Gravitational Microlensing - EPN Campus · Gravitational Microlensing Light Curve of...

5/19/11 Reinhold Dorn ESI 2011 33

Gravitational Microlensing

Gravitational lensing caused by the presence of a star and an exoplanet

Lensing effect of the general relativistic curvature of spacetime based on Einstein's General Theory of Relativity


Gravitational Microlensing

Light Curve of OGLE-2005-BLG-390

Mag

nific

atio

n

Gravitational lensing caused by the presence of a star and an exoplanet

OGLE: Optical Gravitational Lensing Experiment


Astrometry

The astrometry method is similar to radial velocity tracking and is used to detect extrasolar planets by measuring the small regular perturbation in the position of a star due to its unseen companion > measures the position of the star!

Wobble amplitude proportional to:   planet Mass   (star mass )-2/3

  (planet period )2/3

  1 / distance to the star   amplitude does not depend on orbit inclination (sin i)

Reflex motion on the star (caused by the gravitation pull of the planet on the star)

Credit: http://www.mpia-hd.mpg.de


Example: Astrometry in space with GAIA

Gaia will continuously scan the sky for at least five years from a point in space known as the second Lagrangian point (or L2), located at about 1.6 million km away from the Earth, in the direction opposite to the Sun.

Gaia’s goal is to perform the largest census of our Galaxy and build a highly accurate 3D map.

Gaia will also identify as many as 10 000 planets around other stars, and discover several tens of thousands of new bodies - comets and asteroids - in our own Solar System.

Credit:ESA

Gaia will be the most accurate optical astronomy satellite ever built so far. Due for launch in 2011.

Gaia will have an accuracy of 20 microarcseconds for faint stars : (equivalent to measuring the diameter of a human hair at a distance of 1000 km!)

Much more difficult to do from the ground due to atmospheric effects!


Atmospheric turbulence causes a wavefront degradation. The stars twinkle due to intensity fluctuations.

A perfect telescope would have a theoretical diffraction limited angular resolution of

λ is the wavelength and D the diameter of the telescope. is the radius of the first dark ring of the airy disk in radians.

Due to the turbulence, the angular resolu1on is

is the seeing angle, r0 is the Fried parameter

Why do stars twinkle?

Image credit: Claire Max


Ground based Astrometry ……….and it’s limitations

PSFs of an 8 m telescope at a wavelength of 1000 nm with 0.65" seeing.

Diffraction limited

Speckle pattern Seeing disk

Even the largest ground-based astronomical telescopes have no better resolution than a 20 cm telescope due to turbulence of the atmosphere


Adaptive Optics

Closed loop with a 60 element MACAO curvature system and the AO-IR 1k x 1k test camera at 2.2 microns


Astronomical interferometry Astrometry with microarcsecond precision

Light source > Star

Use in Astronomy:

Recall Young's double-slit experiment (Thomas Young 1803) > wave nature of light

Optical interference effects convert the image of the point source into a fringe pattern (fringes).

Holes > Telescopes

Lens > Beamcombiner


Combination of light from a star observed through separated telescopes and combined to produce interference fringes.

Astronomical interferometry Astrometry with microarcsecond precision

Interferometer simulates a large aperture telescope in terms of resolution.


10 nm @ 100m =

~ a coin on the moon

This is what one wants to measure:

Ground based Astronomical interferometry


Aperture can be separate areas spread over a diameter but light needs to be combined coherently! > this is the main technological challenge

Astronomical interferometry - Astrometry with microarcsecond precision

Image: Quirrenbach, 2009

The external delay Dext is compensated by the two delay lines.

The zero-order interference maximum (maximum stellar fringe) occurs when

Dint = D1 − D2 =−Dext.

Astrometric precision: σ = (1/SNR) · (λ/2πB)

> Larger baseline B is better

combination of light from a star observed through separated telescopes and combined to produce interference fringes


The beam combiner compares the phases of the incoming wavefronts from the two telescope.

As a star moves difference in phase changes occur and allow the position of the star to be determined very accurately.

Constructive interference is found when the optical paths are equal.

OPDintern=OPDextern

This allows OPDextern (and hence the angle ) to be determined experimentally.

Image credits: Ralf Launhardt (MPIA)


PRIMA – the VLTI dual feed facility > Narrow angle astrometry

PRIMA picks two stars and feeds it into the Delay Lines and produces the fringes in the VLTI Laboratory OPDint measured with laser metrology • OPDturb averaged by long integration • ΔS B + φ determined by interferometric instruments • ΔS gives the astrometry, φ the imaging

ΔS = angle vector on sky between the stars


Copyright: ESA 2002. Illustration by Medialab.

In nulling interferometry the beam from one telescope is delayed by half a wavelength.

This method removes the stellar light observed with an interferometer by placing a dark interference fringe on it (destructive interference)

Nulling interferometer

Direct method


This composite image represents the close environment of Beta Pictoris as seen in near infrared light.

Observed in 1996 with the ADONIS instrument on ESO's 3.6 m telescope; the inner part is the innermost part of the system, as seen at 3.6 microns with NACO on the Very Large Telescope.

The newly detected source is more than 1000 times fainter than Beta Pictoris, aligned with the disc, at a projected distance of 8 times the Earth-Sun distance.

Both parts of the image were obtained on ESO telescopes equipped with adaptive optics.

Direct Imaging “The holy grail”

Direct imaging is the only way to assess some important physical parameters, such as the amount of water on the surface and the properties of any possible biosphere.


Peak at 0.5 microns: Refected solar spectrum

Peak at 9-20 microns:Thermal emission of the planet

Spectral energy distribution of our sun and planets (as viewed from a distance of 10pc)

Contrast ratio between the sun and the planets is enormous!

Sun is 10 billion times brigther than Jupiter within 2 arcsec.

Contrast ratio gets better at longer wavelength.

>> observe in the infrared.

But also need to block light from the star:

Coronagraph


Method of blocking off the stellar light (originally worked out by Bernard Lyot 1939)

Coronagraphy

Lyot observed the corona of the Sun without having to wait for total solar eclipses.

In the search for planets a coronagraph blocks the starlight to be able to image the faint object nearby.

Image credit : www.lyot.org


Coronagraphy principle (removal of diffraction pattern) The method removes 99% of the stellar light and it leaves out ~50% of light from the hypothetical planet

Telescope pupil

Image is formed

Blocked by a mask (on

axis)

Pupil is reimaged, remaining light of source

forms rings

Rings of light or diffracted light are blocked by Lyot stop and light from surrounding sources can pass to the final image

Final image

Secondary mirror

Lyot stop


Example: SPHERE – The planet finder for the VLT Spectro-Polar-imetric High-contrast Exoplanet REsearch

Challenge consists in the very large contrast between the host star and the planet, larger than 12.5 magnitudes (or 105 in flux ratio) and compensation of atmospheric turbulence ( > AO).

An eXtreme AO system (SAXO) using a 1600 actuator DM with up to 1200 Hz loop frequency

Dual-band imaging camera IRDIS, providing simultaneous imaging in two channels throughout the J, H, and K NIR bands.

3D spectroscopic imager IFS (J-band)

Differential imaging polarimeter ZIMPOL (Z-band (600-900 nm)

feed coronagraph with well corrected WF


Polarimeter principle

•  Light from a star is not polarized •  Reflected light from a planet is polarized

•  Contrast between the star and the planet is much smaller in polarized light


Polarimeter principle (Sphere/Zimpol)

Principle is fast modulation (1 KHz) , using a ferro-electric retarder and demodulation of the polarisation signal using a modified CCD array.

Advantages of this technique are the simultaneous detection of two perpendicular polarisations (the modulation is faster than seeing variations) and the recording of both images on the same pixel.

Polarimetric precision better than 10-5 --> very high contrast capability for polarized reflected light


SPHERE – The planet finder for the VLT


Radial Velocity

Large Surveys

HC & HAR Imaging

µ Lensing

Transits

What will Sphere be able to see? Scientific objectives

Image credit: Jean-Luc Beuzit


Pulsar Timing t ; mp/Ms

Radial Velocity t ; mp *sin I ;

Astrometry t ; mp ; a ; Ds

Transits t ; Ap ; a ; I ; Ds ; atm comp.

Gravitational Microlensing f(m, Ms ,r, Ds, DL )

Direct Imaging t ; Ap ; a ; I ; e ; Ds ; atm comp.

t=period, a=semi-major axis, mp=planet mass, Ap=planet area, I=orbit inclination, e=eccentricity, Ds=distance to star

Summary of Methods:


Where are we today? Known planets 2011

The exoplanets detected up to now are:

•  More massive •  Bigger and hotter •  Closer to the central star

than our Earth.

This is expected as it is limited by the Telescopes/ Instrumentation available.

Credit: Sara Seager (MIT, 2011)

Semi – Major Axis (AU)

Ear

th M

asse

s

5/19/11 59 Reinhold Dorn ESI 2011

With the upcoming E-ELT 2019, (D = 42 m, or maybe a little smaller), for the first time in history, technology may allows us to observe and characterise exoplanets in habitable zones.

The radial velocity down to 1 cm/s accuracy.

Direct imaging: approaching 10–9 contrast


www.earthday.de


All images and animation are ESO credit accept those referenced.

Thanks goes to the people how provided material and advise for this talk:

Francoise Delplancke-Stroebele, Markus Kasper, Luca Pasquini, Frank Eisenhauer, Jean-Luc Beuzit, Suzanne Ramsay, Jim Beletic and Gaspare Lo Curto.

If you want to read more about this and further links:

http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets

The Extrasolar Planets Encyclopaedia: http://exoplanet.eu/

http://exoplanets.org/


Backup slides


Kepler's third Laws of Planetary Motion

Astrocentric coordinates > coordinates centered on the star

The orbital period P, the semi-major axis a and the masses of the planet and the star are described by Kepler’s third law:

Credit: www.astronomynotes.com

Backup slides


ISAAC

NSCO YEPUN

TIMMI2 3.6m

NACO

FLAMES

VIMOS

UVES

2xFORS ANTU/YEPUN

2 x FORS

Extensive Instrumentation at the ESO Very Large Telescope observing from the visible up to 28 microns

and many more……


Wavelength calibration with a laser frequency comb. The LFC emits a periodic pulse train with a period of 1/frep. The associated optical spectrum shows a comb-like structure with a line-spacing of frep and an offset frequency f0. The two radio-frequencies frep and f0 are phase locked to a reference signal such as an atomic clock. This ensures precise knowledge of the frequencies fn of every spectral line in the LFC.

Laser frequency comb

Illustration from Thomas R. Schibli, "Frequency combs: Combs for dark energy", Nature Photonics, Volume 2, Issue 12, pp. 712-713 (2008).

Gravitational Microlensing - EPN Campus · Gravitational Microlensing Light Curve of...

Documents

Transcript of Gravitational Microlensing - EPN Campus · Gravitational Microlensing Light Curve of...