Portions of Cas A’s interior unshocked ejecta
that were surveyed by our near-infrared observations are also visible in previous observations
taken at longer wavelengths. Infrared images
of Cas A taken with the Spitzer spacecraft show
[S III] 33.48-mm and [S IV] 10.51-mm emission
inside the boundary of the main shell at locations coincident with regions where we detect
the strongest [S III] 906.9- and 953.1-nm emissions (19). Follow-up Spitzer infrared spectra in
the central region showed line emission from
interior O, Si, and S ejecta in sheetlike structures
and filaments with inferred radial velocities approaching T5000 km s−1 (20). Those spectroscopic Spitzer observations covered a smaller
50″ × 40″ area of Cas A, whereas our survey en-compasses the entire remnant and reveals a far
larger extent of the remnant’s internal debris.
We interpret Cas A’s main-shell rings of ejecta
to be the cross sections of reverse-shock–heated
cavities in the remnant’s internal ejecta now
made visible by our survey. A cavity-filled interior is in line with prior predictions for the
arrangement of expanding debris created by a
postexplosion input of energy from plumes of
radioactive 56Ni-rich ejecta (21, 22). Such plumes
can push the nuclear burning zones located
around the Fe core outward, creating dense
shells separating zones rich in O, S, and Si from
the Ni-rich material. Compression of surrounding nonradioactive material by hot expanding
plumes of radioactive 56Ni-rich ejecta generates a
“Swiss cheese”–like structure that is frozen into
the homologous expansion during the first few
weeks after the SN explosion, when the radioactive power of 56Ni is strongest.
In this scenario, the decay chain of 56Ni →
56Co → 56Fe should eventually make these
bubble-like structures enriched in Fe. Doppler
reconstruction of Chandra x-ray observations
sensitive to Fe K emissions shows that the three
most significant regions of Fe-rich ejecta are
located within three of Cas A’s main-shell rings
(11, 12). Thus, the coincidence of Fe-rich material with rings of O- and S-rich debris is consistent with the notion of 56Ni bubbles.
However, Fe-rich ejecta associated with the
Ni bubble effect should be characterized by diffuse morphologies and low ionization ages, and
yet the x-ray bright Fe emissions we currently
see are at an advanced ionization age relative to
the other elements (23, 24). Furthermore, not all
main-shell rings have associated x-ray–emitting
Fe-rich ejecta, and there is no clear relationship
between the locations of the internal bubbles
we have detected and the spatial distribution
of 44Ti recently mapped by NASA’s NuSTAR
(Nuclear Spectroscopic Telescope Array) (17).
One solution is that Fe-rich ejecta associated
with some of the remnant’s internal cavities
remain undetected. Low-density Fe could be
present in ionization states between those de-
tectable by optical or infrared line emission and
those in x-rays. The total mass of unshocked
Fe that is potentially contained in the bubbles
is constrained by the total nucleosynthetic
yield of Fe in the original SN explosion, which
is estimated to be less than ∼0.2 solar mass (M☉)
(25), and the amount of shocked Fe that is observed today, which is estimated to be 0.09 to
0.13 M☉ (24). Together these estimates imply
that no more than an additional ∼0.1 M☉ of Fe
could potentially be located within Cas A’s reverse shock.
Whatever their true cause, Cas A’s bubble-like interior and outer ringlike structures observed in the main shell suggest that large-scale
mixing greatly influences the overall arrangement of ejecta in core-collapse SNe. Presently,
the extent of such mixing and how it takes place
are not well known. A variety of potential dynamical processes may contribute to the redistribution
of chemical layers, including uneven neutrino
heating, axisymmetric magnetorotational effects,
and Rayleigh-Taylor and Kelvin-Helmholtz instabilities [e.g. (26, 27)].
Compelling evidence for large-scale mixing
involving considerable nonradial flow was first
observed in the nearest and brightest SN seen
in modern times, SN 1987A. In that case, high-energy gamma rays and x-rays with broad emission line widths from the decay of 56Ni were
detected only months after the explosion, implying that Ni-rich material was near the star’s
surface well before 1D progenitor models had
predicted, assuming spherical symmetry (28).
Since SN 1987A, state-of-the-art 3D computer
simulations of core-collapse explosions have confirmed that large-scale mixing can lead to Ni-dominated plumes overtaking the star’s outer
oxygen- and carbon-rich layers with velocities
up to 4000 km s−1 (29). However, the majority
of these simulations show that the mass density should essentially be unaffected. Although
mixing can affect the species distribution, the bulk
of the Ni mass should remain inside the remnant
with velocities below 2000 km s−1. This is, in fact,
opposite to what we currently see in Cas A, where
the x-ray bright Fe has velocities around the
4000 km s−1 limit (11). Thus, either the simulations are not adequately following the dynamics of mixing or, as we suspect, more Fe remains
to be detected in Cas A’s interior.
An additional consideration in interpreting
a SN debris field is the chemical makeup of the
star at the time of outburst. The evolution of massive stars toward the ends of their life cycles is
likely to be nonspherical and may produce extensive intershell mixing. If strong enough, these
dynamical interactions lead to Rayleigh-Taylor
instabilities in the progenitor structure that can
contribute to the formation of Ni-rich bubbles
and influence the overall progression of the explosion (30). Thus, asymmetries introduced by a
turbulent progenitor star interior, in addition
to those initiated by the explosion mechanism,
could contribute to the bubble-like morphology
observed in Cas A.
Because Cas A’s opposing streams of Si- and
S-rich debris have kinematic and chemical properties indicative of an origin deep within the
progenitor star (12), we searched for evidence of
any structure joining the high-velocity material
with the interior ejecta mapped in our survey.
However, we were not able to find any clear
relationship between them. An indirect association is hinted at by the inferred projected motion of Cas A’s central x-ray point source (XPS)
that is thought to be the remnant neutron star.
Its motion toward the southwest of the center of
expansion is (i) roughly opposite to and moving
away from the direction of the largest internal
cavity in our reconstruction that is coincident
with a sizable concentration of reverse-shocked
Fe and (ii) nearly perpendicular to the axis of the
high-velocity jets (31). The XPS, conserving momentum, could have been kicked in a direction
opposite the largest plume of Fe-rich material
(15) and released an energetic proto–neutron
star wind that shaped the jets shortly after the
core-collapse explosion (32).
The apparent mismatch of Ti-rich and Fe-rich ejecta regions uncovered by NuSTAR is a
reminder that unresolved key issues surrounding
Cas A still linger despite decades of scrutiny. Our
3D map of its interior is an important step forward, as it represents a rare look at the geometry
of a SN remnant’s inner volume of debris unmodified by reverse-shock instabilities. Because
Cas A shows many striking similarities with SNe
young and old (33, 34), its dynamical properties
described in this work are probably not unique
and can be used to help interpret other SN explosions and remnants that cannot be resolved.
Our data make it clear that Cas A’s dominant
ejecta structure is in the form of large internal
cavities whose cross sections are the prominent
rings of the reverse-shock–heated main shell.
What is not clear, however, is why only half a
dozen bubbles—not dozens—are present. SN
explosion models can explore this issue, as well
as better understand how the remnant’s interior bubbles fit into a single coherent picture
with its opposing high-velocity jets. A crucial
test of the origin of these cavities would be a
confirmation of the “missing” internal Fe we
predict to be located in the remnant’s interior,
but conclusive observations may not be possible until the next generation of infrared and
x-ray space telescopes comes online.
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