Radiation thermo-chemical models of protoplanetary disks - II. Line diagnostics

摘要

Aims. In this paper, we explore the diagnostic power of the far-IR fine-structure lines of [O I] 63.2m, 145.5m, [C II] 157.7m, as well as the radio and sub-mm lines of CO J=1-0,2-1 and 3-2 in application to disks around Herbig Ae stars. We aim atunderstanding where the lines originate from, how the line formationprocess is affected by density, temperature and chemical abundance inthe disk, and to what extent non-LTE effects are important. Theultimate aim is to provide a robust way to determine the gas mass ofprotoplanetary disks from line observations. Methods. We use the recently developed disk code P ROD IM Oto calculate the physico-chemical structure of protoplanetary disks andapply the Monte-Carlo line radiative transfer code R ATRAN to predict observable line profiles and fluxes. We consider a series of Herbig Ae type disk models ranging from 10-6to (between 0.5 and 700AU) to discuss the dependency of the linefluxes and ratios on disk mass for otherwise fixed disk parameters.This paper prepares for a more thorough multi-parameter analysisrelated to the Herschel open time key program G ASPS. Results. We find the [C II]157.7m line to originate in LTE from the surface layers of the disk, where .The total emission is dominated by surface area and hence depends strongly on disk outer radius. The [O I] lines can be very bright (>10-16W/m2) and form in slightly deeper and closer regions under non-LTE conditions. For low-mass models, the [O I]lines come preferentially from the central regions of the disk, and thepeak separation widens. The high-excitation [O I]145.5m line, which has a larger critical density, decreases more rapidly with disk mass than the 63.2m line. Therefore, the [O I] 63.2m/145.5m ratio is a promising disk mass indicator, especially as it is independent of disk outer radius for 200$ -->AU. CO is abundant only in deeper layers .For too low disk masses ()the dust starts to become transparent, and CO is almost completelyphoto-dissociated. For masses larger than that the lines are anexcellent independent tracer of disk outer radius and can break theouter radius degeneracy in the [O I]63.2m/[C II]157.7m line ratio. Conclusions. The far-IR fine-structure lines of [C II] and [O I]observable with Herschel provide a promising tool to measure the diskgas mass, although they are mainly generated in the atomic surfacelayers. In spatially unresolved observations, none of these lines carrymuch information about the inner, possibly hot regions <30AU. Key words: astrochemistry - methods: numerical - line: formation - circumstellar matter - stars: formation - radiative transfer