The distribution of radioactive ^(44)Ti in Cassiopeia A

Abstract

The distribution of elements produced in the innermost layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially resolved spectroscopic analyses of the ^(44)Ti ejecta, which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the ^(44)Ti ejecta. We find an initial ^(44)Ti mass of (1.54 ± 0.21) × 10^(−4) M_⊙, which has a present-day average momentum direction of 340° ± 15° projected onto the plane of the sky (measured clockwise from celestial north) and is tilted by 58° ± 20° into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some ^(44)Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to it. Where we observe ^(44)Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe ^(44)Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of ^(44)Ti by at least a factor of two in some regions, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.

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