The COROT mission: From Structure of Stars to ... - Semantic Scholar

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a1, Pierre Barge c, Claude Catala d Magali Deleuil c, Rafael Garrido e, Alain. Leger f, Eric Michel a, .... iM can be several tens of degrees (28o for Saturn). For anĀ ...
The COROT mission: From Structure of Stars to Origin of Planetary Systems. Jean Schneider a , Thierry Appourchoux b, Michel Auvergne a , Annie Baglin a1 , Pierre Barge c, Claude Catala d Magali Deleuil c, Rafael Garrido e, Alain Leger f , Eric Michel a, Daniel Rouan a , Andre Vuillemin c, Werner Weiss g . (a) Observatoire de Paris, France (b) ESA/ESTEC, Nordwijk, The Netherlands (c) Laboratoire d'Astrophysique Spatiale, Marseille, France (d) Observatoire Midi-Pyrennees, Toulouse, France (e) Instituto de Astro sica, Calar-Alto, Spain (f ) Institut d'Astrophysique Spatiale, Orsay, France (g) Vienna University, Austria

Abstract

COROT is a space mission approved by the French Space Agency CNES. Its goal is to make a signi cant step forward in the understandig of the origins and destiny of stars and of planetary systems. 1

Principal Investigator. [email protected]

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1 Scienti c Objectives COROT will make an intensive monitoring of the brightness of appropriate samples of stars with high photometric precision. From that monitoring it will be able to sound the internal constituion of stars, giving a deeper insight on their evolution process and nally on their age. It will also detect a fair number of telluric planets with a few Earth radii by their transits in front of the parent star. 1. Asterosismology. The study of stars and stellar atmospheres constitutes one of the most successful activities in astrophysics in the early 1950's. However, many unknowns still prevent us from having an accurate description of stellar evolution, which is one of the basic eld in astronomy, providing fundamental informations concerning several topics: for instance, the age of the Universe, its chemical evolution, as well as the conditions required to form planetary systems. The main reason was because we had access only to quantities related to the surface of stars: temperature, radiated energy, gravity, chemical composition, etc. Yet, one had to wait until two decades ago before knowing a lot more inside a star. Helioseismology, complemented by measurements of the solar neutrino

ux (still controversial), has recently provided an outstanding tool to probe the interior of this ordinary G2 star. As a matter of fact, our Sun is oscillating in an intricate manner, e.g. observations carried out in the solar atmosphere show a supersposition of periodic oscillations in a quite narrow range of periods close to 5 minutes, corresponding to many (more than 10**7) individual modes. The theoretical interpretation in terms of eigenmodes of the solar model, considered as a self-gravitating object, has been very successful and has improved considerably our understanding of the solar interior, o ering an elegant way to examine the physical processes at work in the solar interior, thermodynamics (equation of state, opacities......) as well as hydrodynamics (turbulent convection, rotation rate and magnetic elds.....). Many e orts are being carried out to improve the observational data (e.g. G.O.N.G , S.O.H.O.) for the Sun. 2

But it is obvious that the Sun, by itself, cannot validate all the ingredients of the stellar structure theory. Analogous studies would be very important to achieve in various types of stars with di erent masses, in di erent stages of evolution, of rotational status.... The theory of stellar oscillations predicts a similar behaviour for other stars. The interior structure of stars of di erent types should then be probed by analyzing their oscillations. The extension of helioseismology to other stars than the Sun, the socalled asteroseismology is then the next century's most exciting challenge to get a signi cant understanding of stellar evolution. Variable stars are known for a long time, and observed from the ground, using various techniques, based either on photometry or on spectroscopy. Unfortunately, due to the earth atmosphere, and also to the faintness of stars, the variability observed from the ground consists in very few modes of large amplitudes, in a few types of stars. Even though this information has been important in the eld of astrophysics, it does not say much about stellar interiors. If one really wants to generalize the work done on the Sun, one should try to detect a large number of modes, with very small amplitudes . Thee amplitudes (a few ppm in light variations, and a few tenth cm/s in Doppler shifts) are very dicult to detect. Spectroscopy in principle is not sensitive to the seeing conditions. But, up to now, it seems that it cannot be used for stars with large rotation rates, due to the enlargement of the spectral lines. In addition, this technique requires very large telescopes. In ground-based photometry, scintillation prevents from reaching very low variations. Furthermore, asteroseismology experiment has to observe continuously the same star for a long period, which is impossible to achieve for periods longer than 2 months, even with networks of telescopes, regularly spaced around the world (WET, STEPHI, DSN.....). 2. Planetary transits. If the orbital plane of the planet is correctly oriented, it produces a drop in the star light during transits of the star disk by the planet. The detection of a transit in the star lightcurve requires three conditions: 3

1. The orbital plane of the planet must be correctly oriented: for random orientations, the geometric p probability is

p = R=aP

(1) For a Jupiter (resp. an Earth) around a 1 R star, this probability is 10?3 (resp. 0.5%). Since, in addition, the star must be photometrically monitored continuously over at least one entire orbital revolution of the planet, this makes the transit method very inecient for large aP and favors small aP since then p is larger and the required time base is shorter. 2. The duration of the transit is

M DT = PP Ra  = 13 M P

!?1=2 

a 1=2 R 1AU 1R

!

h

(2)

i.e. 25 h for a Jupiter and 13 h for an Earth. This duration is not very sensitive to aP . 3. The depth of the drop in the stellar ux F during the transit is

F =  RP 2

F

R

(3)

The photometric precision of the lightcurve must be better than F =F. For a 1 RJup (resp. 1 R) planet the drop is 1% (resp. 10?4). In ground-based observations, the photometric precision is at best 0.1%. In space, it is limited only by the photon noise and the stellar activity noise. The photon noise can be reduced to 10?5 in a 1 minute exposure for a 1 m telescope and a magnitude 5 star. From our knowledge of the Sun, we can infer that the stellar activity noise is of the order of 10?4 in 1 h. Thus, while Jupiter-size planets can be detected in this way from the ground, one can detect Earth-sized planets from space. In fact, it is presently the only method capable of detecting and investigating further Earth-like extrasolar planets. In addition to planets, the transit method has the potential to detect their satellites and rings [1]: 4

1. Moons of extrasolar planets. A transit by a moon light curve would be added to the transit by the giant planet light curve. In addition, since the moon moves back and forth (when projected onto the sky) in the planet frame, it can make multiple transits with temporal features similar to the case of transits of binary stars by planets [11], [10]. The condition for such an event to occur is PM  planet transit time. Even when the transit of the giant planet's moon is not detected for any reason (lack of photometric precision, stellar activity noise, unvafourable orbital inclination, ...) it is possible to detect it indirectly by the timing of the transits of the parent giant planet [1]. The gravitational pull exerted by the moon on the giant planet makes a modulation of its transverse position; it results a periodic modulation q with an 2 amplitude T = aM (MM =MP )=VP = aM (MM :M =MP ) a=GM and with a period equal to the period of revolution of the moon (aM and MM are the distance to the planet of the satellite and its mass; MP is the planet mass). For an Earth-mass moon at 0.1 AU of a Jupitermass giant planet, the amplitude DT is 20 min. The period would give the planet-moon distance; this period would nevertheless be dif cult to determine with precision, since the timing can be made only when the parent planet transits the star, which would give only a few points along the timing curve. From the point of view of the strategy of observations, it is easier to detect the transits of an Earth-like moon than of a single Earth-like planet. Indeed, the detection of the giant parent planet, making a brightness drop of 1%, can be detected from the ground in large wide eld photometric surveys [5], [6]. Then, once a giant planet is detected (or suspected) by a single transit, its approximate orbital period can be determined by astrometry, imaging or radial velocity. From there, a second transit can be predicted and searched. The careful measurement of its ingress and egress light curves from the ground can give the orbital period and the transit duration with a precision of a few tens of minutes. Then high precision photometry can be performed from space in a  2 days run with an existing space observatory (HST for instance), without requiring a dedicated long term (typically more than 3 years) monitoring mission; since the transits are, in a very good approximation, completely achromatic, a run dedicated 5

to the search of the moon can be performed at any wavelength. In another con guration, it is possible to detect the transit of the moon and not the transit of the parent planet. This happens when the moon's orbit makes an inclination iM larger than a limiting inclination iL = RP =aM . For a moon at 0.01 AU from the planet, iL = 3o (I assume that all giant planets have a maximum radius of 1RJ ). For giant planets, iM can be several tens of degrees (28o for Saturn). For an inlination of 30o, the geometric probability of occultation is (R + siniaM )=aP . For aM = 0:05 AU, aP = 1 AU and i = 30o, the probability (integrated over an in nite number of planet orbits) is 3  10? 2 , i.e. 6 times larger than for a single Earth-like planet. But, in this con guration, the moon's transit can be missed if it does not lie at the correct position on its orbit at minor conjunction of the star-planet system. On the other hand, a moon in a Io-orbit (aM = 0:002 AU) with an inclination of 45o around a planet with 10 MJ would have a period of revolution around the planet of 12h. This is equal the duration of the transit. Thus the moon's transit would in this case be seen with a probability 100% if the inlination iP of the planet orbit is within the limit iL. 2. Rings of extrasolar planets. Finally, the method of transits is very ecient for the detection of rings of planets [1]. Indeed, the depth of the luminosity drop F =F = cos iR RR2 =R2 for a ring with radius RR and inclination iR is larger than for planets (1.8 10?2 for Saturn ring + planet), so that the event can be detected even from the ground. In contrast to any other method, it is rather easy to make here the distinction between a giant planet and a ring since the shape of the light curves made by a ring transit and a planet transit are di erent. Indeed a planet, being seen as circular, has a transit light curve L(t), during ingress or egress, LP (t) = R2 ? RP2 arccos(d(t)=RP ), where d(t) = VP :t, VP being the orbital velocity of the planet, while for a ring having an inclination iR , its transit lightcurve is LR (t) = R2 ? RR2 sin iR arccos(d(t)=RR) [7].

2 Payload and Mission Strategy. The main characteristics of the COROT payload are: 6

1. FOV: 4.5 deg2 2. Tel. aperture: 500 cm2 It leads to a noise to signal ratio of 3 10?4 for a mV = 15.5 star in 1 h. 3. Each eld is monitored during 150 days Thus we can see 3 transits only for orbital periods  50 days, i.e. for planets at an orbital distance  0.3 AU. This gives a probability of transit  1.5 % per star. 4. Mission duration: 2.5 years We will therefore monitor 5 elds. 5. Detection threshold: NT  F =F  6 N/S Here NT is the number of transits by a given planet during the mission. This relation means that one can detect either large planets with a few (up to only one) transit or small planets with many transits (i.e. in close orbit). In the COROT context, 6 N/S = 10?3 for mV = 15. Thus COROT an detect planets with  2 R (\ super-Earths ") at a few tens of an AU from the star. From the constraints (1), (2), (3), (4) and (5) above, COROT is capable of monitoring 25600 stars with mV  15 monitored during 150 days each. If 3% of the stars have planets, COROT will thus detect:  by transits: { 15 planets (from super-Earths to Jupiters with their potential moons and rings) at 0.3 AU { 45 planets (from super-Earths to Jupiters with their potential moons and rings) at 0.1 AU { 2 - 3 Jupiters with their potential habitable (super-)moons at 1 AU  by re ection: hundreds of Jupiters in close orbit. It is clear that to detect Earthes in the Habitable Zone of a solar type star a Kepler-like mission is needed. 7

3 Conclusion. COROT will make a signi cant step forward in the understanding of stellar structure and in the discovery of telluric planets and of surroundings of giant planets. For the latter case (rings and satellites), an extensive preliminary search for transits of Jupiters from the ground, to prepare their followup from space, is highly desirable. In addition, a lot of secondary scienti c returns, such as for instance stellar variability and quasar microlensing, make a high precision photometric space mission full of promises.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Schneider J., The surrounding of giant extrasolar planets (submitted) J. Schneider, 1995 Strategies for the Search for Life in the Universe. in D. Williams, J. Kasting and R. Wade, Nature, 385, 234, (1997) It is plausible that such a con guration is not exceptional [1] S. Howell, B. Koehn & E. Bowell, BAAS, 187, #70.20 (1995) C. Woodward et al. Poster at NASA Origins Conference, May 19 - 23 M. Schull et al. Eds. These expressions do not take into account the limb darkening of the star and are only valid when the node of the ring plane is =2. A complete discussion is given in [1]. J. Schneider Astrophys. & Space Sci., 212, 321 (1994) W. Borucki & A. Summers, Icarus, 58, 121 (1984) J. Schneider Planetary. & Space Sci., 42, 539 (1994) Schneider J. & Chevreton M., 1990. The photometric search for earthsized extrasolar planets by occultation in binary systems. Astron. & Astrophys., 232, 251 8

[12] Schneider J., 1996, Strategies for the Search of Life in the Universe in Chemical evolution: physics of the evolution of life, Chela-Flores & Raulin Eds. Kluwer p. 73

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