Insights and
Challenges in Stellar
Evolution
Lars Bildsten
Kavli Institute for Theoretical
Physics
University of California Santa
Barbara
Though much is known about how stars evolve, we
are only just beginning to probe rotation, interior
states and hydrodynamics in a meaningful way.
Most of the observational progress is from the
asteroseismic data from the Kepler and CoRoT
satellites, while theoretical progress is driven by
people and new computational tools.
Matteo Cantiello (KITP), Joergen ChristensenDalsgaard (Aarhus Univ.), Jim Fuller
(Caltech/KITP), Phil Macias (UCSB=>UCSC), Chris
Mankovich (UCSB=>UCSC), Kevin Moore
(UCSB=>UCSC), Bill Paxton (KITP), Dennis Stello
(U. Sydney) & Rich Townsend (U. Wisconsin)
After the Main Sequence:
Red Giant Branch and Clump Stars
Clump
stars
Paxton et al. ‘11
• M< 2 M develop degenerate Helium cores that increase in
mass with time until ignition in a flash => lifting
degeneracy => stable He burning in core (Thomas 1967). Is
Space-Based Photometry
CoRoT
27 cm diameter
Launched December 2006
Kepler
95 cm diameter
Launched March 2009
Non-Radial Stellar Oscillations
• P-modes (acoustic waves):
In the high wavenumber limit=>
Evenly spaced in Frequency (in Envelope!)
• G-modes (gravity waves):
In the high wavenumber limit=>
Evenly spaced in Period (in Core!)
Only Acoustic Waves seen in the Sun
Christensen-Dalsgaard
Acoustic Waves (p-modes) in Giants
• Persistent convection in the outer parts of the
giant excites standing acoustic waves (i.e. modes
with n radial nodes for each l)
=>Measures mean density
• The pulsation amplitudes were estimated (e.g.
Christensen-Dalsgaard; Kjeldsen & Bedding ‘05)
based on earlier solar work (c.f. Goldreich &
Keeley ’77), but ground based tests were a
challenge. . . as amplitudes were low. . .
CoRoT finds p-modes !
Kepler Observations
Bedding et al. ‘10 (Kepler)
Kepler
Bedding et al. ‘10 (Kepler)
• Large frequency
spacing is well
measured and
collapsing these
allows for
identification of
l=0, 1, 2 and often
l=3 acoustic modes
• These give mean
density
measurements
straight away!
• n~10-15. . . WKB
nearly valid
Highest Observed Frequency
Huber et al. 2011
• Highest observed
frequency is at the acoustic
cutoff of the photosphere.
• Higher frequency waves
have large damping due to
wave escape
• Combined with frequency
spacing, M and R inferred!
• However, internal state not
probed. . .
Degenerate Core => Burning Core
Bildsten et al. ‘12
• Time spent on the Red
Giant Branch (RGB)
at L>30L is
comparable to that
spent on the Red
Clump.
• Hard to distinguish a
clump star from an
RGB star in the field,
but let’s see what
seismology can do . .
Propagation Diagrams and Mixed Modes
• Scuflaire ’74; Osaki ’75 and
Aizenman et al. ’77 noted that
the acoustic waves couple to the
non-radial g-modes, which are
uniformly spaced in period at:
• Coupling is strongest for l=1, and
many g-modes exist between
each successive acoustic mode
Burning vs. Degenerate Cores
RGB
Clump
Internal Gravity Waves in the
Stellar Core then Detected
The g-mode spectrum is very dense in the core, but the modes
couple to the envelope well enough to emerge and be detected as
oscillations evenly spaced in period. Very stable and long-lived CLOCK!
A=p-dominated mode (np,ng=8, 476)
B=g-dominated mode (np,ng=7, 505)
Star near the RGB bump.
Bedding et al. 2011
Luminosity
Mosser et al. 2011
• Distinction of
stars on the Red
Giant branch
from those
doing He
burning in the
core (clump
stars)!!
• Discovered now
in nearly all RGB
stars (Stello et
al. 2013)
Q: How does the Electron Degenerate
Helium Core Transition to the Burning
State in a M<2 M Star?
A: Through a series of thermonuclear
instabilities over a 2 million year
period (Thomas 1967) that is often
overlooked in the literature and never
yet tested. . .
MESA is open source:
anyone (over 700
users!) can download
the source code,
compile it, and run it for
their own research or
education purposes.
Bill Paxton, Father of MESA
Temperature Evolution of First Flash
Macias et al. 2013
H Burning
Layer
Macias et al. 2014
Outer Envelope Undergoes
Kelvin Helmholtz Contraction
Macias et al. 2014
• The radial
expansion leads
to adiabatic T
decline in the H
burning layer,
shutting it off.
• Leads to rapid
KH contraction
of the giant to the
clump.
Core Flash Sequence from MESA
Bildsten et al. ‘12
Propagation Diagrams
Relic Layer
Bildsten et al. ‘12
• Contraction leads to outer
envelope profiles during
the flash nearly identical to
the red clump
• Coupling to core modes
during the flash will be as
strong as on clump.
• Core is in an intermediate
state => g-modes dif’t
Bildsten et al. ‘12
Repercussions
Bildsten et al. ‘12
• The period of successive flashes is 2 million
years, the thermal time through the core.
• Implies that one in ~50 stars near the clump
should be in an unusual state with a core
structure intermediate to the two states. .
• ~5000 giants studied by Kepler, so many
examples expected. . . Clear IDs starting to
emerge. .
Mosser et al. 2014
f=possible flashers
Solar Model “Trivially” Evolved
Courtesy C. Mankovich
See also Tayar & Pinsonneault ‘13
• Rotation rate in
the core can
impact the later
evolution of the
star, especially for
massive stars
• Certainly sets
rotation for the He
burning core and
eventually the
white dwarf
RGB Power Spectrum: Rotation!
G
G
Beck et al. ‘12
Stello et al. 2013
P
G
Inferred Core Rotation
Mosser et al. 2012
Core loses 95% of its Angular
Momentum after Leaving MS
Calculations with Magnetic Dynamos
Cantiello et al. ‘14,
Still not enough angular momentum loss from the shrinking core!
Conclusions
• New astronomical tools are revealing the
interiors of stars in ways previously impossible.
• Rotation can now be explored, an important
player in how stars finally collapse.
• Theory remains key in at least three ways:
– Reliable physical modeling (MESA+GYRE) needed
to fully interpret the data
– Novel analytics and mechanisms (Cantiello et al.
2014; Fuller et al. 2014) to transport J
– Suggestions of new ways to look at the data to
reveal rare physical states (e.g. core flash)