Jul 072017
 

By Brian Williams (STSCI).

In 1572 C.E., a “new star” appeared in the sky, gradually brightening to be as bright as Venus at peak brightness, staying visible for nearly two years. This event, now known as Tycho’s supernova, helped to usher in a new age of science where the heavens were no longer fixed. Nearly 450 years later, we know this object as Tycho’s supernova remnant (Figure 1). The shock wave from this supernova has expanded to a radius of nearly 4 parsecs and is still racing into the interstellar medium at over 6000 km/s in places. Because the remnant is so young (and our telescopes are so great!) a particularly exciting aspect of Tycho is that we can watch it expand over baselines of a few years. This video  shows the expansion of the remnant over five epochs of  observations between 2000 and 2015 taken by the Chandra X-ray observatory.

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Figure 1: Snapshot of the video showing Tycho supernova remnant expansion (based on observations with the Chandra X-ray observatory).

Tycho’s supernova remnant is known to be the result of a Type Ia supernova, and while we have a lot of observational data, we do not yet have a great understanding of what the progenitor systems of these supernovae are. It seems that they result from the thermonuclear explosion of a white dwarf star in a binary system, but whether the companion star is another white dwarf or an AGB or red giant star is unknown (both models have pros and cons). We also do not understand how the explosion begins in the degenerate star and propagates through it. However, models of the explosion offer different predictions for the motion of the ejecta after the star has exploded. As an example, Figure 2 shows the velocity distribution of Silicon, an element produced in the supernova, for two different explosion models from Seitenzahl et al. (2013), where the difference between the two is the number of ignition points for the detonation inside the white dwarf. Plotted is the velocity of the ejecta along various cuts in the XYZ coordinate plane. As can be seen, the amount of symmetry in the velocities of the expanding ejecta is not the same between the two models.

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Figure 2: Slices through all three coordinate axis planes for the velocity distribution of Si shortly after the explosion. A model with many ignition points for the detonation is shown on the left; a model with only a few is shown on the right.

The question then becomes whether we can constrain these models with observations. My collaborators and I studied new and archival observations of Tycho’s supernova remnant, identifying nearly sixty dense knots of ejecta from the supernova that had a measurable proper motion between the epochs. Of course, this only gives the two-dimensional motion in the plane of the sky, which is only part of the story. However, these ejecta knots, dominated by emission from silicon and sulfur, are also quite bright and Chandra’s CCD cameras produce a spectrum for every pixel. We were able to use the redshift and blueshift of the Si and S lines in the X-ray spectrum between 1.8 and 2.6 keV to measure the velocity along the line of sight. Measuring an accurate shift in the lines required a significant effort to ensure that the atomic physics was properly accounted for. (The rest energy of X-ray lines can be a function of the temperature and ionization state of the gas; see Williams et al. 2017 for more details on this).

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Figure 3: Histograms of the velocity distribution in the X (green), Y (blue), and Z (red) directions.

Our results showed that there is no measurable asymmetry in the expanding ejecta, meaning that, as far as we can infer from this data, the explosion of Tycho was symmetric. Models with more ignition points inside the white dwarf better reproduce the observations in this case. The knots have velocities ranging from just over 2000 km/s to well over 6000 km/s and the 3-D velocity vectors all point away from the center of the remnant (as one would expect). Our work represents the first 3D map of the expanding ejecta from a Type Ia supernova and is an important step in understanding these stellar explosions. We cannot stretch these results too far: Tycho is just one supernova remnant and there may well be more than one way to explode a Type Ia supernova. Still, results like this are encouraging and with Chandra and the next generation of X-ray telescopes studies of more Ia remnants are possible.

References: 

  • Seitenzahl, Ciaraldi-Schoolmann, Röpke et al. 2013, MNRAS, 429, 1156.
  • Williams, Coyle, Yamaguchi et al. 2017, ApJ, 842, 28.

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