The Secret Solar Neighborhood Superbubble

Title: The Per-Tau Shell: A Giant Star-Forming Spherical Shell Revealed by 3D Dust Observations

Authors:  Shmuel Bialy, Catherine Zucker, Alyssa Goodman, Michael M. Foley, João Alves, Vadim A. Semenov, Robert Benjamin, Reimar Leike, Torsten Enßlin

First Author’s Institution: Center for Astrophysics, Harvard & Smithsonian, 60 Garden st., Cambridge, MA, 02138

Status: Published in ApJ Letters [open access]

Molecular clouds are the beautiful, complex formations of dense gas from which stars form. They represent an important phase of the baryon cycle, in which gas condenses to form stars, the most massive of which shed winds and explode as supernovae to release their material back into space, eventually condensing down to form more clouds. Understanding the structure of these clouds and the way they form is extremely important for constraining the details of this vitally important cycle. Unfortunately, it can be hard to measure their properties accurately when all we get is a 2 dimensional projection of the clouds onto the plane of the sky. 

Fig 1. The well-studied molecular clouds named Taurus and Perseus lie next to each other on the plane of the sky, but that doesn’t mean they are directly connected. The right panel shows the clouds highlighted in color against a greyscale dust map. The right panel shows the line-of-sight distance to different high density components of this part of the sky. It’s clear that the Perseus cloud lies about two hundred parsecs farther from us than Taurus, and the apparent bridge between them is an illusion of line-of-sight projection. 

The Perseus and Taurus molecular clouds are an excellent example: two molecular clouds appear nearby each other on the plane of the sky, although estimates of their distances suggest that they’re actually separated by over two hundred parsecs. While it’s been suggested that their overlap is no coincidence, and that they might be part of the same larger structure, it’s impossible to prove anything of the sort without having a detailed idea of the 3D structure of the clouds and their surroundings. Until recently, that kind of high-accuracy mapping was not feasible, but the authors of today’s paper use a sophisticated 3D modeling technique hinging on data from Gaia to reveal the hidden structure of these cloud complexes and the explosive secret of their origin. 

The technique used by the authors combines two different types of measurements: highly accurate distances to stars and detailed measurements of the column density of gas between us and those stars. One of our best ways of measuring a distance to a star is measuring parallax. If you can carefully measure the motion of a star against the background of more distant objects behind it, you can calculate the distance to that star. The accuracy of that distance is limited mainly by how carefully you can measure the shift of the star against the background. Recent results from Gaia have revolutionized both the quantity and accuracy of parallax measurements for stars in the Milky Way

Fig 2. Two views of the newly identified Per-Tau shell, a supernova remnant originating 6-22 million years ago. The left panel shows the region from our perspective from the Solar system, and the left panel shows a side-on view which better reveals the 3D structure of the bubble. The well-studied molecular clouds Perseus and Taurus appear nearby from the Sun’s perspective, but the 3D map produced by the authors shows that these clouds are really on opposite sides of the supershell. 

The column density can be measured by modeling the photometry of these stars: if you measure the light is being emitted by the star, and you know what you would expect from that type of star in the absence of dust, you can determine the amount of dust in the “column” along line-of sight that is blocking the light. Then, by making some assumptions about the relationship between gas and dust, you can use those dust extinction measurements to calculate the column density of gas between you and the star. But one of the most powerful features of Gaia is the staggering number of sources, and that’s what makes this 3D mapping feasible. With a large enough sample of stars (and therefore data points with measurements of distance and integrated up density along the line-of-sight) scattered throughout the region of interest, it becomes possible to determine the 3D distribution of material. 

Taking a look at the full spatial distribution of that gas in 3D, a coherent structure emerges. The Perseus and Taurus molecular clouds appear to be part of a massive superbubble blown by a cluster of ancient supernovae! The authors have produced some beautiful interactive figures and augmented reality visualization tools, making it easy to dive into the 3D map produced and get a more hands-on look at the structure revealed by their data, it’s definitely worth checking out! Using the size of the bubble and the fact that it seems to no longer be expanding, the authors estimate an age of the cataclysmic cluster of stellar explosions that created the shell as happening somewhere between 6 and 22 million years ago. The force of that explosion displaced the gas in the shell, perhaps leading to the star formation that we can now observe in the Perseus and Taurus molecular clouds. Whether stellar feedback (like supernovae) is responsible for triggering or suppressing new star formation remains an important topic of research. This study highlights the dangers of making assumptions about what we see projected on the plane of the sky, and the exciting revelations that can come from breaking the curse of line-of-sight confusion.

Astrobite edited by Ciara Johnson

Featured image credit: Figure 2 from the article

About H Perry Hatchfield

I'm a PhD candidate in Physics at the University of Connecticut, where I study star formation and gas structure in the Milky Way's Galactic Center. I do this using radio observations of molecular clouds as well as hydrodynamic simulations, and I'm all about trying to find ways to compare these two exciting means of exploring the universe.

1 Comment

  1. you dont give the suns coordinate position 56.75 +- 6.2 light years north of the midplane
    looking down below the midplane 0 -45
    to a sphere 600 light years wide and 6 to 22 million years ago the supernovae or supenova exploded
    so from our position now 2021 to a distance 700 light years away the supernovae exploded 6 to 22 million years ago and expanded out to 300 light years from the source and today we see those taurus and perseus shells 2021
    can you confirm if this is as you detail
    plus the distance to the perseus shell remains local and was never is never was and will never be part of the perseus arm of the milky way galaxy
    so the taurus and perseus clouds are part of the spur of the orion arm which now would extend 1000 light years wide is this also correct according to your modelling


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