We study the observables of 158 relatively normal Type Ia supernovae ( SNe Ia ) by dividing them into two groups in terms of the expansion velocity inferred from the absorption minimum of the Si II \lambda 6355 line in their spectra near B -band maximum brightness .
One group ( “ Normal ” ) consists of normal SNe Ia populating a narrow strip in the Si II velocity distribution , with an average expansion velocity \langle v \rangle = 10 , 600 \pm 400 km s ^ { -1 } near B maximum ; the other group ( “ HV ” ) consists of objects with higher velocities , v \gtrsim 11 , 800 km s ^ { -1 } .
Compared with the Normal group , the HV one shows a narrower distribution in both the peak luminosity and the luminosity decline rate \Delta m _ { 15 } .
In particular , their B - V colors at maximum brightness are found to be on average redder by \sim 0.1 mag , suggesting that they either are associated with dusty environments or have intrinsically red B - V colors .
The HV SNe Ia are also found to prefer a lower extinction ratio R _ { V } \approx 1.6 ( versus \sim 2.4 for the Normal ones ) .
Applying such an absorption-correction dichotomy to SNe Ia of these two groups remarkably reduces the dispersion in their peak luminosity from 0.178 mag to only 0.125 mag .