Is there a difference between an ultracool dwarf and a brown dwarf？

[ZX.Liang]

After reading the introduction of ultracool dwarf and brown dwarf in Wikipedia, I can't see the difference between them.
Besides, do they all belong to the main sequence stars?

 

[256bits]

I found this.
You can read more about from Kurkpatrick it at
https://articles.adsabs.harvard.edu//full/1998ASPC..134..405K/0000405.000.html
if that makes any more sense to you ( or me ).
First time I have heard of ultra cool.
thanks for beinging it up.

I think all brown dwarfs are in the ultra cool class, but not all ultra cool are brown dwarfs.

PS.
One should post the site(s) visited to which you are referencing.
A complete opening post shows a reference for others to 'reference.'
Wiki, though, is easy to find.

 

[ZX.Liang]

I found this.
You can read more about from Kurkpatrick it at
https://articles.adsabs.harvard.edu//full/1998ASPC..134..405K/0000405.000.html
if that makes any more sense to you ( or me ).
First time I have heard of ultra cool.
thanks for beinging it up.

I think all brown dwarfs are in the ultra cool class, but not all ultra cool are brown dwarfs.

PS.
One should post the site(s) visited to which you are referencing.
A complete opening post shows a reference for others to 'reference.'
Wiki, though, is easy to find.

Thank you.
The paper is very helpful.

Two Wikipedia pages.

Ultra-cool dwarfs:
https://en.wikipedia.org/wiki/Ultra-cool_dwarf

Brown dwarf:
https://en.wikipedia.org/wiki/Brown_dwarf

 

[Ken G]

For my own part, I prefer to think in terms of physical differences of objects, moreso than spectral classifications-- though I know the latter generally is an essential step in achieving the former. So for me, the key question is, what kinds of different physics can happen near the bottom of the main sequence that we might want to think about, and how can that different physics connect with spectral differences?

I don't know the answer, but I think a key type of new physics that "ultra-cool dwarfs" connect with is the possibility of having a very low-mass dwarf that is not fully convective. The usual idea is, pre-main-sequence stars start out fully convective, because they are very large and even though they are red, their luminosity is too high (because of their huge surface area) to be carried radiatively, so convection has to pick up the slack. But as the star contracts, its luminosity drops, and its interior also gets hotter, so for both those reasons, radiative diffusion becomes better able to carry the load without as much help from convection. So stars like the Sun become less convective before they get to the main sequence, and stay that way.

But not the cool dwarfs. It turns out that radiative diffusion is very sensitive to the star mass, so low-mass stars are not very good at having radiation carry their luminosity. Hence "red dwarfs" stay fully convective all the way to the main sequence, and stay that way even once fusion initiates. However, and this is the interesting thing about "ultra-cool dwarfs" on the main sequence, as H is converted into He, radiation becomes better able to carry the luminosity. (What happens is a little counterintuitive-- the star expands because it literally has fewer particles in it, so it must reach a lower overall pressure scale and hence a weaker gravity scale, and a more expanded star at the same fusion T carries more photons in it, which carry the luminosity as they diffuse out, and they might encounter less opacity to block them as well.) The bottom line is, at some point during the conversion of H to He, radiative diffusion might be able to carry the luminosity of the star (I don't know just when), so the star is no longer fully convective.

This changes the evolution of the star, because if the core is not convective, it starts to pile up He in the center. It also means the entire star is not able to be converted into He, as would hold if the star stayed fully convective. I'm not sure what all the evolutionary ramifications are (and note they have not been observed because these stars have not had time to do this in the entire age of the universe), but it certainly makes for a very interesting object! (The sources you cite talk about evolving into "blue dwarfs", for example, but I'm not sure exactly how that happens, there must be quite a lot of interesting physics in these objects that the universe has never yet seen!) Thank you for bringing attention to these fascinating objects.

 

[ZX.Liang]

For my own part, I prefer to think in terms of physical differences of objects, moreso than spectral classifications-- though I know the latter generally is an essential step in achieving the former. So for me, the key question is, what kinds of different physics can happen near the bottom of the main sequence that we might want to think about, and how can that different physics connect with spectral differences?

Thank you for providing so much interesting knowledge.

 

[Ken G]

To delve a bit deeper, the Wiki on "blue dwarfs" (https://en.wikipedia.org/wiki/Blue_dwarf_(red-dwarf_stage) )
says that ultracool dwarfs are thought to stay convective while on the main sequence, but they are not restricted (by the usual rules of the "Hayashi track") to stay red, because their surface opacity does not act like the Hayashi track usually does (since they are so dense). A simple scaling law says that to keep the core T at fusion levels, as the number of particles in the star N drops with time (as He is converted to H), the star must expand (roughly with radius proportional to 1/N). So while the surface T stays the same (which would be the Hayashi-like thing to do), the L rises like 1/N^2, so maybe a factor of about 4 while on the main sequence.

But this is where there seems to be considerable potential for the interior to shift to radiative diffusion rather than convection. I would think the radiative luminosity is able to rise like 1/N^7 (it comes from 1/N^3 due to the increasing volume of the light bucket, and 1/N^4 due to the diffusion time falling with Kramers-like opacity with a largely unchanging convective temperature structure. The Wiki statement comes come from https://onlinelibrary.wiley.com/doi/10.1002/asna.200510440, though it might require running down additional sources to get the reasons why, so it is well supported. But I note they are saying the surface T late in the main sequence goes up significantly, more than a factor of 2. So with the necessary R rise like 1/N, that's about a factor of 4 in surface area, which together with the more-than-factor-2 increase in surface T making the surface blue, it implies the L increases like a factor of 100-- which is precisely what I would expect if convection ceases and the luminosity is carried radiatively!

But even though radiation is getting closer to being able to carry that L, it seems that it never quite makes it. So apparently the star does stay fully convective the entire main sequence, but gets blue because the surface opacity does not behave in the Hayashi-type way that keeps the surface of red stars (like red giants) red. This makes ultracool dwarfs sound a whole lot like garden variety red dwarfs that have a bit different behavior of their surface opacity that makes them ultimately get a bit blue in few hundred billion years from now.