Multiple Rings of Millimeter Dust Emission in the HD 15115 Debris Disk

We present observations of the HD 15115 debris disk from ALMA at 1.3 mm that capture this intriguing system with the highest resolution ($0.\!\!^{\prime\prime}6$ or $29$ AU) at millimeter wavelengths to date. This new ALMA image shows evidence for two rings in the disk separated by a cleared gap. By fitting models directly to the observed visibilities within a MCMC framework, we are able to characterize the millimeter continuum emission and place robust constraints on the disk structure and geometry. In the best-fit model of a power law disk with a Gaussian gap, the disk inner and outer edges are at $43.9\pm5.8$ AU ($0.\!\!^{\prime\prime}89\pm0.\!\!^{\prime\prime}12$) and $92.2\pm2.4$ AU ($1.\!\!^{\prime\prime}88\pm0.\!\!^{\prime\prime}49$), respectively, with a gap located at $58.9\pm4.5$~AU ($1.\!\!^{\prime\prime}2\pm0.\!\!^{\prime\prime}10$) with a fractional depth of $0.88\pm0.10$ and a width of $13.8\pm5.6$ AU ($0.\!\!^{\prime\prime}28\pm0.\!\!^{\prime\prime}11$). Since we do not see any evidence at millimeter wavelengths for the dramatic east-west asymmetry seen in scattered light, we conclude that this feature most likely results from a mechanism that only affects small grains. Using dynamical modeling and our constraints on the gap properties, we are able to estimate a mass for the possible planet sculpting the gap to be $0.16\pm0.06$ $M_\text{Jup}$.