the emission azimuthally, however no significant emission is found. In §\[dis:gas\], we use this total flux upper limit to estimate an upper limit on the CO gas mass that could be present in the disc.

Similarly, we search for HCN emission, finding no significant emission. Based on this non detection we place a $3\sigma$ upper limit of 91 mJy km s$^{-1}$, which we also use in §\[dis:gas\] to constrain its abundance in planetesimals in this system. HCN is of particular interest for exocometary studies as, besides being abundant in Solar System comets [@Mumma2011], it has recently been suggested that it could play a key role for prebiotic chemistry in habitable planets [@Patel2015; @Sutherland2017].

Modelling {#sec:model}
=========

In this section we model the data using parametric models to constrain the density distribution of solids in the system (§\[sec:parmodel\]), and N-body simulations of a planet embedded in a planetesimal disc tailored to HD 107146 to constrain the mass and orbit of a putative planet carving the observed gap (§\[sec:nbody\]). In both approaches we model the central star as a G2V type star with a mass of 1 $M_{\sun}$, an effective temperature of 5750 K and a radius of 1 $R_{\sun}$.

Parametric model 