Hot Zombie Stars in Your Area

Title: Chemical abundances in Sgr A East: evidence for a type Iax supernova remnant

Authors: Ping Zhou, Shing-Chi Leung, Zhiyuan Li, Ken’ichi Nomoto, Jacco Vink and Yang Chen

First author’s institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam 

Journal: Submitted to the Astrophysical Journal. Open access on ArXiv.

Disclaimer: The author would first like to publicly state that Black lives and Black Trans lives matter. Secondly, the author condemns all police brutality against people of color. Lastly, the author recognizes that the writing of this article was performed on the stolen land of indigenous people.

Let’s Blow Up a Star!

Almost a decade ago, amongst the heap of exotic transients observed every day in our Universe, a new class of stellar explosion was identified and labeled Type Iax supernovae (SNe Iax). Similar to the infamous Type Ia supernovae (SNe Ia) that are commonly used to measure the expansion rate of the universe, SNe Iax are also thought to arise from the destruction of a white dwarf star as it approaches the Chandrasekhar mass limit. However, these supernovae are physically distinct explosions from SNe Ia given their low observed luminosities and kinetic energies, combined with the slow velocities at which the obliterated white dwarf blasts through interstellar space i.e., the ejecta velocity.

After their identification, theorists were tasked with explaining how the destruction of a white dwarf could produce such a “weak” explosion rather than the typical characteristics seen in SNe Ia. Many now agree that in order to create a SN Iax, the white dwarf needs to explode via a deflagration rather than a detonation: the shock wave that initially ripples through the white dwarf will travel sub-sonically instead of super-sonically. As the weakened shock wave attempts to explode the white dwarf, it will not be strong enough to overcome the pull of gravity and completely unbind the star. Instead, only some of the white dwarf’s stellar structure will be released in the explosion to produce a SN Iax, leaving behind a remnant stellar core. And thus, a zombie star is born!

Of the ~100 SNe Iax known presently, all have occurred in galaxies far from the Milky Way. However, for every 10 SN Ia identified, which make up ~20% of all explosions in the universe, 2-5 SNe Iax are also discovered, making them relatively common explosions. Nevertheless, none of the remnant supernovae that exploded in the Milky Way in the past have been identified as a SN Iax despite the identification of many SNe Ia remnants, as well as those from the collapse of massive stars. And so, since the inception of the SN Iax class, a pressing question has eluded astronomers: where are all the galactic Type Iax supernova remnants?

A “Weak” Explosion in the Milky Way

The authors of today’s paper attempt to answer this burning question by offering up what may be the first confirmed SN Iax remnant in the Milky Way. Using archival data from the Chandra X-ray Observatory, the authors study the X-ray emission from supernova remnant Sgr A East (i.e., G0.0+0.0, shown in Figure 1) that is thought to have exploded at least 2000 years ago near the galactic center. Even after thousands of years, shock waves within the synthesized supernova material will continue to accelerate subatomic particles, which in turn created prominent X-ray emission that can be observed in supernova remnants throughout our own galaxy. Consequently, the strength of this X-ray emission from different elements in the remnant is directly linked to the type of explosion that may have produced the G0.0+0.0 remnant.

Figure 1: False-color image of the Sgr A East supernova remnant. Shown in red is radio emission and in cyan is X-ray emission. Circles/rectangles indicate the Chandra observations.

As shown in Figure 2, the authors constructed an X-ray emission spectrum from over 3 MILLION seconds of X-ray observations with Chandra. Elements such as Calcium, Iron, Manganese and Nickel are identified as the bi-products of a powerful explosion that occurred thousands of years ago. The authors use this emission spectrum to measure the relative abundance of these elements i.e., how much of a certain element is produced with respect to all the others. This precise metric of supernova nucleosynthesis is the key to determining what type of explosion produced the Sgr A East remnant. 

Figure 2: X-ray spectrum of Sgr A East supernova remnant with emission from observed ions present in the supernova ejecta indicated in black. From left to right: Sulfur, Argon, Calcium, Chromium, Manganese, Iron, Nickel + Iron.

With the help of supercomputers, we now have detailed modeling of supernova explosions that are not only capable of simulating the explosion of all types of stars, but can also predict how much of a given element the supernova generates. And so, through the beautiful synthesis of theory and observations, the authors directly compare the simulation-predicted elemental abundances for different explosion types to those derived from X-ray observations of the Sgr A East remnant.

As shown in Figure 3, the abundance ratio of a given element to Iron (Z/Fe), indicates that Sgr A East produced far too little intermediate mass elements (IME) such as Argon and Calcium to match the predicted amounts seen in the explosion of a massive star (i.e., core-collapse supernova). However, the simulated abundances in the deflagration of a white dwarf (lower right panel) appears to be a near perfect match to the observations. A local zombie star has been identified!

Figure 3: A comparison of element abundance for different explosion types. Top left is a core-collapse supernova from a massive star, top right is the double detonation of a white dwarf, and bottom left is a partial deflagration with a detonation of a white dwarf. Shown in the bottom right is the favored model for the Sgr A East remnant which is the pure deflagration of a white dwarf.

As with any astronomical study, there are always sources of statistical error in our observations and models. Therefore, the apparent link between a white dwarf deflagration model and the observations presented in today’s paper may be open for debate. However, this analysis presents one of the most convincing arguments to date that a SN Iax occurred in the Milky Way. This discovery will push astronomers to identify more local SN Iax remnants, which in turn can help uncover the complex origins of these exotic stellar explosions. 

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1 Comment

  1. Thank you for standing up as an anti-racist. My great great grandmother’s land was and still is stolen to do astronomical research and as a trans person who is male presenting i feel alienated in the academic world every day. Not one single person in academia I feel supports my cause which is made even more challenging by my condition of Anderson’s polycythemia.


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