Astrophys. J., 902, 139-139 (2020/October-3)
Direct evidence of two-component ejecta in Supernova 2016gkg from nebular spectroscopy.
KUNCARAYAKTI H., FOLATELLI G., MAEDA K., DESSART L., JERKSTRAND A., ANDERSON J.P., AOKI K., BERSTEN M.C., FERRARI L., GALBANY L., GARCIA F., GUTIERREZ C.P., HATTORI T., KAWABATA K.S., KRAVTSOV T., LYMAN J.D., MATTILA S., OLIVARES F.E., SANCHEZ S.F. and VAN DYK S.D.
Abstract (from CDS):
Spectral observations of the type-IIb supernova (SN) 2016gkg at 300-800 days are reported. The spectra show nebular characteristics, revealing emission from the progenitor star's metal-rich core and providing clues to the kinematics and physical conditions of the explosion. The nebular spectra are dominated by emission lines of [O I] λλ6300, 6364 and [Ca II] λλ7292, 7324. Other notable, albeit weaker, emission lines include Mg I] λ4571, [Fe II] λ7155, O I λ7774, Ca II triplet, and a broad, boxy feature at the location of Hα. Unlike in other stripped-envelope SNe, the [O I] doublet is clearly resolved due to the presence of strong narrow components. The doublet shows an unprecedented emission line profile consisting of at least three components for each [O I]λ6300, 6364 line: a broad component (width ∼2000 km s–1), and a pair of narrow blue and red components (width ∼300 km s–1) mirrored against the rest velocity. The narrow component appears also in other lines, and is conspicuous in [O I]. This indicates the presence of multiple distinct kinematic components of material at low and high velocities. The low-velocity components are likely to be produced by a dense, slow-moving emitting region near the center, while the broad components are emitted over a larger volume. These observations suggest an asymmetric explosion, supporting the idea of two-component ejecta that influence the resulting late-time spectra and light curves. SN 2016gkg thus presents striking evidence for significant asymmetry in a standard-energy SN explosion. The presence of material at low velocity, which is not predicted in 1D simulations, emphasizes the importance of multidimensional explosion modeling of SNe.
© 2020. The American Astronomical Society. All rights reserved.
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<Available at CDS (J/ApJ/902/139): fig1.dat>
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