2018A&A...618A.171R

In this work (Paper I), we analyse Herschel-PACS spectroscopy for a subsample of 23 O-rich and 3 S-type evolved stars, in different evolutionary stages from the asymptotic giant branch (AGB) to the planetary nebula (PN) phase, from the THROES catalogue. (C-rich targets are separately studied in Paper II). The broad spectral range covered by PACS (∼55-210µm) includes a large number of high-J CO lines, from J=14-13 to J=45-44 (v=0), that allow us to study the warm inner layers of the circumstellar envelopes (CSEs) of these objects, at typical distances from the star of ~=10¹⁴-10¹⁵cm and ~=10¹⁶cm for AGBs and post-AGB-PNe, respectively. We have generated CO rotational diagrams for each object to derive the rotational temperature, total mass within the CO-emitting region and average mass-loss rate during the ejection of these layers. We present first order estimations of these basic physical parameters using a large number of high-J CO rotational lines, with upper-level energies from E_up∼580 to 5000K, for a relatively big set of evolved low-to-intermediate mass stars in different AGB-to-PN evolutionary stages. We derive rotational temperatures ranging from T_rot∼200 to 700K, with typical values around 500 K for AGBs and systematically lower, ∼200K, for objects in more advanced evolutionary stages (post-AGBs and PNe). Our values of T_rot are one order or magnitude higher than the temperatures of the outer CSE layers derived from low-J CO line studies. The total mass of the inner CSE regions where the PACS CO lines arise is found to range from M_H2∼10^–6 to ~=10^–2M_☉, which is expected to represent a small fraction of the total CSE mass. The mass-loss rates estimated are in the range M∼10^–7-10^–4M_☉/yr, in agreement (within uncertainties) with values found in the literature. We find a clear anticorrelation between M_H2 and M vs. T_rot that probably results from a combination of most efficient line cooling and higher line opacities in high mass-loss rate objects. For some strong CO emitters in our sample, a double temperature (hot and warm) component is inferred. The temperatures of the warm and hot components are ∼400-500K and ∼600-900K, respectively. The mass of the warm component (∼10^–5-8x10^–2M_☉) is always larger than that of the hot component, by a factor of between two and ten. The warm-to-hot M_H2 and T_rot ratios in our sample are correlated and are consistent with an average temperature radial profile of ∝r^–0.5±0.1, that is, slightly shallower than in the outer envelope layers, in agreement with recent studies.