The progenitor of the supernova imposter AT 2019krl: a SN 2008S-like transient from a blue supergiant

For a recent intermediate-luminosity transient, AT 2019krl in M74 (NGC 628) at a distance of only ∼ 9.8 Mpc, extensive archival Hubble Space Telescope (HST), Spitzer Space Telescope, and Large Binocular Telescope (LBT) imaging reveal a bright optical and mid-infrared progenitor star. While the optical peak of the event was missed, a peak was detected in the infrared with an absolute magnitude of M_{4.5 µm} = −18.4 mag, leading us to infer a visual-wavelength peak absolute magnitude of −13.5 to −14.5. The light curve from the pre-discovery archival data indicated no outbursts over the previous 16 yr. The colors, magnitudes, and inferred temperatures of the progenitor best match a 13–14 M yellow or blue supergiant, if only foreground extinction is taken into account, or a hotter and more massive star, if any additional local extinction is included. A pre-eruption spectrum of the star reveals strong Hα emission having a narrow line core with a width of about 200 km s⁻¹ (FWHM) and with wings extending to ± 2000 km s⁻¹. The post-eruption spectrum is fairly flat and featureless with only Hα, Na I D, [Ca II], and the Ca II near-infrared triplet in emission, with very little change in the shape of Hα over 120 days. As in many previous intermediate-luminosity transients, AT 2019krl shows remarkable similarities to both massive luminous blue variable (LBV) eruptions and SN 2008S-like events. However, in this case, the information about the pre-eruption star allows us to clearly rule out both a super-AGB star and an electron-capture SN as the origin of this SN 2008S-like event. Instead, the data favor either a relatively unobscured blue supergiant (likely viewed pole-on) or a highly extinguished LBV with M > 20 M, confirming that BSGs or LBVs that may undergo mergers are a viable progenitor pathway to produce SN 2008S-like events.