def lam(m, M):
    """the scaling variable, a.k.a sqrt(chi(0))

    :param m: scalar mass [eV]
    :param M: solition mass [Msun]
    :returns: lambda, the scaling variable [1]

    """
    res = 3.6e-4*(m/(1.e-22))*(M/1.e9)
    return res