Measuring an Off-center Detonation through Infrared Line Profiles: The Peculiar Type Ia Supernova SN 2020qxp/ASASSN-20jq

We present and analyze a near-infrared (NIR) spectrum of the underluminous Type Ia supernova SN 2020qxp/ASASSN-20jq obtained with NIRES at the Keck Observatory, 191 days after B-band maximum. The spectrum is dominated by a number of broad emission features, including the [Fe ii] at 1.644 μm, which is highly asymmetric with a tilted top and a peak redshifted by ≈2000 km s-1. In comparison with 2D non-LTE synthetic spectra computed from 3D simulations of off-center delayed-detonation Chandrasekhar-mass (Mch) white dwarf (WD) models, we find good agreement between the observed lines and the synthetic profiles, and are able to unravel the structure of the progenitor's envelope. We find that the size and tilt of the [Fe ii] 1.644 μm profile (in velocity space) is an effective way to determine the location of an off-center delayed-detonation transition (DDT) and the viewing angle, and it requires a WD with a high central density of ∼4 × 109 g cm-3. We also tentatively identify a stable Ni feature around 1.9 μm characterized by a "pot-belly"profile that is slightly offset with respect to the kinematic center. In the case of SN 2020qxp/ASASSN-20jq, we estimate that the location of the DDT is ∼0.3MWD off center, which gives rise to an asymmetric distribution of the underlying ejecta. We also demonstrate that low-luminosity and high-density WD SN Ia progenitors exhibit a very strong overlap of Ca and 56Ni in physical space. This results in the formation of a prevalent [Ca ii] 0.73 μm emission feature that is sensitive to asymmetry effects. Our findings are discussed within the context of alternative scenarios, including off-center C/O detonations in He-triggered sub-MCh WDs and the direct collision of two WDs. Snapshot programs with Gemini/Keck/Very Large Telescope (VLT)/ELT-class instruments and our spectropolarimetry program are complementary to mid-IR spectra by the James Webb Space Telescope (JWST).
P.H., A.F., B.H., D.C., and E.Y.H. acknowledge support by the National Science Foundation (NSF) grant AST1715133. E.B. was supported in part by NASA grant 80NSSC20K0538. C.A. and B.J.S. are supported by NASA grant 80NSSC19K1717 and NSF grants AST-1920392 and AST-1911074. M.S. is supported by grants from the Villum FONDEN (28021) and the Independent Research Fund Denmark (8021-00170B). M.A.T. acknowledges support from the DOE CSGF through grant DE-SC0019323. L.G. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal program RYC2019-027683 and from the Spanish MICIU project PID2020-115253GA-I00. C.R.B., E.Y.H., M. M.P., N.B.S. and P.H. are supported by the NSF grant AST1613472.