Transcript Asteroseismology and the Solar

Asteroseismology and the
Solar-Stellar Connection
Travis Metcalfe (NCAR)
Collaborators: W. A. Dziembowski,
P. G. Judge, M. Snow
Seeing with sound
• Sun-like stars have their
own internal source of
“ultrasound” waves
• Turbulent convection
creates acoustic noise in
a broad frequency range
• Some of the frequencies
are resonant inside the
star’s spherical cavity
Helioseismology
Larger
 Surface spatial scale 
Smaller
Shallower
 Interior sampled 
Deeper
http://sohowww.nascom.nasa.gov/
• Millions of independent
oscillation frequencies
excited simultaneously
• Each mode samples the
interior in a different and
complementary way
• Only the lowest degree
modes are detectable in
distant stars (l < 3)
Asteroseismology
• Oscillations decomposed
into spherical harmonics,
small features cancel out
• Low-degree modes probe
deepest into the interior,
several dozen detectable
• Such data will allow lowresolution inversions of
the inner 30% of radius
Gough & Kosovichev (1993)
Observing techniques
Bouchy et al. (2004)
Velocity variation
(ground)
Fletcher et al. (2006)
Light variation
(space)
Ground-based: a Cen A+B
Butler et al. (2004)
a Cen A
Frohlich et al. (1997)
Sun
Kjeldsen et al. (2005)
a Cen B
• Nearest stellar system,
masses slightly above
and below solar mass
• The range of excited
frequencies scales
inversely with radius
• Amplitudes and mode
lifetimes mostly agree
with our expectations
MOST: differential rotation
• Three seasons of precise
photometry for the young
solar-type star k1 Ceti
• Detailed spot modeling
from 30, 20, and 15 days
of uninterrupted data
Ca HK period
Walker et al. (2007)
• Latitudinal differential
rotation pattern has same
functional form as Sun
CoRoT: CZ depth
• Expected seismic signal
from 5-month observation
of the star HD 49933
• Second differences (d2n)
measure deviations from
even frequency spacing
• Base of the convection
zone and He ionization
create oscillatory signals
Baglin et al. (2006)
Kepler: 105 solar-type stars
• 105 square degrees for
4-6 years, slightly above
galactic plane in Cygnus
• Intensity measurements
and rotation profiles for
100,000 solar-type stars
• Stellar radii and ages
down to 12-14 magnitude
from 1-year seismic data
Christensen-Dalsgaard et al. (2007)
Solar activity
• Active regions on the Sun
are bright in Mg II (UV)
and Ca II (optical)
• Measure ratio of total
emission in line cores to
flux in the wings
• Use disk-integrated time
series measurements to
track magnetic cycles
http://spacescience.spaceref.com/
Stellar activity cycles
Frohlich & Lean (2004)
• Sun-as-a-star data show
10% variation in the Mg II
index through solar cycle
• Similar magnetic activity
cycles can be observed
in other solar-type stars
Dravins et al. (1993)
• Mean activity level and
cycle period scale with
Rossby number (Prot / tc )
Solar p-mode shifts
Salabert et al. (2004)
• Solar p-mode shifts first
detected in 1990, depend
on frequency and degree
• Even the lowest degree
solar p-modes are shifted
by the solar cycle
• Unique constraints on the
mechanism could come
from solar-type stars
Libbrecht & Woodard (1990)
Theoretical interpretation
• Magnetic perturbations
modify the near-surface
propagation speed
Goldreich et al. (1991)
Dziembowski & Goode (2005)
• Also leads to decreased
convective velocity and
change in temperature
• Distinct behavior for solar
f-modes and p-modes
confirms these sources
Observations: b Hydri
Dravins et al. (1993)
• Only star with a known
activity cycle and multiepoch asteroseismic data
• Widely studied as “future
Sun” (age ~ 7 Gyr) with a
relatively low activity level
• Reanalysis of archival
IUE data, including more
recent observations
Metcalfe et al. (2007)
Scaling from the Sun
• Parameterize shifts with
Dn ~ A0 (R / M) Qj(Dc) and
fit the MDI p-mode data
• A0 ~ activity level, while
the depth of the source
Dc ~ Hp ~ L1/4 R3/2 / M
• A0 captures most of the
variation if depth fixed at
Dc = 0.3 Mm for the Sun
Metcalfe et al. (2007)
Frequency dependence
• Solar p-mode shifts show
spread with degree and
frequency dependence
• Normalizing with our
expression removes most
of both dependencies
• Rise at low frequencies
due to fixed Dc but largest
shifts at higher frequency
Metcalfe et al. (2007)
Results: b Hydri
Bedding et al. (2007)
MODEL
OBSERVATIONS
• Asteroseismic data from
2000 (just past maximum)
and 2005 (near minimum)
• Cross-correlation of data
sets yields a systematic
shift of 0.1 + 0.4 mHz
• Single radial mode (n=18)
in both data sets shifted
by 0.17 + 0.62 mHz
Outlook
• Space-based asteroseismology will extend the
calibration of stellar structure/evolution models to
many new sets of physical conditions and ages.
• Measurements of latitudinal differential rotation
and convection zone depths will provide new
constraints for solar/stellar dynamo models.
• Though we will observe each star in less detail,
we will have hundreds of stars to provide a broad
context for our understanding of the Sun.