ZTF18aalrxas: A Type IIb Supernova from a Very Extended Low-mass Progenitor

C. Fremling, H. Ko, A. Dugas, M. Ergon, J. Sollerman, A. Bagdasaryan, C. Barbarino, J. Belicki, E. Bellm, N. Blagorodnova, K. De, R. Dekany, S. Frederick, A. Gal-Yam, D. A. Goldstein, V. Z. Golkhou, M. Graham, M. Kasliwal, M. Kowalski, S. R. KulkarniT. Kupfer, R. R. Laher, F. J. Masci, A. A. Miller, J. D. Neill, D. A. Perley, U. D. Rebbapragada, R. Riddle, B. Rusholme, S. Schulze, R. M. Smith, L. Tartaglia, Lin Yan, Y. Yao

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova (SN) discovered during science validation of the Zwicky Transient Facility. ZTF18aalrxas was discovered while the optical emission was still rising toward the initial cooling peak (0.7 mag over 2 days). Our observations consist of multi-band (ultraviolet and optical) light curves (LCs), and optical spectra spanning from ≈0.7 to ≈180 days past the explosion. We use a Monte-Carlo based non-local thermodynamic equilibrium model that simultaneously reproduces both the 56Ni-powered bolometric LC and our nebular spectrum. This model is used to constrain the synthesized radioactive nickel mass (0.17 M) and the total ejecta mass (1.7 M) of the SN. The cooling emission is modeled using semi-analytical extended envelope models to constrain the progenitor radius (790-1050 R o) at the time of explosion. Our nebular spectrum shows signs of interaction with a dense circumstellar medium (CSM), and this spectrum is modeled and analyzed to constrain the amount of ejected oxygen (0.3-0.5 M) and the total hydrogen mass (≈0.15 M) in the envelope of the progenitor. The oxygen mass of ZTF18aalrxas is consistent with a low (12-13 M) zero-age main-sequence mass progenitor. The LCs and spectra of ZTF18aalrxas are not consistent with massive single-star SN Type IIb progenitor models. The presence of an extended hydrogen envelope of low mass, the presence of a dense CSM, the derived ejecta mass, and the late-time oxygen emission can all be explained in a binary model scenario.

Original languageEnglish (US)
Article numberL5
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Jun 10 2019


  • supernovae: general
  • supernovae: individual (ZTF18aalrxas)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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