LINE IDENTIFICATIONS OF TYPE I SUPERNOVAE: ON THE DETECTION OF Si II FOR THESE HYDROGEN-POOR EVENTS

Abstract

Here we revisit line identifications of type I supernovae (SNe I) and highlight trace amounts of unburned hydrogen as an important free parameter for the composition of the progenitor. Most one-dimensional stripped-envelope models of supernovae indicate that observed features near 6000-6400 Å in typeI spectra are due to more than Si IIλ6355. However, while an interpretation of conspicuous Si IIλ6355 can approximate 6150 Å absorption features for all SNe Ia during the first month of free expansion, similar identifications applied to 6250 Å features of SNe Ib and Ic have not been as successful. When the corresponding synthetic spectra are compared with highquality timeseries observations, the computed spectra are frequently too blue in wavelength. Some improvement can be achieved with Fe II lines that contribute redward of 6150 Å; however, the computed spectra either remain too blue or the spectrum only reaches a fair agreement when the rise-time to peak brightness of the model conflicts with observations by a factor of two. This degree of disagreement brings into question the proposed explosion scenario. Similarly, a detection of strong Si IIλ6355 in the spectra of broadlined Ic and super-luminous events of type I/R is less convincing despite numerous model spectra used to show otherwise. Alternatively, we suggest 6000-6400 Å features are possibly influenced by either trace amounts of hydrogen or blueshifted absorption and emission in Hα, the latter being an effect which is frequently observed in the spectra of hydrogen-rich, SNe II.

Spectropolarimetry of the Type Ib Supernova iPTF 13bvn: revealing the complex explosion geometry of a stripped-envelope core-collapse supernova

“…The feature is significantly polarised in the first two epochs only (−10 and 0 days) before both the degree of polarisation and absorption depth decrease. As has been suggested for other Type Ib SNe (Wheeler et al 1994;Deng et al 2000;Branch et al 2002;Parrent et al 2015), if this feature is Hα at −16,400 km s −1 , this would imply that the H line-forming region lies in the outermost layers of the ejecta, perhaps in a shell at the outer edge of the ejecta, as was suggested for Type Ib SN 2008D (Maund et al 2009). The polarisation of this feature may then reflect asymmetries in the outermost layers of the progenitor or the progenitor's wind swept up by the ejecta, as has been proposed for some Type IIb SNe.…”

“…2). Alternatively, Parrent et al (2015) suggest that features such as this in other Type I CCSNe may be high velocity Hα. If this feature arises instead from hydrogen the corresponding velocity at absorption at -10 days was 16, 700 km s −1 .…”

Comparative analysis of SN 2012dn optical spectra: days −14 to +114

“…those of type IIbIbc and IIPLn, separately, are sometimes viewed as also forming a physically related sequence of progenitor configurations (Nomoto et al 1996;Claeys et al 2011;Dessart et al 2012;Smith et al 2015, see also Maurer et al 2010). For SN IIbIbc, one of the key questions is how many "hydrogen-poor SN IIb" are misidentified as "hydrogen-poor SN Ib" when both events are primarily discovered and typed during a "type Ib" phase (Chevalier & Soderberg 2010;Arcavi et al 2011;Milisavljevic et al 2013;Folatelli et al 2014;Parrent et al 2015). Our estimates of vmin for C II, O I, Si II, and Ca II for SN 2011fe and 2012dn are shown in Figure 13.…”

“…This assumes, however, that some of the under-observed events in either a given sample or pre-defined subclass are not caught-late or "peculiar" outliers for a neighboring, and possibly unrelated, subclass such as Broad Line SN Ia (see also Folatelli et al 2014;Parrent et al 2015). In fact, there is curious overlap between events that have been typed as "high velocity Core Normal" and "normal velocity Broad Line" SN Ia (Blondin et al 2012;Parrent 2014).…”

Revisiting the Lick Observatory Supernova Search Volume-limited Sample: Updated Classifications and Revised Stripped-envelope Supernova Fractions

“…SNe IIb show a strong Hα line at early times, as do normal SNe II, but then the hydrogen fades and the later spectra of SNe IIb resemble those of SNe Ib (e.g., Filippenko 1988;Filippenko et al 1993;Filippenko 1997;Pastorello et al 2008;Chornock et al 2011;Milisavljevic et al 2013). Several authors have claimed the detection of weak high-velocity hydrogen features in SNe Ib and Ic (e.g., Branch et al 2006;Parrent et al 2007Parrent et al , 2016Liu et al 2016). However, Liu et al (2016) argue that the putative weak Hα absorption line often present in SNe Ib is, at all phases, weaker than the Hα line in SNe IIb, and that SNID capably distinguishes between SNe Ib and SNe IIb even after the strong Hα feature of the SNe IIb has faded, so long as spectra were obtained during the photospheric phase.…”