The long wavelength view of GG Tau A: Rocks in the Ring World

Abstract

We present the first detection of GG Tau A at centimeter-wavelengths, made with the Arcminute Microkelvin Imager Large Array (AMI-LA) at a frequency of 16 GHz ({\lambda} = 1.8 cm). The source is detected at > 6 {\sigma}_{rms} with an integrated flux density of S = 249+/-45 {\mu}Jy. We use these new centimetre-wave data, in conjunction with additional measurements compiled from the literature, to investigate the long wavelength tail of the dust emission from this unusual proto-planetary system. We use an MCMC based method to determine maximum likelihood parameters for a simple parametric spectral model and consider the opacity and mass of the dust contributing to the microwave emission. We derive a dust mass of approximately 0.1 solar masses, constrain the dimensions of the emitting region and find that the opacity index at {\lambda} > 7mm is less than unity, implying a contribution to the dust population from grains exceeding 4 cm in size. We suggest that this indicates coagulation within the GG Tau A system has proceeded to the point where dust grains have grown to the size of small rocks with dimensions of a few centimetres. Considering the relatively young age of the GG Tau association, in combination with the low derived disk mass, we suggest that this system may provide a useful test case for rapid core accretion planet formation models.