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WP 3 - Transits

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.

Ground-based transit observations

This WP element describes ongoing and planned synergetic efforts of the Italian community to detect and characterize transiting planetary systems from the ground.

APACHE - A Pathway to the Characterization of Habitable Earths (resp. A. Sozzetti)

APACHE is a project to undertake a high-precision (< 5 mmag) photometric transit search for small-size (Rp < 4R(Earth)), possibly habitable, planets around >3000 bright (J<10), nearby (D<50 pc) M0-M5 dwarfs. The project is a collaboration between INAF-Osservatorio Astrofisico di Torino and the Astronomical Observatory of the Autonomous Region of the Aosta Valley (OAVdA), in the western Italian Alps. APACHE employs an array of five identical Carbon Truss 40-cm f/8.4 Ritchey-Chretien telescopes hosted on a single platform with electronically controlled roll-off enclosure (a site unique in Europe). The survey, started in July 2012 will last for five years, and based on current estimates of planet occurrence rates, APACHE is expected to uncover a significant number of transiting systems. Transiting planet candidates identified by APACHE will be confirmed with HARPS-N@TNG, and APACHE targets have been included in the target lists of the Italy-led GAPS long-term program. Upon its conclusion, the APACHE survey data will thus critically contribute to test theoretical models of planet formation, structure, and evolution across orders of magnitude in radius, mass, and stellar irradiation, complementing observations obtained by ongoing and planned wide-field planet transit surveys, both from the ground and in space. Furthermore, all planets discovered by APACHE will orbit stars bright enough to constitute prime targets for crucial atmospheric characterization at infrared wavelengths with upcoming and future ground-based and space-borne instrumentation, such as GIANO, JWST, SPICA, EChO, and FINESSE.

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.

Supporting atmospheric characterization with dedicated facilities (resp. I. Pagano):

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.


Transit observations from space

This WP element describes ongoing and planned synergetic efforts of the Italian community to detect and characterize transiting planetary systems from the space.

CHEOPS - CHaracterizing ExOPlanets Satellite (resp I. Pagano)

CHEOPS has been selected by ESA as the first S-class mission in Cosmic Vision 2015-2025. Its launch is planned for 2017. The CHEOPS mission is a joint ESA-Switzerland project, with important contribution from Italy and a number of other ESA Member States, cooperating within a dedicated Mission Consortium.
CHEOPS will be the first mission dedicated to search for exoplanetary transits by means of ultrahigh precision photometry on bright stars already known to host planets, as shown in the Figure below. By being able to point at nearly any location on the sky, it will provide the unique capability of determining accurate radii for a subset of those planets in the super-Earth to Neptune mass range, for which the mass has already been estimated from ground-based spectroscopic surveys. It will also provide precision radii for new planets discovered by the next generation of ground-based transits surveys (Neptune-size and smaller). By unveiling transiting exoplanets with high potential for in-depth characterization, CHEOPS will also provide prime targets for the future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres (e.g. JWST, EChO, E-ELT).
Italian scientists based at INAF (OACT, OAPA, OAPD, OATO) and UNIPD are deeply involved in the science preparation and in the development of the payload. As for the payload, the Italian Team based at INAF (OAPD, OACT) is responsible for the CHEOPS Telescope, a very fast instrument whose design is driven by the requirement of very high precision and stable photometry, that can be reached by strongly suppress and control the stray-light. The planned activities include the optical design (mirrors and back-end optics), the analysis of the stray-light, the integration of optics with the mechanical structure (the latter provided by University of Bern), tests and validation. Other contribution is the provision to the project of star trackers for the upgrade of the pointing accuracy performance of the platform accordingly to the science requirements.
The Italian Space Agency (ASI) will provide fund for specific materials and tools required to the Italian participation to CHEOPS, but not for laboratory facilities that are to be provided by INAF.
While most of these facilities are already present in INAF laboratories, this is not the case for a scatterometer for the characterization of properties of optical and mechanical surfaces. The knowledge of the scatter properties (BSDF, TIS ...) of the materials to be used would allow us a better design and simulation of the instrument, and an a-posteriori modelling useful to remove instrumental errors from the measured data. Moreover, the availability of such a facility would allow us a more close collaboration with industrial partners both at the time of development, and at the time of acceptance. Being the delivery date of the Cheops telescope in Aug 2015, the facility should be procured at the beginning of 2014.

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.

PLATO - PLAnetary Transit and Stellar Oscillations (resp. G. Piotto)

The PLAnetary Transit and Stellar Oscillations (PLATO) satellite is a medium size mission presently under study for a selection for the M3 slot of the ESA Cosmic Vision program. The launch is foreseen for 2024. This is a mission devoted to the search of exoplanets through the transit technique, and to the asteroseismology characterization of their host stars. In the 6 years of operation from L2 point, the 34, 120mm PLATO telescopes will monitor about half of the sky to search for exoplanets, focussing on bright stars (V<13) in order to guarantee the ground based radial velocity follow-up for the planet candidate confirmation and characterization (radius from transit, mass from radial velocities, and therefore density). The Italian astronomical community is deeply involved into the mission. Italy is responsible of the telescope optical design and assembling, of the main onboard computer (ICU), and of providing PLATO input catalog. About 50 scientists at INAF, and Italian Universities are involved. A huge effort shall be devoted to the preparation of the input PLATO Catalog, which is under Italian responsibility. As PLATO cannot deliver the images (only a limited number of windowed CCD imagettes can be downloaded), photometry will be done onboard, on pre-selected targets in the ~2400 square degree field covered by PLATO images. Therefore, a careful selection of targets is needed. This implies the selection of stable (low stellar activity) dwarfs and subgiants later than F5 spectral classification. A thoroughly analysis of available catalogs (including future GAIA catalogs), and characterization of targets is needed for a successful mission. Target selection and characterization involve expertise on stellar parameter determination (age, rotation, binarity, metallicity) and stellar activity. This activity is at the basis of the mission success, but only very partially supported by ASI. We need to support researchers based mainly at INAF and the participating Universities to create the input catalog of PLATO, starting from 2016.