Astronomers have for the first time used X-ray imaging to examine a calcium-rich supernova. Their findings, published in the Astrophysical Journal, show that a calcium-rich supernova is a compact star that sheds an outer layer of gas during the final stages of its life; when the star explodes, its matter collides with the loose material in that outer shell, emitting bright X-rays; the overall explosion causes intensely hot temperatures and high pressure, driving a chemical reaction that produces calcium.
These pre-explosion (left panel and right top panel) and post-explosion (right bottom panel) images from Hubble show SN 2019ehk and its host galaxy Messier 100. The location of SN 2019ehk is marked by red lines and the alignment uncertainty is indicated by pink ellipses. Image credit: Jacobson-Galán et al, doi: 10.3847/1538-4357/ab9e66.
Half of all the calcium in the Universe — including the very calcium in our teeth and bones — was created in stellar explosions called calcium-rich supernovae.
These events are so rare that astrophysicists have struggled to find and subsequently study them.
“Calcium-rich supernovae are so few in number that we have never known what produced them,” said Dr. Wynn Jacobson-Galan, a researcher at Northwestern University.
“By observing what this star did in its final month before it reached its critical, tumultuous end, we peered into a place previously unexplored, opening new avenues of study within transient science.”
Dr. Jacobson-Galan and colleagues studied a calcium-rich supernova dubbed SN 2019ehk in Messier 100, a star-forming spiral galaxy located 50 million light-years away in the constellation of Coma Berenices.
This stellar explosion was first spotted on April 29, 2019 by amateur astronomer Joel Shepherd.
NASA’s Neil Gehrels Swift Observatory, Lick Observatory and the W. M. Keck Observatory immediately examined SN 2019ehk in optical wavelengths.
Swift also observed the event in X-rays and ultraviolet. The X-ray emission only lingered for five days before completely disappearing.
“Before this event, we had indirect information about what calcium-rich supernovae might or might not be. Now, we can confidently rule out several possibilities,” said Dr. Raffaella Margutti, also from Northwestern University.
Typical stars create small amounts of calcium slowly through burning helium throughout their lives. Calcium-rich supernovae produce massive amounts of calcium within seconds.
“The explosion is trying to cool down. It wants to give away its energy, and calcium emission is an efficient way to do that,” Dr. Margutti said.
Using the Low Resolution Imaging Spectrometer (LRIS) at the W. M. Keck Observatory, the astronomers discovered SN 2019ehk emitted the most calcium ever observed in a singular astrophysical event.
“The beautiful Keck spectrum revealed it wasn’t just calcium-rich. It was the richest of the rich,” Dr. Margutti said.
The researchers believe the progenitor star of SN 2019ehk — a white dwarf or very low-mass massive star — shed an outer layer of gas in its final days.
When it exploded, its material collided with this outer layer to produce a bright, energetic burst of X-rays.
“Its luminosity tells us how much material the star shed and how close that material was to the star,” Dr. Jacobson-Galan said.
“In this case, the star lost a very small amount of material right before it exploded. That material was still nearby.”
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Wynn V. Jacobson-Galán et al. 2020. SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features. ApJ 898, 166; doi: 10.3847/1538-4357/ab9e66
This article is based on a press-release provided by the Northwestern University.
