Warm CO in evolved stars from the THROES catalogue - I. Herschel-PACS spectroscopy of O-rich envelopes

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 (～55–210 μm) 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 ≈10^{14–1015 cm and ≈1016 cm 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 5000 K, 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 Trot ～ 200 to 700 K, with typical values around 500 K for AGBs and systematically lower, ～200 K, for objects in more advanced evolutionary stages (post-AGBs and PNe). Our values of Trot 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 MH2 ～ 10？6 to ≈10？2 M⊙, which is expected to represent a small fraction of the total CSE mass. The mass-loss rates estimated are in the range ？ ～ 10？7 ？ 10？4 M⊙yr？1, in agreement (within uncertainties) with values found in the literature. We find a clear anticorrelation between MH2 and ？ vs. Trot 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 ～400–500 K and ～600–900 K, respectively. The mass of the warm component (～10？5–8 × 10？2}}