We present the discovery of an optical transient (OT) in Messier 51, designated M51 OT2019-1 (also ZTF 19aadyppr, AT 2019abn, ATLAS19bzl), by the Zwicky Transient Facility (ZTF). The OT rose over 15 days to an observed luminosity of Mr = -13 (νLν = 9 × 106 Lo), in the luminosity gap between novae and typical supernovae (SNe). Spectra during the outburst show a red continuum, Balmer emission with a velocity width of ≈400 km s-1, Ca ii and [Ca ii] emission, and absorption features characteristic of an F-type supergiant. The spectra and multiband light curves are similar to the so-called "SN impostors" and intermediate-luminosity red transients (ILRTs). We directly identify the likely progenitor in archival Spitzer Space Telescope imaging with a 4.5 μm luminosity of M[4.5] ≈ -12.2 mag and a [3.6]-[4.5] color redder than 0.74 mag, similar to those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensive monitoring of M51 with Spitzer further reveals evidence for variability of the progenitor candidate at [4.5] in the years before the OT. The progenitor is not detected in pre-outburst Hubble Space Telescope optical and near-IR images. The optical colors during outburst combined with spectroscopic temperature constraints imply a higher reddening of E(B - V) ≈ 0.7 mag and higher intrinsic luminosity of Mr ≈ -14.9 mag (νLν = 5.3 × 107 Lo) near peak than seen in previous ILRT candidates. Moreover, the extinction estimate is higher on the rise than on the plateau, suggestive of an extended phase of circumstellar dust destruction. These results, enabled by the early discovery of M51 OT2019-1 and extensive pre-outburst archival coverage, offer new clues about the debated origins of ILRTs and may challenge the hypothesis that they arise from the electron-capture induced collapse of extreme asymptotic giant branch stars.
Bibliographical noteFunding Information:
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. DGE-1144469. H.E.B. acknowledges support from program numbers GO-14258 and AR-15005, provided by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This work is part of the research programme VENI, with project number 016.192.277, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). A.G.-Y. is supported by the EU via ERC grant No. 725161, the ISF, the BSF Transformative program and by a Kimmel award. R.D.G. was supported by NASA and the United States Air Force.
- circumstellar matter
- galaxies: individual (M51)
- stars: evolution
- stars: variables: general
- stars: winds, outflows
- supernovae: individual (M51 OT2019-1)