JWST Observations of Calcium-Strong Transients: I. Complex Nebular He Emission in SN 2024uj. Hall, S., Kwok, L. A., Ravi, A. P., Miller, A. A., Dessart, L., Jacobson-Galán, W. V., Sears, H., Andrews, M., Bostroem, K. A., Brink, T. G., Farah, J., Filippenko, A. V., Foley, R. J., Hosseinzadeh, G., Howell, D. A., Jha, S. W., Larison, C., Liu, C., Macrie, C. W., Maguire, K., McCully, C., Meza-Retamal, N. E., Modjaz, M., Newsome, M., Gonzalez, E. P., Sand, D. J., Schulze, S., Terreran, G., Touchard-Paxton, C., Valenti, S., Yang, Y., & Zheng, W. June, 2026. arXiv:2607.00111 [astro-ph.HE]
JWST Observations of Calcium-Strong Transients: I. Complex Nebular He Emission in SN 2024uj [link]Paper  doi  abstract   bibtex   
We present the first JWST observations of a Calcium-Strong Transient (CaST), SN 2024uj, a rare class of supernovae (SNe) with observable properties that are consistent with both thermonuclear explosions of white dwarfs (WDs) and the core collapse of massive stars. SN 2024uj is offset by ${\}sim6.6$ kpc from its host and exhibits a double-peaked light curve consistent with shock cooling of nearby circumstellar material. At early times, its optical spectra resemble those of normal SNe Ib, but strong [Ca II] $λλ$7291, 7324 emission emerges between $+$2 and $+$17 days after maximum light. Radiative-transfer models of a massive stripped He star cannot reproduce this early forbidden Ca emission, even with artificially enhanced surface Ca, whereas it arises naturally in thermonuclear scenarios. The $+$150 d JWST/NIRSpec spectrum reveals highly asymmetric, multicomponent He I at both 1.083 and 2.058 $μ$m. The He extends to ${\}gtrsim+$5000 km/s, with a strong, narrow peak at $+$1500 km/s, indicating that He is distributed throughout the ejecta with a concentration offset from center. This He distribution overlaps central [Ca II] and [O I], implying a degree of mixing difficult to produce in a massive star explosion. The He peak might further trace interaction with a shocked, ejected companion in a thermonuclear system. The NIRSpec spectrum also shows molecular CO emission and a rising continuum that, together with a 10 $μ$m photometric detection, indicates dust emission extending into the mid-infrared. Given the remote environment, early forbidden Ca, mixed He/Ca/O ejecta, and possible companion signature, we favor a thermonuclear origin for SN 2024uj involving at least one low-mass, partially He-rich WD.
@misc{hall_jwst_2026,
	title = {{JWST} {Observations} of {Calcium}-{Strong} {Transients}: {I}. {Complex} {Nebular} {He} {Emission} in {SN} 2024uj},
	shorttitle = {{JWST} {Observations} of {Calcium}-{Strong} {Transients}},
	url = {http://arxiv.org/abs/2607.00111},
	doi = {10.48550/arXiv.2607.00111},
	abstract = {We present the first JWST observations of a Calcium-Strong Transient (CaST), SN 2024uj, a rare class of supernovae (SNe) with observable properties that are consistent with both thermonuclear explosions of white dwarfs (WDs) and the core collapse of massive stars. SN 2024uj is offset by \${\textbackslash}sim6.6\$ kpc from its host and exhibits a double-peaked light curve consistent with shock cooling of nearby circumstellar material. At early times, its optical spectra resemble those of normal SNe Ib, but strong [Ca II] \$λλ\$7291, 7324 emission emerges between \$+\$2 and \$+\$17 days after maximum light. Radiative-transfer models of a massive stripped He star cannot reproduce this early forbidden Ca emission, even with artificially enhanced surface Ca, whereas it arises naturally in thermonuclear scenarios. The \$+\$150 d JWST/NIRSpec spectrum reveals highly asymmetric, multicomponent He I at both 1.083 and 2.058 \$μ\$m. The He extends to \${\textbackslash}gtrsim+\$5000 km/s, with a strong, narrow peak at \$+\$1500 km/s, indicating that He is distributed throughout the ejecta with a concentration offset from center. This He distribution overlaps central [Ca II] and [O I], implying a degree of mixing difficult to produce in a massive star explosion. The He peak might further trace interaction with a shocked, ejected companion in a thermonuclear system. The NIRSpec spectrum also shows molecular CO emission and a rising continuum that, together with a 10 \$μ\$m photometric detection, indicates dust emission extending into the mid-infrared. Given the remote environment, early forbidden Ca, mixed He/Ca/O ejecta, and possible companion signature, we favor a thermonuclear origin for SN 2024uj involving at least one low-mass, partially He-rich WD.},
	language = {en},
	urldate = {2026-07-06},
	publisher = {arXiv},
	author = {Hall, Saarah and Kwok, Lindsey A. and Ravi, Aravind P. and Miller, Adam A. and Dessart, Luc and Jacobson-Galán, W. V. and Sears, Huei and Andrews, Moira and Bostroem, K. Azalee and Brink, Thomas G. and Farah, Joseph and Filippenko, Alexei V. and Foley, Ryan J. and Hosseinzadeh, Griffin and Howell, D. Andrew and Jha, Saurabh W. and Larison, Conor and Liu, Chang and Macrie, Colin W. and Maguire, Kate and McCully, Curtis and Meza-Retamal, Nicolas E. and Modjaz, Maryam and Newsome, Megan and Gonzalez, Estefania Padilla and Sand, David J. and Schulze, Steve and Terreran, Giacomo and Touchard-Paxton, C.-G. and Valenti, Stefano and Yang, Yi and Zheng, WeiKang},
	month = jun,
	year = {2026},
	note = {arXiv:2607.00111 [astro-ph.HE]},
	keywords = {Astrophysics - High Energy Astrophysical Phenomena, WG: Explorable},
}
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