Post on 09-Jan-2016
description
Somak Raychaudhurywww.sr.bham.ac.uk/~somak/Y2SiU/
Two-body problem Binary stars
VisualEclipsingSpectroscopic
How to find extrasolar planets
Lecture 7Guest Lecturer: Dr W. J. Chaplin
Two-body problem: centre of mass
The two bodies must have the same angular speed ω, otherwise one body will catch up with another and the force will no longer be directed towards the common centre of the circles.
Two-body problem: centre of mass
But
Types of binary stars
Visual Binary Stars
Orbit of Star 1
Orbit of Star 2
Centre of Mass
Astrometric binariesAstrometric binaries are unresolved visual binaries Sirius AB was first discovered as an astrometric binary system, but now we can resolve them, so they are visual binaries
Sirius A & B
Eclipsing binaries
In these stars, inclination is close to 90 deg
Eclipsing binaries
Light curve of HIP 53806, from the Hipparcos satellite.This star varies between about 7.58 and 7.84 magnitudes, with a period of 4.535 days
Spectroscopic Binaries
Spectrum of Hydrogen in
Lab
Spectrum of stars- Obs 1
What do these spectra tell us about the system?
Spectrum of stars- Obs 2
Spectrum of stars- Obs 3
Spectrum of stars- Obs 4
Spectroscopic binaries
The absorption lines are redshifted or blueshifted as the star moves in its orbit
Radial Velocity Curve
HDE 226868
Spectroscopic binaries: circular orbits
• The radial velocities are a sinusoidal function of time. The minimum and maximum velocities (about the centre of mass velocity) are given by
ivv
ivv
r
r
sin
sin
2max2
1max1
where i is the angle of inclination
Measuring masses in a binary
Kepler 3:
Observed speeds
We know the value of i only if the binary has eclipses
Methods of finding exoplanets
2004(2M1207)
Direct obs.
2002 ?(Gl 876 b)
Astro-metry
2003(O235/M53)
1995(51 Peg b)
Micro-lensing
Dopplershift
Transit
2000 and 2004
(HD209458b)
Pulsartiming
1992(PSR 1257+12)
Exoplanets are extreme cases of binaries, where one member is much less massive than the other. So we can use some of the same techniques as binaries for detection, and mass measurement. We’ll discuss some of these methods.
Extrasolar planet searches
• As of last Friday, 429 planets have been detected outside our solar system (http://exoplanets.org/)
Most of these have a<1 AU and masses >MJupiter
Direct Detection: very hardThe albedo of the Earth is about AV=0.4.
Issues to think about: How bright is it in visible (reflected) light, relative to the Sun? How do they compare at infrared wavelengths, where Earth emits thermal radiation?
A picture of Earth, from the surface of Mars, just before sunrise.
Doppler Shifts: the #1 method• This technique directly measures the velocity imparted to a star by having planets going around it:
All gravitationally bound systems orbit their Centre of Mass (CM).Here index 1 refers to the Sun, and 2 to the planet
C.M. outside the surface of the Sun!
Here index 1 refers to the Sun, and 2 to the planet
Period of Jupiter’s orbit=13.86 years
Speed of Jupiter around Sun=We expect
Speed of Sun around c.m.=
Sun’s wobble due to Jupiter
Transits
• Imagine viewing the Earth-Sun system from a distant star. By how much will the Sun fade during a transit of the Earth? How about during a transit of Jupiter?
Planetary transits• This is a fairly direct technique that relies on the correct orientation of the solar system to give rise to the transit of a planet across the face of its star.
Transits• Detects a planet's shadow when it transits in front of its host star.
• Can be used to measure the radius of a planet.
Transit method
Space Interferometry mission
• NASA: Would have searched for terrestrial planets around the nearest ~250 stars, with astrometry accurate to 1as. Cancelled in 2010
Kepler
• NASA mission launched in 2009• Searching for large numbers of stars for Earth-sized terrestrial planets using the transit method.
Cygnus-Lyra regions along the Orion arm
Field of view
• Finding Earth-like planets• Formal seismology component:
Kepler Asteroseismology Investigation (KAI)• KAI has an international consortium of ~400 scientists
called the Kepler Asteroseismology Science Consortium (KASC)
http://kepler.asteroseismology.org
NASA Kepler Mission
• Nominal mission 3.5 yr (up to 10 yr)• Will observe Sun-like and classical pulsators:– Survey Phase: 1-month data on ~1500 Sun-like stars!
– Long-Term Phase: data for several years on 100 Sun-like stars
NASA Kepler Mission
Kepler ResultsChaplin et al. (2010)