Two paths to a Type Ia supernova?
in [Physics]
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From: Sam Wormley on 27 Feb 2010 23:17 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.)
From: Steve Willner on 1 Mar 2010 18:02 SW> I don't think there's an obvious timescale problem, at SW> least at moderate redshift. In article <4b8944ec$1(a)news.bnb-lp.com>, Yousuf Khan <bbbl67(a)spammenot.yahoo.com> 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/community/skyblog/newsblog/84771852.html -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 swillner(a)cfa.harvard.edu Cambridge, MA 02138 USA
From: Yousuf Khan on 2 Mar 2010 01:43 Steve Willner wrote: > SW> I don't think there's an obvious timescale problem, at > SW> least at moderate redshift. > > In article <4b8944ec$1(a)news.bnb-lp.com>, > Yousuf Khan <bbbl67(a)spammenot.yahoo.com> 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. What I was referring to was that after the primary has already turned into a white dwarf, then it will be the secondary's turn to go red giant and then white dwarf. During that time that the secondary is in its red giant phase, there is an opportunity for the primary to accrete the gas off of the secondary and go Type Ia. That will happen to the primary while the secondary is still in its red giant phase, but before it enters its own white dwarf phase. A dual white dwarf system means that the two stars weren't close enough initially for the primary to go Type Ia, while the secondary was in its red giant phase. In the red giant phase, it is easiest to steal mass off of a star because, that star is in its least densest form possible, and the star's own gravity has less of a tenuous hold on its own upper atmosphere. So if the primary wasn't able to steal enough gas from the secondary when it was a red giant, then that means that they weren't really close enough. Once the secondary goes white dwarf, then the only drag (since atmosphere has been blown off both by now) that they have that will decay their orbits further is gravitational frame drag, which can take billions of years to be effective. > 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. I wasn't referring to Type Ia progenitors in particular, but to supernova progenitors in general. Of course, the progenitors of Type Ia's are all white dwarfs which will all get to be around the exact same mass just before the end, so there won't be any difference for any Type Ia's. Now if there is a secondary type of Type Ia explosion, i.e. the dual white dwarf progenitors, then the mass won't be the same between normal Type Ia's, since there are now two white dwarfs rather than one. So these dual white dwarf supernovas will be much brighter than a normal Type Ia, as there will be a greater mass involved. Two white dwarfs together could have a much higher mass than the Chandrasekhar Limit. A normal Type Ia is always going to be a mass at the Chandrasekhar Limit, no other choice. Yousuf Khan
From: Steve Willner on 4 Mar 2010 17:58 In article <4b8cb39d$1(a)news.bnb-lp.com>, Yousuf Khan <bbbl67(a)spammenot.yahoo.com> writes: >What I was referring to was that after the primary has already turned >into a white dwarf, then it will be the secondary's turn to go red giant >and then white dwarf. During that time that the secondary is in its red >giant phase, there is an opportunity for the primary to accrete the gas >off of the secondary and go Type Ia. That could happen, depending on the mass of the white dwarf, the mass lost by the (original) secondary, and the fraction of that mass collected by the white dwarf. > That will happen to the primary >while the secondary is still in its red giant phase, but before it >enters its own white dwarf phase. _If_ it happens at all, it will be during that phase, but "will happen" seems an overstatement. >Now if there is a secondary type of Type Ia explosion, i.e. the dual >white dwarf progenitors, then the mass won't be the same between normal >Type Ia's, since there are now two white dwarfs rather than one. So >these dual white dwarf supernovas will be much brighter than a normal >Type Ia It's that last statement I keep having trouble with. Why will the luminosity necessarily be higher if the mass is higher? It could be that way, of course, but why does it have to be so? As far as I know, we don't have any good models of Type Ia explosions, and there isn't any observational knowledge of progenitor masses. So why couldn't luminosity be independent of mass? -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 swillner(a)cfa.harvard.edu Cambridge, MA 02138 USA
From: BURT on 4 Mar 2010 18:39
On Mar 4, 2:58 pm, will...(a)cfa.harvard.edu (Steve Willner) wrote: > In article <4b8cb39...(a)news.bnb-lp.com>, > Yousuf Khan <bbb...(a)spammenot.yahoo.com> writes: > > >What I was referring to was that after the primary has already turned > >into a white dwarf, then it will be the secondary's turn to go red giant > >and then white dwarf. During that time that the secondary is in its red > >giant phase, there is an opportunity for the primary to accrete the gas > >off of the secondary and go Type Ia. > > That could happen, depending on the mass of the white dwarf, the mass > lost by the (original) secondary, and the fraction of that mass > collected by the white dwarf. > > > That will happen to the primary > >while the secondary is still in its red giant phase, but before it > >enters its own white dwarf phase. > > _If_ it happens at all, it will be during that phase, but "will > happen" seems an overstatement. > > >Now if there is a secondary type of Type Ia explosion, i.e. the dual > >white dwarf progenitors, then the mass won't be the same between normal > >Type Ia's, since there are now two white dwarfs rather than one. So > >these dual white dwarf supernovas will be much brighter than a normal > >Type Ia > > It's that last statement I keep having trouble with. Why will the > luminosity necessarily be higher if the mass is higher? It could be > that way, of course, but why does it have to be so? As far as I > know, we don't have any good models of Type Ia explosions, and there > isn't any observational knowledge of progenitor masses. So why > couldn't luminosity be independent of mass? > > -- > Help keep our newsgroup healthy; please don't feed the trolls. > Steve Willner Phone 617-495-7123 swill...(a)cfa.harvard.edu > Cambridge, MA 02138 USA Astro physics is too young of a science to claim it knows about stars lives. Give science millions of years of observation and a better theoretical development. Mitch Raemsch |