Are $\mathsf{^{44}}$Ti-producing supernovae exceptional?

Abstract

According to standard models supernovae produce radioactive 44 Ti, which should be visible in gamma-rays following decay to 44 Ca for a few centuries. 44 Ti production is believed to be the source of cosmic 44 Ca, whose abundance is well established. Yet, gamma-ray telescopes have not seen the expected young remnants of core collapse events. The 44 Ti mean life of τ 89 y and the Galactic supernova rate of 3/100 y imply several detectable 44 Ti gamma-ray sources, but only one is clearly seen, the 340-year-old Cas A SNR. Furthermore, supernovae which produce much 44 Ti are expected to occur primarily in the inner part of the Galaxy, where young massive stars are most abundant. Because the Galaxy is transparent to gamma-rays, this should be the dominant location of expected gamma-ray sources. Yet the Cas A SNR as the only one source is located far from the inner Galaxy (at longitude 112• ). We evaluate the surprising absence of detectable supernovae from the past three centuries. We discuss whether our understanding of SN explosions, their 44 Ti yields, their spatial distributions, and statistical arguments can be stretched so that this apparent disagreement may be accommodated within reasonable expectations, or if we have to revise some or all of the above aspects to bring expectations in agreement with the observations. We conclude that either core collapse supernovae have been improbably rare in the Galaxy during the past few centuries, or 44 Ti-producing supernovae are atypical supernovae. We also present a new argument based on 44 Ca/ 40 Ca ratios in mainstream SiC stardust grains that may cast doubt on massive-He-cap type I supernovae as the source of most galactic 44 Ca.

Galactic annihilation emission from nucleosynthesis positrons

“…These values agree with the compilation of observational constraints done in Martin et al (2010) and with the recently-estimated range for G1.9+0.3 (Borkowski et al 2010). Overall, this corresponds to a mean Galactic 44 Ti production rate of 4.2 ×10 −6 M yr −1 , roughly consistent with the estimate of 5.5 × 10 −6 M yr −1 by The et al (2006) based on the present-day solar 44 Ca abundance and a Galactic chemical evolution model. This 44 Ti production rate can be translated into a positron production rate by applying a factor 0.94 for the β + -decay branching ratio of the decay chain and using an escape fraction of 100%.…”

“…The 56 Ni isotope is synthesised by explosive Si-burning deep in the stellar ejecta during core-collapse and thermonuclear 7 In The et al (2006), the authors found a ∼2 times larger uncertainty range for the Galactic 44 Ti production rate due to unknown parameters in the Galactic chemical evolution model; they also note that the entire range may be shifted to lower values if there is a nucleosynthesis channel to produce 44 Ca directly.…”

Cosmic Gamma-Ray Spectroscopy

“…The characteristics of 44 Ti emission from the Cas A supernova remnant and candidate sources in the Galaxy show that nucleosynthesis in core-collapse supernovae occurs under a diversity of environmental conditions, which result from substantial deviations from spherical symmetry in these explosions [127]. Exploitation of gamma-rays from the 56 Ni decay chain which is responsible for supernova light still awaits a sufficiently-nearby supernova of type Ia [49]; the independent information carried by gamma-rays from this decay chain could help to understand details of explosion physics, which happens in the dense and dynamic initial phase of the exploding star that is otherwise occulted to observations.…”

“…Both the COMPTEL and INTEGRAL suverys fail to detect the expected numbers of 44 Ti sources. In spite of the low number of events involved, this is a significant observation [126,108]. Thus, also from the rate of 44 Ti emitting core-collapse supernovae, it seems that object to object variations must be large, and 44 Ti ejection non-typical, probably much larger than current models predict in case they do happen.…”

“…Again, proximity is key to gamma-ray observations, and more constraining due to the much smaller amounts of 56 Ni compared to SNIa. 44 Ti ejection is much less clear [126], as it is produced predominantly in the supernova's interior region very close to the mass cut separating ejecta from material which will end up in the central neutron star. One-dimensional models of Type II or Type Ib supernovae obtain typical 44 Ti yields of 5 10 −5 M , with large (factors 2-4) variations with progenitor mass [129].…”