The Time Evolution of Md/M? in Protoplanetary Disks as a Way to Disentangle between Viscosity and MHD Winds

As the classic viscous paradigm for protoplanetary disk accretion is challenged by the observational evidence of low turbulence, the alternative scenario of MHD disk winds is being explored as being potentially able to reproduce the same observed features traditionally explained with viscosity. Although the two models lead to different disk properties, none of them has been ruled out by observations-mainly due to instrumental limitations. In this work, we present a viable method to distinguish between the viscous and MHD framework based on the different evolution of the distribution in the disk mass (M-d )-accretion rate ( M? ) plane of a disk population. With a synergy of analytical calculations and 1D numerical simulations, performed with the population synthesis code Diskpop, we find that both mechanisms predict the spread of the observed ratio M-d/M? in a disk population to decrease over time; however, this effect is much less pronounced in MHD-dominated populations compared with purely viscous populations. Furthermore, we demonstrate that this difference is detectable with the current observational facilities: we show that convolving the intrinsic spread with the observational uncertainties does not affect our result, as the observed spread in the MHD case remains significantly larger than in the viscous scenario. While the most recent data available show a better agreement with the wind model, ongoing and future efforts to obtain direct gas mass measurements with Atacama Large Millimeter/submillimeter Array and next-generation Very Large Array will cause a reassessment of this comparison in the near future.