Light-curve studies of nearby Type Ia supernovae with a Multiband Stretch method

We create new U-, B-, V-, R-and I-band light-curve templates of Type Ia supernovae (SNe Ia) and re-analyse 122 nearby (redshift < 0.11) SNe Ia using a new 'Multiband Stretch method', which is a revised Stretch method extended to five bands. We find (i) our I-band template can fit about 90 per cent of SNe Ia I-band light curves, (ii) relationships between luminosity, colours and stretch factors, (iii) possible subgroups of SNe Ia and (iv) the ratio of total to selective extinction R in other galaxies can be consistent with that in the Milky Way under the assumption that SNe Ia have diversity in their intrinsic colour. Based on these results, we discuss how to select subsets of SNe Ia to serve as good distance indicators for cosmology. We find two possibilities: one is to choose 'BV bluest' [-0.14 < (B-V)(max) <= -0.10] objects and the other is to use only SNe Ia which occur in E or S0 galaxies. Within these subsets, we find the root mean square (rms) of peak B-band magnitudes is 0.17 mag ('BV bluest' sample) and 0.12 mag (E or S0 sample).