How Large Is a Disk—What Do Protoplanetary Disk Gas Sizes Really Mean?

The Astrophysical Journal, vol. 954, pp. 41

Abstract

Abstract It remains unclear what mechanism is driving the evolution of protoplanetary disks. Direct detection of the main candidates, either turbulence driven by magnetorotational instabilities or magnetohydrodynamical disk winds, has proven difficult, leaving the time evolution of the disk size as one of the most promising observables able to differentiate between these two mechanisms. But to do so successfully, we need to understand what the observed gas disk size actually traces. We studied the relation between R CO,90%, the radius that encloses 90% of the 12CO flux, and R c , the radius that encodes the physical disk size, in order to provide simple prescriptions for conversions between these two sizes. For an extensive grid of thermochemical models, we calculate R CO,90% from synthetic observations and relate properties measured at this radius, such as the gas column density, to bulk disk properties, such as R c and the disk mass M disk. We found an empirical correlation between the gas column density at R CO,90% and disk mass: N gas ( R CO , 90 % ) ≈ 3.73 × 10 21 ( M disk / M ⊙ ) 0.34 cm − 2 . Using this correlation we derive an analytical prescription of R CO,90% that only depends on R c and M disk. We derive R c for disks in Lupus, Upper Sco, Taurus, and the DSHARP sample, finding that disks in the older Upper Sco region are significantly smaller (〈R c 〉 = 4.8 au) than disks in the younger Lupus and Taurus regions (〈R c 〉 = 19.8 and 20.9 au, respectively). This temporal decrease in R c goes against predictions of both viscous and wind-driven evolution, but could be a sign of significant external photoevaporation truncating disks in Upper Sco.

Authors 4

1. Leon Trapman corresponding

   University of Wisconsin–Madison

   Affiliation as printed

   Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA; ltrapman@wisc.edu

   Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA

2. Giovanni P. Rosotti Aachen Leiden Observatory

   Leiden University · University of Leicester · University of Milan · Leiden Observatory

   Affiliation as printed

   Dipartimento di Fisica, Università degli Studi di Milano, Via Giovanni Celoria, 16, I-20133 Milano, Italy

   Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands

   School of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

3. Ke Zhang

   University of Wisconsin–Madison

   Affiliation as printed

   Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA; ltrapman@wisc.edu

   Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA

4. Benoît Tabone Aachen Leiden Observatory

   Leiden University · Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands

   Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, F-91405 Orsay, France

   Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, F-91405 Orsay, France