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

  • Author(s): Yang, Y;
  • Hoeflich, P;
  • Baade, D;
  • Maund, JR;
  • Wang, L;
  • Brown, PJ;
  • Stevance, HF;
  • Arcavi, I;
  • Burke, J;
  • Cikota, A;
  • Clocchiatti, A;
  • Gal-Yam, A;
  • Graham, ML;
  • Hiramatsu, D;
  • Hosseinzadeh, G;
  • Howell, DA;
  • Jha, SW;
  • McCully, C;
  • Patat, F;
  • Sand, DJ;
  • Schulze, S;
  • Spyromilio, J;
  • Valenti, S;
  • Vinkó, J;
  • Wang, X;
  • Wheeler, JC;
  • Yaron, O;
  • Zhang, J
  • et al.

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.

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