Thierry Appourchaux: “We always use outdated technology to go to space”

Thierry  Appourchaux at the XXII Canary Islands Winter School of Astrophysics. Photo: Miguel Briganti, Servicio MultiMedia (IAC)
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Thierry Appourchaux is a mathematician by training and an expert in statistics. His experience in instrumentation and data analysis in Helio- and Asteroseismology is outstanding: he heads the group dedicated to data analysis for the CoRoT (COnvection ROtation and planetary Transits) mission and the group studying the characterization of the acoustic modes of solar-type stars for the Kepler mission. He also leads the Asteroseismology analysis team within the PLATO (PLAnetary Transits and Oscillations of stars) consortium.
A specialist in the characterization of the acoustic modes of the Sun, he worked on the VIRGO experiment on board SoHO (Solar and Heliospheric Observatory). This scientist, based at the Institutd´Astrophysique Spatiale (IAS) in France, gave a course at a previous Winter School dedicated to 'Payload and Mission Definition in Space Sciences' in 2003.

The European Space Agency (ESA) is planning its "Cosmic Vision" programme for the ten years between 2015 and 2025. As things stand, the programme will include the PLATO (PLAnetary Transits and Oscillations of stars) mission, with which you are very familiar. What would it add to the programme? Do you think it is likely to happen?

PLATO is an essential part of the Cosmic Vision (CV) programme. This mission will contribute to one of the major goals of CV which is "to search for planets around stars other than the Sun". PLATO will search for such planets by detecting the faint decrease of the stellar light when they move in from of their star. Such technique requires being able to detect change in star light of the order of a few part-per-million. This requirement allows not only to detect transiting exoplanets but also to perform Asteroseismology. The combination of these two scientific objectives is ideal to characterize exosystems. The recent report of the Exoplanet Roadmap Advisory Team identified PLATO as a milestone in our quest of habitable planets in exosystems. (The report is available at As such PLATO is a key ingredient of CV and the roadmap set by the Advisory Team, such an ingredient is very likely to be included in the first round of the CV programme.

How might PLATO's discoveries compare with those made by CoRoT (COnvection ROtation and planetary Transits) and expected from Kepler?

The number of stars observed by PLATO will be 10 to 20 times larger than that of Kepler. CoRoT was really a discovery mission showing that the technique works for detecting exoplanets and for doing Asteroseismology. Kepler allowed the community to have a broader view of the Hertzprung-Russell diagram. Therefore it is anticipated that the number of detectable exoplanets will also increase by a factor 10 to 20 on average. PLATO will go beyond not only by the number of stars but also by allowing the observations of open clusters. The observations of such objects are keys to a better understanding of the evolution of stars because cluster stars have the same age and the same initial composition; this aspect imposes severe constraints of the model of the stellar cluster.

There are technological problems involved in trying to analyse large amounts of data. Is there any point launching new missions if we risk being overwhelmed by their output? How much information are we able to deal with from observations?

Up to now, we never encountered technological problems related to the analysis of large amount of data. The reason is quite simple: when one wants to make a space instrument, one needs to have technology that is roughly 5-10 years older than what is available at the time of launch. Therefore, we always use outdated technology to go to space. On the other hand, computer technology foreseen at the time of the mission concept, that is 5 to 10 years before launch, evolves so quickly (thanks to the Moore's laws) that when the mission starts even a larger amount of data than anticipated can be reduced and analysed.

The SoHO solar satellite has been in orbit for 14 years. Would another fourteen years of data add anything to our understanding of the sun? What haven't we found out yet?

Regarding Helioseismology, there is the point of view shared by several colleagues of the School that we have not yet detected the gravity modes on the Sun. The gravity modes (or g modes) are superior probes of the inside of the Sun compared to the pressure modes. So far the search for detecting individual modes have not been successful. With the advent of Kepler, there is the secret goal to do an ensemble search of these g modes on a large amount of stars.

You have a lot of experience of working on solar instruments, which are currently much better developed for space observation than stellar instruments. What do you think it will take for them to catch up?  

Gosh a lot to catch up! First of all, it would be key in the future to be able to measure stellar radial velocities from space in order to avoid the regular gaps due to the Earth's rotation, and allowing also year-long observations which are difficult from the Earth due to the rotation of the Earth around the Sun. The difficulties lie in building a space qualified spectrometer. But the reward will be enormous given the higher signal-to-noise ratio in velocity compared to that of intensity, i.e. the characterisation of the pressure modes is greatly facilitated.
Second it is not unlikely that we will see higher degree modes in the future. There are projects such as the Stellar Imager which aims at making images of our closest stars, thereby making possible the detection of higher degree modes than what is possible with CoRoT or Kepler. We could think of a combination of a stellar spectrometer and with an imaging photometer that could bring a lot of useful information on the whole of the stars, very much like SoHO provided key information on the internal and dynamics of our closest star. This is my current dream that hopefully a future generation of asteroseismologists will be ready to carry out.

XXII Canary Islands Winter School of Astrophysics

Annia Domènech

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