PDA

View Full Version : Interstellar Medium; current leaders, current challanges?



tu144
2011-May-12, 07:26 PM
Who are the current leaders, icons in this part of astronomy? What are they struggling with right now?

antoniseb
2011-May-12, 07:40 PM
You are asking about a topic that is a little on the dry side... no explosions or other fast changes... so no celebrities.
I would recommend looking at arxiv.org, and scan the astrophysics papers to see who is publishing on this topic.

Amber Robot
2011-May-12, 07:56 PM
Who are the current leaders, icons in this part of astronomy? What are they struggling with right now?

That's a bit of a broad topic, can you narrow it down a little?

Icons take time to develop, so you have to look at a certain person's total career. Bruce Draine is a very big name in the theory of interstellar dust. Ed Jenkins is interested in metal depletion and the distribution of thermal pressures in the cold, neutral medium. These are a couple of the big names that pop into my head without much thought.

tu144
2011-May-13, 11:14 AM
I can't really narrow the field. I'm looking for someone who did something significant in this field, whatever the specific part of the field was and is more or less good expalinor like Richard Feynman and is famous enough to be interviewed at least once or gave a videolecture that I can find somewhere on the internet.

Someone like William A. Klemperer

form wikipedia
(...)most widely known for: (...)(3) Pioneering Astrochemistry, including developing the first gas phase chemical models of cold molecular clouds that predicted an abundance of the molecular HCO+ molecular ion that was later confirmed by Radio Astronomy

I've managed to find his lecture at:
http://www.vega.org.uk/video/programme/64

If it comes to Bruce Draine
I've found his lecutre on: http://www.astro.princeton.edu/~draine/dust/lectures.html
But if it comes to Ed Jenkins, he doesn't even exits on wikipedia.

If someone else comes to your mind, I'd be grateful for names I can check.

ngc3314
2011-May-13, 12:39 PM
Lyman Spitzer and Don Osterbrock were major gurus as important parts of ISM study came to maturity. Each left massive legacies of articles and books. Poking around with Google turned up this interview (http://www.aip.org/history/ohilist/4809.html) with Osterbrock. There are several available with Spitzer, mostly in the context of his being an early and forceful proponent of space astronomy (some date Hubble's inception to Spitzer's 1947 "white paper" appendix for the RAND corporation).

Amber Robot
2011-May-13, 12:51 PM
Yeah, Spitzer's name came up when you mentioned Feynman. Osterbrock is another great example of a pioneer in ISM. However, neither are really "current".

tu144
2011-May-13, 06:30 PM
I've checked them. It's kind of odd that there are absolutly no video or audio recordings of these figures. It wasn't that long time ago, there should be something.

Amber Robot
2011-May-13, 06:43 PM
I've checked them. It's kind of odd that there are absolutly no video or audio recordings of these figures. It wasn't that long time ago, there should be something.

Why is that odd? Most astrophysicists don't become known to pop culture and there'd be little reason to record them. I'm not sure anyone outside of astronomy knows who Donald Osterbrock was.

tu144
2011-May-13, 07:12 PM
Why is that odd? Most astrophysicists don't become known to pop culture and there'd be little reason to record them. I'm not sure anyone outside of astronomy knows who Donald Osterbrock was.

I suppose you are right on this one.
But still if somebody is extremely good in his area and he discovered something important, you would want to at least talk with him and record the conversation.
Maybe I'm bad in searching but even with such icon like Edwin Hubble I've only managed to find 3 seconds of archive video in italian documentary about him: http://www.youtube.com/watch?v=SM-f6NJSglI&feature=player_detailpage#t=64s

tu144
2011-May-13, 07:23 PM
Why prohibited lines of emission show up in ionized nebula spectrum?
Why are they so bright in ionized nebulas?
Why they don't appear in normal conditions on Earth?

EigenState
2011-May-13, 07:30 PM
Greetings,


Why prohibited lines of emission show up in ionized nebula spectrum?
Why are they so bright in ionized nebulas?
Why they don't appear in normal conditions on Earth?

By "prohibited lines", do you mean forbidden lines? I ask explicitly because the terminology "prohibited" is not used in spectroscopy whereas "forbidden" has a well defined meaning.

Best regards,
EigenState

tu144
2011-May-13, 07:32 PM
What effects produce interstellar dust in the spectrum of nebulae?
How can I quantify interstellar extinction from the nebulae spectrum?

tu144
2011-May-13, 07:34 PM
Yes. I ment forbidden. Sorry for that.

tu144
2011-May-13, 07:38 PM
Why gas of a nebulae is set in motion when it gets ionized?
Why most of the HII regions we see have an expansion velocity of about 10 km / s?

tu144
2011-May-13, 07:42 PM
Why are these lines so useful for studying molecular clouds?

tu144
2011-May-13, 07:50 PM
What is the critical density of an atom level?
What happens to the intensity of the collisional excitation lines when it exceeds this density?

tu144
2011-May-13, 07:52 PM
What are they?
What could be the examples of such diagram?

tu144
2011-May-13, 08:01 PM
How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

tu144
2011-May-13, 08:05 PM
What are the differences between these two?
What are they characteristic spectrums?

EigenState
2011-May-13, 08:05 PM
Greetings,

The term "forbidden" rigorously means that a specific transition is not an allowed electric dipole transition. Such transitions are allowed as either magnetic dipole or electric quadrupole transitions which are typically 106 or 108 times less probable. Thus "forbidden" transitions certainly do occur but they are weaker.

The quantum mechanical description of photon emission can be conveniently summarized by a set of so-called selection rules which account for the changes in the various angular momentum quantum numbers describing the relevant energy levels. For just one example: the selection rule ∆S = 0 applies to electric dipole transitions, whereas ∆S = 1, ∆S ≠ 0 applies to magnetic dipole and electric quadrupole transitions.

To reiterate, there is nothing forbidden about "forbidden" transitions. They involve different kinds of quantum states with different changes in the quantum numbers. So what "forbidden" really means is that such transitions are less probable, weaker (in the sense that the transition probabilities are very much smaller), and the energy states tend to be much longer lived.

Such emission lines show up in many situations, even here on Earth. The principle component of the night time airglow is an emission feature from OI at 5577.34. This is an electric quadrupole (E2) transition. Such lines would be strong in the spectra of certain astronomical objects because the physical conditions within those objects selectively populate the pertinent energy levels--that is they appear strong because the transitions you are more used to do not come into play.

Hope that helps some.

Best regards,
EigenState

tu144
2011-May-13, 08:07 PM
What are the different processes of formation of molecules in interstellar medium?
How H2 is formed?

Swift
2011-May-13, 08:33 PM
tu144,

10 new threads in an hour, all on Interstellar medium. Are you using BAUT to write your dissertation? You might want to slow it down, just a bit.

Upon further consideration - I've merged the threads

ngc3314
2011-May-13, 10:18 PM
With that many detailed questions, you might well start with tracking down a copy of a book, such as Osterbrock and Ferland's Astrophysics of Gaseous Nebulae and Active Galactic Nuclei or Spitzer's older book on the interstellar medium. Our explanations would only be distilled versions of such pedagogically ordered material.

EigenState
2011-May-13, 10:28 PM
With that many detailed questions, you might well start with tracking down a copy of a book, such as Osterbrock and Ferland's Astrophysics of Gaseous Nebulae and Active Galactic Nuclei or Spitzer's older book on the interstellar medium. Our explanations would only be distilled versions of such pedagogically ordered material.

Bravo!!

@Swift: I believe you missed one thread. Parameter of ionization (http://www.bautforum.com/showthread.php/115575-Parameter-of-ionization)

Best regards,
EigenState

tu144
2011-May-14, 07:47 AM
tu144,

10 new threads in an hour, all on Interstellar medium. Are you using BAUT to write your dissertation? You might want to slow it down, just a bit.

Upon further consideration - I've merged the threads

Nope. I need to learn Interstellar Medium basics in 10 days. BAUT forum is the only place I used so far I can ask questions and go on with a topic until I get a thing. I was going to follow and participate in all 10 discussions I've started.
They are only 10, there was going to be a lot more after solving these.
Merging them will not help me.
As you can see
With that many detailed questions, you might well start with tracking down a copy of a book(...)
I put them separtely so not to make total chaos and make the discussions possible.
It would really help me if you could separate them again.
Thank you.

pzkpfw
2011-May-14, 07:52 AM
It would really help me if you could separate them again.
Thank you.

No.

You will not dominate the Q&A section of BAUT by having half the first page of questions all yours. That's not fair to other BAUT members.

Please use a report or PM to discuss this further, don't post in-thread.

grapes
2011-May-14, 07:59 AM
Nope. I need to learn Interstellar Medium basics in 10 days. Just out of curiosity, why the rush? I ask because it might help motivate us too! :)

tu144
2011-May-14, 08:01 AM
No.

You will not dominate the Q&A section of BAUT by having half the first page of questions all yours. That's not fair to other BAUT members.

Please use a report or PM to discuss this further, don't post in-thread.

The remaining nine would not even be answerd in this case so I don't know who to PM.
Maybe you can tell me what is the speed limit of this forum in units of [posts/unit time(day, hour, week, whatever)],
bring back the allowed minimum and I'll add the rest in a way not to break this speed limit?

astromark
2011-May-14, 08:02 AM
@ the OP Tu 144; You ask a wall of questions..

Who is leading in the fields of spectroscopic study and analysis of light emission signatures..

Cal. tech. University and NASA and almost any astronomy branch of Universities from around the planet...

and almost all you ask can be discussed with them. Most will respond to your inquiring mind.

tu144
2011-May-14, 08:05 AM
Just out of curiosity, why the rush? I ask because it might help motivate us too! :)

I want to pass 5 astrophysical subjects within 2 months one by one.
The time for Interstellar Medium has come.

grapes
2011-May-14, 08:30 AM
I want to pass 5 astrophysical subjects within 2 months one by one.
The time for Interstellar Medium has come.You're taking online school courses? Which ones?

grapes
2011-May-14, 08:35 AM
The remaining nine would not even be answerd in this case so I don't know who to PM.Why wouldn't they be answered?

Maybe you can tell me what is the speed limit of this forum in units of [posts/unit time(day, hour, week, whatever)],
bring back the allowed minimum and I'll add the rest in a way not to break this speed limit?No speed limit. Just like Montana in the good old days, just reasonable and prudent. :)

tu144
2011-May-14, 08:42 AM
You're taking online school courses? Which ones?
Nope. It's highly complicated.
I am doing normal classes on University, but it's neither in my mother language nor in english, so I've got real problems with understanding this stuff.

tu144
2011-May-14, 09:11 AM
Greetings,

Such lines would be strong in the spectra of certain astronomical objects because the physical conditions within those objects selectively populate the pertinent energy levels--that is they appear strong because the transitions you are more used to do not come into play.

Hope that helps some.

That depends on the density of gas, right?
If the density of ionized nebulae is low enough that will happen?
How is that connected?
And what are the transitions that I am more used to?

WayneFrancis
2011-May-14, 02:59 PM
...
I've found his lecutre on: http://www.astro.princeton.edu/~draine/dust/lectures.html
...

Anyone else watch that and not get out of their head Professor Frink during the whole lecture?

Whoops it was the other link...


...
http://www.vega.org.uk/video/programme/64
...

ngc3314
2011-May-14, 04:05 PM
There is some useful material on many of these questions in web class notes here (http://www.astr.ua.edu/keel/galaxies/emission.html) and here (http://www.astr.ua.edu/keel/galaxies/sfr.html).

Amber Robot
2011-May-14, 05:04 PM
I want to pass 5 astrophysical subjects within 2 months one by one.
The time for Interstellar Medium has come.

You are best off using books. The aforementioned Spitzer's book and Osterbrock's books are good starts.

EigenState
2011-May-14, 07:05 PM
Greetings,

There are also the free online course materials from MIT. The physics courses can be found at http://ocw.mit.edu/courses/physics/

There are a couple of astronomy and astrophysics listings at the level of undergraduate courses, and more at the graduate level.

And I most certainly agree with the sentiments of others that you would be best served by pursuing more comprehensive, authoritative, and efficient sources than this forum given you objectives.

Best regards,
EigenState

tu144
2011-May-15, 08:57 AM
Greetings,

There are also the free online course materials from MIT. The physics courses can be found at http://ocw.mit.edu/courses/physics/

Dead end. That's the first place I check, always. Lectures on astrophysics don't have notes, not to mention videos.
By the way, I found something half-connected to the topic. Yale University is also giving lectures online.
Here is the link to the list of topics on astronomy:
http://oyc.yale.edu/astronomy/frontiers-and-controversies-in-astrophysics/content/downloads
Unfortunately it's only an intro and I can't find anything that would be useful for me, right now.


And I most certainly agree with the sentiments of others that you would be best served by pursuing more comprehensive, authoritative, and efficient sources than this forum given you objectives.

I don't have much of a solid physics/astrophysics background. I couldn't cope with "The Sun. An Introduction" by Stix even with help of this forum. Besides, I am not aiming at being able to solve any fancy problems, just need to understand the concepts.

Like I wrote before, the only fully understandable materials I put my eyes on were lectures on Michael Richmond page.
There are loads of stuff, but not everything I need.
If anyone know something on that level of explanation like these: http://spiff.rit.edu/classes/
connected with Interstellar Medium, I would be glad to read it.

Amber Robot
2011-May-15, 02:01 PM
What effects produce interstellar dust in the spectrum of nebulae?
How can I quantify interstellar extinction from the nebulae spectrum?

There are a few ways to measure interstellar extinction. A typical way is to compare the color of a star to a what it's color would be without any extinction. This gives you a measure of the extinction along the line of sight to that star.

Another method is through star counts. This can give the extinction in a general area where there's an interstellar cloud.

Amber Robot
2011-May-15, 02:26 PM
Why are these lines so useful for studying molecular clouds?

CO is the second most common molecule in the ISM. Molecular hydrogen is number one, but because it is a homonuclear molecule, it's long wavelength transitions are much weaker than those of CO. CO is thus used as a proxy for hydrogen when studying molecular clouds and measuring their properties.

tu144
2011-May-15, 09:31 PM
There is some useful material on many of these questions in web class notes here (http://www.astr.ua.edu/keel/galaxies/emission.html) and here (http://www.astr.ua.edu/keel/galaxies/sfr.html).
Thanks for links. I don't understand all that I've read, but it gave me some "Aha!'s".

tu144
2011-May-15, 09:40 PM
There are a few ways to measure interstellar extinction. A typical way is to compare the color of a star to a what it's color would be without any extinction. This gives you a measure of the extinction along the line of sight to that star.

Another method is through star counts. This can give the extinction in a general area where there's an interstellar cloud.

To measure how much of the extinction is caused by a nebulae, there have to be a star behind it, or it can be next to the nebulae heating it or ionizing some atoms and we can calculate the extinction purely from the nebulae spectrum?

tu144
2011-May-15, 09:43 PM
CO is the second most common molecule in the ISM. Molecular hydrogen is number one, but because it is a homonuclear molecule, it's long wavelength transitions are much weaker than those of CO. CO is thus used as a proxy for hydrogen when studying molecular clouds and measuring their properties.
homonuclear molecule long wavelength transitions are much weaker than those of CO in sense that
a) they are less probable, therefore it's intensity is lower
OR
b) energy emited per molecule is lower
?

EigenState
2011-May-15, 09:53 PM
Greetings,


homonuclear molecule long wavelength transitions are much weaker than those of CO in sense that
a) they are less probable, therefore it's intensity is lower
OR
b) energy emited per molecule is lower
?

You need to explicitly state what you mean by "long wavelength". Infrared? Microwave?

This is necessary because the kinds of transitions occurring in different frequency domains are very different.

Best regards,
EigenState

Amber Robot
2011-May-15, 10:24 PM
To measure how much of the extinction is caused by a nebulae, there have to be a star behind it, or it can be next to the nebulae heating it or ionizing some atoms and we can calculate the extinction purely from the nebulae spectrum?

If you are interested in the extinction caused by the dust in a nebular region, it is best to have a star behind it. Of course, the extinction you will see is caused by all of the dust along the line of sight and not just the dust in the nebular region. Disentangling the contributions from various parts of the line of sight is very difficult.

Amber Robot
2011-May-15, 10:29 PM
homonuclear molecule long wavelength transitions are much weaker than those of CO in sense that
a) they are less probable, therefore it's intensity is lower
OR
b) energy emited per molecule is lower
?

The comparable lines, for example the pure rotational lines of the ground state, are weaker in H2 than in CO. So, even though there is 10^4 more times H2 molecules in a molecular cloud than CO, the CO rotational lines, which appear at sub-millimeter to radio wavelengths.

ngc3314
2011-May-15, 11:40 PM
If you are interested in the extinction caused by the dust in a nebular region, it is best to have a star behind it. Of course, the extinction you will see is caused by all of the dust along the line of sight and not just the dust in the nebular region. Disentangling the contributions from various parts of the line of sight is very difficult.

You can frequently get some information from line ratios in the nebula - the emitted ratios of the hydrogen Balmer, Paschen... lines are pretty consistent over relevant temperatures, and there are combinations of much weaker sulphur (or oxygen, IIRC) lines which are exactly consistent, so you get the emissivity-weighted mean extinction in that direction.

On molecular hydrogen - as I understand it, H2 has zero dipole moment ("symmetric rotor") so the ordinary vibrational transitions are forbidden in emission. Thus cold H2 is seen only in absorption (which is nasty, since the Lyman and Werner bands are deep in the UV, which is the first piece of the spectrum absorbed by associated dust at high column densities). FUSE could barely point anywhere outside the solar system without seeing the UV lines of intervening H2.

EigenState
2011-May-16, 12:13 AM
Greetings,



On molecular hydrogen - as I understand it, H2 has zero dipole moment ("symmetric rotor") so the ordinary vibrational transitions are forbidden in emission.

The relevant property is the permanent dipole moment. No homonuclear diatomic has a permanent dipole moment. Thus no pure allowed rotational spectrum.

For pure vibrational transitions, a change in the dipole moment as a function of bond length is required. Homonuclear diatomics fail there as well. Thus no pure vibrational spectrum.

Best regards,
EigenState

Amber Robot
2011-May-16, 04:43 AM
On molecular hydrogen - as I understand it, H2 has zero dipole moment ("symmetric rotor") so the ordinary vibrational transitions are forbidden in emission. Thus cold H2 is seen only in absorption (which is nasty, since the Lyman and Werner bands are deep in the UV, which is the first piece of the spectrum absorbed by associated dust at high column densities). FUSE could barely point anywhere outside the solar system without seeing the UV lines of intervening H2.

Yes, this is essentially correct. H2 has no net dipole moment and as such these transitions are forbidden. The UV transitions, being electronic changes are strong and nearly ubiquitous in the diffuse molecular interstellar medium. CO also has transitions in the ultraviolet and these have been observed on many sightlines.

tu144
2011-May-16, 09:32 AM
FUSE could barely point anywhere outside the solar system without seeing the UV lines of intervening H2.


The comparable lines, for example the pure rotational lines of the ground state, are weaker in H2 than in CO. So, even though there is 10^4 more times H2 molecules in a molecular cloud than CO, the CO rotational lines, which appear at sub-millimeter to radio wavelengths.


CO is the second most common molecule in the ISM. Molecular hydrogen is number one, but because it is a homonuclear molecule, it's long wavelength transitions are much weaker than those of CO. CO is thus used as a proxy for hydrogen when studying molecular clouds and measuring their properties.

Quote from wiki:

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.[1]

Why we focus on radio and infrared?
Give me a feedback on this one, do I get it right.
If we are talking about molecular cloud, there is no star nowhere nearby to ionize nothing, so we don't have any lines from transitions like ripping of the electron from hydrogen atoms or other atomic level to level jumps (I don't know how it looks like with H2 in comparison to single H atom; They have Lyman or Balmer series too?)
That is why we don't focus on UV and visible, because there is nothing emitted at these wavelengths, right?
So we focus on infrared and radio where rotational lines can be seen, and here CO is winning the battle because it has more intense lines.
And that means that if the ratio of H2/CO is 10^4, these rotational lines are comming more than 10^4 times more often from CO than from H2.
Is my understanding correct?

How far we can go into the space with this method?

Amber Robot
2011-May-16, 01:08 PM
That is why we don't focus on UV and visible, because there is nothing emitted at these wavelengths, right?


We can indeed study CO and H2 in the ultraviolet and this is often done for the diffuse and translucent interstellar medium. However, as you look through more and more material, extinction eventually prevents you from seeing through at these wavelengths -- there is much more dust extinction at shorter wavelengths than at longer ones.



So we focus on infrared and radio where rotational lines can be seen, and here CO is winning the battle because it has more intense lines.

These longer wavelength transitions correspond to lower energy transitions, thus they can be excited more easily in colder material. The CO rotational lines are dipole allowed, which means if they get excited they emit light very quickly. For H2, the rotational lines are not allowed and thus it is more likely that the molecule will be collisionally de-excited before it can emit the light and thus you don't really see those lines.



How far we can go into the space with this method?

I know that CO rotational lines have been studied in external galaxies and even at high redshift. H2 and CO have been studied in their ultraviolet absorption lines at high redshift too, where the UV transitions are redshifted down to the visible.

ngc3314
2011-May-16, 01:59 PM
I know that CO rotational lines have been studied in external galaxies and even at high redshift. H2 and CO have been studied in their ultraviolet absorption lines at high redshift too, where the UV transitions are redshifted down to the visible.

With a black-belt radio colleague, we have a couple of CO detections in a group at z=2.4 from the EVLA.

Seeing the H2 UV absorption at high z has proven tricker, because of a built-in bias. The lines of sight in a galaxy with the strongest molecular absorption are also the ones most strongly absorbed in the far-UV. The plot shows data leading to a limit on the strength of H2 absorption in composite spectra of Lyman-break galaxies at z~3 (details (http://adsabs.harvard.edu/abs/2006AJ....131.2755K)).

14994

Amber Robot
2011-May-16, 02:31 PM
Seeing the H2 UV absorption at high z has proven tricker, because of a built-in bias. The lines of sight in a galaxy with the strongest molecular absorption are also the ones most strongly absorbed in the far-UV. The plot shows data leading to a limit on the strength of H2 absorption in composite spectra of Lyman-break galaxies at z~3 (details (http://adsabs.harvard.edu/abs/2006AJ....131.2755K)).

Thanks, this looks interesting. I'll try to give it a read some time today.

There's some cool work of CO and H2 in damped lyman-alpha systems, which is what I was thinking about in my earlier comment.

tu144
2011-May-17, 11:46 PM
thus they can be excited more easily in colder material.
So this works only for cold environments? If so, how cold?


The CO rotational lines are dipole allowed, which means if they get excited they emit light very quickly.
For H2, the rotational lines are not allowed and thus it is more likely that the molecule will be collisionally de-excited before it can emit the light and thus you don't really see those lines.

What density the H2 molecular cloud has to have in the above scenario?
Is there a density at which lines H2 become stronger than CO lines?

BTW.
Is there any strong connection between cloud temperature and it's density in general?

ngc3314
2011-May-18, 12:46 AM
BTW.
Is there any strong connection between cloud temperature and it's density in general?

I'll respond in a slightly Socratic way. For a gravitationally-bound cloud of given mass, supported only by the internal motions of its atoms or molecules, what relation might you expect between its temperature and density? (The story is more complex involving the fact that most cooling processes occur in two-body collisions, but this piece is important).

tu144
2011-May-18, 09:29 AM
I'll respond in a slightly Socratic way. For a gravitationally-bound cloud of given mass, supported only by the internal motions of its atoms or molecules, what relation might you expect between its temperature and density? (The story is more complex involving the fact that most cooling processes occur in two-body collisions, but this piece is important).
Well, the higher the density, the higher would be the temperature. But there is something odd about it. It's in open space and if the density and temperature are high enough, the volume of the gass should increase, thus lowering the density.
Now, the temperature is proportional to the speed of the molecules. I don't see any reason why their speed should fall down, so the temperature stays the same?

ngc3314
2011-May-18, 11:38 AM
Well, the higher the density, the higher would be the temperature.

No - unlike a typical laboratory situation, the cloud mass but not its volume is fixed, so if the temperature is raised it would expand. Now what happens to the density?

tu144
2011-May-18, 12:21 PM
Falls down with rising temperature?
So the cold molecular clouds with this CO must be highly densed.
Back to the main one. For what temperatures this CO study works well and above what temperatures it stops to work?

ngc3314
2011-May-18, 12:30 PM
Falls down with rising temperature?

Yes. Now add one more important factor strengthening that trend - the rate of energy loss to the cloud rises very rapidly toward higher density. Since the internal energy is carried mainly in the motions of particles, what general relation do you expect between the temperature and density in ISM clouds?

tu144
2011-May-18, 01:09 PM
Yes. Now add one more important factor strengthening that trend - the rate of energy loss to the cloud rises very rapidly toward higher density. Since the internal energy is carried mainly in the motions of particles, what general relation do you expect between the temperature and density in ISM clouds?

The temperature would be inversly proportional to the density.
How it happens that the cloud loses energy faster when it is more dense?
Stars the hotter they are the faster they lose energy, right? What is the difference?
Even assuming the logic for the clouds that more dense they are the shorter the average path between molecules and therefore molecules hit each other more often, emitting photones, the photones would be captured again by surrounding molecules, thus staying within the cloud, right?

ngc3314
2011-May-18, 01:31 PM
Even assuming the logic for the clouds that more dense they are the shorter the average path between molecules and therefore molecules hit each other more often, emitting photones, the photones would be captured again by surrounding molecules, thus staying within the cloud, right?

Not for processes that cool the clouds. The important feature is that at most temperature ranges, there are processes which give off photons at wavelengths that escape the clouds (far-infrared "fine structure" lines from C+ and O, optical forbidden lines such as [O III]). Photons will be trapped in the clouds only if they are in a resonance line - the most important is Lyman alpha of hydrogen which is easily trapped in a a cloud - or if there is enough dust to absorb all the optical and UV radiation. For H-alpha, for example, it can be absorbed only by a hydrogen atom already in the first excited state, which is very rare in the ISM because such a state has a very short decay time.

tu144
2011-May-18, 01:58 PM
Not for processes that cool the clouds. The important feature is that at most temperature ranges, there are processes which give off photons at wavelengths that escape the clouds (far-infrared "fine structure" lines from C+ and O, optical forbidden lines such as [O III]). Photons will be trapped in the clouds only if they are in a resonance line - the most important is Lyman alpha of hydrogen which is easily trapped in a a cloud - or if there is enough dust to absorb all the optical and UV radiation. For H-alpha, for example, it can be absorbed only by a hydrogen atom already in the first excited state, which is very rare in the ISM because such a state has a very short decay time.
It makes sense to me now.
Thanks for the explanation.

tu144
2011-May-18, 05:32 PM
Thread checkpoint.

1. Prohibited lines of emission in ionized nebula spectrum (IM) [PARTLY UNDERSTOOD]
Why prohibited lines of emission show up in ionized nebula spectrum?
Why are they so bright in ionized nebulas?
Why they don't appear in normal conditions on Earth?


(...)Such lines would be strong in the spectra of certain astronomical objects because the physical conditions within those objects selectively populate the pertinent energy levels--that is they appear strong because the transitions you are more used to do not come into play.

What are the physical conditions that selectively populate the pertinent energy levels, and what are the conditions that don't do that? Can someone give examples?
What are these transitions I'm more used to?

2. HII regions movement (IM)
Why gas of a nebulae is set in motion when it gets ionized?
Why most of the HII regions we see have an expansion velocity of about 10 km / s?

3. Spectral lines of CO molecule, molecular clouds in interstellar medium (IM)
Why are these lines so useful for studying molecular clouds? [UNDERSTOOD]

4. Interstellar dust (IM)
What effects produce interstellar dust in the spectrum of nebulae?[UNDERSTOOD]
How can I quantify interstellar extinction from the nebulae spectrum? [UNDERSTOOD]

5. Critical density of an atom level (IM)
What is the critical density of an atom level?
What happens to the intensity of the collisional excitation lines when it exceeds this density?

6. Empirical diagrams in context of Interstellar Medium (IM)
What are they?
What could be the examples of such diagram?

7. Temperature in ionized nebulae from collisional excitation lines(IM)
How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

8. Planetary nebulae vs HII region (IM)
What are the differences between these two?
What are they characteristic spectrums?

9. Formation of molecules in interstellar medium (IM)
What are the different processes of formation of molecules in interstellar medium?
How H2 is formed?

StupendousMan
2011-May-18, 09:33 PM
Thread checkpoint.

1. Prohibited lines of emission in ionized nebula spectrum (IM) [PARTLY UNDERSTOOD]
Why prohibited lines of emission show up in ionized nebula spectrum?
Why are they so bright in ionized nebulas?
Why they don't appear in normal conditions on Earth?

What are the physical conditions that selectively populate the pertinent energy levels, and what are the conditions that don't do that? Can someone give examples?
What are these transitions I'm more used to?


The pertinent difference in this case is density. In the atmosphere of the Earth, an excited molecule or atom has only a very brief time to de-excite by radiating away its energy; if it doesn't radiate the energy in a short time, a collision with another atom/molecule can excite or de-excite the molecule/atom instead.

In an interstellar cloud, the density is so low that collisions don't occur very often. Atoms/molecules have much more time to radiate energy when they have been excited; therefore, some specific energy level changes, which take a long time to occur, may happen in space -- but not on Earth.

tu144
2011-May-19, 04:37 PM
Understood. Thank you.

As we are talking about collisions,
How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

Amber Robot
2011-May-19, 04:53 PM
Understood. Thank you.

As we are talking about collisions,
How to calculate the electron temperature in ionized nebulae from the ratio of intensity of collisional excitation lines?

You really should go check out Osterbrock's book. I think you will find that many of your questions will have detailed answers and you will be able to find them quickly.

tu144
2011-May-19, 05:11 PM
You really should go check out Osterbrock's book. I think you will find that many of your questions will have detailed answers and you will be able to find them quickly.

Don't have it. I'll try to scan thorugh The physics of the interstellar medium by Dyson J., Williams D. and come back afterwards.

I'll be back.

ngc3314
2011-May-19, 05:35 PM
You could look here (http://www.astr.ua.edu/keel/galaxies/emission.html), which includes very brief summaries of these points in Osterbrock's notation. Examples of density- and temperature-sensitive line ratios are given.

tu144
2011-May-22, 03:25 PM
You could look here (http://www.astr.ua.edu/keel/galaxies/emission.html), which includes very brief summaries of these points in Osterbrock's notation. Examples of density- and temperature-sensitive line ratios are given.
I'm on it. Reading it over and over again mixing with some other stuff.
Meanwhile
I have spectral distribution of energy for HII Regions & Planetary Nebulas:

http://img5.imageshack.us/img5/2715/spectrumm.jpg
Now, around 10^13.1 Hz looking in the direction of lower and lower frequencies the lines for HII Regions and Planetary Nebulas split. Emited energy rises for HII and fals down for Planetary Nebulas.
The opposite split happens at 10^13.7 Hz.
What is it?
Why these spectrums are so different?
Both HII regions and Planetary Nebulas are ionized hydrogen.
These are the physical characteristics I have from wikipedia:
Planetary Nebula density~100-10 000 particles/cm^3, T~10,000-25,000K
HII density~0-1 000 000 particles/cm^3; T~10,000K
This doesn't make sense, because both of the clouds can have the same characteristics, yet the spectrum is different.
What I am missing here?

ngc3314
2011-May-22, 06:57 PM
I'm on it. Reading it over and over again mixing with some other stuff.
Meanwhile
I have spectral distribution of energy for HII Regions & Planetary Nebulas:

Now, around 10^13.1 Hz looking in the direction of lower and lower frequencies the lines for HII Regions and Planetary Nebulas split. Emited energy rises for HII and falls down for Planetary Nebulas.
The opposite split happens at 10^13.7 Hz.
What is it?
Why these spectrums are so different?
Both HII regions and Planetary Nebulas are ionized hydrogen.
These are the physical characteristics I have from wikipedia:
Planetary Nebula density~100-10 000 particles/cm^3, T~10,000-25,000K
HII density~0-1 000 000 particles/cm^3; T~10,000K
This doesn't make sense, because both of the clouds can have the same characteristics, yet the spectrum is different.
What I am missing here?

That part of the spectrum doesn't come from the gas itself, but is reradiated from heated dust grains. Cooler grains (longer wavelengths) would mostly be farther from the heating source (star or cluster). Planetary nebulae may run out of dust grains before getting to such low temperatures.

At the short-wavelength end, I have to wonder whether the those data include the ionizing star for planetary nebulae but not for star-forming regions. Dust doesn't get hot enough to be an important contributor in the J band before it evaporates.

tu144
2011-May-25, 11:32 PM
(...)

(...)

(...)

(...)

(...)

Ok, thanks to all of you, and the links you've send me, I have most of my questions answered and understood.
With this two I still have problem:

2. HII regions movement (IM)
Why gas of a nebulae is set in motion when it gets ionized?
Why most of the HII regions we see have an expansion velocity of about 10 km / s?
I've found some documents that say the sound speed is 10km/s in HII regions, but I can't connect it in my mind with expansion.

6. Empirical diagrams in context of Interstellar Medium (IM)
What are they?
What could be the examples of such diagram?
This one is unfindable on the internet, lecture presentations, nowhere. I have no idea what is this stuff.