Entrevista com Malcolm Fridlund

Malcolm Fridlund é uma figura central nas missões da ESA relacionadas com a descoberta de exoplanetas. Desde 1996 é Project Scientist para a ambiciosa missão Darwin, cujo objectivo consiste em obter imagens directas e caracterizar exoplanetas em torno de estrelas próximas. Em 2006 assumiu funções de Project Scientist da missão CoRoT e faz parte da equipa que elaborou a proposta para a missão PLATO. O AstroPT contactou o Prof. Fridlund para nos dar uma breve visão sobre o progresso da missão CoRoT e um vislumbre do que será a missão PLATO, caso seja aprovada.

[AstroPT]
All the planets discovered by CoRoT so far have periods under 10 days despite the fact that the mission has collected data over a much longer time frame. Is there some hint in the data of planets with periods of, say, tens of days or more ?

[M. Fridlund]
The short answer is yes. The paper will be published in Nature, probably in a few weeks, so I can not talk about it except to say that this is a very long period planet and these take longer to follow-up and to prove their planetary status.

[AstroPT]
CoRoT is apparently discovering less planets than it was initially expected. Is this a real effect or just a result of complex data analysis and follow up procedures ?

[M. Fridlund]
This is not a real effect.

Extracting data for interpretation from the the raw CCD data gathered by CoRoT’s CCDs is an extremely complex task. It takes several months to reduce the observational data to the point where we can start to look for signals. The data needs to be cleaned from particle hits, spacecraft jitter, etc. It is a very difficult process that requires very sophisticated software.

On the other hand, it takes a very long time to follow-up the candidates we find. For each target field we have literally hundreds of candidates and each needs to be checked. Usually, it takes about 1 year before we can start the checking (because of CoRoT’s orbit, the candidates from a target field disappear behind the Sun a short time after we stop observing).

We have plenty of planets that we are working on and eventually we (or someone else) will publish large numbers of discoveries.

[AstroPT]
The stars observed by CoRoT are, in general, faint (m > 10) which makes radial velocity follow up more difficult and high precision spectrographs in large telescopes are scarce. Can you describe how the follow up program for CoRoT candidates is setup ? What is the typical time-scale involved, from the identification of a candidate until the publication of a paper ?

[M. Fridlund]
The timescale is more than a year.

First we have to process the raw data and identify candidates. These are classified according criteria such as: shape, quality and stellar properties, and then collected into a prioritized list.

For each candidate in the list, our first action is to determine if there is any possible contaminating background object within the photometric mask applied to the star. This is done with a high spatial resolution, ground based, photometric telescope. We take exposures during an eclipse (ON) according to the CoRoT ephemeris and out of eclipse (OFF). If there is a background object some arc-seconds away, it will undergo an eclipse of depth 25-50% or more which will be easily detected. These observations can be difficult in terms of time and logistics. Say we find a candidate with a period of 15.665 days. Then we can take the OFF observation almost anytime, but for the ON we need to have a telescope, at the right time, with an observer, at the right place and with a clear sky.

After the photometric follow up, some of the candidates are rejected. For the remaining candidates we need to do radial velocity observations and these can also take a long time. For example, for the CoRoT-7b planet we used 108 individual observations!

Finally, we have to interpret the data and write the paper – so in total the better part of a year.

[AstroPT]
What are the goals, in terms of exoplanetary science, for the extended CoRoT mission (up to 2013) ? In your opinion, what kind of breakthrough can we expect in the next 3 and a half years ?

[M. Fridlund]
There is a large number of exotic objects that we are working on at the moment. We want to find more rocky planets and study the properties of this class of objects. What I hope for is a CoRoT-7b type of planet in a longer orbit (at least 10-15 days).

[AstroPT]
You are part of the team that is proposing the new PLATO mission to ESA. Can you briefly compare CoRoT, Kepler and PLATO ?

[M. Fridlund]
Kepler and CoRoT are very similar missions. Both have asteroseismology and exoplanet components. The main difference is that Kepler is a 95cm telescope and CoRoT is a 27cm telescope and, naturally, the former has better signal to noise ratio. Kepler also has a larger field of view, but this is compensated somewhat by the fact that CoRoT has multiple target fields. Still, Kepler covers 100 square degrees, while CoRoT so far has covered about 30 square degrees. After 3 more years, the difference will not be that much.

Most of CoRoT’s stars are somewhat fainter than Kepler’s. The end result is that a) Kepler should find more small planets; b) CoRoT and Kepler should find approximately the same number of large planets; c) CoRoT does not have transiting planets around any star that we can measure the asteroseismological structure; d) Kepler will have a total of 2-4 stars with transiting planets where they can measure the asteroseismological signal (further asteroseismological data for 100 stars without planets will be obtained). This data is important because it allows us to know the stellar and planetary physical parameters to an accuracy of 1-2 % and the age to within approximately 250 million years. Without asteroseismology the errors in the physical parameters can exceed 50% and 1-6 Giga-years in the age!

The new proposal, PLATO, will observe at least 50% of the sky for at least 6-8 years, to measure 21000 stars with very good precision (enough to detect the asteroseismological signal) and a further 80000 with somewhat lower precision. We estimate we will find 200 or more planets around stars in the first sample and another 800 around stars in the second sample. This will vastly improve the number of exoplanets and host stars for which precise physical parameters are known, allowing – for the first time – true comparative planetology.

[AstroPT]
Algumas ideias a reter desta entrevista:

1 – o anúncio, dentro de poucas semanas, na revista Nature, de um planeta em trânsito com um período longo (dezenas de dias) descoberto pelo CoRoT;

2 – existem imensos candidatos a exoplanetas que estão actualmente a ser alvo de escrutínio por parte da equipa da missão CoRoT;

3 – o processo de descoberta e confirmação dos planetas envolve uma logística complexa e é demorado, mais de um ano entre a detecção do sinal e a confirmação e publicação;

4 – a extensão da missão até 2013 possibilitará a consolidação dos resultados actuais e permitirá, espera-se, a detecção de Super-Terras do tipo do CoRoT-7b mas com períodos orbitais maiores;

5 – a importância da combinação da asterosismologia com o estudo dos exoplanetas. Para estrelas suficientemente brilhantes e com planetas em trânsito, é possível obter uma precisão fotométrica suficiente para realizar uma análise asterosismológica da estrela e determinar com uma precisão elevada os seus parametros físicos (massa, raio, idade). Por arrastamento, os parâmetros dos planetas que orbitam tais estrelas serão caracterizados com uma precisão semelhante;

6 – a missão PLATO, proposta à ESA e a ser alvo de avaliação para possível financiamento, que permitirá, seguindo a abordagem referida no ponto anterior, caracterizar aproximadamente 200 exoplanetas com uma precisão tal que possibilitará a utilização de técnicas da planetologia comparativa.