We know what a nova is, but how?

  • I work with astrophysicists and require some basic knowledge of many astronomical sources, however research priorities often demand that most of human knowledge on a subject be taken for granted.



    I am currently researching galactic novae, and I find it relevant to summarize their history briefly when presenting my research to certain audiences. Unfortunately, I am unable to find any source material which describes how we know one key aspect of the events: that they are an accreting white dwarf in a stellar binary. This fact appears to be so well founded that no scientific paper feels obligated to cite it when stated, but basic resources like astronomical encyclopedia also make no reference that I've seen.



    How do we know that novae are binary systems?



    E.g., have follow-up observations clearly identified the white dwarf and its companion? Or do other astronomical measurements strongly confirm this binary hypothesis (and make it all but obviously true)? I apologize if it's as simple as "someone looked through a telescope, and it was pretty obvious" -- in my experience no revelation in astrophysics is nearly so simple, but certainly this could be the case.


    Good question. I've noticed that there are some things for which there's a clear consensus, but when you dig down through the papers, the "foundation" is elusive.

  • rob

    rob Correct answer

    4 years ago

    Following a reference to Darley et al., ApJ 746, 61 (2012) from your Wikipedia link gives a (very technical) discussion of nova progenitors, including distinctions between nova systems where the secondary stars are main sequence or supergiant stars, and distinctions among white dwarfs with different chemistries.
    The first sentence of that paper is




    A classical nova (CN) outburst occurs in an interacting
    binary system comprising a white dwarf (WD, the primary) and
    typically a late-type main-sequence (MS) star (the secondary)
    that fills its Roche lobe (Crawford & Kraft, 1956).




    That suggests the 1956 paper is the original proposal for the Roche overflow model of the classical nova.
    Like many original-idea papers, it's a pretty clear read.
    But for your question, Crawford and Kraft seem to hedge about whether the "blue star" in their particular pair needs to be a white dwarf:




    [T]he
    observed
    luminosity
    of
    the
    blue
    star
    is
    essentially
    due
    to
    the
    energy
    released
    by
    the
    accreted
    material.
    This
    view
    is
    strengthened
    also
    by
    the
    fact
    that
    the
    blue
    star
    occupies
    a
    peculiar
    position
    in
    the
    H-R
    diagram.
    It
    lies
    10.5
    vis.
    mag.
    below
    the
    main
    sequence
    but
    about
    4
    mag.
    above
    the
    most
    luminous
    white
    dwarfs,
    whose
    effective
    temperature
    it
    exceeds
    by
    about
    8000°
    K.
    Unless
    the
    blue
    star
    is
    essentially
    degenerate,
    it
    can
    readily
    be
    shown
    that
    the
    small
    radius
    implies
    such
    a
    high
    internal
    temperature
    that
    electron
    scattering
    is
    the
    principal
    source
    of
    opacity.
    A
    simple
    calculation
    based
    on
    the
    standard
    model
    then
    yields
    a
    luminosity
    8
    mag.
    brighter
    than
    is
    observed.




    In other words, Crawford and Kraft don't come out and say "definitely a WD," but if it's a non-degenerate star, it's a very strange one.
    More modern observations of novae are compared to detailed models of the dynamics of the surface dynamics, models which have been debated vigorously for decades; the current generation of comparisons to data are sensitive to details like the amount of helium accumulating on the white dwarf's surface during the nova event. It seems unlikely that such details could even come close if the underlying assumptions about the basic physics of the erupting star were wrong.



    Note that a classical nova system can be thought of as a type of contact binary star.
    For any reasonable estimate of the size of the giant star, a distance of 10 AU between the two members of the pair seems like an overlarge estimate.
    Ten astronomical units of separation viewed from a distance of 50 parsecs is already a gap of 0.1 seconds of arc. I wouldn't expect to see visible-light photographs showing both the giant star and the white dwarf, but rather that all of the information about the binary systems comes from spectroscopy.


License under CC-BY-SA with attribution


Content dated before 7/24/2021 11:53 AM

Tags used