HST survey of ONC in H20 1.4um abs. band. I.

These bandpasses correspond to the 1.4{mu}m H2O absorption feature and an adjacent line-free continuum region. Out of 4504 detected sources, 3352 (about 75%) appear fainter than m130=14 (Vega mag) in the F130N filter, a brightness corresponding to the hydrogen- burning limit mass (M~0.072M_{sun}_) at ~1Myr.

Of these, however, only 742 sources have a negative F130M-F139N 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<~2850K at an assumed 1Myr cluster age. On our color-magnitude diagram (CMD), 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 1Myr old ~3M_Jup_ planetary-mass object under about 2mag of visual extinction.