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Star formation near the Sun is driven by expansion of the Local Bubble

Abstract

For decades we have known that the Sun lies within the Local Bubble, a cavity of low-density, high-temperature plasma surrounded by a shell of cold, neutral gas and dust1,2,3. However, the precise shape and extent of this shell4,5, the impetus and timescale for its formation6,7, and its relationship to nearby star formation8 have remained uncertain, largely due to low-resolution models of the local interstellar medium. Here we report an analysis of the three-dimensional positions, shapes and motions of dense gas and young stars within 200 pc of the Sun, using new spatial9,10,11 and dynamical constraints12. We find that nearly all of the star-forming complexes in the solar vicinity lie on the surface of the Local Bubble and that their young stars show outward expansion mainly perpendicular to the bubble’s surface. Tracebacks of these young stars’ motions support a picture in which the origin of the Local Bubble was a burst of stellar birth and then death (supernovae) taking place near the bubble’s centre beginning approximately 14 Myr ago. The expansion of the Local Bubble created by the supernovae swept up the ambient interstellar medium into an extended shell that has now fragmented and collapsed into the most prominent nearby molecular clouds, in turn providing robust observational support for the theory of supernova-driven star formation.

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Fig. 1: A 3D spatial view of the solar neighbourhood.
Fig. 2: The evolution of the Local Bubble and sequential star formation at the surface of its expanding shell.

Data availability

The datasets generated and/or analysed during the current study are publicly available on the Harvard Dataverse (https://dataverse.harvard.edu/dataverse/local_bubble_star_formation/), including Extended Data Table 1 (https://doi.org/10.7910/DVN/ZU97QD), Extended Data Table 2 (https://doi.org/10.7910/DVN/1VT8BC), per-star data for individual stellar cluster members (https://doi.org/10.7910/DVN/1UPMDX) and the cluster tracebacks (https://doi.org/10.7910/DVN/E8PQOD).

Code availability

The results generated in this work are based on publicly available software packages and do not involve the extensive use of custom code. Given each star’s reported Gaia data, we use the astropy38 package to obtain the Heliocentric Galactic Cartesian positions and velocities. The extreme deconvolution algorithm in the astroML51 package is used to estimate the mean 3D positions and velocities of the stellar clusters. The Orbit functionality in the galpy40 package is used to perform the dynamical tracebacks. The dynesty43 package is used to fit the analytic superbubble expansion model and determine the best-fit parameters governing the Local Bubble’s evolution.

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Acknowledgements

The visualization, exploration and interpretation of data presented in this work were made possible using the glue visualization software, supported under NSF grant numbers OAC-1739657 and CDS&E:AAG-1908419. The interactive figures were made possible by the plot.ly python library. D.P.F. acknowledges support by NSF grant AST-1614941 ‘Exploring the Galaxy: 3-Dimensional Structure and Stellar Streams’. D.P.F., A.A.G. and C.Z. acknowledge support by NASA ADAP grant 80NSSC21K0634 ‘Knitting Together the Milky Way: An Integrated Model of the Galaxy’s Stars, Gas, and Dust’. A.B. acknowledges support by the Excellence Cluster ORIGINS, which is funded by the German Research Foundation (DFG) under Germany’s Excellence Strategy -EXC-2094-390783311. J.A. acknowledges support from the Data Science Research Centre and the TURIS Research Platform of the University of Vienna. J.G. acknowledges funding by the Austrian Research Promotion Agency (FFG) under project number 873708. C.Z. acknowledges that support for this work was provided by NASA through the NASA Hubble Fellowship grant number HST-HF2-51498.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. C.Z., A.A.G., J.A. and S.B. acknowledge Interstellar Institute’s program ‘The Grand Cascade’ and the Paris-Saclay University’s Institut Pascal for hosting discussions that encouraged the development of the ideas behind this work.

Author information

Authors and Affiliations

Authors

Contributions

C.Z. led the work and wrote the majority of the text. All authors contributed to the text. C.Z., A.A.G. and J.A. led interpretation of the observational results, aided by S.B., M.F. and A.B. who helped interpret their significance in light of theoretical models for supernova-driven star formation. C.Z. and A.A.G. led the visualization efforts. J.S.S. and D.P.F. helped shape the statistical modelling of the Local Bubble’s expansion. C.Z., A.A.G. and J.S.S. contributed to the software used in this work. J.G. and C.S. provided data for and the subsequent interpretation of the 3D kinematics of the Orion region. D.K. helped to develop the code used to model the 3D positions and motions of stellar clusters described in the Methods.

Corresponding author

Correspondence to Catherine Zucker.

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The authors declare no competing interests.

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Nature thanks Joanne Dawson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 1D and 2D marginal distributions (“corner plot”) of the model parameters governing the evolution of the Local Bubble’s expanding shell.

Parameters include the time since the first explosion (i.e. the age of the Local Bubble), texp, the density of the ambient medium the bubble is expanding into, n0, the time between supernova explosions powering its growth, 𝞓tSNe, and the thickness/uncertainty on the expanding shell radius 𝞓R. In the 1D distributions, the vertical dashed lines denote the median and 1𝝈 errors, while in the 2D distributions, we show the 0.5𝝈, 1𝝈, 1.5𝝈, and 2𝝈 contours.

Extended Data Fig. 2 Temporal evolution of the Local Bubble, based on the fit to the dynamical tracebacks and the analytic expansion model22 summarized in the Methods section.

Panel a) The evolution of the Local Bubble’s expansion velocity vexp. Panel b) The evolution of the Local Bubble’s shell radius Rshell. Panel c) The evolution of the average momentum injection per supernova . The thick purple line represents the median fit, while the thin purple lines represent random samples. We estimate a current radius of \(165\pm 6\) pc and current expansion velocity of \(6\,.7{\,}_{-0.4}^{+0.5}\) km/s, corresponding to time t=0 Myr (the present day).

Extended Data Fig. 3 PDF of the estimate of the number of supernovae required to power the Local Bubble’s expansion.

The estimate is obtained by comparing the shell’s present-day momentum to the average momentum injected by supernovae.

Extended Data Fig. 4 Analysis of the stellar tracebacks of the UCL and LCC clusters, whose progenitors were likely responsible for the supernovae that created the Local Bubble.

The scatter points indicate the positions of the current cluster members of UCL and LCC, which are colored as a function of time (spanning the present day in pink to 30 Myr ago in black). Panel a: Using Hipparcos data and adopting a solar peculiar motion (U, V, W) = (10.0, 5.2, 7.2) km/s46, previous literature6,7 find that UCL and LCC were born outside the Local Bubble (black trace4) 15 Myr ago and only entered its present-day boundary in the past 5 Myr (reproduced from Fig. 6 in ref. 6). Panel b: We attempt to reproduce the results from previous literature6,7 using the same data and solar motion, but are unable to account for the curvature of the tracebacks, finding the UCL and LCC formed just inside its northern boundary 15 Myr ago. Panel c: Using a different value for the solar motion, (U, V, W) = (10.0, 15.4, 7.8) km/s41 but the same Hipparcos data, we find that UCL and LCC were born near the center of the Local Bubble. Panel d: Finally, using updated Gaia data but the same adopted solar motion used in panel c. (U, V, W) = (10.0, 15.4, 7.8) km/s41, we also find that UCL and LCC were born near the center of the bubble, given an updated model for its surface13.

Extended Data Table 1 Summary of the 3D positions and 3D velocities of young stellar clusters within 400 pc of the Sun
Extended Data Table 2 Temporal evolution of cluster births at the surface of the Local Bubble’s expanding shell

Supplementary information

Peer Review File

Supplementary Figure 1

Interactive 3D visualization of dense gas and young stars on the Local Bubble’s surface. This figure is the interactive 3D counterpart to Fig. 1. The figure supports interactive panning, zooming and rotation. Individual data layers can be toggled on/off by clicking on the layer in the legend on the right-hand side of the figure. The surface of the Local Bubble13 is shown in purple. The short squiggly coloured lines (or ‘skeletons’) demarcate the 3D spatial morphology of dense gas in prominent nearby molecular clouds11. The 3D cones indicate the positions of young stellar clusters, with the apex of the cone pointing in the direction of stellar motion. The Sun is marked with a yellow cross. We also overlay the morphology of the 3D dust (grey blobby shapes9) and the models for two Galactic scale features—the Radcliffe Wave (red)16 and the Split (blue)10. The Per-Tau Superbubble15 (green sphere) is also overlaid.

Supplementary Figure 2

Interactive 3D visualization of the Local Bubble’s expansion. This figure is the interactive 3D counterpart to Fig. 2. The figure supports interactive panning, zooming and rotation. Individual data layers can be toggled on/off by clicking on the layer in the legend on the right-hand side of the figure. Stellar cluster tracebacks are shown with the coloured paths. Before the cluster birth, the tracebacks are shown as semi-transparent circles meant to guide the eye, since our modelling is insensitive to the dynamics of the gas before its conversion into stars. After the cluster birth, the tracebacks are shown with filled circles and terminate in a large dot, which marks the cluster’s current position. For time snapshots ≤14 Myr ago, we overlay a model for the evolution of the Local Bubble (purple sphere), as derived in the Methods. Click ‘Play Forward’ to see the Local Bubble evolve starting 17 Myr ago and progressing forwards to the present day. Click ‘Play Backward’ to see the evolution in reverse. Click ‘Pause’ to stop the animation. Alternatively, drag the time slider back and forth to view the Local Bubble’s expansion at any time. To jump to epochs of particular interest, click on any of the ‘action’ buttons (for example, ‘UCL Born’) on the right-hand side of the figure.

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Zucker, C., Goodman, A.A., Alves, J. et al. Star formation near the Sun is driven by expansion of the Local Bubble. Nature 601, 334–337 (2022). https://doi.org/10.1038/s41586-021-04286-5

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