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Register a new userTitanium and Iron in the Cassiopeia A Supernova Remnant
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Authors
Gregory S. Vance
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Mixing above the proto-neutron star is believed to play an important role in the supernova engine, and this mixing results in a supernova explosion with asymmetries. Elements produced in the innermost ejecta, e.g., ${}^{56}$Ni and ${}^{44}$Ti, provide a clean probe of this engine. The production of ${}^{44}$Ti is particularly sensitive to the exact production pathway and, by understanding the available pathways, we can use ${}^{44}$Ti to probe the supernova engine. Using thermodynamic trajectories from a three-dimensional supernova explosion model, we review the production of these elements and the structures expected to form under the convective-engine paradigm behind supernovae. We compare our results to recent X-ray and $gamma$-ray observations of the Cassiopeia A supernova remnant.
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We report the likely detection of near-infrared 2.29 $mu$m first overtone Carbon Monoxide (CO) emission from the young supernova remnant Cassiopeia A (Cas A). The continuum-subtracted CO filter map reveals CO knots within the ejecta-rich reverse shock. We compare the first overtone CO emission with that found in the well-studied supernova, SN 1987A and find $sim$30 times less CO in Cas A. The presence of CO suggests that molecule mixing is small in the SN ejecta and that astrochemical processes and molecule formation may continue at least ~300 years after the initial explosion.
Phosphorus ($^{31}$P), which is essential for life, is thought to be synthesized in massive stars and dispersed into interstellar space when these stars explode as supernovae (SNe). Here we report on near-infrared spectroscopic observations of the young SN remnant Cassiopeia A, which show that the abundance ratio of phosphorus to the major nucleosynthetic product iron ($^{56}$Fe) in SN material is up to 100 times the average ratio of the Milky Way, confirming that phosphorus is produced in SNe. The observed range is compatible with predictions from SN nucleosynthetic models but not with the scenario in which the chemical elements in the inner SN layers are completely mixed by hydrodynamic instabilities during the explosion.
The death of massive stars is believed to involve aspheric explosions initiated by the collapse of an iron core. The specifics of how these catastrophic explosions proceed remain uncertain due, in part, to limited observational constraints on various processes that can introduce asymmetries deep inside the star. Here we present near-infrared observations of the young Milky Way supernova remnant Cassiopeia A, descendant of a type IIb core-collapse explosion, and a three-dimensional map of its interior, unshocked ejecta. The remnants interior has a bubble-like morphology that smoothly connects to and helps explain the multi-ringed structures seen in the remnants bright reverse shocked main shell of expanding debris. This internal structure may have originated from turbulent mixing processes that encouraged the development of outwardly expanding plumes of radioactive 56Ni-rich ejecta. If this is true, substantial amounts of its decay product, 56Fe, may still reside in these interior cavities.
We use new large area far infrared maps ranging from 65 - 500 microns obtained with the AKARI and the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) missions to characterize the dust emission toward the Cassiopeia A supernova remnant (SNR). Using the AKARI high resolution data we find a new tepid dust grain population at a temperature of ~35K and with an estimated mass of 0.06 solar masses. This component is confined to the central area of the SNR and may represent newly-formed dust in the unshocked supernova ejecta. While the mass of tepid dust that we measure is insufficient by itself to account for the dust observed at high redshift, it does constitute an additional dust population to contribute to those previously reported. We fit our maps at 65, 90, 140, 250, 350, and 500 microns to obtain maps of the column density and temperature of cold dust (near 16 K) distributed throughout the region. The large column density of cold dust associated with clouds seen in molecular emission extends continuously from the surrounding interstellar medium to project on the SNR, where the foreground component of the clouds is also detectable through optical, X-ray, and molecular extinction. At the resolution available here, there is no morphological signature to isolate any cold dust associated only with the SNR from this confusing interstellar emission. Our fit also recovers the previously detected hot dust in the remnant, with characteristic temperature 100 K.
We report, for the first time, the detection of the Mn-K$alpha$ line in the Type IIb supernova (SN IIb) remnant, Cassiopeia A. Manganese ($^{55}$Mn after decay of $^{55}$Co), a neutron-rich element, together with chromium ($^{52}$Cr after decay of $^{52}$Fe), is mainly synthesized at the explosive incomplete Si burning regime. Therefore, the Mn/Cr mass ratio with its neutron excess reflects the neutronization at the relevant burning layer during the explosion. Chandras archival X-ray data of Cassiopeia A indicate a low Mn/Cr mass ratio with values in the range 0.10--0.66, which, when compared to one-dimensional SN explosion models, requires that the electron fraction be 0.4990 $lesssim Y_{rm e} lesssim$ 0.5 at the incomplete Si burning layer. An explosion model assuming a solar-metallicity progenitor with a typical explosion energy ($1 times 10^{51}$ erg) fails to reproduce such a high electron fraction. In such models, the explosive Si-burning regime extends only to the Si/O layer established during the progenitors hydrostatic evolution; the $Y_e$ in the Si/O layer is lower than the value required by our observational constraints. We can satisfy the observed Mn/Cr mass ratio if the explosive Si-burning regime were to extend into the O/Ne hydrostatic layer, which has a higher $Y_{rm e}$. This would require an energetic ($> 2 times 10^{51}$ erg) and/or asymmetric explosion of a sub-solar metallicity progenitor ($Z lesssim 0.5Z_{odot}$) for Cassiopeia A. The low initial metallicity can be used to rule out a single-star progenitor, leaving the possibility of a binary progenitor with a compact companion (white dwarf, neutron star or black hole). We discuss the detectability of X-rays from Bondi accretion onto such a compact companion around the explosion site. We also discuss other possible mass-loss scenarios for the progenitor system of Cassiopeia A.
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