A Way to Other Worlds |
Resp. Isabella Pagano
INAF - Osservatorio Astrofisico di Catania
Transiting planets are the big shots among exoplanets because it is possible for them to gather information both on their internal structure and their atmosphere. By observing the light output of the star when the planet is transiting the stellar surface, the shape and depth of the light curve can tell us about the size and other physical parameters of the planet (and of the star). Specifically, the transit constrains the inclination of the exoplanetary orbit so well that the true mass of the planet can be found from the radial velocity curve. By observing both the occultation of part of the light of the star by the planet, as well as the associated radial velocity curve, exact measurements of the planetary mass and diameter allow a determination of the body's average density and thus its mineralogy. Moreover, when secondary eclipses are observed, information on the planetary albedo (in the optical) and on the planetary emissivity (in the IR) can be obtained.
Transmission spectroscopy acquired during primary transits (with the stellar lights probing the thin atmospheric annulus surrounding the optically thick disk of the planet) and emission spectroscopy obtained during secondary eclipse by difference between the combined stellar and planetary spectrum and the pure stellar spectrum, are two methods used to analyse the planet atmosphere, from its composition to its structure.
Transits happen if the planet orbital plane intersects the line-of-sight
to the star. The probability of a chance alignment varies between
about 0.5% for a 1REarth planet at 1 AU from a solar type star to
several tens of percent for gaseous giant planets that are orbiting
very close to red dwarf stars. Transits are hence detected only by
means of photometric surveys of huge number of stars, with ground-based
transit observations aiming at detecting giants to Super-Earths
orbiting close to their stars, and space based projects having the
goal to detect Earth like planets including those orbiting at 1 AU
around solar type stars.
The timing analysis of a known transit allows searching for variations in either the transit duration or the centre induced by the perturbation of a third body, e.g. a second planet. By applying this transit-based method, the TASTE (The Asiago Search for Transit Timing variations of Exoplanets) project is collecting high-precision, short-cadence light curves for a selected sample of transits by using imaging differential photometry at the Asiago 1.82m telescope and the IAC80 and TCS telescopes at the Teide Observatory, Canary Islands. The TASTE target list can be easily updated to accommodate transiting planets discovered by APACHE
In this context we plan extending the Apache survey at NIR wavelengths: crucial improvements in the measurements of the time of mid-transit and the physical properties of transiting system can be obtained by taking advantage of milli-mag precision, multi-wavelength photometric observations. In fact multi-wavelength transit observations allow us: to probe very small stars (intrinsically red), to detect the presence of an atmosphere (thanks to observations of radius variations with wavelength), and to monitor and correct for the stellar activity.
We want to develop the prototype of a novel, compact imaging system
capable of simultaneous photometric observations in four optical
(e.g., Sloan g', r', i', z') and three NIR (J, H, K) pass-bands.
The full system will be composed of a specifically designed 60-cm
class telescope, equipped with dichroic beam-splitters feeding light
into the NIR channels and the visual channels, each equipped with
its own detector (e.g., back-illuminated E2V CCDs in the optical,
Rockwell HAWAII-1 arrays in the NIR). The final system testing will
be carried out at the OAVdA site, taking advantage of the existing
APACHE infrastructure and data reduction and analysis software. A
timeline of two years is estimated for the opto-mechanical design
of the telescope and its assembly, procurement of the detectors,
CCD and NIR cameras optics, electronics, and cryo-mechanics, their
integration, and full prototype assembly and testing.
Low resolution spectroscopy obtained contemporary in a wide range
from optical to near IR to monitor bright stars with transiting
planets is of paramount interest to clean data from micro stellar
variability and study the spectral variations with the orbital phase
of the planet. Such activity is crucial to select targets to be
observed by new generation dedicated instrumentations, as the ESA
mission EChO (see WP 5). Few instruments are available worldwide
on small-medium class telescope offering spectral coverage from
Optical to NIR. We plan to upgrade CAOS, a UVES-like spectro-polarimeter
operable in hi-res (R 70000) and low-res (R 2500) modes - at moment
between 388 and 1060 nm - by adding an IR branch to be used in
low-res mode (R 2500). CAOS is in operation at Catania Astrophysical
Observatory fibre-fed by a 91cm Cassegrain telescope. Its first
light has been in September 2012, with a nominal performance. The
extension toward NIR can be obtained by inserting a dichroic to
separate the optical light from IR. The optical arm is not changed.
The IR radiation is then dispersed using a prism and focused on an
IR detector. The upgrade requires procurement of optics, optics
holders, IR detectors, IR detector controller, additional vacuum
and cooling systems. A development
plan of 1.5 years is required including time for design, procurement,
assembly and testing.
Figure 4 - CHEOPS is thought to measure radii in the range ~1-10
REarth of planets orbiting stars with apparent visible magnitude
in the range ~6-12 mag, that are typical targets of radial velocity
surveys. Note that NGTS, the next generation transit survey that
will be operated from Chile starting in 2014, is capable to measure
size of a limited subsample of planets with known mass.