Report - School of Physics

ground requires simultaneous observations of the target and an astrometric reference.
In a dual-star interferometer, each telescope accepts two small fields and sends two
separate beams through the delay lines. The delay difference between the two fields
is taken out with an additional short-stroke differential delay line; an internal laser
metrology system is used to monitor the delay difference (which is equal to the
phase difference multiplied with λ/2π). For astrometric observations, this delay
difference ∆D is the observable of interest, because it is directly related to the
coordinate difference between the target and reference stars; it follows that ∆D ≡
D t −D r = B·(ŝ ⃗ t − ŝ r ) = B(cos θ t −cos θ r ), where the subscript t is used for the target,
and r for the reference. To get robust two-dimensional position measurements,
observations of the target with respect to several references and with a number of
baseline orientations are required.
Measurements of the delay difference between two stars give relative astrometric
information; this means that the position information is not obtained in a global
reference frame, but only with respect to nearby comparison stars, which define
a local reference frame on a small patch of sky. This approach greatly reduces
the atmospheric errors, and some instrumental requirements are also relaxed. The
downside is that the information that can be obtained in this way is more restricted,
because the local frame may have a motion and rotation of its own. This makes it
impossible to measure proper motions. Moreover, all parallax ellipses have the same
orientation and axial ratio, which allows only relative parallaxes to be measured.
Specific instrument approaches are discussed in Section 2.1.6 (NAOS-CONICA,
Planet Finder, PRIMA) and Section 2.2.2 (Gaia, SIM, etc.).
No planets have been discovered using this technique to date.
2.1.6 Direct Detection
The light coming from an extra-solar planet is much fainter (of order 10 9 in the
optical, and a factor 10–100 less in the infrared) than the signal from the star.
Therefore the challenge is to build instruments that are able to provide extremely
high contrast and spatial resolution. The different approaches are summarised below
and in Table 4. The first direct detection of a young planet may already have
been achieved by a team using NACO on the VLT. An object detected close to
2MASS WJ1207334–393254 is either a planet or possibly a brown dwarf (Chauvin
et al., 2004). Regardless of the exact nature of this particular object, it is likely that
imaging of more massive, young extra-solar planets will become more feasible in the
near future.
A number of programmes are using, or planning to use, interferometry to achieve
high spatial resolution. Destructive interference can be used to remove most of
the light from the central star (nulling). ESA and ESO are collaborating on a
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