The Nickel Mass Distribution of Stripped-Envelope Supernovae: Implications for Additional Power Sources

Abstract

We perform a systematic study of the 56 Ni mass (M Ni ) of 27 stripped envelope supernovae (SESNe) with both well-constrained rise times and late-time coverage (>60 days) by modeling their light-curve tails. Based on this sample, we find that using "Arnett's rule" with observed peak times (t p ) and luminosities (L p ) will overestimate M Ni for SESN by a factor of ∼2. Recently, Khatami & Kasen (2019) presented a new analytic model relating t p and L p of a radioactive-powered SN to its M Ni that addresses several limitations of Arnettlike models, but depends on a dimensionless parameter β, which is sensitive to details of the progenitor system and explosion mechanism. Using the observed t p , L p , and tail-measured M Ni values for the sample of SESN, we observationally calibrate β for the first time-finding 0.0 < β < 1.71 with a median value of 0.70. Despite scatter, we demonstrate that the model of Khatami & Kasen (2019) coupled with these empiricallycalibrated β values yields a significantly improved measurement of M Ni when only photospheric data is available. However, these observationally-constrained β values are systematically lower than those inferred from numerical simulations, due primarily to the observed sample having significantly higher (0.2-0.4 dex) L p for a given M Ni . We investigate this discrepancy and find that while effects due to composition, mixing, and asymmetry can increase L p none can explain the systematically low β values. However, the discrepancy with simulations can be alleviated if ∼7-50% of L p for the observed sample comes from sources other than the radioactive decay of 56 Ni. Either shock cooling or magnetar spin down could provide the requisite luminosity, with the former requiring that a substantive fraction of SESN undergo late-stage mass loss or envelope inflation. Finally, we find that even with our improved measurements, the M Ni values of SESN are a factor of ∼3 larger than those of hydrogen-rich Type II SN, indicating that these supernovae are inherently different in terms of their progenitor initial mass distributions or explosion mechanisms.

An Early-Time Optical and Ultraviolet Excess in the type-Ic SN 2020oi

“…For stripped-envelope SNe such as the one considered here, this can have a large effect on the reported mass estimates (Khatami & Kasen 2019). We adopt the value of 𝛽 = 0.9 that has been empirically calibrated from a sample of well-studied SNe Ic (Afsariardchi et al 2020).…”

Are stripped envelope supernovae really deficient in $^{56}$Ni?

“…From these analyses, we conclude that our results in section 6 are robust to the different methods of deriving 56 Ni mass. It is true that several works model the incomplete γ-ray trapping when using the tail luminosity of SESNe (Afsariardchi et al 2020;Sharon & Kushnir 2020). Since the 56 Ni masses derived from such methods typically lie between 'Tail mass' and 'Arnett mass' (Sharon & Kushnir 2020), the effect of using such methods on the mock observation is covered by our discussion on the two cases above (i.e., 'Arnett mass' and 'Tail mass').…”

“…Indeed, the 'Arnett-rule', which has been widely used for SESNe have been claimed to overestimate the 56 Ni mass (Dessart et al 2015(Dessart et al , 2016Khatami & Kasen 2019). However, several works have concluded that even by taking into account the different methods to derive the 56 Ni mass and various observational errors, a difference in 56 Ni masses between SNeII and SESNe remains (Afsariardchi et al 2020;Meza & Anderson 2020;Sharon & Kushnir 2020). Meza & Anderson (2020) further noted the possibility that SESNe with a small amount of 56 Ni might have been missed by the existing surveys.…”

The spins of compact objects born from helium stars in binary systems

“…Figure 5 also shows a few magnetar rotation period estimates obtained by modeling "normal" type Ib SNe, which likely arise from He stars formed in binaries (e.g., Zapartas et al 2021). Afsariardchi et al (2020) provided estimates for many events by assuming a typical ejecta mass of either 2 M or 5 M . They found magnetar rotation periods very similar to those predicted by our models were consistent with helping to power the events they analyzed.…”