SN 1994D, a type Ia supernova in the galaxy NGC 4526

Variability in Type 1A Supernovae Has Implications for Studying Dark Energy

12 Aug , 2009 by

[/caption]

The discovery of dark energy, a mysterious force that is accelerating the expansion of the universe, was based on observations of type 1a supernovae, and these stellar explosions have long been used as “standard candles” for measuring the expansion. But not all type 1A supernovae are created equal. A new study reveals sources of variability in these supernovae, and to accurately probe the nature of dark energy and determine if it is constant or variable over time, scientists will have to find a way to measure cosmic distances with much greater precision than they have in the past.

“As we begin the next generation of cosmology experiments, we will want to use type 1a supernovae as very sensitive measures of distance,” said lead author Daniel Kasen, of a study published in Nature this week. “We know they are not all the same brightness, and we have ways of correcting for that, but we need to know if there are systematic differences that would bias the distance measurements. So this study explored what causes those differences in brightness.”

Kasen and his coauthors–Fritz Röpke of the Max Planck Institute for Astrophysics in Garching, Germany, and Stan Woosley, professor of astronomy and astrophysics at UC Santa Cruz–used supercomputers to run dozens of simulations of type 1a supernovae. The results indicate that much of the diversity observed in these supernovae is due to the chaotic nature of the processes involved and the resulting asymmetry of the explosions.

For the most part, this variability would not produce systematic errors in measurement studies as long as researchers use large numbers of observations and apply the standard corrections, Kasen said. The study did find a small but potentially worrisome effect that could result from systematic differences in the chemical compositions of stars at different times in the history of the universe. But researchers can use the computer models to further characterize this effect and develop corrections for it.

A type 1a supernova occurs when a white dwarf star acquires additional mass by siphoning matter away from a companion star. When it reaches a critical mass–1.4 times the mass of the Sun, packed into an object the size of the Earth–the heat and pressure in the center of the star spark a runaway nuclear fusion reaction, and the white dwarf explodes. Since the initial conditions are about the same in all cases, these supernovae tend to have the same luminosity, and their “light curves” (how the luminosity changes over time) are predictable.

Some are intrinsically brighter than others, but these flare and fade more slowly, and this correlation between the brightness and the width of the light curve allows astronomers to apply a correction to standardize their observations. So astronomers can measure the light curve of a type 1a supernova, calculate its intrinsic brightness, and then determine how far away it is, since the apparent brightness diminishes with distance (just as a candle appears dimmer at a distance than it does up close).

The computer models used to simulate these supernovae in the new study are based on current theoretical understanding of how and where the ignition process begins inside the white dwarf and where it makes the transition from slow-burning combustion to explosive detonation.

The simulations showed that the asymmetry of the explosions is a key factor determining the brightness of type 1a supernovae. “The reason these supernovae are not all the same brightness is closely tied to this breaking of spherical symmetry,” Kasen said.

The dominant source of variability is the synthesis of new elements during the explosions, which is sensitive to differences in the geometry of the first sparks that ignite a thermonuclear runaway in the simmering core of the white dwarf. Nickel-56 is especially important, because the radioactive decay of this unstable isotope creates the afterglow that astronomers are able to observe for months or even years after the explosion.

“The decay of nickel-56 is what powers the light curve. The explosion is over in a matter of seconds, so what we see is the result of how the nickel heats the debris and how the debris radiates light,” Kasen said.

Kasen developed the computer code to simulate this radiative transfer process, using output from the simulated explosions to produce visualizations that can be compared directly to astronomical observations of supernovae.

The good news is that the variability seen in the computer models agrees with observations of type 1a supernovae. “Most importantly, the width and peak luminosity of the light curve are correlated in a way that agrees with what observers have found. So the models are consistent with the observations on which the discovery of dark energy was based,” Woosley said.

Another source of variability is that these asymmetric explosions look different when viewed at different angles. This can account for differences in brightness of as much as 20 percent, Kasen said, but the effect is random and creates scatter in the measurements that can be statistically reduced by observing large numbers of supernovae.

The potential for systematic bias comes primarily from variation in the initial chemical composition of the white dwarf star. Heavier elements are synthesized during supernova explosions, and debris from those explosions is incorporated into new stars. As a result, stars formed recently are likely to contain more heavy elements (higher “metallicity,” in astronomers’ terminology) than stars formed in the distant past.

“That’s the kind of thing we expect to evolve over time, so if you look at distant stars corresponding to much earlier times in the history of the universe, they would tend to have lower metallicity,” Kasen said. “When we calculated the effect of this in our models, we found that the resulting errors in distance measurements would be on the order of 2 percent or less.”

Further studies using computer simulations will enable researchers to characterize the effects of such variations in more detail and limit their impact on future dark-energy experiments, which might require a level of precision that would make errors of 2 percent unacceptable.

Source: EurekAlert

,



Sort by:   newest | oldest | most voted
Jon Hanford
Member
Jon Hanford
August 12, 2009 1:24 PM

So type Ia supernovae aren’t quite the ‘standard candles’ they were once thought to be, even after making corrections for elemental abundances (metallicity). Hopefully, these new computer sims will help clarify the observed luminosity range of these SN events. As noted, this will be crucial in teasing out accurate data on the evolution of Dark Energy in the universe and how it may vary over time.

Nereid
Member
Nereid
August 12, 2009 1:42 PM
This seems to be the paper (link is to ArXiv preprint): http://fr.arxiv.org/abs/0907.0708 @JH: Concerns over just how good Ia SNe are as standard candles can be found in the literature right from Day One; systematic effects are the bane of astronomers’ lives, and they are (or should be) acutely aware of them. What I think you’ll find is that the coverage of these objects outside the relevant community tends to downplay or completely ignore this aspect. In general, astronomers address systematic effects in two broad ways: they try to identify, characterise, and study them in as much detail as they can; and they try to circumvent them by using independent techniques, seeking multiple consistency checks, etc. In the… Read more »
Jon Hanford
Member
Jon Hanford
August 12, 2009 2:42 PM
Nereid, thanks for your introduction to the supernova ‘as standard candles’ debate that I first encountered in my frosh year at OSU in 1976. Maybe less informed readers may look into this well-known (in astronomical circles) effect and learn more about what we know about different types of SNe. Four years of college quickly brought to my attention the “constant standard luminosity” problem at the the forefront of distant SNe studies at the time. I do find it encouraging that modern astrophysicists are using all means at their disposal to accurately determine the types and total luminosities of these celestial beacons. I even remember when GRBs were seriously being touted as far more distant ‘standard candles’ a few… Read more »
ND
Member
ND
August 12, 2009 6:29 PM

Type 1a? Cepheid’s? I say parallax is the way to go man. We just need to put two telescopes at either end of the milky way and we should be all set smile

weeasle
Member
weeasle
August 12, 2009 7:49 PM
First let me start my rant by saying that I appreciate the great work of the writers and site maintainers and appreciate that I can read this content free which helps keep some fun in life for me as I cannot watch the intellectuall void called TV…. However, one sentence in this article really bothers me a bit: “The discovery of dark energy, a mysterious force that is accelerating the expansion of the universe, was based on observations of type 1a supernovae” As far as I understand, Dark Energy and Dark Matter are theories brought about by cosmologists to explain and plug gaps in the mathematical and theoretical models of the big bang and accelerating expansion of the… Read more »
Astrofiend
Member
Astrofiend
August 12, 2009 8:18 PM
weeasle Says: August 12th, 2009 at 7:49 pm I agree with what you’re saying to a certain extent, however, there comes a point where prefacing every statement you make in science with a ‘this may or may not be the case’ starts to become a bit tedious. At some point, you have to just accept that everything in science is certain only in so much as it currently provides the best explanation for what we observe – the uncertainly is implicit in any scientific statement of ‘fact’ – there are only degrees of uncertainty. For the most part, I would think that almost anybody who visits this site would realise that there is a fair chance that DM,… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
August 13, 2009 5:17 AM

This is an unfortunate but not unexpected finding. A white dwarf which growsin mass from a companion will reach the C-limit at 1.4M_{sol} . The subsequent implosion is a sort stellar version of a nuclear fusion bomb. It is my understanding the fusion inolves lots of He to C and C to Si and so forth. So the energy released will clearly depend upon the elemental (chemical) composition of the white dwarf.

It is my hope that DE is not ruled out by this. The existence of what we call DE, or the influence of the cosmological constant, has some fascinating consequences for quantum gravity and cosmology.

LC

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
August 13, 2009 5:48 AM
@ weeasle: Not that I’m an astronomer, but these things interest me of course. Standard cosmology (hey, it’s been ~ 5 years or more without replacement! ) is a nifty thing for a layman. To the best of my understanding neither DM nor DE is brought out to plug untestable “gaps” in models, but was suggested as testable hypotheses. Take the Bullet Cluster (and now some more clusters) observations for example, that provide a direct test of DM, albeit with indirect observation. Other hypotheses that matches these observations are now very contrived, and must according to “Starts With A Bang” astrophysicist be tailored specifically to each of them. Ie the alternatives are AFAIU contradictory and so falsified, while… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
August 13, 2009 5:56 AM

My bad, seems my nitpick #2 was invalid of sorts (“that theory was later proven wrong”). Except perhaps that I don’t think duality has been falsified (“proven wrong”) as such, it is just not part of today’s theory.

Manu
Member
Manu
August 13, 2009 6:41 AM

Interesting discussion!
I credit Weeasle with an opinion I share: what you 3 guys explained at lenght here, and Torbjorn summed up best, is THE most important thing that science information should convey to the public, that science writers should write about (they do! but not always…), that science readers should look for.
Until you have come to recognize these major aspects of scientific knowledge (temporary, collective, open), you only come across apparently contradictory definitive statements, and that’s very frustrating.

What makes me mad is that mainstream channels never, ever go into this, so science end up being totally misrepresented.
Bacteria in a Mars stone make headlines, the falsification thereof doesn’t, and whoever heard of Popper anyway?

neoguru
Member
neoguru
August 13, 2009 7:16 AM

A spectacular image….. What’s it’s source?

Nereid
Member
Nereid
August 13, 2009 8:25 AM

It’s from the Hubble, of Supernova 1994D in Galaxy NGC 4526
Source:
http://hubblesite.org/gallery/album/entire/pr1999019i/

Nereid
Member
Nereid
August 13, 2009 8:50 AM
For now, I’d like to comment on just this part of what weeasle wrote: As far as I understand, Dark Energy and Dark Matter are theories brought about by cosmologists to explain and plug gaps in the mathematical and theoretical models of the big bang and accelerating expansion of the universe where less than 5% of matter/energy can be accounted for by observation alone. (bold added) Dark Matter has a fascinating, and intricate history! It began with Zwicky, in 1933 (or 1934), who discovered that there seemed to be more mass in a galaxy cluster (Coma?) than he could account for, based on the light detected from the galaxies in the cluster. Several decades later, x-ray astronomers discovered… Read more »
Jon Hanford
Member
Jon Hanford
August 13, 2009 10:26 AM
@Nereid, belated thanks for that earlier link to the relevant preprint that was eventually published in Nature. The authors point out limitations and possible (mis)interpretations in using Type Ia SN as the only means of determining objects at great distances to us (as you have pointed out, several other methods are used for this determination in your posts above). Kudos for that last post that succinctly describes how astronomers came to realize the ‘missing mass’ problems that initially cropped up in detailed studies of galactic rotation curves and the problems that arose with ‘gravitationally bound’ members of galaxy clusters. And your keen observations on the downright rejection of the existence of DM in light of other ‘remarkable’ events… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
August 13, 2009 12:00 PM

These results will not particularly challenge the existence of dark matter. That is a more local gravitational effect, and there is lots of evidence for DM by other means. This issue might have some bearing on th existence of dark energy. The accelerated expansion of the universe is calibrated with SN1s based on their regular luminosities.

LC

weeasle
Member
weeasle
August 13, 2009 3:47 PM
Wow! I get the feeling a few of you here must work in observatories or some more serious science-related field for your day jobs… You all probably noticed that I was trolling a bit there but in a positive way, ie. to elucidate informative responses from such an educated group as opposed to just wanting to reinforce any of own pet theories or notions… A personal issue I was having was accepting placeholders as theory without solid observational evidence (from more than 1 or 2 distinct fields or methods, ie. not just telescope observations but it would be nice to have the particle physics people discover something that reinforces the idea). I accept Nereid you are right in… Read more »
Astrofiend
Member
Astrofiend
August 13, 2009 4:08 PM

That was a nice little exchange. It is so refreshing to have a civilized discussion about the current state of play in regards to DM and DE, and the various merits and possible issues with these theories. A reasonable dose of scientific skepticism followed by an interesting discussion.

Contrast this approach and outcome to the usual efforts of those who usually hijack such threads with their inane ramblings and crayon-on-the-wall physics.

As I said – refreshing.

Jon Hanford
Member
Jon Hanford
August 13, 2009 10:40 PM

@Astrofiend: Ditto smile

DrFlimmer
Member
DrFlimmer
August 14, 2009 7:30 AM

@ weeasle

We will see, where DM and DE will lead us. But maybe in a few years we will have direct proof of black holes. AFAIK it won’t be long till we have the resolution to resolve the event horizon of the black hole in Sgr A* (the central BH of the Milky Way). Then we will know for sure!

@ Astrofriend:

Indeed, although I didn’t participate, it was really an enjoyment to read this thread!

Nereid
Member
Nereid
August 14, 2009 11:46 AM
I’d like to go back to what weeasle wrote, and throw out some thoughts for further discussion … Dark Energy and Dark Matter are theories brought about by cosmologists to explain and plug gaps in the mathematical and theoretical models of the big bang and accelerating expansion of the universe where less than 5% of matter/energy can be accounted for by observation alone. Here I go … * in astrophysics – indeed, the whole of physics – is there any *theory* that is not mathematical? IOW, as all theories are mathematical, isn’t “mathematical” redundant? * in physics, are there any models which are not built from theories? IOW, isn’t “theoretical” redundant? * in physics, for relationships which are… Read more »
wpDiscuz