Transcript Stellar physics revealed by planet transits

Stellar Physics Revealed by
Planetary Transits
Willie Torres
Harvard-Smithsonian Center for Astrophysics
IAU General Assembly, Special Session 13
High Precision Tests of Physics from High-Precision Photometry
Beijing, 29 August 2012
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Selected Topics
Accurate mass and radius determinations
for low-mass stars
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Known disagreements between observations
and stellar evolution theory
Eclipsing binaries are by-products of transit
surveys: many new light curves available
Circumbinary transiting planets
Spin-orbit alignment for planetary systems
Spots on the host stars of transiting planets:
spot properties and distribution
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Low-Mass Stars and the
Disagreements with Models
Many low-mass stars are both larger and cooler
than predicted by stellar evolution theory
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Evidence has been accumulating for many years, mostly
from double-lined eclipsing binaries (Lacy 1977, Popper 1997,
Clausen 1999, Torres & Ribas 2002, and many others)
The vast majority of these binary systems have short
orbital periods (mostly < 3 days)
Stellar activity has long been suspected as the underlying
cause (tidal synchronization  rapid rotation  activity)
Magnetic fields inhibit convective energy transport
Spot coverage reduces radiating surface area
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CM Dra compared with models (Dotter et al. 2008)
Age and metallicity not
well known
Population II star?
Solar-metallicity
models do not fit
Age is not totally
irrelevant
Detailed studies
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Feiden et al. 2011
Spada & Demarque 2012
MacDonald & Mullan 2012
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CM Dra compared with models (Dotter et al. 2008)
Age and metallicity not
well known
Population II star?
Solar-metallicity
models do not fit
Age is not totally
irrelevant
Detailed studies
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Feiden et al. 2011
Spada & Demarque 2012
MacDonald & Mullan 2012
[Fe/H] = +0.50 fits, but
binary is unlikely to be
that metal-rich
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If short-period binaries show disagreements with
theory, should long-period binaries behave better?
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Tidal forces should be much weaker
In principle the binary components should rotate more
slowly, and should be relatively inactive
However, such long-period systems are rare among
eclipsing binaries (difficult to study)
Two long-period eclipsing binaries recently found,
both as by-products of transit surveys:
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LSPM J1112+7626 (MEarth; Irwin et al. 2011)
Kepler-16 (Doyle et al. 2011, Winn et al. 2011), a
system with a circumbinary transiting planet
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Circumbinary
planet
K5V
LSPM J1112+7626 (Irwin et al. 2011)
P = 41.032 days
P = 41.079 days
[Fe/H] = 0.30
M4V
Kepler-16
(Doyle et al. 2011, Winn et al. 2011)
Dartmouth models (Dotter et al. 2008)
Age and metallicity unknown;
secondary may still be active
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Current models do agree
with the measurements
of at least one low-mass
system: KOI-126 BC
Feiden et al. 2011
KOI-126 B
This same model
fits KOI-126 A
KOI-126 C
Triple System
found by Kepler
Age = 4.1 Gyr
Pair of M dwarfs
with P = 1.77 days
in a 33.9-day orbit
around a G dwarf
Photo-dynamical
modeling of the
Kepler light curve
[Fe/H] = +0.15
(Carter et al. 2011)
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Models are able to explain the larger radii and
cooler temperatures of late-type stars, but add free
parameters that must be tuned to each case
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 (magnetic inhibition parameter; Mullan & MacDonald 2001)
ML , and spot filling factor  (Chabrier et al. 2007)
Systematic effects play an important role in
measuring masses and radii of low-mass stars
(e.g., spots change with time, and can affect the
results)
High-precision photometry and continuous
coverage (e.g., Kepler, CoRoT) is an advantage
Photo-dynamical modeling in multiple systems can
alleviate some of the problems caused by spots
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Spin-Orbit Alignment
Transiting planet observations can provide
information on the orientation of the stellar
spin axis relative to the planetary orbit
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Rossiter-McLaughlin effect
Observation of spot anomalies
Obliquity measurements can tell us about the
efficiency of tidal interactions, energy
dissipation, and have a bearing on planet
migration theories
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Orbital axis
Stellar spin axis
Transiting
planet
The obliquity (or spin-orbit angle ) is the angle
between the spin axis of the host star and the axis of
the orbit of the planet. Typically we can only measure
its projection on the plane of the sky, λ.
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The Rossiter-McLaughlin Effect
Queloz et al. (2000), Ohta, Taruya, & Suto (2005), Gaudi & Winn (2007)
R-M observations are relatively easy for hot Jupiters
transiting bright and rapid rotators
A  (Rp/R*)2  v sin i
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WASP-17
HAT-P-2
WASP-15
WASP-8
HAT-P-1
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HAT-P-7
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Migration and Stellar Properties
Two broadly different migration mechanisms
proposed for hot Jupiters
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Interactions with a flat circumstellar disk
 Low obliquities
Dynamical processes (e.g., planet-planet scattering)
 High obliquities
Winn et al. (2010) first noticed that hot Jupiters
orbiting early-type stars tend to be misaligned,
while those around cool stars are not
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Initial obliquities were nearly random (scattering), and
low obliquities result from subsequent tidal interactions
Albrecht et al. (2012) provided additional support
for the scattering process
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Obliquities measured via the R-M effect
For hot Jupiters, systems with
high obliquities tend to be
associated with hotter stars
Albrecht et al. (2012)
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Spots on the Host Stars of Transiting Planets
A nuisance: they interfere with the determination of
planet properties
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They cause variations in transit depth, biasing the radius
They produce chromatic effects that can be mistaken for
atmospheric absorption
They cause anomalies in individual light curves
They can bias transit timing measurements
An opportunity to learn about the planet, its orbit,
and the parent star
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Stellar rotation period (Silva-Valio 2008)
Spot distribution (Lanza et al. 2009; Désert et al. 2011)
Spin-orbit alignment (Nutzman et al. 2011, Deming et al. 2011;
Sanchis-Ojeda et al. 2011, 2012)
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Porb/Prot = 0.1
Aligned
axes
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3
Time
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Misaligned
axes
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2
3
Time
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Starspots, spin-orbit alignment, and active latitudes
in the HAT-P-11 exoplanetary system
(Sanchis-Ojeda et al. 2011)
Out-of-transit variability from Kepler
Prot  30.5 days
K4V star with a “super-Neptune”
Orbital period = 4.9 days
Rp = 4.7 R
Mp = 26 M
Known to be misaligned (λ = 103º)
from R-M measurements
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Spot anomalies seem to occur at two specific
phases of the transit
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Spot distribution on HAT-P-11
Two preferred phases  Two preferred latitudes?
Latitude = ±19.7º
is = 80º
Latitude = 67º
Planetary transits of active stars allow us to
constrain the three-dimensional stellar obliquity 
(not just λ) based on the observed pattern of spot
anomalies and a simple geometrical model
is = 168º
Sanchis-Ojeda et al. (2011)
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A Butterfly Diagram for HAT-P-11?
HAT-P-11
Spot distribution as a
function of time
If the active latitudes change with
time analogously to the “butterfly
diagram” of the Sun’s activity, future
Kepler observations should reveal
changes in the preferred phases of
spot-crossing anomalies
Sanchis-Ojeda et al. (2011)
The Sun
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Summary
Spot anomalies detected with
high-precision photometry are now
a common and useful tool for
measuring rotation periods and
obliquities in transiting systems
(complementary to the R-M
effect). They can also serve to
characterize the spot distribution.
R. Sanchis-Ojeda
KOI-126
Carter et al. (2011)
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Accurate measurements of stellar properties
(masses, radii) are enabled by the many new light
curves resulting from transit surveys, and photodynamical modeling in special configurations
(triples, circumbinary transiting planets)
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