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2018 Hot Sci @ STScI

The Milky Way IMF Down to Planetary Masses, Observed in Real Time: Near-Infrared Observations of the Orion Nebula Cluster

Presented by: Mario Gennaro  (Space Telescope Science Institute)
Category: Science Colloquia   Duration: 30 minutes   Broadcast date: August 01, 2018
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8/1/18 Mario Gennaro (STScI) Title: The Milky Way IMF Down to Planetary Masses, Observed in Real Time: Near-Infrared Observations of the Orion Nebula Cluster Abstract: We exploit the Hubble Space Telescope’s location above the Earth’s atmosphere, to observe near-infrared water absorption bands in low-mass stars, brown dwarfs and planetary mass objects. Using H20 diagnostics, we create a very pure sample of low-mass Orion Nebula Cluster (ONC) members, not affected by background stars and galaxies contamination which lack water absorption. Thanks to these data we measure the Initial Mass Function (IMF) of the ONC in the $0.005 - 1.4$M$_{\odot}$ mass regime, i.e. from Solar, down to few Jupiter masses. The young age of the ONC allows us to obtain a real snapshot if the "initial" outcome of star formation for the present day conditions (metallicity, temperature, pressure) of typical Milky Way disk molecular clouds. We demonstrate that the IMF of the ONC is well described by either a log-normal or a broken power-law model, with parameters close to the canonical Chabrier or Kroupa IMF forms. Details on the parameter values depend on the adopted stellar models, as well as on the assumed star formation history and binary fraction. However we strongly rule out any large deviation from the canonical IMFs at planetary masses. Our study contradicts findings based on broad-band near-infrared ground-based photometry, which likely suffer from unaccounted background contamination. According to such findings an extraordinary large number of planetary mass objects should exist in the ONC, a number that we do not observe in our deeper, more pure ONC catalog.