A study of the s-process in the carbon-rich post-AGB stars IRAS06530-0213 and IRAS08143-4406 on the basis of VLT-UVES spectra

In an effort to extend the still limited sample of s-process enriched post-AGB stars, high-resolution, high signal-to-noise VLT+UVES spectra of the optical counterparts of the infrared sources IRAS06530-0213 and IRAS08143-4406 were analysed. The objects are moderately metal deficient by [Fe/H]=-0.5 and -0.4 respectively, carbon-rich and, above all, heavily s-process enhanced with a [ls/Fe] of 1.8 and 1.5 respectively. Especially the spectrum of IRAS06530-0213 is dominated by transitions of s-process species, and therefore resembling the spectrum of IRAS05341+0852, the most s-process enriched object known so far. The two objects are chemically very similar to the 21micron objects discussed in Van Winckel & Reyniers (2000). A homogeneous comparison with the results of these objects reveals that the relation between the third dredge-up efficiency and the neutron nucleosynthesis efficiency found for the 21micron objects, is further strengthened. On the other hand, a detailed comparison with the predictions of the latest AGB models indicates that the observed spread in nucleosynthesis efficiency is certainly intrinsic, and proves that different C-13 pockets are needed for stars with comparable mass and metallicity to explain their abundances.Comment: 14 pages, 10 figures, accepted for publication in A&A; Table 4 is available at ftp://ftp.ster.kuleuven.ac.be/dist/maarten/filescds/ pending upload to CD

High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae

High-resolution near-infrared HST NICMOS (F160W, F222M) images and polarization (2 um) observations were made of four bipolar proto-planetary nebulae (PPNs): IRAS 17150-3224, IRAS 17441-2411, IRAS 17245-3951, and IRAS 16594-4656. The first three of these are viewed nearly edge-on, and for the first time the central stars in them are seen. Color maps reveal a reddened torus between the bipolar lobes in the edge-on cases, with bluer lobes. The polarization values are high, with maximum values ranging from 40 to 80%. The polarization patterns are basically centrosymmetric, with some deviations in the low polarization equatorial regions. For IRAS 17150-3224, circumstellar arcs are seen at 1.6 um, along with a newly-discovered loop in the equatorial region. Bright caps are seen at the end of the lobes, indicating that they are not open-ended. A distinct point-symmetric pattern is seen in the strengths of the polarization vectors, especially in IRAS 17150-3224. HST NICMOS observations provide a valuable complement to the WFPC2 visible images in deriving the basic structure of bipolar PPNs.Comment: 20 pages, 12 figures, accepted for publication in Astronomical Journal higher resolution figures are available on http://katherine.as.arizona.edu/~ksu/cv/su_nicmos7840.ps.g

A search for diffuse bands in the circumstellar envelopes of post-AGB stars

In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on the analysis of 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high resolution optical spectroscopy and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. The conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.Comment: Accepted for A&

Pulsations and Long-Term Light Variability of Three Candidates to Protoplanetary Nebulae

We present new photometric data and analysis of the long-duration UBV photoelectric observations for three candidates to protoplanetary objects - F-supergiants with IR-excesses located at large galactic latitudes, IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All three stars have revealed quasiperiodic low-amplitude variabilities caused by pulsations observed against the long-term trends of brightnesses. For IRAS 18095+2704=V887 Her we have found a pulsation period of 109 days and a linear trend of brightness under the constant colours if being averaged over the year timescale. The light curve of IRAS 19386+0155=V1648 Aql over 2000-2008 can be approximated by a wave with a main period of 102 days which is modulated by close frequency, with a period of 98 days, that results in brightness oscillations with a variable amplitude. V1648 Aql has also shown synchronous reddening together with a persistent rise of brightness in the V-band. IRAS 19500-1709=V5112 Sgr experiences irregular pulsations with the periods of 39 and 47 days. The long-term component of the variability of V5112 Sgr may be related to the binary character of this star.Comment: 11 pages, 6 figures, accepted for publication in Pis'ma Astron. Z

Candidate carriers and synthetic spectra of the 21- and 30-mu protoplanetary nebular bands

Computational chemistry is used here to determine the vibrational line spectrum of several candidate molecules. It is shown that the thiourea functional group, associated with various carbonaceous structures (mainly compact and linear aromatic clusters), is able to mimic the 21-$\mu$m band emitted by a number of proto-planetary nebulae. The combination of nitrogen and sulphur in thiourea is the essential source of emission in this model: the band disappears if these species are replaced by carbon. The astronomical 21-$\mu$m feature extends redward to merge with another prominent band peaking between 25 and 30 $\mu$m, also known as the 30-$\mu$m band. It is found that the latter can be modelled by the combined spectra of aliphatic chains, made of CH$_{2}$ groups, oxygen bridges and OH groups, which provide the 30-$\mu$m emission. The absence of oxygen all but extinguishes the 30-$\mu$m emission. The emission between the 21- and 30-$\mu$m bands is provided mainly by thiourea attached to linear aromatic clusters. The chemical software reveals that the essential role of the heteroatoms N, S and O stems from their large electronic charge. It also allows to determine the type of atomic vibration responsible for the different lines of each structure, which helps selecting the most relevant structures. A total of 22 structures have been selected here, but their list is far from being exhaustive; they are only intended as examples of 3 generic classes. When background dust emission is added, model spectra are obtained, which are able to satisfactorily reproduce recent observations of proto-planetary nebulae. The relative numbers of atomic species used in this model are typically H:C:O:N:S=53:36:8:2:1.Comment: 9 pages, 14 figure

The post-AGB evolution of AGB mass loss variations

We present new numerical hydrodynamical modelling of the evolution of Asymptotic Giant Branch (AGB) mass loss fluctuations during the post-AGB/Planetary Nebula phase. These models show that after ionization, the observable effects of the mass loss fluctuations disappear in a few thousand years, consistent with the fact that only few PNe have been found to be surrounded by `rings'. We derive the observational characteristics of these rings, and compare them to reported observations of the rings around NGC 6543, finding a good match of emission properties and line shapes. We predict small variations in the observable electron temperatures.Comment: 12 pages, 13 figures, accepted by A&

New Spectroscopic Observations of the Post-AGB Star V354Lac=IRAS22272+5435

The strongest absorption features with the lower-level excitation potentials $\chi_{\rm low}<1$ eV are found to be split in the high-resolution optical spectra of the post-AGB star V354 Lac taken in 2007--2008. Main parameters, T$_{eff}$=5650 K, $\log g$=0.2, $\xi_t$=5.0 km/s, and the abundances of 22 chemical elements in the star's atmosphere are found. The overabundance of the $s$-process chemical elements (Ba, La, Ce, Nd) in the star's atmosphere is partly due to the splitting of strong lines of the ions of these metals. The peculiarities of the spectrum in the wavelength interval containing the LiI $\lambda$ 6707 \AA{} line can be naturally explained only by taking the overabundances of the CeII and SmII heavy-metal ions into account. The best agreement with the synthetic spectrum is achieved assuming $\epsilon$(LiI)=2.0, $\epsilon$(CeII)=3.2, and $\epsilon$(SmII)=2.7. The velocity field both in the atmosphere and in the circumstellar envelope of V354 Lac remained stationary throughout the last 15 years of our observations.Comment: 16 pages, 6 figures, 2 table