http://arxiv.org/abs/1402.7079
We investigate the correlations between the peak intrinsic colors of Type Ia supernovae (SN Ia) and their expansion velocities at maximum light, measured from the Si II 6355 A spectral feature. We construct a new hierarchical Bayesian regression model and Gibbs sampler to estimate the dependence of the intrinsic colors of a SN Ia on its ejecta velocity, while accounting for the random effects of intrinsic scatter, measurement error, and reddening by host galaxy dust. The method is applied to the apparent color data from BVRI light curves and Si II velocity data for 79 nearby SN Ia. Comparison of the apparent color distributions of high velocity (HV) and normal velocity (NV) supernovae reveals significant discrepancies in B-V and B-R, but not other colors. Hence, they are likely due to intrinsic color differences originating in the B-band, rather than dust reddening. The mean intrinsic B-V and B-R color differences between HV and NV groups are 0.06 +/- 0.02 and 0.09 +/- 0.02 mag, respectively. Under a linear model for intrinsic B-V and B-R colors versus velocity, we find significant slopes of -0.021 +/- 0.006 and -0.030 +/- 0.009 mag/(1000 km/s), respectively. Since the ejecta velocity distribution is skewed towards high velocities, these effects imply non-Gaussian intrinsic color population distributions with skewness up to +0.3. Accounting for the intrinsic color-velocity correlation results in corrections in A_V extinction estimates as large as -0.12 mag for HV SN Ia and +0.06 mag for NV events. Deviance information criteria strongly favor simple functions for intrinsic colors versus velocity over no trend, while higher-order polynomials are disfavored. Velocity measurements from SN Ia spectra have potential to diminish systematic errors from the confounding of intrinsic colors and dust reddening affecting supernova distances.
K. Mandel, R. Foley and R. Kirshner
Mon, 3 Mar 14
19/55