Abstract
We investigate the statistical dependence of the peak intrinsic colors of Type Ia supernovae (SN Ia) on their expansion velocities at maximum light, measured from the Si II 6355 spectral feature. We construct a new hierarchical Bayesian regression model, accounting for the random effects of intrinsic scatter, measurement error, and reddening by host galaxy dust, and implement a Gibbs sampler and deviance information criteria to estimate the correlation. The method is applied to the apparent colors from BVRI light curves and Si II velocity data for 79 nearby SNe Ia. The apparent color distributions of high (HV) and normal velocity (NV) supernovae exhibit significant discrepancies for 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. A linear model finds significant slopes of -0.021 +/- 0.006 and -0.030 +/- 0.009 mag/(1000 km/s) for intrinsic B-V and B-R colors versus velocity, respectively. Since the ejecta velocity distribution is skewed towards high velocities, these effects imply non-Gaussian intrinsic color distributions with skewness up to +0.3. Accounting for the intrinsic color-velocity correlation results in corrections to A_V extinction estimates as large as -0.12 mag for HV SNe Ia and +0.06 mag for NV events. Velocity measurements from SN Ia spectra have potential to diminish systematic errors from the confounding of intrinsic colors and dust reddening affecting supernova distances.
Highlights
We investigate how measurements of the expansion velocity of the SN atmosphere can be used to learn more about the intrinsic color distribution of SNe Ia and improve inferences of host galaxy dust extinction
Recent analyses of multiwavelength light curve and color data, including near-infrared (NIR) observations, have indicated that SNe Ia with low reddening are subject to dust extinction AV with a reddening law closer to RV ≈ 3, whereas highly reddened objects appear extinguished by dust with RV ≈ 1.7 (Folatelli et al 2010; Mandel et al 2011; Burns et al 2014)
We have constructed a hierarchical Bayesian model to estimate the relation between multiple peak intrinsic colors of SNe Ia and their photospheric expansion velocities measured from the Si ii λ6355 absorption feature
Summary
Type Ia supernova (SN Ia) light curves have been used as cosmological distance indicators to trace the history of cosmic expansion, detect cosmic acceleration (Riess et al 1998; Perlmutter et al 1999), and constrain the equation-of-state parameter w of dark energy (Garnavich et al 1998; WoodVasey et al 2007; Astier et al 2006; Kowalski et al 2008; Hicken et al 2009b; Kessler et al 2009; Freedman et al 2009; Amanullah et al 2010; Conley et al 2011; Sullivan et al 2011; Rest et al 2014; Scolnic et al 2014a). Blondin et al (2012) presented the spectroscopy from the CfA Supernova Program and compared the maximum-light Si ii velocities with the intrinsic B − V colors inferred from the BayeSN statistical model for optical and NIR SN Ia light curves (Mandel et al 2011) This comprehensive statistical model analyzes the apparent light curve data, incorporating uncertainties due to peculiar velocities, measurement error, host galaxy reddening, and extinction, and the intrinsic population distribution to infer intrinsic colors, luminosities, and distances. Significant correlations of ejecta velocity with intrinsic colors mean that measurement of the ejecta velocity from the SN Ia spectrum would provide additional information to estimate the intrinsic colors of individual SNe Ia more precisely This in turn would lead to more accurate inferences of the host galaxy dust extinction that should improve luminosity distance estimates.
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