Transcript M sun - X-ray and Observational Astronomy Group

DEPARTMENT OF PHYSICS AND ASTRONOMY
3677 Life in the Universe:
Extra-solar planets
Dr. Matt Burleigh
www.star.le.ac.uk/mrb1/lectures.html
Course 3677 Life in Universe 2013/2014 Academic Year
Course Given by Prof. Mark Sims and Dr. Matt Burleigh
Topics: Life in Universe and Extra-Solar Planets
Lecture Dates and Lecturer
Actual
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Lecture by
Topic
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n
Time
Date
Nominal Course
Order 2013/14
M.R. Sims
M. Burleigh
M. Burleigh
M. Burleigh
M. Burleigh
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M.R. Sims
M. Burleigh
Both
Life in Universe
Extra-Solar Planets
Extra-Solar Planets
Extra-Solar Planets
Extra-Solar Planets
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Life In Universe
Extra-Solar Planets
Continuous
Assessment Answers
Revision Lectures
Phys A
KE LT2
Phys A
Phys A
KE LT2
Phys A
Phys A
KE LT2
Phys B
KE LT2
Phys B
KE LT2
Phys B
1300
1300
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1100
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1100
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R1
R2
M. Burleigh
M.R. Sims
Phys D
Phys B
0900
0900
7/5/14
15/5/14
Please note course is not in nominal order due to availability of Lecturers
Course has been extensively revised from previous years as Prof. Raine is no longer teaching part of
the course, consequently exam format has changed to 4 questions two short (20 marks each), two
long (30 marks each) all compulsory.
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 1
–
–
–
–
Definition of a planet
A little history
Pulsar planets
Doppler “wobble” (radial velocity) technique
• Lecture 2
– Transiting planets
– Transit search projects
– Detecting the atmospheres of transiting planets:
secondary eclipses & transmission spectroscopy
– Transit timing variations
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 3
–
–
–
–
Microlensing
Direct Imaging
Other methods: astrometry, eclipse timing
Planets around evolved stars
• Lecture 4
– Statistics: mass and orbital distributions, incidence of solar
systems, etc.
– Hot Jupiters
– Super-Earths
– Planetary formation
– Planetary atmospheres
– The host stars
Dr. Matt Burleigh
3677: Life in the Universe
Course outline
• Lecture 5
– The quest for an Earth-like planet
– Habitable zones
– Results from the Kepler mission
• How common are rocky planets?
• Amazing solar systems
– Biomarkers
– Future telescopes and space missions
Dr. Matt Burleigh
3677: Life in the Universe
Useful numbers
•
•
•
•
RSun = 6.995x108m
Rjup = 6.9961x107m ~ 0.1RSun
Rnep = 2.4622x107m ~ 4Rearth
Rearth = 6.371x106m ~ 0.1Rjup ~ 0.01RSun
•
•
•
•
MSun= 1.989x1030kg
Mjup= 1.898x1027kg ~ 0.001MSun = 317.8Mearth
Mnep= 1.02x1026kg ~ 5x10-5MSun ~ 0.05Mjup = 17.15Mearth
Mearth= 5.97x1024kg = 3x10-6MSun = 3.14x10-3Mjup
• 1AU = 1.496x1011m
• 1 day = 86400s
Dr. Matt Burleigh
3677: Life in the Universe
Blue: radial velocity, Green: transiting, Red: microlensing,
Orange: direct imaging, Yellow: pulsar timing
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• When a foreground star passes in front of a background star, light
from the background star is bent by the gravitational field of a
foreground lens to create distorted, multiple and/or brightened
images
– Consequence of general relativity
• The milli-arcsecond separation between multiple images is too
small to be resolved by modern telescopes. The combined light of
all images is instead observed as a single image of the source
• The brightness of the combined image is a function of the projected
separation of the source and lens on the sky, and changes as the
source, lens and observer move relative to one another
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• If the lens is a single, isolated object, the lightcurve of the background
source is simple, smooth and symmetric.
• The background star appears to brighten and then dim as the
projected separation between the source and lens first decreases and
then increases.
• For sources and microlenses are in our own Galaxy, a typical
timescale for the detectable rise and fall of the apparent brightness of
the source star is weeks to months.
• The basic shape is the same regardless of the relative path the source
takes on the sky; the amplitude of the the lightcurve is determined by
the minimum angular separation between the lens and source in units
of the Einstein radius, ie θLS/θE .
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• If the star has planets, the magnification pattern experienced by a
background source is no longer circularly symmetric on the sky
• The combined gravitational field of the star and planet can create strong
deviations in the lensing pattern, called caustics
– This means that the changes in the lightcurve of the background source can
be quite dramatic if it does happen to cross the planet-affected area, even
for Earth-sized planets.
– In the diagram, the red patches are the caustics and P indicates the position
of the planet
• Because the planet has a gravitational mass that is much smaller than
that of the lensing star, the percentage of the lensing pattern area
influenced by the planet will be relatively small.
– This means that the probability that the source will cross the planet-affected
area is low, and thus the chance of detecting a planet by microlensing is
also low,
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• Beginning in the 1990s, millions
of stars have been monitored
every night in search of the few
that are microlensed at that time
– 24 planets found
• Microlensing gives the mass ratio
between the planet and its parent
star, q=Mp/M∗ , and the angular
separation between the planet
and star on the sky at the time of
the lensing event, θ∗,p/θE , in units
of the Einstein ring radius
– M* is obtained from the spectral
type of the lensing star
– The star’s proper motion gives
the time to cross the Einstein ring
Dr. Matt Burleigh
3677: Life in the Universe
Gravitational microlensing
• Advantages
– Can detect Earth-size
planets across Galaxy
– Can detect planets in other
galaxies
• Disadvantages
– Must monitor millions of
stars constantly (eg in
galactic bulge)
– Lensing event never
repeats
– Star too far away to study
planet again
Dr. Matt Burleigh
3677: Life in the Universe
Astrometry
• The motion of a star around the centre
of mass of a star-planet system can be
detected by repeatedly measuring the
position of the star on the sky
• The amplitude of the motion in micro
arseconds (10-6 arcsec) is given by:
-1
æ q öæ a öæ d ö
DQ = 0.5ç -3 ÷ç
֍
÷
è 10 øè 5AU øè 10 pc ø
• Where q=(Mpl/M*), a = semi-major axis
of the orbit in AU and d = distance to
star in pc
• For Jupiter at 5AU, the amplitude of the
Sun’s motion as seen from another star
is ~5x10- arcsec (right)
• GAIA will discover ~10,000 Jupiters at
1-4AU around stars up to 200pc away
Dr. Matt Burleigh
3677: Life in the Universe
Other methods
• Pulsation timing:
– Many stars, like white dwarfs, have
very stable pulsation modes. The
presence of a planet will be
revealed in anomalous timings, just
as with pulsar planets
• Eclipse timing:
– Close, eclipsing binary systems
can also reveal the presence of
planets through anomalous timings
of the expected eclipses
– A good example is the close
eclipsing white dwarf + red dwarfs
binary NN Ser, which appears to
have 2 Jupiter mass planets
orbiting it
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection
• Imaging = spectroscopy = physics:
composition & structure
• Difficult
• Why?
– Stars like the Sun are billions of times brighter than
planets
– Planets and stars lie very close together on the sky
• At 10pc Jupiter and the Sun are separated by 0.5”
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection
• Problem 1:
– Stars bright, planets faint
• Solution:
– Block starlight with a coronagraph
• Problem 2:
– Earth’s atmosphere distorts starlight, reduces
resolution
• Solution:
– Adaptive optics, Interferometry – difficult,
expensive
– Or look around very young and/or intrinsically faint
stars (not Sun-like)
Dr. Matt Burleigh
3677: Life in the Universe
First directly imaged planet?
• 2M1207 in TW Hya
association
• Discovered at ESO VLT in
Chile
• 25Mjup Brown dwarf + 5Mjup
“planet”
• Distance ~55pc
• Very young cluster ~10M
years
• Physical separation ~55AU
• A brown dwarf is a failed
star
– Can this really be called a
planet?
– Formation mechanism may
be crucial!
Dr. Matt Burleigh
3677: Life in the Universe
First directly imaged planetary system
• In 2008 3 planets imaged around
the star HR8799
• 130 light years away (40pc)
• Three planets at 24, 38 and 68AU
separation
– In comparison, Jupiter is at 5AU
and Neptune at 30AU
• Masses of 7Mjup, 10Mjup and
10Mjup
• Young: 60Myr
– Earth is ~4.5Gyr
Dr. Matt Burleigh
3677: Life in the Universe
Fomalhaut (alpha Piscis Austrini)
• One of the brightest stars in
the southern sky
• Long known to have a dusty
debris disk
• Shape of disk suggested
presence of planet
• 2Mjup planet imaged by HST
inside disk
• 200Myr old
• Like early solar system
Dr. Matt Burleigh
3677: Life in the Universe
Direct detection: White Dwarfs
• White dwarfs are the end state of stars like the Sun
– What will happen to the solar system in the future?
• WDs are 1,000-10,000 times fainter than Sun-like
stars
– contrast problem reduced
• Over 100 WD within 20pc
– At 10pc a separation of 100AU = 10” on sky
– Planets should be located well away from the host
white dwarf
•
At Leicester we are searching for planets around
nearby WD with 8m telescopes and the Spitzer
space telescope
Dr. Matt Burleigh
3677: Life in the Universe
Log(L/Lsun)
PN ejected
to WD
Thermal Pulse begins
4
AGB
2
He
Helium Flash
C+O
RGB
H
0
He
WD
cooling
4.2
3.8
3.4
Log Teff (K)
Dr. Matt Burleigh
3677: Life in the Universe
The end of our solar system
Dr. Matt Burleigh
3677: Life in the Universe
The end of our solar system
• The inner planets, Mercury, Venus and probably Earth, will be destroyed
by the expanding red giant
– As a red giant (actually, asymptotic giant), the Sun’s radius will be ~1AU
• Mars, the asteroids and outer gas giants will survive
• As the red giant loses mass when it evolves to the planetary nebula
stage, the outer planets orbits evolve outwards by factor:
• Jeans (1924):
M MS
M WD
• Where MMS and MWD are the main sequence and white dwarf masses in solar mass
units (Msun)
• Note: the relationship between a star’s mass and a white dwarf’s mass is
given by:
MWD = 0.12M MS + 0.36
• This is called the “initial-final mass relation”, and is derived from observations of white
dwarfs in clusters (Casewell et al., 2009, MNRAS, 395, 1795)
Dr. Matt Burleigh
3677: Life in the Universe
Spitzer 4.5micron image
GJ3483 (LTT3059 / WD0806-661)
130” /
2500AU
I maybe a planet…
or a brown dwarf
Dr. Matt Burleigh
3677: Life in the Universe
I am the
white dwarf
Dr. Matt Burleigh
3677: Life in the Universe
Dr. Matt Burleigh
3677: Life in the Universe
Proper motion
WD
Companion
• PM error +/-25mas/yr
Dr. Matt Burleigh
3677: Life in the Universe
Calculating the planet’s mass
•
How can we estimate the mass of a directly imaged planet?
– Planets of identical mass are assumed to be born with identical temperatures, &
cool with age (no nuclear burning in core)
– Thus by measuring their brightness, and estimating the host star’s age &
distance, we can use a theoretical “evolutionary model” to convert the brightness
to a mass!
•
Method:
– (1) measure it’s brightness from the image
– (2) determine the star’s distance (eg from it’s spectral type if its main sequence,
or better still from its parallax)
– (3) convert the star’s apparent mag to absolute mag
– (4) estimate the star’s age (eg from it’s rotation period, or if it belongs to an open
cluster or coeval moving group)
– (5) compare the absolute mag to evolutionary model predicted masses and
luminosities for the correct age
•
Caveats:
– Ages of main sequence stars are notoriously difficult to measure
– There is no guarantee that two planets of the same age and mass will have the
same atmospheric chemistry, structure and temperature
– Evolutionary models are only as good as the input physics and assumptions, and
are particularly poor at predicting masses at very young ages (few million years)
Dr. Matt Burleigh
3677: Life in the Universe
Calculating the planet’s mass:
example: GJ3483b
•
Measure apparent magnitude of object in Spitzer’s 4.5micron filter (“Band 2”)
– Find m = 16.75
•
•
We know the distance d to the white dwarf star from its parallax (it’s 19.2parsecs
away)
So we can convert the apparent mag to an absolute mag
– Absolute mag M is magnitude at 10pc
– Use m – M = 5 log d – 5
(Pogson’s equation)
– Find M = 15.33
•
•
We also know how old the white dwarf is from its temperature (white dwarfs cool
steadily with time) – 2Gyr
Look up a theoretical model which predicts brightness of gas giant planets at
different ages
•
•
•
•
•
t (Gyr) = 2.00
-------------------------------------------------------------------------------M/Ms Teff L/Ls gIRS Blue IRS red Band1 Band2 Band3 Band4
0.0080 370. -6.73 4.29 18.25 18.74 18.93 15.44 16.99 18.16
0.0090 391. -6.64 4.35 17.99 18.55 18.65 15.20 16.76 17.87
•
Jupiter is about 0.001M/Ms, so our planet GJ3483b is between 8-9 times the
mass of Jupiter!
Dr. Matt Burleigh
3677: Life in the Universe
What did the original system look
like?
• The white dwarf GJ3483 has a mass of 0.58MSun
– From the intial-final mass relation
•
MWD = 0.12M MS + 0.36
– The progenitor main sequence star had a mass of
1.83MSun (a late A or early F star)
The white dwarf and planet are separated by
130”
– At 19.2pc, 130” = 2500AU
• (Note 1AU = 1” at a distance of 1pc)
– Using the Jean’s relation between the initial and
final separations the planet was originally located
at a separation: 2500AU / (MMS/MWD) = 790AU
– Still a very large solar system!
Dr. Matt Burleigh
3677: Life in the Universe