Paper Title: Failed supernova explosions increase the duration of star formation in globular clusters
Authors: Henriette Wirth, Jaroslav Haas, Ladislav Šubr, Tereza Jerabkova, Zhiqiang Yan, and Pavel Kroupa
First-author institution: Charles University
Status: Accepted by A&A
Globular clusters have long served as important laboratories for studying stellar populations. In particular, the isolation of the stars in a given cluster allows astronomers to study star formation (SF) with minimal concern of external influence. The prototypical model of globular cluster formation involves rapid creation of a population of stars (at relatively the same age) from a dense core of gas. The remaining gas is eventually blown out of the region, quenching further star formation and leaving a relatively homogenous, singular stellar population. Since its launch, improved data from the Hubble Space Telescope have shown that the single stellar population is an imperfect model, and that the majority of globular clusters realistically contain two or more distinct populations. The specific mechanism(s) of multiple stellar population (MSP) formation, however, are still under investigation. Today’s authors use new understandings of supernovae frequency to probe a possible explanation.
Iron Measures SF Duration.
Because globular clusters are isolated, the amount of time during which stars are able to actively form (before the remaining gas is expelled from the system) ought to be finite. The exact duration of SF is an important clue to help astronomers better understand the MSP question. This proves tricky, of course, because globular cluster stars first formed billions of years ago. To estimate the SF duration, today’s authors make use of the present-day observed iron abundance in the stellar populations of 55 nearby globular clusters.
Iron is primarily formed in the cores of very massive stars. These stars, at greater than 8 times the mass of the sun, are massive enough to undergo core-collapse supernova (CCSNe) explosions at the end of their lives. Such very massive stars are also the ones that evolve and die most quickly–in less than 10 million years (as opposed to our Sun’s lifetime of 10 billion years)! As they are dying, they are able to enrich the surrounding gas in the cluster with iron–which then forms into marginally younger, more iron-rich stars.
Because cluster stars are formed in environments at various stages of enrichment, the abundance of iron observed in the present-day stellar population is varied, and this spread is primarily a function of:
- How much iron was in the gas cloud prior to any star formation, measured as the minimum stellar iron abundance observed.
- The number of core-collapse supernova explosions that occurred during star formation. This is largely determined by the range of iron abundances, and can be related to the stellar initial mass function (the distribution of star masses at the time the stars initially formed, IMF) and the all-important SF duration.
Initial Mass Functions and Failed Supernovae
Whether or not a star goes supernova at the end of its life is primarily dependent on how massive it is. Generally, stars greater than 8 times the mass of the sun are expected to be able to experience CCSNe. In the context of globular cluster formation, however, the least massive of these will not reach the end of their lives by the time SF ceases. The authors estimate how many explosions must have occurred during SF, in order to achieve the observed iron abundance. They do so by assuming each CCSNe injects a set amount of iron into the surrounding gas. Because the most massive stars reach the ends of their lives first, the authors are also able to use a number of IMF models to estimate the mass of the last star to supernova during SF (see fig. 1). The age of this final CCSNe, then, corresponds to the total length of star formation in the cluster.
Such methodology, however, assumes that every star greater than 8 solar masses will eventually supernova. Recent studies show that some stars with masses greater than 20 times that of the sun will fail to explode as expected. If these failed supernova explosions are common enough, star formation in the globular cluster must go on for longer than expected, in order to obtain the same maximum iron enrichment. To address this, today’s authors conduct the same modeling, but assume that no star greater than 20 solar masses goes supernova. In this scenario, the stars which contribute to the iron abundance are significantly less massive, and therefore live for longer, increasing the duration of SF necessary to be consistent with observations.
A Broad Range of SF Durations
The two scenarios presented–one where all stars greater than 8 solar masses supernova, and one where only those between 8-20 solar masses supernova–represent the most extreme cases for the influence (or lack thereof) of failed CCSNe on globular cluster star formation histories. With this methodology, today’s authors are able to establish bounds on the expected durations of star formation for 55 nearby globular clusters. They find that, on average, a globular cluster with absolutely no failed CCSNe will experience SF for around 3.5 million years, while the maximum possible number of failed CCSNe will experience SF for around 10.5 million years (see Fig. 2) . They conclude that this difference, nearly a factor of three, is enough that contamination from sources like binary stellar evolution and stellar winds may have enough time to create multiple stellar populations.
Astrobite edited by: Maria Vincent
Featured image credit: NASA, ESA, STScI and A. Sarajedini (University of Florida)