Warm CO in evolved stars from the THROES catalogue

摘要

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 ≈10 14 –10 15 cm and ≈10 16 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 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 500K 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 H 2 ～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 ˙ M ～10 ?7 –10 ?4 M ? yr ?1 , in agreement (within uncertainties) with values found in the literature. We find a clear anticorrelation between M H 2 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 –8×10 ?2 M ? ) is always larger than that of the hot component, by a factor of between two and ten. The warm-to-hot M H 2 and T rot ratios in our sample are correlated and are consistent with an average