A New Look into Cas A’s Past

Title: Detection of Pristine Circumstellar Material of the Cassiopeia A Supernova

Authors: Bon-Chul Koo, Hyun-Jeong Kim, Heeyoung Oh, John C. Raymond, Sung-Chul Yoon, Yong- Hyun Lee, Daniel T. Jaffe

First Author’s Institution: Department of Physics and Astronomy, Seoul National University, Seoul, Korea

Status: Published in Nature Astronomy [open access on arXiv]

Supernova Remnants and Circumstellar Material

Ever wondered what a star looked like before it exploded? Today, with extensive archival data, we can always go back after a supernova (SN) explosion and search for pre-SN images of the progenitor star. However, this method does not work for stars without such prior imaging, and it is certainly impossible for historical SN that are several centuries old and predate the era of telescopic astronomy. In these cases, we can take an alternative approach and study the material left behind after the stellar explosion. This material, generally referred to as a supernova remnant (SNR), interacts with circumstellar material (CSM) that was ejected towards the end of the progenitor star’s lifetime. By studying the physical and chemical characteristics of this material, astronomers can learn more about how the progenitors of core collapse supernovae strip off their hydrogen/helium envelopes and the explosion process itself. Using such an approach, today’s authors report the exciting discovery of a pristine CSM knot in Cassiopeia A (Cas A), one of the most famous and well-studied SNRs in our Galaxy.

Cas A: A Brief History

The light from the stellar explosion responsible for Cas A likely first reached Earth approximately 300 years ago. Due to its relative youth and nearby distance, Cas A is perhaps the most well-studied in remnant in our Galaxy. From light echo observations, we know that a Type IIb SN produced the remnant we see today. Over the last several decades, astronomers have even been able to make a detailed 3D reconstruction of the expansion of the leftover stellar material.

The presence of dense, He- and N-enriched CSM knots in Cas A has been known since the 1950s and are often referred to as “quasi-stationary flocculi” (QSFs) due to their relatively low-velocity (~400 km/s) compared to the faster moving main ejecta (1000s of km/s). These QSFs are believed to be dense CNO-processed CSM clumps that have been recently shocked by the SN blast wave. As they likely originated due to mass-loss from the SN progenitor star, they offer a rare window into the conditions deep inside the progenitor star. However, until now, no one had detected emission from an unprocessed CSM knot, which retains the original physical and chemical conditions of the stellar interior and has not been disrupted by the advancing SN shockwave.

Discovery of a Pristine CSM Knot

Using observations in the near-infrared (NIR), today’s authors detected a strong iron emission line toward a new QSF knot in the south of Cas A, as shown in Figure 1. The presence of emission from a heavy element such as iron is a telltale sign that this material originated deep inside the progenitor star and represents a mass-loss event prior to the SN explosions.

Figure 1. An image of the [Fe II] 1.644 μm line of Cas A with higher intensities shown as brighter white colors. QSFs are indicated by magenta contours. The white box in the south marks the newly-discovered pristine CSM knot and the inset shows the slits used for NIR spectroscopic mapping. The central red star marks the explosion center and the yellow contours indicate the outer boundary of the remnant as seen in radio observations. North is up and east is to the left. The scale bar in the upper right represents an angular scale of 1’, which corresponds to about 1 parsec at the distance of Cas A. Adapted from Figure 1 in paper.

The spectra, as shown in Figure 2, reveal two distinct components: one broad-line component (BLC) with a velocity width of 200 km/s, which indicates that this emission is from shocked gas, and a narrow-line component (NLC) with a width of only 8 km/s.

Figure 2. Average spectra for the pristine CSM knot. The spectra of the entire knot is shown in black (with an arbitrary vertical shift for visual clarity) and those from Clump A in blue. Both broad and narrow components of the [Fe II] 1.644 μm line are clearly visible. Adapted from Figure 2 in paper.

The authors imaged the entire knot and as seen in Figure 3, discover a complex morphology with several distinct clumps. The largest one, ‘Clump A-NLC’, is located in the southeast and is responsible for the large line widths seen in Figure 1. Immediately adjacent to this feature is another emission feature called the ‘Clump A-BLC,’ which as the name implies is the region responsible for producing the narrowest iron emission line widths. When the authors checked available Hubble Space Telescope (HST) imaging, they found a prominent emission feature that was spatially coincident with Clump A-BLC. Moreover, it also showed a sharp decrease of brightness towards Clump A-NLC.

The authors believe that this morphology suggests that a strong shock is currently propagating into Clump A from the NW to SE direction and that Clump A-NLC represents the unshocked part of the clump. A further analysis of Clump A-NLC, using additional detected iron lines and those from excited hydrogen lines indicates that Clump A-NLC is unshocked, pristine CSM photoionized by UV radiation from the NW shock propagating into the clump.

Figure 3. Integrated intensity maps of the [Fe II] 1.644 μm line are shown for the narrow-line (a) and broad-line component (b). The intensity scale is linear in arbitrary units and the white dashed boxes indicate the areas used for deriving physical parameters for Clump A. An HST narrow-band image sensitive to Hα and [N II] lines that trace shocked gas. The red arrow shows the radial direction from the explosion center (c). Adapted from Figure 3 in paper.

Interestingly, the authors also find that the majority of Fe in this CSM knot is in the gas phase, which contrasts with the interstellar medium in general, where less than 5 percent is found in the gas phase. The ‘non-depletion’ of Fe in this CSM reflects the physical and chemical conditions of the stellar material ejected from the progenitor in the pre-SN stage.

Implications for Cas A’s Stellar Progenitor

To deduce when the mass-loss event that ejected the QSFs occurred and what the evolutionary stage of the progenitor was, the authors also analyzed the proper motions of the QSFs. They find a systemic expansion with a velocity of 180 km/s and are able to determine a stellar radius and surface temperature that are 100x and 2x that of our Sun, respectively. Taken together with the high He and N abundances of these QSFs and the high fraction of gas phase Fe, the authors conclude that the Cas A QSFs were likely the result of mass-loss due to stellar winds from a blue supergiant with a thin hydrogen envelope. Further observations are needed not only to better understand the formation and evolution of these dense clumps, but to search for more pristine QSFs, which would help further constrain the explosive process responsible for creating the Cas A remnant.

About Charles Law

Hi! I'm a third-year graduate student at Harvard/CfA. I study chemical complexity in protoplanetary disks and star-forming regions using telescopes such as the SMA, VLA, and ALMA. In my free time, I enjoy hiking, bicycling, and traveling.

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