A Hubble Space Telescope Search for Substellar Companions in the Young Cluster IC 348^*

We present the results of a search for substellar companions to members of the star-forming cluster IC 348. Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have obtained deep, high-resolution images of the cluster through the F791W and F850LP filters. These data encompass 150 known members of IC 348, including 14 primaries that are likely to be substellar (M₁ = 0.015-0.08 M_☉). The detection limits for companions to low-mass stars and brown dwarfs in the PC images are Δm₇₉₁ = 0, 2.5, and 5.5 at separations of 005, 01, and 03, respectively, which correspond to M₂/M₁ = 1, 0.3, and 0.1 at 15, 30, and 90 AU. Meanwhile, for heavily saturated solar-mass primaries in the WFC images, the limits are Δm₇₉₁ = 0 and 6 (M₂/M₁ = 1 and 0.04) at 02 and 04. The sky limiting magnitude of m₇₉₁ ~ 26 at large separations from a primary corresponds to a mass of ~0.006 M_☉ according to the evolutionary models of Chabrier and Baraffe. Point sources appearing near known and candidate cluster members are classified as either field stars or likely cluster members through their positions on the color-magnitude diagram constructed from the WFPC2 photometry. For the two faintest candidate companions appearing in these data, we have obtained 0.8-2.5 μm spectra with SpeX at the IRTF. Through a comparison to spectra of optically classified dwarfs, giants, and pre-main-sequence objects, we classify these two sources as cluster members with spectral types near M6, corresponding to masses of ~0.1 M_☉ with the models of Chabrier and Baraffe. Thus, no probable substellar companions are detected in this survey. After considering all potential binaries within our WFPC2 images, we find that the frequencies of stellar and substellar companions within 04-5'' (120-1600 AU) from low-mass stars (M₁ = 0.08-0.5) in IC 348 agree within the uncertainties with measurements in the field. The factor of ~3-10 deficiency in brown dwarfs relative to stars among companions at wide separations in IC 348 and across the much larger range of separations probed for field stars in previous work is equal within the uncertainties to the deficiency in brown dwarfs in measurements of mass functions of isolated objects. In other words, when defined relative to stars, the brown dwarf "desert" among companions is also present among isolated objects, which is expected if stellar and substellar companions form in the same manner as their free-floating counterparts. Meanwhile, among the 14 substellar primaries in our survey of IC 348, no companions are detected. This absence of wide binary brown dwarfs is statistically consistent with the frequency of wide binary stars in IC 348.