detection of extrasolar planets through gravitational microlensing and timing method technique &...
TRANSCRIPT
Detection of Extrasolar Planets through Gravitational
Microlensingand Timing Method
Technique & Results
Timing Method
A Brief History of Light Deflection
In 1911 Einstein derived:
Einstein in 1911 was only half right !
= 2 GMּס
c2Rּס
= 0.87 arcsec
In 1916 using General Relativity Einstein derived:
= 4 GM
c2r
= 1.74 arcsec
Light passing a distance r from object
Factor of 2 due to spatial curvature which is missed if light is treated like particles
Eddington‘s 1919 Eclipse expedition confirmed Einstein‘s result
A Brief History of Light Deflection
In 1924 Chwolson mentioned the idea of a „factious double star.“ In the symmetric case of a star exactly behind a star a circular image would result
In 1936 Einstein reported about the appearance of a „luminous“ circle of perfect alignment between the source and the lens: „Einstein Ring“
In 1937 Zwicky pointed out that galaxies are more likely to be gravitationally lensed than a star and one can use the gravitational lens as a telescope
Einstein Cross
Einstein Ring
Evidence for gravitational lensing first appeared in extragalactic work
Source
Lens
Observer
SS2 S1
Basics of Lensing:
Basics of Lensing: The Einstein Radius
s
E
S1
S2
Lens
Source off-centered
E
Source centered
≈ 1 milli-arcsecond
=> Microlensing
= 0
= – ()
Magnification due to Microlensing:
Typical microlensing events last from a few weeks to a few months
Time sequence: single star
• Top panel shows stellar images at ~1 mas resolution centered on lens star
• Einstein ring in green• Magnified stellar
images shown in blue• Unmagnified image is
red outline• The observable total
magnification is shown in the bottom panel
Animation by Scott Gaudi:
http://www.astronomy.ohio-state.edu/~gaudi/movies.html
Time sequence: star + planet
• A planet in the shaded (purple) region gives a detectable deviation
A planet lensing event lasts 10-30 hours
Mao & Paczynski (1992) propose that star-planet systems will also act as lenses
• 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 microlensing method
Successful Microlensing Programs
Problem:Only 4 points!
Solution: Multi-site Campaigns
Microlensing Results:
12 Planets so far
Rumor has it that there are another ~20 planet candidates
The First Planet Candidate: OGLE-235-MOA53
OGLEalert
Lightcurve close-up & fit (from Bennet)• Cyan curve is the
best fit single lens model 2 = 651
• Magenta curve is the best fit model w/ mass fraction 0.03 2 = 323
• 7 days inside caustic = 0.12 tE–Long for a
planet,–but mag = only
20-25%–as expected for
a planet near the Einstein Ring
1st definitive lensing planetary discovery
- complete coverage not required for characterization
Real-time data monitoring was critical!
S. Gaudi video
The First Planet Candidate: OGLE-235-MOA53
OGLE 2005-BLG-071
Udalski et al. 2005
The Star:
BASED ON GALACTIC MODEL
M = 0.46 Mּס
d = 3300 pc
I-mag = 19.5
The Planet:
M = 3.5 MJup
a = 3.6 AU
OGLE-06-109L
The Star:
M = 0.5 Mּס
d = 1490 pc
I-mag = 17.17
The Planets:
M1 = 0.71 MJup
a1 = 2.3 AU
M2 = 0.27 MJup
a2=4.6 AU
Gaudi et al. 2008, Science, 319, 927
Features 1,2,3,5 are caused by Saturn mass planet near Einstein radius. Feature 4 by another Jovian planet
Fig. 1.—Top: Data and best-fit model for OGLE-2005-BLG-169. Bottom: Difference between this model and a single-lens model with the same (t0, u0, tE, ρ). It displays the classical form of a caustic entrance/exit that is often seen in binary microlensing events, where the amplitudes and timescales are several orders of magnitude larger than seen here. MDM data trace the characteristic slope change at the caustic exit (Δt = 0.092) extremely well, while the entrance is tracked by a single point (Δt = −0.1427). The dashed line indicates the time t0. Inset: Source path through the caustic geometry. The source size ρ is indicated.
From The Astrophysical Journal Letters 644(1):L37–L40.© 2006 by The American Astronomical Society.For permission to reuse, contact [email protected].
OGLE-2005-BLG-169
The Star:
M = 0.49 Mּס
d = 2700 pc
I-mag = 20.4
The Planet:
M = 0.04 MJ
a = 2.8 AU
Microlensing planet detection of a Super Earth?
OGLE-2005-BLG-390
Mass = 2.80 – 10 Mearth
a = 2.0 – 4.1 AU
Best binary source
q = 7.6 x 10–5 Ratio between planet and star
MOA-2007-BLG-192-L
The Star (brown dwarf):
M = 0.06 Mּס
d = 1000 pc
J-mag = 19.6
The Planet:
M = 3.3 Mearth
a = 0.62 AU
Is it or isn‘t it a Super Earth?
Best fit stellar binary
OGLE-2007-BLG-368
Mass star ~ 0.2 Msun
Mass planet ~ 2.6 MJupiter
To get the mass of the host star one must once again rely on statistics including a galactic model of the distribution of stars in the galaxy
Red line: constraints from galactic model
Black: constraints from observations with the Very Large Telescope
Stellar mass ranges from 0.05 Msun (brown dwarf) to 0.2 Msun (star)
Mplanet = 0.07 – 0.49 MJupiter
Semi-major axis = 1.1 – 2.7 AU
Both at only the 90% confidence level.
Planet Mass
(MJ)
Period
(yrs)
a
(AU)
e M*
(Msun)
Dstar
(pcs)
OGLE235-MOA53 b ~2.6 ~15 ~5 ? 0.63 5200
OGLE-05-071L b ~3.5 ~10 ~3.6 ? 0.64 3300
OGLE-05-169L b 0.04 ~9 ~2.8 ? 0.49 2700
OGLE-05-390L b 0.017 ~9.6 ~2.1 ? 0.22 6500
MOA-2007-BLG-192-L b 0.01 ~2 0.62 ? 0.06 1000
OGLE-06-109L b 0.71 ~5 2.3 ? 0.5 1490
OGLE-06-109L c 0.27 ~14 4.6 0.11 0.5 1490
MOA-2007-BLG-400-L b 0.9 - 0.5 0.35 6000
OGLE-2007-BLG-368L b 0.07 - 3.3
MOA-2008-BLG-310-L b 0.23 - 1.25 0.67 >6000
MOA-2008-BLG-387-L b 2.6 -1.8 3.6 0.19 ~5700
Microlensing Planets
• Microlensing has discovered 4-5 cold Neptunes/Superearths
• Neptune-mass planets beyond the snowline are at least 3 times more common than for Jupiter- mass planets
But….this is based on small number statistics
• No bias for nearby stars, planets around solar-type stars
• Sensitive to Earth-mass planets using ground-based observations: one of few methods that can do this
• Most sensitive for planets in the „lensing zone“, 0.6 < a < 2 AU for stars in the bulge. This is the habitable zone!
• Can get good statistics on Earth mass planets in the habitable zone of stars
• Multiple systems can be detected at the same time
• Detection of free floating planets possible
The Advantages of Microlensing Searches
Microlensing is complementary to other techniques
Fig. 3.— Exoplanet discovery potential and detections as functions of planet mass and semimajor axis. Potential is shown for current ground-based RV (yellow) and, very approximately, microlensing (red) experiments, as well as future space-based transit (cyan), astrometric (green), and microlensing (peach) missions. Planets discovered using the transit (blue), RV (black), and microlensing (magenta) techniques are shown as individual points, with OGLE-2005-BLG-169Lb displayed as an open symbol. Solar system planets are indicated by their initials for comparison.
From The Astrophysical Journal Letters 644(1):L37–L40.© 2006 by The American Astronomical Society.For permission to reuse, contact [email protected].
• Probability of lensing events small but overcome by looking at lots of stars
• One time event, no possibility to confirm, or improve measurements
• Duration of events is hours to days. Need coordinated observations from many observatories
• Planet hosting star is distant: Detailed studies of the host star very dfficult
• Precise orbital parameters of the planet not possible
• Light curves are complex: only one crossing of the caustic. No unique solution and often a non-planet can also model the light curves
• Final masses of planet and host stars rely on galactic models and statistics and are poorly known
• Future characterization studies of the planet are impossible
The Disadvantages of Microlensing Searches
2. The Timing Method
If you have a very stable “clock” that sends a signal with a constant pulse rate andthe capability to measure the time of arrival (TOA) of the signal with very high precisionSearch for systematic deviations in the TOAs that indicate different light travel
times due to orbital motion
The Technique:
time
Due to the orbital motion the distance the Earth changes. This causes differences in the light travel time
Timing Variations:
Change in arrival time =
apmpsini
M*c
ap, mp = semimajor axis, mass of planet
time
Don’t forget to takeinto account yourown motion!!!
A Pulsar: a very stable astronomical clock!
Rotation periods of pulsars < 10 second
The fastest rotators are millisecond pulsars: PSR1257+12: P = 0.00621853193177 +/- 0.00000000000001 s
radiation
Strong magnetic fieldActs like a cosmiclighthouse
The (Really) First Exoplanets:in 1992
Arecibo Radio-telescope
98 d orbit removed, 66 d orbit remains
66 d orbit removed, 98 d orbit remains
PSR 1257+12 system:
Planet A:M = 0.02 M_EarthP = 25.3 d ; a = 0.19 AU
Planet B:M = 4.3 M_EarthP = 66.5 d ; a = 0.36 AU
Planet C:M = 3.9 M_EarthP = 98.2 d ; a = 0.46 AU
fourth companion with very low mass and P~3.5 yrs
Interaction between B & CConfirms the planets andEstablishes true masses!
Other applications ofthe timing method:
• Stably pulsating white dwarfs (P~200s)
• Pulsating sdB stars (P~500s)• Eclipse timing • Transit time variations
NN Ser eclipses
Kepler-9 transits
Timing Method Summary:
• First successful detection technique!• Requires a suitable target (clock) • Lack of large sample => not efficient• In best case (very short periods) is
sensitive to Earth-mass planets