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Type 1a supernovas are not all the same intrinsic brightness


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I have heard it said we can work out distances to galaxies where a type 1a supernova has gone off.

From my data (taken from online official log) there have been 28 galaxies where at least two type 1a supernova have erupted.

Of these 14 or half show at least 1 mag between the magnitudes of the supernova. ;)

Some are really big differences eg NGC3190 2002bo is 13.8 and 2002cv is 16.6. Or NGC 1316 1980N 12.5 2006mr 15.1.:D

So my view from my brief analysis is that type 1a supernova vary in their intrindic brightness significantly and should not be used to assess distance...

What do you think then..:)

Mark

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Just a suggestion. Could the light be dimmed by variable amounts of intervening dust within the host galaxy? NGC 3190 is fairly edge-on, so there is likely to be a fairly dense amount of material between us and the supernova. Obviously, light from a supernova closer to the edge of the galaxy would pass through less dust than one further away.

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Type 1a's are caused by white dwarfs stealing matter from a companion star, until they reach a critical density which causes the degenerate matter it is composed of to suddenly light off and explode.

The reason they all have similar brightness is that they are all roughly the same size, as they reach this critical mass. However there is a trick to using them as standard candles. The light curve of a type 1a goes up rapidly, to a hump like peak, then decays away slowly over the remaining days. Like this:

supernova3a.jpg

However it was noticed that the peak luminosity was not always the same, nor was the width of the hump. However there appears a fairly simple relationship between the height of the peak and the width of the hump.

If you normalise for this, by applying some factors to it, most of the light curves fit each other very well, and so after this the error in the distance can be reduced to about 10%.

There are now ideas that by applying environmental factors, such as position in the galaxy and other factors, it's thought the error can be brought down to just a few percent - like 2-3%.

Anyway, the idea that all type 1a's are exactly the same is rather simplistic.

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Hi Mark,

As Julian says, the often reported approach that "you measure the brightness of a type Ia and it gives you the distance" is a rather simplified one unfortunately. If you did that, you'd get an unusably large scatter, as you've discovered. To correct for this, you need to measure the shape (width) of the light curve, and the colour of the SNe (gives you a handle on the reddening), and fit these into models which include the properties of the SNe and the cosmological parameters at the same time. To do that reliably you need a lot (many hundreds) of SNe, and invest many many years of effort into understanding the different effects.

Certainly though, when the first SNe cosmology results came out, there was a very healthy section of the community engaged in coming up with reasons why the results would be wrong because of effects like these ;)

In your examples, 2002cv and 2006mr are both heavily reddened (according to a quick google search anyway) and hence affected by local dust. Objects like this probably get thrown out from fit, because there is something obviously funny going on with them.

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