ImBaSE17 Garching 3-7 July Maurizio Salaris

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1 ImBaSE17 Garching 3-7 July 2017 Maurizio Salaris

2 How accurately can we predict radii, effective temperatures, chemical stratification (hence surface abundances and evolutionary timescales) of lowintermediate-mass stars? Convection Thermohaline mixing Element transport in radiative regions Mass loss

3 CONVECTION i) How extended is the mixing region beyond the formal convective border (convective boundary mixing -CBM)? Instantaneous mixing in this region? Diffusive mixing (following Freytag et al. 1996)? Diffusion, Rotation, micro-ph t X i t = 1 ρr Mr 2 r is given by D ovρr 2 X i r with D ov = D c exp 2z «fh p ficient inside the convective reg (D c =(1/3)α MLT v c H P ) ssure scale height at the ii) iii) What is the temperature gradient in this CBM region? Adiabatic (overshooting) or radiative (penetration)? How do we reduce to zero the extension of the CBM region when convective core masses approach zero? iv) What is the temperature gradient in surface convective regions? Choices made affect evolutionary times (star counts), luminosities, T eff, loops in the Colour-Magnitude-Diagrams, predicted populations of variable stars in stellar populations, chemical profiles, asteroseismic properties.

4 Helium burning core mixing Core Expansion C produced by He-burning Opacity increases F c Radiative gradient discontinuity at the convective core boundary rad ad Mass of fully mixed core increases See, e.g. Castellani et al. (1971), Gabriel et al. (2015)

5 What happens now? When Yc decreases below ~0.7, a partial mixing may be invoked beyond the boundary of the convective core (called semiconvection). But other options do exist

6 1 M solar initial compositon Basic properties of oscillation modes Lamb frequency: with four tfivedif-, no overhoot(secmaximal ν for Buoyancy frequency: Figure 3. Internal properties of the 1 M models with four different mixing prescriptions when Y = 0.4 in the centre. The helium mass fraction Y,ratiooftemperaturegradients rad Y/ c ad =0.4,andBrunt Väisäläfrequency are shown. The four mixing prescriptions are no overshoot (Section 2.3.1; black), standard overshoot (Section 2.3.2; cyan), classicalsemiconvection (Section 2.3.3; orange),andmaximalovershoot(section 2.3.4; magenta). This colour scheme is used for mixing comparisons throughout thispaper. Gravity mode period spacing (same l and ABC consecutive n) from Kepler stars favours the maximal which is defined as overshoot scenario Rs E = r 1 [ ξ 2 r + l(l +1)ξ 2 h ] ρr 2 dr Mξ r (R s ) 2, (11) Studies also by Bossini et al. (2015, 2017) where R s is the radius at the outermost point, r 1 is the location of the innermost mesh point, and ξ r and ξ h are the radial and horizontal displacement eigenfunctions, respectively, which are both func- Constantino et al. (2015) Different mixing schemes, different C/O stratifications C O Straniero et al. (2003)

7 Superadiabatic gradient : MIXING LENGTH THEORY T eff mismatches/trends between theory and observations might have nothing to do with a variation of the mixing length α MLT

8 3D radiation hydrodynamics simulations predict a variable mixing length α MLT Trampedach et al. (2014)

9 Are stellar models very affected by the variation of α MLT? 3D radiation hydrodynamics calibration (mixing length and boundary conditions) by Trampedach et al. (2014) Solar metallicity only At most just K difference between solar and variable α calibration Salaris & Cassisi (2015)

10 THERMOHALINE MIXING The H-burning front moves outward into the stable region, but preceding the H-burning region proper is a narrow region, usually thought unimportant, in which 3 He burns. The main reaction is 3 He ( 3 He, 2p) 4 He: two nuclei become three nuclei, and the mean mass per nucleus decreases from 3 to 2. Because the molecular weight (µ) is the mean mass per nucleus, but including also the much larger abundances of H and 4 He that are already there and not taking part in this reaction, this leads to a small inversion in the µ gradient. 1M solar composition Eggleton et al. (2006)

11 Field halo stars RGB extra-mixing after first dredge up 1 st dredge up 0.8 M metal poor RGB model Salaris et al. (2002) Bottom conv. envelope at δm=1 Bottom H-burning shell at δm=0 Gratton et al. (2000) 0.8 M [Fe/H]= 1.58 D th = C th K c p ρ «φ δ µ ( rad ad ) being a free parameter related to the aspe C th =(8/3)π 2 α 2, g elements 11 with α free parameter (Charbonnel & Zahn 2007)

12 1.25 M solar initial composition A( 7 Li) C t = 1000 Surface abundances for a given C t are very sensitive to timestep and mesh resolution adopted in the stellar model calculations STARS SE-G-V2.3 MONSTAR SE-V5.5 MESA Also, hydro-simulations of this process do not give definitive results, even though they hint that C t < 1000 log 10 L/L Lattanzio et al. (2015)

13 Atomic diffusion on the Main Sequence Treatment from first principles Richard et al. (2002)

14 Puzzling observations Korn et al. (2007)

15 Inhibition of diffusion from/into the convective envelopes with ad-hoc counteracting diffusive mixing (called generically turbulence) 0.8M, [Fe/H]= 1.3 model, in the latter phase of its MS evolution. The vertical thin line marks the bottom of the convective envelope

16 Data from Vick et al. (2013) Effect of mass loss

17 Rotation inhibits atomic diffusion from surface and also increases evolutionary timescales (rotational mixing counteracts the development of chemical gradients) Georgy et al. (2013) black red Brown et al. (2016) C F275W,F336W,F438W (mag) C F275W,F336W,F438W (mag) NGC 2808 HB NGC 6723 HB [Fe/H]=-1.10 Category 4 NGC 2808 HB NGC 0288 HB [Fe/H]=-1.32 Category 4 (+0.20,+0.05) (+0.40,+0.08) NGC 2808 HB NGC 6341 HB [Fe/H]=-2.31 Category 4 NGC 2808 HB NGC 5904 HB [Fe/H]=-1.29 Category 4 (+0.30,+0.06) (+0.25,+0.04) Atomic diffusion in HB stars Also problem matching Abundances of sdb stars (Hu et al. 2011) C F275W,F336W,F438W (mag) C F275W,F336W,F438W (mag) Figure 5. Continued NGC 2808 HB NGC 6254 HB [Fe/H]=-1.56 Category 4 NGC 2808 HB NGC 4833 HB [Fe/H]=-1.85 Category 4 (-0.30,-0.05) (-0.30,-0.05) m F275W - m F438W (mag) NGC 2808 HB NGC 6779 HB [Fe/H]=-1.98 Category 4 NGC 2808 HB NGC 6681 HB [Fe/H]=-1.62 Category 5 (-0.30,-0.02) (+0.20,+0.00) m F275W - m F438W (mag) M15, M68, M92 Michaud et al. (2011)

18 EFFICIENCY OF ATOMIC DIFFUSION AND AGE OF FIELD STARS MS 8.5 Gyr 6 Gyr 5 Gyr Δ[Fe/H]=±0.10 Black solid Red solid Red dashed Red dotted Large age uncertainty if we do not know the stellar mass

19 Zero-order test with atomic diffusion inhibited from envelopes Spectroscopy of GCs tells us that gravitational settling-levitation are strongly inhibited (at least) for the convective envelope Only below conv env. 1.3 Gyr age difference Red

20 Diffusion and more in open clusters. An example Hyades (Turn off mass ~ 2.3 M ) Surface brakes Increase diff rot. More rotational mixing and Li destruction Charbonnel & Talon (2009) Gravity waves decrease diff. rotation. Li destruction decreases

21 Why are we talking about gravity waves?(internal gravity waves - IGWs) des that behav KIC cillation spectr These IGWs are expected to be generated by the injection of kinetic energy from a turbulent region into an adjacent stable region Inferred rotational profile 0.84M lower RGB star (Deheuvels et al. 2012) = Some extra angular momentum transport needs to be included in the current generation of rotating stellar models, because they predict much larger rotation rates for stellar cores compared to the surface Model from Ekstroem et al. (2012)

22 RGB mass loss From Scl HB Catelan (2009) IR excess RGB globular cluster stars (Origlia et al 2014) Synthetic HB modelling of Sculptor with known SFH (Salaris et al. 2015)

23 AGB stars Mass loss, boundaries of convection UNCERTAIN!!!!!

24 Uncertain yields Different AGB mass loss law Doherty et al. (2014) Super-AGB models only Z=0.001 M=6.5, 7.0, 7.5 M V13 M=6.0, 6.5,7.0, 7.5 M S10 M= 8.0, 8.5, 9.0 M

25 HOPES FOR THE FUTURE. Convection - test Magic et al. (2015) 3D-hydro α MLT calibration (covers a large [Fe/H] range) once they provide their boundary conditions - Asteroseismology to help for boundary mixing and core He-mixing? - Hopefully, increasingly more realistic 3D hydro-simulations - Eclipsing binaries (M-R diagrams) - Asteroseismology of WDs Thermohaline mixing More RGB spectroscopy on clusters of varying age to put stronger observational constraints but also improved hydro simulations and also stellar model calculations following criteria set out by Lattanzio et al. (2015) Element transport in radiative regions?? Mass loss RGBs hopefully more constraints from modelling of HBs in Local Group dwarf galaxies (e.g. Salaris et al. 2015)

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28 Instabilities in non-rotating stars ad χµ χ T µ L

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