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B Brown Dwarf Minimum Mass

  1. Jul 8, 2017 #1
    It has been my understanding that a brown dwarf was defined by when an object has achieved sufficient mass to begin deuterium fusion. Much like a star is defined by when it has achieved sufficient mass to begin hydrogen fusion. The minimum mass to begin fusing deuterium is estimated to be approximately 13 Jupiter masses. However, after reading a paper recently published in Astronomy & Astrophysics, it made references to "giant planets" (not brown dwarfs) with as much as 20 Jupiter masses. Therefore, I am left wondering how a giant gas planet can be greater than 13 Jupiter masses and not be of sufficient mass to begin the deuterium fusion process.

    Or is it that once a brown dwarf has fused all of its deuterium it becomes a giant gas planet?

    Observational evidence for two distinct giant planet populations - Astronomy & Astrophysics, Volume 603, July 2017 (free preprint)
  2. jcsd
  3. Jul 8, 2017 #2
    Just guessing, but expect the density of deuterium present would make a difference.
    A planet 12x Jupiter might initiate fusion if deuterium is present as a large percentage of it's total mass,
    whereas as 20x Jupiter planet might not if it is lacking an abundance of deuterium.
  4. Jul 8, 2017 #3
    Hi rootone:

    I am confused about deuterium being a major component of any star, except perhaps very old 1st generation stars, but I don't think brown dwarfs are in that category.
    23 atoms of deuterium per million hydrogen atoms in undisturbed gas clouds, which is only 15% below the WMAP estimated primordial ratio of about 27 atoms per million from the Big Bang.​
    Can you provide a link that explains how this could happen given the rarity of deuterium?

  5. Jul 8, 2017 #4
    Was just following from the OP question.
    I agree it's unlikely that a gas giant planet a few times the mass of Jupiter could form which is very deuterium rich and therefore would easily go to fusion.
    Not impossible though.
  6. Jul 9, 2017 #5
    A likely reply is that deuterium fusion is just one proposed, but not universally accepted, definition of brown dwarf. Therefore some astronomers can, and do, term more massive objects as "planets".
  7. Jul 9, 2017 #6
    In essence then, it is only a brown dwarf so long as it is fusing deuterium and/or lithium. If the brown dwarf runs out of this material (which it will in just a few hundred million years), then it ceases to be a brown dwarf and becomes a giant gas planet instead.
    Last edited: Jul 9, 2017
  8. Jul 9, 2017 #7
    What other criteria could there be for a brown dwarf if not deuterium and/or lithium fusion?
  9. Jul 9, 2017 #8
    Could the temperature stay below 106 K? Give enough time for the planet to radiate the heat of formation. Then add more mass.

    A binary planet could tidal lock and then cool off. Exchanged gas would be closer to the surface and more able to radiate heat. A lot of the impact energy would be preserved as rotational energy. Would still be toasty warm but I am not sure how to calculate the temperature.
  10. Jul 10, 2017 #9
    If the temperature was below 106 K wouldn't it also mean that the object is smaller than 13 Jupiter masses? I suppose that is the crux of my question: How can an object with more than 13 Jupiter masses maintain an internal temperature that is less than 106 K?

    If, as you suggest in your example, two objects were to eventually collide and their combined mass becomes greater than 13 Jupiter masses, then it should be of sufficient mass to begin deuterium fusion, and a brown dwarf would be born. At least temporarily. After a few million years all the deuterium will be consumed and the now giant gas planet will slowly continue to cool.

    It really isn't that different from a main sequence star, which becomes something else when it ceases to fuse hydrogen. Depending on the mass of the star, it will be either a white dwarf, neutron star, or black hole upon its "death." So it makes sense that a brown dwarf can only be a brown dwarf so long as it has an internal core temperature between approximately 106 K and 107 K.

    This would also mean than brown dwarfs greater than 20 Jupiter masses would eventually become giant gas planets after their internal core temperature drops below 106 K. So there could be giant gas planets with ~69 Jupiter masses floating around out there radiating very little heat. What a scary thought. :wideeyed:
  11. Jul 10, 2017 #10
    I believe the definitions are based on spectra. So if your "thing" does not have deuterium lines then it is a brown dwarf. The low mass brown dwarfs should have lithium.

    The temperature should drop. The surface gases would move toward equilibrium with background radiation. Well below 100K in our neighborhood. Wikipedia puts Jupiter's core temperature at about 36000°K.

    I am not sure but I believe brown dwarfs would cool off somewhat slower but a similar time scale. Metals conduct heat faster. The atmosphere in a brown dwarf will be thinner than Jupiter's.

    There could be a lot of them.
  12. Jul 11, 2017 #11

    Ken G

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    One possibility is that the planet has simply not contracted enough yet to fuse deuterium. For example, Saturn is still contracting now. Saturn will never fuse deuterium, but it shows that a planet might take awhile to finish contracting, so we might call something a giant planet even if it is destined in the future to be a brown dwarf. The same is actually true of stars-- for some silly reason, a star is not regarded as a star if fusion is in its future or its past, it must be in its present. I would never have defined one that way, but it's what you usually find (though not in the Wiki definition). In fact, the definitions of stars and brown dwarfs is even more of a mess than the definitions of planets, it is fortunate that we can study these things without really being capable of defining them clearly.
  13. Jul 12, 2017 #12
    Except that it is the mass and density of an object that determines its internal core temperature. Neither Jupiter nor Saturn could ever fuse deuterium because they lack the mass and density to achieve an internal core temperature of between 106 °K and 107 °K. While giant gas planets may indeed continue contracting for billions of years after they form, their mass does not change significantly.

    I agree with you, upon further reflection. Brown dwarfs are only brown dwarfs while they are fusing deuterium and/or lithium. If the object does not have an internal core temperature between 106 °K and 107 °K, then it cannot be a brown dwarf. This internal core temperature could happen upon formation, or afterwards if something were to alter the mass or change the object's density - such as a collision, for example. Once there is sufficient mass and density to create temperatures in the 106 °K and 107 °K range, then a brown dwarf is born. After the brown dwarf has fused all of its deuterium and lithium in a few million years and begins to cool off, then it ceases to be a brown dwarf once the internal core temperature drops below 106 °K.

    Currently we are making discoveries faster than we can accurately define them. I do agree that we need better definitions, but I also know that those definitions will come eventually. I just hope those definitions are based upon science rather than politics. The Pluto debacle is an example of politics winning out over science. Science doesn't give a damn how many planets are in a solar system, only the politically correct are concerned about such matters.
  14. Jul 12, 2017 #13

    Ken G

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    But don't forget about the most common cause of a rising core temperature-- gravitational contraction.
    Yes, you're right, so you could not only have a gas giant turn into a brown dwarf as it contracts, you could also have a brown dwarf turn back into a gas giant after it runs out of deuterium. If the brown dwarf definition works like the star definition, where many sources require it to have current fusion, not just fusion in the past or future.
    Believe it or not, I think the problem goes deeper-- there are many things people study that really escape a clear definition, and as such, there may never be one. And that turns out not to be that important-- what bonds stellar astronomers, and planetary astronomers, is their common interests, much more than any particular definition of stars or planets. That's why stellar astronomers will say "white dwarf star" without batting an eye, because when they do that, they simply mean "the aspects of white dwarfs that unify them with earlier phases of life when they would have been more formally considered a star", more so than "this is an object that is officially a star by some clear definition." Similarly, the people who specialize in understanding Pluto are always going to call themselves "planetary astronomers" who study "planets", and will even be heard to talk about the "planet Pluto," notwithstanding the formal definition. Really the formal definition is for the popular media, which needs answers to questions like "how many planets are in our solar system"-- planetary astronomers have little need to answer a question like that, it's not what interests them.
  15. Jul 12, 2017 #14
    Hi Glitch:

    I have been trying (unsuccessfully) to find sources that talk about brown dwarfs fusing deuterium (D) and/or lithium (Li). Since the abundances of D (27 parts per million)
    and Li ( 6 parts per billion)
    seem so extremely small, it seems very unlikely that any brown dwarf would have enough D and/or Li to make fusion of them happen.

    From (underlining is mine)
    They are, however, thought to fuse deuterium (2H) and to fuse lithium (7Li) if their mass is above a debated[4] threshold of 13 MJ and 65 MJ, respectively.[2]
    I looked at the references [2] and [4]
    and found no discussion of fusing D and Li.

    I would much appreciate it if anyone can post a link to a reference that discusses why it is "thought" that a brown dwarf might fuse D and/or Li.

  16. Jul 12, 2017 #15
    This was from a few years ago:

    The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets - The Astrophysical Journal, Volume 727, Number 1, January 2011 (free issue)

    They put the minimum mass for deuterium fusion at ~(13.0 ± 0.8) Jupiter masses if you use the same metallicity as our sun. They suggest that an object with as little as ~11.0 Jupiter masses could also begin deuterium fusion if it had three times the metallicity of Sol.

    Furthermore, there was a recent study that suggests brown dwarfs cannot be any larger than 6.7% the mass of the sun, or 70.2 Jupiter masses.

    Individual Dynamical Masses of Ultracool Dwarfs - arXiv free preprint, March 2017
  17. Jul 12, 2017 #16
    Believe it or not, I came across this working group definition from the IAU:
    Source: The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets - The Astrophysical Journal, Volume 727, Number 1, January 2011 (free issue)

    With "sub-brown dwarfs" being thrown into the mix they are really blurring the lines between a planet and a brown dwarf. [​IMG]

    Attached Files:

  18. Jul 12, 2017 #17
    Does it mean that a comet that has been accelerated past escape speed of Solar System is a sub-brown dwarf?
  19. Jul 12, 2017 #18
    Hi Glitch:

    Thank you very much for the link. The article did discuss the assumptions about deuterium abundance that were the same as the ones I quoted. However, I still feel uncomfortable about the article's conclusions since there was no discussion about how much energy the D fusion would produce and how much the temperature of the brown dwarf would increase from this produced energy. I guess I should accept this as a "homework problem". If I am able to calculate an answer I will post it.

    BTW: The article for second link you posted did not include the word "deuterium".

  20. Jul 12, 2017 #19

    Ken G

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    Gold Member

    Yes, the introduction of "sub-brown dwarfs" is exactly the problem you run into if you seek precise definitions. It's tricky, because one does like to have precise terms when possible, but then, what really matters is that people who are interested in similar phenomena have similar objects to talk about. Those people don't care about arbitrary definitions, they only care about the similar phenomena! So if you care about the interior of Jupiter, for example, you really don't care if it is in a solar system and can be called a "giant planet", or if it is free-floating and should be called a "sub-brown dwarf," that distinction is just of no value to you.
  21. Jul 12, 2017 #20
    The second link I posted dealt with stars up to type M7 and T5 brown dwarfs and discusses lithium depletion. Its purpose was to determine the minimum mass for when hydrogen fusion begins. So it was only interested in objects with 65 Jupiter masses or greater.
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