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(Abridged) Using the Zwicky Transient Facility alert stream, we areconducting a large campaign to spectroscopically classify all transientsoccurring in galaxies in the Census of the Local Universe (CLU) catalog. Theaim of the experiment is to construct a spectroscopically complete,volume-limited sample of transients coincident within 100" of CLU galaxies outto 200 Mpc, and to a depth of 20 mag. We describe the survey design andspectroscopic completeness from the first 16 months of operations. We presentresults from a systematic search for Calcium rich gap transients in the sampleof 22 low luminosity (peak absolute magnitude $M > -17$), hydrogen poor eventsfound in the experiment (out of 754 spectroscopically classified SNe). Wereport the detection of eight Calcium rich gap transients, and constrain theirvolumetric rate to be at least $\approx 15\pm5$% of the SN Ia rate. Combiningthis sample with ten events from the literature, we find a likely continuum ofspectroscopic properties ranging from events with SN Ia-like features (Ca-Iaobjects) to SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within theCa-Ib/c events, we find two populations of events distinguished by their red($g - r \approx 1.5$ mag) or green ($g - r \approx 0.5$ mag) spectral colors at$r$-band peak, wherein redder events show strong line blanketing signatures,slower light curves, weaker He lines and lower [Ca II]/[O I] in the nebularphase. Together, we find that the spectroscopic continuum, volumetric rates andstriking old environments are consistent with the explosive burning of Heshells on low mass white dwarfs. We posit that Ca-Ia and red Ca-Ib/c objectsare consistent with the double detonation of He shells with high He burningefficiency, while green Ca-Ib/c objects could arise from less efficient Heburning scenarios such as detonations in low density He shells or He shelldeflagrations.

astrophysical journal/the astrophysical journal

The Zwicky Transient Facility Census of the Local Universe I: Systematic  search for Calcium rich gap transients reveal three related spectroscopic  sub-classes