Transcript bildsten

Hearing the Stars!
Lars Bildsten
Kavli Institute for Theoretical
Physics
University of California Santa
Barbara
Happy Birthday David!!
KITP Residence: Opens January 2017
Though we know how stars evolve, we are only just
beginning to probe rotation, interior states,
hydrodynamics and magnetism in a meaningful way.
Most 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=>Claremont), 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).
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. . .
Revealing the Population (CoRoT)
Miglio et al. 2013
Highlights so Far
• Acoustic waves seen in nearly all evolved stars with
amplitudes of 3-200 parts per million.
• Measured frequency
spacing and maximum
observed frequency
give R, M and D for
>10,000 stars across the
galaxy. Great test for
GAIA and new galactic
science enabled.
• Useful diagnostics for
extra-solar planets.
Where tested, distance estimates are
~10% accurate (Silva Aguirre+ ’13)
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 . .
• More modes are
present than allowed
for by acoustic waves!
• Most prevalent
plethora is near the
l=1 modes, which
scatter away from
simple expectations.
Bedding et al. ‘10 (Kepler)
Inferring the State
of the Stellar Core
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)
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 and Magnetism 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
– Recent theoretical (Fuller et al. 2015) work showed how
dipole suppression arises from strong B in core (Stello et
al. ‘16, Cantiello, Fuller, LB’16)
We now have a New way to Hear!
The width of 10 nuclei over the earth’s diameter!
The width of one atom between earth and sun!