Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs

Journal Article · Tue Jul 02 00:00:00 EDT 2019 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1093/mnras/stz1640· OSTI ID:1612508

Lyutikov, Maxim ^[1]; Toonen, Silvia ^[2]

---

1. Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907-2036, USA
2. Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE, Amsterdam, the Netherlands

We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class, AT2018cow, result from an electron-capture collapse to a neutron star following the merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less-massive WD during the merger and the formation of a shell-burning non-degenerate star incorporating the ONeMg core. During the shell-burning stage, a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after ~10²–10⁴ yr. During the formation of a neutron star, as little as |$${\sim } 10^{-2} \, \mathrm{M}_\odot$$| of the material is ejected at the bounce-off with mildly relativistic velocities and total energy of about a few 10⁵⁰ erg. This ejecta becomes optically thin on a time-scale of days – this is the FBOT. During the collapse, the neutron star is spun up and the magnetic field is amplified. The ensuing fast magnetically dominated relativistic wind from the newly formed neutron star shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high-energy emission in a manner similar to pulsar wind nebulae. If the secondary WD was of the DA type, the wind will likely have |$${\sim } 10^{-4} \, \mathrm{M}_\odot$$| of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high-energy emission of similar luminosity to the optical, and late infrared features.

• Research Organization: Purdue Univ., West Lafayette, IN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: SC0016369
• OSTI ID: 1612508
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 487, Issue 4; ISSN 0035-8711
• Publisher: Royal Astronomical Society
• Country of Publication: United States
• Language: English

Related Subjects

Astronomy & Astrophysics