1208.3781 (Daniel R. van Rossum)

The Nature of the Si II 6150A, Ca II HK, Ca II IR-triplet, and other Spectral Features in Supernova Type Ia Spectra    [PDF]

Daniel R. van Rossum

Spectra of Supernovae of type Ia (SN Ia) are commonly interpreted as a continuum with absorption features. The pseudo equivalent width (pEW) and Doppler shift of absorption features like Si II 6150A, Ca II HK 3750A, and the Ca II IR-triplet 8150A, measures which are commonly interpreted as the optical thickness and the velocity of a shell of corresponding absorbing material, form the basis of common SN Ia spectral interpretation and classification schemes. In this paper, we examine the nature of spectral features in SN Ia spectra using W7 model spectra, and show that Si II 6150A and many other features are largely emission dominated instead of absorption dominated. We show that apparent absorption features (like Si II 6150A) are frequently just coincidental troughs between two (or more) uncorrelated emission features. The pEW measured between such emission peaks is then only little related to true strength of the presumed absorption feature. It shows how the concepts of "absorption troughs" and "continuum" can be misleading and should be used with care. Furthermore, using the same model spectra, we demonstrate for different times post explosion how spectral features overlap each other and how together they compose the total spectrum. This overlap distorts individual line profiles and affects measured absorption velocities. With an explosion model atmosphere that is tuned to match specific observations, the method presented in this paper can in principle be used to quantify all these effects and improve the interpretation and informative value of observed SN Ia spectra.

View original: http://arxiv.org/abs/1208.3781