Improving our understanding
of Stellar Evolution with
PLATO 2.0
Santi Cassisi
INAF - Astronomical Observatory of Teramo, Italy
PLATO2.0
Stellar Evolution
The main focus of the PLATO asteroseismology program will be to support
exoplanet science by providing:



stellar
stellar
stellar
masses with an accuracy of better than 10%;
radii to 1-2%;
what is the current
ages to 10% accuracy;
accuracy
uncertainty?
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)
Stellar Evolution: the “ingredients”
An evolutionary code
Physical inputs
Mixing treatment
• Numerics
• Boundary conditions
• 1D versus 3D
•
•
•
•
•
Equation of State
Radiative opacity
Conductive opacity
Nuclear reaction rates
Neutrino energy losses
•
•
•
•
Overshooting
Superadiabatic convection
Semiconvection
Breathing pulses
Microscopic mechanism
• Atomic diffusion
• Radiative levitation
Additional mechanism
•
•
•
•
Mass loss
Rotation
Magnetic field
Internal gravity waves
what is the <effect> of the “ingredients’s uncertainty”
on age dating of individual stars?
An evolutionary code
Physical inputs
Mixing treatment
• Numerics
• Boundary conditions
• 1D versus 3D
≈4% for LMS - ≈0% for IMS
≈3%
•
•
•
•
•
Equation of State
Radiative opacity
Conductive opacity
Nuclear reaction rates
Neutrino energy losses
•
•
•
•
up to 10% for MS stars- ≈0% for He-burn
Overshooting
≈0%
Superadiabatic convection
Semi-convection
>3%
Breathing pulses
≈10%
Microscopic mechanism • Atomic diffusion
• Radiative levitation
Additional mechanism
≈0%
•
•
•
•
Mass loss
Rotation
Magnetic field
Internal gravity waves
≈2-3% for H-burn – 10% for He-burn
≈6% for LMS - but…
≈0%
≈0%
The impact of updated reaction rate on the transition mass
between the Lower & Upper Main Sequence
14N(p,)15O
-
17O(p,)18F
1.2M – solar composition
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%.
Atomic diffusion
Diffusion velocity
in the solar interior
upward motion
downward motion
Helioseismological
constraints
(Bahcall et al. 97) strongly support
the efficiency of this process
More sophisticated models…
A theoretical evidence:
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)
An observational … suggestion:
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);
It is necessary to account for
atomic diffusion + radiative levitation + extra-mixing!
Who is responsible for this non-canonical mixing?
Non-canonical “transport mechanisms”
• rotational induced mixings;
• internal gravity waves;
Baumann et al. (2010)
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!
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.
RGB stars
core He-burning stars
Mosser et al. (2013) - Deheuvels et al. (2014)
the inferred rotation periods are 10 - 30 days for ≈ 0.2M He degenerate cores
on the RGB and 30 - 100 days for core He-burning stars
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
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?;
 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…;
Can we still rely on Reimers’ “law”?
“modified Reimers
formula”
“Mullan formula”
1.4
dM æ L ö
µç
÷
dt
è gR ø
dM æ g ö
µç 3 ÷
dt
è R2 ø
“Goldberg
formula”
-0.9
“Judge & Stencel
formula”
dM
µ g -1.6
dt
Reimers’ law is rejected at the 3 level…
but we can’t distinguish among the alternatives!
dM
µ R 3.2
dt
RGB mass loss & Asteroseismology
< ΔM>/M=0.09±0.03 (random) ± 0.04 (systematic)
More data are mandatory!
Miglio et al. (2012)
PLATO 2.0 will lead to a
major progress in this area!!!
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…
Hot HB (sdB/sdO) stars: the great prospect of Asteroseismology
The B subdwarfs are hot and compact (Teff ∼ 22000
– 40000 K and log g ∼ 5.2−6.2) in the core Heburning 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);
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;
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
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 0.0004
…very low but…still non-negligible;
•A large amount of matter processed by both Hand 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
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…