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 ( ${L}_{{\rm{x}}}\approx {10}^{41}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ at 3 days; Lx ∝ t-3), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≍ 500 $\mathrm{km}\,{{\rm{s}}}^{-1}$ ) 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 ), and a Shane/Kast spectral detection of narrow Hα and He II emission lines (v ≍ 500 $\mathrm{km}\,{{\rm{s}}}^{-1}$ ) 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 15 cm and the resulting cooling emission. We calculate a total CSM mass of ∼7 × 10-3 ${M}_{\odot }$ (MHe/MH ≍6) with particle density n ≍ 109 cm-3. Radio observations indicate a significantly lower density n . Radio observations indicate a significantly lower density n 4 cm-3 at larger radii r > (0.1-1) × 1017 cm. The photometric and spectroscopic properties during the second light-curve peak are consistent with those of Ca-rich transients (rise-time of tr = 13.4 ± 0.210 days and a peak B-band magnitude of MB = -15.1 ± 0.200 mag). We find that SN 2019ehk synthesized (3.1 ± 0.11) × 10-2 ${M}_{\odot }$ of ${}^{56}\mathrm{Ni}$ and ejected Mej = (0.72 ± 0.040) ${M}_{\odot }$ total with a kinetic energy Ek = (1.8 ± 0.10) × 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 ${M}_{\odot }$ ) 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.
