1. Presented by: Suklima Guha Niyogi Leiden Observatory 13/02/2013 USING INFRARED OBSERVATIONS OF CIRCUMSTELLAR DUST AROUND EVOLVED STARS TO TEST DUST FORMATION HYPOTHESES

2. In my thesis 1.Introduction, 2.Stellar evolution and Asymptotic Giant Branch (AGB) stars, 3.Optical properties of dust grains, 4.An introduction of dust mineralogy and morphology around O-rich AGB stars, 5.Investigating the dust properties of O-rich AGB star: T Cep, 6.Investigating the spectral dust features of O-rich AGB stars using spatially resolved spectroscopy, 7.Effect of composition-temperature-grainshape on the IR laboratory spectra of crystalline silicate minerals.

3. Life cycle of a star

4. AGB stars  Low and intermediate mass stars (0.8 – 8 M  ) lose 40 – 80% of original mass (10 -8 – 10 -4 M  per year) due to stellar pulsation.  That leads to dense, cool, extended stellar atmosphere where dust can be formed.  AGB stars surrounded by dusty envelopes.  Dust absorb stellar radiation and reemit photon in infrared (IR) region.

5. Stellar pulsation

6. Chemical evolution of AGB Stars

7. Types of AGB stars  In stellar atmosphere, C and O atoms become bound into carbon monoxide (CO), which is extremely stable.  The less abundant element is consumed by the formation of CO, leaving the more abundant element to dominate the chemistry and mineralogy of the dust. Oxygen-rich AGB star (C/O < 1) Carbon-rich AGB star (C/O > 1) S type AGB star (C/O = 1)

8. Circumstellar dust around Oxygen-rich AGB stars O-rich AGB star silicates olivinespyroxenes oxidessilica spinel alumina hibonite

9. Amorphous vs. crystalline silicate Ref: Molster & Kemper (2004)

10. Type of silicates  Olivine (Mg, Fe) 2 SiO 4 : Forsterite (Mg 2 SiO 4 ) Fayalite (Fe 2 SiO 4 )  Pyroxene (Mg, Fe)SiO 3 : Enstatite (MgSiO 3 ) Ferrosilite (FeSiO 3 ) Si O O O Mg O Si O O OO OO Mg

11. Observational spectral features of O-rich AGB stars Ref: Sloan et al. 2003

12. Dust formation hypotheses (i) Thermodynamic equilibrium condensation (Lodders & Fegley 1999), (ii) Formation of chaotic solids in a supersaturated gas followed by annealing (Stencel et al. 1990), (iii) Formation of seed nuclei in a supersaturated gas followed by mantle growth (Gail & Sedlmayr 1999). Ref: Grossman (1972) Predicted dust condensation sequence amorphous oxides amorphous silicates Sloan et al. 2003

13. Main questions  What are the main properties of dust (composition, lattice structure, shape, size, density, temperature)?  How do these properties change with time and location?  How can the dust formation process be understood by using these dust properties?

14. Investigating the dust properties of O-rich AGB star: T Cep

15. T Cep Light curve of T Cep Ref: Data of light curve taken from AAVSO, solid lines indicate the dates of ISO observations.

16. Spectral dust features of T Cep

17. Dust mineralogy around T Cep Ref: Fo9: Fayalite (Fe 2 SiO 4 ); Pitman et al. (2010) En1: Ferrosilite (FeSiO 3 ); Hofmeister et al. in prep. Cor: Corundum (Al 2 O 3 ); Gervais 1991

18. Model calculation  Optical constants  Material used : Forsterite (Mg 2 SiO 4 ) Fayalite (Fe 2 SiO 4 )  Size: 0.1 μm  Shape: SPH (Mie theory); CDS, CDE, DHS (Min et al. 2003)

19. Dust morphology around T Cep

20. Conclusions  Overall changes in dust features with pulsation cycle – mostly due to underlying dust continuum temperature.  The peak features at 9.7, 11.3, 13.1 μ m can be explained by mixture of crystalline silicates.  Long wavelength features (20, 32 μ m) suggest the presence of Fe-rich silicates, rather than expected Mg-rich silicates. Guha Niyogi et al. 2011-a (Astrophysical Journal)

21. Investigating the spectral dust features of O-rich AGB stars using spatially resolved spectroscopy

22. Long slit spectroscopy It is a technique used to obtain both spatial & spectral information simultaneously.

23. Long slit spectroscopy of AGB envelope Offset position # 3 Offset position # 5 Offset position # 7 10 9 7 8 5 6 3 4 1 2 0.36” 1 pixel projects=0.18’’ 0.54” The slit is 2 pixels wide and extraction region is 30 pixels long; Each slice consists 3 pixels along the slit. E N

24. 10 X 10 grid E N For SW Vir: d = 143 pc, v exp = 8.9 km/sec Each grid = 51AU X 77AU Dust shell region mapped = 515 AU X 722 AU Time taken dust to traverse from the star = 351 yr X 526 yr 1 pc = 3.08x10 16 m 1 AU = 1.49x10 11 m

25. Spatially resolved dust spectra of SW Vir

26. Monochromatic flux intensities of SW Vir

27. The ``broad” feature of SW Vir

28. Conclusions  The dust shell is not homogenous or isotropic, but axisymmetric in structure; may be caused by binarity of the system.  The spatial variation of the ``broad’’ features are consistent with having classic amorphous silicate dominate in the dense torus and chaotic mixtures of oxides and silicates dominate in the less dense regions away from the torus. Guha Niyogi et al. 2011-b (Astronomical Review)

29. How do these conclusions fit with dust formation hypotheses? Classic dust condensation sequence [(i) & (iii)]Chaotic solid formation [(ii)] Woitke (2006)

30. Thank You.