Type Ia Supernovae (SNe Ia) have been used as standardizable candles to discover the accelerated expansion of the universe, leading to the revelation of dark energy and the award of 2011 Nobel Prize in Physics to astronomers. However, important as they are, SNe Ia are not fully understood yet. Generally speaking, two contrasting origins of SNe Ia have been suggested: a single degenerate (SD) origin in which a white dwarf (WD) accretes material from a non-degenerate normal star companion until its mass nears the Chandrasekhar limit, and a double degenerate (DD) origin that results from the merger of two WDs. In the SD scenario, the interaction between the WD and the normal star companion strips the mass of the companion to form a circumstellar medium (CSM), and the companion can survive the SN explosion and be detected. In the DD scenario, both WDs are destroyed and no dense CSM or detectable stellar remnant is expected. Therefore, if a surviving companion or a dense CSM is detected near the explosion center of a Type Ia SN remnant (SNR Ia), the SD origin of this SN can be affirmed.
To date, no surviving companion has been unambiguously identified near explosion centers of SNRs Ia in the Milky Way Galaxy. In 2017, within the young Type Ia SNR N103B in the Large Magellanic Cloud (LMC), Postdoc Fellow Chua-Jui Li in ASIAA led a team to spectroscopically identify the presence of a dense forbidden-line-emitting CSM and confirmed the SD origin of its SN progenitor, and photometrically identified a candidate for a surviving companion within this SNR. It is the first time that a candidate of a surviving companion near the center of a SNR Ia is identified outside the Milky Way Galaxy! In 2019, advancing forward from previous work, Li and his team used Multi-Unit Spectroscopic Explorer (MUSE) observations obtained with the Very Large Telescope (VLT) and found that a star in SNR 0519-69.0 and a star in SNR DEM L71 are possible candidates of surviving companions in the LMC. It has been said that a picture is worth a thousand words but a spectrum is worth a thousand images in astronomy. In this work, candidates of surviving companions of SN Ia progenitors are spectroscopically identified near the centers of SNRs Ia outside the Milky Way Galaxy for the first time; furthermore, he found that 20%– 60% of these five young LMC Type Ia SNRs may originate from SD progenitors, significantly higher than the 20% that people previously suggested in the Milky Way Galaxy.
Young SNRs Ia show filamentary shells with optical spectra dominated by Balmer lines without forbidden-line counterparts. In the case of N103B, bright forbidden lines are detected from dense knots that represent the CSM ejected by the progenitor before the SN explosion, implying that the SN progenitor must be of SD origin. Intrigued by the CSM in N103B, Li and his team further used VLT MUSE and Advanced Technology Telescope (ATT) Wide Field Integral Spectrograph (WiFeS) observations to search for forbidden-line emission from five Type Ia SNRs ( 0509-67.5, 0519-69.0, N103B, DEM L71 and 0548-70.4) containing Balmer-dominated shells in the LMC. Among these five SNRs, only 0509-67.5 has no plausible candidate surviving companion and a confirmed DD origin. Thus, it would be interesting to make comparisons between SNR 0509-67.5 and other young SNRs Ia.
Contrary to expectations, they found bright forbidden-line emission from small dense CSM embedded in four of these SNRs (0519-69.0, N103B, DEM L71 and 0548-70.4). Dense nebular knots were discovered in 0519-69.0, DEM L71, and 0548-70.4 for the first time! SNR 0509–67.5 has no knots at all, consistent with the expectation for a DD origin. They conclude that 80% of these five young SNRs Ia contain dense CSM and may originate from SD progenitors, consistent with their previous finding based on the search for surviving companions in 2017 and 2019. In this work, the proportion of SD and DD origins is given by searching for the CSM in SNRs Ia outside the Milky Way Galaxy for the first time. Furthermore, physical property variations of the CSM in the SNRs appear to reflect an evolutionary effect. It indicates that the presence of CSM in SNRs Ia could be more prevalent than people previously thought.