HST survey of the Orion Nebula Cluster in the H${}_2$O 1.4 $\hat{I}¼$m absorption band: I. A census of substellar and planetary mass objects

Abstract

In order to obtain a complete census of the stellar and sub-stellar population, down to a few M${}_{Jup}$ in the $\sim 1$ 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 $1.4\hat{I}¼$m H${}_2$O absorption feature and an adjacent line-free continuum region. Out of $4,504$ detected sources, $3,352$ (about $75\%$) appear fainter than m${}_{130}=14$ (Vega mag) in the F130N filter, a brightness corresponding to the hydrogen-burning limit mass (M$\simeq 0.072M_{\odot }$) at $\sim 1$ Myr. Of these, however, only $742$ sources have a negative F130M-139N color index, indicative of the presence of H${}_2$O vapor in absorption, and can therefore be classified as bona-fide M and L dwarfs, with effective temperatures T$\lesssim 2850$ K at an assumed $1$ Myr cluster age. On our color-magnitude diagram, this population of sources with H${}_2$O 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, m${}_{130}\simeq 21.5$, corresponding to a $1$ Myr old $\simeq 3$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 $\sim 1$M${}_{Jup}$) fail to reproduce the observed H${}_2$O color index at M$\lesssim 20$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.

Accepted for publication in the Astrophysical Journal. The resolution of several figures has been downgraded to comply with the size limit of arXiv submissions