Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae

Abstract

S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0e-6 Msol yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5e-5, which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3e-6, and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6e-6, and distribution of CN in W Aql's circumstellar envelope using ground-based data. We find an SiO abundance of 3e-6, and an NH3 abundance of 1.7e-5, confined to a small envelope.

arXiv comment17 pages, 15 figures

References in corpus (32)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory
• An atomic and molecular database for analysis of submillimetre line observations
• Rotational quenching of CO due to H${}_2$ collisions
• AGB Variables and the Mira Period-Luminosity Relation
• Photoprocesses in protoplanetary disks
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae
• Models of circumstellar molecular radio line emission: Mass loss rates for a sample of bright carbon stars
• Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
• `Thermal' SiO radio line emission towards M-type AGB stars: a probe of circumstellar dust formation and dynamics
• On the reliability of mass-loss-rate estimates for AGB stars
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles I. Theoretical model -- Mass-loss history unravelled in VY CMa
• Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
• Numerical methods for non-LTE line radiative transfer: Performance and convergence characteristics
• The first INTEGRAL-OMC catalogue of optically variable sources
• Dust driven mass loss from carbon stars as a function of stellar parameters - I. A grid of Solar-metallicity wind models
• Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
• The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
• Circumstellar water vapour in M-type AGB stars: Radiative transfer models, abundances and predictions for HIFI
• Molecular abundances in carbon-rich circumstellar envelopes
• Herschel/HIFI observations of O-rich AGB stars : molecular inventory
• Modeling molecular hyperfine line emission
• The wind of W Hya as seen by Herschel - I. The CO envelope
• Probing the mass loss history of carbon stars using CO line and dust continuum emission
• Mass-loss properties of S-stars on the AGB
• Imaging the circumstellar dust around AGB stars with PolCor
• Large-scale environments of binary AGB stars probed by Herschel - I. Morphology statistics and case studies of R Aquarii and W Aquilae
• The widespread occurence of water vapor in the circumstellar envelopes of carbon-rich AGB stars: first results from a survey with Herschel/HIFI
• W Hya through the eye of Odin Satellite observations of circumstellar submillimetre H${}_2$O line emission
• Herschel/HIFI deepens the circumstellar NH3 enigma
• Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
• H2O vapor excitation in dusty AGB envelopes. A PACS view of OH 127.8+0.0

Cited by in corpus (34)

• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• New observations and models of circumstellar CO line emission of AGB stars in the Herschel SUCCESS programme
• Sulphur molecules in the circumstellar envelopes of M-type AGB stars
• A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling
• Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
• ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results
• Photodissociation of CO in the outflow of evolved stars
• Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at $159.0-368.5$ GHz
• Sulphur-bearing molecules in AGB stars II: Abundances and distributions of CS and SiS
• Search for aluminium monoxide in the winds of oxygen-rich AGB stars
• SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme
• ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H${}_2$O and OH lines
• ALMA observations of the vibrationally-excited rotational CO transition $v=1,J=3-2$ towards five AGB stars
• ATOMIUM: Molecular inventory of 17 oxygen-rich evolved stars observed with ALMA
• Water isotopologues in the circumstellar envelopes of M-type AGB stars
• Circumstellar ammonia in oxygen-rich evolved stars
• ATOMIUM: Halide molecules around the S-type AGB star W Aquilae
• Molecular line study of the S-type AGB star W Aquilae. ALMA observations of CS, SiS, SiO and HCN
• The surprisingly carbon-rich environment of the S-type star W Aql
• Chemical tracers of a highly eccentric AGB-main sequence star binary
• Classifying the secondary component of the binary star W Aquilae
• On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?
• Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
• H12CN and H13CN excitation analysis in the circumstellar outflow of R Scl
• Multi-frequency high spectral resolution observations of HCN toward the circumstellar envelope of Y CVn
• PACS and SPIRE range spectroscopy of cool, evolved stars
• ATOMIUM: Continuum emission and evidence of dust enhancement from binary motion
• CO and HCN isotopologue ratios in the outflows of AGB stars
• The impact of third dredge-up on the mass loss of Mira variables
• Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes
• Morphology and kinematics of the gas envelope of the Mira binary W Aquilae
• Molecular line polarisation from the circumstellar envelopes of Asymptotic Giant Branch stars
• Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars. II. Abundances and spatial distributions of CS
• A rotational line CO cooling rate prescription for AGB outflows