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

Authors: Catherine Zucker, Alyssa A. Goodman, João Alves, Shmuel Bialy, Michael Foley, Joshua S. Speagle, Josefa Groβschedl, Douglas P. Finkbeiner, Andreas Burkert, Diana Khimey, Cameren Swiggum

First Author’s Institutions: Center for Astrophysics, Harvard & Smithsonian; Space Telescope Science Institute

Status:  Published in Nature [closed access]

Astronomers have known for some time that we live in a bubble–the Local Bubble.  To be precise, we are passing through this big bubble on our grand orbital tour of the Milky Way.  The Local Bubble can be described as a low-density ‘cavity’ dug out of the local interstellar medium.  We have suspected for some time that the Local Bubble was carved out by a series of supernovae about 14 million years ago, but its precise size, cause, and effect on our nearby stellar neighbors has been difficult to determine.  Today’s authors use the power of Gaia, alongside careful analysis of 3-dimensional maps of nearby interstellar dust, to model the surface of this Bubble.  Astonishingly, they find strong evidence that the expansion of the Local Bubble is directly responsible for almost all star formation within 200 pc of the Sun since the Bubble was first formed.  Let’s dive into some of the details of their work and its exciting implications for star formation.

Constraining the Size of the Bubble

The authors first set off to provide new estimates for the size and dynamical state of the Local Bubble.  To understand the size of the Bubble, we need to understand the three dimensional space it occupies.  However, studying interstellar gas in three dimensions is notoriously difficult!  While we can use spectroscopy to determine the radial velocity of the gas and imaging to observe its projected spatial distribution, we can not trace its proper motion (apparent transverse velocity) like we can stars’.  And we need this proper motion to get the full picture of the gas’ three dimensional position and velocity distribution.  To address this, today’s authors turn to young stellar groups.  Young stellar groups are effective tracers of the position and velocity of their birth cloud because they adopt these properties from their parent cloud.  In other words, we can look at recently born stars to get a general picture of the state of the gas from which they were born.

The authors selected 34 nearby (within 400 parsecs of the Sun) stellar clusters with ages less than 20 million years for this analysis.  The 20 million year age cut off coincides with previous estimates of the Local Bubble’s age, as stars older than this would not have been affected by the Local Bubble.  Leveraging the power of Gaia Early Third Data Release (eDR3), the authors map out the three dimensional positions and velocities of these young stellar associations and use that data to trace their trajectories through time using the galpy Python package.

The Local Bubble is Responsible for Much of Nearby, Recent Star Formation

Astonishingly, the authors find that almost all star-forming clouds within 200 pc, as traced by the youngest stellar groups, line the surface of the Bubble (Figure 1).  Furthermore, these young stellar groups are expanding outward, clearly reacting to a centralized event.  By tracing the trajectories of the stellar groups backwards in time, the authors are able to constrain the epicenter of the explosion.  This is clear, observational evidence that 1) a series of centralized explosive events drove the creation and expansion of the Bubble, and 2) that the Local Bubble is directly responsible for driving nearly all recent, nearby star formation!  In their analysis, the authors find that just one star-forming complex, the Perseus Molecular Cloud, does not line the surface of the Bubble and instead lies about 100 pc beyond its surface.  Fascinatingly, this can be explained by the fact that the Perseus Cloud was displaced from the surface of the Local Bubble by its own mini-bubble (see this bite for more!).

What Set Off the Bubble?

Given these observational constraints on the size and expansion of the Local Bubble, the authors set out to calculate what sort of event could have carved out such a huge chunk of the local interstellar medium.  Many works have suggested that supernovae from two stellar populations near the center of the Bubble, Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (UCC), were likely the source of the Bubble’s expansion.  The observational constraints provided by today’s work support this, and the authors estimate that it is likely that about 14 supernovae went off at the center of the Bubble, with one million years between each supernova explosion.

This is a major result because it validates the theory of supernova-triggered star formation.  When a supernova explodes, it violently expels surrounding gas outward.  A shock front forms at the interface of the expansion, triggering the star formation process.  This work provides important observational constraints on this process, ones that we can reference and apply when studying other star forming regions, both within and outside the Milky Way.  Understanding the star formation process is central to unveiling the mechanisms that build up much of our visible Universe.  With these new results, we are led to ask—how common is supernova-triggered star formation, and what role has it played in building up the galaxies we observe today?

Fun fact—works estimate that we entered the Local Bubble about 5 million years ago on our orbit around the Galactic center.  About 2.6 million years ago, one of the many supernovae powering the Bubble’s expansion went off.  This aligns with the Pliocene marine megafauna extinction, a mass extinction event where 36% of the Earth’s marine megafauna disappeared.  Scientists wonder if radiation from the supernova(e) could have played an important role in this extinction event.

Astrobites Edited By: Luna Zagorac

Featured Image Credit: Catherine Zucker

• About the Author

About Catherine Manea

Catherine is a 3rd year PhD student at the University of Texas at Austin. Her research is in Galactic Archaeology, the practice of using the dynamical and chemical information of individual stars to study the evolution of our Milky Way. She is particularly interested in pushing chemical tagging, the practice of tracing stars back to their birth sites, to new limits.

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