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Seeing pristine material from the donor star in a Type Ia supernova (SN Ia)explosion can reveal the nature of the binary system. In this paper, we presentphotometric and spectroscopic observations of SN 2020esm, one of thebest-studied SNe of the class of "super-Chandrasekhar" SNe Ia (SC SNe Ia), withdata obtained $-12$ to +360 days relative to peak brightness, obtained from avariety of ground- and space-based telescopes. Initially misclassified as aType II supernova, SN 2020esm peaked at $M_{B} = -19.9$ mag, declined slowly($\Delta m_{15}(B) = 0.92$ mag), and had particularly blue UV and opticalcolors at early times. Photometrically and spectroscopically, SN 2020esmevolved similarly to other SC SNe Ia, showing the usual low ejecta velocities,weak intermediate mass elements (IMEs), and the enhanced fading at late times,but its early spectra are unique. Our first few spectra (corresponding to aphase of $\gtrsim$10~days before peak) reveal a nearly-pure carbon/oxygenatmosphere during the first days after explosion. This composition can only beproduced by pristine material, relatively unaffected by nuclear burning. Thelack of H and He may further indicate that SN 2020esm is the outcome of themerger of two carbon/oxygen white dwarfs (WDs). Modeling its bolometric lightcurve, we find a $^{56}$Ni mass of $1.23^{+0.14}_{-0.14}$ M$_{\odot}$ and anejecta mass of $1.75^{+0.32}_{-0.20}$ M$_{\odot}$, in excess of theChandrasekhar mass. Finally, we discuss possible progenitor systems andexplosion mechanisms of SN 2020esm and, in general, the SC SNe Ia class.

A Carbon/Oxygen-dominated Atmosphere Days After Explosion for the "Super-Chandrasekhar" Type Ia SN 2020esm