Gas in Globular Clusters I: Gas Retention and Its Possible Consequences

TL;DR

This paper develops a cohesive, time-dependent framework for gas in young globular clusters, showing that low-velocity AGB winds can be gravitationally retained in clusters above a few times $10^5\,M_\odot$, with wind–wind collisions triggering efficient retention. The retained gas is rapidly altered by encounters, preventing new star formation and instead accreting onto pre-existing stars and compact objects via Bondi–Hoyle accretion, which can rejuvenate some stars and grow compact objects to larger masses. The evolving gaseous reservoir reaches a quasi-steady state but is ultimately cleared by feedback processes (accretion luminosity, novae, pulsar winds, SNe) on a timescale of about $1\ \text{Gyr}$, aligning with present-day observations of gas-poor globular clusters. The model links wind physics, cluster dynamics, and accretion-driven pathways to the two chemically distinct populations, and sets the stage for Paper II to quantify the detailed production and distribution of second-population stars. Overall, the work provides testable predictions about gas retention thresholds, central concentration of enriched stars, and the dynamical evolution of binaries and compact objects in massive clusters.

Abstract

Globular clusters host complex stellar populations whose chemical signatures suggest early (3 Myr - 1 Gyr) retention and reprocessing of stellar ejecta, yet direct evidence for intracluster gas is lacking. Here we present a unified theoretical framework for the evolution of gas in young globular clusters, and its implications for the production of multiple stellar populations. We show that low-velocity AGB winds are gravitationally retained in clusters more massive than a few 10^5 MSun. In addition, AGB winds in such clusters collide with each other and the previously retained winds, triggering a rapid `switch' to efficient gas retention. Expected gas retention fractions agree well with the observed second population fractions in Galactic globular clusters. Furthermore, the accumulated gas cannot form new stars because protostellar cores are disrupted by encounters with pre-existing stars. Instead, the gas is accreted onto pre-existing main-sequence stars and compact objects. Time-dependent core-halo models indicate that compact objects can grow and collapse within 100 Myr - 1 Gyr, while lower-mass main-sequence stars can be `rejuvenated' into the 4 - 6 MSun range required to reproduce key abundance patterns. Therefore, in our model, the multiple populations will be found in sufficiently massive clusters, with the second-population stars being formed from the inner subset of first-population stars that accreted large fractions of their mass from the AGB-processed retained gas. Finally, we argue that a combination of feedback processes, including accretion luminosity onto compact objects, novae, pulsar winds, and binary supernovae, will clear the gas by 1 Gyr, thus reproducing the gas-poor conditions observed for present-day clusters.

Figures (10)

• Figure 1: The cumulative amount of stellar mass lost through winds shown as a function of the wind mass loss rate $\dot{M}$, based on MIST stellar evolution tracks at $\text{[Fe/H]}=-1.5$. The cumulative mass loss is calculated by dividing the tracks into small time intervals and re-ordering these intervals in increasing order of $\dot{M}$. One can see that, according to the MIST stellar tracks, stars with initial masses above $2\,\text{M}_\odot$ lose most of their mass at rates higher than approximately $\dot{M}\gtrsim10^{-5}$ -- $10^{-4}\,\text{M}_\odot/\text{yr}$, corresponding to timescales of a few tens of kiloyears, which is relatively short compared to the final ages of the stars.
• Figure 2: The duration of the active mass-losing phase for stars at $\text{[Fe/H]}=-1.5$ shown as a function of stellar lifetime. This phase is defined as the time interval between the points (in Figure \ref{['fig:MDotSorted']}) when a star loses between $25\,\%$ and $75\,\%$ (dark blue line) or $10\,\%$ and $90\,\%$ (light blue line) of the total mass it is going to lose. Additionally, the green line in the plot shows the time required for the stellar winds to traverse the central $4\,\text{pc}$ region of the cluster. It can be concluded that, in the first $1\,\text{Gyr}$, the wind has a thick shell geometry relative to the cluster scale. In comparison, at later times, the wind outflow from a single star can engulf the entire cluster.
• Figure 3: The number of interacting stellar wind ejecta in a globular cluster with a Kroupa population of $N=10^6$ stars at a metallicity of $\text{[Fe/H]}=-1.5$, as a function of cluster age. The stellar winds of two stars are assumed to interact if either both stars are in the active mass-losing phase simultaneously or if the time for a wind ejecta shell to traverse the cluster is shorter than the time until the next star enters the active mass-losing phase. We can see that the number of interacting stellar wind ejecta in the globular cluster remains greater than one for most of the time until it reaches $10\,\text{Gyr}$. The collisions of stellar wind ejecta enhance gas retention in globular clusters.
• Figure 4: The fraction of stellar mass lost into winds that stays gravitationally bound to the host cluster as a function of enclosed mass within the cluster, normalised by the total mass. The figure is based on the Plummer model for $10^6\,{\rm M}_\odot$, $3 \times 10^5\,{\rm M}_\odot$ and $10^5\,{\rm M}_\odot$ clusters, shown by red, yellow and green lines, respectively. The cluster half mass radius is assumed to be $4\,{\text{pc}}$ and the wind speed is set to $20\,{\text{km}/\text{s}}$. More massive clusters of a given physical size have deeper potential wells, making it progressively easier for stellar winds to be retained within the cluster. At the cluster mass of $10^6\,{\text{M}}_\odot$, most of the stellar winds are gravitationally bound throughout the cluster.
• Figure 5: Gas retention fraction shown in the cluster initial mass -- half-mass radius plane. The yellow dotted lines are for gravitational gas retention fractions, $f_{\text{ret}}$, of $10\,\%$, $25\,\%$, $50\,\%$, $75\,\%$, and $90\,\%$ (from lower to upper lines) assuming a Plummer model for the cluster and all mass is lost via stellar winds having a speed of $20\,\text{km}/\text{s}$. The purple horizontal dot-dashed line represents the boundary above which multiple stars are likely to be undergoing high wind mass-loss rates at the same time, in which case collisions between winds will likely enable gas retention. Thus, gas will likely be retained in clusters above the horizontal dashed line or above the lowest of the dotted lines.