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

• Published in: The Astrophysical journal Vol. 902; no. 1; pp. 46 - 80
• Main Authors: Yang, Yi, Hoeflich, Peter, Baade, Dietrich, Maund, Justyn R., Wang, Lifan, Brown, Peter. J., Stevance, Heloise F., Arcavi, Iair, Burke, Jamison, Cikota, Aleksandar, Clocchiatti, Alejandro, Gal-Yam, Avishay, Graham, Melissa. L., Hiramatsu, Daichi, Hosseinzadeh, Griffin, Howell, D. Andrew, Jha, Saurabh W., McCully, Curtis, Patat, Ferdinando, Sand, David. J., Schulze, Steve, Spyromilio, Jason, Valenti, Stefano, Vinkó, József, Wang, Xiaofeng, Wheeler, J. Craig, Yaron, Ofer, Zhang, Jujia
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.10.2020; IOP Publishing; Institute of Physics (IOP)
• Subjects: ASTRONOMY AND ASTROPHYSICS; Astrophysics; Companion stars; Ejecta; European Southern Observatory; Explosions; Galaxies; individual galaxies; individual supernovae; Optical observations; Polarimetry; Polarization; Spectro-polarimetry; Spectrophotometry; Spectroscopy; Spectrum analysis; Stellar envelopes; Supernova; Type Ia supernovae; Very Large Telescope; White dwarf stars
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/aba759

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 λ 6355 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.