arXiv subjects

Frank J. Masci

Publications and source records attributed to Frank J. Masci.

At least 109 records · Page 6Linked to original sources

arXiv2020

An 8.8 minute orbital period eclipsing detached double white dwarf binary

We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just $8.8$ minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its $6.91\, \rm min$ counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of $d=2120^{+131}_{-115}\,\rm pc$, LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of $87\pm5$ after four years. We find component masses of $M_A= 0.349^{+0.093}_{-0.074}\,M_\odot$ and $M_B=0.384^{+0.114}_{-0.074}\,M_\odot$, radii of $R_A=0.0308^{+0.0026}_{-0.0025}\,R_\odot$ and $R_B = 0.0291^{+0.0032}_{-0.0024}\,R_\odot$, and effective temperatures of $T_A=22200^{+1800}_{-1600}\,\rm K$ and $T_B=16200^{+1200}_{-1000}\,\rm K$. We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint ($r>21 \, m_{\rm AB}$) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.

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arXiv2020

The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics

We present a public catalog of transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS), a magnitude-limited (m<19 mag in either the g or r filter) survey for extragalactic transients in the ZTF public stream. We introduce cuts on survey coverage, sky visibility around peak light, and other properties unconnected to the nature of the transient, and show that the resulting statistical sample is spectroscopically 97% complete at <18 mag, 93% complete at <18.5 mag, and 75% complete at <19 mag. We summarize the fundamental properties of this population, identifying distinct duration-luminosity correlations in a variety of supernova (SN) classes and associating the majority of fast optical transients with well-established spectroscopic SN types (primarily SN Ibn and II/IIb). We measure the Type Ia SN and core-collapse (CC) SN rates and luminosity functions, which show good consistency with recent work. About 7% of CC SNe explode in very low-luminosity galaxies (M_i > -16 mag), 10% in red-sequence galaxies, and 1% in massive ellipticals. We find no significant difference in the luminosity or color distributions between the host galaxies of Type II and Type Ib/c supernovae, suggesting that line-driven wind stripping does not play a major role in the loss of the hydrogen envelope from their progenitors. Future large-scale classification efforts with ZTF and other wide-area surveys will provide high-quality measurements of the rates, properties, and environments of all known types of optical transients and limits on the existence of theoretically predicted but as of yet unobserved explosions.

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arXiv2020

The ZTF Source Classification Project: II. Periodicity and variability processing metrics

The current generation of all-sky surveys is rapidly expanding our ability to study variable and transient sources. These surveys, with a variety of sensitivities, cadences, and fields of view, probe many ranges of timescale and magnitude. Data from the Zwicky Transient Facility (ZTF) yields an opportunity to find variables on timescales from minutes to months. In this paper, we present the codebase, ztfperiodic, and the computational metrics employed for the catalogue based on ZTF's Second Data Release. We describe the publicly available, graphical-process-unit optimized period-finding algorithms employed, and highlight the benefit of existing and future graphical-process-unit clusters. We show how generating metrics as input to catalogues of this scale is possible for future ZTF data releases. Further work will be needed for future data from the Vera C. Rubin Observatory's Legacy Survey of Space and Time.

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arXiv2020

Optical follow-up of the neutron star-black hole mergers S200105ae and S200115j

LIGO and Virgo's third observing run (O3) revealed the first neutron star-black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements creating optical/near-IR "kilonova" (KN) emission. The joint gravitational-wave (GW) and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter, and independently measure the local expansion rate of the universe. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility (ZTF). ZTF observed $\sim$\,48\% of S200105ae and $\sim$\,22\% of S200115j's localization probabilities, with observations sensitive to KNe brighter than $-$17.5\,mag fading at 0.5\,mag/day in g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art KN models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with depths of $\rm m_{\rm AB}\approx 22$ mag, attainable in meter-class, wide field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high BH spins, and large neutron star radii.

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arXiv2020

A systematic search of Zwicky Transient Facility data for ultracompact binary LISA-detectable gravitational-wave sources

Using photometry collected with the Zwicky Transient Facility (ZTF), we are conducting an ongoing survey for binary systems with short orbital periods ($P_{\rm b}<1\rm \,hr)$ with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). Here, we present a sample of fifteen binary systems discovered thus far, with orbital periods ranging from $6.91\rm\,min$ to $56.35\rm\,min$. Of the fifteen systems, seven are eclipsing systems which do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum (AM CVn) systems and six systems exhibiting primarily ellipsoidal variations in their light curves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios (SNR), and a discussion of their physical characteristics.

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arXiv2020

A large fraction of hydrogen-rich supernova progenitors experience elevated mass loss shortly prior to explosion

Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the spatial extent of such material is compact, and hence imply an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility (ZTF) survey. We find that at least six out of ten events for which a spectrum was obtained within two days of estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction ($>30\%$ at $95\%$ confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.

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arXiv2020

Characterization of Temporarily-Captured Minimoon 2020 CD$_3$ by Keck Time-resolved Spectrophotometry

We present time-resolved visible spectrophotometry of minimoon 2020 CD$_3$, the second asteroid known to become temporarily captured by the Earth-Moon system's gravitational field. The spectrophotometry was taken with Keck I/LRIS between wavelengths 434 nm and 912 nm in $B$, $g$, $V$, $R$, $I$ and RG850 filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The spectrophotometry of 2020 CD$_3$ most closely resembles the spectra of V-type asteroids and some Lunar rock samples with a reddish slope of ~18$\%$/100 nm between 434 nm and 761 nm corresponding to colors of $g$-$r$ = 0.62$\pm$0.08, $r$-$i$ = 0.21 $\pm$ 0.06 and an absorption band at ~900 nm corresponding to $i$-$z$ = -0.54$\pm$0.10. Combining our measured 31.9$\pm$0.1 absolute magnitude with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD$_3$'s diameter to be ~0.9$\pm$0.1 m making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect periodic lightcurve variations with a $<$10$^{-4}$ false alarm probability corresponding to a lightcurve period of ~573 s and a lightcurve amplitude of ~1 mag implying 2020 CD$_3$ possesses a $b/a$ axial ratio of ~2.5. In addition, we extend the observational arc of 2020 CD$_3$ to 37 days between 2020 February 15 UTC and 2020 March 23 UTC. From the improved orbital solution for 2020 CD$_3$, we estimate its likely duration of its capture to be ~2 y, and we measure the non-gravitation perturbation on its orbit due to radiation pressure with an area-to-mass ratio of 6.9$\pm$2.4$\times$10$^{-4}$ m$^2$/kg implying a density of 2.3$\pm$0.8 g/cm$^3$, broadly compatible with the densities of other meter-scale asteroids and Lunar rock. We searched for pre-discovery detections of 2020 CD$_3$ in the ZTF archive as far back as 2018 October, but were unable to locate any positive detections.

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arXiv2020

Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-ray Burst Triggers

The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found during optical surveys of the sky, independent of gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded $>11,200$ candidates, 24 of which passed quality checks and strict selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star-black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting, catalog cross-matching, and study of their color evolution. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. In addition, we identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB190106A, and the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat and linearly decaying light curves and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be $R < 1775$ Gpc$^{-3}$ yr$^{-1}$ at 95% confidence level by requiring at least 2 high-significance detections. By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of $R < 4029$ Gpc$^{-3}$ yr$^{-1}$.

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arXiv2020

A new class of Roche lobe-filling hot subdwarf binaries

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\rm sdOB}=0.41\pm0.04$ M$_\odot$ and a WD mass of $M_{\rm WD}=0.68\pm0.05$ M$_\odot$. The weak eclipses imply a WD black-body temperature of $63,000\pm10,000$ K and a radius $R_{\rm WD}=0.0148\pm0.0020$ M$_\odot$ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for $\approx1$ Myrs at a rate of $10^{-9} M_\odot {\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $\approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase. Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with $T_{\rm eff}\approx25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD.

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arXiv2020

A non-equipartition shockwave traveling in a dense circumstellar environment around SN2020oi

We report the discovery and panchromatic followup observations of the young Type Ic supernova, SN2020oi, in M100, a grand design spiral galaxy at a mere distance of $14$ Mpc. We followed up with observations at radio, X-ray and optical wavelengths from only a few days to several months after explosion. The optical behaviour of the supernova is similar to those of other normal Type Ic supernovae. The event was not detected in the X-ray band but our radio observation revealed a bright mJy source ($L_ν \approx 1.2 \times 10^{27} {\rm erg\,s}^{-1} {\rm Hz}^{-1}$). Given, the relatively small number of stripped envelope SNe for which radio emission is detectable, we used this opportunity to perform a detailed analysis of the comprehensive radio dataset we obtained. The radio emitting electrons initially experience a phase of inverse Compton cooling which leads to steepening of the spectral index of the radio emission. Our analysis of the cooling frequency points to a large deviation from equipartition at the level of $ε_e/ε_B \gtrsim 200$, similar to a few other cases of stripped envelope SNe. Our modeling of the radio data suggests that the shockwave driven by the SN ejecta into the circumstellar matter (CSM) is moving at $\sim 3\times 10^{4}\,{\rm km\,s}^{-1}$. Assuming a constant mass-loss from the stellar progenitor, we find that the mass-loss rate is $\dot{M} \approx 1.4\times 10^{-4}\,{M}_{\odot}\,{\rm yr}^{-1}$, for an assumed wind velocity of $1000\,{\rm km\,s}^{-1}$. The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard $r^{-2}$.

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arXiv2020

Kilonova Luminosity Function Constraints based on Zwicky Transient Facility Searches for 13 Neutron Star Mergers

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run. We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization of 4480 deg^2, median distance of 267 Mpc and false alarm rates ranging from 1.5 to 1e-25 per yr. The ZTF coverage had a median enclosed probability of 39%, median depth of 20.8mag, and median response time of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UVOIR photometric points, 64 OIR spectra, and 3 radio. We find no promising kilonova (radioactivity-powered counterpart) and we convert the upper limits to constrain the underlying kilonova luminosity function. Assuming that all kilonovae are at least as luminous as GW170817 at discovery (-16.1mag), we calculate our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6mag (extrapolated peak magnitude of GW170817) and fade at 1 mag/day (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, <57% (<89%) of putative kilonovae could be brighter than -16.6mag assuming flat (fading) evolution, at 90% confidence. If we further account for the online terrestrial probability for each GW trigger, we find that <68% of putative kilonovae could be brighter than -16.6mag. Comparing to model grids, we find that some kilonovae must have Mej < 0.03 Msun or Xlan>1e-4 or phi>30deg to be consistent with our limits. (Abridged)

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arXiv2020

SN2019dge: a Helium-rich Ultra-Stripped Envelope Supernova

We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova with a fast-evolving light curve indicating an extremely low ejecta mass ($\approx 0.3\,M_\odot$) and low kinetic energy ($\approx 1.2\times 10^{50}\,{\rm erg}$). Early-time (<4 d after explosion) photometry reveal evidence of shock cooling from an extended helium-rich envelope of $\sim0.1\,M_\odot$ located at $\sim 3\times 10^{12}\,{\rm cm}$ from the progenitor. Early-time He II line emission and subsequent spectra show signatures of interaction with helium-rich circumstellar material, which extends from $\gtrsim 5\times 10^{13}\,{\rm cm}$ to $\gtrsim 2\times 10^{16}\,{\rm cm}$. We interpret SN2019dge as a helium-rich supernova from an ultra-stripped progenitor, which originates from a close binary system consisting of a mass-losing helium star and a low-mass main sequence star or a compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the range of 1400--8200$\,{\rm Gpc^{-3}\, yr^{-1}}$ (i.e., 2--12% of core-collapse supernova rate). This can be compared to the observed coalescence rate of compact neutron star binaries that are not formed by dynamical capture.

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arXiv2020

Recurring Outbursts of P/2019 LM$_4$ (Palomar)

We present a preliminary analysis of comet P/2019 LM$_4$ (Palomar) as observed by the Zwicky Transient Facility (ZTF) survey in 2019 and 2020. We find that the discovery of the comet in 2019 and the recovery in 2020 is largely attributed to two separate outbursts that are $\gtrsim2$ and $\gtrsim3.9$ mag in strength. The outbursts occurred around the end of April to early May of 2019 as well as between 2020 May 8.31 and 9.52 UTC, respectively.

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arXiv2020

Zwicky Transient Facility constraints on the optical emission from the nearby repeating FRB 180916.J0158+65

The discovery rate of fast radio bursts (FRBs) is increasing dramatically thanks to new radio facilities. Meanwhile, wide-field instruments such as the 47 deg$^2$ Zwicky Transient Facility (ZTF) survey the optical sky to study transient and variable sources. We present serendipitous ZTF observations of the CHIME repeating source FRB 180916.J0158+65, that was localized to a spiral galaxy 149 Mpc away and is the first FRB suggesting periodic modulation in its activity. While 147 ZTF exposures corresponded to expected high-activity periods of this FRB, no single ZTF exposure was at the same time as a CHIME detection. No $>3σ$ optical source was found at the FRB location in 683 ZTF exposures, totalling 5.69 hours of integration time. We combined ZTF upper limits and expected repetitions from FRB 180916.J0158+65 in a statistical framework using a Weibull distribution, agnostic of periodic modulation priors. The analysis yielded a constraint on the ratio between the optical and radio fluences of $η\lesssim 200$, corresponding to an optical energy $E_{\rm opt} \lesssim 3 \times 10^{46}$ erg for a fiducial 10 Jy ms FRB (90% confidence). A deeper (but less statistically robust) constraint of $η\lesssim 3$ can be placed assuming a rate of $r(>5$ Jy ms)= hr$^{-1}$ and $1.2\pm 1.1$ FRB occurring during exposures taken in high-activity windows. The constraint can be improved with shorter per-image exposures and longer integration time, or observing FRBs at higher Galactic latitudes. This work demonstrated how current surveys can statistically constrain multi-wavelength counterparts to FRBs even without deliberately scheduled simultaneous radio observation.

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arXiv2020

Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations

We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2019 November 29 using the GROWTH, the APO ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery observations of 2I obtained by ZTF from 2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System comets shows an object very similar to mildly active Solar System comets with an out-gassing rate of $\sim$10$^{27}$ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of $\sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H$_2$O sublimation with the comet being active with super volatiles such as CO at heliocentric distances $>$6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I's nucleus of $\lesssim$1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size-distribution resulting in a slope of $\sim$3.4$\pm$1.2, similar to Solar System comets and bodies produced from collisional equilibrium.

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arXiv2020

A Search for Extra-Tidal RR Lyrae in the Globular Cluster NGC 5024 and NGC 5053

Recently, Kundu et al (2019) reported that the globular cluster NGC 5024 (M53) possesses five extra-tidal RR Lyrae. In fact, four of them were instead known members of a nearby globular cluster NGC 5053. The status of the remaining extra-tidal RR Lyrae is controversial depending on the adopted tidal radius of NGC 5024. We have also searched for additional extra-tidal RR Lyrae within an area of $\sim8$~deg$^2$ covering both globular clusters. This includes other known RR Lyrae within the search area, as well as stars that fall within the expected range of magnitudes and colors for RR Lyrae (and yet outside the cutoff of 2/3 of the tidal radii of each globular clusters for something to be called "extra-tidal") if they were extra-tidal RR Lyrae candidates for NGC 5024 or NGC 5053. Based on the the proper motion information and their locations on the color-magnitude diagram, none of the known RR Lyrae belong to the extra-tidal RR Lyrae of either globular clusters. In the cases where the stars satisfied the magnitude and color ranges of RR Lyrae, analysis of time series data taken from the Zwicky Transient Facility do not reveal periodicities, suggesting that none of these stars are RR Lyrae. We conclude that there are no extra-tidal RR Lyrae associated with either NGC 5024 or NGC 5053 located within our search area.

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arXiv2020

ZTF Early Observations of Type Ia Supernovae III: Early-Time Colors as a Test for Explosion Models and Multiple Populations

Colors of Type Ia supernovae in the first few days after explosion provide a potential discriminant between different models. In this paper, we present $g-r$ colors of 65 Type Ia supernovae discovered within 5 days from first light by the Zwicky Transient Facility in 2018, a sample that is about three times larger than that in the literature. We find that $g-r$ colors are intrinsically rather homogeneous at early phases, with about half of the dispersion attributable to photometric uncertainties ($σ_\mathrm{noise}\simσ_\mathrm{int}\sim$ 0.18 mag). Colors are nearly constant starting from 6 days after first light ($g-r\sim-0.15$ mag), while the time evolution at earlier epochs is characterized by a continuous range of slopes, from events rapidly transitioning from redder to bluer colors (slope of $\sim-0.25$ mag day$^{-1}$) to events with a flatter evolution. The continuum in the slope distribution is in good agreement both with models requiring some amount of $^{56}$Ni mixed in the outermost regions of the ejecta and with "double-detonation" models having thin helium layers ($M_\mathrm{He}=0.01\,M_\odot$) and varying carbon-oxygen core masses. At the same time, six events show evidence for a distinctive "red bump" signature predicted by "double-detonation" models with larger helium masses. We finally identify a significant correlation between the early-time $g-r$ slopes and supernova brightness, with brighter events associated to flatter color evolution (p-value=0.006). The distribution of slopes, however, is consistent with being drawn from a single population, with no evidence for two components as claimed in the literature based on $B-V$ colors.

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arXiv2020

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

(Abridged) Using the Zwicky Transient Facility alert stream, we are conducting a large campaign to spectroscopically classify all transients occurring in galaxies in the Census of the Local Universe (CLU) catalog. The aim of the experiment is to construct a spectroscopically complete, volume-limited sample of transients coincident within 100" of CLU galaxies out to 200 Mpc, and to a depth of 20 mag. We describe the survey design and spectroscopic completeness from the first 16 months of operations. We present results from a systematic search for Calcium rich gap transients in the sample of 22 low luminosity (peak absolute magnitude $M > -17$), hydrogen poor events found in the experiment (out of 754 spectroscopically classified SNe). We report the detection of eight Calcium rich gap transients, and constrain their volumetric rate to be at least $\approx 15\pm5$% of the SN Ia rate. Combining this sample with ten events from the literature, we find a likely continuum of spectroscopic properties ranging from events with SN Ia-like features (Ca-Ia objects) to SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within the Ca-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 nebular phase. Together, we find that the spectroscopic continuum, volumetric rates and striking old environments are consistent with the explosive burning of He shells on low mass white dwarfs. We posit that Ca-Ia and red Ca-Ib/c objects are consistent with the double detonation of He shells with high He burning efficiency, while green Ca-Ib/c objects could arise from less efficient He burning scenarios such as detonations in low density He shells or He shell deflagrations.

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