Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Yuan Qi Ni
Dae-Sik Moon
Maria R. Drout
Abigail Polin
David J. Sand
Santiago Gonzalez-Gaitan
Sang Chul Kim
Youngdae Lee
Hong Soo Park
D. Andrew Howell
Peter E. Nugent
Anthony L. Piro
Peter J. Brown
Lluis Galbany
Jamison Burke
Daichi Hiramatsu
Griffin Hosseinzadeh
Stefano Valenti
Niloufar Afsariardchi
Jennifer E. Andrews
Abstract： Type Ia Supernovae are thermonuclear explosions of white dwarf stars. They play a central role in the chemical evolution of the Universe and are an important measure of cosmological distances. However, outstanding questions remain about their origins. Despite extensive efforts to obtain natal information from their earliest signals, observations have thus far failed to identify how the majority of them explode. Here, we present infant-phase detections of SN 2018aoz from a brightness of -10.5 absolute AB magnitudes -- the lowest luminosity early Type Ia signals ever detected -- revealing a hitherto unseen plateau in the $B$-band that results in a rapid redward color evolution between 1.0 and 12.4 hours after the estimated epoch of first light. The missing $B$-band flux is best-explained by line-blanket absorption from Fe-peak elements in the outer 1% of the ejected mass. The observed $B-V$ color evolution of the SN also matches the prediction from an over-density of Fe-peak elements in the same outer 1% of the ejected mass, whereas bluer colors are expected from a purely monotonic distribution of Fe-peak elements. The presence of excess nucleosynthetic material in the extreme outer layers of the ejecta points to enhanced surface nuclear burning or extended sub-sonic mixing processes in some normal Type Ia Supernova explosions.
Peer Review Status:
Awaiting Review
Subjects: Astronomy >> Astrophysical processes submitted time 2023-02-19
Yuan Qi Ni
Dae-Sik Moon
Maria R. Drout
Abigail Polin
David J. Sand
Santiago GonzÁlez-GaitÁn
Sang Chul Kim
Youngdae Lee
Hong Soo Park
D. Andrew Howell
Peter E. Nugent
Anthony L. Piro
Peter J. Brown
LluÍs Galbany
Jamison Burke
Daichi Hiramatsu
Griffin Hosseinzadeh
Stefano Valenti
Niloufar Afsariardchi
Jennifer E. Andrews
Abstract： SN~2018aoz is a Type Ia SN with a $B$-band plateau and excess emission in the infant-phase light curves $\lesssim$ 1 day after first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN~2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show the SN is intermediate between two subtypes of normal Type Ia: Core-Normal and Broad-Line. The excess emission could have contributions from the radioactive decay of surface iron-peak elements as well as ejecta interaction with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on H$\alpha$ and He~I favour a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [O~I] and He~I disfavours a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN~2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Fe~II] and [Ni~II]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1-D models are incompatible with the infant-phase excess emission, $B_{\rm max}-V_{\rm max}$ color, and absence of nebular-phase [Ca~II]. Although the explosion processes of SN~2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear normal after $\sim$ 1 day.
Peer Review Status:Awaiting Review
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