What Causes a White Dwarf Star to Exceed the Chandra Mass Limit and Explode?

[Labguy]

Clearly, a white dwarf cannot survive beyond the mass limit, so there must be some kind of explosion. It's hard for me to imagine that a significant fraction of white dwarfs explode without producing an observable amount of light.

Other posts and discussion have left at least one thing clear. That is that only Carbon-Oxygen white dwarf stars will become Type Ia supernovae if enough mass is accreted to exceed M_Chandra, about 1.39 solar masses. Others can use the more common 1.44 M_solar since it isn't important to detail here.

But, the composition of any white dwarf will depend on the mass (and some other factors) of the progenitor star. (The bolded highlights in the following were inserted by Labguy to emphasize certain phrases)

http://72.14.207.104/search?q=cache...e+dwarf"&hl=en&gl=us&ct=clnk&cd=49&lr=lang_en

White dwarf: composition depends on mass of progenitor.
helium
carbon
carbon-oxygen
oxygen-neon-magnesium

And:http://www.geocities.com/tonylance/dwarf.html

The interior of a typical white dwarf is mostly composed of carbon and oxygen nuclei, though white dwarfs formed by smaller stars may be mostly helium and those formed by bigger stars may be formed of oxygen, neon, and magnesium.

So, for the many white dwarfs that formed from the far more numerous small stars there must be a large number of He white dwarfs. And the larger stars also do not form not carbon-oxygen rich dwarfs. Therefore:

A typical white dwarf has half the mass of the Sun yet is only slightly bigger than Earth; this makes white dwarfs one of the densest forms of matter (109 kg·m−3), surpassed only by neutron stars and hypothetical quark stars. The higher the mass of the white dwarf, the smaller the size. There is an upper limit to the mass of a white dwarf, the Chandrasekhar limit (about 1.4 times the mass of the Sun). When this limit is exceeded, the pressure exerted by electrons is no longer able to balance the force of gravity, and the star continues to contract, eventually forming a neutron star. A white dwarf which exceeds this limit (known as the Chandrasekhar limit), typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as "carbon detonation".

and: http://chandra.harvard.edu/chronicle/0400/sirius.html

If the mass of the white dwarf becomes greater than about 1.4 times the mass of the Sun -- called the Chandrasekhar limit -- it will collapse to become a neutron star or black hole, or blow itself apart in a supernova.

And: http://online.itp.ucsb.edu/online/gravity_c03/lindblom/pdf/Lindblom.pdf

The Accretion Induced Collapse of a White Dwarf star can lead to the formation of neutron stars with large angular momenta. These rapidly rotating neutron stars may be subject to non-axisymmetric instabilities which could provide an interesting source of gravitational radiation.

Therefore, from the handy links and quotes provided I would conclude that:

(1) Since smaller stars are far more numerous in the universe than large stars, more white dwarf stars are formed with an He composition.

(2) The largest white dwarf progenitors also do not lead to a carbon-oxygen composition.

(3) Regardless of mass accreted, most white dwarfs will not result in a Type Ia supernova even if M_Chandra is exceeded.

(4) For all white dwarf stars existing, the required C-O composition needed for a Type Ia supernova could be considered "rare" regardless of accreted mass.

I am very interested in this and related stellar evolution subjects, so if any of this seems unclear or invalid please provide me with a link or two for consideration.

 

[SpaceTiger]

Other posts and discussion have left at least one thing clear. That is that only Carbon-Oxygen white dwarf stars will become Type Ia supernovae if enough mass is accreted to exceed M_Chandra, about 1.39 solar masses.

Given their rarity, it's possible we've never observed an explosion from a helium white dwarf passing the chandrasekhar limit. However, if it did occur, it would almost certainly have a different spectral signature, so you're right that it wouldn't be classified as a Type Ia.

http://72.14.207.104/search?q=cache...e+dwarf"&hl=en&gl=us&ct=clnk&cd=49&lr=lang_en And:http://www.geocities.com/tonylance/dwarf.html So, for the many white dwarfs that formed from the far more numerous small stars there must be a large number of He white dwarfs.

Think a little harder about this one. Helium white dwarfs are thought to form from progenitors with M <~ 0.4 M_sun. How long do those stars live on the main sequence? How old is the universe?

Helium white dwarfs can, however, form from binary systems.

Wikipedia

You should know better than to quote Wikipedia for these discussions.

The Accretion Induced Collapse of a White Dwarf star can lead to the formation of neutron stars with large angular momenta. These rapidly rotating neutron stars may be subject to non-axisymmetric instabilities which could provide an interesting source of gravitational radiation.

The AIC scenario might happen, but population synthesis models suggest that it occurs a factor of >~ 20 times less frequently than Type Ia supernovae. (http://lanl.arxiv.org/abs/astro-ph/0601603")From what I've read, we can check your points:

(1) Since smaller stars are far more numerous in the universe than large stars, more white dwarf stars are formed with an He composition.

No.

(2) The largest white dwarf progenitors also do not lead to a carbon-oxygen composition.

Yes.

(3) Regardless of mass accreted, most white dwarfs will not result in a Type Ia supernova even if M_Chandra is exceeded.

No.

(4) For all white dwarf stars existing, the required C-O composition needed for a Type Ia supernova could be considered "rare" regardless of accreted mass.

No.

 

[Labguy]

Obviously there won't be any agreement on this subject. My opinion is:
(1) Yes
(2) Yes
(3) Yes
(4) Yes

Did you know also that every large star with a "short" lifetime always becomes a Type II supernova.:zzz:

That's a broad statement, I'm sure that there couldn't possibly be any exceptions. But like you said elsewhere, you post yours and I'll post mine, but not here again. If I see obvious errors I'll just post my objections/corrections anyway. I have no doubt whatsoever that you will do the same. Odd how so many threads end with the same name, even when the topic has already been discussed, agreed upon and settled..

 

[SpaceTiger]

Obviously there won't be any agreement on this subject.

This isn't a matter of opinion; in fact, I'm relatively certain that your claims are wrong since your references are vague (they don't even directly support your statements) and are mostly from pop-sci websites. In the future, if you think something I say is wrong or want to expand on it, just address it directly in the thread, don't get all hostile and rant about how I'm always ignoring important details. I'm not always right, but I can assure you that I know what I'm doing.

Odd how so many threads end with the same name, even when the topic has already been discussed, agreed upon and settled..

I don't know what you're getting at here, but the reason topics are repeated has to do mostly with the fact that new posters don't search the entire post database before asking a question. I can understand this, so if something has been addressed before, I have no problem with linking to the old discussion.

 

[Astronuc]

Dr. Andy Howell has discovered a distant supernova, or exploded star, so large that it will force scientists to question their understanding of how certain older stars disintegrate. Howell and co-authors Peter Nugent and Richard Ellis will be publishing results and analysis of the discovery in Nature this week.

http://today.reuters.com/news/articlenews.aspx?type=scienceNews&storyID=2006-09-20T205836Z_01_N20216834_RTRUKOC_0_US-SPACE-SUPERNOVA.xml&WTmodLoc=NewsArt-L3-Science+NewsNews-3

Scientists have believed that dying stars known as "white dwarfs" can't expand to more than 1.4 times the size of our sun without exploding in a massive thermonuclear blast.

That rule, known as the "Chandrasekhar Limit," has served as the foundation of decades of astrophysical research and helped scientists estimate the size of the universe.

But a team of astronomers said on Wednesday that they have found a supernova in a galaxy 4 billion light years away that reached a mass twice that of the sun before exploding.

"It should not be possible to break this limit but nature has found a way," said Andy Howell, the University of Toronto researcher who discovered the supernova.

http://www.news.utoronto.ca/bin6/060920-2575.asp
'Champagne supernova' challenges understanding of how supernovae work
A type Ia supernova breaks Chandrasekhar limit, astronomy's 'standard candles' suddenly variable

An international team of astronomers led by a group at the University of Toronto has discovered a supernova more massive than previously believed possible. This has experts rethinking our basic understanding of how stars explode as supernovae, according to a paper to be published in Nature on September 21.

University of Toronto postdoctoral researcher Andy Howell, lead author of the study, identified a Type Ia supernova named SNLS-03D3bb in a distant galaxy 4 billion light years away that originated from a dense evolved star, termed a 'white dwarf,' whose mass is far larger than any previous example. Type Ia supernovae are thermonuclear explosions that destroy carbon-oxygen white dwarf stars that have accreted matter from a companion star.

Researchers say SNLS-03D3bb’s “obesity” has opened up a Pandora’s box on the current understanding of Type Ia supernovae and how well they can be used for precision cosmology.

Current understanding is that Type Ia supernova explosions occur when the mass of a white dwarf approaches 1.4 solar masses, or the Chandrasekhar limit. This important limit was calculated by Nobel laureate Subrahmanyan Chandrasekhar in 1930, and is founded on well-established physical laws. As such, decades of astrophysical research have been based upon the theory. Yet, somehow the star that went supernova as SNLS-03D3bb reached about two solar masses before exploding.

"It should not be possible to break this limit," says Howell, "but nature has found a way. So now we have to figure out how nature did it."

In a separate News & Views article on the research in the same issue of Nature, University of Oklahoma professor David Branch has dubbed this the “Champagne Supernova,” since extreme explosions that offer new insight into the inner workings of supernovae are an obvious cause for celebration.

The team speculates that there are at least two possible explanations for how this white dwarf got so fat before it exploded. One is that the original star was rotating so fast that centrifugal force kept gravity from crushing it at the usual limit. Another is that the blast was in fact the result of two white dwarfs merging, such that the body was only briefly more massive than the Chandrasekhar limit before exploding. Observations of the supernova were obtained at the Canada-France-Hawaii telescope and the Keck telescope, both located on Mauna Kea in Hawaii.

Since Type Ia supernovae usually have about the same brightness, they can be used to map distances in the universe. In 1998 they were used in the surprising discovery that the universe is accelerating. While the authors are confident that the discovery of a supernova that doesn't follow the rules does not undermine this result, it will make them more cautious about using them in the future.

University of Toronto postdoctoral fellow Mark Sullivan, a coauthor on the research, says, “This supernovae muddies the waters. We now know these rogue supernovae are out there which might throw off our cosmology results if we aren't careful about identifying them.”

Snls-03d3bb: An Overluminous, Low Velocity Type Ia Supernova Discovered At Z=0.244

Elsewhere -

Towards a Cosmological Hubble Diagram for Type II-P Supernovae

Authors: Peter Nugent (1), Mark Sullivan (2), Richard Ellis (3), Avishay Gal-Yam (3 and 4), Douglas C. Leonard (3 and 5), D. Andrew Howell (2), Pierre Astier (6), Raymond G. Carlberg (2), Alex Conley (2), Sebastien Fabbro (7), Dominique Fouchez (8), James D. Neill (9), Reynald Pain (6), Kathy Perrett (2), Chris J. Pritchet (9), Nicolas Regnault (6) ((1) Lawrence Berkeley National Laboratory, (2) University of Toronto, (3) California Institute of Technology, (4) Hubble Postdoctoral Fellow, (5) NSF Astronomy and Astrophysics Postdoctoral Fellow, (6) LPNHE, CNRS-IN2P3 and University of Paris VI & VII, (7) CENTRA, (8) CPPM, CNRS-IN2P3 and University Aix Marseille II, (9) University of Victoria)
Comments: 36 pages, 16 figures, accepted for publication in ApJ

http://arxiv.org/PS_cache/astro-ph/pdf/0603/0603535.pdf