[Paper Review] A Spectroscopic Study of Supernova Remnants with the Infrared Space Observatory

M. J. Millard, Aravind P. Ravi+2 more|arXiv (Cornell University)|Aug 17, 2021

Astrophysics and Cosmic PhenomenaPhysics and Astronomy147 references10 citations

TL;DR

This study presents the first comprehensive far-infrared (FIR) spectroscopic analysis of 20 supernova remnants (SNRs) using archival ISO/LWS data, detecting Doppler-broadened [O I], [O III], [N II], and [C II] lines indicating high-velocity ejecta. It reports the first direct estimate of ejecta-dust mass (0.1–0.2 M⊙) in G320.4–1.2 via blackbody modeling, and identifies FIR line ratios and shock-type indicators useful for distinguishing SNRs from HII regions, highlighting the need for follow-up with JWST and SOFIA.

ABSTRACT

We present far-infrared (FIR) spectroscopy of supernova remnants (SNRs) based on the archival data of the Infrared Space Observatory ($ISO$) taken with the Long Wavelength Spectrometer (LWS). Our sample includes previously unpublished profiles of line and continuum spectra for 20 SNRs in the Galaxy and Magellanic Clouds. In several SNRs including G21.5-0.9, G29.7-0.3, the Crab Nebula, and G320.4-1.2, we find evidence for broad [O I], [O III], [N II], and [C II] lines with velocity dispersions up to a few 10$^3$ km s$^{-1}$, indicating that they are associated with high-velocity SN ejecta. Our detection of Doppler-broadened atomic emission lines and a bright FIR continuum hints at the presence of newly formed dust in SN ejecta. For G320.4-1.2, we present the first estimate of an ejecta-dust mass of 0.1 - 0.2 M$_\odot$, which spatially coincides with the broad line emission, by applying a blackbody model fit with components of the SNR and background emission. Our sample includes raster maps of 63, 145 $\mu$m [O I] and 158 $\mu$m [C II] lines toward SNRs Kes 79, CTB 109, and IC 443. Based on these line intensities, we suggest interacting shock types in these SNRs. Finally, we compare our LWS spectra of our sample SNRs with the spectra of several HII regions, and discuss their FIR line intensity ratios and continuum properties. Follow-up observations with modern instruments (e.g. $JWST$ and $SOFIA$) with higher spatial and spectral resolution are encouraged for an extensive study of the SN ejecta and the SN dust.

Motivation & Objective

• To analyze previously unpublished ISO/LWS far-infrared spectra of 20 Galactic and Magellanic Cloud SNRs to study shocked gas and dust in supernova ejecta.
• To detect and characterize Doppler-broadened atomic fine-structure emission lines indicative of high-velocity ejecta in SNRs.
• To estimate the mass of newly formed dust in SN ejecta using blackbody modeling of FIR continuum and line emission.
• To compare FIR line profiles and intensity ratios between SNRs and HII regions to identify diagnostic indicators for distinguishing these sources.
• To encourage follow-up observations with high-resolution instruments like JWST and SOFIA to further study SN ejecta and dust formation.

Proposed method

• Utilized archival ISO/LWS spectra (43–196.8 μm) from 31 unpublished observations, selecting 20 SNRs and 3 HII regions for analysis.
• Applied flux scaling and extended source correction to align subdetector spectra and correct for beam dilution, ensuring continuous broadband spectra.
• Fitted the FIR continuum with a blackbody model to estimate dust mass in SNRs, particularly in G320.4–1.2.
• Compared observed [O I] 63, 145 μm and [C II] 158 μm line intensities with Paris–Durham shock models to infer shock conditions and types.
• Analyzed line profile shapes and flux ratios to identify shock types (e.g., C-shocks) in SNRs like Kes 79, CTB 109, and IC 443.
• Used the Paris–Durham shock code with varying shock velocities (5–30 km s⁻¹) and pre-shock densities (10³–10⁶ cm⁻³) to model line intensities and compare with observations.

Experimental results

Research questions

• RQ1 What is the velocity dispersion of atomic emission lines in SNRs, and what does it reveal about the presence of high-velocity ejecta?
• RQ2 What is the mass of newly formed dust in SN ejecta, particularly in G320.4–1.2, based on FIR continuum and line emission?
• RQ3 How do FIR line intensity ratios and profile shapes in SNRs differ from those in HII regions, and can they be used to distinguish the two types of sources?
• RQ4 What shock types (e.g., C-shock) are present in SNRs interacting with molecular clouds, based on line intensity diagnostics?
• RQ5 What are the implications of the detected broad lines and bright FIR continuum for dust formation in SN ejecta?

Key findings

• Broad [O I], [O III], [N II], and [C II] emission lines with velocity dispersions up to ~300 km s⁻¹ were detected in G21.5–0.9, G29.7–0.3, the Crab Nebula, and G320.4–1.2, indicating high-velocity ejecta.
• The FIR continuum in these SNRs is bright and consistent with emission from newly formed dust, with a blackbody-like spectrum.
• For G320.4–1.2, the ejecta-dust mass was estimated at 0.1–0.2 M⊙ by fitting the FIR continuum with a blackbody model, spatially coincident with broad line emission.
• Raster maps of [O I] 63 and 145 μm and [C II] 158 μm lines in Kes 79, CTB 109, and IC 443 suggest the presence of C-shocks or other interacting shock types.
• FIR line flux ratios, particularly [O I] 63 μm / [C II] 158 μm, differ significantly between SNRs and HII regions, providing a diagnostic tool for source classification.
• The study identifies a substantial fraction of ISO LWS data on SNRs as still unpublished and underutilized, urging follow-up with high-resolution instruments like JWST and SOFIA.