Can a T-tauri star degenerate into a Brown dwarf?

[Florens]

Can T-tauri stars, due to the fact that they loose matter, become a Brown dwarf?

 

[Ken G]

Well, a T Tauri star doesn't usually lose that much mass, and it is a fairly short-lived phase of the star's life so if the star will lose mass, it's more likely to happen much later. But your basic question is true-- if a young star that has enough mass to initiate H fusion should, for some reason, lose a significant amount of that mass, then yes, it could be forced to expand enough to no longer be hot enough to undergo fusion. It would then either be, or soon go, degenerate, and look a lot like a brown dwarf (except for the He it has already fused, so the composition would be a little different). But it's not so easy to make a T Tauri star lose mass. Probably you'd need a close binary companion, and even then the mass loss would be much later on when the T Tauri star enters the main sequence and starts to expand. Then it could overflow its Roche lobe and become, in essence, a brown dwarf.

So your question does bring up an interesting possibility-- if someone observes a brown dwarf in a close binary, they might want to look for evidence that the brown dwarf has a surprising excess of helium, that would be a clear sign it was once a main-sequence star that underwent mass transfer. But there is another issue here-- for the star to be of such low mass that this is a possibility, it will evolve extremely slowly, so there might not have been enough time in the galaxy, or even the entire universe, for that to have happened yet.

 

[Bandersnatch]

I'm not clear on the low-mass cut-off, or whatever other property is used, for when a collapsing object can no longer be considered a TTS. Shouldn't brown dwarfs have a TTS phase as well?

 

[Vanadium 50]

I read the question like @Bandersnatch - do brown dwarfs have a T Tauri phase. And related, where do you draw the line?

 

[Ken G]

Probably just convention, but to be a TTS it has to have H fusion, so it's the same cutoff as the "main sequence."

 

[Bandersnatch]

Wait, does it? I thought there was no fusion throughout the phase.

 

[Ken G]

What I mean is, purely by definition a T Tauri star has to be a star that is about to undergo fusion, even though it hasn't started yet-- it needs a mass larger than the "bottom of the main sequence." Hence, if the question is interpreted as, "do brown dwarfs have a T Tauri phase", the answer is "no, by definition a brown dwarf has insufficient mass to be a main-sequence star, so cannot be a T Tauri star, since the latter must be on its way to H fusion." But, if the brown dwarf started out with enough mass, then it could have been a T Tauri star, but not a brown dwarf. It would have had to transition from a TTS to a BD by losing mass, which is hard to do.

But perhaps the issue is not one of classification, but rather, "can the physics that leads to T Tauri star disks and so forth also occur in brown dwarfs." That's an interesting question also, I don't know what is required for a star to make a disk but I presume brown dwarfs do it too-- even Saturn did!

 

[Florens]

"But, if the brown dwarf started out with enough mass, then it could have been a T Tauri star, but not a brown dwarf. It would have had to transition from a TTS to a BD by losing mass, which is hard to do." that was my question. Is it possible that the tts looses so much mass that he transitions into a bd

 

[Ken G]

The fusion already starts in the protostellar phase. First the Deuterium fusion and than in the end of the protostellar phase the H-fusion. Due to the start of H-Fusion the protostar becomes an pre-main sequence star.

Not quite-- the start of H-fusion ends the pre-main sequence phase and begins the main sequence phase.

And to my question about T-Tauri stars: I have read that Tts can loose about 0,4 sunmasses. So if there is an tts with a mass of f.e 0.47 sunmasses and he looses 0.4 in form of jets and other things; he is under the H-Fusion limit and so an brown dwarf. Could that happen?

The issue of what constitutes mass loss from the T Tauri star is a tricky issue. Once the mass is deep in the potential well of the star, it won't be lost, but there is a lot of gas outside the star that has not officially joined the star yet, and a lot of that can be lost before it ever gets into the star. So what constitutes mass loss from the star itself? A T Tauri star is defined as a star headed to the main sequence, so after all the mass infall and outflow is over, we have to end up with the minimum mass of some 0.1 solar masses. So that's not something that can turn into a brown dwarf without something additional happening, like binary mass transfer. But it's mostly convention-- I think you are more interested in the physics of systems like that, which probably don't much care if the star is going to end up fusing or not.

 

[Vanadium 50]

A T Tauri star is defined as a star headed to the main sequence

That sort of answers the OPs question. But if you allow collapsing gas to a brown dwarf, how do you draw the line between a below-Main Sequence T Tauri and just a plain old blob of gas?

 

[Ken G]

It's the bizarre way a T Tauri star is defined, not as what it is, but what it is going to be! If the physical distinction that will separate it from a brown dwarf hasn't even happened yet, then why should we care? So I think the issue is not so much if one can turn into the other, but rather why we would care to make the distinction anyway, at a phase of its life where the distinction doesn't really matter yet. If we care about the present state, we don't need the distinction, if we care about the future, the distinction makes a huge difference. But in the latter case, one cannot turn into the other, because each has only one future...

 

[Vanadium 50]

Yeah, but you astromers like that.

When Pluto was deplaneted, the "clear their neighborhood" requirement meant that there were objects that at one time weren't planets because they hadn't cleared their neighborhoods, but at a later time would be, because that process had completed.

If I ran the zoo, I'd de-star them and call them "T Tauri Objects". Yes, that would mean T Tauri, despite the name, is not a star (yet). It can join BL Lacertae and Omega Centauri in that respect.

 

[Ken G]

Yeah, but you astromers like that.

When Pluto was deplaneted, the "clear their neighborhood" requirement meant that there were objects that at one time weren't planets because they hadn't cleared their neighborhoods, but at a later time would be, because that process had completed.

True enough. We have a way of using pretty strange definitions sometimes, like even the definition of a "star" often requires there be nuclear fusion going on-- so "white dwarfs" aren't stars, even when they look like what's in the center of this:
https://cdn.britannica.com/57/2957-050-678417DA/Ring-Nebula-Lyra-gases-star-centre.jpg

Tell me that's not a star!

If I ran the zoo, I'd de-star them and call them "T Tauri Objects". Yes, that would mean T Tauri, despite the name, is not a star (yet). It can join BL Lacertae and Omega Centauri in that respect.

In the past when I've looked for the astronomical definition of a "star", it generally mentioned fusion. But I just looked again and I'm not seeing that now (although a top hit says:
"any object that is sufficiently massive that it can ignite the fusion of elements in its core"
https://www.space.com/what-is-a-star-main-sequence
which used to be the norm.) So I would have to say at the moment, the official definition seems pretty unclear on just what a star is, and if it requires fusion or not! But as usual, the active researchers have no difficulty with it, pretty much everything studied by a "stellar astronomer" is a star (including protostars, pre-main-sequence stars, neutron stars, and even white dwarfs), and pretty much anything studied by a "planetary astronomer" is a planet (including moons, and certainly Pluto!).

 

[Vanadium 50]

sufficiently massive

I don't like that. Black holes are not stars. SMBH's are definitely not stars. I would include white dwarfs, and would like a definition that includes "primarily powered by fusion, including fusion that occurred in the past". That would exclude T Tauri obhects and brown dwarfs (and random gas clouds) and I am OK with that.

PS I was complaining about the revised definition of planet being time dependent, and how objects can move in and out o f this category in a sort of "Brownian motion". You shoukd have heard the boos and groans.

 

[Ken G]

It ends up being remarkably difficult to actually define these objects, and even harder to get consensus on the definitions! So it's not at all like mathematics-- in mathematics, you couldn't get anywhere without careful definitions, but astronomy functions better by intentionally leaving these meanings vague. There is generally something about the shared interests of stellar astronomers that defines the kinds of objects they study, so they end up knowing a star by those shared interests, even though they can't tell you precisely what a star is without getting into these difficulties!

 

[Drakkith]

I don't know about the low end, but here's some info on the high end.
Per this link: https://chandra.harvard.edu/edu/formal/stellar_ev/story/index5.html

A T Tauri star is a very young, lightweight star, less than 10 million years old and under 3 solar masses, that it still undergoing gravitational contraction;

Why 3 solar masses as a cutoff? Apparently that's when Young Stellar Objects, YSO's, cease to transition through the Hayashi track, a near-vertical track on the Hertzsprung–Russell diagram signifying a constant temperature but changing luminosity vs time. Above 3 solar masses YSO's move directly to the Henyey track, where the YSO maintains near-constant luminosity but changing temperature vs time. These YSO's are known as Herbig Ae/Be-type stars.

I was only able to find one reference to the lower mass end of the Hayashi track and brown dwarfs, from wikipedia's article on the Hayashi track:

Lower-mass stars follow the Hayashi track until the track intersects with the main sequence, at which point hydrogen fusion begins and the star follows the main sequence. Even lower-mass 'stars' never achieve the conditions necessary to fuse hydrogen and become brown dwarfs.

 

[Ken G]

The low end is the "bottom of the main sequence", typically cited to be at 0.08 solar masses in a lot of places. Below that, electron degeneracy sets in at a lower energy scale than nuclear fusion, preventing the further heat loss and contraction that is necessary to reach fusion temperature.

 

[snorkack]

So:
Do young brown dwarfs, and for the matter young gas giant planets, follow Hayashi track?
Do young brown dwarfs display T Tauri like variability?

 

[Ken G]

I didn't know the answer to that, so for the first time ever, I asked ChatGPT an astronomy question for the purposes of increasing my knowledge. It said:
"Once the brown dwarf's temperature rises to around 2,000 Kelvin, it enters the "Class I" phase, where it emits both infrared and visible light. At this stage, the brown dwarf is still contracting and heating up, and it follows the Hayashi track in the Hertzsprung-Russell (HR) diagram.

As the brown dwarf continues to contract, it becomes denser and hotter. When the temperature reaches around 3,000 Kelvin, the brown dwarf enters the "Class II" phase and begins to emit mainly visible light. At this stage, the brown dwarf has stopped contracting and has reached its final size and mass."

That sounds pretty reasonable to me. I would only use this is a starting point for further investigation, as ChatGPT is far from reliable at this stage, but I certainly don't see any obvious signs this is incorrect. What I do know is that the Hayashi track functions when the surface opacity behaves in such a way as to keep the surface temperature in the general vicinity of 3000 K, which tends to be a property of H minus opacity (the same opacity as at the surface of the Sun, though the Sun's surface is hotter because it reaches a higher luminosity using radiative diffusion than it could with pure convection on the Hayashi track). If the mass is low enough, some other opacity source could take over and a young Jupiter might never be on the Hayashi track.

I didn't know, so for the second time ever, I asked ChatGPT, again to get an answer I did not already know. It said that Jupiter was never on the Hayashi track, but as an example of the issues with ChatGPT, it also said "Jupiter's core accreted a significant amount of gas from the protoplanetary disk, but it never reached the point where it could sustain nuclear fusion and become a star. Therefore, it did not follow the Hayashi track, which refers specifically to the early stages of low-mass star and brown dwarf formation." This is confused and muddled, because we just heard that brown dwarfs were on the Hayashi track, even though they never sustain H fusion either, nor does being on the Hayashi track relate in any direct way with fusion. So one cannot expect logical consistency from ChatGPT, but I do think it plausible that Jupiter was never on the Hayashi track-- though it would take more investigation. (Probably the forum will soon need a policy on use of AI in generating answers, we certainly cannot regard them as authoritative but can be a good place to start the conversation.)

 

[Drakkith]

@Ken G I don't know if it will help, but check out this paper, specifically figure 7 where they show a plot of the luminosity vs time of low mass stars, brown dwarfs, and giant planets.

 

[Ken G]

@Ken G I don't know if it will help, but check out this paper, specifically figure 7 where they show a plot of the luminosity vs time of low mass stars, brown dwarfs, and giant planets.

That paper is certainly everything you could want to know about gas giants! It's hard to tell if they have a Hayashi phase (the central part of the blue curves in Figure 7), but the very early parts of the highest-mass gas giants (and middle parts of highest-mass brown dwarfs), have the same slope as the Hayashi-track stars. So probably it's rare for gas giants to be on the Hayashi track, but maybe young high-mass ones. After all, planets are not self-luminous in the optical for very long.