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Thread: Distinguishing an IK Pegasi

  1. #1
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    Distinguishing an IK Pegasi

    From Wikipedia:
    The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about 5 108 km (3 AU) in the case of the pulsating AGB star Mira.[30] This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope. As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.[5]

    Suppose there were two evolved stars at 10 pc from Earth at identical stages, except one was alone and the other was like IK Pegasi of long ago.
    Could we tell anything unusual about the latter?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  2. #2
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    In the early phase, you have a peanut-shaped star, which would be evident in the luminosity curve unless the orbital plane was in the plane of the sky.
    In the late phase, the in-spiral is releasing as much energy as the fusion processes of the stars, so we'd see a freakishly large star, and then a luminous red nova.

    Grant Hutchison

  3. #3
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    Why didn't IK Pegasi turn into a blue straggler?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Why didn't IK Pegasi turn into a blue straggler?
    To be a blue straggler, it needs to have something to straggle behind - that is, it has to be part of a cluster of stars of the same age, characterized by a main sequence turn-off point. Is it a member of a cluster? I can't seem to find any suggestion that it is.

    Grant Hutchison

  5. #5
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    No, no, Grant.
    I meant that AFAIK Blue stragglers are results of luminous red novae that are single stars that merged together.
    Why is IK Pegasi a binary star instead of one bigger star?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  6. #6
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    Quote Originally Posted by Tom Mazanec View Post
    No, no, Grant.
    I meant that AFAIK Blue stragglers are results of luminous red novae that are single stars that merged together.
    Why is IK Pegasi a binary star instead of one bigger star?
    1) It still needs to be part of a cluster with a main-sequence turn-off in order to qualify as a blue straggler.
    2) I haven't seen any evidence that a merger is required to produce a blue straggler, rather than simply mass transfer (which is what we see in IK Pegasi).
    3) IK Pegasi apparently avoided a merger by blowing off the common envelope before it got as far as the luminous red nova stage.

    Grant Hutchison

  7. #7
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    I think it's a bit odd that the wiki says that since the orbital separations are within the red giant radius, it means there has to have been a common envelope phase. There are many stars that have the former situation without the latter, such as the Algol binary. You just get Roche lobe overflow, but no common envelope. To have a common envelope, you need the mass transfer to occur so fast that the star receiving the mass is unable to remain within its own Roche lobe, and gas spills out into the common envelope. But presumably there are other reasons to think IK Peg has been through a common envelope phase, perhaps evidence of a large loss of angular momentum (removed when the stars were dragging through the common envelope and then the envelope was ejected). As for elemental abundance anomalies, there are so many different ways to get that it's hard to believe a common envelope phase could be established that way. The bottom line is, most of what we know about stars works well in statistical bunches of stars, but we cannot with much certainty point to a particular star and say with any precision what it's evolution history has been. We don't even know how much mass leaves a system as mass transfer occurs.

  8. #8
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    I see a lot of text suggesting that all, or almost all, close compact binary systems must have gone through a common envelope phase. "Compact" in this setting is white dwarf / neutron star / black hole, but I can't find a definition of "close", which seems absolutely critical to the associated narrative.

    Grant Hutchison

  9. #9
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    Right, it's not even completely clear what is the boundary between going supernova, or creating a white dwarf. And as for type Ia supernovae, it's not known if it's even possible to get that with mass transfer-- it might require direct merger. It's like being a detective and trying to piece together a crime just from the evidence left behind, without any witnesses to ask.

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