Has a Supernova type Ia progenitor finally been found?

Prof. Sion and collaborators at Villanova think they have found a genuine progenitor system for type Ia supernova, in our neighbourhood, in the Milky Way.

Type Ia supernovae are thermonuclear detonations of white dwarfs which acquire more mass, somehow, and go over the Chandrasekhar limit (about 1.4 solar masses).

Type Ia supernovae are not as common as the core-collapse type II/Ib/Ic supernovae from massive young stars.
They are astrophysically important, since most should involve the detonation of a Chandrasekhar mass of light elements, probably O/Ne/Mg mixture, to iron, with associated explosive disassembly - and since the mass involved is close to constant, the resultant explosion is, mostly, of fixed luminosity (there are complications with overluminous and underluminous detonations, but those are correctable from systematic duration and spectral variations).
Hence type Ia are used as standard candles and since they are bright, they work to large cosmological distances, and as such provide a key, but not critical, piece of evidence for an accelerating universe and hence dark energy.
And they are also intrinsically interesting.

But, how do you make them? It is not as easy as it sounds.

One way would be to smash two white dwarfs together but would tend to produce overluminous supernovae. The resultant system would tend to exceed the Chandrsekhar mass, briefly.
It is also conceivable that such systems would collapse to a rapidly rotating neutron star rather than detonate - detonation turns out to be slightly non-trivial.
No bound white dwarfs with the large enough combined mass are known. But they are hard to see. It is not clear that white dwarfs merge with each other at high enough a rate to account for the observed type Ia rate - in particular the back-and-forth mass transfer between the intermediate mass stellar progenitors makes it hard to end up with white dwarfs of the right mass in orbits close enough to ever merge. Some leverage can be gained by putting the white dwarfs in dense globular clusters and using three or four-body dynamics to push up the merger rate, but not all type Ia are in globular clusters, or anywhere near globulars.

Alternatively, you can take a massive white dwarf, but one less massive than the Chandrasekhar mass, and dribble mass onto it. Slowly.
You can take the mass from a companion star, either a (sub)giant or another, lower mass, white dwarf.

But, this is also tricky. It turns out that dribbling mass onto a white dwarf is mostly a net loss - episodic surface deflagrations - nova explosions - tend to blow off more mass than is gained by accretion, and so the white dwarf's mass tends to decrease, not increase.
But, theoretically, for some combinations of white dwarf mass and mass accretion rates, the deflagrations are weak enough that less mass is blown off than is accreted and the white dwarf mass increases steadily.
Then, one day, it crosses the Chandrasekhar limit and KABLOOIE.

The problem is that no accreting white dwarf system has been identified which is positively consistent with a type Ia progenitor - some x-ray emitting binary systems are broadly consistent with leading to type Ia but not really any we can definitely point to as definitely being ready to blow.
Also the rates (inverse lifetimes) of such systems consistent with maybe becoming Ia are too low to account for the Ia rate - suggesting that multiple channels lead to type Ia, which is certainly possible but inelegant, and makes the argument for a robust standard candle a bit weaker.



Pyxis

Now, Sion and collaborators havetaken a fresh look at T Pyxidis, a recurrent nova in Pyxis (the Compass Box).
T Pyxidis erupted in 1890, 1902, 1920, 1944 and 1967. But not since.
It was thought to be in a "mass-loss" deflagration mode, with the white dwarf shrinking a bit each time.

They took archival IUE utlraviolet spectra and fit new-and-improved accretion disk models to the observations. Hence they concluded T Pyxidis is closer than previously thought, at about a kpc, and hence is less luminous, the accretion rate is lower than previous estimates, and the system is consistent with gaining mass, not losing it per cycle. The shells of ejecta are closer than we thought, hence occupy less volume and therefore contain less mass - since their density is relatively well known.

The revised mass accretion rate is about 3*10-9 solar masses per year.



T Pyxidis

This also implies the white dwarf is more massive than previously thought, in fact very close to the Chandrasekhar mass. And there has not been a deflagration for a while, so mass has accumulated...

Supernovae are bright, very very bright. Dangerously bright.
And this one could go KABLOOIE - tomorrow - or sometime in the next 10-30 million years or so, depending on how close to the Chandrasekhar mass the white dwarf actually is.

So, how bad would that be?
Well, some models suggest that any type Ia within a kpc or so would zap the upper atmosphere with enough γ-rays to create enough NO in the upper atmosphere to seriously damage the ozone layer and hence the biosphere.
Uh-oh.

Of course other papers suggest the supernova might have to be more like 10 pc away to do any real damage. The factor of 10,000 uncertainty in the theoretical estimates is not bad, for theorists.

see wikipedia for discussion and sources

or go straight to the Spaceman Spiff for the straight dope

Interesting if confirmed. Preprint is not yet out.

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As a practitioner I must say that I am very skeptical of any accretion rate or compact-object mass determinations based on fits of such models to observed spectra (especially IUE spectra). There are some notorious uncertainties and internal inconsistencies in this game. The best example is that when such models reproduce the slope of the UV spectra of such systems, they fail to reproduce the normalization by close to two orders of magnitude. I will be convinced only when T Pyx explodes as a SN Ia.

By Dr. Skepto (not verified) on 08 Jan 2010 #permalink

I'm shocked.
One must not doubt theory, especially not new and improved theory!

Anyway, how else are you going to have a like, OMG! We're all gonna die! press release?