Brown Dwarf Minimum Mass

  • #26
jim mcnamara
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New paper: https://arxiv.org/abs/1706.08781

The EBLM project III. A Saturn-size low-mass star at the hydrogen-burning limit

Alexander von Boetticher, Amaury H.M.J. Triaud, Didier Queloz, Sam Gill, Monika Lendl, Laetitia Delrez, David R. Anderson, Andrew Collier Cameron, Francesca Faedi, Michaël Gillon, Yilen Gómez Maqueo Chew, Leslie Hebb, Coel Hellier, Emmanuël Jehin, Pierre F.L. Maxted, David V. Martin, Francesco Pepe, Don Pollacco, Damien Ségransan, Barry Smalley, Stéphane Udry, Richard West
(Submitted on 27 Jun 2017 (v1), last revised 12 Jul 2017 (this version, v2))

We report the discovery of an eclipsing binary system with mass-ratio q ~ 0.07. After identifying a periodic photometric signal received by WASP, we obtained CORALIE spectroscopic radial velocities and follow-up light curves with the Euler and TRAPPIST telescopes. From a joint fit of these data we determine that EBLM J0555-57 consists of a sun-like primary star that is eclipsed by a low-mass companion, on a weakly eccentric 7.8-day orbit. Using a mass estimate for the primary star derived from stellar models, we determine a companion mass of 85±4MJup (0.081M?) and a radius of 0.84+0.14-0.04RJup (0.084R?) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity logg2=5.50+0.03-0.13 and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.

 
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  • #27
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New paper: https://arxiv.org/abs/1706.08781

The EBLM project III. A Saturn-size low-mass star at the hydrogen-burning limit

Alexander von Boetticher, Amaury H.M.J. Triaud, Didier Queloz, Sam Gill, Monika Lendl, Laetitia Delrez, David R. Anderson, Andrew Collier Cameron, Francesca Faedi, Michaël Gillon, Yilen Gómez Maqueo Chew, Leslie Hebb, Coel Hellier, Emmanuël Jehin, Pierre F.L. Maxted, David V. Martin, Francesco Pepe, Don Pollacco, Damien Ségransan, Barry Smalley, Stéphane Udry, Richard West
(Submitted on 27 Jun 2017 (v1), last revised 12 Jul 2017 (this version, v2))

We report the discovery of an eclipsing binary system with mass-ratio q ~ 0.07. After identifying a periodic photometric signal received by WASP, we obtained CORALIE spectroscopic radial velocities and follow-up light curves with the Euler and TRAPPIST telescopes. From a joint fit of these data we determine that EBLM J0555-57 consists of a sun-like primary star that is eclipsed by a low-mass companion, on a weakly eccentric 7.8-day orbit. Using a mass estimate for the primary star derived from stellar models, we determine a companion mass of 85±4MJup (0.081M?) and a radius of 0.84+0.14-0.04RJup (0.084R?) that is comparable to that of Saturn. EBLM J0555-57Ab has a surface gravity logg2=5.50+0.03-0.13 and is one of the densest non-stellar-remnant objects currently known. These measurements are consistent with models of low-mass stars.
I read about that. It was originally thought to be a planet because it was discovered by transiting its parent star so they knew its diameter, they just didn't know how dense it was until later. It is also a good example of why diameter is not included in the criteria for defining stars. Interestingly, they just recently (March 2017) theorized that the smallest mass a star can have is 70.2 Jupiter masses. Now we have observational evidence of a star with only 85 ± 4 Jupiter masses.

The paper states "[t]he mass and radius of J0555-57Ab are consistent with models of a metal-poor, low-mass star", and they give an estimated Fe/H -0.24 ± 0.16 dex (40% to 83% of Sol's metallicity). However, I was unable to find anything about the estimated age of the star.
 
  • #28
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That's true, officially. But all the same, the only people who will care about what happens to a brown dwarf, or a star, when it "dies" are people who will call themselves "stellar astronomers." Those people are likely to call the objects "stars" anyway, simply because their starlike attributes are the reasons they are of interest to them. So those people are never going to care what the official definition of a star or brown dwarf is, they're still going to think of a white dwarf as a kind of star, or a "sub-brown dwarf" as a kind of brown dwarf, all the same.
I think more than just "stellar astronomers" care about the correct application of labels because it is how we communicate. If we want to be understood we have to use labels that everyone understands, or at least are "officially accepted." If we call a single Jupiter mass planet, that has never had any hope of fusing deuterium, a "sub-brown dwarf" just because it happens to be a rogue planet in a cluster that becomes very confusing and is clearly a misnomer.

We don't necessarily have to be able to clearly define something before we label it, but we do have to agree with the label or communication becomes problematic.
 
  • #29
Ken G
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I think more than just "stellar astronomers" care about the correct application of labels because it is how we communicate.
But the point is "we" are many different people, and we communicate many different things. An elementary school teacher has to tell her/his class what a planet is and how many there are. Someone calling themself a "planetary astronomer" most likely has no interest at all in what that teacher is going to say, because they are going to be interested in much more than 8 objects, and they are going to be interested in attributes of those objects that don't come into the formal definition of a "planet." So how we communicate is exactly the point-- the way planetary astronomers communicate about the objects of their interest has very little to do with the official definition of a planet, and the way stellar astronomers communicate has very little to do with the official definition of a star (in particular, the role of fusion will be just as interesting if it is currently happening, as if it is about to start or has recently finished-- no stellar astronomer is going to say "but fusion has finished in that object, so it's not a star any more, so I don't care about it.")
If we want to be understood we have to use labels that everyone understands, or at least are "officially accepted."
And just as important as having those labels, is the need to, quite often, ignore them. That's my point, that's what actually happens in research areas-- the labels are widely ignored because they just aren't helpful in understanding these objects. There may be things we discover about Pluto that help us understand Mercury, and vice versa, and if so, no one is going to care if one is officially a "planet" and the other isn't-- except in those elementary school classrooms where it's going to be the "right answer" to some question the teacher asks.
If we call a single Jupiter mass planet, that has never had any hope of fusing deuterium, a "sub-brown dwarf" just because it happens to be a rogue planet in a cluster that becomes very confusing and is clearly a misnomer.
I agree that is a distinction that might be of no interest to someone studying Jupiter-mass planets, or it might be a distinction that is important to someone interested in using stellar transits to understand the atmosphere. Either way, the labels are sometimes useful, and more often ignored. So we probably do need to have these kinds of labels like "brown dwarf" or "sub-brown dwarf", but we should not imagine the labels are terribly important, or that anyone will pay much attention to them. It's much ado about nothing, really.
We don't necessarily have to be able to clearly define something before we label it, but we do have to agree with the label or communication becomes problematic.
Although that's true, I would tend to focus on how important it is to not rely on labels, given all the stress that currently is aimed at the need for them. No doubt it's nice to have good labels, as they can allow faster communication, but imagining that our labels can really be relied on to replace careful communication can actually create miscommunication. So better to say what you mean in some detail when it is needed, and only use labels when the communication is allowed to be rather imprecise. All you have to do is find some random definition of "star" to see what I mean, it's never going to substitute for really saying what you are talking about.
 
  • #30
Buzz Bloom
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Hi @|Glitch|:

After thinking about the "homework" problem (my post #18) for a while, and unsuccessfully trying to research facts needed to do calculations, I have given up on it. I have located the title, etc., of a recent book which seems promising that it might have some of the facts I need to do the calculation. I have asked my local research librarian to try to borrow a copy for me.

I do have a new related question that has not so far been included in this thread's discussion. From all I have read I think I understand the concept discussed here of creating a definition of a "brown dwarf" in terms of theoretical criteria about the possibility that such an astronomical body can burn deuterium:
H+D->3He.​
Apparently such a definition does not include a requirement that an observation of such a body actually detect the burning of deuterium. It also seems that the definition does not require even the theoretical possibility of making such an observation with current (or expected near future) observational technology.

Q: Does anyone know if it is theoretically possibility to make such an observation with current (or expected near future) observational technology?

Regards,
Buzz
 
  • #31
stefan r
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Hi @|Glitch|:

After thinking about the "homework" problem (my post #18) for a while, and unsuccessfully trying to research facts needed to do calculations, I have given up on it. I have located the title, etc., of a recent book which seems promising that it might have some of the facts I need to do the calculation. I have asked my local research librarian to try to borrow a copy for me.

I do have a new related question that has not so far been included in this thread's discussion. From all I have read I think I understand the concept discussed here of creating a definition of a "brown dwarf" in terms of theoretical criteria about the possibility that such an astronomical body can burn deuterium:
H+D->3He.​
Apparently such a definition does not include a requirement that an observation of such a body actually detect the burning of deuterium. It also seems that the definition does not require even the theoretical possibility of making such an observation with current (or expected near future) observational technology.

Q: Does anyone know if it is theoretically possibility to make such an observation with current (or expected near future) observational technology?

Regards,
Buzz
Measuring fusion in the sun is challenging. Theoretically you could put a neutrino detector inorbit around another star. However, proton to deuterium does not kick out a neutrino.

Easiest measurement is the surface. If there is no deuterium then it is likely that it burned. Taking measurements for a few thousand years should tell you something about the stars internal dynamics.
 
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  • #32
Buzz Bloom
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Taking measurements for a few thousand years should tell you something about the stars internal dynamics.
Hi stefan:

I like your sense of humor.

Regards,
Buzz
 

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