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Thread: The Dividing Line Between Red Dwarfs and Brown Dwarfs

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    The Dividing Line Between Red Dwarfs and Brown Dwarfs

    Here's how cool a star can be and still achieve lasting success: 1500 to 1700 K.

    by Ken Croswell

    The motions of nearby stars reveal the dividing line between red dwarfs and brown dwarfs.

    Read more at Science News

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    This implies there is a population of currently undetectable old brown dwarfs.

    I'm not too sure about this, because I seem to recall reading the galaxy isn't old enough for this to be true?

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    Quote Originally Posted by kzb View Post
    This implies there is a population of currently undetectable old brown dwarfs.

    I'm not too sure about this, because I seem to recall reading the galaxy isn't old enough for this to be true?
    The universe is not old enough for black dwarfs, the burnt out remains of white dwarfs. Brown dwarfs are defined by size and temperature, not age. There can be young brown dwarfs.
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    Quote Originally Posted by kzb View Post
    This implies there is a population of currently undetectable old brown dwarfs.

    I'm not too sure about this, because I seem to recall reading the galaxy isn't old enough for this to be true?
    As Noclevername alluded to, it sounds like you are confusing white dwarfs and brown dwarfs. White dwarfs are what’s left of a star that was on the main sequence, but isn’t massive enough to produce a neutron star or black hole. They are very dense, hot, electron degenerate objects around the diameter of earth or smaller, but with the mass of around that of the sun. It takes many billions of years to radiate that heat away from a small surface, and the universe isn’t nearly old enough for them to get cold.

    Brown dwarfs are much less massive objects that can fuse deuterium, but deuterium is rare, and more massive ones get a bit more interesting but ultimately they can’t sustain fusion for long and most of their heat comes from gravitational contraction, a limited source. Older ones in today’s universe can be quite cool by now and they aren’t nearly as dense as white dwarfs, and tend to have a diameter similar to Jupiter’s (very hot ones can be a bit larger).

    Cool objects less massive than red dwarfs are pretty easy to miss if they are any significant distance from us.

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    The article is basically saying all observable brown dwarfs are young.

    That means there is a population of undetected old brown dwarfs out there.

    I am questioning it because I've never heard that before. I thought they had a good handle on how many brown dwarfs there are.

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    Quote Originally Posted by kzb View Post
    The article is basically saying all observable brown dwarfs are young.
    Not sure where in the article you are getting that from.

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    Quote Originally Posted by glappkaeft View Post
    Not sure where in the article you are getting that from.
    My bold:

    the orbital paths of stars can reveal their approximate age.

    Most red dwarfs are fairly old; their predicted lifetimes are far longer than the current age of the universe. But because brown dwarfs cool and fade, any that are still warm are young.

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    Quote Originally Posted by kzb View Post
    My bold:

    the orbital paths of stars can reveal their approximate age.

    Most red dwarfs are fairly old; their predicted lifetimes are far longer than the current age of the universe. But because brown dwarfs cool and fade, any that are still warm are young.
    Well, of course that says the same thing we previously discussed - older brown dwarfs are cool. But that doesn’t say all observable brown dwarfs are young. Do you have a quote for that? The thing is, cooler and older brown dwarfs have been observed, but only relatively close ones - they are hard to observe, to be sure. Even cool sub-brown dwarfs have been discovered. See:

    https://en.wikipedia.org/wiki/WISE_0855%E2%88%920714

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    Let me add that in our search for planets we have also observed (detected) many old brown dwarfs that orbit other stars.
    Forming opinions as we speak

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    Quote Originally Posted by Van Rijn View Post
    Well, of course that says the same thing we previously discussed - older brown dwarfs are cool. But that doesn’t say all observable brown dwarfs are young. Do you have a quote for that? The thing is, cooler and older brown dwarfs have been observed, but only relatively close ones - they are hard to observe, to be sure. Even cool sub-brown dwarfs have been discovered. See:

    https://en.wikipedia.org/wiki/WISE_0855%E2%88%920714
    Yeah when you put it like that it kind of makes sense. Is it the case then that there is an unbiased census of nearby brown dwarfs including the old and cold ones?

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    Quote Originally Posted by kzb View Post
    Yeah when you put it like that it kind of makes sense. Is it the case then that there is an unbiased census of nearby brown dwarfs including the old and cold ones?
    By coincidence, there just happens to be a UT article touching on this right now, and a paper - a discovery of a cool brown dwarf, just barely detectable, and with implications on the brown dwarf population (substantially fewer brown dwarfs have been discovered than would be expected). “The accident” has an odd spectra suggesting it has little methane which in turn suggests it is so old that there was little carbon in the medium that formed it. It gives astronomers a clue to look for objects with its spectral signature which may lead them to find more old brown dwarfs. Stay tuned.

    UT article:
    https://www.universetoday.com/152373...usly-believed/

    Paper (pdf):
    https://iopscience.iop.org/article/1...213/ac0437/pdf

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    Oh, just as a note, the UT article has some mistakes that I noticed and mentioned them here:

    https://forum.cosmoquest.org/showthr...22#post2545922

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." — Abraham Lincoln

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    This is from the introduction of the paper linked above:

    “The coldest brown dwarfs are a difficult population to characterize. What little flux they emit is concentrated near 5 μm, and, because this region is largely unobservable from the ground, space-based missions offer the best chance of discovery and follow-up. Studies at other wavelengths are hampered by intrinsically faint magnitudes. For known Y dwarfs—the coldest brown dwarfs with effective temperatures below ∼450K (Cushing et al. 2011)—absolute J-band magnitudes range from 19.4 to 28.2 mag (Kirkpatrick et al. 2021).

    Despite these difficulties, the number of known, very cold brown dwarfs is slowly growing, with 50 confirmed or suspected Y dwarfs now cataloged (Kirkpatrick et al. 2021).”

    (Bolding added) So currently there are 50 cold (and therefore old) brown dwarfs cataloged (though some of these may be below the cutoff mass for actual brown dwarfs, so called sub-brown dwarfs).

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." — Abraham Lincoln

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    Quote Originally Posted by Van Rijn View Post
    By coincidence, there just happens to be a UT article touching on this right now, and a paper - a discovery of a cool brown dwarf, just barely detectable, and with implications on the brown dwarf population (substantially fewer brown dwarfs have been discovered than would be expected). “The accident” has an odd spectra suggesting it has little methane which in turn suggests it is so old that there was little carbon in the medium that formed it. It gives astronomers a clue to look for objects with its spectral signature which may lead them to find more old brown dwarfs. Stay tuned.

    UT article:
    https://www.universetoday.com/152373...usly-believed/

    Paper (pdf):
    https://iopscience.iop.org/article/1...213/ac0437/pdf
    Thanks, that is interesting. Notice this new object also has a high transverse velocity, which is related to the non-circular orbit argument in the OP article.

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