Download Diapositiva 1 - Dipartimento di Fisica

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
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
Main Characteristics
ones (10-20 days each), in order to maximize the scientific return of the mission. It will
Weight of the satellite. . . . . . . . . . . . . .600 kg
observe near the galactic equator to satisfy the requirements of the extrasolar planets
Sixe of the satellite . . . . . . . . . . . . . 4.2x9.6 m
search, which needs to observe in densely populated regions.
Diameter of the telescope . . . . . . . . . . . 27 cm
The long runs are best suited for the exoplanet search and for the detailed
CCDs . . . . . . . . . . . . . . 2 asteroseismological
asteroseismological measurements of ~60 stars, which have been carefully selected in the
. . . . . . . . . . . . . . 2 for exoplanet search
framework of the ground-based preparatory work. The whole main-sequence will be
CCDs size . . . . . . . . . . . . . .2048x2048 pixels
investigated, from solar-like to massive B stars. The short runs instead will be focused on
Field of view . . . . . . . . . . . . . . . . .3.05° x
specific scientific topics.
2.7°
Moreover, a large number of additional scientific programs has been proposed in order to
Pointing precision . . . . . . . . . . . . . .0.2 arcsec
complete the observational program of the satellite and exploit its capabilities (highHeight of the orbit . . . . . . . . . . . . . . . .896 km
precision photometry, long time-baseline,...). The main proposals focus on the study of
Inclination of the orbit . . . . . . . . . . . . . . .polar
http://corot.oamp.fr/ binary systems, pre-main sequence stars, activity and rotation of stars, classical pulsators
Telemetry volume . . . . . . . . . . . 1.5 Gbits/day
as RR Lyrae and Cepheids.
Lifetime of the mission . . . . . . . . . . . ~3 years
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.
Low degree modes cross the
entire star, while high degree
modes are trapped in the
surface layers
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.
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 mainsequence stars are poorly observed due to
the low amplitude of their pulsations:
COROT will make up for this lack of
information.
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.
THE GAUDI DATABASE
0
CCD
CCD E1
The observing zones of COROT were chosen in order to satisfy the requirements of the scientific
A1
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 0
2.70°
0
CCD
CCD
launch a vast program of ground-based observations, both spectroscopical and photometrical, in
A2
E2
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 0
objects as secondary targets, with mv < 9.5). It was not possible to carry on similar observations
3.05°
for the stars involved in the planet search, simply because of their huge number: the two exoplanet Left: the 2 seismological CCDs. Right: the 2
CCDs will observe up to 12000 stars each run (~ 60000 stars during the whole mission). At first, exoplanet CCDs, with colour information
obtained via a small dispersion prism .
the ground-based surveys covered all the stars in the two COROT's eyes with mv < 8, but it did not
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
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
[7] and are available to the scientific community (http://sdc.laeff.esa.es/gaudi/).
the solar light and will change its zone of observation.
E1
A1
E2
A2
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]:
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 ε between 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
The position ratio of the first two zeroes of the Fourier transforms of understand whether this is a real mark of anti-solar differential rotation or some
the symmetrized mean line profiles plotted against temperature. The
other effects which changes the rotational broadening in a similar way.
straight lines show the values compatible with rigid rotation.
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
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 The transit of Mercury on the Sun in the 2003 as
by the SOHO satellite: COROT will use this
photometry in order to detect a large number of seen
kind of phenomenon to detect the presence of
extrasolar planets.
planets.
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.
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:
I8498Å = log a
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