PHYS178QAP-LEC2-3 by xuyuzhu


									            Extrasolar planets
Lecture 2: Planetary formation theory and
            detection techniques
                 A/Prof. Quentin A Parker

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   Formation of a Star and proto-planetary disk

(a) Dense cores form within a molecular cloud.
(b) A protostar with a surrounding disk of material forms at the centre, accumulating
additional material from the molecular cloud through gravitational attraction.
(c) A stellar wind breaks out, confined by the disk to flow along the stellar poles.
(d) Eventually this wind sweeps away the cloud and halts the accumulation of
additional material, and a newly formed star, surrounded by a disk, becomes visible.
The diameter of a typical accreting envelope is about 5000 astronomical units.
The typical diameter of the disk is about 100 AU.
Disks around protostars
These Hubble Space Telescope infrared images show disks around
young stars in the constellation of Taurus, in a region about 450 LY

In some cases we can see the central star (or stars—some are
binaries). In other cases, the dark horizontal bands indicate
regions where the dust disk is so thick that even infrared radiation
from the star embedded within it cannot make its way through.

The bright glowing regions are starlight reflected from the upper
and lower surfaces of the disk, which are less dense that the
central regions.
Fig 20-12, p.450
•Tracks are plotted on the H–R diagram to show how stars of
different masses change during the early parts of their lives.

•The numbers next to each dark point on a track are the rough
number of years it takes an embryo star to reach that stage.

•You can see that the more mass a star has, the shorter the time it
takes to go through each stage.

•Stars that lie above the dashed line would typically still be
surrounded by infalling material and would be hidden by it.
                  Disks around protostars

These HST images show 4 disks around young stars in the Orion Nebula.

The dark, dusty disks are seen silhouetted against the bright backdrop of the
glowing gas in the nebula.

The size of each image is about 30 times the diameter of our planetary system;
this means the disks we see here range in size from two to eight times the orbit of Pluto.

The red glow at the center of each disk is a young star, no more than a million years

(credit: M. McCaughrean, C. R. O’Dell, and NASA)
• The currently favoured scenario for planet formation is that of core
• Initially planetary cores form from condensed material in the
protoplanetary disc around a star
• In an inner hotter zone only grains of dust and small particulates
aggregate together
• Planet formation is further supported by the presence of icy snowballs in
a cooler zone outside the so-called
“ice boundary”
• Planets forming there are likely to grow to gas giants by accreting
hydrogen and helium
• They can then migrate inwards
• A decent fraction end up in close orbits
• These constitute the hot inner planets.

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                    Dust Ring Around a Young Star

This near infrared HST image shows a narrow ring of dust around the very young
star HR 4796A, ~220 lyr away in the constellation of Centaurus. The ring is very
narrow, spanning the same distance as that which separates Mars from Uranus
Though the ring is much further from its star, lying at what would be about twice
the distance of Pluto from our Sun. The image was taken with a coronagraph, a
device that covers the bright star which allows faint structures to be seen.
(B. Smith, U. of Hawaii; G. Schneider, U. of Arizona; and NASA)
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           (at least not at the moment)

Planets do not produce (much of )their own light

            They are very far away

       The stars they orbit are too bright

    We have to rely on indirect
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   The Doppler shift
   Astrometry
   Planet transits
   Gravitational microlensing
   Direct imaging

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Planets do not orbit stars,
  they orbit each other
  around the common
     centre of mass

                                               This causes the
                                              star to “wobble”

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   In 1842, the Austrian physicist Christian Johann Doppler
    noted that the wavelength of light, sound, or any other kind of
    propagating energy measured by a moving observer will be
    shifted by a factor of: v/c where v is the velocity at which the
    observer is approaching or receding from the source and c is
    the speed at which the wave propagates
   This effect occurs for any kind of radiation not just
    electromagnetic radiation
   We may all be familiar with the effect with sound – a
    mechanical wave transmitted through air
   We note the doppler effect with sound by a change in pitch of
    the sound

   Consider truck approaching with constant velocity clanging bell
    once a sec.
   When bell clangs first the sound reaches our ears and 1 sec later
    the truck has moved forward and the sound from the bell has a
    shorter distance to travel
   Note the circles of expanding sound from the position of each bell
    causing a compression of sound ahead of the truck and an
    expansion behind due to the truck’s movement
    (a)                  v                            (b)

   light waves emanate from origin
   If source is moving forward relative to observer (a) then the
    wavefront is compressed – frequency is increased and
    wavelength decreased (blue shift)
   If source receding from observer (b) then wavefront is
    stretched out, i.e. the frequency is decreased and wavelength
    is increased (red shift)
Measure this wobble
using spectroscopy

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  Convert Doppler shift into velocity

Derive Period,
Eccentricity &
minimum Mass

 Velocity measured in metres per second
     Required precision is ~3 m/s or
          1 part in 100,000,000!
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   Conception  Reality ~ 40 years
   Use very stable spectrographs, either
       Very temperature/pressure stable; or
       With very precise reference

   Several long-term programmes

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   Measure the absolute position
    of an object over time
   Look for a regular wobble as
    the star drifts through space
   Very hard to do - need to
    account for all other effects

                               Need accuracy to 1 part in
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   Similar to a solar eclipse
   A star dims when a planet passes in front
   Brightness change depends on the planet’s size
       Small planet  small change
       Large planet  large change

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   Only a small fraction of planets transit
   Almost any telescope can be used to find them
       An amateur astronomer discovered a planet around
        HD149026 with a 14” Celestron

   Space telescopes have a huge advantage

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From Einstein’s Theory
 of General Relativity

                                                The gravity of
                                              massive objects can
                                                act as a “lens”

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                                         Stars can do this on a
                                             smaller scale

A background star will
 brighten when a star
  passes in front of it

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   More sensitive than other techniques to small-mass earth-like
   Most sensitive to planets that have orbits of just several AU’s
    (such as for Mars or Jupiter/Saturn)
   The most common stars will be the most likely candidates for
   Capable of detecting multiple planets in a single light curve
   Can be used to study the statistical abundance of extra solar
    planets in our own Galaxy with properties akin to those in the
    Solar System.

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   Millions of stars must be monitored to find the few that are microlensing
    at any given time
   Planetary deviations in a light curve are short-lived and could be easily be
   Quite high probability that any planet will not be detected in a lensed
    system, even if present
   Deviations in microlensing light curves due to planets will not repeat (as
    they are due to a chance alignment that will not recur
   Planetary parameters (such as mass, orbit size, etc) depend on the
    properties of the host star, which are typically unknown
   The microlensing technique requires intensive use of telescope time, and
    is unsuitable for continued detailed study of individual extra solar

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Table 20-2, p.457
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    Astronomers would, however, prefer to obtain a direct image of an
    exoplanet, allowing them to better characterize the object's physical
    nature. This is an exceedingly difficult task, as the planet is generally
    hidden in the "glare" of its host star.

   To partly overcome this problem, astronomers study very young objects.
    Indeed, sub-stellar objects are much hotter and brighter when young and
    therefore can be more easily detected than older objects of similar mass.

   Based on this approach, it might well be that last year's detection of a
    feeble speck of light next to the young brown dwarf 2M1207 by an
    international team of astronomers using the ESO Very Large Telescope
    (ESO PR 23/04) is the long-sought bona-fide image of an exoplanet. A
    recent report based on data from the Hubble Space Telescope seems to
    confirm this result. The even more recent observations made with the
    Spitzer Space Telescope of the warm infrared glows of two previously
    detected "hot Jupiter" planets is another interesting result in this context.
    This wealth of new results, obtained in the time span of a few months,
    illustrates perfectly the dynamic of this field of research.
   On several occasions during the past years, astronomical images revealed faint objects, seen near
    much brighter stars. Some of these have been thought to be those of orbiting exoplanets, but after
    further study, none of them could stand up to the real test. Some turned out to be faint stellar
    companions, others were entirely unrelated background stars. This one may well be different.

   In April of this year, the team of European and American astronomers detected a faint and very red
    point of light very near (at 0.8 arcsec angular distance) a brown-dwarf object, designated
    2MASSWJ1207334-393254. Also known as "2M1207", this is a "failed star", i.e. a body too small for
    major nuclear fusion processes to have ignited in its interior and now producing energy by
    contraction. It is a member of the TW Hydrae stellar association located at a distance of about 230
    light-years. The discovery was made with the adaptive-optics supported NACO facility [3] at the 8.2-
    m VLT Yepun telescope at the ESO Paranal Observatory (Chile).

   The feeble object is more than 100 times fainter than 2M1207 and its near-infrared spectrum was
    obtained with great efforts in June 2004 by NACO, at the technical limit of the powerful facility. This
    spectrum shows the signatures of water molecules and confirms that the object must be comparatively
    small and light.

   None of the available observations contradict that it may be an exoplanet in orbit around 2M1207.
    Taking into account the infrared colours and the spectral data, evolutionary model calculations point
    to a 5 jupiter-mass planet in orbit around 2M1207. Still, they do not yet allow a clear-cut decision
    about the real nature of this intriguing object. Thus, the astronomers refer to it as a "Giant Planet
    Candidate Companion (GPCC)" [4].

   Observations will now be made to ascertain whether the motion in the sky of GPCC is compatible
    with that of a planet orbiting 2M1207. This should become evident within 1-2 years at the most.
 ESO PR Photo 26a/04 is a composite
image of the brown dwarf object 2M1207
(centre) and the fainter object seen near it,
at an angular distance of 778 milliarcsec.
Designated "Giant Planet Candidate
Companion" by the discoverers, it may
represent the first image of an exoplanet.
Further observations, in particular of its
motion in the sky relative to 2M1207 are
needed to ascertain its true nature. The
photo is based on three near-infrared
exposures (in the H, K and L' wavebands)
with the NACO adaptive-optics facility at
the 8.2-m VLT Yepun telescope at the ESO
Paranal Observatory.
   ESO PR Photo 26b/04 shows near-
    infrared H-band spectra of the
    brown dwarf object 2M1207 and the
    fainter "GPCC" object seen near it,
    obtained with the NACO facility at
    the 8.2-m VLT Yepun telescope. In
    the upper part, the spectrum of
    2M1207 (fully drawn blue curve) is
    compared with that of another
    substellar object (T513; dashed line);
    in the lower, the (somewhat noisy)
    spectrum of GPCC (fully drawn red
    curve) is compared with two
    substellar objects of different types
    (2M0301 and SDSS0539). The
    spectrum of GPCC is clearly very
    similar to these, confirming the
    substellar nature of this body. The
    broad dips at the left and the right
    are clear signatures of water in the
    (atmospheres of the) objects.
   "If the candidate companion of 2M1207 is really a planet, this would be the first time that a gravitationally bound exoplanet has been imaged around
    a star or a brown dwarf" says Benjamin Zuckerman of UCLA, a member of the team and also of NASA's Astrobiology Institute.

   Using high-angular-resolution spectroscopy with the NACO facility, the team has confirmed the substellar status of this object - now referred to as
    the "Giant Planet Candidate Companion (GPCC)" - by identifying broad water-band absorptions in its atmosphere, cf. PR Photo 26b/04.

   The spectrum of a young and hot planet - as the GPCC may well be - will have strong similarities with an older and more massive object such as a
    brown dwarf. However, when it cools down after a few tens of millions of years, such an object will show the spectral signatures of a giant gaseous
    planet like those in our own solar system.

   Although the spectrum of GPCC is quite "noisy" because of its faintness, the team was able to assign to it a spectral characterization that excludes a
    possible contamination by extra-galactic objects or late-type cool stars with abnormal infrared excess, located beyond the brown dwarf.

   After a very careful study of all options, the team found that, although this is statistically very improbable, the possibility that this object could be an
    older and more massive, foreground or background, cool brown dwarf cannot be completely excluded. The related detailed analysis is available in
    the resulting research paper that has been accepted for publication in the European journal Astronomy & Astrophysics (see below).

   Implications

   The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW
    Hydrae Association, it is about eight million years old.

   Because our solar system is 4,600 million years old, there is no way to directly measure how the Earth and other planets formed during the first tens
    of millions of years following the formation of the Sun. But, if astronomers can study the vicinity of young stars which are now only tens of millions
    of years old, then by witnessing a variety of planetary systems that are now forming, they will be able to understand much more accurately our own
    distant origins.

   Anne-Marie Lagrange, a member of the team from the Grenoble Observatory (France), looks towards the future: "Our discovery represents a first
    step towards opening a whole new field in astrophysics: the imaging and spectroscopic study of planetary systems. Such studies will enable
    astronomers to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."
ESO PR Photo 10a/05 shows
the VLT NACO image, taken in
the Ks-band, of GQ Lupi. The
feeble point of light to the right
of the star is the newly found
cold companion. It is 250 times
fainter than the star itself and it
located 0.73 arcsecond west. At
the distance of GQ Lupi, this
corresponds to a distance of
roughly 100 astronomical units.
North is up and East is to the
 ESO PR Photo 10c/05 shows
the NACO spectrum of the
companion of GQ Lupi (thick
line, bottom) in the near-
infrared (around the Ks-band at
2.2 microns). For comparison,
the spectrum of a young M8
brown dwarf (top, in red) and
of a L2 brown dwarf (second
line, in brown) are shown. Also
presented is the spectrum
calculated using theoretical
models for an object having a
temperature of 2,000 degrees.
This theoretical spectrum
compares well with the
observed one.

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