Type Ia supernovae (SNe Ia) are widely believed to originate from thermonuclear runaway explosions of carbon-oxygen (CO) white dwarfs (WDs) in binary systems, and their essentially identical peak luminosities make them indicators of extragalactic distances and important probes for the study of cosmic expansion. There is a diversity of Type Ia supernovae, and different progenitor systems and explosion mechanisms may affect their accuracy as distance probes. Studies suggest that HV SN Ia and NV SN Ia may come from two different populations. Observational studies of Type Ia supernovae of different velocities contribute to the study of SN Ia diversity and provide insight into the explosion mechanism of SN Ia and their progenitor systems. Researchers from the Optical Astronomy Research Laboratory of Xinjiang Astronomical Observatory with their collaborators from domestic and abroad have confirmed that SN 2017fgc is a HV SN Ia with a photospheric expansion velocity of about 15000±150 km/s, which is significantly higher than that of a NV SN Ia. The results are published in the Astrophysical Journal (ApJ, 2021，919，49).

The researchers jointly utilize multiple telescopes conducting multi-band photometric and spectroscopic observations of SN 2017fgc. The multi-band photometric observations cover 12.3 days before to 204.7 days after the maximum light, while a total of 38 spectra range from 11.7 days before to 388.9 days after the light maximum. By using light curve template fitting and flux density distribution analysis, the researchers obtained the light curve shape parameters, absolute magnitude and quasi-bolometric luminosity of the supernova. The gaussian fitting is employed to estimate the velocities of the photospheric and some intermediate mass elements of SN 2017hpa from the local absorption profiles in the spectra. The bulk statistical studies suggested that there is a positive correlation between the equivalent widths of Fe II blind lines and Mg II blind lines in the spectra of SN Ia and the photospheric expansion velocity. It is suggested that compared with NV SN Ia, HV SN Ia may experience more complete burning, and the high-speed expansion characteristics of SN Ia may be related to the deep mixing of heavier elements in the ejecta. 

Figure 1: Comparison of different phase spectra of SN 2017fgc and well observed type Ia supernovae

Figure 2: Scatter plot of the observed maximum light velocity of well observed type Ia supernovae and the equivalent width of Fe II blind lines and Mg II blind lines

Contact: ZENG Xiangyun

Xinjiang Astronomical Observatory, Chinese Academy of Sciences

Email: zengxiangyun@xao.ac.cn

Web: https://ui.adsabs.harvard.edu/abs/2021ApJ...919...49Z/abstract