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

TL;DR

This study uses Hubble Space Telescope imaging to analyze luminous infrared transients and variables identified by the SPIRITS survey, revealing their nature, origins, and classifications in nearby galaxies.

Contribution

It provides detailed optical and near-IR follow-up of IR transients and variables, clarifying their classifications and origins, especially distinguishing between different types of dusty stellar objects.

Findings

Most variables are associated with star-forming regions, indicating massive star origins.

Many initially classified as transients are actually periodic or irregular variables.

Some SPRITEs are dusty classical novae or AGB stars with specific pulsation periods.

Abstract

SPIRITS--the SPitzer InfraRed Intensive Transients Survey--searched for luminous infrared (IR) transients and variables in nearly 200 nearby galaxies from 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 in ground-based optical surveys. We classify them as (1) transients, (2) periodic variables, and (3) irregular variables. The transients include "SPRITE"s (eSPecially Red Intermediate-luminosity Transient Events), having maximum luminosities fainter than supernovae, red IR colors, and a wide range of outburst durations (days to years). Here we report deep optical and near-IR imaging with the Hubble Space Telescope (HST) of 21 SPIRITS variables. They were initially considered SPRITE transients, but many eventually proved instead to be periodic or irregular variables as more data were collected. HST images show most of these cool and dusty variables are associated with star-forming regions in late-type galaxies, implying an origin in massive stars. Two SPRITEs lacked optical progenitors in deep pre-outburst HST images; however, one was detected during eruption at J and H, indicating a dusty object with an effective temperature of ~1050 K. One faint SPRITE turned out to be a dusty classical nova. About half the HST targets proved to be periodic variables, with pulsation periods of 670-2160 days; they are likely dusty asymptotic-giant-branch (AGB) stars with masses of ~5-10 Msun. A few of them were warm enough to be detected in deep HST frames, but most are too cool. Out of six irregular variables, two were red supergiants with optical counterparts in HST images; four were too enshrouded for HST detection.