Supernova 2019ehk
Last year, a peculiar transient called SN 2019ehk was discovered within half a day of explosion in the nearby galaxy M100. This supernova (SN) belongs to the “Calcium-rich” transient class, which are distinguished by dominant Calcium emission in their spectra and a rapidly evolving, low luminosity light curve. These objects represent a relatively new class of explosions whose progenitors are completely unknown. Thanks to a global collaboration, we were able to study this SN with observations spanning the electromagnetic spectrum (x-ray to radio). This analysis placed the tightest constraints on the progenitors of this class to date.
Press:
Below is a summary of my recent paper on this intriguing object:
Figure 1
SN 2019ehk’s light curve has two peaks, which is unique for Ca-rich transients as well as SNe in general. This SN also emitted luminous X-rays that were coincident with the first peak, suggesting a linked emission mechanism. Theoretical predictions suggest that the first peak can arise from a SN shock colliding with extended material in the pre-explosion environment.
Figure 2
The name “Calcium-rich” is derived from an observation properties of SNe in this class. As shown to the right, the spectra of Ca-rich objects are dominated by Ca II and [Ca II] emission once the SN has become optically thin i.e. "nebular.” A SN is “Ca-rich” if the ratio of [Ca II] to [O I] emission is greater than 2. In SN 2019ehk, the ratio of [Ca II] to [O I] is over 30 making it the most “calcium-rich” SN ever discovered!
Figure 3
As soon as SN 2019ehk was discovered, a spectrum was obtained of the explosion (left). As shown to the left, this spectrum showed “flash-ionized” Hydrogen and Helium emission lines that form when the SN shock collides with gas in the progenitor environment. This observational signature had never been seen before in a Ca-rich SN and revealed that their progenitors eject mass into their local environment before exploding.
Figure 4
SN 2019ehk is the first Calcium-rich transient with pre-explosion imaging. We searched 2 decades worth of archival Hubble Space Telescope images but found no visible progenitor of the SN. As shown in the Hertzsprung-Russel diagram to the right, we can exclude all single, massive stars >8 Msun (pink region) as the progenitor of SN 2019ehk.
Figure 5
Left, a visual representation of SN 2019ehk derived from different explosion properties. From pre-explosion imaging, this system is either a massive star in a binary (9-10 Msun) or a white dwarf with a binary companion. The double-peaked light curve, coincident with X-ray emission and “flash-ionized” spectral lines, is strong evidence for circumstellar material (CSM, sea foam green) made of Hydrogen and Helium surrounding the progenitor star. When the star exploded, the SN shock interacted with this material, which caused these different observation signatures.