Optical spectroscopy of Post-AGB stars

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

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Limitations

• There is a limited number of galactic sources to
  study (326 in the most recent compilation)
• The Torun 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

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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

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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)

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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)
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Recent progress

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

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Recent progress

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

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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 193860155 (Pereira et al. 2004)
• IRAS 13266-5551, IRAS 17311-4924 (Sarkar et al.
  2005)
• IRAS 053811012 (Pereira Roig 2006)
• IRAS 08281-4850, IRAS 14325-6428 (Reyniers et al.
  2007)

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Recent progress
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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 Relativ
ely high galactic latitude (b10.0 deg)
a Per
Variable and complex H-alpha profile interpreted
as shock waves estimulating mass outflow
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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 (Mgt3-3.5
  M?)
• Rb overabundances confirms the activation of the
  22Ne neutron source in massive AGB stars (Mgt4 M?)

Li I 6707
Rb I 7800
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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

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Recent progress

• Diffuse Interstellar band studies indicators of
  circumstellar reddening?

(Reyniers et al. 2007)
(Luna et al. 2007)
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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 M?) C-rich PNe?

Hen 3-401
(from van Winckel 2003)
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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 lt -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?)

IRAS 19590-1249
Hen 3-401
IRAS 204623416
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Thin disk post-AGB stars
IRAS 220365306

• 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?
  (Mgt3-3.5 M?)
• May never become observable in the optical as PNe
  in the most extreme cases

IRAS 17347-3139
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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.)
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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.)
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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 (Mlt1.0 M?) belonging to
  the halo population may never become PNe.
• Low-mass AGB stars (Mlt1.0-1.4 M?) will remain
  O-rich (optically bright Miras) during the whole
  AGB evolution (progenitors of type II O-rich PNe)
• Intermediate-mass AGB stars (Mgt 1.0-1.4 M?) may
  turn into C-rich Miras, leading eventually to
  C-rich PNe (s-process enriched as well)
• High-mass AGB stars (Mgt3.0-3.5 M?) 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

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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?)