Detection of CI line emission from the detached CO shell of the AGB star R Sculptoris

Abstract

Stars on the asymptotic giant branch (AGB) lose substantial amounts of matter, to the extent that they are important for the chemical evolution of, and dust production in, the universe. The mass loss is believed to increase gradually with age on the AGB, but it may also occur in the form of bursts, possibly related to the thermal pulsing phenomenon. Detached, geometrically thin, CO shells around carbon stars are good signposts of brief and intense mass ejection. We aim to put further constraints on the physical properties of detached CO shells around AGB stars. The photodissociation of CO and other carbon-bearing species in the shells leads to the possibility of detecting lines from neutral carbon. We have therefore searched for the CI(${}^3{P}_1-{}^3{P}_0$) line at 492 GHz towards two carbon stars, S Sct and R Scl, with detached CO shells of different ages, about 8000 and 2300 years, respectively. The CI(${}^3{P}_1-{}^3{P}_0$) line was detected towards R Scl. The line intensity is dominated by emission from the detached shell. The detection is at a level consistent with the neutral carbon coming from the full photodissociation of all species except CO, and with only limited photoionisation of carbon. The best fit to the observed ${}^{12}$CO and ${}^{13}$CO line intensities, assuming a homogeneous shell, is obtained for a shell mass of about 0.002 $M_{\odot }$, a temperature of about 100 K, and a CO abundance with respect to H${}_2$ of 10${}^{-3}$. The estimated CI/CO abundance ratio is about 0.3 for the best-fit model. However, a number of arguments point in the direction of a clumpy medium, and a viable interpretation of the data within such a context is provided.

References in corpus (4)

• Unexpectedly large mass loss during the thermal pulse cycle of the red giant R Sculptoris!
• New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme
• The detached dust shells around the carbon AGB stars R Scl and V644 Sco
• Dusty shells surrounding the carbon variables S Scuti and RT Capricorni

Cited by in corpus (8)

• The impact of stellar companion UV photons on the chemistry of the circumstellar environments of AGB stars
• A detailed view of the gas shell around R Sculptoris with ALMA
• Detection of CI line emission towards the oxygen-rich AGB star omi Cet
• PACS and SPIRE range spectroscopy of cool, evolved stars
• Ionized carbon around IRC +10216
• Probing the dynamical and kinematical structures of detached shells around AGB stars
• Extended view on the dust shells around two carbon stars
• Detached Shell Carbon Stars: Tracing Thermal Pulses on the Asymptotic Giant Branch