Abstract

Methods. We use the recently developed disk code ProDiMo to calculate the physico-chemical structure of protoplanetary disks and apply the Monte-Carlo line radiative transfer code RATRAN to predict observable line profiles and fluxes. We consider a series of Herbig Ae type disk models ranging from 10^-6 M_Sun to 2.2 10^-2 M_Sun (between 0.5 and 700 AU) to discuss the dependency of the line fluxes and ratios on disk mass for otherwise fixed disk parameters. Results. We find the [CII] 157.7 mum line to originate in LTE from the surface layers of the disk, where Tg > Td . The total emission is dominated by surface area and hence depends strongly on disk outer radius. The [OI] lines can be very bright (> 10^-16 W/m^2) and form in slightly deeper and closer regions under non-LTE conditions. The high-excitation [OI] 145.5 mum line, which has a larger critical density, decreases more rapidly with disk mass than the 63.2 mum line. Therefore, the [OI] 63.2 mum/145.5 mum ratio is a promising disk mass indicator, especially as it is independent of disk outer radius for Rout > 200 AU. CO is abundant only in deeper layers A_V >~ 0.05. For too low disk masses (M_disk <~10^-4 M_Sun) the dust starts to become transparent, and CO is almost completely photo-dissociated. For masses larger than that the lines are an excellent independent tracer of disk outer radius and can break the outer radius degeneracy in the [OI] 63.2 mum/[CII]157.7 mum line ratio. Conclusions. The far-IR fine-structure lines of [CII] and [OI] observable with Herschel provide a promising tool to measure the disk gas mass, although they are mainly generated in the atomic surface layers. In spatially unresolved observations, none of these lines carry much information about the inner, possibly hot regions < 30 AU.

Highlights

  • We find the [Cii] 157.7 μm line to originate in LTE from the surface layers of the disk, where Tg Td

  • The far-IR fine-structure lines of [Cii] and [Oi] observable with Herschel provide a promising tool to measure the disk gas mass, they are mainly generated in the atomic surface layers

  • Observations of gas in protoplanetary disks are intrinsically difficult to interpret as they reflect the interplay between a complex chemical and thermal disk structure, statistical equilibrium and optical depth effects

Read more

Summary

Introduction

Observations of gas in protoplanetary disks are intrinsically difficult to interpret as they reflect the interplay between a complex chemical and thermal disk structure, statistical equilibrium and optical depth effects. This is true if non-thermal excitation such as fluorescence or photodissociation dominate the statistical equilibrium. Beckwith et al 1986; Koerner et al 1993; Dutrey et al 1997; van Zadelhoff et al 2001; Thi et al 2004) Those lines originate in the outer regions of disks, r > 100 AU, where densities are at most n ∼ 107 cm−3.

Methods
Results
Discussion
Conclusion

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.