Two paths to a Type Ia supernova?

Common knowledge about what creates a Type Ia supernova is that a white
dwarf siphons off matter from a companion star, usually a red giant. It
siphons off so much matter that its mass goes over the Chandrasekhar
limit (approx. 1.4 solar masses), and this creates a thermonuclear
explosion so big that all of the white dwarf's matter gets dispersed
leaving no further core behind. This scenario is known as the "accreting
white dwarf".

The second theorized method to Type Ia supernovas is when there are two
white dwarfs, and the two of them merge which takes them over the
Chandrasekhar limit and blows them both to smithereens too. One problem
with this is that if a Type Ia is produced this way, then depending on
the mass of the two original white dwarfs, they might be well over the
Chandrasekhar limit and the explosion would be bigger than a regular
Type Ia. Another problem with this theory is that another possible
theoretical path for them is that the two white dwarfs merge to form a
neutron star, instead of blowing up.

In this article, the researchers say that if most SNIa's are of the
accreting white dwarf type, then there should be a lot of X-rays
produced from the infalling matter prior to the explosion. If it's
merging white dwarfs, then there would be no X-rays produced priorly.
Their studies conclude that there is less X-rays coming out of various
galaxies than can be expected if there were lots of accreting white
dwarfs inside them. I personally think this is way too indirect of an
observation to be conclusive about anything. The article author's
conclusion is pretty much the same.

If there are enough binary white dwarfs that can merge, I don't think
the universe is old enough yet to have seen them yet. It takes a long
time to create a white dwarf, let alone two of them together. But if the
path to Type Ia's includes merging white dwarfs, then that would mean
that using Type Ia's as standard candles is unreliable. That would
affect distance measurements of Dark Energy in the universe.

Yousuf Khan

***
SkyandTelescope.com - News from Sky & Telescope - Supernova Mystery
Remains Just That
http://www.skyandtelescope.com/news/84771852.html

Yousuf Khan wrote:

Common knowledge about what creates a Type Ia supernova is that a white
dwarf siphons off matter from a companion star, usually a red giant. It
siphons off so much matter that its mass goes over the Chandrasekhar
limit (approx. 1.4 solar masses),

No. That would mean it does a collapse into a black hole with minimal energy
release.

and this creates a thermonuclear
explosion so big that all of the white dwarf's matter gets dispersed
leaving no further core behind. This scenario is known as the "accreting
white dwarf".

No. A Type 1a is the surface explosion of accumulated layers of Hydrogen on
the surface of a white dwarf.


The second theorized method to Type Ia supernovas is when there are two
white dwarfs, and the two of them merge which takes them over the
Chandrasekhar limit and blows them both to smithereens too.

No. The type of a supernova isn't something that is meaningless, this would
be a different (and not known to be seen) type of supernova. Plus *again*
things that reach the Chandrasaekhar limit implode - not explode.

[snip rest]

In article ,
Yousuf Khan writes:
YK If there are enough binary white dwarfs that can merge, I don't think
YK the universe is old enough yet to have seen them yet. It takes a long
YK time to create a white dwarf, let alone two of them together.

Stars up to something like 2-4 solar masses produce white dwarfs, and
they evolve in something under a billion years. Equal-mass binaries
are pretty common, and mass transfer shortens the lifetime of the
secondary. I don't think there's an obvious timescale problem, at
least at moderate redshift.

Of course to get a real answer, one would have to make a quantitative
comparison of supernova rates with actual lifetimes and stellar
numbers. I certainly haven't done that.

YK if the path to Type Ia's includes merging white dwarfs, then that
YK would mean that using Type Ia's as standard candles is unreliable.

Not necessarily. It might turn out, for example, that the visual
magnitudes don't much depend on the masses. Nevertheless, having
multiple mechanisms would certainly justify some suspicion.

There is strong evidence that _local_ type Ia SNe are good standard
candles, especially after correction for light curve "stretch." I
think the question is whether distant Ia's are the same kinds of
objects -- or at least have the same magnitudes -- as the local ones.
Stay tuned.

On Feb 22, 2:12 am, eric gisse wrote:

Usually Eric knows what he's writing about, but he seems to have had
a bad day here.

YK Common knowledge about what creates a Type Ia supernova is that a white
YK dwarf siphons off matter from a companion star, usually a red giant. It
YK siphons off so much matter that its mass goes over the Chandrasekhar
YK limit (approx. 1.4 solar masses),

EG No. That would mean it does a collapse into a black hole with
EG minimal energy release.

Yousuf's statement is correct. Core-collapse supernovae (Type II,
for example) may produce black hole remnants, but there is still
plenty of energy released.

EG A Type 1a is the surface explosion of accumulated layers of Hydrogen on
EG the surface of a white dwarf.

Eric here describes a nova, not a supernova.

YK The second theorized method to Type Ia supernovas is when there are two
YK white dwarfs, and the two of them merge which takes them over the
YK Chandrasekhar limit and blows them both to smithereens too.

EG No. The type of a supernova isn't something that is meaningless,
EG this would be a different (and not known to be seen) type of
EG supernova.

The type of a supernova is based on its spectrum and light curve.
It's conceivable that different underlying processes could produce
the same observed "type." In any case, Yousuf's description is a
possible origin for Type 1a SNe.

EG things that reach the Chandrasaekhar limit implode - not explode.

Type Ia SNe are thought to leave no remnant; the entire star is
destroyed (or both stars, if it was a merger).

[Eric: I tried emailing you before posting the above, but the email
bounced.]

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Help keep our newsgroup healthy; please don't feed the trolls.
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On Feb 21, 10:57*pm, Yousuf Khan wrote:
Common knowledge about what creates a Type Ia supernova is that a white
dwarf siphons off matter from a companion star, usually a red giant. It
siphons off so much matter that its mass goes over the Chandrasekhar
limit (approx. 1.4 solar masses), and this creates a thermonuclear
explosion so big that all of the white dwarf's matter gets dispersed
leaving no further core behind. This scenario is known as the "accreting
white dwarf".

The second theorized method to Type Ia supernovas is when there are two
white dwarfs, and the two of them merge which takes them over the
Chandrasekhar limit and blows them both to smithereens too. One problem
with this is that if a Type Ia is produced this way, then depending on
the mass of the two original white dwarfs, they might be well over the
Chandrasekhar limit and the explosion would be bigger than a regular
Type Ia. Another problem with this theory is that another possible
theoretical path for them is that the two white dwarfs merge to form a
neutron star, instead of blowing up.

In this article, the researchers say that if most SNIa's are of the
accreting white dwarf type, then there should be a lot of X-rays
produced from the infalling matter prior to the explosion. If it's
merging white dwarfs, then there would be no X-rays produced priorly.
Their studies conclude that there is less X-rays coming out of various
galaxies than can be expected if there were lots of accreting white
dwarfs inside them. I personally think this is way too indirect of an
observation to be conclusive about anything. The article author's
conclusion is pretty much the same.

If there are enough binary white dwarfs that can merge, I don't think
the universe is old enough yet to have seen them yet. It takes a long
time to create a white dwarf, let alone two of them together. But if the
path to Type Ia's includes merging white dwarfs, then that would mean
that using Type Ia's as standard candles is unreliable. That would
affect distance measurements of Dark Energy in the universe.

* * * * Yousuf Khan

***
SkyandTelescope.com - News from Sky & Telescope - Supernova Mystery
Remains Just Thathttp://www.skyandtelescope.com/news/84771852.html

Besides Sirius(B) going supernova and taking Sirius(A) out at the same
time, Can LHC create a local nova?
http://blogs.physicstoday.org/newspi...hy-we-exi.html

So far, those mostly public funded wizards at LHC are only missing or
having lost track of 98% proton mass, and if they keep going and going
with this colliding process should create either black holes, hot
quark soup or ice-9.

How paramagnetic or diamagnetic are these protons?
How paramagnetic or diamagnetic are those quarks?
With trillions upon trillions of spare/rogue quarks running lose, what
could possibly go wrong?

~ BG

Steve Willner wrote:
In article ,
Yousuf Khan writes:
YK If there are enough binary white dwarfs that can merge, I don't think
YK the universe is old enough yet to have seen them yet. It takes a long
YK time to create a white dwarf, let alone two of them together.

Stars up to something like 2-4 solar masses produce white dwarfs, and
they evolve in something under a billion years. Equal-mass binaries
are pretty common, and mass transfer shortens the lifetime of the
secondary. I don't think there's an obvious timescale problem, at
least at moderate redshift.

I understand what you're saying, but I was referring to the whole
process of not just creating the two white dwarfs, but also the amount
of time to get their orbits to decay enough to collide. This would be
especially less likely in the early universe. Assuming the "early
universe" refers to anything in the first 5 billion years of the universe.

The only way that I can think of for two white dwarfs to be close enough
to spiral into each other, is if they were extremely close already when
they were normal stars. And if they were already close, then whichever
star went white dwarf first, would be already close enough to produce a
Type Ia supernova through the normal gas accretion method beforehand.

YK if the path to Type Ia's includes merging white dwarfs, then that
YK would mean that using Type Ia's as standard candles is unreliable.

Not necessarily. It might turn out, for example, that the visual
magnitudes don't much depend on the masses. Nevertheless, having
multiple mechanisms would certainly justify some suspicion.

I can't see how that could be possible. There's always a strong
correlation between the mass of the progenitor stars and the magnitude
of their supernova. That's why Type Ia supernovas were chosen in the
first place as standard candles, because at the point that they blow,
each of them are within ounces of each other in mass, i.e. near the
Chandrasekhar mass limit.

There is strong evidence that _local_ type Ia SNe are good standard
candles, especially after correction for light curve "stretch." I
think the question is whether distant Ia's are the same kinds of
objects -- or at least have the same magnitudes -- as the local ones.
Stay tuned.

Actually, I think I remember posting something about a study which
showed that early type Ia's would be of a different magnitude than
current type Ia's, due to the lack of certain heavy elements in their
cores. This would of course mean, very early white dwarfs, probably
within the first billion years of the universe. Let me see if I can find
that article again. Ah, here it is:

Were early-generation Type Ia supenovae intrinsically brighter than
today's? - sci.physics | Google Groups - Mozilla Firefox
http://groups.google.com/group/sci.p...d25a5d7408 8c
or, http://tinyurl.com/yfbswly

Here's the original article:

Star burst | COSMOS magazine
http://www.cosmosmagazine.com/featur...rst?page=0%2C1

On Feb 22, 2:12 am, eric gisse wrote:

Usually Eric knows what he's writing about, but he seems to have had
a bad day here.

YK Common knowledge about what creates a Type Ia supernova is that a white
YK dwarf siphons off matter from a companion star, usually a red giant. It
YK siphons off so much matter that its mass goes over the Chandrasekhar
YK limit (approx. 1.4 solar masses),

EG No. That would mean it does a collapse into a black hole with
EG minimal energy release.

Yousuf's statement is correct. Core-collapse supernovae (Type II,
for example) may produce black hole remnants, but there is still
plenty of energy released.

Well, I puzzled over that statement too, but then I gave him the benefit
of the doubt and thought maybe he was being pedantic about my wording. I
thought he took issue with my statement about the white dwarf going
"over the Chandrasekhar Limit", whereas I should've said "approaches the
Chandrasekhar Limit". I answered him too, but it looks like Eric does
newsgroup pruning, and it only made it over to sci.physics group, not to
sci.astro too.

EG A Type 1a is the surface explosion of accumulated layers of Hydrogen on
EG the surface of a white dwarf.

Eric here describes a nova, not a supernova.

Yup.

YK The second theorized method to Type Ia supernovas is when there are two
YK white dwarfs, and the two of them merge which takes them over the
YK Chandrasekhar limit and blows them both to smithereens too.

EG No. The type of a supernova isn't something that is meaningless,
EG this would be a different (and not known to be seen) type of
EG supernova.

The type of a supernova is based on its spectrum and light curve.
It's conceivable that different underlying processes could produce
the same observed "type." In any case, Yousuf's description is a
possible origin for Type 1a SNe.

It's possible that a merger of white dwarfs would produce the same light
curve and spectrum as a gas accretion supernova, but just a larger
magnitude of explosion.

Also I had previously posted an article, which seems to imply that a
white dwarf on white dwarf explosion has already been observed. But in
this case, it didn't create a larger explosion, but a smaller one! Also
it didn't result in complete destruction of the white dwarfs, only
partial destruction of a good deal of the surfaces of both of them. They
called it a "Type .Ia" supernova, meaning it was somewhere between a
nova and Type Ia supernova.

New Type of Supernova Discovered
http://news.nationalgeographic.com/n...nova-type.html

Also, in this case the white dwarfs were of unequal mass. I assume that
to produce a merger-type Type Ia, the dwarfs have to be close in mass.

Yousuf Khan

Brad Guth wrote:
Besides Sirius(B) going supernova and taking Sirius(A) out at the same
time,

Congrats Brad, this is actually a plausible outcome of the Sirius
system, billions of years down the road. After Sirius A goes red giant,
it won't be close enough to Sirius B to create a Type Ia supernova
through gas accretion. However, it might produce occasional novas on
Sirius B, as some small amount of Sirius A's gas might fall on to Sirius
B. Eventually, Sirius A will settle down into its own white dwarf
retirement. Then tens of billions of years later, the two of them might
spiral in close enough to each other to collide and create a dual-white
dwarf Type Ia supernova.

Yousuf Khan

On Feb 27, 9:16*am, Yousuf Khan wrote:
Brad Guth wrote:
Besides Sirius(B) going supernova and taking Sirius(A) out at the same
time,

Congrats Brad, this is actually a plausible outcome of the Sirius
system, billions of years down the road. After Sirius A goes red giant,
it won't be close enough to Sirius B to create a Type Ia supernova
through gas accretion. However, it might produce occasional novas on
Sirius B, as some small amount of Sirius A's gas might fall on to Sirius
B. Eventually, Sirius A will settle down into its own white dwarf
retirement. Then tens of billions of years later, the two of them might
spiral in close enough to each other to collide and create a dual-white
dwarf Type Ia supernova.

* * * * Yousuf Khan

Sirius(B) is currently gaining mass, so perhaps we got thousands of
years at best.

~ BG

Brad Guth wrote:
On Feb 27, 9:16 am, Yousuf Khan wrote:
Brad Guth wrote:
Besides Sirius(B) going supernova and taking Sirius(A) out at the same
time,
Congrats Brad, this is actually a plausible outcome of the Sirius
system, billions of years down the road. After Sirius A goes red giant,
it won't be close enough to Sirius B to create a Type Ia supernova
through gas accretion. However, it might produce occasional novas on
Sirius B, as some small amount of Sirius A's gas might fall on to Sirius
B. Eventually, Sirius A will settle down into its own white dwarf
retirement. Then tens of billions of years later, the two of them might
spiral in close enough to each other to collide and create a dual-white
dwarf Type Ia supernova.

Yousuf Khan

Sirius(B) is currently gaining mass, so perhaps we got thousands of
years at best.

No, it's not.

Yousuf Khan

On 2/27/10 9:41 PM, Brad Guth wrote:
Sirius(B) is currently gaining mass

No it's not! "...a large orbital eccentricity carrying them
Sirius A & B] from 31.5 AU apart to 8.1 AU and back again".

http://stars.astro.illinois.edu/sow/sirius.html

SIRIUS (Alpha Canis Majoris). From Orion, look south and to the east to
find brilliant Sirius, as if one really needs directions to find the
brightest star in the sky. Its name comes from the Greek word for
"searing" or "scorching," certainly appropriate for a star that shines
at the bright end of the "minus-first" (-1.47) magnitude. Sirius is the
luminary of the constellation Canis Major, the Greater Dog, which
represents Orion's larger hunting dog, and as such is commonly referred
to as the "Dog Star." So great is its prominence that it has two
"announcer stars" that from the mid- northern hemisphere rise before it,
Procyon and Mirzam. Famed from times long past, the first glimpse of
Sirius in dawn announced the rising of the Nile in ancient Egypt. (It no
longer does because of precession, the 26,000-year wobble of the Earth's
axis.) Sirius is also part of a large asterism, the Winter Triangle, the
other two stars of which are Betelgeuse in Orion and Procyon in the
smaller dog, Canis Minor. Because of its brilliance, Sirius is the
champion of all twinklers, the effect caused by variable refraction in
the Earth's atmosphere. The star, a white class A (A1) hydrogen-fusing
dwarf with a temperature of 9880 Kelvin, is bright in part because it is
indeed rather luminous, 26 times more so than the Sun, but mostly
because it is nearby, a mere 8.6 light years away, just double that of
the closest star to the Earth (Alpha Centauri) and the fifth closest
star system. Sirius is "metal rich," its iron content perhaps double
that of the Sun, most likely from some sort of elemental diffusion. With
a radius of 1.75 solar (in agreement with the measured angular diameter)
and a minimum equatorial rotation speed of 16 kilometers per second,
Sirius rotates in under 5.5 days. The star's greatest claim to fame may
be its dim eighth magnitude (8.44) companion, Sirius B, which is
visually nearly 10,000 times fainter than the bright star, Sirius A.
Sirius B, however, is actually the hotter of the two, a blue-white
24,800 Kelvin. Though typically separated from each other by a few
seconds of arc, Sirius B is terribly difficult to see in the glare of
Sirius A. The only way the companion star can be both hot and dim is to
be small, only 0.92 the size of Earth, the total luminosity (including
its ultraviolet light) just 2.4 percent that of the Sun. The two orbit
each other with a 50.1 year period at an average distance of 19.8
Astronomical Units, about Uranus's distance from the Sun, a large
orbital eccentricity carrying them from 31.5 AU apart to 8.1 AU and back
again. They were closest in 1994 and will be again in 2044, while they
will be farthest apart in 2019. From the orbit (and spectroscopic data),
we find that Sirius A and B have respective masses of 2.12 and 1.03
times that of the Sun. Sirius B is the chief member of a trio of classic
white dwarfs, the others Procyon B and 40 Eridani B. Its high mass and
tiny radius lead to an amazing average density of 1.7 metric tons per
cubic centimeter, roughly a sugar cube. White dwarfs are the end
products of ordinary stars like the Sun, tiny remnants that were once
nuclear-fusing cores that have run out of fuel. Most are balls of carbon
and oxygen whose fates are merely to cool forever. To have evolved
first, Sirius B must once have been more massive and luminous than
Sirius A. That its mass is now lower is proof that stars lose
considerable mass as they die. Given the mass of the white dwarf and the
250 million year age of the system, Sirius B may once have been a hot
class B3-B5 star that could have contained as much as 5 to 7 solar
masses, the star perhaps losing over 80 percent of itself back into
interstellar space through earlier winds. (Thanks to Steve Ash for
prompting a rewrite.)

SW I don't think there's an obvious timescale problem, at
SW least at moderate redshift.

In article ,
Yousuf Khan writes:
The only way that I can think of for two white dwarfs to be close enough
to spiral into each other, is if they were extremely close already when
they were normal stars. And if they were already close, then whichever
star went white dwarf first, would be already close enough to produce a
Type Ia supernova through the normal gas accretion method beforehand.

I don't see how that last follows. When the initial primary enters
its red giant phase, it will lose mass. Some of this mass will be
accreted onto the secondary, which will still be in its main sequence
phase. It seems to me that the mass loss _cannot_ be enough to cause
the secondary to supernova. Even if the secondary captures _all_ the
mass lost by the primary, its mass cannot exceed the initial mass of
the primary, which by construction of this whole scenario is going to
end up as a white dwarf.

SW It might turn out, for example, that the visual
SW magnitudes don't much depend on the masses.

There's always a strong correlation between the mass of the
progenitor stars and the magnitude of their supernova.

Do you have a reference for that statement? I wasn't aware that
masses of Type 1a progenitors had ever been measured.

There's very good commentary on this subject, including quotes from
real experts (which I am not!), at:

http://www.skyandtelescope.com/commu.../84771852.html


--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA