The Initial Mass Function of Low‐Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-ObjectSpectrometer (NICMOS). Combining these data with new ground-based K-bandspectra (R=800) and existing spectral types and photometry and the models ofD'Antona & Mazzitelli, we find that the distributions of ages of comparablesamples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of\~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-massIMFs in these sites of clustered star formation are similar over a wide rangeof stellar densities and other environmental conditions. With current data, wecannot rule out modest variations in the substellar mass functions among theseclusters. We then make the best estimate of the true form of the IMF in theTrapezium by using the evolutionary models of Baraffe et al. and an empiricallyadjusted temperature scale and compare this mass function to recent results forthe Pleiades and the field. All of these data are consistent with an IMF thatis flat or rises slowly from the substellar regime to about 0.6 Msun, and thenrolls over into a power law that continues from about 1 Msun to higher masseswith a slope similar to or somewhat larger than the Salpeter value of 1.35. Forthe Trapezium, this behavior holds from our completeness limit of ~0.02 Msunand probably, after a modest completeness correction, even from 0.01-0.02 Msun.These data include ~50 likely brown dwarfs. We test the predictions of theoriesof the IMF against various properties of the observed IMF.