APN 4, La Palma, June 2007 VG # 1 Optical spectroscopy of Post-AGB stars Pedro García-Lario European Space Astronomy Centre ESA,Villafranca del Castillo, Madrid

APN 4, La Palma, June 2007 VG # 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?

APN 4, La Palma, June 2007 VG # 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

APN 4, La Palma, June 2007 VG # 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: –3 rd 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

APN 4, La Palma, June 2007 VG # 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)

APN 4, La Palma, June 2007 VG # 6 Recent progress A flatter distribution of spectral types? Classical post-AGB stars Obscured post-AGB stars (from Suárez et al. 2006) Evidence for two different mass populations?

APN 4, La Palma, June 2007 VG # 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)

APN 4, La Palma, June 2007 VG # 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)

APN 4, La Palma, June 2007 VG # 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 , IRAS (Reyniers et al. 2003); –IRAS (Pereira et al. 2004) –IRAS , IRAS (Sarkar et al. 2005) –IRAS (Pereira & Roig 2006) –IRAS , IRAS (Reyniers et al. 2007)

APN 4, La Palma, June 2007 VG # 10 Recent progress

APN 4, La Palma, June 2007 VG # 11 Recent progress Atlas of ~1500 spectral lines/features in HD 56126, the ‘canonical’ post-AGB star (Klochkova et al; 2007) Variable and complex H-alpha profile interpreted as shock waves estimulating mass outflow 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) HD α Per

APN 4, La Palma, June 2007 VG # 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 M ʘ ) Rb overabundances confirms the activation of the 22Ne neutron source in massive AGB stars (M>4 M ʘ ) Li I 6707 Rb I 7800

APN 4, La Palma, June 2007 VG # 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 or BD –Binarity promotes the formation of circumbinary stable, Keplerian dust disks

APN 4, La Palma, June 2007 VG # 14 Recent progress Diffuse Interstellar band studies: indicators of circumstellar reddening? (Reyniers et al. 2007) (Luna et al. 2007)

APN 4, La Palma, June 2007 VG # 15 Thick disk post-AGB stars ‘Classical’ post-AGB stars Usually C-rich and s-process enriched (efficient dredge- up; consistent with a 13 C 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 H 2 emission for stars earlier than A-type Precursors of intermediate progenitor mass ( M ʘ ) C-rich PNe? Hen (from van Winckel 2003)

APN 4, La Palma, June 2007 VG # 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 M ʘ ) Hen IRAS IRAS

APN 4, La Palma, June 2007 VG # 17 Thin disk post-AGB stars 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 ( 22 Ne neutron source) overabundances Progenitors of O-rich (N-rich) type I PNe? (M>3-3.5 M ʘ ) May never become observable in the optical as PNe in the most extreme cases IRAS IRAS

APN 4, La Palma, June 2007 VG # 18 A population of infrared PNe? Rapidly evolving, heavily obscured post-AGB stars (some showing already nebular emission) Perea Calderón et al. (in prep.) IRAS

APN 4, La Palma, June 2007 VG # 19 A population of infrared PNe? The missing population of massive PNe with 4-8 Msun progenitor masses? Perea Calderón et al. (in prep.) IRAS

APN 4, La Palma, June 2007 VG # 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 3 rd dredge up). –Very low-mass AGB stars (M<1.0 M ʘ ) belonging to the halo population may never become PNe. –Low-mass AGB stars (M< M ʘ ) will remain O-rich (optically bright Miras) during the whole AGB evolution (progenitors of type II O-rich PNe) –Intermediate-mass AGB stars (M> M ʘ ) may turn into C-rich Miras, leading eventually to C-rich PNe (s-process enriched as well) –High-mass AGB stars (M> M ʘ ) will activate the HBB and eventually become type I PNe However, this branching is strongly dependent on the metallicity of the progenitor star –3 rd dredge-up is more efficient at low metallicity –HBB becomes activated at a lower mass limit at low metallicity

APN 4, La Palma, June 2007 VG # 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?)