Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes

Abstract

This is the second paper of a series making use of Herschel/PACS spectroscopy of evolved stars in the THROES catalogue to study the inner regions of their circumstellar envelopes (CSEs). We analyze the CO emission spectra, including a large number of high-$J$ CO lines (from $J$=14-13 to $J$=45-44), as a proxy for the warm molecular gas in the CSEs of a sample of bright carbon-rich stars spanning different evolutionary stages from the Asymptotic Giant Branch (AGB) to the young planetary nebulae (PNe) phase. We use the rotational diagram (RD) to derive rotational temperatures ($T_{\mathrm{rot}}$) and masses ($M_{{\mathrm{H}}_2}$) of the envelope layers where the CO transitions arise. We also obtain a first order estimate of the mass-loss rates and assess the impact of the opacity correction for a range of characteristic envelope radii. We use multi-epoch spectra for the well studied C-rich envelope IRC+10216 to investigate the impact of CO flux variability on the values of $T_{\mathrm{rot}}$ and $M_{{\mathrm{H}}_2}$. PACS sensitivity allowed the study of higher rotational numbers than before, indicating the presence of a significant amount of warmer gas ($\sim$200-900 K) not traceable with lower-$J$ CO observations at sub-mm/mm wavelengths. The masses are in the range $\sim 10^{-2}-10^{-5}{\mathrm{M}}_{\odot }$, anti-correlated with temperature. For some strong CO emitters we infer a double temperature (warm $T_{\mathrm{rot}}\sim$400 K and hot $T_{\mathrm{rot}}\sim$820 K) component. From the analysis of IRC+10216, we corroborate that the effect of line variability is perceptible on the $T_{\mathrm{rot}}$ of the hot component only, and certainly insignificant on $M_{{\mathrm{H}}_2}$ and, hence, the mass-loss rate. Therefore, the parameters derived from the RD are robust even when strong line flux variability occurs, with the major source of uncertainty in the estimate of the mass-loss rate being the size of the CO-emitting volume.

accepted in A&A

References in corpus (13)

• Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds
• Molecular shells in IRC+10216: tracing the mass loss history
• Discovery of Time Variation of the Intensity of Molecular Lines in IRC+10216 in The Submillimeter and Far Infrared Domains
• The wind of W Hya as seen by Herschel - I. The CO envelope
• Further ALMA observations and detailed modeling of the Red Rectangle
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Dor: Non-LTE abundance analysis of CO, SiO, and HCN
• Herschel/PACS observations of the 69 $\mu m$ band of crystalline olivine around evolved stars
• A Study of H2 Emission in Three Bipolar Proto-Planetary Nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22
• Deep Herschel PACS point spread functions
• Spatially resolved kinematic observations of the planetary nebulae Hen 3-1333 and Hen 2-113
• Monitor variability of millimeter lines in IRC +10216
• On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?

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