Strong Near-Infrared Carbon Absorption in the Transitional Type Ia SN 2015bp
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Publisher version: S. D. Wyatt et al. Strong Near-Infrared Carbon Absorption in the Transitional Type Ia SN 2015bp. 2021 ApJ 914 57 [https://doi.org/10.3847/1538-4357/abf7c3]
SponsorshipNational Science Foundation (NSF) AST-1821967 AST-1821987 AST-1813708 AST-1813466 AST-1908972 PHY-1607611 AST1813176 AST-2008108; Heising-Simons Foundation 20201864; National Science Foundation (NSF) AST-1008343 AST-1613426 AST-1613455 AST-1613472; Danish Agency for Science and Technology and Innovation; Florida Space Grant Consortium; European Commission 839090; Spanish grant within the European Funds for Regional Development (FEDER) PGC2018-095317-B-C21; Villum Foundation 13261 28021; Independent Research Fund Denmark 8021-00170B; TABASGO Foundation; Christopher R. Redlich Fund; Miller Institute for Basic Research in Science (U.C. Berkeley); International Gemini Observatory GN-2015A-Q-8 GS-2015A-Q-5; W.M. Keck Foundation; University of Hawaii 80HQTR19D0030; National Aeronautics & Space Administration (NASA); ESO Telescopes at the La Silla Paranal Observatory 188.D-3003 191.D-0935: PESSTO
Unburned carbon is potentially a powerful probe of Type Ia supernova (SN) explosion mechanisms. We present comprehensive optical and near-infrared (NIR) data on the "transitional" Type Ia SN 2015bp. An early NIR spectrum (t=−9.9 days with respect to B-band maximum) displays a striking C I λ1.0693μm line at 11.9×103~km s−1, distinct from the prominent Mg II λ1.0927μm feature, which weakens toward maximum light. SN 2015bp also displays a clear C II λ6580A notch early (t=−10.9 days) at 13.2×103~km s−1, consistent with our NIR carbon detection. At MB=−18.46, SN 2015bp is less luminous than a normal SN Ia and, along with iPTF13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the C I feature is misidentified He I λ1.0830μm because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times. Intrigued by these strong NIR carbon detections, but lacking NIR data for other SNe Ia, we investigated the incidence of optical carbon in the sample of nine transitional SNe Ia with early-time data (t≲−4 days). We find that four display C II λ6580A, while two others show tentative detections, in line with the SN Ia population as a whole. We conclude that at least ∼50% of transitional SNe Ia in our sample do not come from sub-Chandrasekhar mass explosions due to the clear presence of carbon in their NIR and optical spectra.