The Young and Nearby Normal Type Ia Supernova 2018gv: UV-optical Observations and the Earliest Spectropolarimetry

Yi Yang, Peter Hoeflich, Dietrich Baade, Justyn R. Maund, Lifan Wang, Peter J. Brown, Heloise F. Stevance, Iair Arcavi, Jamison Burke, Aleksandar Cikota, Alejandro Clocchiatti, Avishay Gal-Yam, Melissa L. Graham, Daichi Hiramatsu, Griffin Hosseinzadeh, D. Andrew Howell, Saurabh W. Jha, Curtis McCully, Ferdinando Patat, David J. SandSteve Schulze, Jason Spyromilio, Stefano Valenti, József Vinkó, Xiaofeng Wang, J. Craig Wheeler, Ofer Yaron, Jujia Zhang

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Abstract

The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at-13.6 days relative to the B-band maximum light, or ∼5 days after the estimated explosion-the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si ii λ6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.

Original languageEnglish (US)
Article number46
JournalAstrophysical Journal
Volume902
Issue number1
DOIs
StatePublished - Oct 10 2020

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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