Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old ‘globular’ clusters—those with ages greater than ten billion years and masses several hundred thousand times that of the Sun—often harbour multiple stellar populations1,2,3,4, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars5,6,7 are often suggested to be triggers of second-generation star formation. For this to occur8, the initial cluster masses need to be greater than a few million solar masses. Here we report observations of three massive relatively young star clusters (1–2 billion years old) in the Magellanic Clouds that show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could have accreted sufficient gas to form new stars if they had orbited in their host galaxies’ gaseous disks throughout the period between their initial formation and the more recent bursts of star formation. This process may eventually give rise to the ubiquitous multiple stellar populations in globular clusters.
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We thank F. Ferraro, P. Kroupa and X. K. Liu for discussions. Partial financial support for this work was provided by the National Natural Science Foundation of China through grants 11073001, 11373010 and 11473037. C.L. was also partially supported by the Chinese Academy of Sciences (grant XDB09000000) and the 973 Program (grant 2014CBB45700). A.M.G. was funded by a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award no. AST-1302765. C.-A.F.-G was supported by a National Science Foundation grant AST-1412836.
The authors declare no competing financial interests.
Extended data figures and tables
Here ρ is the number density of stars at a given radius R. a, NGC 1783. The vertical solid line indicates the cluster’s core radius, that is, the position where the number density decreases to half the central value. The ±1σ uncertainties shown are due to Poisson noise. b, c, As a, but for NGC 1696 (b) and NGC 411 (c).
Red and blue points represent cluster stars and the adopted field stars, respectively. The black dots are all observed stars except for the cluster and field stars. RA, right ascension; dec., declination. a, NGC 1783; b, NGC 1696; and c, NGC 411.
Extended Data Figure 3 Field-star-decontaminated colour–magnitude diagrams for samples of stars at different radii in NGC 1783.
a, R ≥ 30″; b, R ≥ 60″; and c, R ≥ 90″.
Extended Data Figure 4 Field-star-decontaminated colour–magnitude diagrams of NGC 1783 for three different adopted reference fields.
a, b, Resulting colour–magnitude diagram (a) based on the field-star sample drawn from the image containing the cluster (b). c, d, As a and b, but for a representative field region taken from a separate image. e, f, As c and d, but for a different field region taken from the same, separate image. The black points are stars which are located in the cluster region, whereas the red points are the adopted field stars.
Extended Data Figure 5 Field-star-decontaminated colour–magnitude diagrams of NGC 1783 for three different grid sizes.
a, 0.30 mag × 0.15 mag; b, 0.40 mag × 0.20 mag; and c, 0.50 mag × 0.25 mag.
a–d, NGC 1783; e–h, NGC 1696; i–l, NGC411. First column (a, e, i), raw colour–magnitude diagrams; second column (b, f, j), field-decontaminated colour–magnitude diagrams (also shown in Fig. 1); third column (c, g, k), colour–magnitude diagrams of the representative field regions; and fourth column (d, h, l), stellar colour–magnitude distributions that were removed from the raw catalogues.
a, Purple and dark green squares, stars in NGC 1783 sequences A and B, respectively. Dark green circles, corresponding red-giant-branch and red-clump stars, used for comparison with sequence B. The combination of dark green and purple circles represents the sample used for comparison with sequence A. b, Purple squares, NGC 1696 young-sequence stars; red circles, corresponding red-giant-branch and red-clump stars used for comparison. c, As b, but for NGC 411.
V is the relative velocity of the cluster with respect to the gas, and n represents the gas density. The shaded regions indicate the parameter space where an NGC 1783-like cluster can accrete the required mass to form the two additional generations of stars, namely one of 250M⊙ over 520 Myr (blue, corresponding to sequence B in Fig. 1), and a second of 370M⊙ over 440 Myr (green, corresponding to sequence A in Fig. 1). We have assumed a star-formation efficiency of 10% for all calculations in this figure. The regions to the right of the ‘Gravity’ curves correspond to where Bondi accretion can accumulate at least the required mass, and the regions to the right of the ‘Sweep’ curves correspond to where accretion by collisional sweeping up of ambient interstellar gas by seed intracluster gas can accumulate at least the required mass. The parameter space above the ‘Ram’ curves are excluded because ram pressure strips clusters of their gas in those regions. See Methods for details.
We have adopted a star-formation efficiency of 10% and calculated representative interstellar gas-accretion frameworks that can explain the stellar masses in the secondary sequences A and B in NGC 1783. For the Bondi regime (solid lines), we used a relative velocity of 4 km s–1 and a density of 0.3 cm–3. For the sweeping regime (dashed lines), we used a velocity of 50 km s–1 and a density of 0.05 cm–3. The blue and green filled regions indicate the stellar masses and age offsets of NGC1783 sequences A and B, respectively. The vertical extent of each region provides an estimate of the range in allowed masses. Specifically, for each sequence we plot a region centred on the mass derived using the Kroupa initial mass function, as given in the main text; the range to higher and lower masses is equal to the mean of the differences between the total masses derived using a Salpeter and Kroupa initial mass function of 180M⊙ (multiplied by the assumed 10% star formation efficiency).
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Li, C., de Grijs, R., Deng, L. et al. Formation of new stellar populations from gas accreted by massive young star clusters. Nature 529, 502–504 (2016). https://doi.org/10.1038/nature16493