Title: On the Pair-instability Supernova Origin of J1010+2358

Authors: Ása Skúladóttir, Ioanna Koutsouridou, Irene Vanni, Anish M. Amarsi, Romain Lucchesi, Stefania Salvadori, and David S. Aguado

First Author’s Institution: Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via G. Sansone 1, I-50019 Sesto Fiorentino, Italy

Status: Published on Astrophysical Journal Letters [open access]

A link to the first stars?

Our 4.6-billion-year-old Sun is part of a long lineage of stars, the first generation of which was born when the Universe was only 30-100 million years old. Searching for this first generation of stars, called Population III (Pop III), is one of the most exciting pursuits in astronomy today. 

We wonder about these ancient stars the same way we wonder about our own human ancestors: What were they like? How did they live? How did they die? How did they influence the way we are? Right now, we have some ideas about the answers to these questions. We believe Pop III stars were supremely massive (as much as 1000 times the mass of the Sun in extreme cases), and, as a result, lived for only a few million years. When they died, they probably exploded as immensely powerful supernovae called pair-instability supernovae (PISNe). For many reasons, these ancient stars were likely key players in the evolution of the universe. 

Unfortunately, all we have on Pop III stars are ideas. After decades of searching, we still haven’t found definitive evidence of them or of their direct descendants. Well, that was what we thought until last year, when Xing and colleagues (hereinafter X23) announced on Nature that they caught a big break in the search for Pop III. 

While combing through data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, they found a peculiar star (J1010+2358) that seemed to be a direct descendant of a Pop III star. Could this be our direct link to the very first stars?

“A PISN was here”

Just like how we trace our human ancestry with DNA, we trace the ancestry of stars with their chemical composition. When a star forms from a gas cloud, we can assume that it takes roughly the same chemical composition as its birth cloud. What sets the chemical composition of the cloud are the processes, usually supernova explosions, that “enrich” the gas with chemical elements. Different types of supernovae leave distinct chemical signatures in the stars born from enriched gas. We then see these signatures when we look at stellar spectra.

When the most massive Pop III stars (140-260 M⊙) ended their lives as PISNe, they produced many more nuclei with even nuclear charges (e.g., Mg, Si, S, Ar) than those with odd nuclear charges (e.g., Na, Al, P, V, Mn). This chemical signature shows up as a sharply jagged zigzag pattern on a plot of atomic number (Z) versus chemical abundance (as shown in Figure 2). Such a strong “odd-even ratio” is a strong sign of PISN enrichment—kind of like a cheeky chemical graffiti saying “A PISN was here.” Other types of supernovae do not leave the same strong odd-even signature.

When X23 obtained J1010+2358’s spectrum and measured its chemical abundances, they found that it was born from gas enriched not by multiple supernovae, but by a singular PISN. X23 had not only detected the first ever PISN—a tremendously exciting finding by itself—they also found a pure signature of one. This purity is important because it tells us that what we’re measuring is only due to a PISN and its Pop III progenitor. We do not want our “signal” to be contaminated by the contributions of other supernovae (e.g., core-collapse supernovae).

However, the excitement around this long-awaited “discovery” was short-lived. When Skúladóttir and colleagues (hereinafter S24) obtained their own spectra of the star, their results contradicted the original claim: J1010+2358 is “not a pure PISN descendant” after all.

The claim debunked

Was a PISN the sole enricher of J1010+2358’s birth cloud? Was it a core-collapse supernova instead? Or was it a mixture of both? If it was a mixture, was it a 50-50 split or 70-30 mixture? Distinguishing between these scenarios requires carbon and aluminum abundances—elements which were not reported in the original abundance analysis by X23. 

To clarify the star’s ancestry, S24 obtained high-resolution spectra with the UVES instrument on the European Southern Observatory’s Very Large Telescope. By specifically measuring the abundances of carbon and aluminum, S24 obtained more data to arrive at a clearer interpretation of J1010+2358’s origins. Besides carbon and aluminum, they also measured abundances for 20 other chemical species as shown in Figure 3.

With their new chemical abundance measurements (red stars in Figure 3), they found that the odd-even ratio was not as strong as previously claimed (orange squares in Figure 3). For instance, they found the odd element Na to be severely underestimated and the even element Si to be overestimated. From this, it was clear that J1010+2358 is not a pure PISN descendant.

If not a PISN, then what kind/s of supernovae enriched the birth cloud of J1010+2358? Based on supernova models, we predict that a particular change in the progenitor’s mass changes the supernova’s yield (i.e., how much of some element it makes). After fitting the star’s chemical abundance pattern (red stars in Figure 4) with the predicted yields of many different supernova scenarios (lines in Figure 4), S24 found that the star’s birth cloud was most likely enriched by a combination of two core-collapse supernovae (CCSNe): one whose progenitor was a 13 M⊙ Pop II star (a younger generation than Pop III) and another whose progenitor was a 39 M⊙ Pop III star. In Figure 4, note how far the gray dashed line indicating 100% enrichment from a 260 M⊙ Pop III star—the original interpretation by X23—deviates from data.

There is still a slight possibility that half of the star’s metals came from a 260 M⊙ PISN but this is less likely than the dual CCSN scenario. What is clear, however, is that a PISN was not the sole or even dominant (>70%) enricher of J1010+2358’s birth gas.

Therefore, J1010+2358 is not the pure PISN descendant that we once thought. The relentless and exciting search for PISNe and their Pop III progenitors continues!

Astrobite edited by Shalini Kurinchi-Vendhan and Junellie Perez

Featured image credit: NOIRLab/NSF/AURA/J. da Silva/Spaceengine