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
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
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
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
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
: 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
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
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
• 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
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
• 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
• 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
• 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
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