SN 2017fzw: A Fast-Expanding Type Ia Supernova with Transitional Features

In this study, we analyzed the optical observations of a subluminous Type Ia supernova (SN Ia) 2017fzw, which exhibited high photospheric velocity (HV) at <i>B</i>-band maximum light. The absolute <i>B</i>-band peak magnitude was determined to be <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>M</mi><mrow><mi>m</mi><mi>a</mi><mi>x</mi></mrow><mi>B</mi></msubsup><mo>=</mo><mo>−</mo><mn>18.65</mn><mo>±</mo><mn>0.13</mn></mrow></semantics></math></inline-formula> mag, similar to 91bg-like SNe Ia. An estimation of the rate of decline for the <i>B</i>-band light curve was determined to be <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>Δ</mo><msub><mi>m</mi><mn>15</mn></msub><mrow><mo>(</mo><mi>B</mi><mo>)</mo></mrow><mo>=</mo><mn>1.60</mn><mo>±</mo><mn>0.06</mn></mrow></semantics></math></inline-formula> mag. The spectra of SN 2017fzw were similar to those of 91bg-like SNe Ia, with prominent Ti <span style="font-variant: small-caps;">ii</span> and Si <span style="font-variant: small-caps;">ii</span> <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>λ</mi><mn>5972</mn></mrow></semantics></math></inline-formula> features at early phases, gradually transitioning to spectra resembling normal (mainly HV subclass) SNe Ia at later phases, with a stronger Ca <span style="font-variant: small-caps;">ii</span> NIR feature. Notably, throughout all phases of observation, SN 2017fzw displayed spectral evolution characteristics that were comparable to those of HV SNe Ia, and at peak brightness, the Si <span style="font-variant: small-caps;">ii</span> <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>λ</mi><mn>6355</mn></mrow></semantics></math></inline-formula> velocity was determined to be 13,800 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>±</mo><mo> </mo><mn>415</mn></mrow></semantics></math></inline-formula> km s<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></semantics></math></inline-formula> and a more pronounced Ca <span style="font-variant: small-caps;">ii</span> NIR feature was also detected. Based on these findings, we classify SN 2017fzw as a transitional object with properties of both normal and 91bg-like SNe Ia, providing support for the hypothesis of a continuous distribution of supernovae between these two groups.