The Temperature and Ionization of Unshocked Ejecta in Cas A

Abstract

The supernova remnant Cassiopeia A (Cas A) is one of the few remnants in which it is possible to observe unshocked ejecta. A deep 1.64 µm image of Cas A shows a patch of diffuse emission from unshocked ejecta, as well as brighter emission from Fast-Moving Knots and Quasi-Stationary Flocculi. Emission at 1.64 µm is usually interpreted as [Fe II] emission, and spectra of the bright knots confirm this by showing the expected emission in other [Fe II] lines. We performed NIR spectroscopy on the diffuse emission region and found that the unshocked ejecta emission does not show those lines, but rather the [Si I] 1.607 µm line. This means that the 1.64 µm line from the unshocked ejecta may be the [Si I] 1.645 line from the same upper level, rather than [Fe II]. We find that the [Si I] line is formed by recombination, and we use the [Si I] to [Si II] ratio to infer a temperature about 100 K, close to the value assumed for analysis of low frequency radio absorption and that inferred from emission by cool dust. Our results constrain estimates of Cas A's total mass of unshocked ejecta that are extremely sensitive to temperature assumptions, but they do not resolve the ambiguity due to clumping.

The dust content of the Crab Nebula

“…Compared to the heavy element mass predicted by nucleosynthesis models (0.42 M , for a 11 M progenitor star, Woosley & Weaver 1995), the modelled dust masses for "a-C" grains correspond to a dust condensation efficiency of 8-12%. The inferred dust condensation efficiency is consistent with estimates of the dust condensation efficiency for Cassiopeia A (0.1-0.17, Nozawa et al 2010;Raymond et al 2018;Priestley et al 2019).…”

Dust destruction and survival in the Cassiopeia A reverse shock

“…The choice of temperature, ionisation state, and geometry follow the assumptions made in DeLaney et al (2014). The T = 100 K assumption was later confirmed by Raymond et al (2018). Moreover, we assume that the unshocked ejecta is composed mostly of threetimes ionised oxygen ([O IV], as observed by Isensee et al (2010) among others), whereas we know that there are low ionisation, heavier species, such as [Si II] and [S III] (Smith et al 2009;Isensee et al 2010;Milisavljevic & Fesen 2015), and possibly also Fe.…”

The mass, location, and heating of the dust in the Cassiopeia A supernova remnant

“…The ejecta dust-to-gas ratios are ∼ 20× higher than in the ISM, implying that a significant fraction of the metals are condensed into dust grains (0.17, assuming all the ejecta mass is condensible material). Raymond et al (2018) estimated a similar effiency of ∼ 0.1 from the gas and dust masses from Arias et al (2018) andDe Looze et al (2017) respectively, combined with their measurement of the preshock gas temperature. Owen & Barlow (2015) found lower ratios (0.026 − 0.038) for the Crab Nebula, which has no reverse shock processing the ejecta -however, the Crab Nebula contains significant quantities of hydrogen and helium, unlike Cas A, so the fraction of condensible material locked up in dust grains is higher than the value inferred from this ratio.…”

“…The SNR also contains ejecta which has not yet encountered the reverse shock, and is consequently much cooler. Smith et al (2009) estimated a maximum electron density of ne 100 cm −3 for the unshocked ejecta based on forbidden line ratios, while observations of radio absorption by DeLaney et al (2014) and Arias et al (2018) give ne ∼ 10 cm −3 and T ∼ 100 K. Raymond et al (2018) inferred a preshock temperature of ∼ 100 K from [Si I] IR emission lines. Krause et al (2008) determined that the Cas A SN was of type IIb from a spectrum of its light echo, meaning that the progenitor star must have lost most of its hydrogen envelope pre-explosion.…”