Skip to main content

Evolved stars: clues to the chemical evolution of galaxies

Final Report Summary - POSTAGBINGALAXIES (Evolved stars: clues to the chemical evolution of galaxies)

The late stages (Asymptotic Giant Branch - AGB and post-AGB) of the life of intermediate-mass stars are of the greatest importance for the chemical evolution of galaxies. Pulsations, dust formation, and mass loss through massive stellar winds are characteristic phenomena for AGB stars. Stellar pulsations cause waves, which travel into the outer layers of the atmosphere and develop shocks, compressing matter to such a high density that dust formation occurs and mass loss is triggered. However, the nature of the mass loss is still not well understood. The chemistry of gas and dust lost by AGB stars depends heavily on dredge-up processes that bring the products of stellar nucleosynthesis to the stellar surface. The ejected material, enriched in heavy elements, is later incorporated into new generations of stars and planets.

To achive the project’s objectives a comprehensive study of the physical properties of evolved stars (AGB and post-AGB), their atmospheres and environments was carried out. Radial velocities and brightness variations were monitored for selected stars and a time series of high-resolution spectra were obtained to better understand the dynamical phenomena in the stellar atmospheres. Extensive modelling of photospheric and circumstellar chemistry and radiative transfer in molecular lines were performed. The unique capabilities of the Herschel Space Observatory (HSO) were used to probe the inner region of circumstellar envelopes around evolved stars. The scientific goal of this project is to understand better the following properties of evolved stars: (1) the pulsation and dynamical phenomena in the atmospheres, (2) the structure and mass-loss history, (3) the chemistry, and (4) the kinematics of the outflows. The existing Torun catalogue of Galactic post-AGB and related objects was extended and catalogues for post-AGB sources and candidates in Large and Small Magellanic Clouds (LMC and SMC) were created to facilitate research on unsolved problems of stellar evolution. The research, transfer of knowledge, and training activities lead to 11 peer reviewed publications, 23 presentation at scientific conferences/workshops, 2 PhD Thesis and 6 Bachelor/Master projects.

HSO HIFI observations of C-rich AGB stars provided data for rotational transitions of many molecules which are unavailable with the ground-based telescopes. Modeling of CO lines allowed the determination of the physical conditions in circumstellar envelopes, and these conditions can be employed during the modeling of other molecular lines. Surprisingly water and ammonia, whose existence was not predicted by the hitherto existing models, were detected in many of the C-rich sources. We have performed a detailed modeling of 9 rotational transitions of ortho- and para-ammonia in IRC+10216 using developed by us code MOLEXCSE, and showed that near-infrared (NIR) pumping has to be taken into account to properly determine the ammonia abundance. Thus the previously existing discrepancy between ammonia abundance determination from rotational and from inversion lines has been removed. The method will be applied to the analysis of water rotational lines in IRC+10216.

To facilitate investigations of the post-AGB phase of stellar evolution in galaxies with different metallicities, we have created catalogues of post-AGB objects and candidates in our Milky Way Galaxy (MWG) as well as in the LMC and SMC. All catalogues are available online. The LMC and SMC post-AGB candidates were selected using Spitzer Space Telescope and 2MASS photometry. Period analysis has been performed for all post-AGB candidates identified in the OGLE database. The results showed that only some small fraction of them have well-determined periods. The largest frequency of periodic behavior was found for C-rich post-AGB sources. However, among 22 C-rich LMC/SMC post-AGB stars only 8 show periodic variations of their light curves, with periods range from 49 to 157 days. The other 14 all show evidence of variability on shorter timescales. They have likely hotter central stars, and if so, this may suggest faster evolution during the post-AGB phase in the LMC/SMC in comparison with the MWG. With the aim to better investigate simultaneously the dust and gas properties for post-AGB objects in different galaxies, we have developed a method of such analysis for Galactic AGB, post-AGB objects and planetary nebulae (PNe). We have compared the CO 2-1 line strengths with the infrared dust emission measured at 25 microns. The post-AGB objects are found to be located between AGB stars and PNe, and are segregated into three distinctive groups on the proposed diagram. This method is a powerful tool for investigating the co-evolution of circumstellar gas and dust during the shortly lasting post-AGB evolution.

A sample of cool stars, candidates for cool R Caronae Borealis (R CrB) stars, was analyzed using spectroscopic methods to better understand the late stages of stellar evolution, e.g. the apparent lack of cool R CrB stars in comparison with the warm ones. Effective temperatures and C and N abundances were determined in the atmospheres of the selected candidate stars ES Aql, SV Sge, Z UMi, and NSV 11154 by modeling their spectral energy distributions (SEDs) in the optical and NIR regions. H-deficient model atmospheres were used. Temperatures in the range from 4600 to 5200 K and the C abundances typical for R CrB stars were found. Diverse N abundances were determined, which are lower about 30 times on average in comparison with the mean value for the warm R CrB stars. The SED modeling procedure can be applied for other post-AGB stars. The grids of models calculated for H-deficient and H-rich atmospheres are available online.

The detailed study of the light and color variations for two bright C-rich post-AGB stars revealed four significant periods (P) for each, with the primary period for IRAS22223+4327 and IRAS22272+5435 being 88 and 132 day, respectively. For each of them, the ratio of secondary to primary period is 0.95 a value that has recently been found for several other post-AGB stars but which is much different from that found in Cepheids. Comparison of the observed periods and amplitudes with those of post-AGB pulsation models shows poor agreement. Thus our observational data provide an excellent benchmark for new pulsation models of post-AGB stars. A time series of high-resolution optical spectra for IRAS22272+5435 show significant changes in spectral features during the light cycle. The computed self-consistent atmospheric models showed that the observed changes in molecular line intensities cannot be explained solely by the temperature variations of the pulsating star. The formation site of these molecular features appears to be a cool outflow. Radial velocity measurements, BVR photometry, and high-resolution spectroscopy are employed in order to clarify the evolutionary status of the C-enhanced metal-poor star IRAS12570+3805 which possesses a unique ratio of C isotopes in its atmosphere. An LTE abundance analysis was carried out using the method of spectral synthesis and new self-consistent 1D atmospheric models. C to O and C isotope ratios are found to be extremely high, C/O ~ 12.6 and 12C/13C > 1500, respectively. IRAS12570+3805 could be a single low mass halo star in its AGB phase of evolution.

For more details please contact Laimons Zacs (zacs@latnet.lv) and/or Ryszard Szczerba (szczerba@ncac.torun.pl).