The Story of Pulsational Pair-Instability SNe Briana Ingermann & Parke Loyd

Meet Pulsational Pair-Instability SN Progenitor

After a short MS life, our progenitor senses something is wrong

A runaway thermonuclear explosion ejects a shell, and our progenitor feels better

Fig S5 from Woosley et al plotting the light curve resulting from the first pulse of the 110 M ʘ model..

...but the respite from instability is brief

With a final gasp, our progenitor dazzles spectators with a display like few others

Fig 1 of Woosley et al plotting the velocity and interior mass as a function of radius when the second ejected shell is beginning to impact the first in the 110 M ʘ model.

Fig 2 from Woosley et al plotting the cumulative lightcurve predicted by their simulation of a 110 M ʘ low-metallicity star.

So it goes with the pulsational pair- instability supernova

Fig 3 from Woosley et al plotting the R-band absolute magnitude of SN 2006gy as the red data points, predicted values with 30-day smoothing for the 110 M ʘ model as the purple curve, and predicted values for the same model but with 4x the KE of the ejecta as the dashed curve.

Quit faking your lightcurves When they attempt to model the observations of SN 2006gy they find much better agreement when they artificially increase the explosion energy by a factor of 4 (which gives an energy above all the values in their supplementary table 1). Is this higher energy still consistent with the pulsational pair instability mechanism they describe? If so, what parameters of the star would have to be changed? Excerpt from Table S1 of Woosley et al showing how much the kinetic energy and mass released in a pulse can vary just as a function of the core mass.

Energy of the second pulse Why is the second pulse so much higher energy than the first? Is the runaway reaction of Helium just more efficient? Or when they say "more energetic" do they mean energy per unit mass ejected? The energy release they refer to in their paper is the kinetic energy of the ejected material. The total energy generated by the runaway thermonuclear reactions depends on the specifics of those reactions (but note a very strong dependence on temperature). The paper is not consistent on whether the second pulse is always strong and ejects less mass (see right). But wait, doesn't the first ejected shell lose most of its KE leaving the star while the second hardly loses any? Why? Excerpt from Table S1 of Woosley et al contradicting their statement in the caption that "in each case, the first pulse is the weakest," as well as their statement in the main text of the article that "later ejections have lower mass."

Shall we compare the reactions occurring in the first and second pulse?

First Pulse Interior mass / solar masses Log mass fraction

First Pulse Interior mass / solar masses Log mass fraction

Second Pulse Interior mass / solar masses Log mass fraction

Second Pulse Interior mass / solar masses Log mass fraction

Comparative supernovatology How does the driving mechanism for a PPI SN differ from a core collapse (type II and Ib-c) or a type Ia?

Have we ever seen a SN of a >120 M ʘ Progenitor? Maybe: Next week's paper (Smith et al. 2007) suggests an initial mass of the 2006gy progenitor of ~150 M ʘ. Cooke et al claim to have observed superluminous supernovae (10 51 erg) with estimated progenitor masses of M ʘ at high redshift. Plus some possible future SNe: Crowther et al claim there are four Wolf-Rayet stars in the R136 cluster with masses of M ʘ. Fig. 1 from Crowther et al showing a 12'' x 12", infrared ( μm bandpass) image of the R136 cluster.

Eta Carinae

Enter: the IMF What rate would we expect for such events based on the IMF? What do we observe? I would expect them to be rare compared to regular supernovae because they require such massive progenitors. Fig 2 from Cooke et al plotting the lightcurve of a superluminous SN at redshift z = 3.9.

How does metallicity fit in? Our questions: Pair instability can only occur in enormously massive stars of low to moderate metallicity. Why this metallicity requirement? What are the implications for observation? And a related question from you: They say in the paper that they use a 110 solar mass star with solar composition, but with a mass loss rate at a fraction of the standard value. Do they have any justification for the validity of doing this?

Why PPI? Do pair instability ejections have to be the mechanism by which the envelope is removed? It seems like any stellar wind followed by any explosion would lead to high speed collisions of gas. What is special about the pair instability ejections that make them more likely than other scenarios? Last week we talked about WR stars, which I think are less massive than the stars discussed, collapsing directly to a black hole with no explosion. So I guess I am just wondering what is fundamentally different in this case which cause these stars to make super luminous supernovae as opposed to simply collapsing to a black hole with no explosion? In Table 1. in the birth mass range of M ʘ row they state that there is an explosion but with no remnant. How is this possible, and if so what happens to all the remaining mass that would have been a black hole or neutron star? I would think that all that excess mass would have to be directly converted to energy. This would leave at least an excess energy of E ~ mc 2 ~ (Minimum neutron star/black hole mass)(c 2 ) that could go into the explosion.

Does the first pulse resemble a type-II SN? So it seems from plot S5 and figure 2 that the light curve from the first mass ejection has a peak luminosity around ergs, which I think is around the peak luminosity of a regular core collapse supernova. So my question is can we detect the first outburst and what supernova types might it look like? When a pair-instability star first explodes and sheds its envelope, would that explosion likely resemble a nova or a supernova itself? Suppose that we were only able to observe the spectra of a pair-instability supernova during one of its pulses and not its light curve. What would the spectra of this type of supernova look like? Are there any unique features in its spectra, apart from its extreme luminosity, that would differentiate it from the more common types of supernovae?

Fig 2 from Kasen & Woosley 2009 plotting bolometric light curves predicted by a SN model with varying amounts of ejected 56 Ni.

Observe the SN in it's natural habitat Pre-SN The paper describes how the temperature in the star after that first pair- instability explosion will determine how long it will take the star to re-ignite and continue burning until the next pair-instability explosion (from anywhere between days to decades [or even centuries?!]). If a star's burning continued after a long period of time (like closer to the decades range) and thus it didn't explode again until much later than the first expulsion of the envelope, would we still observe the same high-energy SN like what we saw in SN2006gy? Or could we possibly miss it, maybe suggesting that more pair-instability SN occur but we do not detect them? Will pulsational PISN ever go through a LBV or WR phase? Post-SN A PI SN site will be heavily obscured by several solar masses of ejected material, so is it feasible to search for progenitors or remnants to these SN?

References Cooke, J., Sullivan, M., Gal-Yam, A., et al. 2012, Nature, 491, 228 Crowther, P. A., Schnurr, O., Hirschi, R., et al. 2010, MNRAS, 408, 731 Kasen, D., & Woosley, S. E. 2009, ApJ, 703, 2205 Smith, N., Li, W., Foley, R. J., et al. 2007, ApJ, 666, 1116 Nakamura, F. & Umemura, M. 2001, ApJ, 548, 19

Table 1 from Woosley et al showing the notional fates of stars (but at what metallicity?).