Answer to the Call for Mission Proposals for two Flexi-missions
_________________________________________________________________________________
From Stars
to Habitable Planets
The COROT challenge
22/9/00
presented by A. BAGLIN (France),
T. Appourchaux (SSD), R. Garrido (Spain), W.
Weiss (Austria).
Contacts:
annie.baglin@obspm.fr (PI), frederic.bonneau@cnes.Fr (Project Manager)
gerard.epstein@obspm.fr (System group)
Table
of Content
Executive Summary......................................................................................................................................................................
I. SCIENTIFIC
OBJECTIVES..............................................................................................................................................................
I.1.
The asteroseismology programme..........................................................................................................................
I.1.1 The exploratory programme................................................................................................................................................
I.1.2. The Central programme.......................................................................................................................................................
I.2.
The exoplanet programme..............................................................................................................................................
I.2.1. COROT can detect Telluric exoplanets............................................................................................................................
I.2.2. Expected number of transits observed by
COROT.........................................................................................................
I.3.
The Additional Programmes and the COROT stellar photometry data base............................
I.4.
Summary of the observing Programme...............................................................................................................
II.
INTERNATIONAL PARTNERS..................................................................................................................................................
III.THE
PAYLOAD CONCEPT.........................................................................................................................................................
III.1. The detection efficiency..............................................................................................................................................
III.2.
The Payload architecture..........................................................................................................................................
III.2.1.Optical Scheme of the telescope COROTEL...............................................................................................................
III.2.2. The defocused spot, the readout rate
and the pointing stability............................................................................
III.3.
The Camera Detector assembly : COROTCAM..................................................................................................
III.4.
The Charges Coupled Device Detectors............................................................................................................
III.4.1. The requirements..............................................................................................................................................................
III.4.2. The CCD space evaluation.............................................................................................................................................
III.4.3. The CCD photometric calibration................................................................................................................................
III.5.
The Photometric Performances of the Instrument.................................................................................
III.5.1. Noises and perturbations................................................................................................................................................
III.5.2. In the Seismology channel.............................................................................................................................................
III.5.3. In the Exoplanet channel...............................................................................................................................................
IV. MISSION
REQUIREMENTS.......................................................................................................................................................
IV.1.
COROT field of view.........................................................................................................................................................
IV.2.
Mission profile...................................................................................................................................................................
IV.3.
Chronology of the observations.......................................................................................................................
V. SCIENCE
OPERATIONS AND ARCHIVING...........................................................................................................................
V.1.
The scientific council.....................................................................................................................................................
V.2.
The ground segment.........................................................................................................................................................
V.3.
Project for the data Distribution Policy.......................................................................................................
VI.
TECHNOLOGICAL DEVELOPMENTS...................................................................................................................................
VII.
MANAGEMENT AND FUNDING..............................................................................................................................................
VII.1.
Funding....................................................................................................................................................................................
VII.1.1. Total cost..........................................................................................................................................................................
VII.1.2. Instrument........................................................................................................................................................................
VII.2.
Management.........................................................................................................................................................................
VII.2.1. At the Satellite level.......................................................................................................................................................
VII.2.2. At the Instrument level...................................................................................................................................................
VIII.
COMMUNICATION AND OUTREACH.................................................................................................................................
Annex I:
SCIENTIFIC SPECIFICATIONS OF THE COROT MISSION................................................................................
Annex
II: A
PROPOSAL FOR THE INTEGRATION OF SCIENTISTS OF ESA MEMBER STATES IN THE
COROT SCIENTIFIC COMMUNITY.......................................................................................................................................................................................
Annex
III: A
PROPOSAL FOR A POSSIBLE CONTRIBUTION OF ESA TO THE COROT MISSION......................
COROT on the PROTEUS platform
Executive Summary
COROT , which stands for COnvection, ROtation
and planetary Transits,
is dedicated to
Ultra High precision, wide field, relative
stellar photometry for very long
continuous observing runs on the same field of view.
It has two main scientific
programmes working simultaneously on adjacent regions of the sky:
ASTEROSEISMOLOGY,
and
the SEARCH
FOR EXTRASOLAR PLANETS .
Following the
recommendations of SSAC on January 25th 2000, the project is proposed to the
Flexi-Missions Programme. COROT has been proposed and studied in the framework
of the CNES mini-satellite programme using the PROTEUS platform; it has already
passed successfully Phase A and B and is ready to start Phase C. Its launch is foreseen in 2004.
The COROT instrument is a
white-light wide-field photometer, with a set of 4 frame-transfer CCD as
detectors. The entrance pupil is 27cm. COROT will be launched on a low-earth
polar inertial orbit, allowing it to monitor stellar fields near the pole of
the orbit for about 5 months continuously. The mission lifetime will be
nominally of 3 years.
What is asked ESA is a
scientific evaluation, and a small financial participation ( 2 to 3 Meuros). If
this is accepted, the ESA community will participate to the mission and have
access to the data (see Annex II).
COROT
addresses major scientific domains:
- In stellar physics, the
asteroseismology technique will allow a new understanding of the internal
structure and evolution of stars. COROT
will perform asteroseimic measurements on 50 stars during 150 days each; for
the 5 to 10 brightest ones (solar-type and delta Scuti) oscillations of ppm amplitudes will be
detected. An exploratory programme
will monitor 50 to 100 stars during 20 days each.
- In the search of
extra-solar planets, the detection of telluric exoplanets is the next
significant challenge after the discovery of Jovian-sized exoplanets. COROT will be able to detect Earth-size
planets close to their parent star, similar to Mercury, and planets slightly
larger than the Earth in the Habitable Zone.
- The photometric database
of COROT, which will contain almost 100 thousand stars, observed continuously
during 150 days, with an accuracy of 100 ppm, will be of extreme interest for
many subjects e.g. stellar variability and specially activity, discovery of
eclipsing binaries, new types of variability below the millimagnitude
level, comets and Kuiper belt
objects......
COROT scientific organisation
is centred on a Scientific Council (CS), which is responsible in particular for
the observation programme and the data
distribution policy. Scientific activities concerning the two main science
objectives (asteroseismology and exoplanet search) are organised around
Associated Scientists (AS). The AS belong to working teams under the responsibility
of a Co-I member of the CS.
Guest Investigators (GI) are
scientists who have answered to the Call for Opportunity of the Additional
programmes and whose programmes have been selected by the Scientific Council.
The AS and the GI are
members of the different countries participating to COROT (including ESA member
states is ESA contribution is granted).
Shortly after the end of an
observation sequence, the reduced and qualified data will be distributed to the
AS and the GIs, then released in the community after an additional delay of
about 1 year.
COROT
is a precursor mission:
The SSAC of ESA on April 26,
1996 stated that it would be very much worthwhile to first execute a smaller
stellar seismology project before embarking on a full scale mission.
- AWG_ Recommendation of the
Next Medium-size Mission M3 April 25th 1996......The AGW therefore found it difficult to reach a decision.....The AGW recommends
unanimously to SSAC the COBRAS/SAMBA as the third medium mission of Horizon
2000 but hopes that means can be found to realise a mission like STARS as soon
as possible.
- SSAC_Selection of M3
mission: April 26th 1996: ......Concerning STARS, the SSAC also confirms its
great scientific interest. It believes that
it would be very much worthwhile to first execute a smaller stellar
seismology project, as being considered in some member states of ESA, before
embarking on a full scale "Stars" mission.
COROT is such a precursor mission. In asteroseismology, oscillations will be detected for the first
time in various types of stars. For
exoplanets search, it will provide the first detection of exoplanets long
before DARWIN or GAIA can come into operation.
COROT
is a truly European mission:
The long history of proposed
space-based asteroseismology missions makes Europe the leader in the domain,
well suited to continue the development of such missions and finally to
harvest the scientific results by executing
the COROT project.
COROT is basically an European mission. The instrument
hardware and ground segment involve
France, Spain, Austria, Scientific Department of ESA, and likely Belgium and
Italy. Many European scientists are ready to become Co-investigators.
COROT will then provide
European scientific leadership in the areas of asteroseismology and search for
telluric planet. The COROT microvariability database will provide wide research opportunities for the
astronomers community.
COROT will also contribute
to the preparation of ESA cornerstones GAIA and DARWIN, by providing prior information on the
statistics of hot giant exoplanets, and on the existence of telluric or super -Earth exoplanets.
COROT will also prepare for
future high precision photometry "STARS-class" missions.
The directive given to CNES
by its ministerial authority last summer was to widen the co-operative frame of
the COROT project, especially looking towards European countries. This
political willingness can be transmuted into a unique opportunity for embarking
European countries and the European Space Agency onboard a first class project.
And the European leadership
will be preserved in these two hot subjects.
I. SCIENTIFIC OBJECTIVES
These have been described in
details in the document "Scientific
Specifications of the COROT Mission" (see Annex I) . They are
briefly summarised below.
I.1. The asteroseismology programme
COROT will realise both an
exploratory programme, to detect
oscillations in a large variety of stars and to classify the
asteroseimologic properties of stars in the HR diagram ,and a more specific
programme, centred on a detailed study of a few stars, specially chosen
to test the hydrodynamics of the internal layers and the physical state
of stellar cores.
I.1.1 The exploratory programme
As COROT is the first
mission dedicated to asteroseismology, a preliminary programme called "exploratory" will determine the domain of stellar
parameters for which oscillations are detectable, and the relation between the
amplitudes of the solar-like oscillators and their characteristics.
Though several theoretical
predictions of the amplitudes of oscillations excited stochastically by
convective motions exist, a large uncertainty remains, in particular on the
treatment of the superadiabatic external layers and on the scaling of the
turbulent velocity field.
To achieve this, one has to
observe a sample of objects with a variety of stellar parameters, i.e. mass,
age, chemical composition, state of rotation...but with moderate signal to noise ratio. A frequency resolution of
0.5mHz is sufficient
for this purpose, corresponding to observing runs of 10 to 20 days. Stars down
to the 9th magnitude are appropriate targets; 5 to 10 targets will be
observable at the same time.
Several tens of stars will
have to be followed, corresponding to a total observing time of at least 2 to 4
months. COROT is up to now the only project which is able to observe several
(ten) stars simultaneously in its seismological mode.
I.1.2. The Central programme
The "central programme" is more ambitious and more time
consuming than the exploratory one; it corresponds to the second step in the
development of space asteroseimology.
It aims at observing very
precisely a small set of objects, selected for their diagnostic power. The
choice of these targets will be partly based on the results of the exploratory
phase.
Based on the solar case, we
fix the accuracy of the frequency measurement at 0.1mHz to have access
to mode profiles and rotational splitting and
to measure precisely the distribution of the mode frequencies. For a 6th
magnitude star, the detection threshold will be less than 1 ppm. For A and F
stars close to the main sequence, it will then be possible to measure the size
of the convective cores, the size of the outer convective zones and their
helium content or the rotation profile
of d Scuti stars.
A least 5 runs are planned,
during which one bright star (the main target) and several fainter ones in the
surrounding field of view will be followed.
To realise this programme it has been required that the mission fulfils
the following specifications:
* Frequency resolution: 0.1 m Hz
* S/N (in Fourier space) : 0.6 ppm over 5 days at mv=6
* Magnitude of the main targets: ~ 6
* Number of main targets : ≥ 5
* Duty cycle: ≥
90% over 5 days
≥
80% over 150 days
I.2. The exoplanet programme
The detection of a telluric
planet is a major challenge and is expected to be the next big step in
astronomy, and recent discoveries of a few tens of giant planets have upset our
vision of the formation of planetary systems. Let us recall that the discovery
of new terrestrial planets is the first action listed by the "Long Term Space Policy
Committee" SP-2000, May 1999.
A sensible approach of the
search and study of extra-solar planets around
stars is:
- searching for Giant exoplanets,
- searching for Telluric ones,
- spectroscopically analysing them, with an emphasis on
Telluric ones.
* The first step has been
made with the discovery of 51 Peg b in 1995. In fact, it has been preceded by a
photometric observation by Hipparcos of a planetary transit in front of the
star HD 209458, several years before, but not recognised as such. However, presently we do not have an
unbiased statistics of these planets.
* The second objective is
ONLY accessible to the COROT mission (or to similar ones, such as KEPLER in the
US but which is not approved), as developed below.
* The third one will be the
main goal of future very ambitious mission which will probably fly in a few
decades.
I.2.1. COROT can detect Telluric exoplanets
This detection of telluric
planets around solar type stars is very difficult because of their small
masses. Possible methods are: coronography, astrometry, radial velocity,
timing, gravitational amplification, occultations.
Before the achievement of
ambitious space projects (interferometers, coronographs), there are only 2
possible methods on a short or mid term for the telluric planet detection,
gravitational amplification and occultations. As emphasised in the Townes Report
(Comments on the Blue Ribbon Panel on ExNPS, 1996), gravitational
amplifications can not allow one to determine the statistical frequency of
planets at several kpc from the Sun (galactic bulge), while we would need to
detect earths, located a few parsecs from the Sun, in order to subsequently
study them in a more detailed way.
The occultation method or
transit method, allows on the contrary the precise determination of the orbital
period and the size of the planet. Concerning close enough planets, it would
enable to study their atmospheric chemical composition thanks to the
differential absorption of the stars radiance. Yet this method needs a high
precision photometry (10-3 to 10-4) and continuous
observations during a long period (several months).
The photometric method, particularly
well adapted to the telluric planets can also detect giant extra-solar planets
(detectable by spectroscopy from the ground) and determine their albedo,
increasing the scientific return.
As COROT is devoted to
stellar photometry, aiming both a high precision and a long observation time, the
search for exoplanets by the transit method can easily be integrated in the
payload and in the mission profile.
However COROT is not a
mission devoted to telluric exo-planets detection and is not optimised for that
purpose, especially because of its small telescope diameter. Nevertheless, we
will show that we can hope to achieve a significant breakthrough in this field,
as COROT will give a first estimate of the frequency of the appearance of these
planets in the planetary systems, as well as their role in the subsequent
system evolution.
Presently, we do not know
the statistical distribution of Telluric exoplanets as function of their
size. In fact, we do not even know if
they exist! COROT will demonstrate the "existence theorem".
This piece of information is
crucial for the future projects which will aim to perform the spectroscopy of such objets. COROT will
not provide a target list for such missions because it will detect only
Telluric planets with orbit almost edge on.
To do so, it will monitor a large number of stars in a given field of
view (few square degrees) and therefore distant ones, typically at 500 pc.
But if we make the plausible
assumption that the planet distribution is the same in the solar neighbourhood
and at 500 pc, we will know from COROT the fraction of stars that have planets
of a given size, a most valuable piece of information for optimising the search
procedure.
I.2.2. Expected number of transits
observed by COROT
One major difficulty is to
get rid of false alarms due to photometric variations of stellar origin, as
stellar activity. To do so a dispersive
element (prism) has been included in the exoplanet field (see III.2). giving,
instead of a defocused image, a little spectrum (3-4 resolution) with the same
number of pixels as the nominal configuration. It has been shown that this
measurement, possible only on sources brighter than 14, decreases significantly
the false-alarm probability.
This chromaticity criterion
will be particularly useful if only one or two transits are identified within
150 days (a field observation time with COROT) since the periodicity criterion
becomes useless.
In order to estimate the
number of possible transits due to telluric planets, we have to estimate the
number of solar type stars escorted a priori with at least one planet.
Concerning Jupiter-like planets, this number seems to be of a few %.
Qualitatively, it is in agreement with observations which claimed that about 10
% of young stars have got an hydrogen cloud. Around the others, a stellar wind
has dispersed the hydrogen, preventing Jupiters formation.
Regarding the telluric
planets, their formation from Planetesimals is not inhibited by this stellar
wind mechanism because it can not disperse the asteroďd like bodies . Since 50
% of the young stars have a dust disc, it is reasonable to assume a priori that
half of the stars have got telluric planets and that 20 % of them have got
planets whose radius is superior to the earths. Our quantitative estimates are
based upon this hypothesis.
The number of events depends
on the radius and of the distance of the planet to its parent star, so it is
difficult to give numbers. With a photometry optimised up to mv=15,5 (which
corresponds to the COROT field with more than 30000 stars observed during the
mission, i.e. the minimum value ) the expected number of events, assuming that 20% of the targets are associated with such
planets, is:
* 25 planets Rpl = 1.58 R_earth at 0.3 a.u.
* 40 planets Rpl = 2 R_earth during at 0.3 a. u
* A few planets with Rpl > 2.R_earth, in the
"habitable" zone thanks to the chromatic information.
* Several hundreds of hot Jupiters and Uranus like
planets, with detailed light curves.
Complementary specifications have been added to realise the
exoplanet programme:
|
* Noise level : 7
10 -4 in 1 hour for mv=16 *
Magnitude of the targets 11
< m< 16 *
Number of targets : >
30 000 *
Coloured information: m
< 14 |
I.3. The Additional Programmes and
the COROT stellar photometry data base
COROT will provide extremely
long and uninterrupted sequences of photometric data of more than hundred
thousand stars, of magnitude between 12
and 16, acquired by the exoplanet field. The time sampling is 15 minutes, the
accuracy on an individual measurement is a few 10-4, and the duration varies
from 10 to 150 days. For the brightest ones two colours will be available, with
the same accuracy.
The only available data of
this type come from the microlensing surveys, with looser time sampling (1 day)
and very rough photometry (1%). They have shown that these sets of wide
complete samples are extremely useful to probe scenarios of evolution and
stellar physics in general. They nicely complement seismology of a much smaller
samples of targets.
For instance, COROT will
obtain light curves of binary stars, with a level of precision which will allow to study tidal modulations and
tidal lags which can provide a direct measurement of the viscosity of the stellar
material.
Colour photometry can
supply, through the use of surface tomography, relevant information concerning
the evolution of cold spots and differential rotation axes of single and binary
stars and can give a direct measurement of limb and gravity darkening laws.
Concerning stellar activity, the high photometric
precision and long time series provided by COROT will allow us to extend the
analysis of the dependence of magnetic activity upon rotation to moderately
active (and therefore little variable) stars and moderate rotators, thus
providing powerful observational tests to dynamo theories long time series.
In a analogous way as
planets, comets will produce transits detectable by COROT. Kuiper Belt objects of very small size could also be
observed.
The concept of Additional Programme is defined as follows:
|
*Address different scientific
questions than the Core Programme but using the same data * Asks for different targets and/or
observing procedures than the
Core Programme in the same fields of
view and/or eventually in different fields of view |
The selection of the Additional
Programmes will be made by the
Scientific Council
An
AO will be issued by 2001,
for a selection : in 2002 or 2003 depending on the launch date.
Authors of selected AP will become Guest Investigators (GI)
I.4. Summary of the observing
Programme
|
Programme |
Duration
of a run (days) |
Number of
runs |
Number of
objects |
mv |
Time
sampling |
|
Exploratory |
10 to 20 |
10 |
|
|
|
|
Sismo |
|
|
50 to 100 |
≤ 9 |
1 s. |
|
Exo |
|
|
30000 to 60000 |
≤ 16.5 |
15 min. |
|
Central |
150 |
≥ 5 |
|
|
|
|
Sismo |
|
|
≥ 5 |
≤ 6 |
1 s. |
|
|
|
|
30 to 50 |
≤ 9 |
1 s. |
|
Exo |
|
|
30000 to 60000 |
≤ 16.5 |
15 min. |
II.
INTERNATIONAL PARTNERS
III.THE PAYLOAD CONCEPT
III.1. The detection efficiency
Photometric stability constraints come from the
Asteroseismology channel needs: to achieve a line level detection of 0.6 ppm
referred to the star brightness in 5 days integration, limited only by photon
noise, 6 106 photoelectrons per second per star spot have to be
collected.
This specification gives the input for the optical
entrance pupil, the optical transmission and the detector quantum efficiency.
For a 6th magnitude target, this specification is reached with a 600
cm2 pupil, a global optical transmission of 70% and an average CCD
quantum efficiency better than 50% integrated on the overall emission spectrum
of G Star.
The Fourier spectrum of
solar oscillations, observed by IPHIR on PHOBOS, during 8 days, in the
green channel
III.2. The Payload architecture
III.2.1.Optical Scheme of the telescope COROTEL
The telescope design is strongly constrained by the
need to minimise straylight from the Earth entering the telescope; baffle
efficiency should be in order of 10-13. The best protection is
reached with an off-axis afocal parabolic system (2 mirrors). The light is
collected by a f/4 telescope of 27-cm entrance pupil to achieve the goal of a
2.8° x 2.8° FOV. The parallel beam at the output of the telescope is re-imaged
on the focal plane by a dioptric optic. A very good relative photometric
precision of defocused stars is reached by CCDs working in a frame transfer
mode in order to avoid a mechanical shutter.
Pointing stability of a fraction
of an arcsecond is obtained, thanks to Proteus modified AOCS :
during the
observation, the stellar sensor information is replaced by the Corot
payload measurements of the position of targets in the
focal plane focal . This method avoids effects of axis misalignment
between the experiment and the stellar
sensor, whose fluctuations induced
photometric noise when images on CCD detectors are not stable within a portion
of a pixel .
Thermal stability of
mechanical structure, optical parts, detector associated electronics
will be carefully controlled in order not to introduce periodic fluctuations on
the photometric measurements induced by eclipses and orbital variation of terrestrial aspect. This goal will be
reached using efficient positioning of
passive radiators and some
thermal regulation .
Numerous Housekeeping temperature measurement
channels and a well calibrated thermal model of the payload will permit to correct the remaining errors coming
from thermal fluctuations.
III.2.2. The defocused
spot, the readout rate and the pointing stability
The
Asteroseismology channel
The limited CCD full well capacity (105 electrons in MPP mode), the acquisition speed
and the noise jitter define the size of the spot image: a minimum 250 pixel
spot for the brightest targets.
PRNU constrains the shape of the spot image.
|
|
The dispersed
point spread function of the Seismology channel
While two measurements per minute would be enough for
scientific data reduction, CCDs have to work at a much higher rate in order to
use the stars position on the focal plane in the pointing control system. The
pointing jitter induced by the photometric measurement fluctuation is due to
the pixel to pixel response non-uniformity. With a 1.5% rms PRNU and a 250
pixel spot size, a pointing stability of 0.25 pixel is needed to maintain this
jitter noise below the photon noise.
One image per second for this channel is a compromise
to give an accurate photometric and a barycentric calculation on the
Asteroseismology image spots, but the windowed readouts are needed to reach
this rate when reading at least five defocused stars per CCD.
The Exoplanet channel
·
In order not to saturate the CCDs with magnitude 12
stars and a 32 seconds integration time, 40 pixels spot areas are needed. These
spot dimensions avoid confusion between the observed stars.
The dispersed
point spread function of the Exoplanet channel
The transits are essentially achromatic so the colour
information is required to resolve the ambiguity between a stellar activity and
a planet occultation. A prism in the beam of the Exoplanet CCDs creates a mini
spectrum of each star.
Full readout rate of 2kpixels by 2kpixels with the
two output registers at 100 kpixels per second is compatible with the 32
seconds integration time.
The read-out
noise
When observing bright stars, the readout noise is not
our main concern compared to the star photon noise (the acquisitions are photon
noise limited) while the photometric chain transfer function fluctuations or
the pointing jitter modulation are the main sources of the instrumental noise.
But for the faint stars observed by the Exoplanet channel, both the readout
noise and the background will be the main noise contributors. A 10 electrons
rms readout noise specification is needed in order to achieve a precise
measurement of the background level (the dark current, the parasitic component
due to the terrestrial albedo or the zodiacal light).
While the white noise perturbation is equally spread
all over the frequency spectrum, the electromagnetic interference may create
undesirable artefacts. In order to avoid them, a high frequency synchronisation
clock is distributed and the four CCDs clock rate are synchronised on the
spacecraft in real time.
The CCD thermal requirements
The orbital
thermal fluctuations may introduce periodic components in the signal via the
quantum efficiency and the dark current (both depending on temperature) which
could be confused with the stellar oscillations. Therefore the temperature of
the CCD should be stabilised below –40°C and regulated at better than 0.05°C
per hour. The CCD temperatures should be measured with 5.10-3°C
resolution to store in the data package.
III.3. The Camera Detector assembly : COROTCAM
COROTCAM
is the core of scientific photometric chain: the
assembly of the optics, the focal box and the proximity electronics. The thermal
stability is the main requirement for this subsystem. The focal box has a
dedicated radiator, which cools and stabilises the CCDs temperature. The local
thermal management should obtain this specification. The temperature range of
the optics is 20±0.5°C. For the proximity electronics, the stability is also
1°C but the range is from 0°C to +40°C.
The defocusing of the Asteroseismology channel is
obtained by the mechanical design. Only the Exoplanet channel uses a prism,
which is inside the focal box.
A 12-mm aluminium shield from the radiation effects
protects the back and the sides of the focal box, whereas in the front the CCDs
are protected by several centimetres of
lens glasses.
... 
The CorotCam
Focal box
III.4. The Charges Coupled Device
Detectors
III.4.1. The requirements
Four 2k x 2k image zone CCDs are necessary to cover
the FOV and allow a 2.3 arc second spatial sampling. Two CCDs are devoted to
the Asteroseismology channel while the two others are used for the Exoplanet
channel. For the Asteroseismology channel, a maximum of 5 defocused stars per
CCD and 5 sky reference windows should be read out every second. So the line
transfer time should be the shortest possible. In addition, the dump drain is
mandatory for windowing readout. For the Exoplanet channel 3000 stars per CCD
are recorded as well as a few dark reference windows. For the Asteroseismology
channel, the useful pixels are less than 1% of the image zone and this number
reaches 5% for the Exoplanet channel.
In order to achieve the photometry aim, the maximum
number of photons should be detected during the 5 observation months. So the
high quantum efficiency of a thinned CCD, from 0.37 µm to 1 µm (the optical
bandwidth) is required and a frame transfer mode is chosen for the CCDs. The
high relative photometry is not obtained with an individual image. The
precision of each image is limited by the quantum efficiency variation in terms
of temperature and wavelength; this value is about a few 103. As the
CCD radiator allows only –40°C, an MPP
mode is required and the dark current stability also generates a fluctuation at
the 10-2 level. The last factor needed to reach the photometric
accuracy is achieved by soft correction.
The shortest time transfer for a frame transfer, MPP
mode, back illuminated, 2kpixels by 4kpixels is 100 µs for the EEV 4280 CCD.
For COROT, we specify a non-standard dark current analysis: the mean value into
a window of 32x32 pixel should be less 0.5 electron per second at minus 40°C.
In order to save money and to satisfy the COROT requirements, we accept a
maximum defect pixel number (6 column defects, 750 black and white spots, 20
traps greater than 200 electrons and no trap allowed into registers) and a
local PRNU value (7% peak to peak from 450 nm to 650 nm, 9% at 370 nm and 10%
at 850 nm).
The four CCDs are read out with the two outputs. For
the Asteroseismology channel, about 5 star windows of 32x32 pixels are recorded
per CCD. In addition the sky reference windows are read out. In order to
provide 1 second integration, a windowed readout is a mandatory. For each
window (star and sky), a mirror window is read in the other side of the CCD but
unfortunately without a bright star: the sky is not perfect!
For the Exoplanet channel, about 3000 stars of 50
pixels are recorded per CCD. The whole readout of the store zone requires about
22 seconds, which is less than the 32 seconds image zone integration time. Soft
windowing selects the good data.
III.4.2. The CCD space evaluation
We decided to choose the EEV 4280 CCD with it
standard package without any modification in order to stay in the EEV's
standard process and reduce risks and cost due to new development; we only add
a thermal sensor (AD590) under the invar block. For the COROT space program,
CNES decided to check the quality of this component and to undertake a special
space evaluation.
The evaluation program, made with Frame transfer
42-90 CCDs (a silicon area largest with the same packaging) is demonstrating
all the capability of this device to complete the COROT mission. The limits of
device endurance and performances have significant margins above the
requirements of the COROT mission as well.
The Lot Acceptance Tests for the flight models are
carried out based on the ESA/SCC specification n°9020. These tests will be
carried out with 17 components issued from the flight model batch.
We will also test the CCD under irradiation,
measuring characteristics before and after exposure to proton flux.
III.4.3. The CCD photometric
calibration
For our high precision photometry application, we
need special CCD calibration.
The CCD calibration process falls in two parts: the
characterisation and the measurement of CCD characteristics, and the flight
model selection in accordance with the scientific criterion.
The main goal of the calibration is to be able to
correlate the CCD response with external parameters like temperature and supply
voltage, to a very high accuracy. Knowledge of these characteristics should
help us to select the CCD flight models and to correct the raw data. So we will
test the CCD in the flight conditions (as far as we can) and change the
external parameters to record the CCD response. The characteristics we plan to
measure are of very low amplitude and we need to increase the measurement range
in order to record the CCD response in terms of temperature (we make the
assumption of the linearity in the response). The pixel to pixel response is
also an other important point due to the fact that a star is defocused over 250
pixels. So for some characteristics we
want to know the pixel-to-pixel response more than the global characteristic of
the CCD.
The process of calibration starts with CCD
calibration and carries on with the whole photometry chain. So we will
calibrate the COROT camera (the focal plane and the optics) and couple this
camera with the flight readout electronics to achieve a complete calibration.
The readout electronics is also independently calibrated.
During the camera calibration, the optical and
mechanical stability of the focal plane will be checked, the thermal stability
will be monitored and the global pixel response non-uniformity recorded.
After these calibrations we will put the camera and
the flight model of readout electronics with a simulator of the onboard
processor. During one month this photometry chain will work without any
interruption. We will monitor both the video signal and all the housekeeping
(about 40 temperatures and 20 voltages) to establish the ultimate performance
of our instrument (we make the assumption that the camera is the most sensitive
part of our instrument). As it is impossible to have during such a time a
sufficient test bench stability (temperature stability, light emission…), the
housekeeping will be monitored. The housekeeping and the results from previous
calibrations of the camera's components will allow correcting the data from the
external fluctuations and to reach the ultimate performances of COROTCam.
III.5. The Photometric Performances of the Instrument
We present in the following
a brief discussion of the perturbators analysis.
The results of an instrument
model are summarised for both channels and are compared to the specifications.
III.5.1. Noises and perturbations.
The main white noise
contributions are the photon noise, the CCD readout noise, the pointing noise
due to the jitter of the stellar image on the detectors, the scattered light
from the Earth and from the interplanetary and human origin dusts.
The external perturbators
are associated to the orbit. Orbital
periodic perturbators are unavoidable (P0 = 103 mn) and we will try
to keep their amplitude as small as possible (less than 2 ppm for each components).
Nevertheless two lines will probably remain in the spectrum a frequencies f =
1/ P0 and 2f .
The following table summarises
the noise components and their time scale.
|
Noise |
White noise high freq. |
f, 2f |
Drift, Aging |
||
|
Photon |
X |
|
|
||
|
CCD
dark current |
X |
X |
X |
||
|
CCD
Read out |
X |
|
|
||
|
CCD
CTE |
X |
|
X |
||
|
ACS
(attitude fluctuations) |
X |
X |
|
||
|
Scattered
light |
X |
X |
|
||
|
Radiations |
X |
X |
X |
||
|
Thermal fluctuations |
Telescope CCD
(QE and dark) Readout
electronic |
|
X X X |
X X X |
|
|
Sky
background |
Faint
stars Dusts Zodiacal
light |
X X X |
|
X |
|
Some noise sources
contributes at the same times to several time scale as for instance the
scattered light. Its average level I0 can be measured and subtracted.
Measurements errors produce a residual periodic error and the difference I- I0
contributes to the white noise.
III.5.2. In the Seismology channel
For the seismology programme
each random noise component is specified to be ten times smaller than the
photon noise of a mv = 6 target which produces 5.3 106
e-/s on the CCD. The periodic perturbations associated to the orbit must be
less than 2 ppm for each components. This result will be obtained both with
hardware architecture and with ground corrections.
An optimal method close to
an aperture photometry method is applied to perform photometry.
Sorting the pixels by
decreasing intensity order, the sum is made from the brightest to the faintest
pixel. The summation end when the intensity of the kth pixel is
equal to the variance background.
This process gives an
optimal signal to noise ratio photometry.
Noise
Budget for the seismology channel.
The instrumental white
noise, as shown below, is for stars with magnitude between 5 and 9 ten times
smaller than the photons noise.
In the Fourier space the
high frequency noise level is 0.7 ppm in five days (specification at 0.6 ppm)
due to a slightly low optical transmission. On the figure the main contributor
are shown as a function of the magnitude. The specifications are fulfilled with
margins.
There will be in the
spectrum two lines at frequency f = 1/ P0 and 2/ P0 ,
respectively with 20 and 15 ppm amplitude. The main contribution to that lines
are temperature (acting on CCD quantum efficiency and readout electronic gain)
and attitude satellite error. Though corrections on ground will help reduce
these perturbators, a degradation of the detection threshold at these frequencies
has been accepted.
White noise contributions of
main components as a function of the star magnitude
III.5.3. In the Exoplanet channel
The exoplanet programme
compares all noises to the photons noise but the requirements take into account
the signal to be detected. For faint stars (mv ~ 15) the total rms noises must
remain less or equal the photons noise of the white light curve. For brightest
stars (mv ≤ 14) the requirements is the same but for the blue light
curve.
The photometric measurements
process should be quickly described before the noise discussion.
For each stars an aperture
is defined in an optimal way. The dispersion lies along the CCD rows and column
are summed to give a one dimensional spectrum. The selected pixels are divided
in two groups, the red pixels with 60% of the flux and the blue pixels with 20%
of the flux. With the attitude information coming form the seismology channel,
an interpolation is performed to take into account the depointing. The fixed
aperture imply that the defocus due to temperature induce photometric
variations, which will be corrected on ground. The correction is efficient as
the defocus is slow and it is measured on the seismology channel with a very
good precision.
The main noise contributors
are the attitude fluctuations (0.1 arc second), the readout noise, the defocus
and the background produce by scattered light from earth and zodiacal light.
Noise
Budget for the exoplanet channel.
Figure below shows that the
main total noise is smaller than the photons noise on white light curves, for
stars with magnitude less than 15.5.
For the detection of
occultations light curves will be averaged on exactly one orbital period
(occultations last more than three hours) smoothing all periodic orbital
noises.
|
|
Noise as a function of the
magnitude after defocus corrections.
The noises taken into
account are background, readout noise, satellite attitude errors and defocus.
IV. MISSION REQUIREMENTS
To achieve the scientific
objectives the mission life-time has to reach at least 3 years.
We have seen that the
programme asks for long and continuous exposures on the same targets. To reach
the frequency resolution required by advanced seismology studies up to 150 days are necessary to reach 0.1 microHz .
The very low cost of the
missions of the Small Mission Programme imposes to remain on a low altitude
orbit.
The polar inertial orbits
are the only ones to allow 150 days continuously on the same field.
IV.1. COROT field of view
IV.2. Mission profile
IV.3. Chronology of the observations
The selection of targets inside fields A and B is the responsibility
of the Scientific Council
The working group
on "Ground Based Programme" is responsible for gathering all the data
necessary to make this choice.
V. SCIENCE OPERATIONS AND ARCHIVING
The scientific organisation
is described in the organigramme below.
V.1. The scientific council
It is responsible for the
observing programme, and the data distribution policy
It is chaired by the PI and
contains
- one representative of each laboratory or country funding the experiment
- the chairs of
the working groups , asteroseismology,
exoplanets, ground based programmes
The creation of another
working group on Additional Programmes is under discussion
- the instrument scientist
More details can be found in
Annexe II, where Co-Is, and Associated Scientists (AS) are defined.
V.2. The ground segment
A preliminary
organisation of the ground segment is shown on the following scheme.
The upper box is
under the responsibility of CNES, the lower one under the responsibility of the
PI, and worked out by the scientific laboratories.
TTC: Telecommand and Telemetry Centre (CNES)
CCC: Command and
Control Centre (CNES)
CMC: COROT Mission Centre (CNES)
STS : Scientific
Treatment System (Scientific Laboratories)
V.3. Project for the data Distribution Policy
It has not yet been decided
formally by the Scientific Council.
What is proposed presently
is that after the end of each long run To, they will
- 6 months devoted to the instrumental corrections
- one year of restricted access to Co-Is, AS and GIs (
< To+1.5y)
- a public release in two steps
- to the
scientists of collaborating countries during one year (between To+1.5y and
To+2.5y
- to the
scientists worldwhile (> To + 2.5y)
VI. TECHNOLOGICAL DEVELOPMENTS
They have been already done.
The feasibility has been
demonstrated, see Chapter III.4.
VII. MANAGEMENT AND FUNDING
VII.1. Funding
VII.1.1. Total cost
The estimated cost at the
end of Phase B is (in MF):
|
PROTEUS Platform |
127 |
|
Launcher |
90 |
|
Instrument |
110 |
|
Ground-Segment |
20 |
|
Ground based observations |
~15 |
|
TOTAL |
~ 362 |
* Platform and launch are
funded by CNES France. Indians may contribute to the launcher if PSLV is chosen
* 50 % of the Ground segment
is funded by Spain.
* The ground-based
observations are carried out by an international team including, England, Ireland,
Spain, Italy, Denmark and France. Other
countries might participate as Netherlands, Brazil, Switzerland.....Each
country supports its own activities.
VII.1.2. Instrument
The cost at the end of Phase B is 110 MF.
It is shared
between France (53 %), Austria (6%), SSD/ESTEC (7%); the contribution of
Belgium TBC (13%) and Italy TBC (12%) is under discussion in the corresponding
countries.
The last 9% could be provided by ESA.
It corresponds to the optics of the telescope (0.8 Meuros), the realisation
of the onboard software (1.5 to 2 Meuros), and the mechanical and thermal
architecture.
It is described in the letter of R. Bonneville (CNES) to R. Bonnet and
the attached cards describing the proposed sub-systems (see Annex III).
VII.2. Management
VII.2.1. At the
Satellite level
VII.2.2. At the
Instrument level
VIII. COMMUNICATION AND OUTREACH
The COROT team values the
importance of reaching the public in its communication. Poor communication means that the science of
a space project may come unnoticed, and as a result the taxpayer may not value
as high the outcome of such a project.
The main mean of
communication is nowadays the Internet.
We will of course set-up a web page for the COROT project where various
informations on the COROT science will be available.
We will need to make various
pointer for various levels of interests such as:
- Materials for other scientists
- Materials for teachers and journalists
- Materials for children and students
For scientists, we may
include the list of targets together with the scientific objectives. For teachers, we may include materials for
classroom activities (e.g. asteroseismology and musical instruments, or an example
of stellar system: the solar system).
For children, we may include more fun aspects such as understanding
musical instrument, for instance; while for student the material may be more
elaborated depending on the level of education.
Of course, reaching out
people with the Internet is quite passive, in other words only interested
people will look at the COROT home page.
We are thinking about a more direct way of getting at the public. We will make a competition for naming the
planets that COROT will discover.
This is the most interesting (and fascinating) aspect of the
science objectives of COROT. A similar
competition was performed for XMM which proved to be extremely effective. Other active aspects of the communication
could be done by contacting directly the television channel of Europe, and
offering them our possible contribution for any scientific programme on stars
and exoplanets (e.g. `La nuit des etoiles' on France 2, scientific programmes
on the Discovery channel, or BBC open university).
Last but not least, the
COROT team members are prepared to go to schools and universities to explain
the science of COROT, on their own free time.
The COROT team realises that
starting early in the communication process is a key element in making a
project known to the public. This is
even more so for COROT because we will not have any sexy pictures of stars or
planets to show. For instance, artistic
impression of other stellar systems will be an important way of showing our
results to the public during press conferences and/or with press releases.
We are determined to make
COROT science communication a success.
Annex I: SCIENTIFIC SPECIFICATIONS OF THE
COROT MISSION
This document, proposed only
in paper format, has been written in 97 and issued in December 97, at the end
of Phase A .
Some scientific events occurred
since that date, which reinforce the problematics of COROT.
Let us cite:
- the photometric data acquired by the stellar sensor of
WIRE, which will certainly help the preparation of the photometric chains and
the precise definition of housekeepings
- the first
observation of a transit of a hot Jupiter from the ground, which confirms the
ability to measure precisely the planet radius using the photometric variations
during the transit, even with a low accuracy,
- the
considerable increase of the number of known giant planets, confirming the very
frequent occurrence of planetary systems. Table 2.1. counted only 10 entries;
the updated list, which can be found at http://www.obspm.fr/planets has to-day
28 entries.
Annex II: A
PROPOSAL FOR THE INTEGRATION OF SCIENTISTS OF ESA MEMBER STATES IN THE
COROT SCIENTIFIC COMMUNITY.
I- The organisation of the scientific programme
of COROT
It is presently organised as
follows:
- A core programme (CP), which is the basis of the
definition of the instrument and of the global mission. It addresses the two
scientific principal objectives, i.e. asteroseismology and search for
extrasolar planets.
It contains two modes
- the exploratory programme (sessions of 10 to 20 days)
- the central programme (sessions of 150 days)
It is under the responsibility
of the leaders of the working groups (see below II.2).
|
-
Additional programmes
(AP)
or serendipity programmes 1- Address different scientific
questions from the core programme but use the same data 2- Ask for different targets
and/or observing procedures from the core programme 2.1. In the same
fields of view 2.2. In different
fields of view |
The selection of the
additional programmes will be made by the Scientific Council
Authors of selected AP will
become Guest Investigators (see below II.3)
II- The organisation of the scientific
community
II.1 The Scientific Council (SC)
It is in charge of the Observing
Program (COP, for COROT Observing Programme), of the Data Distribution Policy
(CDDP) and of the publication policy. The COROT PI is the chairperson of the
SC. The other members of the SC are named Co-Is. The composition of the SC has
been agreed as follows :
- Principal Investigator
(PI)
- Instrument scientist
- Responsible of the
Seismology working group ¸
- Responsible of the
Exoplanet working group ˝(see below II.2)
- Responsible of the Ground
based observations working group ˛
- A representative of each
French major laboratory contributing to the mission : presently 7 (this number could be reduced to obtain a better
representation of the non-French colleagues)
- A representative of each
non-French laboratory or country contributing to the mission (payload / ground
segment / satellite / launch / operations) : presently Austria, Spain, ESA/SSD
; Italy and Belgium should enter this group.
- A representative of the
CNES scientific programme division.
We
propose to add one ESA representative to the Scientific Council.
The project manager and the
system engineer are permanently invited.
II.2. The working groups
Presently, three working
groups organise the scientific activities, before, during and after launch.
- The seismology group is in charge of preparing the
seismology programme.
It has defined several teams, responsible for different
scientific issues.
- The exoplanet group is in charge of preparing the
exoplanet programme.
- The ground based programme group organises all the
ground based observations necessary for the mission, before, during and after
the launch.
The creation of a fourth
working group in charge of the additional programmes will be discussed at the
next Scientific Council.
II.3. The Co-Is, AS and GIs
The Co-Is are the members of
the Scientific Council (see II.1). They are few; no more than 20 if possible.
The Associated scientists
(AS) are originally members of the different countries participating to COROT.
To work on COROT, an AS has to belong to a team of a working group (see II.2.)
under the responsibility of a Co-I.
We
propose to invite all the ESA member states to designate AS.
The Guest Investigators
(GIs) are scientists who have answered the Call for Opportunity for the
additional programmes and who have been selected by the Scientific Council.
We
propose that the A.O. release be co-ordinated by ESA.
III- Access to the data
The Scientific Council,
represented by the PI, is globally responsible for the Ground Segment and the
data distribution. The following timing is proposed:
After the end T0 of each
observation run (20 or 150 days) there will be 3 phases :
Phase 1 : 6 months devoted
to the instrumental corrections (end : T0+0.5y)
performed by the project
team, under the responsibility of the instrument scientist.
Phase 2 : 1 year of
restricted access for the Co-Is, AS and GIs (from T0+0.5y to T0+1.5y)
- For the core programme :
Every Co-I, i.e. member of
the Scientific Council, with a team of Associated Scientists (AS), will propose
a programme of data interpretation, and make a requirement to the SC for the
necessary data.
The Co-I will have the responsibility
of the data he has access to and of the publication of the results of the team
(the publication policy has not been more precisely defined yet).
-For the additional programmes, data will be delivered to
GIs directly at T0+0.5y.
Phase 3 : Public release in
two steps
- Phase 3.1: to the scientists of contributing countries
during one year (from T0 + 1.5 y to T0+2.5y)
- Phase 3.2: to the
scientists world-wide after T0+2.5y.
Annex III: A PROPOSAL FOR A POSSIBLE CONTRIBUTION OF ESA
TO THE COROT MISSION.
* Letter from R. Bonneville to R. Bonnet of January
19th 2000
* Description
of the proposed sub-systems:
-
optics of the telescope
-
Onboard software
-
Mechanical and thermal architecture