Congresso del Dipartimento di Fisica Highlights in Physics 2005

1. Congresso del Dipartimento di Fisica Highlights in Physics 2005 11–14 October 2005, Dipartimento di Fisica, Università di Milano Asteroseismology and search for extrasolar planets with COROT M. Rainer*, E. Poretti†, L. Mantegazza†, E. Antonello†, M. Bossi†, and L.E. Pasinetti* * Dipartimento di Fisica, Università di Milano † INAF – Osservatorio Astronomico di Brera 0E1 CCD A1 CCD E1 2.70° 0A1 0E2 CCD A2 CCD E2 0A2 3.05° The photometric satellite COROT (COnvection, ROtation and planetary Transits) [1] will be launched in summer 2006: it is the result of a French-led international cooperation with support of the European Space Agency. COROT will carry on two main scientific programs: asteroseismological measurements and detection of extrasolar planets with the transit method. Its scientific outcome will be very high: for the first time it will be possible both to test the diagnostic power of asteroseismology on the internal structure of solar-like stars (particularly on the convective zone and on the internal differential rotation) and to detect the presence of telluric planets around main-sequence stars. COROT will alternate long observational runs of about 150 consecutive days with short ones (10-20 days each), in order to maximize the scientific return of the mission. It will observe near the galactic equator to satisfy the requirements of the extrasolar planets search, which needs to observe in densely populated regions. The long runs are best suited for the exoplanet search and for the detailed asteroseismological measurements of ~60 stars, which have been carefully selected in the framework of the ground-based preparatory work. The whole main-sequence will be investigated, from solar-like to massive B stars. The short runs instead will be focused on specific scientific topics. Moreover, a large number of additional scientific programs has been proposed in order to complete the observational program of the satellite and exploit its capabilities (high-precision photometry, long time-baseline,...). The main proposals focus on the study of binary systems, pre-main sequence stars, activity and rotation of stars, classical pulsators as RR Lyrae and Cepheids. Main Characteristics Weight of the satellite. . . . . . . . . . . . . .600 kg Sixe of the satellite . . . . . . . . . . . . . 4.2x9.6 m Diameter of the telescope . . . . . . . . . . . 27 cm CCDs . . . . . . . . . . . . . . 2 asteroseismological . . . . . . . . . . . . . . 2 for exoplanet search CCDs size . . . . . . . . . . . . . .2048x2048 pixels Field of view . . . . . . . . . . . . . . . . .3.05° x 2.7° Pointing precision . . . . . . . . . . . . . .0.2 arcsec Height of the orbit . . . . . . . . . . . . . . . .896 km Inclination of the orbit . . . . . . . . . . . . . . .polar Telemetry volume . . . . . . . . . . . 1.5 Gbits/day Lifetime of the mission . . . . . . . . . . . ~3 years http://corot.oamp.fr/ SEARCH FOR EXTRASOLAR PLANETS The first extrasolar planet around a main-sequence star was found in 1995 around 51 Pegasi, a G5V star, analyzing the radial velocity variations of the parent star due to its motion around the barycenter of the system [3]. The radial velocity method allow to detect only Jupiter-like planets, in particular the so-called hot Jupiters (like 51 Pegasi b), which are very close to the parent star. Until now, no telluric planet has been found around main-sequence stars: we have no statistical information about their formation rate. COROT will be able to observe for the first time this kind of objects: even if it will find difficult to detect earth-like planets in the habitable zone, COROT will help to confirm the presence of telluric planets. The planet detection will be performed using the transit method, i.e. studying the variation of the stellar flux due to the transit of the planet in front of the ASTEROSEISMOLOGY The asteroseismology is the study and analysis of stellar pulsations, arising from acoustic waves trapped inside the star [2]. The oscillation eigenmodes of an autogravitating gaseous sphere possibly rotating can be described with a spheric harmonic plus a radial function: Each pulsation mode causes the regular expansion and contraction of different stellar regions. Ψ(r,θ,ϕ,t) = Kn,l,m(r) Ylm(θ,ϕ)e-iωt r = radial number or number of the nodes in the stellar interior; l = degree or number of nodal circonferences on the surface; m = azimuthal number or number of the longitudinal nodal circonferences. The pulsation modes observed until now from ground-based observations. The radial mode, which is the one with the biggest amplitude, is described by (n,l,m) = (0,0,0), i.e. is a regular expansion and contraction of the whole star. In the p modes the dominating force is the pressure, while in the g modes is the gravity. The main-sequence stars are poorly observed due to the low amplitude of their pulsations: COROT will make up for this lack of information. The simultaneous observations of different pulsation modes, which travel in different stellar regions, allow to define the internal structure of the star, in a similar way as the seismic waves are used to study the structure of the interior of the Earth (hence the name Asteroseismology). COROT will observe the luminosity variations of the stars due to the expansion and contraction of different stellar region. stellar disk. The probability to observe a transit is strictly geometric and decreases with the increasing size of the planet orbit: this method requires a good statistics and high-precision photometry in order to detect a large number of planets. Extrasolar planets found (up to August 2004): the blue dots represent planets found with the radial velocity method, the red ones with ground-based transit surveys and the yellow with micro-lensing. The lines show the detection threshold for several methods and missions, including COROT. Low degree modes cross the entire star, while high degree modes are trapped in the surface layers The transit of Mercury on the Sun in the 2003 as seen by the SOHO satellite: COROT will use this kind of phenomenon to detect the presence of extrasolar planets. THE GAUDI DATABASE STELLAR ACTIVITY: A NEW ACTIVITY INDEX Waiting for the COROT data, many interesting scientific researches can be performed with the help of the GAUDI archive, in which a great amount of good quality, homogeneous data is stored. We focused our work on the spectroscopical data, especially on the FEROS spectra, and started to search for the presence of stellar activity and strong magnetic fields. At first glance, our excellent data would seem quite suitable for this purpose: each FEROS spectrogram covers almost completely the range from ~3800 Å to ~9200 Å with a resolution of 48,000 and signal-to-noise ratio greater than 100. Nevertheless, we met some significant problems in deriving the classical activity indices, based on the fillings in H and CaII line cores. In fact, accurate examinations of the Hα and Hβ profiles are seriously hindered by the location of these features across the border between different Echelle orders. The remaining possibilities are the ultraviolet doublet and the infrared triplet of the ionized Calcium: unfortunately, the former is located across the border between two orders and the latter is lacking of the most sensitive component at ~8542 Å, which falls in a gap between two different spectral orders. So we decided to calibrate some original chromospheric indices fitted for FEROS spectrograms, based mainly on the activity index proposed by Foing [8] using the ionized Calcium infrared triplet. The observing zones of COROT were chosen in order to satisfy the requirements of the scientific programs: not too distant high-density populated regions with a majority of dwarf stars. Unfortunately, the stars in these two regions were not very well known, so it was decided to launch a vast program of ground-based observations, both spectroscopical and photometrical, in order to choose and better define the asteroseismological targets (10 stars for each run in the two seismological CCDs: 1 or 2 primary targets with mv ~ 6.5 and other interesting seismological objects as secondary targets, with mv < 9.5). It was not possible to carry on similar observations for the stars involved in the planet search, simply because of their huge number: the two exoplanet CCDs will observe up to 12000 stars each run (~ 60000 stars during the whole mission). At first, the ground-based surveys covered all the stars in the two COROT's eyes with mv < 8, but it did not Left: the 2 seismological CCDs. Right: the 2 exoplanet CCDs, with colour information obtained via a small dispersion prism . prove sufficient for the choice of all the secondary targets, so it has been recently decided to extend the survey up to mv~9.5 in the regions near the main targets. Our team has been responsible for almost all the spectroscopical observations performed with the high-resolution Echelle spectrograph FEROS (ESO-LaSilla Observatory, Chile) [4], and for the reduction of all the FEROS observations (more than 600 spectra) [5]. We are now cooperating with the Catania Observatory in order to complete the new surveys at the Serra La Nave Observatory (Mt. Etna) [6]. All the reduced high-resolution spectra along with the photometric parameters and other data (Teff, vrad, vrot,...) are currently stored in the GAUDI (Ground-based Asteroseismology Uniform Database Interface) archive [7] and are available to the scientific community (http://sdc.laeff.esa.es/gaudi/). COROT's eyes: in the figure are shown the two observing zones of the satellite. Every six months COROT will rotate in order to avoid the solar light and will change its zone of observation. In his work, Foing proposed to use the area F beneath the bottom of the infrared 8542 Å CaII line to show the filling of the line due the presence of stellar activity. He used a chord of 0.5 Å to define the bottom of the line. We worked on the 8498 Å line and we tried to come up with an index as independent from the intensity of the line as possible: DIFFERENTIAL ROTATION The differential rotation is an important feature in all the stars with a convective envelope: it arises from the interaction between convection and rotation and it is linked to the presence of stellar activity. The main observational methods (the study of the variations of photometric periods, the identification of individual features on Doppler maps and the line profile analysis) are very time-consuming or rather cumbersome. Recently, however, a new method [11] has been proposed that, even if it is less precise than the classical ones, allow a quick and easy check on the presence of differential rotation. This is particularly useful when working on a great number of data, as in our situation. We selected from the FEROS spectra of the GAUDI archive 104 A-type stars and 9 F-type stars with symmetrical line profiles and values of vsini high enough to consider the rotational broadening as dominating. We estimated the mean line profiles using the LSD technique [12] and, for each profile, checked its symmetry by mirroring it at the center and averaging the two mirror images obtained: we considered as symmetrical only the stars for which the result fell inside the error bar of the original profile. Then we Fourier transformed the symmetrized profile and studied the position of the first and second zero of the transform, when they can be found above the noise level. In fact, if the vsini is high enough, the first two zeroes of the Fourier transform will depend only on the rotational broadening and their positions will not be affected by other kinds of broadening. In the case of rigid rotator, the ratio between these zero positions can be expressed by the empirical law [13]: log a I8498Å = log b We used the logarithmic area of the bottom of the line (region a in the figure) on the logarithmic area between the bottom of the line and the continuum (region b in the figure). We used a 0.7 Å chord, which makes our results more stable. We also extimated the average strength of the photospheric magnetic fields adjusting the Stenflo-Lindegren statistical method [9] to our material. In spite of the large errors afflicting this method, this work confirmed the reliability of our new activity index: we found solid estimates of strong magnetic fields only in the more active stars of our sample [10], i.e. the stars with lower values of the index. REFERENCES [1] Baglin A., Auvergne M., Barge P., et al. the COROT Team, 2002. Proceedings of the first Eddington Workshop on Stellar Structure and Habitable Planet Finding. Ed. B. Battrick. Sc. eds. F. Favata, I.W. Roxburgh & D. Galadi. ESA SP-485, 17 [2] Brown T. M., Gilliland R. L., 1994, ARA&A 32, 37 [3] Mayor M., Queloz D., 1995, Nature 378, 355 [4] Kaufer A., Stahl O., Tubbesing S., et al., 1999, ESO Messenger, 95, 8 [5] Rainer M., 2003, Analisi spettrofotometriche di stelle da usare come targets per la missione spaziale COROT, Laurea thesis in Physics, Faculty of Science of the Università degli Studi di Milano, Milan, Italy. [6] Cutispoto G., Distefano E., Rainer M., “Contribution to the COROT ground-based preparatory work from Serra La Nave (Catania) Observatory”, 8th COROT Week, 23-27 May 2005, Tolouse [7] Solano, E., Catala, C., Garrido, R., et al. 2005, AJ, 129, 547 [8] Foing B. H., et al., 1989, A&A Suppl. Ser. 80, 189 [9] Stenflo J. O., Lindegren L., 1977, A&A 59, 367 [10] Rainer M., Bossi M., Mantegazza L., Pasinetti L. E., “Active G-K stars in the GAUDI sample”, 7th COROT Week, 14-17 Dec. 2004, Granada [11] Reiners A., Schmidtt J. H. M. M. 2002, A&A, 384, 155 [12] Donati, J.-F., Semel, M., Carter, B. D., et al. 1997, MNRAS, 291, 658 [13] Dravins, D., Lindegren, L., Torkelsson, U. 1990, A&A, 237, 137 [14] Rainer M., Mantegazza L., “Search for differential rotation in the A- and F-type stars of the GAUDI database”, PhD Conference on Astrophysics of Variable Stars, 5-10 Sep. 2005, Pècs q2/q1 = 1.831 - 0.108ε - 0.022ε2 + 0.009ε3 + 0.009ε4 Which means that, in the case of rigid rotator and for whatever value of the limb darkening parameter εbetween 0 and 1, the ratio will always be 1.72≤q2/q1≤1.83. Ratio values lower than 1.72 show the presence of solar-like differential rotation with the equator rotating faster than the poles, while values higher than 1.83 may point at the presence of anti-solar differential rotation (the poles rotate faster than the equator). We found possible presence of solar-like differential rotation in at least 7 of the 113 stars examined so far. For most of these stars the temperatures can be found in the GAUDI archive: searching for some correlation between differential rotation and temperature, we spotted an excess of anti-solar differential rotation in stars with T ≥ 7900°K, i.e. without outer convective zone [14]. We still have to understand whether this is a real mark of anti-solar differential rotation or some other effects which changes the rotational broadening in a similar way. The position ratio of the first two zeroes of the Fourier transforms of the symmetrized mean line profiles plotted against temperature. The straight lines show the values compatible with rigid rotation.