the consequences of these results for the self-enrichment scenario via fast-rotating massive stars (FRMS). Methods. We analysed the interaction of FRMS with the intra-cluster medium (ICM), in particular where, when, and how the second generation of stars may form. From the results, we developed a timeline of the first ≈40 Myr of GC evolution. Results. Our previous results imply three phases during which the ICM is in a fundamentally different state, namely the wind bubble phase (lasting 3.5 to 8.8 Myr), the supernova phase (lasting 26.2 to 31.5 Myr), and the dark remnant accretion phase (lasting 0.1 to 4 Myr): (i) Quickly after the first-generation massive stars have formed, stellar wind bubbles compress the ICM into thin filaments. No stars may form in the normal way during this phase because of the high Lyman-Werner flux density. If the first-generation massive stars have equatorial ejections however, as we proposed in the FRMS scenario, accretion may resume in the shadow of the equatorial ejecta. The second-generation stars may then form due to gravitational instability in these disc, which are fed by both the FRMS ejecta and pristine gas. (ii) In the supernova phase the ICM develops strong turbulence, with characteristic velocities below the escape velocity. The gas does not accrete either onto the stars or onto the dark remnants in this phase because of the high gas velocities. The strong mass loss associated with the transformation of the FRMS into dark remnants then leads to the removal of the secondgeneration stars from the immediate vicinity of the dark remnants. (iii) When the supernovae have ceased, turbulence quickly decays, and the gas can once more accrete, now onto the dark remnants. As discussed previously, this may release sufficient energy to unbind the gas, and may happen fast enough so that a large fraction of less tightly bound first-generation stars are lost. Conclusions. Studying the FRMS scenario for the self-enrichment of GCs in detail reveals the important role of the physics of the ICM for our understanding of the formation and early evolution of GCs. Depending on the level of mass segregation, this sets constraints on the orbital properties of the stars, in particular high orbital eccentricities, which likely has implications on the GC formation scenario.

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.…”

“…Observations of young massive star clusters by Bastian, Hollyhead & Cabrera-Ziri (2014) and Hollyhead et al (2015) show that they have cleared out their natal gas within a few Myrs. If it was the case for GCs as well, then it poses a serious challenge for the scenario proposed by Krause et al (2012Krause et al ( , 2013 and would imply that either stellar winds and supernova explosions do have to expel the gas or that the gas is completely consumed into stars after a few Myr in which case the scenario suggested here would not work.…”

“…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).…”

The morphology of the sub-giant branch and red clump reveal no sign of age spreads in intermediate-age clusters

“…Longmore et al 2014). Globular clusters, on the other hand, host chemical abundance anomalies within their stellar populations (e.g., Gratton et al 2012), and while age spreads cannot be discerned at these old ages, some scenarios to explain the anomalies invoke multiple star-forming epochs, spanning tens to hundreds of Myr (e.g., D'Ercole et al 2008;Conroy & Spergel 2011), although other scenarios have little or no age spreads (e.g., Krause et al 2013;Bastian et al 2013b). …”