2.9. ANÁLISIS E INTERPRETACIÓN DE RESULTADOS
2.9.1. Análisis e interpretación de la entrevista aplicada al docente
The Next Generation Transit Survey (NGTS) is a new project designed to discover Neptune-sized planets around bright (V<13) and nearby stars. NGTS will deploy an array of fully-robotic small telescopes on independent mounts operating in approximately the 600-900nm band, thereby maximizing sensitivity to bright but relatively small host stars (K and early-M spectral type). This project builds from hardware and software heritage from the world-leading SuperWASP project
[Pollacco et al., 2006], and has recently secured full funding and authorisation to be commissioned early 2013 at ESO’s Paranal Observatory in order to benefit from one of the world’s premier sites.
NGTS has the potential to find Earth-like planets around smaller stars and, because the covered area is larger than that of Kepler [Koch, 2010] (>1500deg2), it will find transiting planets around bright stars, which are those where atmospheric follow-up studies are possible with facilities such as the Hubble Space Telescope, the Very Large Telescope (Chile) and all the planned large telescope facilities (ESO’s E- ELT, ESA’s ECHO mission and NASA’s concept FINESSE). It will achieve this by monitoring the brightness of all the visible targets within its field-of-view in search of periodic dips, which are the tell-tale signs of the existence of transiting planets. Contrary to the SuperWASP project (see section 1.4.1), which is an almost all- sky survey that samples several pointings every night, NGTS will employ astaring
strategy in which it will observe one field for as long as possible (typically around 4 months) before changing. This has the added advantage of increased precision in the time-scales of single transits (around 1-3 hours) but also, crucially, brings the potential to discover planets on longer orbits. The vast majority of transiting planets found to date orbit their host stars in typically less than 10 days (based on exoplanet.eu) mostly due to an observational bias, and only recent surveys, such as the Kepler mission, have observed a single location for a long enough time-scale to be sensitive to longer period planets [e.g. Borucki, 2012].
As with any transiting planet survey, careful follow-up studies are required to confirm the planetary candidates. As discussed by Evans & Sackett [2010], there are many astronomical events that can mimic the transit signature, such as grazing eclipsing binaries, blended objects with deep eclipses, low-mass eclipsing binaries or simply systematic e↵ects, and further observations are necessary to disentangle these scenarios and filter the real planets. In those cases where either there is doubt that a transit was observed or that the signature could be caused by a background source contaminating the flux inside the typically large apertures used on low resolution wide-field imaging surveys, photometric follow-up on 1-2m class telescopes is often sufficient. The location of the NGTS facility is advantageous in this context, since there is a whole host of nearby telescopes such as the TRAPPIST telescope at the La Silla Observatory and the Henrietta Swope telescopes at the Las Campanas Observatory for these follow-up procedures. Spectroscopic follow-up helps identify binary systems rapidly, and is invaluable for this kind of research. Moreover, if a planet is confirmed, radial velocity measurements are necessary to measure the mass and instruments such as the HARPS spectrograph [e.g. Dumusque et al., 2011;
S´egransan et al., 2011; Pepe et al., 2004] currently mounted on ESO’s 3.6m telescope in La Silla have proven to be extremely successful in these enterprises.
The final NGTS facility will be composed of 12 fully-robotic wide-field 20cm telescopes, equipped with deep-depleted CCDs developed to the project’s specifica- tions by industrial partners (Andor Technology plc, Belfast and e2v, Chelmsford, UK). The CCD model, now available at Andor’s general range, is capable of achiev- ing high precision photometric measurements within the desired wavelength range of the instrument (600-900nm). The facility is housed in a custom designed building with a rolling roof, that will comprise of an inner chamber containing the telescopes and two side rooms for the computer servers required to control the facility and communicate with the UK. Each telescope is assembled on an individual mount, in order to achieve precise guiding and maximise the flexibility of the experiment in terms of observing strategy.
Science goals
The main objective of the survey is to search for transiting planets of Neptune- size and below around bright stars. The many recent discoveries of planetary systems harbouring Neptune-mass planets and super-Earths clearly indicate that low-mass planets around solar-type stars must be very common [Traub, 2012; Wittenmyer et al., 2011; Mordasini et al., 2009a,b]. However, very little information is available regarding the structure and composition of these planets. Moreover, planetary sys- tems such as the Kepler-11 case show that, as is the case for gas giants, there is likely to be a large diversity of earth-sized planets [Lissauer et al., 2011]. Figure 1.14 shows the optimal sensitivity range of the facility. It shows that this project is designed to explore a region of parameter space that is relatively unpopulated.
As described in Section 1.3, the geometry of transiting exoplanets places tight constrains on the orbital inclination of these systems, thereby providing the oppor- tunity to accurately measure their radii and masses. Moreover, multi-wavelength observations of the transits provide the chance to probe the atmospheres of exoplan- ets (see Section 1.5.4), and secondary eclipse observations can give an indication of the brightness temperatures and overall atmospheric circulation. NGTS aims at providing a set of bright targets for follow-up studies with facilities such as the Spitzer and HST, and also from the ground with large telescopes like the VLT using FORS or HAWK-I.
Figure 1.14: Parameter space for transit detection with the shaded region indicating the optimal parameter space of the NGTS facility in terms of the smallest planets that can be found for each stellar type. This Figure shows the transit depth as a function of planet and star radius. Known transiting systems are shown in cases where they were discovered in ground-based surveys (green), from space (red) and those in blue represent planets detected through radial velocity measurements and later found to transit. Approximate spectral types of stars are also indicated, as well as the radii of representative Solar System planets.
The Prototype
The ability to achieve 0.1% precision across the wide-field of view is very de- manding and was demonstrated using a prototype system operated on the La Palma Observatory (Spain) during the 2009-2010 season. A smaller version of the e2v CCD (1k⇥1k) was deployed on an 8 inch Takahashi telescope and tests were performed to determine if the desired sub-mmag precision was possible to be achieved. This puts this facility in a superb position to explore a region of parameter space that is currently relatively unpopulated, as shown in Figure 1.14. This Figure also contains information on the typical stellar types associated with each radius as well as radii of representative Solar System planets for reference. The analysis of the data taken by the prototype instrument have shown that 1mmag precision is indeed achieved, as presented in Figure 1.15. This plot displays the fractional RMS for stars in a field observed for one night with the prototype instrument as well as the expected noise based on the model to be described in Section 2.3. Transits of the Hot-Neptune GJ436b were easily recovered and a precision of 0.5 mmag is reached in one hour time scales for a star of magnitude I=10.5, which corresponds to V=12, fulfilling the science requirement.
Other tests were performed on the prototype instrument, such as the guiding capability of the telescope and flat-fielding, and results have shaped the design of the project, with the majority of telescope components already purchased. A testing system will be assembled in the Summer of 2012 by a team at the Geneva Obser- vatory for software and operations testing before the complete system is integrated on the mountain in early 2013. Other elements crucial to the project, such as data storage and mining, analysis software and knowledge of infrastructure are largely based on those developed for the SuperWASP project, making this facility science ready as soon as it is commissioned.