def mass_tau(df):
    """
    Returns a calculated meteor mass as derived from using a constant luminous efficiency and the error in this value.

    :param df: dataframe containing all processed DAF data
    :return:
    """
    tau = 0.007
    intg_luminosity = 10 ** (-0.39026606 * df['avg_abs_mag'] + 1.96769668
        ) * 2.5
    mass = 2.0 / (tau * df['vel_avg'] ** 2) * intg_luminosity
    mass_err = abs(4 * tau ** -1 * df['vel_avg'] ** -3 * intg_luminosity
        ) + abs(2 * tau ** -1 * df['vel_avg'] ** -2 * df['intg_luminosity_err']
        )
    return mass, mass_err
