Near-Infrared Knots and Dense Fe Ejecta in the Cassiopeia A Supernova Remnant

Abstract

We report the results of broadband (0.95-2.46 µm) 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, 200 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 ∼ 2000 km s −1 indicating that they are supernova ejecta material known as fastmoving 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.

Near-infrared [Fe ii] and H2 Emission-line Study of Galactic Supernova Remnants in the First Quadrant

“…NIR [Fe II] and H 2 emission lines have been studied extensively in several bright Galactic SNRs: Kepler (Oliva et al 1989;Gerardy & Fesen 2001), G11.2−0.3 (Koo et al 2007;Moon et al 2009), 3C 391 (Reach et al 2002), W44 (Reach et al 2005), 3C 396 (Lee et al 2009), W49B (Keohane et al 2007), Cygnus loop (Graham et al 1991), Cassiopeia A (Gerardy & Fesen 2001;Lee et al 2017;Koo et al 2018), Crab nebula (Graham et al 1990;Loh et al 2011), IC 443 (Treffers 1979;Graham et al 1987;Burton et al 1988Kokusho et al 2013), MSH 15-52 (Seward et al 1983), RCW 103 (Oliva et al 1990;Burton & Spyromilio 1993). According to these studies, SNRs bright in [Fe II] emission lines may be divided into two groups ): (1) young SNRs interacting with their dense CSMs (e.g., G11.2−0.3, W49B, Cassiopeia A) and (2) middle-aged SNRs interacting with dense atomic gas or MCs (e.g., IC 443, W44).…”

Aspherical Supernovae: Effects on Early Light Curves

“…Linear polarization of the line and continuum emission has long provided evidence of non-spherical ejecta, especially at low velocities (Leonard et al 2001(Leonard et al , 2006 and for stripped-envelope progenitors (Leonard & Filippenko 2005;Wang & Wheeler 2008). Likewise, young SN remnants like Cassiopeia A show a complicated distribution of ejecta (e.g., Lee et al 2017). The stellar envelope may also be distorted by rapid rotation or tides from a companion.…”

The two promising scenarios to explode core collapse supernovae

“…In any case, I argue that strong pre-explosion outbursts require the presence of a binary companion. Grefenstette et al (2017) present the distribution of 44 Ti in the Cassiopeia A SNR (also Lee et al 2017). Wongwathanarat et al (2015) present numerical simulations based on neutrino-driven explosion, and argue that they reproduce the protrusions and the distribution of some metals in Cassiopeia A.…”