More Evidence of Diversity in Supernova Ia - Page 6

Jerry · #**151** (**permalink**) 17-September-2007, 05:08 AM

Coping with Type Ia Supernova “Evolution” When Probing the
Nature of the Dark Energy

http://arxiv.org/PS_cache/astro-ph/p.../0109070v1.pdf

Quote:

Originally Posted by Branch etal

Observations of high–redshift Type Ia supernovae (SNe Ia) have provided strong evidence that the dark energy is real, and making further accurate observations of high–redshift SNe Ia is the most promising way to probe the nature of the dark energy. We discuss one of the concerns about such a project — that of coping with SN Ia evolution...

A frequently expressed concern is that SN Ia evolution — a systematic variation in the properties of SNe Ia as a function of redshift — may cause an erroneous distance–redshift relation to be inferred from the data.

We emphasize that SN Ia evolution differs in an important respect from the kind of evolution that has foiled some past projects in observational cosmology, and we outline empirical strategies that will take it into account. The supporting role of physical models of SNe Ia also is discussed.

SN Ia evolution is unlike the kind of cosmic evolution that presents such a challenge for some other projects in observational cosmology. SN Ia evolution boils down to a modest amount of progenitor population drift. Because we expect a large overlap between the properties of the immediate progenitors of the SNe Ia at various redshifts, we expect the empirical strategies for making luminosity corrections to nearby SNe Ia to apply generally.

SN Ia “evolution” is quite different, and the term can be misleading. A fundamantal distinction between a SN Ia and a galaxy is that the SN Ia is an event that doesn’t know the time — it just knows the properties of its immediate progenitor star. Similar progenitors at different times should produce similar SNe Ia.

From calculations of spectra and light curves of explosion models (e.g., Nugent et al. 1997; H¨oflich et al. 1998; Lentz et al. 2001) we have quite a good idea of what a normal SN Ia ejects: about a Chandraskhar mass, including MNi ~ 0.6 solar masses, with a kinetic energy of 1051 erg so that the velocity at the boundary between the iron–peak core and the intermediate–mass elements is near 9000 km s−1. Models that have acceptable light curves and spectra include the deflagration model W7 of Nomoto et al. (1984), and the delayed–detonation models DD4 of Woosley (1991) and M36 of H¨oflich (1995). Models that differ substantially from these have spectra and light curves that are inconsistent with the observations of normal SNe Ia.

Explosion at the Chandrasekhar mass (in the single degenerate scenario) or at least not very far from it (double degenerate scenario) is a plausible reason for the impressive homogeneity of SNe Ia.

Thus SN Ia evolution should be controllable to high accuracy.

Our conclusion is that systematic errors due to SN Ia evolution will be small.

As discussed in several talks at this conference, Type Ia supernovae are, as
a class, highly homogeneous. They are explosion events that are apparently
triggered under very similar physical conditions. Their “light curves” scat-
ter by less then 25% RMS in brightness (Vaughan et al 1995a, 1995b),
and less than 15% RMS in full-width-at-half-maximum (Perlmutter 1996),
in a sample of “normal” Type Ia supernovae, after rejecting the abnormal
15% with red colors (see Vaughan et al 1995a). Their spectral signatures
also follow a well-defined evolution in time. A paper giving all the photo-
metric and spectroscopic measurements for our SNe

Kwalish Kid · #**152** (**permalink**) 17-September-2007, 03:37 PM

Jerry, did you reference that paper without reading it? At the bottom of the very first page of the document, the authors write:

Quote:

Originally Posted by Branch et al

The present discussion is partially motivated by what we see as a fairly common misconception that controlling SN Ia evolution will require a thorough understanding of the origin and physics of SNe Ia.

Later they write:

Quote:

The exact nature of the immediate progenitors of SNe Ia is not yet firmly established.

Or are you posting this as a mea culpa?

Jerry · #**153** (**permalink**) 17-September-2007, 04:42 PM

My computer died in the middle of an edit - what is posted is incomplete and accidental. (Grrrrrr)

Branch's argument is interesting - the bulk of the paper stresses that 'type Ia' properties are a function of the history of a single type of star, and are therefore not influenced by cosmic evolution.

I disagree strongly with Branch’s assertion you quoted from the paper. If there is no restriction upon the progenitors mass, how can anyone argue that the light-curve/magnitude relationship is highly constrained? The restrictions of the progenitor's mass rest wholly upon the assumption that a star with the proper ingredients to create a type Ia will denote at a critical mass, and grow no larger. Without this caveat, one has no way of assuring the scale of distant events is not influenced by Malmquist bias. Branch wants it all three ways: No cosmic evolution messing with his pet star, absolute confidence in the magnitude/curve relationship; and yet a final assertion that knowledge of the prototype is irrelevant. You can't make those kinds of claims: You either understand the physics or you don't.

A lot of work has been done that tries to discriminate events like 1991T from the family of supernova events that follow the canonical light-curve/magnitude relationship. When 1991T-like spectral discriminators show up in several distant supernova (as they did in this latest survey), is the light curve extra long, like 1991T, or is it as short as it appears to be after time-dilation correction? What rules are used to decide?

Each exceptionally bright, long lasting event observed locally with a ‘type Ia’ spectrum adds heat to this fire: If cosmic evolution is not a factor and the progenitor star type is not important; how do we know that the distant sample is not more like this growing population of ultra bright local exceptions?

Kwalish Kid · #**154** (**permalink**) 18-September-2007, 12:59 PM

Quote:

Originally Posted by Jerry

Branch's argument is interesting - the bulk of the paper stresses that 'type Ia' properties are a function of the history of a single type of star, and are therefore not influenced by cosmic evolution.

The bulk of the paper, and the basis for every SNIa cosmology paper out there, is that these are the same kind of events. The physics can be determined from the nature of the events, but the astronomy is, at this point, relatively independent.

Quote:

I disagree strongly with Branch’s assertion you quoted from the paper. If there is no restriction upon the progenitors mass, how can anyone argue that the light-curve/magnitude relationship is highly constrained?

Why don;t you read their papers and see? They use empirical arguments that have nothing to do with mass, but with the observed nature of the events.

Quote:

The restrictions of the progenitor's mass rest wholly upon the assumption that a star with the proper ingredients to create a type Ia will denote at a critical mass, and grow no larger.

This may be part of an argument about the physics underlying the events.

Quote:

Without this caveat, one has no way of assuring the scale of distant events is not influenced by Malmquist bias.

These papers often discuss Malmquist bias, so you might wish to look at their discussion of the limit of Mamlquist bias and how it might change the results.

Of course, there is always a host of possible errors for every observation. Yet we don't discard a scientific theory because of flights of fancy.

If Malmquist bias is important, then we have to find out the impact that it would be likely to have. This has been considered since the earliest SNIa cosmology papers.
[quote]Branch wants it all three ways: No cosmic evolution messing with his pet star, absolute confidence in the magnitude/curve relationship; and yet a final assertion that knowledge of the prototype is irrelevant. You can't make those kinds of claims: You either understand the physics or you don't. /quote]
If we are ever to know anything about the physics, it will be through the observations the Branch et al. rely upon. Thus it is important to know the strength of the observations in their own right. This is what Branch et al. address. They point out that the light-curve relationship is not one from physics but from observation. It is a regularity in the data that, from induction, they expect to continue in the data. Like anything derived from induction, it may be utterly false.

Nereid · #**155** (**permalink**) 18-September-2007, 03:15 PM

I think it's time to close this thread. It has been, for some time, largely a presentation of ATM ideas, in the Astronomy section.

Jerry, please stop using this Astronomy section as a place to promote ATM ideas. In this thread you have presented a considerable amount of material, in support of some ideas that are poorly articulated, perhaps in part because they are so clearly ATM. Other BAUT members have struggled to address the misunderstandings, misrepresentations, faulty logic, and so on in many of your posts. Note that most of these are not simply narrow, perhaps technical, aspects of some papers you reference.

You are, of course, welcome to start a new ATM thread, and present in a clear and consistent fashion the strongest case for a well-articulated ATM idea. You may use any material from this thread, or any other in BAUT, in support of such a well-articulated ATM idea.

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