Download Exoplanets: prospects for Gaia

Lecture program
1. Space Astrometry 1/3: History, rationale, and Hipparcos
2. Space Astrometry 2/3: Hipparcos scientific results
3. Space Astrometry 3/3: Gaia
4. Exoplanets: prospects for Gaia
5. Some aspects of optical photon detection
Michael Perryman
Friday, 15 November 13
1
Astrometry in the context of exoplanet
detection/characterisation
Star accretion (?)
Exoplanet Detection
November 2013
1030 exoplanets
(784 systems, 170 multiple)
Colliding
planetesimals (1?)
Miscellaneous
Methods
[numbers from exoplanet.eu]
Radio emission
Dynamical
Microlensing
Photometry
Astrometry
Astrometry
Timing
X-ray emission (1)
Protoplanetary/
debris disks
Imaging
Rad velocity
sub
dwarfs
decreasing
planet mass
10MJ
MJ
optical
1
free-floating
space
10M
5
M
Friday, 15 November 13
photometric
15 planets
(12 systems,
2 multiple)
ground
24
projected (10–20 yr)
2 planets
(2 systems,
0 multiple)
24 planets
(22 systems,
2 multiple)
n = planets known
ground
ground
(adaptive
optics)
bound
space
535
535 planets
(403 systems,
93 multiple)
space
39
1
reflected
light
Doppler
variability
2
millisec
existing capability
radio
astrometric
8
Discovered:
Transits
eclipsing
binaries
white
dwarfs
pulsars
slow
TTVs
space
(coronagraphy/
interferometry)
39 planets
(36 systems,
1 multiple)
discoveries
173
244
timing
residuals
(see TTVs)
417 planets
(318 systems,
68 multiple)
follow-up detections
2
Photometry
Friday, 15 November 13
3
Hipparcos: a posteriori transit detection
HD 209458 P=3.524739(14) days
(Robichon & Arenou 2000; Söderhjelm et al 2000)
89 discrete observations around 1990, with 5 at transit epochs
(P=3.5d and Δt = 0.1 d
3% transit probability)
–4
Normalised residuals
Hipparcos magnitude Hp
7.70
7.74
7.78
7.82
7800
8000
8200
8400
8600
8800
9000
Observation date (JD – 2440000)
Normalised residuals
–4
–2
0
2
4
–0.5 –0.4 –0.3 –0.2 –0.1
0.0
Phase
Friday, 15 November 13
0.1
0.2
0.3
0.4
0.5
–2
0
HD 189733
also:
P = 2.218574(8) days
(Bouchy et al 2005;
Hébrard & Lecavelier des Etangs 2006)
2
4
–0.5 –0.4 –0.3 –0.2 –0.1
0.0
0.1
0.2
Other studies set the scene for Gaia:
Phase
• Castellano et al 2000, 2001
• Laughlin 2000
• Koen & Lombard 2002
• Hoeg 2002
• Jenkins et al 2002
• Robichon 2002
• Hebrard et al 2006
• Gould & Morgan 2003
• Hatzes et al 2003, 2006
• Beatty & Gaudi 2008
0.3
0.4
0.5
4
Number of field of view transits
Friday, 15 November 13
5
Gaia: estimates for (very) hot Jupiters
(Dzigan & Zucker 2012)
advantages: 1 mmag photometric accuracy
disadvantages: n(measures), low cadence
Simulations account for planet
frequency, detection probability,
stellar density, false detections, etc
assumes 2-hr transit duration
Conclusion: few hundred to a few thousand discoveries
(with the need for high-precision RV follow-up)
Friday, 15 November 13
6
Photometry: planetary accretion events?
FH Leo (Dall et al. 2005):
•
•
•
•
•
suggestion: a planetary accretion event
rapid magnitude rise suggests asteroid mass such as Pallas or Vesta
decay over ~ 17 days
abundances of α-elements and Li
but may be a nova (Vogt 2006)
8.2
mag
8.3
8.4
8.5
8.6
BJD - 2440000
Friday, 15 November 13
7
Distances and space motions
Friday, 15 November 13
8
Distances and motions
Examples:
• distances provide stellar parameters
•
e.g. transit planet diameters ∝ stellar diameters
• verification of seismology models for M, R
• proper motions characterise population(s)
e.g. HIP 13044 low-metallicity Galactic halo stream (Helmi+1999, Setiawan+2010)
• Galactic birthplace based on metallicity-age
e.g. Wielen (1996) inferred that the Sun’s birthplace was at R=6.6 kpc
Friday, 15 November 13
9
Asteroseismology
Can hope to discriminate:
• primordial or self-enriched metallicity (bulk/surface): μ Ara (Pepe 2007), ι Hor (Laymand 2007)
• planet radii calibrated wrt stellar radii: HAT-P-7 (Christensen-Dalsgaard+ 2010)
• mass estimates cf isochrone models: β Gem, HD13189 (Hatzes+ 2008)
• etc
For this, verification of the models by comparing parallax-based L with asteroseismology
Asteroseismology of η Boo, Z=0.04.
Left: M=1.70±0.005 M⊙ Right: with overshooting
(Di Mauro et al 2003)
Friday, 15 November 13
10
Galactic birthplace (cont.)
0.4
Sun
0.2
0
Ri (kpc)
[Fe/H]
–0.2
4.5
–0.4
6.5
–0.6
8.5
10.5
–0.8
12.5
–1.0
0
2
4
6
8
10
12
14
16
Age (Gyr)
Friday, 15 November 13
11
Hipparcos distances to exoplanet host stars
100 brightest radial velocity host stars (end 2010)
(versus RA, independent of dec)
(a)
(b)
50 pc
ground-based: van Altena et al (1995)
(unknown assigned π = 10±9 mas)
Friday, 15 November 13
100 pc
50 pc
100 pc
Hipparcos parallaxes
(Perryman et al 1997)
12
Gaia distances to exoplanet host stars
Transit host stars (~280, October 2013)
Friday, 15 November 13
13
New astrometric detections
Friday, 15 November 13
14
Discovery from ‘astrometric signature’
and provides M directly
(not simply M sin i)
Declination (mas)
Unseen planets perturb
the photocentre, which
moves with respect to the
barycentre
(as for Doppler measures)
150
Parameters (x50):
d = 50 pc
μ = 50 mas/yr,
over three years;
orbited by a planet with
Mp = 15 MJ, e = 0.2,
a = 0.6 AU
100
2012.8
2011.6
50
2010.4
0
0
50
100
Right ascension (mas)
Friday, 15 November 13
15
∆δ (milli-arcsec)
Astrometric analysis: principles
50
• as the satellite traces out a series of great
circles on the sky, each star is (effectively)
instantaneously stationary
0
• each star has a 2d position (abscissa and
ordinate) projected onto that great circle
• in principle one should solve for both
coordinates
–50
• in practice, only the projection along the
great circle (abscissa) dominates the ‘greatcircle solution’
–100
• least-squares adjustment gives the alongscan position of each star at that epoch
• all great circles over the entire mission are
then ‘assembled’
–150
–200
–150
–100
–50
0
50
• a star’s position at any time t is represented
by just five parameters: position (xy), proper
motion components (μx, μy), parallax (π)
∆α cos δ (milli-arcsec)
Friday, 15 November 13
16
Direct access to planet mass
descending
node
orbiting
body
z r
pericentre
ν(t)
ω
ellipse focus ≡
centre of mass
ϒ=
reference
direction
i
Ω=
longitude of
ascending node
orbit plane
apocentre
reference plane
ascending node
⇓
Keplerian orbit in 3d determined by 7 parameters:
a, e: specify size and shape
P: related to a and masses (Kepler’s 3rd law)
tp: the position along orbit at some reference time
i, Ω,ω: represent projections wrt observer
to observer
Radial velocity measures:
• cannot determine Ω,
• only determine the combination a sin i
• only determine Mp sin i if M* can be estimated
• cannot determine Δi for multiple planets
All 7 parameters are determinable by astrometry (±180º on Ω). Conceptually:
• xy(t) yields max and min angular rates, and hence the line of apsides (major axis)
• then appeal to Kepler’s third law fixes the orbit inclination
Friday, 15 November 13
17
The ‘tragic history’ of
astrometric planet detection
•
Jacob (1855) 70 Oph: orbital anomalies made it ‘highly probable’ that there was a
‘planetary body’; supported by See (1895); orbit shown as unstable (Moulton 1899)
•
Holmberg (1938): from parallax residuals... `Proxima Centauri probably has a companion’
of a few Jupiter masses
•
•
•
•
•
•
Reuyl & Holmberg (1943) 70 Oph: planetary companion of ~10 MJ
•
early discussions of space astrometry/Hipparcos exoplanet capabilities:
Strand (1943) 61 Cyg: companion of ~16 MJ
lengthy disputes about planets around Barnard’s star: van der Kamp (1963, 1982)
similarly for Lalande 21185 (e.g. Lippincott 1960)
Pravdo & Shaklan (2009) vB10 with Palomar-STEPS, later disproved (Bean 2010)
Muterspaugh+ (2010) HD~176051, only current detection: ‘may represent either the
first such companion detected, or the latest in the tragic history of this challenging approach.’
•
Friday, 15 November 13
Couteau & Pecker (1964), Gliese (1982)
18
Astrometric signature
Hipparcos astrometry
is marginal for detection
(and mass determination)
Friday, 15 November 13
19
Gaia: number of astrometric detections
•
•
Gaia was studied since 1997; accepted in 2000; launch: 20 December 2013
•
more recent estimates (Castertano+ 2008):
•
early estimates of 10−30,000 detectable exoplanets were based on target accuracies
at time of acceptance (Lattanzi et al 2000, Perryman et al 2001, Quist 2001, Sozzetti
et al 2001), and are no longer applicable
•
•
•
•
•
single measurement error for bright stars σψ ~ 8 μas
reliable detections for P<5 years and α > 3σψ
at 2x this limit, errors on orbits and masses are ~ 15−20%
> 70% of 2-planet systems with 0.2 < P < 9 yr and e < 0.6 are identified
typical uncertainties on Δi for favourable systems are <10o
bottom line:
•
•
Friday, 15 November 13
discover/measure several thousand giant planets with a =3−4 AU and d < 200pc
characterise hundreds of multiple systems with meaningful tests of coplanarity
20
For multiple planet systems...
Drawing on extensive radial velocity work, multiple systems can be fit
by (e.g.) recursive decomposition
Non-linearity of the fitting for Gaia/SIM: considered by Casertano et al
(2008), Wright & Howard (2009), Traub et al (2010)
5-planet system 55 Cnc
(Fischer et al 2008)
Friday, 15 November 13
Periodograms wrt:
(i) 2-planet model
(ii) 3-planet model
(iii) 4-planet model
Periodicity of the fifth
planet in the Keck data
21
Astrometric motion for multiple planets...
(assuming orbits are co-planar)
4 planets, 60 yr
(a) Sun
2020
4 planets, 3
(b) µ Ara
The sun’s motion guides us:
• Newton: ‘since that centre of gravity
2010
1990
is continually at rest, the Sun,
according to the various positions of
the planets, must continuously move
every way, but will never recede far
from that centre’ (Cajori 1934)
1970
2030
• barycentre frequently extends
1980
2000
beyond the solar disk
• periods when the Sun’s motion is
‘retrograde’ with respect to the
barycentre (~1990, 1811,1632)
–0.008
–0.004
0
0.004
Displacement (AU)
Friday, 15 November 13
0.008
–0.01
0
0.01
Displacement (AU)
22
Motion of host star around barycentre
for multiple exoplanets
(assuming coplanarity)
2 yr
HD 69830
–4e–05
3 planets, 2 yr
0
Displacement (AU)
Friday, 15 November 13
4e–05
HD 155358
–0.001
2 planets, 10 yr
0
0.001
Displacement (AU)
23
Motion for multiple planets (cont.)
HIP 14810
–0.001
3 planets, 2 yr
0
0.001
Displacement (AU)
Friday, 15 November 13
0.002
HD 40307
–4e–06
3 planets, 2 yr
0
4e–06
HD 69
–4e
Displacement (AU)
24
Astronomia Nova (1609) includes Kepler’s hand
drawing of the orbit of Mars viewed from Earth
...designated as ‘mandala’
(Sanskrit for circle)
by Wolfram (2010)
Friday, 15 November 13
25
Exoplanets and the solar dynamo
Friday, 15 November 13
26
Now for something contentious...
•
solar axial rotation is invoked in models of the solar cycle (e.g. turbulent
dynamo operating in or below the convection envelope)
•
precise nature of the dynamo, and details of associated solar activity (sun
spot cycles, and the prolonged Maunder-type solar minima) are unexplained
•
empirical investigations have long pointed to a link between the Sun’s
barycentric motion and various solar variability indices (e.g. Wolf, 1859;
Brown, 1900; Schuster, 1911; Jose, 1965; Ferris, 1969), specifically:
•
•
•
•
•
•
•
•
•
•
Friday, 15 November 13
the Wolf sun spot number counts (Wood & Wood, 1965)
climatic changes (Mörth & Schlamminger, 1979; Scafetta, 2010)
the 80-90-yr secular Gleissberg cycles (Landscheidt, 1981, 1999)
prolonged Maunder-type minima (Fairbridge & Shirley, 1987; Charvátová, 1990, 2000)
short-term variations in solar luminosity (Sperber et al., 1990)
sun spot extrema (Landscheidt, 1999)
the 2400-yr cycle seen in 14C tree-ring proxies (Charvátová, 2000)
hemispheric sun spot asymmetry (Juckett, 2000)
torsional oscillations in long-term sun spot clustering (Juckett, 2003)
violations of the Gnevishev–Ohl sun spot rule (Javaraiah, 2005)
27
Just one example...
Abreu et al (2012), A&A 548, 88 (ETH Zürich)
Gleissberg
Amplitude
120
88
de Vries
104
150
solar modulation potential over
9400 yr from 10Be and 14C
208
506
80
solar
activity
40
Amplitude
0
torque
modulus
0.015
0.01
0.005
0
50
100
150
200
250
300
350
Period (years)
400
450
500
550
600
Proposed coupling mechanisms between the solar axial rotation and orbital revolution:
• Zaqarashvili (1997), Juckett (2000), contested by Shirley (2006)
• Abreu (2012): time-dependent torque exerted by the planets on a non-spherical tachocline
• Callebaut & de Jager (2012): effect considered as negligible
Friday, 15 November 13
28
Exoplanets can arbitrate
(Perryman & Schulze-Hartung 2011)
•
behaviour cited as correlated with the Sun’s activity includes
•
•
changes in orbital angular momentum, dL/dt
intervals of negative orbital angular momentum
•
•
these are common (but more extreme) in exoplanet systems
•
activity monitoring should therefore offer an independent
test of the hypothetical link between:
HD 168443 and HD 74156 have dL/dt exceeding that of the
Sun by more than 105
•
•
Friday, 15 November 13
the Sun’s barycentric motion
and the many manifestations of solar activity
29
HD 168433
Lz
(Msun AU2 d-1)
dLz/dt
(Msun AU2 d-2)
•
•
•
Friday, 15 November 13
two massive planets (8−18MJ) at 0.3−3 AU, e1~0.5
most extreme negative Lz and largest dL/dt
periodicity of ~58 days
30
Coplanarity of orbits
and
transit geometry
Friday, 15 November 13
31
Exoplanet detection with HST−FGS
quite a long history, starting with Benedict et al (1993)
[HST−FGS yields relative parallaxes based on assumed luminosities of reference stars]
4
ν And d
1.0
ν And c
0.5
ν And b
0.0
0
8
16
24
Inclination (˚)
32
40
(b)
2
2
0
(c)
2004
1
–2
y (mas)
1.5
X residuals (mas)
(a)
Y residuals (mas)
Astrometric signature, α (mas)
2.0
4
2
0
2002
2005
2003
2006
–1
0
ν And
–2
–2
400
800
1200
1600
JD – 2 452 000 (d)
• radial velocity observations determine only Mp sin i
• astrometric measurements determine Mp directly
• and hence relative inclinations (van der Kamp 1981):
–2
–1
0
1
2
x (mas)
For ν And, McArthur et al (2010) found:
• Mp (ν And c) = 14.0 MJ
• Mp (ν And d) = 10.2 MJ
• Δi = 29.9°±1°
the first direct determination of relative orbit inclinations
Friday, 15 November 13
32
Importance of Δi
•
Δi ~ 0 in the solar system
• various evidence that this is not necessarily the rule in exoplanets:
(2) Rossiter-McLaughlin effect
used to measure the (sky
projection) of the orbital and
stellar rotation axes indicates
that many orbits are
misaligned wrt stellar equator
(some retrograde)
Rad velocity (m s−1)
(1) long-term dynamical stability (via numerical integrations)
b = – 0.5, λ = 0˚
some cannot be coplanar
b = – 0.5, λ = 30˚
b = – 0.5, λ = 60˚
+ 60
0
– 60
–2
–1
0
Time (h)
1
2
–2
–1
0
Time (h)
1
2
–2
–1
0
1
2
Time (h)
(3) models of formation and evolution admit the possibilities of: (a) asymmetric protoplanetary
infall; (b) gravitational scattering during the giant impact stage of protoplanetary collisions; (c)
Kozai resonance/migration in which Lz is conserved, and hence i and e can be ‘traded’ (explains
high e in triple systems, and hot Jupiters when combined with tidal friction)
Friday, 15 November 13
33
Baluev 2008
optimal strategies of radial velocity observations in search surveys
20
J12
15
(inefficient)
preceding observations
observation particularly efficient
HD 208487
observations
not possible
10
5
0
2005
2006
2007
2008
Year
example:
• how to decide between two orbit solutions from 35 measurements (Butler 2006)
• Wright (2007) identified a second planet at P=28.6d or 900d
• J12 estimates the maximum information from the two predictions
• maxima give the most promising times for new observations
• here:
• predictions differed by 20m/s
• actual observations during 2005 were all at epochs of low information content
• one observation in 2005 would have resolved the degeneracy, those in 2006 could not
Friday, 15 November 13
34
Transit geometry
HD 189733
•
R* = 370
normally, there is no information
on the position angle of a
transiting planet’s orbit plane
181
95
120
•
valuable for higher-order light
curve effects?
provided by Gaia if the
astrometric signature of the
transiting planet is high
E1S1
Visibility
•
provided (in principle) by 2d
interferometry: due to asymmetry
in the source brightness
introduced by the planet
Closure phase (˚)
•
Rp =
64
W1E1
S1W1
0.008
0.000
–0.008
0.2
0.1
0.0
–0.1
–0.2
0.04
0.08
Time (d)
0.12
0.04
0.08
0.12
Time (d)
(CHARA)
V = 7.7, K1, d = 19pc
Friday, 15 November 13
35
Summary
•
•
•
•
accurate distances:
calibration of host star parameters, including R for transiting
calibration of asteroseismology models
accurate proper motions: Galactic dynamics and population
multi-epoch high-accuracy photometry:
new transiting systems (several hundred?)
calibration of photometric jitter vs spectral type
multi-epoch astrometry:
discovery of new (massive, long-period) planets (3000?)
co-planarity of systems: evolutionary models
position angle of planet transits (some multiple?)
•
•
•
•
•
•
•
Friday, 15 November 13
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End
Friday, 15 November 13
37