AstroNote 2024-240

We observed SN 2021yfj (Munoz-Arancibia et al. 2021, TNS Discovery Report 122721) with the Low-Resolution Imaging Spectrometer (LRIS) at the 10m Keck Telescope on 8 September 2021 (1.16 observer-frame days after the first ZTF detection), using the B600/4000 blue-side grism and the R400/8500 red-side grating, dichroic 5600, and a 1''-wide slit.

The spectrum shows a large number of emission lines (pure emission and P Cygni profiles) from

-S III: 3324.0, 3324.8, 3367.2, 3369.5, 3370.4, 3497.3, 3499.2, 3632.0, 3661.9, 3710.4, 3717.7, 3747.9, 3750.7, 3794.6, 3837.7, 3838.3, 3860.6, 3928.6, 3983.7, 3985.9, 4091.2, 4253.5, 4284.9, 4354.5, 4361.5, 4364.7, 4418.8, 4677.6, 5160.1, 5219.3

- S IV: 3097.3, 3117.6, 3119.9, 3338.6, 3340.4, 3341.5

- Si III: 3096.8, 3185.1, 3241.6, 3486.8, 3590.5, 3796.1, 3806.5, 4552.6, 4567.8, 4574.8, 4716.7, 4813.3, 4819.7, 4828.95, 5739.73, 7461.9, 7466.3

- Si IV: 3149.6, 3165.7, 4088.9, 4116.1

- Ar III: 3285.8, 3301.9, 3311.2, 3336.2, 3344.8, 3480.5, 3511.2, 3514.2

All wavelengths (unit: AA) refer to the Ritz air wavelengths in the NIST database.

In addition, the spectrum also shows lines from C III 4647, He I 3889, 5876, and Mg II 2796, 2803.

All lines are at a common redshift of 0.13685.

The Si, S and Ar emission lines had never been observed in any supernova before and are the defining features of the new SN class: Ien (Gal-Yam, Yaron, Schulze, 2024; AstroNote 2024-239).

For more details, we refer to Schulze et al. (submitted).