The Coupled Physical Structure of Gas and Dust in the IM Lup Protoplanetary Disk

TL;DR

This study uses ALMA observations and modeling to explore the coupled distribution of gas and dust in the IM Lup protoplanetary disk, revealing how dust removal influences gas exposure to radiation and CO distribution.

Contribution

It provides a detailed physical and chemical model of the gas and dust structure in IM Lup, highlighting the impact of dust removal on gas chemistry and external radiation effects.

Findings

Gas extends to 970 AU, dust is truncated at 313 AU.

Outer disk dust removal exposes gas to higher radiation.

Estimated gas-phase CO abundance is 5% of ISM value.

Abstract

The spatial distribution of gas and solids in protoplanetary disks determines the composition and formation efficiency of planetary systems. A number of disks show starkly different distributions for the gas and small grains compared to millimeter-centimeter sized dust. We present new Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the dust continuum, CO, $^{13}$CO, and C$^{18}$O in the IM Lup protoplanetary disk, one of the first systems where this dust-gas dichotomy was clearly seen. The $^{12}$CO is detected out to a radius of 970 AU, while the millimeter continuum emission is truncated at just 313 AU. Based upon this data, we have built a comprehensive physical and chemical model for the disk structure, which takes into account the complex, coupled nature of the gas and dust and the interplay between the local and external environment. We constrain the distributions of gas and dust, the gas temperatures, the CO abundances, the CO optical depths, and the incident external radiation field. We find that the reduction/removal of dust from the outer disk exposes this region to higher stellar and external radiation and decreases the rate of freeze-out, allowing CO to remain in the gas out to large radial distances. We estimate a gas-phase CO abundance of $5\%$ of the ISM value and a low external radiation field ($G_0\lesssim4$). The latter is consistent with that expected from the local stellar population. We additionally find tentative evidence for ring-like continuum substructure, suggestions of isotope-selective photodissociation, and a diffuse gas halo.