Download Searching for planets around evolved stars with COROT

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
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
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.
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
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.
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
TABLE 1
BASIC STELLAR DATA OF HD 47536 AND HD 122430
Parameter
unity
HD 47536
HD122430
K1 III
K2/K3 III
Spectral Type
mv
mag
5.25
5.47
r
pc
121
133
Teff
K
4380
4266
R
R
23.5
25
vrot sin i
km/s
1.9
<1
Prot/sin i
days
219
Data of HD 47536 and HD122430 systems
Parameter
unity
HD 47536
HD 122430
P
days
712
345
0.20
0.68
e
f(m)
M
0.991 10exp(-7)
0.353 10exp(-7)
m2 sin i
MJup
4.94 (with m1 = 1.1Ms)
3.71 (with m1 = 1.2Ms
a
AU
1.61
1.02
TABLE 2
BASIC DATA OF STARS IN THE COROT DETECTION LIMIT
Radius
Mass
log g
log Teff
Log L
To detect a Jupiter Like Planet
14
(quiet star)
1
2
3
2.15
2.45
2.63
3.636
3.663
3.689
1.73
1.88
1.99
6
(active star)
1
2
3
2.88
3.18
3.36
3.664
3.700
3.930
1.16
1.30
2.23
To detect Largest Brown Dwarfs
63
(quiet star)
1
2
3
0.84
1.14
1.32
3.559
3.592
3.613
2.78
2.92
2.99
25
(quiet star)
1
2
3
1.64
1.94
2.12
3.608
3.641
3.648
2.18
2.31
2.51
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)