Searching for planets around evolved stars with COROT

1. Searching for planets around evolved stars with COROT • Licio da Silva1,Renan De Medeiros2, José Dias do Nascimento Jr.2, Ramiro de la Reza1, Luca Pasquini3 , Cláudio H. F. Melo3 and Evgueni Jilinski1 • 1Observatório Nacional, Rio de Janeiro, Brazil • 2Departamento de Física Teórica e Experimental/UFRN, Natal, Brazil • 3European Southern Observatory

2. Figura 1 HR diagram of the targets of our precise Radial Velocity measurements of G and K Giants. HD 47536 and HD 122430 are the largest stars known hosting giant planets

3. 1. Introduction Recently we have undertaken the detection of two new extrasolar planets, orbiting the giant stars HD 47536 (ref b) and HD 122430 (ref c). These results were consequences of our precise radial velocity (RV) measurements of G and K giants (ref a). A number of stars from our list of 80 targets have been observed for 14 months, using the fibre-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. Long-term accuracy better than 10 m/s is required to detect Jovian planets around dwarfs at about 5 AU. The situation is more complicated for giant stars, due to other mechanism that may also contribute for observed radial velocity variability. The COROT mission offers, certainly, an unique possibility for the search of planetary systems around dwarfs stars, now looking for planets with sub-Jovian masses. We present here an analysis about the possibility of using COROT also to search planets around evolved stars. The main goal of this project is the search for planetary systems orbiting solar-type evolved stars, namely stars with solar metallicity and masses around 1-3 solar mass, at different evolutionary stages, from the turn-off to the red giant branch (RGB). Such an unprecedented procedure should enable us also to study the role of stellar evolution on the planets life. In particular, it would be possible to check the theoretical predictions on the engulfment of planets by the hosting stars. Of course, the "main COROT programme" will already provide us with planets around stars of the main sequence and the turn-off. We analysis here the possibility of including the RGB stars in this research.

4. 2. COROT Constrains: From A. Baglin et al (ref d), the radius of the smallest detectable planet around a solar like star 20 times more active than the Sun is 1.8 Earth radius. For a “quiet” star, i. e. so active as the Sun, this value is 0.8 Earth radius. As F/F = (Rp/R*)², we can detect planets with the Jupiter radius (11 Earth radius), for a homogeneous disk star (i.e., without luminosity variation from the center to the limb), only if the active star radius is R* (11/1.8) R ~ 6 R For a star so active as the quiet sun we have R*(11/0.8)R 14R From the known expression log g* = -log R* + log M* + log g , where M* is the stellar mass in solar unit, we can get the stellar gravity. 3. Results: Using log g and the values given by the tables and the evolution tracks of Padova Observatory (ref d), we can determine the corresponding evolution stage (Teff and L/L) of stars having the limit radius above for a given mass (fig 2). The results, for a solar abundance, are present in table 2 and the position of the stars in the Padova's evolution tracks are shown in Fig 2.  It is accepted that the largest brown dwarf, that is in the limit of the nuclear burning, has 0.08 solar mass ~90 mJup. Then its radius is ~4.5 RJup (Supposing that the densities are the same). Like above, we can determine the characteristics of the most evolued solar abundance stars that are in the limit of COROT for the detection of such kind of planets. They are in table 2 and their position on the evolution tracks of Padova are

5. shown in Fig.2. But as the giant disk stars are not homogeneous (in the sens above, we are not talking about activity), it could be possible to detect planets even for stars more evolved then those shown in Fig 2. Possibly in the next COROT-WEEK we will present the analysis of that more realistic case. A very intersting sub-product will be obtained if a very big planet, or even a brown-dwarf, passes just in front of the center of an evolved star and we can follow the resulting luminosity variation to study the limb-darkening law of a such amazing star.

6. References: a) Precise radial velocity measurements of G and K giants. First results Setiawan, J.; Pasquini, L.; da Silva, L.; von der Lühe, O.; Hatzes, A. , 2003a A&A, 397, 1151-1159. b) Evidence of a sub-stellar companion around HD 47536 Setiawan, J., Hatzes, A. P., von der Lühe, O., Pasquini, L., Naef, D., da Silva, L.Udry, S., Queloz, D., Girardi, L. , 2003b, A&A, 398, .L19-L23.  b) Evidence for a planetarry companion around HD 122430 Setiawan, J., Hatzes, A. P., von der Lühe, O., Pasquini, L., Udry, S., da Silva, L., Naef, D., Queloz, D., Girardi, L., Santos, N. C., De Medeiros, J. R. (to be submitted to A&A ) c) COROT: Asteroseismology and Planet Finding Baglin, A., Auverne, M., Barge, P., Buey, J.-T., Catala, C., Michel, E., Weiss, W. and the COROT Team, in "Stellar Structure and Habitable Planet Finding", ed. F. Favata, I. W. Roxburgh and D. Galardí-Enriquez (ESA SP-485, january 2002) d)Evolutionary tracks and isochones for low and intermediate-mass stars Girardi, L., Bressan, A., Bertelli, G. and Chiosi, C., 2000, A&AS, 141, 371

7. Parameter Parameter unity unity HD 47536 HD 47536 HD 122430 HD122430 TABLE 1 BASIC STELLAR DATA OF HD 47536 AND HD 122430 Spectral Type P days 712 K1 III 345 K2/K3 III mv e mag 0.20 5.25 0.68 5.47 f(m) r M pc 0.991 10exp(-7) 121 0.353 10exp(-7) 133 m2 sin i Teff MJup K 4.94 (with m1 = 1.1Ms) 4380 3.71 (with m1 = 1.2Ms R a AU R 1.61 23.5 1.02 25 vrot sin i km/s 1.9 <1 Prot/sin i days 219 4266 Data of HD 47536 and HD122430 systems

8. TABLE 2BASIC DATA OF STARS IN THE COROT DETECTION LIMIT Radius Mass log g log Teff Log L To detect aJupiter Like Planet To detectLargest Brown Dwarfs

9. Figura 2 Padova Evolution tracks for 1, 2 and 3 solar mass stars shown the largest stars whose “planets” can be detected by COROT. Red points are for Jupiter like planets and orange for the largest possible brown dwarf (M ~ 0.08solar mass)