SPIRITS: SPitzer InfraRed Intensive Transients Survey

The dynamic mid-infrared sky is hitherto largely unexplored. We propose the SPitzer InfraRed Intensive Transients Survey (SPIRITS) --- a systematic search of 242 nearby galaxies within 20 Mpc, on timescales ranging between a day to a year, to a depth of 20 mag. In preparation for SPIRITS, we undertook three pilot programs: searching the WISE data stream for variables in nearby galaxies, mining the Spitzer Heritage Archive, and Spitzer follow-up of optically discovered transients. Our results are encouraging and motivate our design of SPIRITS to fill in missing pieces in our understanding of the end points of stellar evolution. We expect to discover explosive transients (ILRT, LRN, CNe, SNe), eruptive variables (LBV, RSG, YSG, AGB, RSG) and possibly new phenomena. SPIRITS will be the definitive study to ascertain the rate and origin of two new classes of red gap transients, quantify the contribution of classical novae to galactic chemical evolution and uncover supernovae buried in starbursts. SPIRITS will also systematically probe mass loss rates and dust formation in the most massive stars. SPIRITS will yield a census of supergiant variability and asymptotic giant branch variability in diverse galaxy environments. SPIRITS will likely discover the first extragalactic 'Born Again Giant' stars. The SPIRITS team is committed to a concomitant ground-based NIR and optical survey and extensive, panchromatic follow-up: 110 nights of near-IR imaging, 66 nights of optical imaging and 60 nights of spectroscopy annually. Follow-up will serve to maximize the discovery potential of our requested 994 hrs of Spitzer/IRAC observing time. We believe it is time that the Spitzer Great Observatory add another time-domain jewel in its crown.