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Turbulence is the leading candidate for angular momentum transport inprotoplanetary disks and therefore influences disk lifetimes and planetformation timescales. However, the turbulent properties of protoplanetary disksare poorly constrained observationally. Recent studies have found turbulentspeeds smaller than what fully-developed MRI would produce (Flaherty et al.2015, 2017). However, existing studies assumed a constant CO/H2 ratio of 0.0001in locations where CO is not frozen-out or photo-dissociated. Our previousstudies of evolving disk chemistry indicate that CO is depleted byincorporation into complex organic molecules well inside the freeze-out radiusof CO. We consider the effects of this chemical depletion on measurements ofturbulence. Simon et al. (2015) suggested that the ratio of the peak line fluxto the flux at line center of the CO J=3-2 transition is a reasonablediagnostic of turbulence, so we focus on that metric, while adding someanalysis of the more complex effects on spatial distribution. We simulate theemission lines of CO based on chemical evolution models presented in Yu et al.(2016), and find that the peak-to-trough ratio changes as a function of time asCO is destroyed. Specifically, a CO-depleted disk with high turbulent velocitymimics the peak-to-trough ratios of a non-CO-depleted disk with lower turbulentvelocity. We suggest that disk observers and modelers take into account thepossibility of CO depletion when using line peak-to-trough ratios to constrainthe degree of turbulence in disks. Assuming that CO/H2 = 0.0001 at all diskradii can lead to underestimates of turbulent speeds in the disk by at least0.2 km/s.

astrophysical journal/the astrophysical journal

The Effects of Protostellar Disk Turbulence on CO Emission Lines: A  Comparison Study of Disks with Constant CO Abundance vs. Chemically Evolving  Disks