Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes. (arXiv:1812.07815v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Santos_J/0/1/0/all/0/1">J. M. da Silva Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ramos_Medina_J/0/1/0/all/0/1">J. Ramos-Medina</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Contreras_C/0/1/0/all/0/1">C. Sánchez Contreras</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Garcia_Lario_P/0/1/0/all/0/1">P. García-Lario</a>
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 ($T_{rm rot}$) and masses ($M_{rm H_2}$) 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 $T_{rm rot}$ and $M_{rm H_2}$.
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 $sim10^{-2}-10^{-5},rm M_{odot}$, anti-correlated
with temperature. For some strong CO emitters we infer a double temperature
(warm $T_{rm rot}sim$400 K and hot $T_{rm rot}sim$820 K) component. From
the analysis of IRC+10216, we corroborate that the effect of line variability
is perceptible on the $T_{rm rot}$ of the hot component only, and certainly
insignificant on $M_{rm H_2}$ 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.
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 ($T_{rm rot}$) and masses ($M_{rm H_2}$) 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 $T_{rm rot}$ and $M_{rm H_2}$.
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 $sim10^{-2}-10^{-5},rm M_{odot}$, anti-correlated
with temperature. For some strong CO emitters we infer a double temperature
(warm $T_{rm rot}sim$400 K and hot $T_{rm rot}sim$820 K) component. From
the analysis of IRC+10216, we corroborate that the effect of line variability
is perceptible on the $T_{rm rot}$ of the hot component only, and certainly
insignificant on $M_{rm H_2}$ 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.
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