Extensive archival textit{Hubble Space Telescope}, textit{Spitzer Space Telescope}, and Large Binocular Telescope imaging of the recent intermediate-luminosity transient, AT~2019krl in M74, 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 $M_{4.5,mu {rm m}} = -18.4$ mag, leading us to infer a visual-wavelength peak absolute magnitude of $-$13.5 to $-$14.5. The pre-discovery light curve indicated no outbursts over the previous 16,yr. The colors, magnitudes, and inferred temperatures of the progenitor best match a 13--14 M$_{sun}$ yellow or blue supergiant (BSG), 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 H$alpha$ and [N~{sc ii}] emission with wings extending to $pm 2000$,km,s$^{-1}$. The post-eruption spectrum is fairly flat and featureless with only H$alpha$, ion{Na}{1}~D, [ion{Ca}{2}], and the ion{Ca}{2} triplet in emission. As in many previous intermediate-luminosity transients, AT~2019krl shows remarkable observational similarities to luminous blue variable (LBV) giant eruptions, SN~2008S-like events, and massive-star mergers. However, the information about the pre-eruption star favors either a relatively unobscured BSG or a more extinguished LBV with $M > 20$,M$_{sun}$ likely viewed pole-on.
تحميل البحث