REACTION RATE SENSITIVITY OF ⁴⁴Ti PRODUCTION IN MASSIVE STARS AND IMPLICATIONS OF A THICK TARGET YIELD MEASUREMENT OF ⁴⁰Ca(α, γ)⁴⁴Ti

Abstract

We evaluate two dominant nuclear reaction rates and their uncertainties that affect 44 Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the 40 Ca(α,γ) 44 Ti reaction at E α = 4.13, 4.54, and 5.36 MeV using γ-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower 44 Ti production in scenarios for the α−rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the 44 Ti(α,p) 47 V reaction rate. Using these two rates we carry out a sensitivity survey of 44 Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in 44 Ti synthesis as that produced by different treatments of stellar physics.

New measurement of the astrophysically important40Ca(α,γ)44Ti reaction

“…We find that previous PGS measurements [11,[16][17][18][19][20] are approximately a factor of 3.3 smaller than our reaction rate over the astrophysically relevant temperature range of T = 1 -5.5 × 10 9 K. However our rate lies between the more recently published results of AMS and offline counting measurements outlined in [12,30]. Our rate also shows close agreement with the IKS studies outlined in [13] which is smaller on average by a factor of 1.33, and similarly to this work indicates an increase in identified resonance structure.…”

“…Conversely the AMS [12] rate is greater by an average factor of 1.44 with an increasingly larger deviation visible above T = 3.5 × 10 9 K. The PGS work is smaller by an average factor of 3.3. The reaction rate work by Hoffman et al [30] is slightly different due to the use of a normalized theoretical cross-section for their reaction rate calculations. From this they stress an assigned factor of 2 uncertainty in their rate (not shown on figure comparisons), bringing it possibly much closer to the measured rate in this work.…”

“…Most recently a thick target yield measurement was performed at the Lawrence Livermore National Laboratory (LLNL) Center for Accelerator Mass Spectrometry (CAMS) by Hoffman et al [30]. An alpha beam incident on nat CaO was used for 3 thick target yield measurements at E α = 4.13, 4.54 and 5.36 MeV.…”

“…It is therefore of particular interest to understand the production mechanism which typically is assumed to be associated with the α-rich freeze out phase of the expanding shock front [7]. A number of reaction rate sensitivity studies have been performed in the broader framework of model parametrization for the α-rich freeze out expansion phase [8,9], seeking to identify the role of critical reaction rates for 44 Ti production at thermal equilibrium and quasi-equilibrium conditions. A recent parameter study [10] uses different expansion trajectories to investigate the role of particular reactions decoupling from equilibrium.…”

Revisiting INTEGRAL/SPI observations of44Ti from Cassiopeia A

“…44 Ti and 56 Ni arise from nuclear burning in the innermost parts of supernovae very close to the mass-cut separating ejecta from the compact remnant (Diehl & Timmes 1998). Therefore 44 Ti and 56 Ni are expected to also be co-spatial when they decay in the expanding SNR Hoffman et al 2010;Magkotsios et al 2010). But as this inner region is probably characterized by dynamical instabilities and simultaneous inflows and outflows of material, it remains unclear how and where this separation between material accreting onto the compact remnant and the ejecta occurs, and thus how much of the 44 Ti can end up in the ejecta (Fryer et al 2008;Wongwathanarat et al 2013;Popov et al 2014).…”