Searching for Supernova Survivors

Title: Search for Surviving Companions of Progenitors of Young LMC Type Ia Supernovae Remnants

Authors: Chuan –Jui Li, Wolfgang E. Kerzendorf, You-Hua Chu, Ting-Wan Chen, Tuan Co, Robert A. Gruendl, Abigail Holmes, R. Ishioka, Bruno Leibundgut, Kuo-Chuan Pan, Paul M. Ricker, Daniel Weisz

First Author Institution: Graduate Institute of Astrophysics, National Taiwan University

Status: Accepted to ApJ, open access

Supernovae Survivors?

Surviving a supernova (SN) may sound crazy, since supernovae (SNe) are among the most energetic events in space. Type Ia SNe result from the explosion of a white dwarf, and just one of these events can temporarily outshine an entire galaxy. So how could anything survive such an explosion?

Well, there are two types of Type Ia SNe, both caused by white dwarfs hitting the Chandrasekhar mass limit – single degenerate (SD) and double degenerate (DD). DD Type Ia’s are caused by the merger of two white dwarfs, which upon merging, will pretty much annihilate one another and cause a SN. However, a SD Type Ia SN only involves one white dwarf. In this case, there is no merger – instead, the white dwarf has a non-degenerate (a.k.a., not a white dwarf) companion which it has drawn too much mass from, causing the white dwarf to explode. Since only one star (called the ‘progenitor’) is doing the exploding in this SD scenario, perhaps that companion will live long enough to tell its story…

Searching for A Companion

The authors of today’s paper set out to look for potential companions dancing around SN remnants, the shell of material left over by a SN explosion. These companions stars, which could be main sequence (MS) stars, red giant stars, or helium stars, may have lost their outer layers in the deadly explosion but could live on as a dense core. These surviving cores should be identifiable – they probably move differently as a result of the explosion, and likely look different in color.

Figure 2 (Figure 4]9 in the paper): SN 0519-69.0. The authors fit the SN shell in H\alpha (taken by HST) to an ellipse marked in white, with the white cross at its center. Averaging this with the center determined by another publication, the authors take the red ‘x’ as the explosion center. The red dashed circle marks the runaway distance for a MS companion (0.2 pc) and the cyan circle marks this distance for helium companions (0.6 pcs). The green circle denotes the search radius for background stars taken between the cyan and green circles. Similar figures for the other supernovae are available in the paper.

Knowing that these companion cores will stand out from background stars, the authors choose three Type Ia supernovae remnants to investigate for survivors: SN 0519-69.0, DEML71, and SN 0548-70.4. Because SN remnants in our own galaxy can be tough to look at through the galactic plane, these remnants are all located in the Large Magellanic Cloud (LMC). The first two SNe on the list have been examined before with no luck, but the authors hope that their new Hubble Space Telescope data will shed new light on these areas of the sky.

Today’s authors use those two methods, analyzing the color and motion of stars surrounding the chosen SNe to search for surviving companions. Before they can do this though, they need to determine a proper area to search.

Where to Look?

SNe remnants have a generally circular or elliptical shape, as the shock from the explosion propagates outward in all directions and interacts with the interstellar medium. By finding the geometrical center of the remnant’s visible shell, the authors estimate an explosion site (see Figure 1).

If a star survives a SN explosion, its velocity after the supernova should be the sum of its own orbital velocity and the velocity of the progenitor’s translational velocity. Previous studies have determined the maximum speed that a MS or helium star could be traveling after a Type Ia SNe. Using these velocities, the authors calculate just how far a companion core could have traveled away from the SN center since the explosion and narrow their search for survivors to this area (called the ‘runaway distance’). And of course, there has to be a control – the author’s determine a set of background stars to which they can compare their potential survivors (see Figures 1 & 2).

Figure 2 (Figure 5 in the paper): SN 0519-69.0. Stars with V mag < 23.0 (most likely cutoff for potential companions from Schaefer et al. 2012) that lie within the runaway bounds. These are analyzed as potential survivors in the CMDs and RV plots. Red for MS, cyan for helium stars.

Method 1: Examining Color

To examine the color of their potential survivors, the authors plot the stars’ color and absolute magnitude on a very useful diagram called a Color-Magnitude Diagram (clever name, right?). Included on these plots are all the candidate companions and background stars, as well as several ‘post-impact evolutionary tracks‘ (see Figure 3). These tracks are merely paths on this diagram that show how a MS or helium companion star, after a SN explosion, should change in color (which depends on its temperature) and brightness according to its initial mass. Therefore, if there are any true surviving companions, they should lie on these tracks.

You may have noticed that red giant stars, although a potential type of companion, have not been included in the search up to this point. Astronomers do not yet have evolutionary tracks for red giants unfortunately. More on why that is unfortunate in just a second.

Figure 3 (Figure 6 in the paper): CMDs for SN 0519-69.0. Left: The HST equvialent of a V vs B-V CMD. Right: The HST equivalent of an I vs V-I CMD. Evolutionary tracks are shown in green, with the helium star tracks situated in the left of each diagram.

Method 2: Examining Motion

The second method for identifying surviving companions is to examine their radial velocity (RV), the speed of their motion away from or towards the Earth. Astronomers need spectral data to get this, which the author’s only have for SN 0519-69.0 and DEML71. Now, although we don’t have a great idea of what that RV should be, it clearly should be different from the RV of background stars not involved in the SNe. The authors look at the distributions of RV for relevant stars (candidates or candidates+background – Figure 4) to determine which stars have abnormal RVs, and these are considered candidate survivors.

Figure 4 (Figure 7 in the paper): RV for stars with V mag < 21.6 (limiting magnitude for reliable spectral fits). For SN 0519-69.0, there were only a few candidates, so the authors included the background stars to establish a distribution. Star #5 is the strange one – it is not moving with the rest of the group! Again, the same figures for the other SNe are available in the paper.

Results

So what came of this survivor search? Let’s take a look at each supernova.

SN 0519-69.0 : The CMD search did not return any potential companions. The stars within the runaway radii have colors that do not fall on one of the corresponding evolutionary tracks. However, there is a star with a strange (> 2.5\sigma away from the mean) RV, as shown in Figure 4. This oddball star may be considered a candidate if it also fell on the evolutionary tracks, but it does not. Why, you ask? Well, it seems that this star is likely a red giant, as it falls on the red giant branch in the CMDs. So, this star could very well be a candidate, but red giant evolutionary tracks must be developed for the authors to confirm either way (that’s the unfortunate part).

DEML71 : This SN has a very similar story to SN 0519-69.0. No stars can be considered candidates from the CMDs, but there is indeed a star with a strange RV. However, as we saw before, it seems to be a red giant and therefore cannot be considered a candidate due to the lack of theoretical data. Boo.

SN 0548-70.4 : Inspection of the CMDs show that there is indeed a star that falls on one of the MS evolutionary tracks! Great! … But wait… there’s more. This star does not appear on evolutionary tracks for both colors, so the authors remain skeptical. A true candidate should fall on tracks for both CMDs Furthermore, the part of the evolutionary track that the candidate does fall on indicates an age of only ~110 years. This SN remnant is about 10,000 years old, so obviously this star is unrelated to the explosion and is likely not the candidate the authors were looking for.

As with all science, null results are still results. Even though no surviving cores were identified, the authors still gained valuable information- like, we really need some red giant post-impact evolutionary tracks. Or perhaps these SNe are not what they seem; if the SD and DD models are drastic oversimplifications, then our predictions for them won’t lead us to surviving stars. Many other types of Type 1a supernova have been proposed, such as sub-/super-Chandrasekhar or spin-up/spin-down. All in all, astronomers rely on models quite often, since we can’t go grab a star. With comparison to more models, we will have a better picture of reality.

About Lauren Sgro

I am a PhD student at the University of Georgia and, as boring as it may sound, I study dust. This includes debris disk stars and other types of strange, dusty star systems. Despite the all-consuming nature of graduate school, I enjoy doing yoga and occasionally hiking up a mountain.

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