Author Topic: Supernova Science  (Read 36881 times)

Richard Scalzo

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Re: Supernova Science
« Reply #15 on: October 15, 2009, 12:47:45 am »
Zeus -- thanks, and enjoy!

XYZoo -- Well, I'm not an expert on SN 1987A but I'll try to say something helpful:  Usually the ejected matter from a SN explosion is at first moving very fast (sometimes up to a tenth of the speed of light, 30,000 km/s -- imagine the ticket the police would give you on the highway), so it has a lot of kinetic energy which can be converted into light and heat.  The glowing spots you see are not actually SN ejecta themselves, but gas which had been floating around near the supernova and was heated to very high temperatures after colliding with the SN ejecta.  The reason you see bright spots is because the gas surrounding the SN isn't totally smooth, but is kind of lumpy -- in some places it is denser than others, meaning there's more stuff to heat up and glow.  So parts which are denser than others will appear to glow more brightly in these images.

In fact, far from being even close to spherical, this cartoon suggests that given the elliptical shape the lumps take on the sky, they probably lie in a 2-D ring which we're viewing at an angle:  http://hubblesite.org/newscenter/archive/releases/2007/10/image/i/
See also http://findarticles.com/p/articles/mi_m1200/is_n9_v145/ai_14878130/ for a brief exposition.

-- Richard
« Last Edit: October 15, 2009, 02:33:34 pm by Richard Scalzo »

Wowi680

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Re: Supernova Science
« Reply #16 on: October 15, 2009, 03:01:03 pm »
 The glowing spots you see are not actually SN ejecta themselves, but gas which had been floating around near the supernova and was heated to very high temperatures after colliding with the SN ejecta.

Could it be gas ejected by the star when it became a red giant? Or is it for sure :)
 "Only two things are infinite, the Universe, and human stupidity, and I'm not so sure about the Universe"--Albert Einstein

bportlock

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Re: Supernova Science
« Reply #17 on: October 15, 2009, 09:09:03 pm »
I'm quite fond of SN1987A because I know one of the co-discovers - Colin Henshaw. Our company actually hosts a website on one of Colin's other pet projects on Light Pollution (run jointly with a chappie called Graham Cliff) and its effect on insects and disrupted sleep patterns in the now 24 hour day.

It always annoyed me that it was too far south to be seen from Manchester, however I have hopes that Betelguese will blow itself to kingdom come one winter night and we'll all have a great view if the neutrino shock doesn't wipe us all out.

Lovethetropics

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Re: Supernova Science
« Reply #18 on: October 15, 2009, 11:38:47 pm »
But Betelgeuse's explosion will not be felt here, it will explode in another direction...or at least that's what I understood from what I read.  We need the expertise of Richard on this one.  ???

 *and find lots of asteroids  ;D

Lovethetropics

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Re: Supernova Science
« Reply #19 on: October 15, 2009, 11:40:59 pm »
When it explodes will we see the explosion first and then feel whatever it is we will feel or everything will happen at once?

 *and find lots of asteroids  ;D

Richard Scalzo

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Re: Supernova Science
« Reply #20 on: October 16, 2009, 12:45:04 am »
Wowi -- Maybe!  If the progenitor star had a strong wind, then it would have thrown a lot of material into the surrounding area, and you certainly would expect the ejecta to interact with that material when the SN blew up.  You can often identify SNe with a lot of gas around them by a very narrow hydrogen line, as opposed to the fat ones in the type II spectra on Dan Kasen's website.  These are usually called "SNe IIn" where the 'n' is for "narrow".  SN 1987A was certainly a "core-collapse" SN, produced when the iron core of a massive star collapses under its own weight, which usually means it's either type Ib, Ic or II.  But I'm not sure whether it belongs to the IIn subtype.

Aida -- I'm going out on a limb here, but if we're talking about core-collapse SNe, the first thing to reach us would be a burst of neutrinos, which have no electric charge and travel near the speed of light, so they can escape the dense inner SN ejecta easily.  I doubt we'd notice them, but after that we would probably see the light from the SN, which would take a while to diffuse outwards.  There would be a lot of it and we'd undoubtedly be toasted.  If we were really close to the SN we might eventually be hit by the blast wave, but I doubt anything would be left of us by then in that case.

There are some people who write papers about the potential impact of a nearby SN on Earth's biosphere -- here's an example:  http://xxx.lanl.gov/abs/0809.0899
I think it's kind of silly to worry about trying to prepare for a nearby SN or anything, but the science can be fun to think about.

-- Richard

Lovethetropics

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Re: Supernova Science
« Reply #21 on: October 16, 2009, 02:22:59 am »
Thank you very much Richard.  I also think it's silly to worry about things you can't change or prevent.  Let's hope Betelgeuse has a very long life. ;D ;D ;D ;D

 *and find lots of asteroids  ;D

EigenState

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Re: Supernova Science
« Reply #22 on: October 16, 2009, 03:35:08 am »
Greetings,

If I may, I would like to ask a question regarding Kasen's discussion of P-Cygni line profiles, which are "quasi-first derivative" in general appearance.  Clearly that is the anticipated result of an effective convolution of differentially Doppler shifted absorption and emission contributions.

The basic physical model is represented by Kasen as follows:



Inspection of his tutorial content leads one to infer that the emission component of the blended line profile will always manifest a maximum intensity at the rest-frame frequency.  Indeed Kasen states explicitly:

Quote
The emission peak, on the other hand, always remains at the rest wavelength (unless more complicated physical effects are present).

Kasen also states that the observed spectral feature depends upon the value of the line source function, that is the ratio of the population densities of excited electronic states to ground state.

All of this appears to depend upon the assumption that what Kasen defines as the "absorption region" has a line source function value much less than unity.  That is to say that the direct line of sight cannot be dominated by emission.  It is not obvious to me that such a physical restriction is an a priori given.  For higher line source function values and conditions not leading to strong self-absorption, I would think that the so-called "absorption region" could in fact yield an emission feature.  If that is indeed possible, then I would expect that the overall observed spectral feature would be an emission feature that is asymmetrically broadened to the blue, and that has a maximum intensity that is blue-shifted from the rest-frame frequency.

So what am I missing regarding the dynamical properties of the supernova atmosphere?

Best regards,
EigenState

Richard Scalzo

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Re: Supernova Science
« Reply #23 on: October 16, 2009, 05:17:11 am »
Dear Eigenstate,

Whew, that's a pretty technical question, but I think I can explain.  As you point out, the line source function is just the ratio of the level populations.  Statistical mechanics tells us how to calculate those.  I don't know whether you were trained as a physical scientist or engineer, but I'll provide a little bit of math here.  Disclaimer:  I'm not a theorist and provide no warranty for this information, just a little physical insight (maybe).  :)

Often in supernova work, you use an approximation called "local thermodynamic equilibrium" (LTE), which means you can approximate the atomic levels in any chunk of gas as though the atoms have a well-defined temperature.  This makes things easier since you can approximate the transfer of energy in the SN atmosphere in terms of heat diffusion.  It breaks down in cases where the SN ejecta can't transfer heat efficiently -- for example, when the density is very low (so that collisions between atoms don't happen often enough to exchange energy between them) -- or when you have a lot of non-thermal radiation e.g. from accelerated particles in the ejecta.  You call those cases "NLTE" and it means you have to solve the full radiation transport equation, which is very complicated and takes lots of time to solve numerically even on very powerful computers.  But for a lot of SN work, LTE is good enough.

So let's try an example, say, in a SN Ia atmosphere around maximum light.  Let's estimate the source function for a particular line, Si II 6355, which would have a wavelength of lambda = 6355 angstroms or 635.5 nm; this is the line most commonly used to identify type Ia supernovae.  If you pull out your thermal physics book (e.g. Kittel & Kroemer), you'll read that the level populations in thermal equilibrium are given by the Boltzmann factor exp(-(E_upper - E_lower)/kT).  The SN photosphere, which is hotter and denser than the region where the lines are emitted, is probably about 10000 K, and the energy level difference is just hc/lambda, where h = Planck's constant and c = speed of light.  If you copy and paste the expression

exp(-(h*c)/(6355 angstroms * k * 10000 K))

into Google (Calculator), you'll get about 0.1.  This is a plausible upper bound to the source function, and if you watch Dan's animation again, you'll see that there's still a healthy absorption trough at 0.1 or so.  At 6000 K, the value is more like 0.02.

So I think the answer (in our example) is just that the line-forming region is cool enough that absorption dominates over emission, if you have a photosphere backlighting it.  Of course, in the limbs there is no photosphere behind for us to view, but light from the photosphere at the wavelength of the line will be scattered into our line of sight, creating the emission part of the P Cygni profile.  You've got a point though, you wouldn't expect this always to be true in general; it depends on the temperature, the energy levels, the density, and possibly other things.

-- Richard
« Last Edit: October 16, 2009, 02:34:46 pm by Richard Scalzo »

EigenState

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Re: Supernova Science
« Reply #24 on: October 16, 2009, 03:55:07 pm »
Greetings Richard,

Thank you.  I do high resolution laser spectroscopy of atoms and diatomic molecules, mainly focusing on hyperfine structure interactions.  I even know Prospect Street and Science Hill.   ;)

Certainly I understand Local Thermodynamic Equilibria and reasonable physical conditions within the expanding atmosphere clearly do yield values of the line source function small enough to expect a dominant absorption feature.  I just wanted to check to make sure it was all in the plasma dynamics and that I was not missing something fundamental about the spectroscopy itself.

Nice to meet you by the way.

Best regards,
ES


Richard Scalzo

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Re: Supernova Science
« Reply #25 on: October 16, 2009, 05:14:24 pm »
Great, then I won't skimp on technical details next time.  :)

I'll still do my best to unfold jargon or approximations that are peculiar to an astronomical context, of course.  And hopefully what I wrote above is accessible to anyone who's taken an undergraduate modern physics course, whether they do physics for a living now or not.

Are you actually at Yale also?  Small world!
RS
« Last Edit: October 16, 2009, 05:18:02 pm by Richard Scalzo »

NGC3314

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Re: Supernova Science
« Reply #26 on: October 16, 2009, 05:50:16 pm »
For comparison, here are the optical spectra of fairly typical supernovae of types Ia and II.

For type Ia (destruction of a white dwarf), we don't see hydrogen lines but instead very broad features from Mg (4300 A, just at the blue end), Ca (way out in the red around 8500 A), Fe (4900 A), Si (6200), and so on.

In Type II, the defining feature is hydrogen lines, which are very strong and very broad (Doppler shifts from the rapidly expanding envelope). H-alpha is especially prominent. Many of these emission lines show a dip to the blue side - that's where material lies right between us and the hot fireball so it absorbs background light. The liens are so broadened by velocity that the emission from one line can overlap the absorption from another, complicating interpretation of the spectra and driving investigators to detailed numerical modeling of the whole thing rather than individual spectral lines.

In both cases, the spectrum changes with time. As the fireball expands and thins, we see progressively deeper into the debris, looking in some cases at layers of elements originally fused deeper and deeper inside the star.

EigenState

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Re: Supernova Science
« Reply #27 on: October 16, 2009, 07:05:27 pm »
Great, then I won't skimp on technical details next time.  :)

I'll still do my best to unfold jargon or approximations that are peculiar to an astronomical context, of course.  And hopefully what I wrote above is accessible to anyone who's taken an undergraduate modern physics course, whether they do physics for a living now or not.

Are you actually at Yale also?  Small world!
RS

Greetings Richard,

No need to avoid the real science from my point of view--that is why I am here after all.  There is a significant difference between doing fundamental spectroscopy on what are effectively isolated atoms or molecules under close to ideal and controllable laboratory conditions and observing distant samples under rather extreme physical conditions that are not well characterized by comparison.  Thus, here I am to learn some astrophysics.

I will say that I think it extremely important that assumptions be stated explicitly.  Had Kasen done that--I admit I might have missed it if he actually did--regarding the assumption of LTE, then I never would have asked the question.

No, I let Yale years ago.  In a pinch, I could probably still find my way to Naples Pizza and Claire's Cornercopia.   :D  Were I still there I would have wandered over to Gibbs to find you and Kevin.

Best regards,
ES

EigenState

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Re: Supernova Science
« Reply #28 on: October 16, 2009, 07:19:20 pm »
...The liens are so broadened by velocity that the emission from one line can overlap the absorption from another, complicating interpretation of the spectra and driving investigators to detailed numerical modeling of the whole thing rather than individual spectral lines.

In both cases, the spectrum changes with time. As the fireball expands and thins, we see progressively deeper into the debris, looking in some cases at layers of elements originally fused deeper and deeper inside the star.

Greetings NGC3314,

What assumptions go into the detailed numerical models?  I am thinking chemical composition including relative abundances, temperature assuming LTE, density.  Surely I have missed something.

Also, what is the generally expected temporal evolution of the spectrum of one of these beasts?  Again I am assuming decreasing effective temperature, decreasing radial velocities, and decreasing densities as a function of time.  That would lead me to expect that the spectrum becomes increasingly less effected by Doppler broadening and increasingly dominated by absorption features.

Best regards,
ES

Richard Scalzo

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Re: Supernova Science
« Reply #29 on: October 16, 2009, 07:32:47 pm »
No need to avoid the real science from my point of view--that is why I am here after all.  There is a significant difference between doing fundamental spectroscopy on what are effectively isolated atoms or molecules under close to ideal and controllable laboratory conditions and observing distant samples under rather extreme physical conditions that are not well characterized by comparison.  Thus, here I am to learn some astrophysics.

Okay, sounds good!  We seem to have a neat group of people with quite diverse backgrounds here.  General note to others reading that if I say anything in one of these exchanges that sounds like it might be interesting if only I had provided enough background, please follow up with more questions!  :)

I will say that I think it extremely important that assumptions be stated explicitly.  Had Kasen done that--I admit I might have missed it if he actually did--regarding the assumption of LTE, then I never would have asked the question.

Well, I think Dan wanted it to be more general -- there are certainly occasions where LTE does not obtain, but you can still talk about what the source function is, even if you can't compute it from a Boltzmann factor.  Similarly, I think that's why he shows another animation showing changes in the all-encompassing "optical depth", instead of listing other parameters like mass of material, oscillator strengths, etc.  So I don't think he intended to be misleading, but it is certainly true that LTE works pretty well in a variety of situations and in the denser ejecta.

Here's an example of where LTE doesn't work:  http://adsabs.harvard.edu/abs/2007ApJ...654L..53T
My colleague Rollin Thomas at LBL here uses PHOENIX, a powerful radiation-transfer Monte Carlo code, to show that C II 4267 probably isn't in LTE in this SN (or else the C distribution is clumpy, unlike elements like Si deeper in the ejecta).  I never quite understood all this as my atomic physics is pretty spotty, but you might be better able to appreciate the implications.

No, I let Yale years ago.  In a pinch, I could probably still find my way to Naples Pizza and Claire's Cornercopia.   :D  Were I still there I would have wandered over to Gibbs to find you and Kevin.

Let us know if you're ever in town, and we'll go get coffee and deconstruct all this.  :)
RS
« Last Edit: October 17, 2009, 02:40:42 am by Richard Scalzo »