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Register a new userNear-Infrared Knots and Dense Fe Ejecta in the Cassiopeia A Supernova Remnant
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We report the results of broadband (0.95--2.46 $mu$m) near-infrared spectroscopic observations of the Cassiopeia A supernova remnant. Using a clump-finding algorithm in two-dimensional dispersed images, we identify 63 knots from eight slit positions and derive their spectroscopic properties. All of the knots emit [Fe II] lines together with other ionic forbidden lines of heavy elements, and some of them also emit H and He lines. We identify 46 emission line features in total from the 63 knots and measure their fluxes and radial velocities. The results of our analyses of the emission line features based on principal component analysis show that the knots can be classified into three groups: (1) He-rich, (2) S-rich, and (3) Fe-rich knots. The He-rich knots have relatively small, $lesssim 200~{rm km~s}^{-1}$, line-of-sight speeds and radiate strong He I and [Fe II] lines resembling closely optical quasi-stationary flocculi of circumstellar medium, while the S-rich knots show strong lines from O-burning material with large radial velocities up to $sim 2000~{rm km~s}^{-1}$ indicating that they are supernova ejecta material known as fast-moving knots. The Fe-rich knots also have large radial velocities but show no lines from O-burning material. We discuss the origin of the Fe-rich knots and conclude that they are most likely pure Fe ejecta synthesized in the innermost region during the supernova explosion. The comparison of [Fe II] images with other waveband images shows that these dense Fe ejecta are mainly distributed along the southwestern shell just outside the unshocked $^{44}$Ti in the interior, supporting the presence of unshocked Fe associated with $^{44}$Ti.
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Dense, fast-moving ejecta knots in supernova remnants are prime sites for molecule and dust formation. We present SOFIA far-IR spectrometer FIFI-LS observations of CO-rich knots in Cas A which cover a ~1 square arc minute area of the northern shell, in the [O III] 52 and 88 micron and [O I] 63 micron lines. The FIFI-LS spectra reveal that the line profiles of [O III] and [O I] are similar to those of the Herschel PACS [O III] and CO lines. We find that the [O III] maps show very different morphology than the [O I] map. The [O III] maps reveal diffuse, large-scale structures and the ratio of the two [O III] lines imply the presence of gas with a range of density 500 - 10,000 per cm^3 within the mapped region. In contrast, the [O I] map shows bright emission associated with the dense CO-rich knots. The 63 micron [O I] line traces cooled, dense post-shocked gas of ejecta. We find that IR-dominated [O III] emission is from post-shocked gas based on its morphology, high column density, and velocity profile. We describe multi-phase ejecta knots, a lifetime of clumps, and survival of dust in the young supernova remnants.
We present a long-exposure (~10 hr) image of the supernova (SN) remnant Cassiopeia A (Cas A) obtained with the UKIRT 3.8-m telescope using a narrow band filter centered at 1.644 um emission. The passband contains [Fe II] 1.644 um and [Si I] 1.645 um lines, and our `deep [Fe II]+[Si I] image provides an unprecedented panoramic view of Cas A, showing both shocked and unshocked SN ejecta together with shocked circumstellar medium at subarcsec (~0.7 arcsec or 0.012 pc) resolution. The diffuse emission from the unshocked SN ejecta has a form of clumps, filaments, and arcs, and their spatial distribution correlates well with that of the Spitzer [Si II] infrared emission, suggesting that the emission is likely due to [Si I] line not [Fe II] line as in shocked material. The structure of the optically-invisible western area of Cas A is clearly seen for the first time. The area is filled with many Quasi-Stationary Flocculi (QSFs) and fragments of the disrupted ejecta shell. We suggest that the anomalous radio properties in this area could be due to the increased number of such dense clumps. We identified 309 knots in the deep [Fe II]+[Si I] image and classified them into QSFs and fast-moving knots (FMKs). The total H+He mass of QSFs is ~0.23 Msun, implying that the mass fraction of dense clumps in the progenitors red-supergiant wind is 4--13%. The spatial distribution of QSFs suggests that there had been a highly asymmetric mass loss $10^4$--$10^5$ yr before the SN explosion. The mass of the [Fe II] line-emitting, shocked dense Fe ejecta is ~3x$10^{-5}$ Msun. The comparison with the ionic S-line dominated Hubble Space Telescope WFC3/IR image suggests that the outermost FMKs in the southeastern area are Fe-rich.
We present results from {it XMM-Newton/RGS} observations of prominent knots in the southest portion of Tychos supernova remnant, known to be the remnant of a Type Ia SN in 1572 C.E. By dispersing the photons from these knots out of the remnant with very little emission in front of or behind them, we obtained the nearly uncontaminated spectra of the knots. In the southernmost knot, the RGS successfully resolved numerous emission lines from Si, Ne, O He$alpha$ and Ly$alpha$, and Fe L-shell. This is the first clear detection of O lines in Tychos SNR. Line broadening was measured to be $sim 3$ eV for the O He$alpha$ and $sim 4.5$ eV for Fe L lines. If we attribute the broadening to pure thermal Doppler effects, then we obtain kT$_{O}$ and kT$_{Fe}$ to be $sim 400$ keV and 1.5 MeV, respectively. These temperatures can be explained by heating in a reverse shock with a shock velocity of $sim 3500$ km s$^{-1}$. The abundances obtained from fitting the RGS and MOS data together imply substantially elevated amounts of these materials, confirming previous studies that the knots are heated by a reverse shock, and thus contain ejecta material from the supernova. We are unable to find a Type Ia explosion model that reproduces these abundances, but this is likely the result of this knot being too small to extrapolate to the entire remnant.
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.
We present the results of extinction measurements toward the main ejecta shell of the Cassiopeia A supernova (SN) remnant using the flux ratios between the two near-infrared (NIR) [Fe II] lines at 1.26 and 1.64 $mu {rm m}$. We find a clear correlation between the NIR extinction ($E(J-H)$) and the radial velocity of ejecta knots, showing that redshifted knots are systematically more obscured than blueshifted ones. This internal self-extinction strongly indicates that a large amount of SN dust resides inside and around the main ejecta shell. At one location in the southern part of the shell, we measure $E(J-H)$ by the SN dust of 0.23$pm$0.05 mag. By analyzing the spectral energy distribution of thermal dust emission at that location, we show that there are warm ($sim$100 K) and cool ($sim$40 K) SN dust components and that the latter is responsible for the observed $E(J-H)$. We investigate the possible grain species and size of each component and find that the warm SN dust needs to be silicate grains such as MgSiO$_{3}$, Mg$_{2}$SiO$_{4}$, and SiO$_{2}$, whereas the cool dust could be either small ($leq$0.01 $mu {rm m}$) Fe or large ($geq$0.1 $mu {rm m}$) Si grains. We suggest that the warm and cool dust components in Cassiopeia A represent grain species produced in diffuse SN ejecta and in dense ejecta clumps, respectively.
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