optical spectroscopy of post-agb stars

APN 4, La Palma, 18-22 June 2007 VG # 1

Optical spectroscopy of Post-AGB stars

Pedro García-LarioEuropean Space Astronomy Centre ESA,Villafranca del Castillo, Madrid


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?


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


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


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)


Recent progress

• A flatter distribution of

spectral types?

Cla

ssic

al p

ost

-AG

B s

tars

Ob

scu

red

po

st-A

GB

sta

rs

(from Suárez et al. 2006)

• Evidence for two different mass populations?


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)


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)


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)


Recent progress


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 56126

α Per


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


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


Recent progress

• Diffuse Interstellar band studies: indicators of circumstellar reddening?

(Reyniers et al. 2007)

(Luna et al. 2007)


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)


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)

Hen 3-401

IRAS 19590-1249

IRAS 20462+3416


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 (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 22036+5306

IRAS 17347-3139


A population of infrared PNe?• Rapidly evolving, heavily

obscured post-AGB stars (some showing already nebular emission)

Perea Calderón et al. (in prep.)

IRAS 17347-3139


A population of infrared PNe?• The missing population of

massive PNe with 4-8 Msun progenitor masses?

Perea Calderón et al. (in prep.)

IRAS 17347-3139


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


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?)

optical spectroscopy of post-agb stars

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