Grouping Normal Type Ia Supernovae by UV to Optical Color Differences

Observations of many SNe Ia with the UVOT instrument on the Swift satellite has revealed that there exists order to the differences in the UV-OPT colors of normal SNe. We examine UV-OPT color curves for 25 SNe Ia, dividing them into 4 groups, finding that ~1/3 of these SNe Ia have bluer UV-OPT colors than the larger group, with these NUV-blue SNe Ia 0.4 mag bluer than the NUV-red SNe Ia in u-v. Another group of events feature colors similar to NUV-red SNe Ia in the u-v to uvw1-v colors, but similar to the NUV-blue SNe Ia in the uvm2-v color. We name these events MUV-blue. The last group initially has colors similar to NUV-red SNe Ia, but with color curves that feature more modest changes than the larger NUV-red group. These irregular events are comprised of all the NUV-red events with the broadest optical peaks, which leads us to consider this minor group a subset of the NUV-red group. When so separated and the accounting is made for the rapid time evolution of the UV-OPT colors, we find that the scatter in two NUV-OPT colors, u-v & uvw1-v, is at the level of the scatter in b-v. This finding is promising for extending the cosmological utilization of SNe Ia into the NUV. We generate spectrophotometry of SNe Ia that have been observed with HST and argue that there is a fundamental spectral difference in the 2900-3500A wavelength range, a range suggested to be dominated by absorption from iron-peak elements. The NUV-blue SNe Ia feature less NUV absorption than the NUV-red SNe Ia. We show that all the NUV-blue SNe Ia in this sample have also featured evidence of unburned carbon in optical spectra, whereas only one NUV-red SN Ia features that absorption line. Every NUV-blue event also exhibits a low gradient of the SiII 6355A absorption feature, but many NUV-red events also exhibit a low gradient, perhaps suggestive that NUV-blue events are a subset of the larger LVG group.