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AriAstronomer
2010-Nov-16, 07:39 PM
So I know that Type Ia happen to have a stringent peak luminosity, and are very valuable tools for distance measurements, but my question is how do we know? How did we first come to the conclusion that type Ia are constant in brightness, and why do we have so much confidence in this? Are there alternative explanations for the phenomena, or is a constant brightness the only conclusion?
Thanks.

Ari

caveman1917
2010-Nov-16, 07:50 PM
The reason for the constant luminosity lies in the specific way type Ia's are produced. You have a white dwarf, which can have a maximum mass of about 1.4 solar mass, this is the maximum that the electron degeneracy pressure (the thing that keeps the white dwarf from collapsing) can withstand. When it accretes material from a companion, and at some point it reaches this mass limit, it explodes in a type Ia supernova. So these are always produced at the same mass, and thus same luminosity.

Cougar
2010-Nov-17, 04:58 AM
So these are always produced at the same mass, and thus same luminosity.

Ha ha, I expect you'll get some comment there. In fact, I'll give it some. You correctly explain fundamentally the idea behind Ia supernovas, but apparently in practice, care and analysis must be paid to the rise and fall of the supernova's light curve, which allows one to further 'normalize' its luminosity for a more accurate reading.

Not to mention spectra, which distinguishes the Ia's from other types...

Ken G
2010-Nov-17, 03:06 PM
Yes, I suspect the additional variable, other than the all-important Chandrasekhar mass of 1.4 solar masses, is the composition of the white dwarf. Type Ia supernovae require a lot of carbon, because carbon is the most volatile for going thermonuclear, so it tends to happen to carbon-oxygen white dwarfs. But the amount of oxygen can vary, as can the amount of other stuff, and that can effect the yield of the blast. As Cougar says, in practice I think what really happens is we use the shape of the light curves (the brightness of the SN as a function of time) to correct for compositional differences, and then use that to normalize the yield to give a "standard candle." Without that step, it can make the cosmologists using Ia supernovae sound a bit naive, even though it does put the proper focus on the mass.

AriAstronomer
2010-Nov-18, 03:34 PM
I see. So there are a few different flavours for the type Ia supernova, but by fitting the light curve, and measuring the rate of diminish, on can find out which flavour (i.e. what the relative composition of oxygen and carbon were) it was. I realize that an accumulating white drawf is currently the best explanation for type Ia supernova, but is it conclusive at this point? My textbook, 'An introduction to Modern Astrophysics' by Caroll and Ostlie (which I'm told by my professor is quite good) says that it's still up for debate....

trinitree88
2010-Nov-18, 06:20 PM
Even Bob Kirshner of the Harvard Smithsonian Center for Astrophysics has capitulated on the claim of standard candles which he once championed. By his own words..."they vary by at least a factor of three"....and standard candles..."they are not".

cjl
2010-Nov-18, 07:27 PM
They do vary, but they are indeed standard candles. By fitting the light curve, you can get quite a good estimate of their intrinsic luminosity. They are better than just about anything else we have at that kind of distance.

Cougar
2010-Nov-18, 08:53 PM
I see. So there are a few different flavours for the type Ia supernova, but by fitting the light curve, and measuring the rate of diminish, on can find out which flavour (i.e. what the relative composition of oxygen and carbon were) it was.

Well, the oxygen-carbon is determined from spectral analysis, AFAIK. The variation in the light curve is, I believe, more analogous to a cepheid variable type variation. Among other things, there are questions whether these supernovas are fully symmetric or whether the observation angle yields slightly different results.

Something that varied by "a factor of three," however, would be an extreme outlier and typically thrown out of any sample population. Seems like it would not just be a different "flavor" but an entirely different species....


My textbook, 'An introduction to Modern Astrophysics' by Caroll and Ostlie (which I'm told by my professor is quite good) says that it's still up for debate....

What specifically do they say is up for debate? No one believes they are perfect standard candles. With analysis, they are very good ones. They don't debate whether Sne Ia's result from a white dwarf accreting mass from a binary neighbor, do they?

forrest noble
2010-Nov-18, 08:56 PM
So I know that Type Ia happen to have a stringent peak luminosity, and are very valuable tools for distance measurements, but my question is how do we know? How did we first come to the conclusion that type Ia are constant in brightness, and why do we have so much confidence in this? Are there alternative explanations for the phenomena, or is a constant brightness the only conclusion?
Thanks.
Ari

Although it has been asserted that there are varieties of them, or even if we really don't understand the exact circumstance or mechanism concerning their creation, the average consistency of their peak luminosities and similar form of their light curves over many similar redshift observations/ ranges is what gives us the confidence that they are at least close to being standard candles concerning their averages. Also as Cougar mentioned, a small percentage (roughly 4%) are thrown out of the calculations because of their inconsistencies concerning peak luminosities, and another percentage is thrown out initially based upon light curve inconsistencies.

caveman1917
2010-Nov-18, 09:04 PM
Ha ha, I expect you'll get some comment there. In fact, I'll give it some.

Thanks. I'd say one of the nicest features of this forum is that when you make a mistake or leave something out, someone will come along correcting you and give you a nice opportunity to learn something. Pedantry is highly underestimated :)

AriAstronomer
2010-Nov-18, 11:32 PM
What specifically do they say is up for debate? No one believes they are perfect standard candles. With analysis, they are very good ones. They don't debate whether Sne Ia's result from a white dwarf accreting mass from a binary neighbor, do they?

Firstly it mentions that although they are pretty certain the trigger involves exceeding the chandrasekhar limit, it is still unclear the exact mechanisms that trigger the explosion itself.
Secondly there are a few models (double and single degenerate models). Double degenerate deals with a binary white dwarf system where the less massive one exceeds its Roche Lobe and be torn apart in a few orbits. A single degenerate involves a white drawf and an evolving star. Mass falls into the white dwarf, and specifically helium. Once enough falls on, a helium flash will actually occur, and helium --> carbon/oxygen, and also sends a shockwave into the degenerate C-O white drawf igniting it.

It touches on a few other minor theories, but I think the conclusion from this book is that although the supernova's themselves may be fairly understood, the mechanism leading to it is still up in the air.

AstroRockHunter
2010-Nov-18, 11:44 PM
Firstly it mentions that although they are pretty certain the trigger involves exceeding the chandrasekhar limit, it is still unclear the exact mechanisms that trigger the explosion itself.
Secondly there are a few models (double and single degenerate models). Double degenerate deals with a binary white dwarf system where the less massive one exceeds its Roche Lobe and be torn apart in a few orbits. A single degenerate involves a white drawf and an evolving star. Mass falls into the white dwarf, and specifically helium. Once enough falls on, a helium flash will actually occur, and helium --> carbon/oxygen, and also sends a shockwave into the degenerate C-O white drawf igniting it.

It touches on a few other minor theories, but I think the conclusion from this book is that although the supernova's themselves may be fairly understood, the mechanism leading to it is still up in the air.

OK. If the companion is "an evolving star", then why would enough helium fall onto the white dwarf to cause a helium flash?

AriAstronomer
2010-Nov-20, 02:12 AM
Not sure. I quoted from the book directly though, meaning that's specifically the choice of words they use. Maybe they are trying to distinguish a star + dwarf as opposed to two dwarfs which are no longer evolving and are at the end of their lives.

parejkoj
2010-Nov-20, 03:34 AM
For more on the latest understanding of how SN Ia are "standardizeable candles," see the Union2 Supernova Compilation (http://www.supernova.lbl.gov/Union/) from the Supernova Cosmology Project, in particular their latest paper, Amanullah et al. 2010 (PDF) (http://www.supernova.lbl.gov/Union/figures/hst-2001_arXiv.pdf) and the references therein. The main website also includes links to tarballs of the lightcurves and associated data, if you want to do your own analysis.

In some sense, it doesn't matter whether SN Ia are produced by white dwarf mergers, helium flash white dwarfs, or angry unicorns with flashlights, because the data (in particular, the spectroscopy) show that we can calibrate them to a consistent, fairly tight, standard. Dig around for some of my BAUT posts from a year or two ago for links to nice papers analyzing different SN Ia spectral features.