It’s gonna blow! Chasing the next Milky Way supernova

Title: Runaway BN supergiant star HD 93840: Progenitor of an imminent core-collapse supernova above the Galactic plane

Authors: D. Weßmayer, M. A. Urbaneja, K. Butler, N. Przybilla

Authors’ Institutions: Universität Innsbruck, Ludwig-Maximilians-Universität München

Status: Published in A&A [open access]

We are made of starstuff.

At the beginning of the Universe, baryonic (i. e. non-dark) matter was composed almost entirely of hydrogen, helium, and small amounts of lithium. Every element other than those three was created later in stars and in supernovae. Astronomers are therefore very interested in studying how exactly these work, and in particular are extremely interested in observing supernovae in action.

Betelgeuse is often quoted as the probable next star in our galaxy to explode due to its high variability, including an exceptional dimming event observed in 2019. However, it’s not the only star in our galaxy close to the end of its life. Today’s authors describe another star whose explosion seems imminent: the blue supergiant HD 93840.

Tracking down evolution

In order to tell how close HD 93840 is to a supernova, the authors have to find its position on the Hertzsprung-Russel diagram, a diagnostic tool that astronomers use to infer a star’s properties and evolutionary stage. To do this, they infer the star’s effective temperature and surface gravity by fitting its spectrum using two different models. They also compare with the results of a third analysis, done by a different paper in 2010. Using photometric measurements as well as the distance to the star, they can then determine its luminosity, allowing them to place it on the HR diagram in Figure 1.

Although the three models agree well for each value individually, on the HR diagram it’s clear that the derived parameters are very different. The derived luminosity also  depends  strongly on whether the authors compute it using the spectroscopic or Gaia distance, with the spectroscopic distance being roughly a kiloparsec less than the Gaia distance estimate. All of this uncertainty and degeneracy means that the evolutionary tracks in Figure 1, which assume that the star is normally evolving and not interacting with a binary companion, are inadequate to describe the evolution of HD 93840. So, the authors turn to other methods.

More detective work

One way of investigating a star’s evolutionary status when the HR diagram fails is to study the relative abundances of elements in its atmosphere. Stars fuse elements at their cores where the pressure is highest, so ordinarily the heaviest elements are found at the center of the star, with concentric shells of lighter elements moving out towards the surface. However, HD 93840 has an unusually high amount of carbon, nitrogen, and oxygen at its surface, where we would expect to see mostly lighter elements, indicating that its internal structure doesn’t follow the concentric shell model, but instead that heavy elements are mixed with the lighter ones to a considerable degree.

The authors also compare the HD 93840’s luminosity with what would be expected for a star with its mass and temperature, and find that HD 93840 is much brighter than a normal star. A star’s luminosity depends on its mean molecular weight, such that brighter stars have more heavy elements. Because HD 93840 is so bright, therefore, it likely contains far more heavy elements than expected for a star with its mass and temperature, and probably has a large helium-burning core.

The authors therefore propose that HD 93840 formed as the primary (higher-mass) star in a binary star system, but began to transfer mass to the secondary companion from its outer atmosphere, leaving only the helium core. The secondary star, meanwhile, began to evolve faster due to this mass transfer, burning helium to create carbon, nitrogen, and oxygen. This material then transferred back onto HD 93840, leading to the high CNO abundances and mixing that the authors observe. Finally, the secondary star exploded in a supernova, ejecting HD 93840 out of the Galactic plane into its current position 500-600 pc above the Milky Way disk.

While the authors can’t be sure that all of the details in their proposed scenario are correct, one thing is clear: HD 93840’s heavy helium core will not be able to sustain its own weight for much longer. The heavy elements in its atmosphere will press down towards the center, keeping the star from expanding into a red supergiant like Betelgeuse. Instead, the core will collapse, triggering a violent explosion that in turn will create even more heavy elements. These atoms will become part of the gas and dust that surrounds and suffuses our Galaxy, and perhaps will one day become the stuff of a new star!

Astrobite edited by Karthik Yadavalli

Featured image credit: NASA Goddard Space Flight Center