The chemical composition of the post-asymptotic giant branch F supergiant CRL 2688

Abstract

We present an analysis of a high‐resolution (R ∼ 50 000) optical spectrum of the central region of the protoplanetary nebula CRL 2688. This object is thought to have recently moved off the asymptotic giant branch (AGB) and displays abundance patterns of CNO and heavy elements that can provide us with important clues to help us understand the nucleosynthesis, dredge‐up and mixing experienced by the envelope of the central star during its AGB stage of evolution. Analysis of the molecular features, presumably originating from circumstellar matter, provides further constraints on the chemistry and velocity of the expanding shell, expelled as a consequence of the strong mass loss experienced by the central star. We confirm that the central star shows a spectrum typical of an F‐type supergiant with Teff = 7250 ± 400 K, log g = 0.5 and [Fe/H] = −0.3 ± 0.1 dex. We find that the abundance pattern of this object is characterized by enhancements of carbon ([C/Fe] = 0.6 ± 0.1), nitrogen ([N/Fe] = 1.0 ± 0.3) and Na ([Na/Fe] = 0.7 ± 0.1), similar to other previously known carbon‐rich post‐AGB stars. Yttrium is also enhanced, while the [Ba/Y] ratio is very low (−1.0), indicating that only the light s‐process elements are enhanced. The zinc abundance is found to be normal, [Zn/Fe] = 0.0 ± 0.3, suggesting that there is no depletion of refractory elements. The Hα, Na i and K i resonance lines show prominent emission components, the heliocentric radial velocities of which are offset by −41 ± 3 km s−1 relative to the photospheric metal‐absorption lines. The molecular features of C2 and CN also show emission components, the velocities of which are consistent with the emission components of the Hα, Na i and K i lines. On the other hand, their absorption components are more highly blueshifted than the corresponding emission components, which suggests that the regions where the emission and absorption components arise are expanding at different velocities.

ExoMol line lists XXXI: spectroscopy of lowest eights electronic states of C2

“…Figure 18 shows a simulated Philips band (2,0) compared to the spectrum of AGB remnants of HD 56126 observed by Bakker et al (1996). Similar spectra of this band were reported by Schmidt et al (2013) and Ishigaki et al (2012). Figure 19 gives detailed, high resolution emission spectra of the (0,0), (1,0) and (0,1) Swan bands computed using our line list and the empirical line list by Brooke et al (2013).…”

“…Unusually, astronomical spectra of C 2 have been observed via several different electronic bands; those considered in this work are summarised in Figure 1. The spectroscopy of C 2 is an important tool for stellar classifications (Keenan & Morgan 1941;Vartya 1970;Fujita 1980;Keenan 1993;Gonneau, A. et al 2017;De Mello et al 2009) and determining the chemical composition of stars (Querci et al 1971;Lambert et al 1984;Bakker et al 1996;Gonneau, A. et al 2017;Hall & Maxwell 2008;Green 2013;Zamora et al 2009;Ishigaki et al 2012;Schmidt et al 2013) and of the Sun (Lambert 1968;Grevesse & Sauval 1973;Lambert 1978;Brault et al 1982).…”

A MOLECULAR LINE SURVEY OF THE CARBON-RICH PROTOPLANETARY NEBULA AFGL 2688 IN THE 3 mm AND 1.3 mm WINDOWS

“…The dynamical age of the nebula and mass loss rate were estimated to be ∼350 yr and 3 × 10 −5 M ⊙ yr −1 , respectively (Ueta et al 2006;Lo & Bechis 1976). The central star of AFGL 2688 is a completely obscured F-type supergiant with an effective temperature of about 7250 K and enhanced abundances of carbon, nitrogen, neon, and yttrium (Ishigaki et al 2012). The distance to this PPN is highly uncertain.…”

“…HCN, HNC, HC 3 N, HC 5 N, HC 7 N, HC 9 N, HCP, PH 3 , PN, NH 3 , SiS, SiO, SiC 2 , H 2 , H 2 O, NaCN, MgNC, AlF, and NaCl (see Cernicharo et al 2001Cernicharo et al , 2011Highberger et al 2001Highberger et al , 2003Milam et al 2008;Park et al 2008;Ishigaki et al 2012;Wesson et al 2010, and references therein).…”

The detection of heavy metals in the circumstellar envelopes of post-AGB stars

“…Overabundances of carbon and heavy-metal [s/Fe] overabundance (or lack thereof) in the atmosphere of the central star were published earlier by [19,22,25,33] for IRAS 04296+3429, 07134+1005, 08005−2356, and 23304+6147 respectively; [21] for IRAS 20000+3239; [23] and [34] for RAFGL 2688; [27] for V448 Lac and [28] for V354 Lac. Chemical abundances for five stars from Table 1 were also published by [35].…”