Report - School of Physics

ferential phase observations with a near-IR interferometer offer a way to obtain
spectra of extra-solar planets. The method makes use of the wavelength dependence
of the interferometer phase of the planet/star system, which depends both on
the interferometer geometry and on the brightness ratio between the planet and the
star. The differential phase is strongly affected by instrumental and atmospheric
dispersion effects. Difficulties in calibrating these effects might prevent the application
of the differential phase method to systems with a very high contrast, such as
extra-solar planets. A promising alternative is the use of spectrally resolved closure
phases, which are immune to many of the systematic and random errors affecting
the single-baseline phases.
Figure 3 shows the predicted response of the AMBER instrument at the VLTI to
a realistic model of the 51 Peg system, taking into account a theoretical spectrum
of the planet as well as the geometry of the VLTI. Joergens & Quirrenbach (2004)
have presented a strategy to determine the geometry of the planetary system and the
spectrum of the extra-solar planet from such closure phase observations in two steps.
First, there is a close relation between the nulls in the closure phase and the nulls in
the corresponding single-baseline phases: every second null of a single-baseline phase
is also a null in the closure phase. This means that the nulls in the closure phase
do not depend on the spectrum but only on the geometry, so that the geometry of
the system can be determined by measuring the nulls in the closure phase at three
or more different hour angles. In the second step, the known geometry can then be
used to extract the planet spectrum directly from the closure phases.
ALMA: ALMA is an interferometer in the mm-wavelength range, which will consist
of sixty-four 12 m diameter antennae located in northern Chile, at 5050 m altitude.
The antennae can be spaced from a compact configuration with a maximum separation
of 150 m to a very extended configuration where the maximum spacing is
16 km, providing a resolution of 10 milli-arcsec at shortest wavelengths. The receivers
will cover the atmospheric windows in the 35–1000 GHz range (350 µm –
7 mm) with a bandwidth of 8 GHz in two polarizations, and a resolution of 32000
channels. ALMA will be powerful for studying the disks around young stars, able to
image such disks out to several hundred parsecs, providing density and temperature
profiles (through measurements of thermal dust emission), and providing constraints
on disk dynamics and chemistry (through measurements of spectral lines). In the
case of protoplanetary disks, ALMA will be able to image gaps and holes caused by
protoplanets.
In terms of direct detection of the planet themselves, however, ALMA is rather
limited. At its best resolution (widest configuration), the system will be able to
resolve a Jupiter-like planet from its star out to 100–150 pc. The main limitation
comes from the flux of the planet. The best frequency for planet observation, which
optimises the combination of expected detector noise characteristics, the spectrum of
the objects, and the site characteristics, is at 350 GHz. The flux density of the planet
is directly related to their temperature, size and distance as F 350 = 6.10 −8 T R 2 J/D 2 ,
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