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Brown dwarfs and close binary systems

  1. Jan 30, 2016 #1
    (I honestly don't know whether this is Intermediate or Advanced. I'm guessing the latter since I don't know the answers and I've done second year astrophysics, but maybe I'm just an idiot.)

    Let's say you've got a high mass (~60 Jupiter masses) brown dwarf in relatively close orbit around a true star. Let's further say that it's an F or G star that eventually climbs the Red Giant Branch.

    1) Can that actually happen, or is there some effect that forbids it?

    If the brown dwarf is in a close enough orbit, it will presumably be engulfed by the expanding envelope of its companion.

    2) Is a brown dwarf's gravity sufficient to accrete matter from a red giant envelope, or would it ablate?
    3) Presumably the drag from the envelope would cause the brown dwarf's orbit to inspiral. Over what sort of timescale would that occur?
    4) If the brown dwarf would, in fact, accrete, is it possible for that accretion to ignite fusion? Could it happen before the inspiral caused it to merge with the star's inner layers?
    5) If it did ignite, what would the result be? Would it become a red dwarf star in its own right, or would it explode in a runaway reaction like that in a type-Ia supernova? Or would there be some other result?

    Thanks in advance.
  2. jcsd
  3. Jan 31, 2016 #2


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    There's all sorts of binary systems which have been observed. There's two stars of roughly the same mass and spectral type; there's one giant and one smaller companion star; there's a normal star and either a white dwarf or neutron star in orbit around one another; just about any combination you can imagine. Some of these stars orbit so close that material from one star is transferred to the companion; some stars are so far apart that it may take decades to complete one orbit. It is possible for one star to be engulfed by its dying companion if they are orbiting close enough. In some binaries, a large companion may have gone supernova while the other star was still on the main sequence; that's one mechanism to get a neutron star orbiting a regular star.

    A lot of the questions in your post cannot be answered in a general fashion. When two stars merge, it is unlikely that one or both will not be affected by this process. Remember, stars are just balls of hot gas. If you put a strong enough gravitational source near by one, its envelope can deform in extreme fashion.
  4. Jan 31, 2016 #3


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    A super jupiter would still be massively out massede by any ordinary star. It would fare the worse in any close binary situation, It is difficult to imagine a scenario where it could accrete enough mass to self ignite.
  5. Jan 31, 2016 #4


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  6. Jan 31, 2016 #5
    I am aware that answers will vary with parameters. That is why I gave the mass of the brown dwarf and the approximate mass of the primary. I am also aware that dense bodies can accrete matter from giant stars' expanded envelopes, but was unsure if a brown dwarf (being significantly less dense than a true compact stellar object such as a white dwarf, neutron star, or black hole) was dense enough to do so.

    But if you want more specifications, let's say the primary's a population II star of 1 solar mass and the orbit is 0.8 AU.

    I have read it already. It did not answer my question.
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