Download Secondary Transits

Transit Searches: Results
I. Results from individual transit searche programs
II. Interesting cases
III. Spectroscopic Transits
IV. In-transit spectroscopy
V. Radiated light (secondary transits)
CoRoT results will be presented during the Space
Missions lecture
The first time I gave this lecture (2003) there was one transiting
extrasolar planet
There are now 52 transiting extrasolar planets
First ones were detected by doing follow-up photometry of radial
velocity planets. Now transit searches are discovering
exoplanets
Radial Velocity Curve for HD 209458
Period = 3.5 days
Msini = 0.63 MJup
Charbonneau et al. (2000): The observations that started it all:
•
•
•
•
Proof that RV variations are due to planet
Mass = 0,63 MJupiter
Radius = 1,35 RJupiter
Density = 0,38 g cm–3
Transit detection was made with the 10 cm STARE Telescope
A light curve taken by amateur astronomers…
..and by the Profis ( Hubble Space Telescope).
Successful Transit Search Programs
• OGLE: Optical Gravitational Lens Experiment
(http://www.astrouw.edu.pl/~ogle/)
• 1.3m telescope looking into the galactic bulge
• Mosaic of 8 CCDs: 35‘ x 35‘ field
• Typical magnitude: V = 15-19
• Designed for Gravitational Microlensing
• First planet discovered with the transit method
• 7 Transiting planets discovered so far
The first planet found with the transit method
Konacki et al.
Until this discovery radial velocity surveys only found planets with
periods no shorter than 3 days. About ½ of the OGLE planets have
periods less than 2 days.
M = 1.03 MJup
R = 1.36 Rjup
Period = 3.7 days
Successful Transit Search Programs
• WASP: Wide Angle Search For Planets (http://www.superwasp.org). Also
known as SuperWASP
• Array of 8 Wide Field Cameras
• Field of View: 7.8o x 7.8o
• 13.7 arcseconds/pixel
• Typical magnitude: V = 9-13
• 15 transiting planets discovered so far
Coordinates
RA 00:20:40.07 Dec
+31:59:23.7
Constellation
Pegasus
Apparent Visual Magnitude
11.79
Distance from Earth
1234 Light Years
WASP-1 Spectral Type
F7V
WASP-1 Photospheric
Temperature
6200 K
WASP-1b Radius
1.39 Jupiter Radii
WASP-1b Mass
0.85 Jupiter Masses
Orbital Distance
0.0378 AU
Orbital Period
2.52 Earth Days
Atmospheric Temperature
1800 K
Mid-point of Transit
2453151.4860 HJD
Successful Transit Search Programs
• TrES: Trans-atlantic Exoplanet Survey (STARE is a member of the network
http://www.hao.ucar.edu/public/research/stare/)
• Three 10cm telescopes located at Lowell Observtory, Mount Palomar
and the Canary Islands
• 6.9 square degrees
• 4 Planets discovered
Successful Transit Search Programs
• HATNet: Hungarian-made Automated Telescope
(http://www.cfa.harvard.edu/~gbakos/HAT/
• Six 11cm telescopes located at two sites: Arizona and Hawaii
• 8 x 8 square degrees
• 8 Planets discovered
HAT 1b
Follow-up
with larger
telescope
Special Transits: GJ 436
Host Star:
Mass = 0.4 M‫( סּ‬M2.5 V)
Butler et al. 2004
Special Transits: GJ 436
Butler et al. 2004
„Photometric transits of the planet across the star are ruled out for gas giant
compositions and are also unlikely for solid compositions“
The First Transiting Hot Neptune
Gillon et al. 2007
Special Transits: GJ 436
Star
Stellar mass [ M‫] סּ‬
0.44 ( ± 0.04)
Planet
Period [days]
2.64385 ± 0.00009
Eccentricity
0.16 ± 0.02
Orbital inclination
86.5
Planet mass [ ME ]
22.6 ± 1.9
Planet radius [ RE ]
3.95 +0.41-0.28
0.2
Mean density = 1.95 gm cm–3, in between Neptune (1.58) and Uranus (2.3)
Mean density
is first hints of
the internal
composition
Special Transits: HD 17156
M = 3.11 MJup
Probability of a transit ~ 3%
Barbieri et al. 2007
R = 0.96 RJup
Mean density = 4.88 gm/cm3
Mean M2 star ≈ 4.3 gm/cm3
Companion is probably
more like a brown dwarf
Special Transits: HD 149026
Sato et al. 2005
Rp = 0.7 RJup
Mp = 0.36 MJup
Mean density = 2.8 gm/cm3
~70 Mearth core mass is
difficult to form with
gravitational instability
Mass Radius Relationship
HD 209458b and HAT-P-1b have anomalously large radii that still
cannot be explained by planetary structure and evolution models
Mazeh et al. 2008 found a mass-period relationship for transiting planets.
Suggest this is evidence of evaporation, only the most massive planets can
survive
Results from the Rossiter-McClaughlin Effect
The RM effect causes a distortion in the radial velocity curve during a
transit whose strength depends on the radius of the planet and the rotation
rate of the star
So far all transiting planets for which an
RM effect has been measured has
shown prograde orbits
What about misalignment of the spin
axis?
HD 147506
Best candidate for misalignment is HD 147506 because of the high
eccentricity
Two possible explanations for the high eccentricities seen in exoplanet orbits:
• Scattering by multiple giant planets
• Kozai mechanism
Planet-Planet Interactions
Initially you have two giant
planets in circular orbits
These interact gravitationally.
One is ejected and the
remaining planet is in an
eccentric orbit
This mechanism has been invoked to explain the „massive eccentrics“
Recall that there are no massive planets in circular orbits
Kozai Mechanism
Two stars are in long period orbits around each other.
A planet is in a shorter period orbit around one star.
If the orbit of the planet is inclined, the outer planet can „pump up“ the
eccentricity of the planet. Planets can go from circular to eccentric orbits.
This was first investigated by Kozai who showed that satellites in orbit
around the Earth can have their orbital eccentricity changed by the
gravitational influence of the Moon
Kozai Mechanism
The Kozai mechanism has been
used to explain the high orbital
eccentricity of 16 Cyg B, a planet
in a binary system
If either mechanism is at work, then we should expect that planets in eccentric
orbits not have the spin axis aligned with the stellar rotation. This can be checked
with transiting planets in eccentric orbits
Winn et al. 2007: HD 147506b (alias HAT-P-2b)
Spin axes are aligned within 14 degrees (error of measurement). No
support for Kozai mechanism or scattering
What about HD 17156?
Narita et al. (2007) reported a large (62 ± 25 degree) misalignment between
planet orbit and star spin axes!
Cochran et al. 2008: l = 9.3 ± 9.3 degrees → No misalignment!
In-transit Spectroscopy
• Take a spectrum of the star
during the out-of-transit time
• Take a spectrum of the star
during the transit
• Subtract the two and what
remains is the spectrum of the
planet atmosphere
In practice this is very difficult. One
requires high signal-to-noise ratio
data (≈ 1000) which means repeated
measurements that have to be coadded.
Problem: In transit spectra can only
be made during transits (infrequent)
and only for about 3 hours!
From The Astrophysical Journal 537(2):916–921.
© 2000 by The American Astronomical Society.
For permission to reuse, contact [email protected].
In-transit Spectroscopy
Sasselov & Seager 2004
Fig. 1.— Flux of HD 209458 a (upper curve) and the transmitted flux through the planet’s transparent atmosphere (lower curve). Superimposed on the
transmitted flux are the planetary absorption features, including the He i triplet line at 1083 nm. The other bound-bound lines are alkali metal lines (see Fig. 2 for
details). The H2O and CH4 molecular absorption dominates in the infrared. The dotted line is a blackbody of 1350 K representative of the CEGP’s thermal
emission, but the thermal emission can be larger than a blackbody blueward of 2000 nm.
From The Astrophysical Journal 537(2):916–921.
© 2000 by The American Astronomical Society.
For permission to reuse, contact [email protected].
Fig. 2.—Upper plot: The normalized in-transit minus out-of-transit spectra, i.e., percent occulted area of the star. In this model the cloud base is at bar. Rayleigh
scattering is important in the UV. Lower plot: A model with cloud base at 0.2 bar. The stellar flux passes through higher pressures, densities, and temperatures
of the planet atmosphere compared to the model in the upper plot. In addition, a larger transparent atmosphere makes the line depth larger. Observations will
constrain the cloud depth. See text for discussion.
From The Astrophysical Journal 568(1):377–384.
© 2002 by The American Astronomical Society.
For permission to reuse, contact [email protected].
Charbonneau et al. 2001
Fig. 4.—Top: Unbinned time series nNa (Fig. 2, top panel). Bottom: These data binned in time (each point is the median value in each bin). There are 10 bins,
with roughly equal numbers of observations per bin (42). The error bars indicate the estimated standard deviation of the median. The solid curve is a model for
the difference of two transit curves (described in § 3), scaled to the observed offset in the mean during transit, ΔnNa = −2.32 × 10−4.
Redfield et al. 2007
Sodium
Calcium
An element not expected to show excess absorption shows none
Vidal-Majar et al. 2003
Picture of the geocorona taken
by the Apollo astronauts
HD 209458 shows
excess abroption in
Hydrogen Lyman a
Evidence for an
evaporating
atmosphere of
Hydrogen?
From The Astrophysical Journal Letters 671(1):L61–L64.
© 2007 by The American Astronomical Society.
For permission to reuse, contact [email protected].
Ben-Jaffel 2007
Or not?
Fig. 3.— Comparison between Lyα line profiles in and out of transit period. The sky background spectral window is indicated by two dashed vertical lines. (a) The in-transit line profile (thin solid line)
is accumulated for the time period starting 3900 s before TCT and ending 3900 s after it. To correct for the ∼8.9% obscuration derived in this study, the corresponding intensity is scaled by 1.098.
The resulting line profile (dotted curve) properly recovers the unperturbed line profile (histogram). (b) The first in-transit line profile, B1 (thin solid line), was accumulated over the time period starting
4000 s before the TCT and ending ∼600 s after it. The second in-transit line profile, B2 (dotted line), was accumulated over the time period starting ∼1800 s before TCT and ending ∼3900 s after it.
From The Astrophysical Journal Letters 676(1):L57–L60.
© 2008 by The American Astronomical Society.
For permission to reuse, contact [email protected].
Vidal-Majar et al. 2008
Claim is that difference is due to different wavelength
range used to calculate absorption depth
Fig. 1.— Observed HD 209458 Lyα profiles as observed by VM03 before and during the planetary transit. The BJ07 reanalysis of nearly the same data set
produced a similar Lyα line profile. The two vertical dashed lines define the limits and of the line core where H i planetary absorption takes place. In VM03 as
well as in BJ07, the central part of the line (noted “Geo”) possibly perturbed by the Earth geocoronal emission is omitted from the analysis. The line wings are
used by VM03 as a flux reference to correct for the stellar Lyα intrinsic variations.
Secondary Transits: The Planet Albedo
The planet reflects light, so one should see a modulation in the light
curve, plus an eclipse of the planet
Secondary Transits: The Planet Albedo
The planet reflects light, so one should see a modulation in the light
curve, plus an eclipse of the planet
Rowe et al. 2008
Albedo < 0.12
Jupiter: 0.5
Sudarsky et al. 2005.
900 < T < 1500 K
Fig. 7.— (a) Spherical albedo of a class III clear EGP. In addition to the isolated (thin curve) and modified (thick curve) T-P profile models, the dashed curve
depicts what the albedo would look like in the absence of the alkali metals. (b) Spherical albedo of a class IV roaster. Theoretical albedo spectra of isolated (thin
curve) and modified (thick curve) T-P profile class IV models are depicted.
From The Astrophysical Journal 538(2):885–903.
© 2000 by The American Astronomical Society.
For permission to reuse, contact [email protected].
T > 1500 K
Better upper limits will be found by CoRoT. Kepler may be able to
detect the second transit.
Fig. 8.— Spherical albedo of a class V roaster. A silicate layer high in the atmosphere results in a much higher albedo than a class IV
roaster. No ionization is assumed in this model.
Secondary Transits with Kepler
For a short period giant in a 4 day orbit Kepler will observe
more than 250 transits. It will be able to detect secondary
transits (eclipses) for Albedos as low as 0.08
Secondary Transits: Infrared Measurements with
Spitzer
The „hot Jupiters“ have temperatures of ~ 1000 K. The radiated
light can be much higher than the reflected light:
Reflected light =
Lstar 1
2
ApR 4pd2 L =
star
AR2
4d2
A = geometric albedo, R = planet radius, d = distance from star
For A = 0.1, d=0.05 AU, R = 1 RJup
Reflected light ≈ 10–5
Secondary Transits: Infrared Measurements with
Spitzer
In radiated light however, for a planet with Teff ≈ 1000 K:
Flux from star =
Flux from planet =
Fp/F* =
2phc2/l–5 2pR 2
*
hc/klT
e
* –1
Only looking at half
the star
2phc2/l–5 2pR 2
p
hc/klT
e
p –1
ehc/klT* –1
Rp2
ehc/klTp –1
R*2
For a 1.5 RJup planet with Tp = 1000 K
around a solar-type star (5800 K) at 8
mm:
Fp/F* ≈ 0.0016
Spitzer is a 0.85m telescope that
can measure infrared radiation
between 3 and 180 mm
HD 209458 secondary
transit (eclipse) at 24
mm
Teff = 1130 K
From The Astrophysical Journal 626(1):523–529.
© 2005 by The American Astronomical Society.
For permission to reuse, contact [email protected].
Fig. 3.— Solid black line shows the Sudarsky et al. (2003) model hot Jupiter spectrum divided by the stellar model spectrum (see text for details). The open diamonds show the predicted flux ratios
for this model integrated over the four IRAC bandpasses (which are shown in gray and renormalized for clarity). The observed eclipse depths at 4.5 and 8.0 μm are overplotted as black diamonds.
No parameters have been adjusted to the model to improve the fit. The dotted line shows the best-fit blackbody spectrum (corresponding to a temperature of 1060 K), divided by the model stellar
spectrum. Although the Sudarsky et al. (2003) model prediction is roughly consistent with the observations at 8.0 μm, the model overpredicts the planetary flux at 4.5 μm. The prediction of a
relatively large flux ratio at 3.6 μm should be readily testable with additional IRAC observations.
Spitzer Measurements of Radiated Light at 8 mm of HD 189733
Knutson et al. 2007
Tmax = 1211 K
Tmin = 973 K
Spitzer Measurements of Radiated Light at 8 mm of HD 189733
Primary
Secondary
Predicted time of secondary transit is off by 120 s → eccentricity?
Brightest point is shifted by 16 degrees from the sub-stellar point
GJ 836 Spitzer measurements
Radius = 4.33 ± 0.18 RE
Tp = 712 K
Eccentricity = 0.15
Summary
1. 52 Transiting planets have been discovered so far. This is the Golden
Era of transit detections
2. In 5 years more transiting planets than non-transiting planets will be
known. My guess: 500
3. The measurement of the mean density is putting constraints on planet
formation and structure theories
4. In-transit spectroscopy is yielding the first chemical composition of an
extrasolar planet
5. Albedo measurements are placing contraints on atmospheric models
6. First indication of exoplanet „weather“
7. We are actually measuring the phyisical properties of the planets
themselves: exoplanetary science
These are exciting times!