import numpy as np
import yt

from yt.data_objects.particle_filters import \
     particle_filter

@particle_filter("star", requires=["creation_time"])
def _dm_filter(pfilter, data):
    dt = data.ds.quan(20, "Myr")
    return data['creation_time'] >= data.ds.current_time - dt

def _sfr_density(field, data):
    dt = data.ds.quan(20, "Myr")
    return data['deposit', 'star_density'] / dt

if __name__ == "__main__":
    ds = yt.load("DD0046/DD0046")
    ds.add_particle_filter('star')
    ds.add_field(('gas', 'sfr_density'),
                 function=_sfr_density,
                 units='Msun/yr/pc**3',
                 sampling_type='cell')