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SPIRITS--the SPitzer InfraRed Intensive Transients Survey--searched forluminous infrared (IR) transients and variables in nearly 200 nearby galaxiesfrom 2014 to 2019, using the warm Spitzer telescope at 3.6 and 4.5 microns.Among the SPIRITS variables are IR-bright objects that are undetected inground-based optical surveys. We classify them as (1) transients, (2) periodicvariables, and (3) irregular variables. The transients include "SPRITE"s(eSPecially Red Intermediate-luminosity Transient Events), having maximumluminosities fainter than supernovae, red IR colors, and a wide range ofoutburst durations (days to years). Here we report deep optical and near-IRimaging with the Hubble Space Telescope (HST) of 21 SPIRITS variables. Theywere initially considered SPRITE transients, but many eventually proved insteadto be periodic or irregular variables as more data were collected. HST imagesshow most of these cool and dusty variables are associated with star-formingregions in late-type galaxies, implying an origin in massive stars. Two SPRITEslacked optical progenitors in deep pre-outburst HST images; however, one wasdetected during eruption at J and H, indicating a dusty object with aneffective temperature of ~1050 K. One faint SPRITE turned out to be a dustyclassical nova. About half the HST targets proved to be periodic variables,with pulsation periods of 670-2160 days; they are likely dustyasymptotic-giant-branch (AGB) stars with masses of ~5-10 Msun. A few of themwere warm enough to be detected in deep HST frames, but most are too cool. Outof six irregular variables, two were red supergiants with optical counterpartsin HST images; four were too enshrouded for HST detection.

Hubble Space Telescope Imaging of Luminous Extragalactic Infrared Transients and Variables from the SPIRITS Survey