Table 5. Open in new tab Summary of the...

	Ultrastripped	Accretion-induced	Double WD	Merger of a WD
Peak brightness	Bad	Possible^a	Good	Possible^b
Light-curve evolution	Bad	Possible^c	Good	Good
Velocity of ejecta	Good	Bad	Good	Good^d
Ejecta mass	Good	Good	Good	Possible^e
Bulk composition	Good	Bad	Bad	Good
Fe/Ni ratio	Bad	Bad	Bad	Good

	Ultrastripped	Accretion-induced	Double WD	Merger of a WD
Peak brightness	Bad	Possible^a	Good	Possible^b
Light-curve evolution	Bad	Possible^c	Good	Good
Velocity of ejecta	Good	Bad	Good	Good^d
Ejecta mass	Good	Good	Good	Possible^e
Bulk composition	Good	Bad	Bad	Good
Fe/Ni ratio	Bad	Bad	Bad	Good

Table 5.

Summary of the pros and cons of each of the progenitor systems considered for AT2018kzr.

	Ultrastripped	Accretion-induced	Double WD	Merger of a WD
	core-collapse SN	collapse of an ONe WD	merger systems	with an NS/BH
Peak brightness	Bad	Possible^a	Good	Possible^b
Light-curve evolution	Bad	Possible^c	Good	Good
Velocity of ejecta	Good	Bad	Good	Good^d
Ejecta mass	Good	Good	Good	Possible^e
Bulk composition	Good	Bad	Bad	Good
Fe/Ni ratio	Bad	Bad	Bad	Good

	Ultrastripped	Accretion-induced	Double WD	Merger of a WD
	core-collapse SN	collapse of an ONe WD	merger systems	with an NS/BH
Peak brightness	Bad	Possible^a	Good	Possible^b
Light-curve evolution	Bad	Possible^c	Good	Good
Velocity of ejecta	Good	Bad	Good	Good^d
Ejecta mass	Good	Good	Good	Possible^e
Bulk composition	Good	Bad	Bad	Good
Fe/Ni ratio	Bad	Bad	Bad	Good

^aCapable of producing a magnetar, which can power the light curve.

^bCapable of reaching the observed luminosity of AT2018kzr, with the help of high-velocity winds launched off the disc.

^cLight-curve evolution too fast in the models discussed, but a higher ejecta mass would act to extend the light curve.

^dThe bulk velocity of the ejecta material agrees with our photospheric estimates for ejecta velocity.

^eThe ejecta mass obtained from light-curve modelling is lower than that estimated from the models discussed, but these values are dependent on the specific merger, which make it difficult to get good approximations.

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