Gas Mass Tracers in Protoplanetary Disks: CO is Still the Best

Abstract

Protoplanetary disk mass is a key parameter controlling the process of planetary system formation. CO molecular emission is often used as a tracer of gas mass in the disk. In this study we consider the ability of CO to trace the gas mass over a wide range of disk structural parameters and search for chemical species that could possibly be used as alternative mass tracers to CO. Specifically, we apply detailed astrochemical modeling to a large set of models of protoplanetary disks around low-mass stars, to select molecules with abundances correlated with the disk mass and being relatively insensitive to other disk properties. We do not consider sophisticated dust evolution models, restricting ourselves with the standard astrochemical assumption of 0.1 µm dust. We find that CO is indeed the best molecular tracer for total gas mass, despite the fact that it is not the main carbon carrier, provided reasonable assumptions about CO abundance in the disk are used. Typically, chemical reprocessing lowers the abundance of CO by a factor of 3, compared to the case of photo-dissociation and freeze-out as the only ways of CO depletion. On average only 13% C-atoms reside in gas-phase CO, albeit with variations from 2 to 30%. CO 2 , H 2 O and H 2 CO can potentially serve as alternative mass tracers, the latter two being only applicable if disk structural parameters are known.

A Three-phase Approach to Grain Surface Chemistry in Protoplanetary Disks: Gas, Ice Surfaces, and Ice Mantles of Dust Grains

“…This efficient conversion of sCO to sCO 2 has also been reported in several recent studies, e.g. Furuya & Aikawa (2014), Aikawa et al (2015), Reboussin et al (2015), Molyarova et al (2017), Eistrup et al (2016), Bosman et al (2018).…”

The First Detection of 13C17O in a Protoplanetary Disk: A Robust Tracer of Disk Gas Mass

“…The inconsistencies in these mass measurements and g/d from different models may be explained by trying to recover the gas density structure with optically thick lines. CO remains the best and most accessible tracer of mass that we have for disks (Molyarova et al 2017), but robust lower limits to the gas mass can only be made by targeting the most optically thin isotopologues ( 12 C 17 O, 13 C 18 O, and 13 C 17 O).…”

Unlocking CO Depletion in Protoplanetary Disks. II. Primordial C/H Predictions inside the CO Snowline

“…There is currently some debate in regards to whether disks inherent their initial abundances from the ISM or if the chemistry is at least partially reset by heating during collapse. However, after several million years of chemical evolution, models with atomic initial abundances have a very similar composition to models with molecular initial abundances (Eistrup et al 2018;Molyarova et al 2017). More generally, a reduction in the initial amount of H 2 O ice will slow down the conversion of CO to CO 2 ice.…”