High-velocity Line Forming Regions in the Type Ia Supernova 2009ig

Abstract

We report measurements and analysis of high-velocity (HVF) (>20,000 km s(-1)) and photospheric absorption features in a series of spectra of the Type Ia supernova (SN) 2009ig obtained between -14 days and +13 days with respect to the time of maximum B-band luminosity (B-max). We identify lines of Si II, Si III, S II, Ca II, and Fe II that produce both HVF and photospheric-velocity (PVF) absorption features. SN 2009ig is unusual for the large number of lines with detectable HVF in the spectra, but the light-curve parameters correspond to a slightly overluminous but unexceptional SN Ia (M-B = -19.46 mag and Delta m(15)(B) = 0.90 mag). Similarly, the Si II lambda 6355 velocity at the time of B-max is greater than "normal" for an SN Ia, but it is not extreme (upsilon(Si) = 13,400 km s(-1)). The -14 days and -13 days spectra clearly resolve HVF from Si II lambda 6355 as separate absorptions from a detached line forming region. At these very early phases, detached HVF are prevalent in all lines. From -12 days to -6 days, HVF and PVF are detected simultaneously, and the two line forming regions maintain a constant separation of about 8000 km s(-1). After -6 days all absorption features are PVF. The observations of SN 2009ig provide a complete picture of the transition from HVF to PVF. Most SNe Ia show evidence for HVF from multiple lines in spectra obtained before -10 days, and we compare the spectra of SN 2009ig to observations of other SNe. We show that each of the unusual line profiles for Si II lambda 6355 found in early-time spectra of SNe Ia correlate to a specific phase in a common development sequence from HVF to PVF.