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

Abstract

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 present photometric and spectroscopic observations of SN 2020esm, one of the best-studied SNe of the class of "super-Chandrasekhar" SNe Ia (SC SNe Ia), with data obtained $-12$ to +360 days relative to peak brightness, obtained from a variety of ground- and space-based telescopes. Initially misclassified as a Type 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 optical colors at early times. Photometrically and spectroscopically, SN 2020esm evolved 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 a phase of $\gtrsim$10~days before peak) reveal a nearly-pure carbon/oxygen atmosphere during the first days after explosion. This composition can only be produced by pristine material, relatively unaffected by nuclear burning. The lack of H and He may further indicate that SN 2020esm is the outcome of the merger of two carbon/oxygen white dwarfs (WDs). Modeling its bolometric light curve, we find a ${}^{56}$Ni mass of $1.23_{-0.14}^{+0.14}$ M${}_{\odot }$ and an ejecta mass of $1.75_{-0.20}^{+0.32}$ M${}_{\odot }$, in excess of the Chandrasekhar mass. Finally, we discuss possible progenitor systems and explosion mechanisms of SN 2020esm and, in general, the SC SNe Ia class.

21 pages, 15 figures, accepted for publication in APJ

References in corpus (15)

• Measurements of the Hubble Constant: Tensions in Perspective
• The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star
• CfA3: 185 Type Ia Supernova Light Curves from the CfA
• The Luminous and Carbon-Rich Supernova 2006gz: A Double Degenerate Merger?
• SOUSA: the Swift Optical/Ultraviolet Supernova Archive
• On the Origin of the Type Ia Supernova Width-Luminosity Relation
• The Type II-P Supernova 2017eaw: from explosion to the nebular phase
• Nebular-Phase Spectra of Nearby Type Ia Supernovae
• The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution
• Spectral modelling of the "Super-Chandra" Type Ia SN 2009dc - testing a 2 M_sun white dwarf explosion model and alternatives
• Evidence for Sub-Chandrasekhar Mass Type Ia Supernovae from an Extensive Survey of Radiative Transfer Models
• A Type Ia supernova at the heart of superluminous transient SN 2006gy
• Type Ia supernovae within dense carbon-oxygen rich envelopes: a model for 'Super-Chandrasekhar' explosions?
• Supernova SN 2012dn: A spectroscopic clone of SN 2006gz
• ASASSN-15hy: an under-luminous, red 03fg-like type Ia supernova

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