Optical spectroscopy of Post-AGB stars

1. Optical spectroscopy of Post-AGB stars Pedro García-LarioEuropean Space Astronomy Centre ESA,Villafranca del Castillo, Madrid

2. Why post-AGB stars? • Key objects in the study of the dramatic morphological and chemical changes which take place at the latest stages of stellar evolution in low- and intermediate-mass stars (0.8 – 8 solar masses) • From the spectroscopic point of view: ideal probes to test and study stellar nucleosynthesis • much easier to study than AGB stars • more complete information, on a wide number of atomic species • Is there any correlation between the physical properties derived from spectroscopic observations and the observed morphology?

3. Limitations • There is a limited number of galactic sources to study (~326 in the most recent compilation) “The Toruń catalogue of Galactic post-AGB and related objects” (Szczerba et al. 2007) • Strong observational bias towards optically bright (‘classical’) post-AGB stars • Lack of information from those strongly obscured in the optical: (‘IRAS selected’ post-AGB stars); need to go to infrared wavelengths • Need to explore the whole parameter space of masses and metallicities

4. What can we learn? • Fundamental physical parameters of the stellar photosphere (log Teff, log g); from which masses can be inferred • Detailed chemical abundances for a large number of atomic species: • [Fe/H] • CNO abundances • s-process elements and their connection with known nuclear processes, such as: • 3rd dredge up • hot bottom burning • neutron source dominating the production of s-elements • Evolutionary links with AGB and PNe based on the statistical analysis of their chemical properties

5. Recent progress • Atlas of low-resolution spectra of an infrared (IRAS) selected sample of 124 post-AGB stars and pre-PNe (Suárez et al. 2006)

6. Recent progress • A flatter distribution of spectral types? Classical post-AGB stars Obscured post-AGB stars • Evidence for two different mass populations? (from Suárez et al. 2006)

7. Recent progress • Compilation of stellar parameters and chemical abundances derived from high-resolution optical spectroscopy for ~125 sources (Stasińska et al. 2006) • Toruń Catalogue of post-AGB stars, including SEDs covering from the optical to the infrared (Szczerba et al. 2006)

8. Recent progress • Compilation of stellar parameters and chemical abundances derived from high-resolution optical spectroscopy for ~125 sources (Stasińska et al. 2006) • Toruń Catalogue of post-AGB stars, including SEDs covering from the optical to the infrared (Szczerba et al. 2006)

9. Recent progress • Low-resolution spectroscopy of individual sources: 10 confirmed identifications (Pereira & Miranda, 2007) • Detailed chemical abundance analysis of an increasing number of sources using high spectral resolution: • IRAS 06530-0213, IRAS 08143-4406 (Reyniers et al. 2003); • IRAS 19386+0155 (Pereira et al. 2004) • IRAS 13266-5551, IRAS 17311-4924 (Sarkar et al. 2005) • IRAS 05381+1012(Pereira & Roig 2006) • IRAS 08281-4850, IRAS 14325-6428 (Reyniers et al. 2007)

10. Recent progress

11. Recent progress • Atlas of ~1500 spectral lines/features in HD 56126, the ‘canonical’ post-AGB star (Klochkova et al; 2007) HD 56126 F5Iab spectral type [Fe/H] = -1.0 Strong excess of C and s-process elements Double-peaked SED; 21 micron feature at infrared wavelengths Relatively high galactic latitude (b=+10.0 deg) α Per Variable and complex H-alpha profile interpreted as shock waves estimulating mass outflow

12. Recent progress • Spectroscopic studies of massive O-rich AGB stars: the precursors of heavily obscured post-AGBs? Determination of lithium, zirconium and rubidium abundances (Garcia Hernandez et al. 2006a; 2006b) • Strong Li interpreted as a HBB indicator (M>3-3.5 Mʘ) • Rb overabundances confirms the activation of the 22Ne neutron source in massive AGB stars (M>4 Mʘ) Li I 6707 Rb I 7800

13. Binary post-AGB stars • Depletion in post-AGB stars surrounded by Keplerian dust disks: (Maas et al. 2005, 2007; de Ruyter et al. 2006) • Refractory elements get locked in dust grains • Radial velocity variations of spectral lines with a period of a few hundred days are interpreted as the signature of binarity (van Winckel et al. 1995); • Some of them confirmed as binary stars: among them, HR 4049, HD 44179, HD 52961, HD 46703 or BD +39 4926 • Binarity promotes the formation of circumbinary stable, Keplerian dust disks

14. Recent progress • Diffuse Interstellar band studies: indicators of circumstellar reddening? (Reyniers et al. 2007) (Luna et al. 2007)

15. Thick disk post-AGB stars • ‘Classical’ post-AGB stars • Usually C-rich and s-process enriched (efficient dredge-up; consistent with a 13C neutron source) • Mildly metal-deficient [Fe/H]= -0.5 to -1.0 • Observations with HST reveal aspherical shapes in scattered light • Little to moderate reddening in the optical • Strong 21 micron emitters • Fluorescent H2 emission for stars earlier than A-type • Precursors of intermediate progenitor mass (1.0 - 1.4 Mʘ) C-rich PNe? Hen 3-401 (from van Winckel 2003)

16. Galactic halo post-AGB stars • Optically bright, usually hot spectral types • Usually C-poor and non s-process enriched (no dredge-up) • Very low metallicities [Fe/H] < -1.0 • Observations with HST reveal only slight departures from round morphologies in scattered light • Little reddening in the optical • Most of them not detected by IRAS • May never develop a PN (progenitor mass below 1.0 Mʘ) IRAS 19590-1249 Hen 3-401 IRAS 20462+3416

17. Thin disk post-AGB stars IRAS 22036+5306 • Usually heavily obscured and strong bipolar morphology • Shocked excited H2 emission • Most of them O-rich, non s-process enriched; usually OH masers; sometime CO as well (massive molecular envelopes) • Solar metallicities • Large dust grains including water ice • Sometimes show nebular emission • Their AGB progenitors show strong Li (HBB indicator) and Rb (22Ne neutron source) overabundances • Progenitors of O-rich (N-rich) type I PNe? (M>3-3.5 Mʘ) • May never become observable in the optical as PNe in the most extreme cases IRAS 17347-3139

18. A population of infrared PNe? • Rapidly evolving, heavily obscured post-AGB stars (some showing already nebular emission) IRAS 17347-3139 Perea Calderón et al. (in prep.)

19. A population of infrared PNe? • The missing population of massive PNe with 4-8 Msun progenitor masses? IRAS 17347-3139 Perea Calderón et al. (in prep.)

20. Links to PNe: observational facts • The nucleosynthesis pattern observed in galactic post-AGB stars is the consequence of the chemical branching experienced by their progenitor stars at the end of the AGB as a consequence of the 3rd dredge up). • Very low-mass AGB stars (M<1.0 Mʘ) belonging to the halo population may never become PNe. • Low-mass AGB stars (M<1.0-1.4 Mʘ) will remain O-rich (optically bright Miras) during the whole AGB evolution (progenitors of type II O-rich PNe) • Intermediate-mass AGB stars (M> 1.0-1.4 Mʘ) may turn into C-rich Miras, leading eventually to C-rich PNe (s-process enriched as well) • High-mass AGB stars (M>3.0-3.5 Mʘ) will activate the HBB and eventually become type I PNe • However, this branching is strongly dependent on the metallicity of the progenitor star • 3rd dredge-up is more efficient at low metallicity • HBB becomes activated at a lower mass limit at low metallicity

21. Summary • Post-AGB stars are chemically much more diverse than initially thought • Mass is the main driver of the diversity observed, which implies that the spectral properties observed, if understood, can be used as a mass indicator but… • Metallicity play a crucial role as well and it is a necessary ingredient to fully understand and interpret the wide variety of spectroscopic properties observed • Mass loss and dust production is also depending on metallicity and introduce another complication in the analysis: visibility is an issue • Massive post-AGB stars tend to show increasing degree of bipolarity • Need to extend the analysis of post-AGB stars to other metallicity environments (LMC, SMC?)