A chemical inventory of the S-type AGB starχ Cygni based onHerschel/HIFI observations of circumstellar line emission

Abstract

Context. S-type AGB stars (C/O ≈ 1) are thought to be transition objects from M-type (O-rich) AGB stars to carbon stars and as such are interesting objects in themselves. Of particular interest is to determine accurate circumstellar properties and molecular abundances, due to their predicted sensitivity to the photospheric C/O-ratio. Aims. Presented here are new sensitive sub-millimetre line observations of molecules towards the S-type AGB star χ Cyg, using the HIFI instrument on-board the Herschel Space Observatory. The observed lines predominantly probe warm gas relatively close to the central star. Methods. Detailed, non-LTE, radiative transfer modelling has been used in order to interpret the circumstellar molecular line observations performed using HIFI, assuming a spherically symmetric, smooth, accelerating wind. Results. Lines from common molecules such as H 2 O, CO and SiO, which are expected to be abundant in an S-type AGB star, are clearly detected (as well as some of their isotopologues) in the HIFI spectra. In addition, we detect lines from carbon-bearing molecules such as HCN and CN. The CO line modelling indicates that the mass-loss rate has not undergone any significant modulations during the past ≈1000 yr. The derived o-H 2 O fractional abundance is ≈7 × 10 −6 , i.e., lower than those obtained for a small sample of M-type AGB stars but higher than what has been derived for a few carbon stars. We further obtain a p-H 2 O fractional abundance of ≈5 × 10 −6 giving an o/p-ratio of ≈1.4. Molecular line cooling is dominated by H 2 O only in a region close to the star ( 6 × 10 14 cm). The SiO abundance is estimated to be ≈1 × 10 −5 . The 12 CO/ 13 CO ratio is 43 ± 6. The high-excitation rotational lines clearly probe the acceleration region of the stellar wind ( 2 × 10 15 cm) and put constraints on dynamical wind models. We are unable to fit consistently the combined ground-based and HIFI data for HCN and CN. Conclusions. The derived H 2 O abundance is reasonably consistent with recent chemical model predictions and so is the SiO abundance. The o/p-ratio of ≈1.4 supports a chemical formation under non-LTE conditions for the H 2 O molecules, and the presence of carbon-bearing molecules at relatively large abundances is also indicative of the importance of non-LTE chemical processes in regulating the circumstellar chemistry. The velocity field derived from the molecular line modelling is consistent with that obtained from solving for the wind dynamics through the coupled momentum equations of the dust and gas particles.

A HIFI view on circumstellar H2O in M-type AGB stars: radiative transfer, velocity profiles, and H2O line cooling

“…The energy balance in this inner part of the CSE is very sensitive to the amount of H 2 O line cooling included in the model, quickly leading to extreme cooling. Models of the S-type stars χ Cyg and W Aql, that include H 2 O line cooling, had H 2 O abundances factors 2−35 lower than the abundances estimated for the M-type stars here (Schöier et al 2011;Danilovich et al 2014). For our high mass-loss-rate objects, H 2 O line cooling is too strong to be included in the energy balance.…”

“…Although detailed modelling of these parameters for individual sources may result in different values, the uncertainties in these individual models would complicate the comparative study that is the goal of this work. This basic modelling strategy has been thoroughly tested and used previously (e.g., Schöier & Olofsson 2001;Olofsson et al 2002;Ramstedt et al 2008;Maercker et al 2008Maercker et al , 2009Schöier et al 2011;Danilovich et al 2014).…”

“…The basic modelling method has been used previously for models of H 2 O lines (Maercker et al 2008(Maercker et al , 2009, and has been upgraded to also include velocity profiles and the effect of H 2 O line cooling on the energy balance (Schöier et al 2011;Danilovich et al 2014). In addition to the HIFI observations, we use spectrally unresolved lines of CO and H 2 O observed with the Photoconductor Array Camera and Spectrometer (PACS) aboard Herschel to further constrain the models.…”

The abundance of HCN in circumstellar envelopes of AGB stars of different chemical type

“…Their data corroborates the findings of Marvel (2005) as to the origin of the HCN emission, and also probes gas still accelerating in the inner wind of the star. The recent analysis of the HCN Herschel/HIFI data (in combination with groundbased data) of the S-star χ Cyg (Schöier et al 2011) also suggest a more intricate HCN distribution.…”

“…The dust temperature structure and dust density profile are obtained from detailed radiative transfer modelling using the publicly available code Dusty (Ivezić & Elitzur 1997 Schöier et al 2011, for an example of such modelling). In the modelling, where the spectral energy distribution (SED) provides the observational constraint, the dust optical depth specified at 10 μm, τ 10 , the dust temperature at the inner radius of the dust envelope, T d (r i ), and the effective stellar blackbody temperature, T , are the adjustable parameters.…”