Type II Supernova Spectral Diversity. II. Spectroscopic and Photometric Correlations

Abstract

We present an analysis of observed trends and correlations between a large range of spectral and photometric parameters of more than 100 type II supernovae (SNe II), during the photospheric phase. We define a common epoch for all SNe of 50 days post-explosion where the majority of the sample is likely to be under similar physical conditions. Several correlation matrices are produced to search for interesting trends between more than 30 distinct light-curve and spectral properties that characterize the diversity of SNe II. Overall, SNe with higher expansion velocities are brighter, have more rapidly declining light-curves, shorter plateau durations, and higher 56 Ni masses. Using a larger sample than previous studies, we argue that 'P d' -the plateau duration from the transition of the initial to 'plateau' decline rates to the end of the 'plateau' -is a better indicator of the hydrogen envelope mass than the traditionally used optically thick phase duration (OP T d: explosion epoch to end of plateau). This argument is supported by the fact that P d also correlates with s 3 , the light-curve decline rate at late times: lower P d values correlate with larger s 3 decline rates. Large s 3 decline rates are likely related to lower envelope masses that enables gamma-ray escape. We also find a significant anticorrelation between P d and s 2 (the plateau decline rate), confirming the long standing hypothesis that faster declining SNe II (SNe IIL) are the result of explosions with lower hydrogen envelope masses and therefore have shorter P d values.

The Early Detection and Follow-up of the Highly Obscured Type II Supernova 2016ija/DLT16am∗

“…The correlation between absolute peak magnitudes and expansion velocity was recently confirmed by Gutiérrez et al (2017), who also confirmed the previous claim of Anderson et al (2014a) and Valenti et al (2016) that brighter SNe II show shorter plateau phases and steeper decline rates. Similar results were also reported by Galbany et al (2016) by analyzing the light curves of a large sample of SNe II.…”

SN 2015an: a normal luminosity type II supernova with low expansion velocity at early phases

“…This SN II sample exhibits a range of Hα expansion velocities (1500 < vHα < 9600 km s −1 ) and pEWs of lines, presumably linked to the diversity in the explosion energies, progenitor radius and metallicity of SNe II. As noted in Gutiérrez et al (2017b) Ca II H&K H 4340 Fe II 4629 H 4861 Fe II 4924 Fe II 5018 Fe II 5169 Sc II 5527 Na I D Ba II 6142 Sc II 6245 Si II 6355 H 6563 O I 7770 Ca II 8662 Ca II weaker metal lines. The continuum of SN 2015an matches well with that of SN LSQ13fn and neither of them has a conspicuous Na i D line at this phase, as compared to the other SNe.…”

“…SN 2015an displays some peculiar characteristics, in addition to the usual properties of SNe II, thereby necessitating the cognizance of the status of SN 2015an in the SNe II population before heading for the comparison with a sample of events. To this aim, we plotted the pseudo-equivalent widths (pEW) of the prominent lines vs Hα velocity at 50 d of SN 2015an along with a sample of 122 SNe II from Gutiérrez et al (2017b) in Fig. 10.…”

“…Most of these metal lines disappears in the last two spectra (150.6 and 158.4 d), except Na i d and Ca ii NIR triplet.The evolution of Hα and Hβ lines from 23.8 to 60.6 d is shown inFig. 6in the velocity domain and centered at the rest wavelengths of these lines along with the syn++ A comparison of the velocity of SN 2015an to SNe 2005cs, LSQ13fn and the mean velocity (grey) of 122 IIP/IIL SNe(Gutiérrez et al 2017b) for Hα (top panel), Hβ (middle panel) and Fe II (λ5169; bottom panel). The light grey regions represent the standard deviations of the mean velocities of the sample.…”

“…shows the Hα, Hβ and Fe ii 5169 velocity evolution of SNe 2005cs, LSQ13fn and 2015an overplotted on the estimated mean velocities of the 122 IIP/L SNe fromGutiérrez et al (2017b). The Hα velocity of SN 2015an lies in the lower edge of the velocity range of SNe II, with significantly low velocity (∼5500 km s −1 ) at early phases as compared to the median velocity of the sample (∼7900 km s −1 ).…”