Evolution of gas and dust in AGB stars Kay Justtanont Chalmers - PowerPoint PPT Presentation

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 H 2 O 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 H 2 O and other oxides.  In C-rich stars, we detect carboneceous molecules, e.g., C 2 H 2 , HCN, … (H 2 O)

HIFISTARS GTKP

Molecules II  Molecules detected depend on the C/O ratio of the photosphere.  In M- (O-rich) stars , we observe H 2 O and other oxides.  In C-rich stars, we detect carboneceous molecules, e.g., C 2 H 2 , HCN, … (H 2 O)  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 5 µ m.  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

H 2 O 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, H 2 O, SiO can mase.

 OH masers are thought to be radiatively pumped by the OH absorption doublet at 35 µ m.

 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 10 µ m 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 (Mg 2 Sio 4 ) in post-AGB stars De Vries et al. 2011

Waters et al., 1996, A&A 315, L361

21 µ m feature in C-rich post-AGB stars Justtanont et al., 1996, A&A 309, 612

30 µ m 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.