Toward Planetesimals in the Disk around TW Hydrae: 3.5 Centimeter Dust Emission

We present Very Large Array observations at λ = 3.5 cm of the nearby young star TW Hya that show the emission is constant in time over weeks, months, and years, and spatially resolved with peak brightness temperature ~10 K at ~0.″25 (15 AU) resolution. These features are naturally explained if the emission mechanism at this wavelength is thermal emission from dust particles in the disk surrounding the star. To account quantitatively for the observations, we construct a self-consistent accretion disk model that incorporates a population of centmeter-size particles that matches the long-wavelength spectrum and spatial distribution. A substantal mass fraction of orbiting particles in the TW Hya disk must have agglomerated to centmeter size. These data provide the first clear indication that dust emission from protoplanetary disks may be observed at centmeter wavelengths, and that changes in the spectral slope of the dust emission may be detected, providing constraints on dust evolution and the planet formation process.