THE COUPLED PHYSICAL STRUCTURE OF GAS AND DUST IN THE IM Lup PROTOPLANETARY DISK

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 4). 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.

First evidence of external disc photoevaporation in a low mass star forming region: the case of IM Lup

“…Interior to some outer disc radius R d we do not allow the conditions to evolve over time. For these fixed disc conditions we use the parameters derived by Cleeves et al (2016). The disc's gas surface density profile follows that of LyndenBell & Pringle (1974), i.e.…”

“…where Cleeves et al (2016) find Σ c = 25 g cm −2 , R c = 100 AU, and γ = 1. The scale height is set by…”

“…Although CO emission likely associated with the disc is detected out to ∼ 1000 AU, it is only detected in millimetre continuum out to about 313 AU so the gas disc is more extended than the dust (Lommen et al 2007;Pinte et al 2008;Panić et al 2009;Cleeves et al 2016). The disc is also very massive, with estimates of 0.1 and 0.17 M from Pinte et al (2008) and Cleeves et al (2016) respectively. The mass accretion rate is currently about 10 −8 M yr −1 (Alcala' et al 2016).…”

“…The mass accretion rate is currently about 10 −8 M yr −1 (Alcala' et al 2016). The most recent analysis of this system by Cleeves et al (2016) combined new 12 CO, 13 CO and C 18 O ALMA observations with a broad array of modelling resources to provide a very comprehensive chemical and radiative transfer model of IM Lup, which could describe many features of the disc very successfully. They also included the effect of external irradiation on the composition and thermal structure of the disc.…”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…For this reason, modelling a protoplanetary disk or hot core with varying physical conditions would have proved problematic. 5,61 Since these are the environments these new processes may prove to be important this is an issue that needs to be addressed.…”