Warm CO in evolved stars from the THROES catalogue I. Herschel-PACS spectroscopy of O-rich envelopes (vol 618, A171, 2018)

J. M. da Silva Santos, J. Ramos-Medina, C. Sánchez Contreras, P. García-Lario. 2018. Warm CO in evolved stars from the THROES catalogue I. Herschel-PACS spectroscopy of O-rich envelopes (vol 618, A171, 2018). Astronomy and Astrophysics 619, DOI: 10.1051/0004-6361/201833177e

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 (Trot) and masses (MH2) 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 Trot and MH2. PACS sensitivity allowed the study of higher rotational numbers than before, indicating the presence of a significant amount of warmer gas (∼200-900 K) not traceable with lower-J CO observations at sub-mm/mm wavelengths. The masses are in the range ∼10−2−10−5M⊙, anti-correlated with temperature. For some strong CO emitters we infer a double temperature (warm Trot∼400 K and hot Trot∼820 K) component. From the analysis of IRC+10216, we corroborate that the effect of line variability is perceptible on the Trot of the hot component only, and certainly insignificant on MH2 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.

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