Asymptotic Giant Branch Intro Talk

Intro talk by Giada Pastorelli & Steven Goldman

1. Asymptotic Giant Branch stars Intro Talk ISM*@ST Group Meeting Giada Pastorelli & Steve Goldman gpastorelli@ stsci.edu sgoldman@stsci.edu October 19, 2020
2. Outline 1. Introduction: AGB stars 2. Metallicity 3. Current Observations 4. AGB samples 5. Future work 2 Pastorelli & Goldman STScI 10/19/20 Credit: ALMA (ESO/NAOJ/NRAO)/M. Maercker et al. • AGB basics • Modeling • Observations • State of the field • Open questions
3. AGB basics & modeling
4. AGB stars in the HRD 4 • Initial masses: ~ 0.8 – 6-8 M☉ • High L: up to a few 104 L ☉ • Low Teff : below 3000 K • Radius: a few 100 R ☉ • Mass-loss: 10-8 – 10-4 M☉ /yr • Variability: ~ 100 – 1000 days • Rich Nucleosynthesis • Molecules and dust form in the extended atmosphere • Fate: White Dwarfs Pastorelli & Goldman STScI 10/19/20
5. AGB schematic structure 5 Pastorelli & Goldman STScI 10/19/20
6. Early-AGB Phase 6 O.R. Pols 2011 (Lect. Notes) Pastorelli & Goldman STScI 10/19/20
7. Early-AGB Phase 7 O.R. Pols 2011 (Lect. Notes) Pastorelli & Goldman STScI 10/19/20
8. Early-AGB Phase 8 • Central He exhaustion: CO core contracts • He-shell burning • Short double shell burning (H, He) • H-shell extinguished (He-rich zone expansion) • Entire envelope expands (H-K) • CO core becomes degenerate O.R. Pols 2011 (Lect. Notes) Pastorelli & Goldman STScI 10/19/20
9. Second dredge-up 9 • Mi ≳ 4 M☉ : 2nd dredge-up • Mi ≲4 M☉ : H-shell is active à no 2nd dredge-up • Effects of 2DU: 1. Increase of He- and N-rich surface abundance 2. Reduction of the mass of the H-exhausted core ØLimits the mass of the WD remnant O.R. Pols 2011 (Lect. Notes) Pastorelli & Goldman STScI 10/19/20
10. TP-AGB stars: schematic internal structure 10 Giada Pastorelli STScI 5/21/20 Karakas & Lattanzio (2014) PASA
11. Thermal Pulses: He-shell instability 11 • Degenerate CO core • He-shell instability • Intershell convection • H-shell extinguished • Penetration of CE Adapted from N. Langer Pastorelli & Goldman STScI 10/19/20
12. Thermal Pulses: third dredge-up 12 Third Dredge-Up (Mi > 1.5 M☉ depending on Zi) He and H burning products reach the surface: Ø C ↗ Series of 3DU events during the TP-AGB phase Ø # C atoms > # O atoms Adapted from N. Langer Pastorelli & Goldman STScI 10/19/20 C/O<1 → C/O>1 M-stars → C-stars
13. Thermal Pulses: Hot Bottom Burning 13 Hot Bottom Burning (Mi > 3 M☉ depending on Zi) H-burning through CNO cycle 1) CN cycle → C C/O>1 → C/O<1 C-stars → M-stars 2) ON cycle → O ↘ C/O<1 → C/O>1 M-stars → C-stars 7Li, 23Na, and 25,26Mg HBB produces Nitrogen but also Adapted from N. Langer Pastorelli & Goldman STScI 10/19/20
14. Thermal pulses: s-process 14 Slow-neutron capture reaction on Fe nuclei Neutron sources: 1. 22Ne(⍺, n)25Mg 14N à 22Ne by He burning in massive AGB stars ( ~ 3 Msun) 2. 13C(⍺, n)16O formation of a 13C pocket in low mass AGB stars Elements heavier than Fe: Zr, Y, Sr, Tc, Ba, La and Pb observed in AGB spectra Adapted from N. Langer Pastorelli & Goldman STScI 10/19/20
15. TP-AGB: 3DU & HBB 15 Pastorelli & Goldman STScI 5/21/20 No 3DU 3DU HBB 3DU Models from Marigo+08
16. Carbon-to-Oxygen ratio 1 Msun 3 Msun 5 Msun Carbon Oxygen Relative Abundance Oxygen rich Carbon rich Oxygen rich 16 Pastorelli & Goldman STScI 10/19/20 C/O < 1 C/O > 1 C/O < 1
17. AGB stars: chemical type evolution 17 Pastorelli & Goldman STScI 10/19/20 C/O > 1 C/O< 1
18. Modeling the third dredge-up 18 1) Onset → Tb dred : minimum temperature to be reached at the base of the convective envelope → Mc,min : minimum core mass for the onset of the sequence of 3DUP events 2) Efficiency Marigo+13 Karakas & Lattanzio (2014) PASA Pastorelli & Goldman STScI 10/19/20
19. Modeling the third dredge-up 19 Giada Pastorelli STScI 5/21/20 1) Onset → Tb dred : minimum temperature to be reached at the base of the convective envelope → Mc,min : minimum core mass for the onset of the sequence of 3DUP events 2) Efficiency Pastorelli+19 Karakas & LaQanzio (2014) PASA
20. Third dredge-up: carbon star luminosity function 20 Pastorelli & Goldman STScI 10/19/20
21. Third dredge-up: consequences 21 Onset • Minimum initial mass for C-star formation Efficiency 1. Amount of dredged-up material à Chemical enrichment of the photosphere 2. Core-mass growth à Initial-to-final mass relation (IFMR) 3. Maximum initial mass for C-star formation Pastorelli & Goldman STScI 10/19/20
22. AGB variability 22 Long Period Variables: • Multiple radial modes • Periods: a few days – a few years • Amplitude: milli-magnitude up to several magnitudes • Classified according to their variability amplitude: Ø Mira Ø Semi-regular Ø Irregular Pulsation mechanism not fully understood : coupling between stellar oscillation and convection https://www.aavso.org/vsots_rrlyr Pastorelli & Goldman STScI 10/19/20
23. Time scales 23 1. Dynamical oscillation of the envelope → Time required for a soundwave to traverse the envelope 2. Instability that triggers thermal pulses → Time required for nuclear energy to double the thermal energy in the He-burning region 3. Thermal relaxation timescale → the return to a quiescent phase after a thermal pulse 4. Building up of the conditions for the helium burning runway hundreds of days - few years less than a few years a few 10^5 yr to 30 yr depending on the core mass Stellar pulsations (i.e. variability) and thermal pulses are two different phenomena Very different timescales!!! Pastorelli & Goldman STScI 10/19/20
24. Mass-loss 24 Several empirical, semi-empirical relations as function of stellar parameters Two distinct regime: • Pulsation assisted dust driven winds: based on dynamical model atmospheres (Höfner+15, Ma>sson+10) • Pre-dust winds mechanism not fully understood: magnetoacustic waves in the cool cromosphere of evolved giants? (Schröder & Cuntz +05, Cranmer & Saar+11) Credit: I. McDonald Pastorelli & Goldman STScI 10/19/20
25. Mass-loss: dust-driven regime 25 Credits: S. Hoefner Pastorelli & Goldman STScI 10/19/20
26. Mass-loss: consequences 26 • AGB lifetime and number of thermal pulses: Ø Number of 3DU episodes and the duration of HBB Ø Level of chemical enrichment Ø Remnant mass (IFMR) Pastorelli & Goldman STScI 10/19/20
27. Dusty circumstellar envelopes 27 Adapted from J. Hron Pastorelli & Goldman STScI 10/19/20
28. Dusty circumstellar envelopes 28 Silicate emission From DUSTY models (Ivezic & Elitzur1997) Pastorelli & Goldman STScI 10/19/20
29. Circumstellar dust effects on CMDs 29 No dust LMC Spitzer TRILEGAL simulation Marigo+10 (IAUS) Pastorelli & Goldman STScI 10/19/20
30. Circumstellar dust effects on CMDs 30 LMC No dust Dust Spitzer TRILEGAL simulation TRILEGAL simulation Marigo+10 (IAUS) C-stars O-rich Pastorelli & Goldman STScI 10/19/20
31. TP-AGB lifetimes 31 • Total lifetimes up to a few Myr • Initial mass and metallicity dependence Pastorelli+20 Pastorelli & Goldman STScI 10/19/20
32. Contribution of TP-AGB stars to integrated luminosities 32 Pastorelli & Goldman STScI 10/19/20 Pastorelli+20
33. Main uncertainties 33 • Uncertainties of all previous evolutionary phases • Treatment of convection → HBB and 3DUP • Formation and growth of dust grains • No satisfactory theory for mass loss • Interplay between mass loss, 3DUP, HBB Pastorelli & Goldman STScI 10/19/20
34. Main uncertainties 34 • Uncertainties of all previous evolutionary phases • Treatment of convection → HBB and 3DUP • Formation and growth of dust grains • No satisfactory theory for mass loss • Interplay between mass loss, 3DUP, HBB Pastorelli & Goldman STScI 10/19/20 Calibration of mass-loss and 3DU as a function of Mi and Zi using observation of resolved AGB stars
35. Observations
36. Identifying AGBs Pastorelli & Goldman STScI 10/19/20 36 Near-IR IR SMC (Boyer et al. 2019) dusty LMC (data from Riebel et al. 2012)
37. Chemical types Pastorelli & Goldman STScI 10/19/20 37 (C/O)f 8 8.5 9 9.5 10 10.5 log(age/yr) -2.2 -1.7 -1.2 -0.6 -0.3 0.0 0.4 [Fe/H] 1 10 (C/O)f 8 8.5 9 9.5 10 10.5 log(age/yr) -2.2 -1.7 -1.2 -0.6 -0.3 0.0 0.4 [Fe/H] C/O < 1 Ventura et al. (2012) Boyer et al. 2013, Marigo et al. 2013 Boyer et al. 2019 Carbon star mass ranges determined by: • C/M raSos • Stellar evoluSonary models • White dwarf iniSal-final mass relaSons
38. Dust and metallicity Pastorelli & Goldman STScI 10/19/20 38 Increased dust Boyer et al. 2015a,b
39. Pulsation Pastorelli & Goldman STScI 10/19/20 39 • Radial pulsations • Evolve through sequences • Gives us stage of evolution • Long secondary period remains a mystery • Seemingly unaffected by metallicity Trabucchi et al. 2017 D OGLE pulsation sequences AGB evolutionary sequence (orange) Trabucchi et al. 2017
40. Wind speeds Pastorelli & Goldman STScI 10/19/20 40 Engels et al. 2012 Ramstedt et al. 2009 oxygen carbon S-type oxygen
41. Wind-driving mechanism Pastorelli & Goldman STScI 10/19/20 41 Winds require delicate balance of: • Condensation radius • Condensation temperature • Gas density • Absorption coefficient • Phase Höfner et al. 2015 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Time [yr] Radial distance [R_star] dust-driven outflow pulsating atmosphere Observations: • 0.3 um grains at 2 R* • Silicates at a few R* • condensation radius and grain composition • Wind speeds
42. Mass loss & dust production (recent findings) Pastorelli & Goldman STScI 10/19/20 42 1. A third of AGB show asymmetric circumstellar envelope 2. Incorrect geometry made lead to incorrect mass-loss rate by 2 orders of magnitude. 3. All grids for determining mass loss rates are spherically symmetric. Decin et al. 2019