1. Improving our understanding of Stellar Evolution with PLATO 2.0
Santi Cassisi
INAF - Astronomical Observatory of Teramo, Italy

2. PLATO2.0 Stellar Evolution
The main focus of the PLATO asteroseismology program will be to support exoplanet science by providing:
stellar masses with an accuracy of better than 10%;
stellar radii to 1-2%;
stellar ages to 10% accuracy;
what is the current uncertainty?
stellar ages to 10% accuracy
The PLATO mission will provide the observational framework in order to:
improve understanding of stellar structure/physics;
identify “missing physics”;
improve our knowledge on stellar evolution;
Rauer’s talk & Rauer et al. (2014)

3. Stellar Evolution: the “ingredients”
Numerics
Boundary conditions
1D versus 3D
An evolutionary code
Equation of State
Radiative opacity
Conductive opacity
Nuclear reaction rates
Neutrino energy losses
Physical inputs
Overshooting
Superadiabatic convection
Semiconvection
Breathing pulses
Mixing treatment
Atomic diffusion
Radiative levitation
Microscopic mechanism
Mass loss
Rotation
Magnetic field
Internal gravity waves
Additional mechanism

4. what is the <effect> of the “ingredients’s uncertainty” on age dating of individual stars?
Numerics
Boundary conditions
1D versus 3D
An evolutionary code
≈0%
Equation of State
Radiative opacity
Conductive opacity
Nuclear reaction rates
Neutrino energy losses
≈4% for LMS - ≈0% for IMS
≈3%
≈2-3% for H-burn – 10% for He-burn
Physical inputs
Overshooting
Superadiabatic convection
Semi-convection
Breathing pulses
up to 10% for MS stars- ≈0% for He-burn
≈0%
>3%
≈10%
Mixing treatment
Atomic diffusion
Radiative levitation
≈6% for LMS - but…
≈0%
Microscopic mechanism
Mass loss
Rotation
Magnetic field
Internal gravity waves
≈0%
Additional mechanism

5. The impact of updated reaction rate on the transition mass between the Lower & Upper Main Sequence
1.2M – solar composition
14N(p,)15O - 17O(p,)18F
The LUNA reaction rate predicts a larger total mass for the stars showing a convective core during the central H-burning stage. The global effect on the age dating is of the order of 7%.

6. Atomic diffusion Diffusion velocity in the solar interior
downward motion
upward motion
Helioseismological constraints (Bahcall et al. 97) strongly support the efficiency of this process

7. Atomic diffusion: the effect on stellar models
to account for atomic diffusion leads to a reduction in the core H-burning lifetime of ~ 0.7Gyrs
atomic diffusion makes cooler the models (the effect depending on the Mass and Z…)
An uncertainty of a factor of 2 is still present in the estimates of the diffusion coefficients:
A change of a factor of 2 in the efficiency of atomic diffusion
causes a change in age estimates of old stellar systems of ≈2.5Gyr

8. Who is responsible for this non-canonical mixing?
More sophisticated models…
radiative accelerations can be up to 40% of gravity below the solar convection zone… for metal-poor stars, their value should be larger… (Turcotte et al. 98)
A theoretical evidence:
Slow extra-mixing at the bottom of convective envelope would improve the agreement concerning:
the observed abundances of Be and Li in the Sun (Richard et al. 96);
predicted sound speed profile and that derived by helioseismological data (Brun et al. 99);
spectroscopical measurements in Globular Cluster Stars (Gratton et al. 2003);
An observational … suggestion:
It is necessary to account for
atomic diffusion + radiative levitation + extra-mixing!
Who is responsible for this non-canonical mixing?

9. Non-canonical “transport mechanisms”
rotational induced mixings;
Baumann et al. (2010)
internal gravity waves;
Talon (2005)
Asteroseismology of stars showing mixing modes
can provide this information!
These processes to be included in stellar models require the calibration of free parameters that are poorly constrained by “standard” observations
Since rotational mixings affect not only chemicals but also the angular momentum distribution… to retrieve information about internal rotation profiles is mandatory!

10. Asteroseismic constraints from observations of RG Stars
The measurement with Kepler of the core rotation rates for hundreds of red giants is allowing an unprecedented view in the internal rotation evolution of evolved stars.
Mosser et al. (2013) - Deheuvels et al. (2014)
RGB stars
core He-burning stars
the inferred rotation periods are days for ≈ 0.2M He degenerate cores on the RGB and days for core He-burning stars

11. The state-of-the-art of theoretical modelling
Cantiello et al. (2014)
Some evidences:
the envelope slows down as expected due to expansion and AM conservation;
the core rotates about 10 – 103 times faster than the values inferred by asteroseismology;
the trend of the core rotational rate with the radius is reversed with respect the observed ones;
the amount of torque between the core and envelope is underestimated by RGB stellar models

12. A query for (additional) non canonical physical mechanisms?
Internal gravity waves
IGWs could be excited by the convective envelope during the RGB;
the details of their excitations and propagations are very poorly understood…;
Large scale magnetic field
if a magnetic field is present at the boundary of He core at the end of the MS stage, it could provide some coupling between core and envelope;
what is its origin? fossil origin? generated by a convective dynamo during the central H-burning stage?;
Dynamo action is fa- vorable as, given the typical rotational velocities of M⊙ stars during the main sequence, Rossby numbers are usually smaller than 1, implying an αΩ-dynamo could be at work in the core (10^4 – 10^5 Gauss)
Mass loss along the Red Giant Branch
Mass loss can significantly reduce the total mass and contribute to remove angular momentum;
a still poorly understood physical mechanisms…;

13. Can we still rely on Reimers’ “law”?
“Goldberg formula”
“Mullan formula”
“Judge & Stencel formula”
“modified Reimers formula”
Reimers’ law is rejected at the 3 level…
but we can’t distinguish among the alternatives! 13

14. RGB mass loss & Asteroseismology
Miglio et al. (2012)
< ΔM>/M=0.09±0.03 (random) ± 0.04 (systematic)
More data are mandatory!
PLATO 2.0 will lead to a major progress in this area due to its large-sky accessibility and its step-and-stare phase
PLATO 2.0 will lead to a major progress in this area!!!

15. The rotational rate pattern of low-mass HB stars (Behr 00 + 03, Recio-Blanco et al. 02, 04)
Globular Clusters
Field Stars
Some embarrassments:
they rotate…
some of them rotate fast…
it seems to exist a discontinuity…

16. Rotation in HB stellar models: the state-of-the-art
Models fail to simultaneously predict the high-rotation rate observed around 11,000K and the low rotation observed at higher Teff
Vrot(Turn-off)~1Km/s
Vrot(Turn-off)~4Km/s
Sills & Pinsonneault (2000)

17. Hot HB (sdB/sdO) stars: the great prospect of Asteroseismology
The B subdwarfs are hot and compact (Teff ∼ – K and log g ∼ 5.2−6.2) in the core He-burning stage;
Two groups of sdB pulsators offer favorable conditions for asteroseismology:
sdBVr (or V361 Hya, or EC 14026) stars oscillate with periods in the 100–600 s range, corresponding mostly to low-order, low-degree p-modes;
sdBVs (or V1093 Her) stars pulsate more slowly with periods of ∼1–2 h, corresponding to mid-order g-modes;
a few stars belong to both classes and are called hybrid pulsators, showing both p- and g-modes;
these modes are driven by a κ-mechanism induced by the so-called Z-bump in the mean Rosseland opacity, due to radiative levitation (Charpinet et al. 1996);

18. sdB/sdO stars: very peculiar properties
Heber (2009)
The problem about the origin of Extreme HB stars is how some kind of mass-loss mechanism in the progenitor manages to remove all but a tiny fraction of the H envelope at about the same time as the helium core has attained the mass (∼0.5 M⊙) required for the He flash;
This requires enhanced mass loss…
The link between diffusive processes, mass loss along the HB & rotation is an open issue;

19. sdB/sdO stars: evolutionary channels
Depending on the amount of the residual H-rich envelope mass, the star will still ignite He burning:
or at the top of the cooling sequence (Early Hot Flashers)
or along the WD cooling sequence (Late Hot Flashers)
The He Flash Induced mixing

20. Spectroscopical analysis are mandatory!
The occurrence of the He Flash-Induced Mixing Cassisi et al. (2003) – Miller Bertolami et al. (2008)
The final H abundance in the envelope is
…very low but…still non-negligible;
A large amount of matter processed by both H-
and He-burning is dredged to the surface…;
The final total metallicity is equal to 0.036, while
the surface is strongly Carbon- (XC0.029) and
He-enhanced (Y  0.96)
Spectroscopical analysis are mandatory!
To obtain accurate estimates of the total mass and of the envelope size is pivotal
PLATO 2.0 will increase the number of sdB stars that can be investigated in deep detail with the tools of asteroseismology

21. PLATO2.0 …What else…?
PLATO can actually push a step forward current generation of stellar models…
…maybe… a “new” more critical and ‘aggressive’ approach is required…