Anomalies among the daughter nuclei of the extinct short-lived radionuclides in calcium–aluminium-rich inclusions indicate that the Solar System must have been born near a source of the short-lived radionuclides so that they could be incorporated before they decayed away1. γ-rays from one such living short-lived radionuclide, 26Al, are detected in only a few nearby star-forming regions. Here we employ multiwavelength observations to demonstrate that one such region, Ophiuchus, containing many prestellar cores that may serve as analogues for the emerging Solar System2, is inundated with 26Al from the neighbouring Upper Scorpius association3, and so may provide concrete guidance for how short-lived radionuclide enrichment proceeded in the Solar System, complementary to the meteoritics. We demonstrate via Bayesian forward modelling drawing on a wide range of observational and theoretical results that this 26Al probably (1) arises from supernova explosions, (2) arises from multiple stars, (3) has enriched the gas before the formation of the cores and (4) gives rise to a broad distribution of core enrichment spanning about two orders of magnitude. This means that if the spread in calcium–aluminium-rich inclusion ages is small, as it is in the Solar System, protoplanetary disks must suffer a global heating event.
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Observational data underlying Fig. 1 and Extended Data Fig. 1 will be made available upon reasonable request from J.A., with the γ-ray data subject to approval by R. Diehl. Posterior samples and weights obtained in our inference problem are available as a pickle file at https://github.com/jcforbes/ophiuchus-al26.
Code to create the plots and generate posterior samples is available at https://github.com/jcforbes/ophiuchus-al26.
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J.A. acknowledges support from the University of Vienna and the Radcliffe Institute. J.C.F. acknowledges funding from an ITC Fellowship and a Flatiron Research Fellowship. D.N.C.L. thanks the Institute for Theory and Computation, Harvard University, for support while this work was initiated. We thank S. Woosley, R. Diehl, S. Portegies Zwart, D. Foreman-Mackey, C. Zucker and F. Bartolić for useful conversations and R. Diehl for providing the COMPTEL γ-ray map.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Martin Krause and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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The images are color composites from the VISIONS ESO public survey, where blue, green and red are mapped to the NIR bands J (1.2 μm), H (1.6 μm), and Ks (2.2 μm) respectively. In all panels the scale bar assumes a distance of 140 pc from the Sun.
The supernova prior, based on supernova explosion and yield calculations14. In the upper panel we show, as points near one or zero respectively, which models exploded or did not under different explosion mechanisms. We then fit a probability of explosion as a function of mass to these results (red and blue lines). The light grey lines show interpolation between (and extrapolation slightly beyond) these two scenarios, each of which represents our prior on whether a supernova of a given mass will actually explode conditioned on ζ. The bottom panel shows the 26Al and 60Fe yields from these same calculations. We adopt the linear interpolation shown in the Figure of the N20 yields (the blue line) as the yields given an explosion has occurred. Between 8 and 12 M⊙ we assume all stars explode and use the Z9.6 yields.
Following previous work15 we show the total lifetime production of 26Al from various yield calculations as a function of a star’s mass from Langer63, Palacios64, Limongi & Chieffi39, and Ekström65. These are powerlaw fits to the actual simulation results. We then characterize these powerlaws by two numbers: the yield at 20 M⊙ and the yield at 120 M⊙, which we call respecitvely A20 and A120. Each study’s result is plotted as a point in the lower panel. To assign a joint prior to the yields at these two masses, we fit a 2D gaussian to these points, essentially taking this collection of models to be a reasonable characterization of the possible values of A20 and A120.
The opacity shows the marginal probability of each age, and at a given age, the fraction of the graph covered by each shaded region indicates the probability of that scenario. For all plausible values of the age of Upper-Sco, the living 26Al is likely to come from just a small handful of individual massive stars, but likely more than one.
The present-day observed value3 is shown as a point with errorbars representing the standard error on the mean on the right side of the plot. Each line is a different posterior realization of Upper-Sco, with transparency proportional to its weight. The colors show different scenarios. Blue lines have their 26Al today dominated by Wolf–Rayet stars, and red lines are dominated by Supernovae. The characteristic sawtooth pattern of the lines comes about as individual sources of 26Al produce the radioactive isotope in a relatively short period of time (or instantaneously for supernovae), followed by exponential decay, as expected for young star-forming regions22.
The blue shaded region shows the contribution from Wolf Rayet winds, and the red shaded region shows the contribution from supernovae. The thick lines show the median, and the regions show the central 68% of the posterior contribution from each source at that time. Upper-Sco itself is at an age where the dominant contribution is shifting from Wolf Rayet winds to supernovae. The two conspire to generate a roughly constant quantity of living 26Al as a function of age in an average sense, though individual realizations of the cluster oscillate substantially as individual sources produce their 26Al, which then decays away until the next 26Al-producing event (see Extended Data Fig. 5).
The data for 21 stars are shown in blue and red (the 3D distances and their on-sky component at the fiducial distance of L1688, respectively). In light blue and light red, we show the maximum a posteriori estimate for a single-component 3D gaussian model taking into account distance uncertainties on the individual massive stars.
The shaded area shows the central 68% of the posterior distribution at that time (that is this distribution is a prediction for Upper Sco in particular). 60Fe peaks later than 26Al both because of its longer half-life and the lack of an early contribution from Wolf–Rayet winds.
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Forbes, J.C., Alves, J. & Lin, D.N.C. A Solar System formation analogue in the Ophiuchus star-forming complex. Nat Astron 5, 1009–1016 (2021). https://doi.org/10.1038/s41550-021-01442-9
Nature Communications (2022)