Evolution of gas and dust in
AGB stars
Kay Justtanont
Chalmers University of Technology
Stellar evolution
  For low- and intermediate-mass stars, they
enter the red giant and subsequently,
asymptotic giant branch (AGB) phases.
  During the AGB, a star loses a significant
of its initial main-sequence mass.
  Mass loss can be observed by studying
dust and/or gas which form an extended
circumstellar shell (CSE) around the star.
  As the star evolves off the AGB, fast winds develop
which impact on the AGB mass loss => shocks
  The central star becomes hotter and ionize the CSE
around it => PDR.
Molecules in CSEs
  Since the photosphere is relatively cool,
molecules can form and ejected via the
mass-loss process.
  CSE environment also promote chemistry
due to the cool, dense and warm conditions
in the inner CSE.
  Molecules can be photodissociated in the
outer part due to interstellar radiation
field.
Molecules II
  Molecules detected depend on the C/O ratio
of the photosphere.
  In M- (O-rich) stars , we observe H2O and
other oxides.
R Dor – an O-rich CSE
Justtanont et al., 2012, A&A 537, A144
Molecules II
  Molecules detected depend on the C/O ratio
of the photosphere.
  In M- (O-rich) stars , we observe H2O and
other oxides.
  In C-rich stars, we detect carboneceous
molecules, e.g., C2H2, HCN, … (H2O)
HIFISTARS GTKP
Molecules II
  Molecules detected depend on the C/O ratio
of the photosphere.
  In M- (O-rich) stars , we observe H2O and
other oxides.
  In C-rich stars, we detect carboneceous
molecules, e.g., C2H2, HCN, … (H2O)
  In S-star (C/O ~ 1), both oxides and C-base
molecules have been seen.
Schöier et al., 2011, A&A 530, A83
Habing et al., 1994, A&A 286, 523
IRC+10 216
  C-star which is the brightest source at 5µm.
  At a distance of 120 pc (Schöier & Olofsson
2000), it is one of the nearest C-stars with
moderately high mass-loss rate and shows
remarkable inventory of molecules.
  > 40 molecules have been detected, many
carbon chains but also metal salts and
hydrides.
Patel et al. 2011,
ApJS 192, 17
H2O in IRC+10216
Decin et al., 2010, Nature 467, 64
Multiple shells seen in IRC+10216
Decin et al., 2011, A&A 534, 1
Masers
  Seen in O-rich CSEs
  Some transitions of OH, H2O, SiO can mase.
  OH masers are thought to be radiatively
pumped by the OH absorption doublet at 35
µm.
  From maser observations => magnetic field.
OH44.8-2.3
Amiri et al., 2011, A&A, in press
  High angular resolution observations of proper
motion of maser spots => distance
Dusty circumstellar envelopes
  The species of dust formed also reflect the
chemistry in the photosphere.
  Oxides are formed in O-rich environment
Sloan et al., 2011, ApJ 729, 121
Silicate 10µm feature and mass-loss rate
Dusty circumstellar envelopes
  The species of dust formed also reflect the
chemistry in the photosphere.
  Oxides are formed in O-rich environment
  Carbonaceous dust are observed in C-rich
stars.
Speck et al., 2005,
ApJ 634, 426
Dusty circumstellar envelopes
  The species of dust formed also reflect the
chemistry in the photosphere.
  Oxides are formed in O-rich environment
  Carbonaceous dust are observed in C-rich
stars.
  For S-stars, the dust is mainly oxides
Water-ice in extreme OH/IR stars
OH 32.8-0.3
Waters et al., 1996 A&A 315, L361
Justtanont et al., 2006, A&A 450, 1051
Crystalline silicates
OH 32.8-0.3 ISO-SWS
HD161796
Hoogzaad et al., 2002, A&A 389, 547
Fosterite and enstatite in post-AGB stars
Molster et al., 2002, A&A 382, 241
Fosterite (Mg2Sio4) in post-AGB stars
De Vries et al. 2011
Waters et al., 1996, A&A 315, L361
21µm feature in C-rich post-AGB stars
Justtanont et al., 1996, A&A 309, 612
30µm feature in post-AGB stars : MgS
Hrivnak et al., 2009, ApJ 694, 1147
PAHs in post-AGB stars and PNe
Beintema et al., 1996, A&A 315, L369
Molecules in PPNe
SUCCESS - Hershel open-time
program
Bujarrabal et al.,
2010 ,A&A 521, L3
Bujarrabal et al.,
2010, A&A 521, L3
Shock chemistry and PDR
  Gas heating can be due to either shock or FUV
photons.
  Line widths of molecular lines - shock?
  Cooling is done via atomic fine structure lines of [OI]
and [CII]
  [OI] 63 / [CII] 158 is an indication of shock / PDR.
  [SI] 25 an indication of shock chemistry.