IRAS08281-4850 and IRAS14325-6428: two A-type post-AGB stars with s-process enrichment
M. Reyniers, G.C. Van de Steene, P.A.M. van Hoof, H. Van Winckel
aa r X i v : . [ a s t r o - ph ] J un Astronomy&Astrophysicsmanuscript no. 7607 c (cid:13)
ESO 2018November 7, 2018
IRAS 08281-4850 and IRAS 14325-6428:two A-type post-AGB stars with s-process enrichment ⋆ M. Reyniers ,⋆⋆ , G.C. Van de Steene , P.A.M. van Hoof , and H. Van Winckel Instituut voor Sterrenkunde, Departement Natuurkunde en Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, 3001 Leuven,Belgium Koninklijke Sterrenwacht van Belgi¨e, Ringlaan 3, 1180 Brussel, BelgiumReceived 4 April 2007 / Accepted 6 June 2007
ABSTRACT
Aims.
One of the puzzling findings in the study of the chemical evolution of (post-)AGB stars is why very similar stars (in terms ofmetallicity, spectral type, infrared properties, etc. . . ) show a very di ff erent photospheric composition. We aim at extending the stilllimited sample of s-process enriched post-AGB stars, in order to obtain a statistically large enough sample that allows us to formulateconclusions concerning the 3rd dredge-up occurrence. Methods.
We selected two post-AGB stars on the basis of IR colours indicative of a past history of heavy mass loss: IRAS 08281-4850 and IRAS 14325-6428. They are cool sources in the locus of the Planetary Nebulae (PNe) in the IRAS colour-colour diagram.Abundances of both objects were derived for the first time on the basis of high-quality UVES and EMMI spectra, using a criticallycompiled line list with accurate log(gf) values, together with the latest Kurucz model atmospheres.
Results.
Both objects have very similar spectroscopically defined e ff ective temperatures of 7750 - 8000 K. They are strongly carbonand s-process enriched, with a C / O ratio of 1.9 and 1.6, and an [ls / Fe] of + + Conclusions.
IRAS 08281-4850 and IRAS 14325-6428 are prototypical post-AGB objects in the sense that they show strong post 3rddredge-up chemical enrichments. The neutron irradiation has been extremely e ffi cient, despite the only mild sub-solar metallicity. Thisis not conform with the recent chemical models. The existence of very similar post-AGB stars without any enrichment emphasizesour poor knowledge of the details of the AGB nucleosynthesis and dredge-up phenomena. We call for a very systematic chemicalstudy of all cool sources in the PN region of the IRAS colour-colour diagram. Key words.
Stars: AGB and post-AGB – Stars: abundances – Stars: carbon – Stars: individual: IRAS 14325-6428 – Stars: individual:IRAS 08281-4850
1. Introduction
Post-AGB stars are key objects in the study of the dramaticchemical and morphological changes of objects on their ascenton the Asymptotic Giant Branch (AGB) and subsequent evolu-tion. In this paper we report on our ongoing research to studythe AGB chemical evolution by a systematic study of post-AGB photospheres. Spectra of post-AGB stars are much easierto study than their AGB precursors for several reasons. First,their atmospheres do not show the large amplitude pulsationsand the large mass loss rates that characterise AGB atmospheres.Second, their photospheres are hotter, so atomic transitions pre-vail in post-AGB spectra, while molecular transitions prevail inAGB star spectra. This allows us to quantify the chemical con-tent of a very wide range of trace elements. Unfortunately, post-AGB stars evolve on a very fast track and whether the currentGalactic sample is representative is one of the most di ffi cult is-sues when interpreting the results of single post-AGB stars inthe broader context of stellar evolution. Therefore, extending thestill limited sample of post-AGB stars is indispensable in orderto attain a statistically large enough sample that allows us to for- Send o ff print requests to : M. Reyniers ⋆ based on observations collected at the European SouthernObservatory, Chile (programmes 70.D-0278(A) and 73.D-0241(A)) ⋆⋆ Postdoctoral fellow of the Fund for Scientific Research, Flanders mulate conclusions concerning the (post-)AGB evolution in gen-eral.During the past decennia, it has been realised that post-AGBstars are chemically much more diverse than previously thought.
Binary objects tend to have a totally di ff erent photospheric com-position than single objects, showing some degree of depletion of refractory elements in their photosphere (see e.g. Maas et al.2005). The single objects, on their side, are also far from chem-ically homogeneous. Some objects are the most s-process en-riched objects known to date (e.g. Reyniers et al. 2004, and ref-erences therein), while others are not enriched at all. This di-chotomy is very strict, in the sense that mildly enhanced objectsdo not exist, except for a few rather atypical objects. Chemicalevolutionary AGB models do predict that there is a minimum ini-tial mass for the 3rd dredge-up to occur, at around 1.4 M ⊙ (e.g.Straniero et al. 2003). Hence post-AGB stars without any heliumburning products in their photosphere are theoretically expected.However a strict dichotomy is not expected, since a more grad-ual transition between non-enriched and enriched objects is pre-dicted if the transition from an O-rich AGB to a C-rich AGB staroccurs over many thermal pulses. Furthermore, the s-process en-riched sub-class of post-AGB objects imposes another unsolvedproblem. These stars exhibit a large spread in s-process e ffi -ciency, as they do not obey the expected anti-correlation betweenmetallicity and s-process e ffi ciency (see Fig. 4 of Van Winckel2003). In other words, examples exist of very similar post-AGB M. Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428
Table 1.
Basic parameters of the two objects discussed in this study.IRAS 08281-4850 14325-6428Equatorial coord. α
08 29 40.552 14 36 34.375 δ −
49 00 04.33 −
64 41 31.09Galactic coord. l 266.08 313.87b − − stars (in metallicity, spectral type, infrared excess, etc. . . ), butwith a totally di ff erent photospheric abundance pattern (see forexample Fig. 1 in Van Winckel & Reyniers 2001). This resultdramatically illustrates that the 3rd dredge-up phenomenon isnot yet fully understood.In this paper, we selected two objects with far infraredcolours typical of PNe from the IRAS point source catalogue.Apart from PNe, only post-AGB stars are typically found inthis part of the colour-colour diagram (van Hoof et al. 1997).Furthermore, the selected objects were not detected in the radiocontinuum above a detection limit of 3 mJy (Van de Steene &Pottasch 1993, 1995). Hence they do not seem to have evolvedto the PN stage as yet and are very likely post-AGB stars. Weobtained JHKL images of the candidates with CASPIR on the2.3-m telescope at Siding Spring Observatory in Australia in or-der to assure the correct identification of the IRAS counterpartsand obtain accurate positions (Van de Steene et al. 2000). Two ofthese infrared selected post-AGB stars which are presented here(see Table 1) have counterparts in the USNO catalogue so thattheir visual magnitudes are also known. They were su ffi cientlybright to be observed in the optical at high resolution.The paper is organised as follows: in the next section, wediscuss the spectral energy distribution of the two objects, andquantify the total reddening towards the two sources. In Sect.3 we briefly discuss the observation and reduction of the high-resolution spectra, while in Sect. 4 we deal with the abundanceanalysis. Sect. 5 is a section devoted to the di ff use interstellarbands in the spectrum of IRAS 14325-6428. In the discussion(Sect. 6), we mainly focus on the s-process abundances. We endthe paper with conclusions.
2. Spectral energy distribution
Images were obtained for both objects with the Swiss Euler tele-scope at La Silla (ESO) in 2005. The observing log is presentedin Table 2. The objects were unresolved in all images. The im-ages were bias subtracted and flatfielded in iraf . The photome-try package was used to do the aperture photometry. In case ofcrowded fields daophot was used to subtract the neighbours. Oneto three standard stars per band per star were available for thephotometric calibration. The resulting magnitudes are presentedin Table 3.
We collected additional photometry from various surveys: theTycho-2 Catalogue (Høg et al. 2000), the 3 rd data release ofDENIS (Epchtein et al. 1994), 2MASS (Skrutskie et al. 1997),MSX (Egan et al. 2003), and the IRAS point source catalogue Table 2.
Observational log for the images obtained at the Euler tele-scope at ESO, La Silla.IRAS Date filter exptime airmass08281-4850 2005-10-07 UG 300.2 1.473BG 20.9 1.423VG 150.5 1.408RG 60.6 1.392IC 98.9 1.35114325-6428 2005-08-04 UG 399.3 1.322BG 299.5 1.311VG 241.6 1.292 (Beichmann 1985). We also obtained literature data from Garcia-Lario et al. (1997, hereafter GL97) and Van de Steene et al.(2000, hereafter VdS00). The data are presented in Table 3.Near-IR data of IRAS 08281-4850 were present in the 2 nd re-lease of the DENIS data set, but was deleted in the 3 rd release.The data from the 2 nd release have not been retained in the anal-ysis. In order to construct the spectral energy distribution (SED),we first converted the Tycho-2 magnitudes to the Johnson systemusing the formulas:B J = B T − . T − V T )V J = V T − . T − V T )which were derived from Perryman et al. (1997). The Gunn-r magnitude from the Geneva system was converted to theJohnson system using the formula:R J = r − . − . J − V J )(Kent 1985) where B J − V J can be calculated from the Genevaphotometry using Eqs. 12 and 13 from Harmanec & Boˇzi´c(2001). After that, the photometry was converted to λ F λ usingthe absolute flux calibrations given in Table 4. For the GL97 ob-servations we adopt the Johnson photometric calibration, whilefor the VdS00 observations we adopt the MSSO photometriccalibration. The resulting spectral energy distribution is shownin Table 5. The SEDs of the post-AGB stars su ff er from interstellar as wellas circumstellar extinction. We assumed that this total extinctioncan be described by an R V = A V and the bolometric flux L / (4 π D ) by minimizing the quadratic residuals of all observa-tions between 0.4 and 4 µ m. The results are given in Table 6.Plots of the Kurucz stellar atmosphere models combined withthe dereddened observed fluxes are shown in Figs. 1 and 2.
3. High-resolution spectra: observation andreduction
High-resolution, high signal-to-noise optical spectra of the twoprogram stars were taken in the framework of our ongoing pro-gram to study the photospheric chemical composition of starsin their last stages of evolution (e.g. Reyniers et al. 2004; . Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428 3
Table 3.
Broadband photometry for IRAS 08281-4850 andIRAS 14325-6428. IRAS 14325-6428 is contained twice in theDENIS catalogue. The data sets do not agree within the error margins,which may point to variability of the source. Both data sets have beenretained in the analysis.phot. band 08281-4850 14325-6428mag ∆ mag mag ∆ magEuler Geneva U 17.81 0.15 14.58 0.25Euler Geneva B 14.76 0.10 12.08 0.10Euler Geneva V 14.09 0.05 11.89 0.05Euler Gunn-r 13.23 0.15Euler I C T T s s s ν ∆ F ν F ν ∆ F ν (Jy) (Jy) (Jy) (Jy)MSX A 8.28 0.72 0.03MSX C 12.13 3.35 0.18MSX D 14.65 7.1 0.4MSX E 21.34 17.7 1.1IRAS 12 2.23 0.11 3.31 0.20IRAS 25 9.83 0.59 30.6 1.2IRAS 60 3.62 0.36 17.1 1.9 Table 6.
The total extinction A V and the bolometric luminosity L of theprogram stars. IRAS A V L at D = ⊙ -0.5 0.0 0.5 1.0 1.5log( λ ) ( µ m) -13.5-13.0-12.5-12.0-11.5 l og ( λ F λ ) ( W m - ) Fig. 1.
The spectral energy distribution of IRAS 08281-4850. -0.5 0.0 0.5 1.0 1.5log( λ ) ( µ m) -13.0-12.5-12.0-11.5-11.0 l og ( λ F λ ) ( W m - ) Fig. 2.
The spectral energy distribution of IRAS 14325-6428.
Fig. 3.
The spectra of IRAS 14325-6428 and IRAS 08281-4850 com-pared with the A7Iab standard HD 81471 and the non-enriched post-AGB star HD 133656.
Reyniers & Van Winckel 2003; Van de Steene & van Hoof2003). IRAS 14325-6428 is observed with UVES on the VLTUT2 telescope (Kueyen), as a member of a larger sample ofseven post-AGB objects that were observed in service mode dur-ing ESO period ∼ ∼ ∼ M. Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428
Table 7.
Log of the high-resolution observations. For IRAS 08281-4850, a spectral gap occurs between 493 nm and 500 nm. ForIRAS 14325-6428, spectral gaps occur between 577 nm and 583 nm andbetween 854.4 nm and 864.5 nm due to the spatial gap between the twoUVES CCDs.date UT exp.time wavelength S / Nstart (sec) interval (nm)IRAS 08281-4850 (NTT + EMMI)2003-02-04 01:11 9000 395 −
795 75IRAS 14325-6428 (VLT-UT2 + UVES)2004-05-13 07:13 1800 374.5 −
498 1302004-05-13 07:13 1800 670.5 − −
681 190
The reduction of our UVES spectra was performed in thededicated “UVES context” of the midas environment and in-cluded bias correction, cosmic hit correction, flat-fielding, back-ground correction and sky correction. We used average extrac-tion to convert frames from pixel-pixel to pixel-order space. Thereduction of our EMMI spectrum was done within the echellepackage in iraf following the user’s guide by D. Willmarth andJ. Barnes (1994). The spectra were normalised by dividing theindividual orders by a smoothed spline function defined throughinteractively identified continuum points. For a detailed descrip-tion of the reduction procedure, we refer to Reyniers (2002). InTable 7, we also list some indicative signal-to-noise values of thefinal data product.Sample spectra of our programme stars can be found inFigs. 3 and 6. In Fig. 3, the spectra of IRAS 08281-4850and IRAS 14325-6428 are compared with the spectra of theA7Iab standard HD 81471 and the non-enriched post-AGBstar HD 133656. The spectrum of HD 81471 is retrieved fromUVESPOP, the library of high-resolution UVES spectra of starsacross the Hertzsprung-Russell diagram (Bagnulo et al. 2003).HD 133656 is discussed in Van Winckel et al. (1996). The spec-trum of HD 133656 is an ESO1.5-m + FEROS spectrum taken onJune 27, 2001. This object has atmospheric parameters com-parable with the program stars (T e ff , log g , ξ t ) = (8000 K, 1.0,3.0 km s − ) but is slightly more metal deficient ([Fe / H] = − ii line. IRAS 08281-4850 is the strongest s-enriched one,but the stronger lines are also due to a slightly lower temperatureof this object.
4. Abundance analysis and results
The general methodology is already extensively discussed in ourprevious papers (e.g. Reyniers et al. 2004; Deroo et al. 2005) andwill not be repeated here. Here we only remind that we make useof the latest Atlas models (Castelli & Kurucz 2004) in combina-tion with the latest version (April 2002) of Sneden’s LTE lineanalysis program MOOG (Sneden 1973). The value of the mi-croturbulent velocity of IRAS 14325-6428 derived from the Fe ii lines is high ( ξ t =
10 km s − ), but this high value is also supportedby the results of other ions as can be seen in Fig. 4.Since the spectral resolution of the UVES spectra is muchhigher than that of the EMMI spectrum, we started the analy-sis with IRAS 14325-6428. In order to avoid undetected blendsthat are caused by the lower spectral resolution of the EMMIspectrum, we took the line list of IRAS 14325-6428 as our start- Fig. 4.
The relatively high microturbulent velocity ( ξ t =
10 km s − ) forIRAS 14325-6428 is also supported by the results of ions other thanFe ii . ing point for the analysis of IRAS 08281-4850. As a conse-quence, all lines that we used for IRAS 08281-4850 are alsopresent in the line list of IRAS 14325-6428. This choice en-sures the consistency of the analyses, implying that a compar-ison between the abundance results is highly reliable. Due tothe stronger s-process enrichment of IRAS 08281-4850 com-pared to IRAS 14325-6428, the spectrum of IRAS 08281-4850shows significantly more lines of s-process elements. Therefore,we performed an extra search for s-process lines in the spectrumof IRAS 08281-4850. This explains why we have used 12 La ii lines and 1 Nd ii in the analysis of IRAS 08281-4850, againstonly 5 La ii lines and no Nd ii lines for IRAS 14325-6428. The final results of our stellar atmosphere parameter determina-tion and our abundance analysis are listed in Table 8. The firstcolumn of this table gives the actual ion. Then, for each star,the following columns are listed: the number of lines used; themean equivalent width in mÅ; the absolute abundances by num-ber log ǫ = log(N(el) / N(H)) +
12; the line-to-line scatter σ ltl ; theabundance relative to iron [el / Fe], and an estimate of the totaluncertainty on the abundance σ tot (see Sect. 4.3). For the ref-erences of the solar abundances (the middle column) needed tocalculate the [el / Fe] values: see Reyniers et al. (2007).The abundances are also graphically presented in Fig. 5. Onthis figure, the di ff erent groups of elements are marked with dif- . Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428 5 Table 8.
Abundance results for IRAS 08281-4850 and IRAS14325-6428. For the explanation of the columns: see Sect. 4.2.
IRAS 08281-4850 IRAS 14325-6428 T e ff = g = . ξ t = . − T e ff = g = . ξ t = . − ion N W λ log ǫ σ ltl [el / Fe] σ tot sun N W λ log ǫ σ ltl [el / Fe] σ tot C i
12 117 9.17 0.24 0.93 0.15 8.57 30 64 9.09 0.12 1.07 0.13N i i i i ii i − i i ii ii i ii ii i
19 79 7.29 0.14 0.11 0.11 7.51 34 37 7.07 0.13 0.11 0.11Fe ii
15 98 7.18 0.11 0.00 7.51 22 74 6.96 0.11 0.00Ni ii − ii ii ii ii
12 75 2.93 0.18 2.13 0.08 1.13 5 54 1.89 0.10 1.31 0.06Ce ii ii iii ii / H] = − / H] = − / O = / O = α / Fe] = + α : Mg, Ca, Ti) [ α / Fe] = + α : Mg, S, Ca, Ti)[ls / Fe] = + / Fe] = + / Fe] = + / Fe] = + / ls] = + / ls] = + ferent symbols. We will summarize the main results for each ofthese groups. Metallicity
Both stars are mildly metal deficient, with ironabundances of [Fe / H] = − − CNO-elements
Both stars are clearly carbon enriched, with anenrichment around [C / Fe] ≃ + / O number ratios for both stars.One has to note, however, that in IRAS 08281-4850 the un-certainty on the oxygen abundance prevents an accurate C / Onumber ratio for this star. α -elements The simple mean of the [el / Fe] values of the(available) α -elements yields [ α / Fe] =+ + s-process elements It is clear that the s-process enrichment ofthe two objects under study is very strong.The s-process elements observed in evolved stars can be dividedinto two groups: the light s-process elements around the magicneutron number 50 (Sr, Y, Zr) and the heavy s-process elements around the magic neutron number 82 (Ba, La, Ce, Pr, Nd, Sm).Three s-process indices are generally defined: [ls / Fe], [hs / Fe]and [hs / ls]. To be consistent with our earlier papers on simi-lar stars (Van Winckel & Reyniers 2000; Reyniers et al. 2004;Reyniers & Cuypers 2005), we define the ls-index as the meanof the Y and Zr abundances and the hs-index as the mean of theBa, La, Nd and Sm abundances, with unavailable elements esti-mated using the tables of Malaney (1987). All indices are listedin Table 8. For the error analysis, we followed the same method as describedin Deroo et al. (2005). We slightly changed formula (1) of thispaper, in the sense that for the uncertainty induced by the model σ mod , we confined the parameter space to consistent models, i.e.models for which there is ionisation equilibrium between Fe i and Fe ii . To be more precise, in order to calculate σ mod , we stud-ied the abundance changes for two di ff erent consistent models(T e ff = g = e ff = g = ξ t = − . Thetotal uncertainty on the [el / Fe] abundances σ tot can be found inTable 8, and is the quadratic sum of the uncertainty on the mean M. Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428
Fig. 5. [el / Fe] values for IRAS 14325-6428 ( upper panel ) andIRAS 08281-4850 ( lower panel ). The uncertainty on the [el / Fe] valuesis the total uncertainty σ tot as listed in Table 8. due to line-to-line scatter, the uncertainty induced by the model,and the uncertainty on the Fe abundance: σ tot = r ( σ ltl √ N el ) + ( σ mod ) + ( σ Fe √ N Fe ) If less than 5 lines were available, a line-to-line scatter of 0.2 dexwas applied.
5. Diffuse Interstellar Bands Di ff use Interstellar Bands (DIBs) are broad absorption lines ofinterstellar origin that are seen in the spectra of reddened objects.The carriers of these DIBs are still not known, but polycyclicaromatic hydrocarbons (PAHs) are amongst the most probablecandidate carriers. Due to the severe mass loss in the precedingAGB phase, post-AGB stars are often enshrouded by carbon-richcircumstellar dust, causing severe reddening. Therefore, post-AGB stars are ideal testlabs to search for possible circumstel-lar DIBs. If, on the other hand, the DIBs are detected to be in-terstellar, a rough division can be made between the interstellarand circumstellar component of the total reddening towards thepost-AGB star, since some DIBs correlate quite well with theinterstellar reddening.
During our analysis of IRAS 14325-6428, we realised that strongDIBs are indeed present in this spectrum. We initiated a system-atic search for DIBs. First, we selected eight well-known andwidely studied DIBs from the list in Herbig (1995). For each ofthese DIBs, we made a spectral synthesis in the vicinity of theDIB wavelength, based on the abundances found in our abun-dance analysis (Table 8). The DIB can then easily be identified asthe spectral feature that is not fitted. Four DIBs are shown in Fig.6. The studied DIBs in the spectrum of IRAS 14325-6428 are
Fig. 6.
Four clear di ff use interstellar bands in the spectrum ofIRAS 14325-6428. A spectral synthesis is overplotted in red, to facil-itate the DIB-detection. listed in Table 9. The columns of this table represent: rest wave-length of the DIB; the observed wavelength in IRAS 14325-6428; the heliocentric radial velocity of the DIB; the measuredequivalent width; the intrinsic strength of the DIB as seen in theDIB standard HD 183143; and the (interstellar) reddening de-rived from the latter relation. The reference for columns (1) and(5) is Herbig (1995).The heliocentric radial velocities of the DIBs show somescatter, but they are definitely di ff erent from the heliocentricvelocity of the star ( −
87 km s − ) and show velocities very dif-ferent from any expected outflow of the circumstellar mate-rial. Therefore, the DIBs seen in the spectrum of IRAS 14325-6428 are the imprint of an interstellar cloud between the objectand the observer. A rough estimate for the distance towards theDIB producing cloud is obtained through the formula of Lang(1980), which describes the rotation of the Galactic Plane, yield-ing 1.6 kpc. Obviously, this distance is also a lower limit for thedistance towards IRAS 14325-6428. The scatter in the DIB ve-locities could be caused by di ff erent DIB producing clouds in theline of sight towards IRAS 14325-6428, but also the rest wave-lengths of the DIBs are di ffi cult to quantify due to their complexfine structure (e.g. Galazutdinov et al. 2003).For most DIBs, there is a correlation between its strengthand the reddening that is caused by the DIB producing cloud.The DIB standard that is often used to quantify this relation, isHD 183143 (E(B-V) = =
1, are given in column5, together with the inferred reddening for the DIB producingcloud towards IRAS 14325-6428 (column 6). A mean interstellarreddening of E(B-V) = total reddening towards IRAS 14325-6428 of E(B-V) tot = . Reyniers et al.: Abundance analysis of IRAS 08281-4850 and IRAS 14325-6428 7 Table 9.
Studied DIBs in the spectrum of IRAS 14325-6428.DIB λ obs. λ V helio EW EWE(B − V) E(B-V)(Å) (Å) (km s − ) (Å) HD 183143 from DIB − − − − − − − − ± σ ) − ± ± Due to the lower resolution and the lower S / N ratio of theIRAS 08281-4850 spectra, we were not able to do a similar studyfor this source. A qualitative comparison of the strength of thestrong DIB at 6196 Å indicates that IRAS 08281-4850 has cer-tainly weaker DIBs than IRAS 14325-6428. The total reddeningtowards IRAS 08281-4850 is, however, stronger than the one to-wards IRAS 14325-6428, implying that the circumstellar com-ponent of the total reddening of IRAS 08281-4850 is probablylarge.
6. Discussion
IRAS 08281-4850 and IRAS 14325-6428 were selected on thebasis of their position in the IRAS colour-colour diagram(Pottasch et al. 1988) and the lack of free-free radio continuumemission (Van de Steene & Pottasch 1993, 1995). Both objectsremained poorly studied until now. Our chemical analysis pre-sented in this paper shows that both objects are among the hottestmembers of the s-process enhanced post-AGB stars known todate and illustrates that the rich IRAS legacy of objects in thetransition between the AGB and the PN phase is far from beingharvested. Although the distance to the objects is not well con-strained, both objects do not show an extremely high luminosity(Table 6) and, given the sub-solar metallicity, both objects repre-sent the final evolutionary phase of a star with a low initial mass(M i < ⊙ ).There is general agreement that the source of the neutronsfor the s-process in low and intermediate AGB stars is the “ Csource”. A long standing problem is the formation of the Cpocket itself. Stellar models using a standard treatment of mixingcannot reproduce the C pocket at a level which is high enoughfor the s-process to take place (Herwig 2005). Until now, there isno satisfying description for this phenomenon, and the di ff erentmodeling groups use di ff erent descriptions. Many models usesome kind of overshoot mechanism (e.g. Herwig et al. 1997).Also di ff erential rotation has recently been studied as a possiblemixing mechanism driving the proton engulfment, but this rota-tionally induced mixing alone cannot account for the formationof a large enough C pocket (Herwig et al. 2003). Other groupsavoid this formation problem by assuming an ad-hoc C pocketin the He intershell (models by e.g. Gallino et al. 1998) or an ad-hoc proton density profile (models by e.g. Goriely & Mowlavi2000). The observed spread in s-process e ffi ciency (see Sect. 1)is then reproduced by a variable C pocket strength. Since the C pocket is build upon primary synthesised C, the formationis thought to be largely independent of the initial metallicity anda richer s-process nucleosynthesis is expected for lower metallic- ity stars, because more neutrons become available per iron-seed.In practise, such a trend has not been found for post-AGB stars.The very recent results of the population synthesis mod-els by Bonaˇci´c Marinovi´c et al. (2006) and Bonaˇci´c Marinovi´cet al. (2007) are very interesting in this context. Their approachis to combine stellar population synthesis with a rapid stellarevolution code including AGB nucleosynthesis and evolution.Interestingly, their models do reproduce both the observed di-chotomy (see Fig. 1d in Bonaˇci´c Marinovi´c et al. 2006), and theobserved spread. Contrary to previous models, only a limitedspread in the strength of the C pocket is needed to reproducethe observed e ffi ciency spread. With the new results presentedin this paper, we are able to add two important datapoints toconstrain this promising new generation of models. Indeed, thetwo stars discussed here show a strong s-process enrichment,although they are only mildly metal deficient. Particularly theresults of IRAS 08281-4850 are on the predicted boundaries ofboth the [hs / ls] index (Fig. 3 of Bonaˇci´c Marinovi´c et al. 2007)and the Zr overabundance (Fig. 8 of Bonaˇci´c Marinovi´c et al.2007).
7. Conclusion
With the comprehensive abundance analysis presented in thispaper, the objects IRAS 08281-4850 and IRAS 14325-6428 jointhe group of post-AGB stars with a clear post third dredge-upsignature, since they do not only show a clear carbon enhance-ment, but also a strong enrichment in s-process elements. Thisenrichment is surprisingly strong with a high ratio of heavy ver-sus light s-process elements, despite the only mild metal defi-ciency of both objects. This provides additional evidence of theintrinsic s-process e ffi ciency spread in (post-)AGB stars. A sys-tematic analysis of post-AGB stars in the PNe locus of the IRAScolour-colour diagram would be a very rewarding program tostudy the relation between the AGB nucleosynthesis, the dredge-up e ffi ciency and the overall stellar evolution of the central star. Acknowledgements.
MR is grateful to Sara Regibo for the preliminary analysisof IRAS 14325-6428 in an early stage of the paper. The Geneva sta ff is thankedfor observation time on the Euler telescope. MR acknowledges financial supportfrom the Fund for Scientific Research - Flanders (Belgium). PvH acknowledgessupport from the Belgian Science Policy O ffi ce through grant MO / / References
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Table 4.
Absolute flux calibration for the various photometric bandsused in this paper.phot. band 0-mag flux referenceW m − µ m − × − Cohen et al. 20032MASS H 1.133 × − ”2MASS K s × − ”Cousins I C × − Lamla 1982DENIS Gunn-i 1.20 × − Fouqu´e et al. 2000DENIS J 3.17 × − ”DENIS K s × − ”Geneva U 5.754 × − Rufener & Nicolet 1988Geneva B 2.884 × − ”Geneva V 3.736 × − ”Johnson B 7.20 × − Johnson 1965, 1966Johnson V 3.92 × − ”Johnson R 1.76 × − ”Johnson J 3.4 × − ”Johnson H 1.26 × − ”Johnson K 3.9 × − ”MSSO J 3.03 × − Thomas et al. 1973MSSO H 1.17 × − ”MSSO K 4.02 × − ”MSSO L 6.18 × − ” Table 5.
The spectral energy distribution (sorted by wavelength) givenas λ F λ in SI units. λ λ F λ ∆ λ F λ λ F λ ∆ λ F λ µ m W m − W m − W m − W m − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × − × −−