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Unusual White Dwarf Explosions Displaying Exceptional Ultraviolet Excess
Unusual White Dwarf Explosions Displaying Exceptional Ultraviolet Excess
A study led by the NCU Transient Group investigates a mysterious class of white dwarf explosions known as “Super-Chandrasekhar” explosions (or 03fg-like SNe Ia) and reveals their extraordinary ultraviolet (UV) properties. Compared to normal SNe Ia, these events exhibit significantly brighter and broader light curves, along with distinct spectral features. Using UV spectra obtained with the Swift satellite, we find that 03fg-like SNe show a pronounced UV excess, which is challenging to explain with current explosion models for normal SNe Ia. Our analysis suggests that interaction with circumstellar material (CSM) could provide a promising explanation for their unique UV characteristics. This work has been published as Bhattacharjee & Pan et al., 2025, MNRAS, 542, 2752.
A Possible Evolution of Type Ia Supernova Populations
A Possible Evolution of Type Ia Supernova Populations
An international team led by Prof. Pan studied the data obtained from Pan-STARRS1 Medium Deep Survey (PS1-MDS) and found the type Ia supernovae (SNe Ia) could evolve with time. As one of the most mature cosmological probes, SNe Ia have homogeneous properties, and they were expected to be consistent and similar during the evolution of our Universe. However, this work suggests that SNe Ia are likely to have at least two different populations, as separated by their ejecta velocities. The fractions between different populations tend to evolve with time. This result could impact the precision of SNe Ia in probing the cosmic expansion and dark energy. This work has been published as Pan et al., 2024, MNRAS, 532, 1887. This research is also highlighted by NV5. Please see the press release here.
A "Closer" Look at the Type Ia Supernova Environments
A "Closer" Look at the Type Ia Supernova Environments
The team led by Prof. Pan investigated the relation between SN Ia ejecta velocity and their host-galaxy environment, and found that there could exist a relationship between the two parameters. This study focused on the local environmental properties of SNe Ia and revealed possible trend that high-velocity SNe Ia tend to be associated with old stellar populations, while normal-velocity SNe Ia can be found in both old and young stellar populations. This result implies that the normal SNe Ia are likely produced by more than one channel, challenging the previous consensus that they are associated with a single and uniform population. This work has been published as Lin & Pan et al., 2024, MNRAS, 531, 1988.
Witnessing the Explosion of a Dying Red Supergiant
Witnessing the Explosion of a Dying Red Supergiant
Young Supernova Experiment, YSE, presents panchromatic observations of supernova SN 2020tlf at ~130 days from mass-loss episodes to the explosion, which involved Prof. Yen-Chen Pan at the graduate institute of astronomy in NCU. This project was led by Ph.D. student Wynn Jacobson-Galán at UC Berkeley to detect a supernova explosion in 2022, by using the Pan-STARRS telescope. SN 2020tlf is far from earth about one hundred million light-years. YSE detected “flash” spectroscopy about one days before its explosion, which is the first time revealed the light curve fluctuated before the explosion of a supernova. Astronomers considered it was due to the mass-loss episodes. So, the tiny changes can only be detected by a sensitive telescope. This may be the reason why it has never been revealed before. This observation can also help us understand the dying process of the massive star. Observations of SN 2020tlf were also obtained with the 1 m Lulin telescope located at Lulin Observatory, which contributed to BVgr bands before the explosion. Lulin observatory played a key role with its excellent location. It can contribute to those events in the future. This important result has been also reported by CNN. This result is published as Jacobson-Galán et al. 2022, ApJ, 924, 15 (Including Prof. Yen-Chen Pan). Please also see the press release by CNN here.
High Velocity Type Ia Supernovae Likely Arise from Unique Progenitor Channels
High Velocity Type Ia Supernovae Likely Arise from Unique Progenitor Channels
Ejecta velocity of SNe Ia is one powerful tool to differentiate between progenitor scenarios and explosion mechanisms. In this paper we investigate the relation between ejecta velocity (using photospheric Si II 6355 line) and host-galaxy properties with ∼280 SNe Ia. We find a significant trend that SNe Ia with faster ejecta velocities tend to explode in massive environments, whereas their slower counterparts can be found in both lower-mass and massive environments. We suggest this relation is likely caused by at least two populations of SNe Ia. We conclude metallicity is likely the dominant factor in forming high-velocity SNe Ia. This also implies their potential evolution with redshift and impact on the precision of SN Ia cosmology. This result has been published as Pan 2020, ApJL, 897, L5.
Linking Ultraviolet Properties of Type Ia Supernovae to Their Progenitors
Linking Ultraviolet Properties of Type Ia Supernovae to Their Progenitors
A study led by Prof. Yen-Chen Pan analyzed the largest ultraviolet (UV) spectroscopic sample of nearby SNe Ia obtained with the Swift UVOT grism, aiming to probe the metallicity of their progenitor systems. As one of the most mature cosmological probes, SNe Ia have been expected to be homogeneous. However, theoretical models predict that SNe Ia could show much larger diversity in the UV, and their UV spectra could be sensitive to the progenitor metallicity. By using host-galaxy metallicity as a proxy, this study found evidence that SNe Ia in more metal-rich galaxies show systematically lower UV flux than those in metal-poor environments. This result supports theoretical predictions and provides a new way to constrain SN Ia progenitor properties, with potential implications for SN Ia cosmology. This work has been published as Pan et al., 2020, MNRAS, 491, 5987.
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