Some colliding white dwarfs can reignite fusion rather than go supernova

On May 17, 11:31*am, Sam Wormley wrote:
On 5/16/11 6:21 PM, Eric Gisse wrote:









On May 15, 10:36 pm, Yousuf *wrote:
Colliding white dwarfs give each other a new lease on life
"Astronomers recently discovered the rather memorably named SDSS
J010657.39 100003.3, which is a binary star system composed of two white
dwarfs, one of which is 17% the Sun's mass, while the other is about
43%. The two orbit each other at a distance of just 140,000 miles, which
is even closer than the distance between the Earth and the Moon. The two
rotate each other at about a million miles per hour.

A factor of 4 or so closer in separation compared to the Hulse-Taylor
pulsar system, but with significantly less mass. And no pulsar for
timing.

This system will decay via gravitational radiation too. Wonder how
long it'd take.

When white dwarfs collide, one of two things can happen: if the combined
masses is greater than 140% of the Sun, the collision creates a
supernova. But in this case, the white dwarfs will actually reignite
nuclear fusion, creating a brand new star just like our Sun that will,
after another few billion years of renewed life, cool down into yet
another white dwarf."http://ca.io9.com/5801997/colliding-white-dwarfs-give-each-other-a-ne...

I wonder what type of supernova that would be. Not a 1a - that's a
helium flash from accretion, and not a II which is a core
collapse...probably be rather noisy, perhaps a candidate the fast
variety of GRB's?

* *If exceeding the Chandrasekhar limit of 1.44 solar masses creates
* *a Type IA supernova, I'm betting that two colliding white dwarfs
* *will do similar.

In the op's case simple arithmetic says that the resulting WD will be
about 60 percent of the Sun's mass. No where near the Chandrasekhar
limit.

On 5/16/11 7:08 PM, palsing wrote:
On May 16, 4:35 pm, Sam wrote:

If exceeding the Chandrasekhar limit of 1.44 solar masses creates
a Type IA supernova, I'm betting that two colliding white dwarfs
will do similar.

Sam, I would agree with this, but aren't we talking about a merger in
this case that is considerably less than the 1.44 mass limit, a total
which is, in fact, is less than 50% of that limit?

By the way, correct or not, the Wiki article...

http://en.wikipedia.org/wiki/Type_Ia_supernova

... makes it quite clear that both carbon and oxygen fusion (and not
fission) have a prominent role in type IA supernovae, although it also
makes it quite clear that not all the details are, well, quite
clear :-)

\Paul A

There is a good probability that the two white dwarf stars combined
will exceed the 1.44 Chandrasekhar limit. And furthermore, we are
dealing with degenerate matter and not the plasma of typical stars,
so the behaviors are quite different.

Sorry, I mistakenly used the word "fission".

From: http://en.wikipedia.org/wiki/Type_Ia_supernova

"Physically, white dwarfs with a low rate of rotation are limited to
masses that are below the Chandrasekhar limit of about 1.38 solar
masses. This is the maximum mass that can be supported by electron
degeneracy pressure. Beyond this limit the white dwarf would begin to
collapse. If a white dwarf gradually accretes mass from a binary
companion, the general hypothesis is that its core will reach the
ignition temperature for carbon fusion as it approaches the limit. If
the white dwarf merges with another star (a very rare event), it will
momentarily exceed the limit and begin to collapse, again raising its
temperature past the nuclear fusion ignition point. Within a few seconds
of initiation of nuclear fusion, a substantial fraction of the matter in
the white dwarf undergoes a runaway reaction, releasing enough energy
(1–2×10^44J) to unbind the star in a supernova explosion".

On May 16, 7:18*pm, Sam Wormley wrote:

* *There is a good probability that the two white dwarf stars combined
* *will exceed the 1.44 Chandrasekhar limit. And furthermore, we are
* *dealing with degenerate matter and not the plasma of typical stars,
* *so the behaviors are quite different.

Good probability? How can this be possible? This is the part I'm
having trouble understanding, Sam. Just how could 17% and 43% solar
mass objects possibly exceed 1.38 or 1.44 solar masses after merging?
It seems to me that degenerate matter can support a .6 solar mass,
unless you are asking me to believe that the mass of these 2 objects
will essentially double when merged? The hand is quicker than the eye?

I can't find any reference anywhere describing what might happen when
2 white dwarfs totaling less than 1.38 solar masses merge, nothing at
all, so to me this result is still up in the air. I'll keep looking...

\Paul A

On 5/16/11 9:57 PM, palsing wrote:
On May 16, 7:18 pm, Sam wrote:

There is a good probability that the two white dwarf stars combined
will exceed the 1.44 Chandrasekhar limit. And furthermore, we are
dealing with degenerate matter and not the plasma of typical stars,
so the behaviors are quite different.

Good probability? How can this be possible? This is the part I'm
having trouble understanding, Sam. Just how could 17% and 43% solar
mass objects possibly exceed 1.38 or 1.44 solar masses after merging?
It seems to me that degenerate matter can support a .6 solar mass,
unless you are asking me to believe that the mass of these 2 objects
will essentially double when merged? The hand is quicker than the eye?

I can't find any reference anywhere describing what might happen when
2 white dwarfs totaling less than 1.38 solar masses merge, nothing at
all, so to me this result is still up in the air. I'll keep looking...

\Paul A


To the best of my knowledge, there are no observed white dwarfs of
less than 0.593 solar masses and none more massive than 1.33 solar
masses, yet it is believed that white dwarfs accumulating hydrogen
reach 1.44 solar masses (the Chandrasekhar limit) and collapse.

Relevant Paper: White dwarf mass distribution in the SDSS
http://articles.adsabs.harvard.edu/f...NRAS.375.1315K

On 5/16/11 9:05 AM, Sam Wormley wrote:
On 5/16/11 12:36 AM, Yousuf Khan wrote:
Colliding white dwarfs give each other a new lease on life
"Astronomers recently discovered the rather memorably named SDSS
J010657.39–100003.3, which is a binary star system composed of two white
dwarfs, one of which is 17% the Sun's mass, while the other is about
43%. The two orbit each other at a distance of just 140,000 miles, which
is even closer than the distance between the Earth and the Moon. The two
rotate each other at about a million miles per hour.

When white dwarfs collide, one of two things can happen: if the combined
masses is greater than 140% of the Sun, the collision creates a
supernova. But in this case, the white dwarfs will actually reignite
nuclear fusion, creating a brand new star just like our Sun that will,
after another few billion years of renewed life, cool down into yet
another white dwarf."
http://ca.io9.com/5801997/colliding-...-lease-on-life



Since white dwarf contain little or no hydrogen the later ain't gonna
happen. Two white dwarfs would likely create a Type I supernova or
form a neutron star.



Related Paper: On Type Ia supernovae from the collisions of two
white dwarfs
http://arxiv.org/abs/0907.3915

On May 16, 8:16*pm, Sam Wormley wrote:

* * To the best of my knowledge, there are no observed white dwarfs of
* * less than 0.593 solar masses and none more massive than 1.33 solar
* * masses, yet it is believed that white dwarfs accumulating hydrogen
* * reach 1.44 solar masses (the Chandrasekhar limit) and collapse.

* * Relevant Paper: White dwarf mass distribution in the SDSS
* * *http://articles.adsabs.harvard.edu/f...NRAS.375.1315K

Huh? What does this have to do with the subject at hand? This whole
thread was started by a referenced article which stated that that 17%
and 43% stellar mass white dwarfs were in the process of merging, and
would then happily continue on as "a brand new star just like our
Sun...", so I don't understand your statement that "there are no
observed white dwarfs of less than 0.593 solar masses...", there are 2
of them in this article, and dozens and dozens of articles Google
found that claim the lightest of all WD's are around 15% solar mass.

I can only assume that you disagree with the conclusions of this
article and instead believe that the result of this merger will be a
supernova... is that about right?

I still don't understand why you say that 17% and 43% solar masses can
merge and end up above 138% solar masses, because I can't follow that
logic at all, and I would look forward to more of your thoughts on
this matter.

\Paul A

On 5/16/11 11:39 PM, palsing wrote:
On May 16, 8:16 pm, Sam wrote:

To the best of my knowledge, there are no observed white dwarfs of
less than 0.593 solar masses and none more massive than 1.33 solar
masses, yet it is believed that white dwarfs accumulating hydrogen
reach 1.44 solar masses (the Chandrasekhar limit) and collapse.

Relevant Paper: White dwarf mass distribution in the SDSS
http://articles.adsabs.harvard.edu/f...NRAS.375.1315K

Huh? What does this have to do with the subject at hand? This whole
thread was started by a referenced article which stated that that 17%
and 43% stellar mass white dwarfs were in the process of merging, and
would then happily continue on as "a brand new star just like our
Sun...", so I don't understand your statement that "there are no
observed white dwarfs of less than 0.593 solar masses...", there are 2
of them in this article, and dozens and dozens of articles Google
found that claim the lightest of all WD's are around 15% solar mass.

I can only assume that you disagree with the conclusions of this
article and instead believe that the result of this merger will be a
supernova... is that about right?

I still don't understand why you say that 17% and 43% solar masses can
merge and end up above 138% solar masses, because I can't follow that
logic at all, and I would look forward to more of your thoughts on
this matter.

\Paul A

If this paper is accurate
http://adsabs.harvard.edu/abs/2011MNRAS.tmpL.233K

then that certainly pushes the range for low mass white dwarfs that
is hard for me to swallow. It's good when I am wrong, for I learn
something in the process.

-Sam

On 5/16/11 11:53 PM, Sam Wormley wrote:
On 5/16/11 11:39 PM, palsing wrote:
On May 16, 8:16 pm, Sam wrote:

To the best of my knowledge, there are no observed white dwarfs of
less than 0.593 solar masses and none more massive than 1.33 solar
masses, yet it is believed that white dwarfs accumulating hydrogen
reach 1.44 solar masses (the Chandrasekhar limit) and collapse.

Relevant Paper: White dwarf mass distribution in the SDSS
http://articles.adsabs.harvard.edu/f...NRAS.375.1315K

Huh? What does this have to do with the subject at hand? This whole
thread was started by a referenced article which stated that that 17%
and 43% stellar mass white dwarfs were in the process of merging, and
would then happily continue on as "a brand new star just like our
Sun...", so I don't understand your statement that "there are no
observed white dwarfs of less than 0.593 solar masses...", there are 2
of them in this article, and dozens and dozens of articles Google
found that claim the lightest of all WD's are around 15% solar mass.

I can only assume that you disagree with the conclusions of this
article and instead believe that the result of this merger will be a
supernova... is that about right?

I still don't understand why you say that 17% and 43% solar masses can
merge and end up above 138% solar masses, because I can't follow that
logic at all, and I would look forward to more of your thoughts on
this matter.

\Paul A

If this paper is accurate
http://adsabs.harvard.edu/abs/2011MNRAS.tmpL.233K

then that certainly pushes the range for low mass white dwarfs that
is hard for me to swallow. It's good when I am wrong, for I learn
something in the process.

-Sam


Looks like I was wrong.
http://arxiv.org/abs/astro-ph/0611498

"Extremely low mass white dwarfs are very rare objects likely formed in
compact binary systems. We present MMT optical spectroscopy of 42 low
mass white dwarf candidates serendipitously discovered in a survey for
hypervelocity B-type stars. One of these objects, SDSS J0917+46, has
Teff= 11,288 \pm 72 K and log g = 5.48 \pm 0.03; with an estimated mass
of 0.17 M_sun, it is the lowest gravity/mass white dwarf currently
known. However, 40 of the low mass candidates are normal DA white dwarfs
with apparently inaccurate SDSS g magnitudes. We revisit the
identification of low mass white dwarf candidates previously found in
the SDSS, and conclude that four objects have M 0.2 M_sun. None of
these white dwarfs show excess emission from a binary companion, and
radial velocity searches will be necessary to constrain the nature of
the unseen companions".

On May 16, 9:53*pm, Sam Wormley wrote:

* *If this paper is accurate
* * *http://adsabs.harvard.edu/abs/2011MNRAS.tmpL.233K

Completely off-topic for this thread, but in that paper it says that
some of the data was obtained using the 2.1 meter telescope at
McDonald in Texas. I have had the pleasure of using that same
telescope optically from dusk to dawn on 3 occasions, once in 2000 and
a full weekend in 2006. 82" of glass, 812X at the lowest power and sub-
arc-second seeing on one night makes for very memorable and
remarkable experiences.

If you like such observing reports, mine is here...

http://www.pnalsing.com/82-report

\Paul A

On 16/05/2011 10:05 AM, Sam Wormley wrote:
Since white dwarf contain little or no hydrogen the later ain't gonna
happen. Two white dwarfs would likely create a Type I supernova or
form a neutron star.

Depending on the size of the white dwarf remnants, they would contain a
lot of helium at the very least, and they can restart helium fusion
rather than hydrogen fusion.

Besides, when a white dwarf goes Type Ia supernova, what do you think
makes it go kablooey? It was a runaway fusion process.

Yousuf Khan