PDA

View Full Version : Looks and fate of a brown subdwarf



chornedsnorkack
2009-Feb-27, 07:50 AM
What would a brown subdwarf look like, if any existed?

The Sun radiates light as a black body because, at 6000 K, it is hot enough that quite some of the hydrogen is ionized or excited so that there are allowed emission lines. You need 13,6 eV to get a free electron, and then H- ion... about 20 eV to excite helium, 10,2 eV to get Lyman alpha, well over 10,2 eV to get other Lymal nines and Balmer etc. series, still quite some energy to ionize or excite a hydrogen molecule.

Brown dwarfs containing metals like carbon tend to have lines of methane. Methane has internal vibrations that possess dipole moment and therefore can radiate and absorb infrared.

But what about brown subdwarf?

Once it is cool enough that hydrogen is no longer ionized or excited, it has no way to radiate and no way to absorb visible or infrared - it absorbs no electromagnetic waves less energetic than far UV Lyman alpha and whatever the lowest excitation level of diprotium is. Diprotium does have vibrational and rotational excitations, but those have no dipole moment and therefore hydrogen can neither absorb nor emit IR.

The one metal found in brown subdwarfs is lithium (absent in the highest mass brown subdwarfs, and red subdwarfs). But the small quantities of lithium should at low temperatures form solid lithium hydride and sink to the interior. Then again, the outer layers of the subdwarf must be transparent for the IR radiation of the hot interior.

In even lower mass brown subdwarfs, deuterium should occur. HD has a very small but nonzero dipole moment, thus it should be able to absorb and emit very small amounts of IR at its vibrational frequencies.

Would a brown subdwarf cool over time, seeing as it is not a black body and therefore cannot radiate electromagnetic waves?

EDG
2009-Feb-27, 08:07 AM
They should have helium in them too, right? And they'd still be generating heat through gravitational contraction (like Jupiter is). Though I don't really follow what you're saying - are you suggesting that somehow they wouldn't emit their heat? I can't really imagine how that could be possible - wouldn't it just convect up to the surface where it can radiate from there?

WayneFrancis
2009-Feb-27, 08:08 AM
I can't wait for the response on this one. I should learn a thing or 2.

chornedsnorkack
2009-Feb-27, 08:13 AM
They should have helium in them too, right?
Yes. Actually, I mentioned that. Lowest excitation level at far UV, 20 eV or so. Absolutely no vibrations or rotations possible.

And they'd still be generating heat through gravitational contraction (like Jupiter is). Though I don't really follow what you're saying - are you suggesting that somehow they wouldn't emit their heat? I can't really imagine how that could be possible - wouldn't it just convect up to the surface where it can radiate from there?

How can it radiate from the surface, seeing that there are no spectral lines whereby the surface could emit or absorb electromagnetic waves?

EDG
2009-Feb-27, 08:17 AM
That just makes no sense to me. Hydrogen can obviously be warmed and cooled at lower temperatures (below 1000K), or we wouldn't be able to do in in labs here - so why does it need to have emission lines to do it? All the heat coming from normal BDs isn't coming just from the carbon and other metals in it.

chornedsnorkack
2009-Feb-27, 08:20 AM
That just makes no sense to me. Hydrogen can obviously be warmed and cooled at lower temperatures (below 1000K), or we wouldn't be able to do in in labs here - so why does it need to have emission lines to do it?

We do this by molecule-molecule collisions, or molecule-solid surface collisions. Not by radiation. Infrared and visible radiation pass through hydrogen without warming it, and hot hydrogen does not emit radiated heat before it is extremely hot.

EDG
2009-Feb-27, 08:35 AM
We do this by molecule-molecule collisions, or molecule-solid surface collisions. Not by radiation. Infrared and visible radiation pass through hydrogen without warming it, and hot hydrogen does not emit radiated heat before it is extremely hot.

So why can't it just convect from the inside until it reaches a surface layer that is cool enough to radiate? What does hot hydrogen do with the heat energy before it gets to the point where it can radiate it?

chornedsnorkack
2009-Feb-27, 08:46 AM
So why can't it just convect from the inside until it reaches a surface layer that is cool enough to radiate?

Because the surface layer is too cool to radiate. Hot hydrogen can radiate because there are excited states like free electrons, H- ions, excited hydrogen atoms. The surface layer, however, is too cool for those things.

grant hutchison
2009-Feb-27, 12:24 PM
And yet cool bodies still manage to radiate approximations to a black-body spectrum (I'm doing it now).

Adam Burrows has written extensively (http://www.astro.princeton.edu/~burrows/pub-html/adam_bp.html) on the chemical properties, spectra, and temperature histories of brown dwarfs.
There's a good treatment in his Theory of brown dwarfs and extrasolar giant planets (http://arxiv.org/abs/astro-ph/0103383).

Grant Hutchison

chornedsnorkack
2009-Feb-27, 12:40 PM
And yet cool bodies still manage to radiate approximations to a black-body spectrum (I'm doing it now).

Because you have vibrational excitations that have nonzero dipole moment and therefore can absorb and radiate in infrared. Like O-H bonds in water.

But the problem is that diprotium does not have a dipole moment and therefore CANNOT absorb nor emit infrared. In order to radiate as a black body, a substance must first be black, and absorb as black body. Which diprotium cannot do, because it is transparent and clear for both visible and infrared.

neilzero
2009-Feb-27, 01:11 PM
I don't understand much of the question, but Jupiter has about 1% of the mass of a sub dwarf star, and radiates in far and medium IR = infrared. Jupiter also absorbs energy throughout the electromagnetic spectrum. My guess is all 4 of our gas giants have a heat source we have not identified; we don't know for sure that they are still cooling, or shrinking. Sub dwarf stars are born about as hot as class M stars. Both cool for a billion years or so, until they appear orange or pink according to an other thread. Sub dwarf stars likely continue to cool, but very slowly, so likely none are invisible to a far infrared telescope, if they are close enough. Neil

chornedsnorkack
2009-Feb-27, 01:19 PM
I don't understand much of the question, but Jupiter has about 1% of the mass of a sub dwarf star, and radiates in far and medium IR = infrared. Jupiter also absorbs energy throughout the electromagnetic spectrum.
Yes, because Jupiter contains metals. Notably carbon. Methane is a gas at the temperatures of Jupiter (does not precipitate out), and it has C-H bonds that are able to absorb and emit infrared.

Methane also is able to absorb small amounts of visible light. Compare with water, which is a blue liquid. Red light can get through thin layers of water, but thick layers of water absorb red and yellow light, while blue remains. This is because the overtones of O-H bond vibration extend into visible red and provide feeble absorption there.

We do not see hydrogen or helium lines in the spectrum of Jupiter.

grant hutchison
2009-Feb-27, 01:45 PM
But the problem is that diprotium does not have a dipole moment and therefore CANNOT absorb nor emit infrared. In order to radiate as a black body, a substance must first be black, and absorb as black body. Which diprotium cannot do, because it is transparent and clear for both visible and infrared.My point is that many other species are also present. See the references I provided.
Unless you are invoking some sort of Population III brown dwarf, your "brown subdwarf" will have some level of metallicity which will allow radiation.

Grant Hutchison

WayneFrancis
2009-Feb-27, 11:06 PM
Because the surface layer is too cool to radiate. Hot hydrogen can radiate because there are excited states like free electrons, H- ions, excited hydrogen atoms. The surface layer, however, is too cool for those things.

ummm but if the inside is hot it will radiate outward. The molecules will move near by molecules. The heat won't just stay in the core.

chornedsnorkack
2009-Feb-28, 09:49 PM
ummm but if the inside is hot it will radiate outward. The molecules will move near by molecules. The heat won't just stay in the core.

Yes, but the problem is that the surface cannot radiate.

However, clear gases do cause Rayleigh scattering.

EDG
2009-Feb-28, 11:03 PM
Yes, but the problem is that the surface cannot radiate.

However, clear gases do cause Rayleigh scattering.

Well put it this way - what do you think it would look like? You seem to be telling us what it isn't, so tell us what you think it is! How do you think a substellar, low metallicity object is supposed to behave if it can't radiate anything?

timb
2009-Mar-01, 01:14 AM
What would a brown subdwarf look like, if any existed?

The Sun radiates light as a black body because, at 6000 K, it is hot enough that quite some of the hydrogen is ionized or excited so that there are allowed emission lines. You need 13,6 eV to get a free electron, and then H- ion... about 20 eV to excite helium, 10,2 eV to get Lyman alpha, well over 10,2 eV to get other Lymal nines and Balmer etc. series, still quite some energy to ionize or excite a hydrogen molecule.

Brown dwarfs containing metals like carbon tend to have lines of methane. Methane has internal vibrations that possess dipole moment and therefore can radiate and absorb infrared.

But what about brown subdwarf?

Once it is cool enough that hydrogen is no longer ionized or excited, it has no way to radiate and no way to absorb visible or infrared - it absorbs no electromagnetic waves less energetic than far UV Lyman alpha and whatever the lowest excitation level of diprotium is. Diprotium does have vibrational and rotational excitations, but those have no dipole moment and therefore hydrogen can neither absorb nor emit IR.

The one metal found in brown subdwarfs is lithium (absent in the highest mass brown subdwarfs, and red subdwarfs). But the small quantities of lithium should at low temperatures form solid lithium hydride and sink to the interior. Then again, the outer layers of the subdwarf must be transparent for the IR radiation of the hot interior.

In even lower mass brown subdwarfs, deuterium should occur. HD has a very small but nonzero dipole moment, thus it should be able to absorb and emit very small amounts of IR at its vibrational frequencies.

Would a brown subdwarf cool over time, seeing as it is not a black body and therefore cannot radiate electromagnetic waves?

I'm not sure what your point is. Obviously brown subdwarfs cool over time. Just search for "ultracool subdwarf" in your favorite astrophysics paper database. Eventually they will get "cold" and stop radiating. Your claim that a brown subdwarf cannot radiate electromagnetic waves is wrong even if your claim the surface isn't radiating were true. What happens to the radiation coming from the core?

chornedsnorkack
2009-Mar-02, 06:58 PM
So the outer layers would cool only till they recombine and become transparent. Further cooling would be hindered...

If the outer layers of the brown dwarf are unable to absorb or emit radiation, but do cause Rayleigh scattering, the effect of Rayleigh scattering should be much the same as that of geometric scattering due to refraction in clouds. High albedo, white body in incident light.

timb
2009-Mar-02, 10:10 PM
So the outer layers would cool only till they recombine and become transparent. Further cooling would be hindered...

If the outer layers of the brown dwarf are unable to absorb or emit radiation, but do cause Rayleigh scattering, the effect of Rayleigh scattering should be much the same as that of geometric scattering due to refraction in clouds. High albedo, white body in incident light.

I don't know what a brown extreme subdwarf looks like, but a transparent atmosphere is usually associated with a very low albedo. Your suggestion that BDs stop radiating when their surface temperature drops to 1000K or some similar temperature isn't right. When that happens the core will still be much hotter and radiating across a broad spectrum. This radiation will get scattered, absorbed and re-emitted until it reaches the surface and escapes, so the BD is still radiating.

I don't think that your claims about the emissions of H are right either.

10,2 eV to get Lyman alpha, well over 10,2 eV to get other Lymal nines and Balmer etc. series

The further you go down the series the lower the energy. eg the Paschen lines (http://en.wikipedia.org/wiki/Hydrogen_lines#Paschen_series_.28n.27_.3D_3.29) are all in the IR. H can also radiate in the microwave band via hyperfine transitions (http://en.wikipedia.org/wiki/21_cm_line). Lastly molecular H2 is only a component of a BD's atmosphere. It exhibits a pressure induced dipole under sufficient pressure. The interior is various states of liquid or solid metallic hydrogen, some of which may have dipole moments.

chornedsnorkack
2009-Mar-03, 05:04 PM
I don't think that your claims about the emissions of H are right either.

10,2 eV to get Lyman alpha, well over 10,2 eV to get other Lymal nines and Balmer etc. series

The further you go down the series the lower the energy. eg the Paschen lines (http://en.wikipedia.org/wiki/Hydrogen_lines#Paschen_series_.28n.27_.3D_3.29) are all in the IR.
So what?
Balmer lines are emitted when hydrogen atom goes from higher excited states to first excited state. They should be accompanied by radiation of Lyman alpha (first excited state to ground state) and other Lyman lines. Therefore, if the gas is not hot enough to excite hydrogen atoms to excited states, they should not appear. And Balmer lines are only absorbed by hydrogen in first excited state. Cold hydrogen (with all atoms in ground state) should be completely unable to absorb Balmer lines. Same logic goes for Paschen series - no IR radiation, unless hydrogen is so hot that it also radiates in far UV Lyman series.


H can also radiate in the microwave band via hyperfine transitions (http://en.wikipedia.org/wiki/21_cm_line). Lastly molecular H2 is only a component of a BD's atmosphere. It exhibits a pressure induced dipole under sufficient pressure.

Ah, so there is that as well.

What is the pressure induced dipole spectrum of hydrogen like?