HST survey of the Orion Nebula Cluster in the H₂O 1.4 µm absorption band: I. A census of substellar and planetary mass objects

In order to obtain a complete census of the stellar and sub-stellar population, down to a few M_Jup in the ∼ 1 Myr old Orion Nebula Cluster, we used the infrared channel of the Wide Field Camera 3 of the Hubble Space Telescope with the F139M and F130N filters. These bandpasses correspond to the 1.4 µm H₂O absorption feature and an adjacent line-free continuum region. Out of 4,504 detected sources, 3,352 (about 75%) appear fainter than m₁₃₀ = 14 (Vega mag) in the F130N filter, a brightness corresponding to the hydrogen-burning limit mass (M ≃ 0.072 M⊙) at ∼ 1 Myr. Of these, however, only 742 sources have a negative F130M-139N color index, indicative of the presence of H₂O vapor in absorption, and can therefore be classified as bona-fide M and L dwarfs, with effective temperatures T ≲ 2850 K at an assumed 1 Myr cluster age. On our color-magnitude diagram, this population of sources with H₂O absorption appears clearly distinct from the larger background population of highly reddened stars and galaxies with positive F130M-F139N color index, and can be traced down to the sensitivity limit of our survey, m₁₃₀ ≃ 21.5, corresponding to a 1 Myr old ≃ 3 M_Jup planetary mass object under about 2 magnitudes of visual extinction. Theoretical models of the BT-Settl family predicting substellar isochrones of 1, 2 and 3 Myr down to ∼ 1 M_Jup fail to reproduce the observed H₂O color index at M ≲ 20 M_Jup. We perform a Bayesian analysis to determine extinction, mass and effective temperature of each sub-stellar member of our sample, together with its membership probability.