Type Ia supernova involving binary white dwarfs?

[bbbl67]

Okay, so for years the typical mechanism used to describe a white dwarf supernova explosion was that a white dwarf accumulates matter from a main sequence binary companion. Then a few years later, the concept of two white dwarfs crashing into each other came up. With the main sequence companion, at least you could see how the mass would stop accumulating at 1.4 solar masses (the Chandrasekhar Limit). But with two white dwarfs, don't both white dwarfs completely obliterate each other? What if the combined mass of the two white dwarfs was above 1.4 solar masses? How can you call the resultant Type Ia supernova the same as any other Type 1a? Wouldn't it be a far bigger explosion?

 

[rootone]

Stars crashing into each other is very unlikely.
If there is any angular momentum at all they will orbit
It's not impossible, if it did happen though I think the result would be an untypical type 2 SN,

 

[phyzguy]

Okay, so for years the typical mechanism used to describe a white dwarf supernova explosion was that a white dwarf accumulates matter from a main sequence binary companion. Then a few years later, the concept of two white dwarfs crashing into each other came up. With the main sequence companion, at least you could see how the mass would stop accumulating at 1.4 solar masses (the Chandrasekhar Limit). But with two white dwarfs, don't both white dwarfs completely obliterate each other? What if the combined mass of the two white dwarfs was above 1.4 solar masses? How can you call the resultant Type Ia supernova the same as any other Type 1a? Wouldn't it be a far bigger explosion?

There are currently two models for Type 1A SN. The first is what you said, where a white dwarf accumulates matter from a main sequence companion. This is called the "Singly Degenerate"(SD) scenario. The second model is two white dwarfs which are orbiting and their orbit decays until they coalesce. This is called the "Doubly Degenerate"(DD) model. There is data supporting both models, and it is not clear at present whether one model or the other dominates, or whether Type 1A SN are a mixture of the two. Personally I favor the DD model, but other people passionately support the SD model. If the SD model is correct, the main sequence star should survive the explosion and still be there after the SN 1A fades. However, searches for the remaining main sequence star in older SN 1A have all come up empty.

Stars crashing into each other is very unlikely.

This is wrong. The "doubly degenerate" model of Type 1A SN doesn't envision two stars just randomly crashing into each other. The model is that two WD stars are orbiting each other and the orbit decays due to gravitatoinal wave emission, causing the two stars to coalesce.

 

[bbbl67]

There are currently two models for Type 1A SN. The first is what you said, where a white dwarf accumulates matter from a main sequence companion. This is called the "Singly Degenerate"(SD) scenario. The second model is two white dwarfs which are orbiting and their orbit decays until they coalesce. This is called the "Doubly Degenerate"(DD) model. There is data supporting both models, and it is not clear at present whether one model or the other dominates, or whether Type 1A SN are a mixture of the two. Personally I favor the DD model, but other people passionately support the SD model. If the SD model is correct, the main sequence star should survive the explosion and still be there after the SN 1A fades. However, searches for the remaining main sequence star in older SN 1A have all come up empty.

Yes, but does the DD model produce a bigger explosion than the SD model? Therefore Type Ia supernovae aren't reliable standard candles.

This is wrong. The "doubly degenerate" model of Type 1A SN doesn't envision two stars just randomly crashing into each other. The model is that two WD stars are orbiting each other and the orbit decays due to gravitatoinal wave emission, causing the two stars to coalesce.

Yes, I know, I'm just being colloquial here.

 

[phyzguy]

Yes, but does the DD model produce a bigger explosion than the SD model? Therefore Type Ia supernovae aren't reliable standard candles.

This is a common point of confusion. SN 1A are not all of the same brightness. Look at the figure below, from this paper, showing the absolute magnitude distribution of SN 1A. The brightness varies by more than a factor of 100. It is better to refer to them as "standardizable candles". There is a relationship, called the "Philips relationship" which relates the peak brightness of the SN to the shape of the light curve. So by observing the light curve, you can calibrate out differences in brightness to determine the peak intrinsic brightness. Why the Philips curve is followed, and what type of explosion leads t the SN, are topics of current research.

 

[stefan r]

...What if the combined mass of the two white dwarfs was above 1.4 solar masses? How can you call the resultant Type Ia supernova the same as any other Type 1a? ...

A Type 1 super nova does not have a hydrogen line. It is type 1a when there is a silicon line.

The merging white dwarfs lack hydrogen so their resulting supernova will also lack hydrogen. The fusing oxygen atoms create the silicone which will then show up in the spectrum.

model of Type 1A SN doesn't envision two stars just randomly crashing into each other...

Someone should make those models and render the pictures. Not just two white dwarfs. Jupiter vs white dwarf, brown dwarf vs white dwarf etc.

 

[bbbl67]

This is a common point of confusion. SN 1A are not all of the same brightness. Look at the figure below, from this paper, showing the absolute magnitude distribution of SN 1A. The brightness varies by more than a factor of 100. It is better to refer to them as "standardizable candles". There is a relationship, called the "Philips relationship" which relates the peak brightness of the SN to the shape of the light curve. So by observing the light curve, you can calibrate out differences in brightness to determine the peak intrinsic brightness. Why the Philips curve is followed, and what type of explosion leads t the SN, are topics of current research.

Ah, okay, so that has had me fooled all of these years. The brightnesses vary, but the light curve remains the same no matter what brightness.

So prior to the discovery of DD supernova, did they know about the Philips relationship? Or did they just use the standard SD brightness by itself?

 

[phyzguy]

Ah, okay, so that has had me fooled all of these years. The brightnesses vary, but the light curve remains the same no matter what brightness.

No, that's not what I said. There is a relationship between the peak brightness and the width of the light curve that allows you to determine the peak intrinsic brightness by measuring the light curve.

So prior to the discovery of DD supernova, did they know about the Philips relationship? Or did they just use the standard SD brightness by itself?

I think all of the analyses that use SN1A as "standard candles" use the Philips relationship to calibrate the brightness.

 

[|Glitch|]

There is a third type of Type Ia SN that has not been mentioned - Type Iax SN. This is where the progenitor is not completely destroyed by the deflagration. They are also a sub-Chandrasekhar limit deflagration. Meaning the supernova results before the Chandrasekhar limit is reached. These Type Iax SN produce the same light curve as Type Ia SN, but are considerably dimmer. The absolute magnitude range of Type Iax SN is between -14.2 and -18.9.

We have known since at least 2003 with the discovery of SNLS-03D3bb (SN 2003fg) that Type Ia SN are not the Standard Candle everyone thought they were during the 1990s. Nor is the "Chandrasekhar limit" really the "limit" since all of these superluminous Type Ia SN have exceeded that limit and the Type Iax SN fall well below that limit. The Type Iax SN classification was created in 2013, and it claims that between 18% and 48% of all Type Ia SN classified prior to 2013 have been misclassified and should actually be the much dimmer Type Iax SN.

Sources:
The Birth Rate of Subluminous and Overlumious Type Ia Supernovae - Astronomy & Astrophysics, Volume 525, January 2011
Type Iax Supernove: A New Class of Stellar Explosion - The Astrophysical Journal, Volume 767, Number 1, March 2013
'Super-Chandrasekhar' Type Ia Supernovae at Nebular Epochs - Monthly Letters of the Royal Astronomical Society, Volume 432, Issue 4, July 2013
Analytical Expressions for Light Curves of Ordinary and Superluminous Type Ia Supernovae - The Astrophysical Journal, Volume 809, Number 1, August 2015