The Blue Supergiant Progenitor of the Supernova Imposter AT 2019krl

Abstract

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

Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions

“…Our observational arguments disfavour a LBV-like eruption scenario for ILRTs. However, we note that Andrews et al (2020) proposed that a recent gap transient, AT 2019krl, was an unobscured ILRT eruption from a relatively massive star. Luminous red novae are another subclass of gap transients, and show double-peaked (or triple-peaked) light curves and a remarkable spectral evolution.…”

“…However, their origin remains mysterious and different scenarios are contemplated. These transients have been proposed to be LBV-like outbursts of moderate-mass stars (e.g., Smith et al 2009;Andrews et al 2020) or stellar mergers (e.g., Kasliwal et al 2011). A third suggested scenario is that we are actually observing faint CC SN explosions triggered by electron captures in the core of super-asymptotic giant branch (S-AGB) stars, and these are therefore labelled electroncapture (EC) SNe (see, e.g., Botticella et al 2009;Pumo et al 2009).…”

The PHANGS-MUSE survey

“…Finally, the DRP and DAP described in this paper were used, in a preliminary form, for the development of tools internally to the PHANGS collaboration. The associated preliminary dataset for NGC 0628 was specifically used by Andrews et al (2021). The main limitation of these early data versions is in the astrometric calibration, and it did not significantly impact the results in that paper.…”

A Rapidly Fading Star as a Type II Obscuring Intermediate Luminosity Optical Transient (ILOT) in a Triple-star System

“…There are several different names to the heterogeneous group of transients with typical luminosities between those of classical novae and supernovae and their subclasses (e.g., Mould et al 1990;Bond et al 2003;Rau et al 2007;Ofek et al 2008;Mason et al 2010;Kasliwal 2013;Tylenda et al 2013;Kaminski et al 2018;Pastorello et al 2018;Boian & Groh 2019;Cai et al 2019;Jencson et al 2019;Kashi et al 2019;Howitt et al 2020;Jones 2020;Kamiński et al 2020;Pastorello et al 2021aPastorello et al , 2021bAndrews et al 2021;Blagorodnova et al 2021;Kamiński et al 2021;Klencki et al 2021). These names include intermediate luminosity (red) transients, red novae, luminous red novae (e.g., Jencson et al 2019;Blagorodnova et al 2021), and gap transients (e.g., Kasliwal 2011;Blagorodnova et al 2017;.…”