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Astron. Astrophys. 357, 669-676 (2000)

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1. Introduction

In advanced asymptotic giant branch (AGB) evolutionary stages, H and He burning occur in two shells. The He-rich zone between them (hereafter defined as the He intershell) is periodically swept by convective instabilities induced by He-burning runaways (thermal pulses, TP), where 12C is synthesized by partial He burning. In these layers heavy elements are built up by slow neutron captures (s process) on seed nuclei, driven by the 13C([FORMULA],n)16O and the 22Ne([FORMULA],n)25Mg reactions, as will be discussed in Sect. 4. After a few TPs, at the quenching of a thermal instability the H shell is inactive and the convective envelope penetrates in the upper region of the He intershell, bringing to the surface newly synthesized 12C and s-processed elements. This recurrent phenomenon is known as third dredge up (TDU) (e.g., Busso et al. 1999).

In AGB stars of mass lower than [FORMULA] 3 [FORMULA] , the maximum temperature at the bottom of the convective instability barely reaches [FORMULA] K in advanced TPs. At this temperature the 22Ne neutron source is marginally activated. On average, about 1% of 22Ne nuclei are burned, and rather small neutron fluxes are generated. The bulk of the neutron flux for the production of the heavy elements comes instead from the 13C neutron source, which is activated at T [FORMULA] 1 [FORMULA] 108 K and consumes all 13C nuclei in radiative conditions during the interpulse period (Straniero et al. 1997). In contrast, in intermediate mass AGB stars ([FORMULA] 5 to 8 [FORMULA] ), the bottom temperature in the convective TPs reaches a peak value of 3.5 [FORMULA] 108 K and the 22Ne([FORMULA],n)25Mg reaction is efficiently activated (Iben 1975). Here, on average 30% of 22Ne nuclei are consumed in the He intershell. On the other hand, in intermediate mass AGBs the mass of the He intershell is an order of magnitude lower than in AGB stars of lower mass, and the duration of the post-flash dip is shorter. Thus the 13C pocket is expected to be comparatively less efficient (Vaglio et al. 1998; Straniero et al. 2000).

The s-process isotopes synthesized by the two neutron bursts described above belong to heavy elements from Sr to Pb (e.g., Truran & Iben 1977; Iben & Truran 1978; Gallino et al. 1998). Observationally, evidence of this dates back to the measurement of Tc in S stars (Merrill 1952). At the s-process efficiencies typical of S and C (type N) stars in the galactic disc the s-elements at the Zr or Ba abundance peaks are enhanced by up to one or two orders of magnitude (Busso et al. 1995). Unfortunately, this cannot be verified here, as no spectrocopic analysis is possible for the photosphere of CW Leo, the central object of the very dusty source IRC+10216.

In contrast, all nuclei lighter than Fe, though capable of capturing up to 50% of the available neutrons and hence to act as filters, or poisons for the neutron captures, reach only small enhancements, below 10 - 50 in the He intershell and below 2 - 3 in the envelope (Lugaro et al. 1999). This is so because of their high initial abundance and small neutron capture cross sections (the heavy isotopes beyond A [FORMULA] 90 have cross sections that are larger by up to three orders of magnitude). Despite this fact, intermediate mass nuclei from Ne to Fe can be used as indicators of the s-process conditions. Indeed, their final concentration is sensitive to the activation of the 22Ne neutron source, hence to the maximum temperature achieved in the convective TPs. The Cl isotopic ratio shares this property. In particular, the neutron magic 37Cl is relatively enhanced by the s process while the lighter 35Cl, whose neutron capture cross section is a factor of 5 larger than that of 37Cl, is depleted. Also the Mg isotope ratios are of interest. Both 25Mg and 26Mg are enhanced in the He intershell as a combination of [FORMULA]-captures on 22Ne and of neutron captures, while 24Mg remains almost untouched.

Thanks to the properties outlined above, the measurement of relative isotopic abundances of intermediate mass elements in the circumstellar envelopes of enshrouded AGB stars appears a powerful tool for studying the details of the s processing during thermal pulses, in particularly constraining the progenitor stellar mass.

Except for the lightest elements, isotopic shifts of the atomic lines are smaller than the thermal linewidths so that isotopic abundances are mainly derived from molecular lines, for which isotope shifts are easily resolved. The molecular rotational transitions of circumstellar molecules, observed in the radio millimeter range, have proved to be extremely useful for such determinations. In particular, towards the high mass-loss stars belonging to the end of the AGB phase, the optically thick dusty envelopes prevent any optical observation of the photospheric molecules, but the circumstellar molecules provide intense radio emission. Up to now, systematic measurements of the silicon, sulfur and chlorine isotopic ratios have been performed in a single circumstellar envelope, the carbon-rich IRC+10216. The central star, CW Leo, is a long period Mira-type variable, near the end of its AGB stage (Skinner et al. 1998) with a high mass loss rate of [FORMULA] 1.5 [FORMULA] 10-5 [FORMULA] yr-1. The luminosity is rather low, between 1.1 and 1.9 [FORMULA] 104 [FORMULA] (Groenewegen et al. 1998; Weigelt et al. 1998), depending on the precise value adopted for the distance, which is estimated to be in the range 130 to 170 pc (Le Bertre 1997; Winters et al. 1994). From models of the circumstellar emission, a C/O ratio [FORMULA] 1.4 and a total mass below 2 [FORMULA] were estimated (Winters et al. 1994). However, if we accept for the distance a value close to the maximum allowed limit (170 pc), the derived luminosity might imply a more massive star, up to 4 - 4.5 [FORMULA] (Weigelt et al. 1998). This higher mass estimate was found by Guélin et al. 1995to be compatible with some of the observed isotopic ratios. On the basis of new measurements for Cl isotopes and of a reanalysis of published data for other species, we plan to readdress here the problem of the progenitor mass of IRC+10216.

The chlorine isotopic ratio derived from the previous 2mm survey showed a quite large uncertainty. Additional observations have been performed and are presented in Sect. 2. The chlorine isotopic ratio, based on a careful compilation of all existing measurements, is derived with better accuracy and analyzed in Sect. 3, together with those of other intermediate atomic mass elements. Sects. 4 and 5 present an analysis of the envelope isotopic composition of intermediate mass elements, as expected from AGB nucleosynthesis models, to be compared with the observed ratios. Finally, Sect. 6 summarizes the main results of this research.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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