def krueger12_eta(lpc):
    """Ratio of 56Ni to total iron-peak yield from Krueger+ 2012

    Fitting formula for K12 central density results.  Based on looking at
    iron-peak yields from Khokhlov, Mueller & Hoflich 1993, I assign a
    flat prior eta = 0.9 below a central density of 1e+9 g/cm^3.
    Could probably do better by incorporating other results e.g. from the
    MPA group (Seitenzahl+ 2013).  Input lpc (log10 of central density),
    output eta = MNi/(MNi + MFe).
    """
    pc9 = 1e-9 * 10**lpc
    return min(0.95, 0.95 - 0.05*pc9), max(0.025, 0.03*pc9)