2020ApJ...898..166J

We present panchromatic observations and modeling of the Calcium-rich supernova (SN) 2019ehk in the star-forming galaxy M100 (d ≃ 16.2 Mpc) starting 10 hr after explosion and continuing for ∼300 days. SN 2019ehk shows a double-peaked optical light curve peaking at t = 3 and 15 days. The first peak is coincident with luminous, rapidly decaying Swift-XRT-discovered X-ray emission (L_x~10⁴¹ergs^–1 at 3 days; L_x ∝ t^–3), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≃ 500 kms^–1) originating from pre-existent circumstellar material (CSM). We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at r < 10¹⁵ cm and the resulting cooling emission. We calculate a total CSM mass of ∼7 x 10^–3 M_☉ (M_He/M_H ≃6) with particle density n ≃ 10⁹ cm^–3. Radio observations indicate a significantly lower density n < 10⁴ cm^–3 at larger radii r > (0.1-1) x 10¹⁷ cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of t_r = 13.4 ± 0.210 days and a peak B-band magnitude of M_B = -15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) x 10^–2 M_☉ of ⁵⁶Ni and ejected M_ej = (0.72 ± 0.040)M_☉ total with a kinetic energy E_k = (1.8 ± 0.10) x 10⁵⁰ erg. Finally, deep HST pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (∼10 M_☉) in binaries that lost most of their He envelope or white dwarfs (WDs). The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD+CO WD binaries.