def select_formed_stars(ds, data, t_min, t_max):
    """
    Select stars formed in a given time range - useful for determining
    'instantaneous' properties of formed stars...
    """
    lifetime = data[('io','particle_model_lifetime')].to('Myr')
    t_o      = data[('io','creation_time')].to('Myr')

    pcut     = (t_o >= t_min) * (t_o <= t_max)

    return pcut