Download Looking inside stars and looking for planets

Meeting report
T
he RAS discussion meeting on “Stellar
seismology and extrasolar planet-finding
and the Eddington mission”, organized
by Alan Penny (Rutherford Appleton
Laboratory) and Ian Roxburgh (Queen Mary,
University of London), was held at the Scientific
Societies Lecture Theatre on 11 January 2002.
The morning session was chaired by Alan Penny
and opened with a general review of the
Eddington mission and then covered the science
of asteroseismology, with particular respect to
that possible with Eddington.
The first talk was by Fabio Favata (ESTEC),
the Eddington Study Scientist, who presented a
general review of Eddington. Eddington is a
high-precision, long-duration space photometry
mission in the ESA science programme. Two
key science goals are to provide the data needed
for an empirically based theory of stellar
structure and evolution, through asteroseismology, and to detect a significant sample of
extrasolar planets through transits. Eddington
will find planets with a wide range of characteristics, including Earth-like, habitable
worlds. The instrumental configuration is being
studied in detail, the favourite one being a multiple (three or four) set of 600 mm Schmidt
cameras with mosaic CCD cameras operating
in frame transfer mode, resulting in a highperformance mission.
Asteroseismology
The first of the talks devoted to asteroseismology was by Ian Roxburgh (Queen Mary,
University of London) who spoke on the use of
asteroseismology with Eddington. He first
described the principles of
asteroseismology – namely
that as the frequencies of
oscillation of a star are determined by its internal structure, so measurements of a set
of frequencies for a star
encode information on that
internal structure, primarily
on the variation of density
with radius inside the star. He
went on to describe some of the methods available to extract this information ranging from
the use of the small separations between frequencies of degree 0 and 2 to place a global
constraint on the interior structure, to detailed
inversion techniques based on the phase shift of
the oscillation modes, that is the departure of a
particular oscillation wave from a pure sine
wave. In these inversion techniques the
unknown structure of the surface layers of a
star is represented by a frequency-dependent
surface phase shift which is independent of
degree, while the central regions of the star produce a phase shift that depends on degree 1.
Matching the internal and external phase shifts
beneath the surface layers gives algorithms for
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Looking inside
stars and looking
for planets
Asteroseismology and extrasolar planets are the main science goals of the
Eddington mission, now approved by ESA for a 2007 launch. Alan Penny
presents a summary of the January 2002 RAS meeting that discussed the
sciences of this wide-field high-precision photometric space telescope.
Since the date of this meeting, ESA has decided to implement the mission
in the framework of a 2007–08 launch.
1: Sir Arthur Eddington, the
astronomer and physicist known
for his research on the motion,
internal constitution and
luminosity of stars, and in
whose memory ESA’s space
photometry mission is
named.
2: Artist’s impression of the Eddington
craft/satellite due for
launch in 2007–08
(ESA).
determining the variation of density with radius
in the bulk of the stellar interior. Three examples of such inversions were described, one for
the Sun using data obtained by the Birmingham
Oscillation Network, the others using artificial
data for stars of 0.8 and 1.45 M. He stressed
that with the data from the Eddington mission
we should be in a position to test the
predictions of the theory of stellar evolution and
to improve that theory on the basis of asteroseismic measurements.
Photometric studies
Next Don Kurtz (University of Central
Lancashire) discussed some current results in
asteroseismology, mostly from photometric
studies. PG 1159, a DOV star, is the current asteroseismic record holder with
101 identified modes; its mass is
known to a precision of ±0.003 M
and atmospheric stratification has
been detected. Studies of the
EC 14026 stars are important for
understanding extreme horizontal
branch evolution. One EC 14026
star, PG 1336-018, is in an eclipsing
binary where tomography during eclipse
is allowing a unique method of mode identification, and possibly has a tidally governed
pulsation axis. Study of the DAV star
BPM 37093 has the potential to test theories of
C–O crystallization in cool white-dwarf interiors. New theoretical understanding of the
interaction of pulsation, rotation and global
stellar magnetic fields are being tested in the
rapidly oscillating Ap stars. All photometric
studies of pulsating stars would be revolutionized by the one-to-two orders of magnitude
improvement in sensitivity over ground-based
observations that Eddington could achieve.
Model callibration
Douglas Gough (Institute of Astronomy,
Cambridge) addressed how accurately one can
calibrate a main-sequence model of one solar
mass, assuming radius and luminosity to be
known, using the Sun as an example. Perhaps
the most frequently used method for inferring
the internal properties of stars from Eddington
data will be to use seismic oscillation frequencies to calibrate stellar models. Such calibrations will best be carried out in conjunction
with other astronomical data, such as metallicity, position on the HR diagram – which
December 2002 Vol 43
Meeting report
determines a star’s radius – and, in the case of
binary systems, the mass. Given the uncertainty
in the structure of the outer boundary layer of
the convection zone, whose influence on the
oscillations must be eliminated by considering
only suitably chosen combinations of the data,
and given the fact that changes in the age and
the presumed initial helium abundance have a
rather similar influence on those data combinations, the precision with which one can ascertain the age is less than might naively be
suspected. Nevertheless, it will be far better
than any current method. Indeed, it will raise
our knowledge of the structure and evolution of
stars to a completely new level.
Michael Thompson (Imperial College of
Science Technology and Medicine) spoke on
asteroseismic inversion for solar-type stars.
These are, broadly speaking, F, G and K stars
on the main sequence, or subgiants, which are
expected to display turbulently excited modes:
a defining feature is the expected low amplitudes of pulsation. He described how experiments (including blind tests) to invert
frequencies of stellar models, with
the anticipated mode coverage and
noise properties from planned spaceborne observations, have been carried out to determine what could be
achieved with the expected data.
The sound speed profile throughout
most of the core of a solar-type star
was estimated to within a few percent with a SOLA-type inversion method, with
three months of data and assuming that l = 0–3,
mode frequencies were determined. The
recently reported observed spectra of oscillations in a Cen and b Hyd, which are qualitatively like the solar low-degree p-mode
spectrum, look very encouraging for the
prospects of such inference.
reflected light studies for transiting and nontransiting planets would give planet albedoes.
There would be Announcements of
Opportunity for the wider community to be
involved. The consequences of the selection of
the NASA Kepler planet transit search mission
were described.
Keith Horne (University of St Andrews) discussed Eddington planet catch simulations.
Eddington aims to detect transits of Earthanalogue planets 8 ×10–5 mag deep, lasting ~13
hours, with one year period. The baseline
design – a space telescope with area A ~ 0.6 m2
and CCD cameras observing a W ~ 10 deg2
starfield for t > 3 years – does the job. The
planet catch can be boosted at fixed cost by
using N co-aligned telescopes with smaller A
but wider W. For the nearby main-sequence
stars surveyed, the planet catch scales as
W (N A t)3/2, M–3/8 T3 r6, with M the star mass, T
and r the planet temperature and radius,
strongly favouring large, hot planets. For a
power-law planet mass function ∂n/∂m µm–1.5,
Monte Carlo simulations predict that baseline
and thereby provides a greater number of stars
at a given magnitude than does the optical
design used in Eddington. Because Kepler stares
at a single FOV throughout the mission, it does
much less asteroseismology than Eddington.
Other comparisons were also discussed.
Planets around variable stars
Suzanne Aigrain (Institute of Astronomy,
Cambridge) spoke about the impact of stellar
variability on planetary transit detection. The
variability of the parent star is a major source
of non-Gaussian noise. Characterizing it will
help to refine instrument design, develop data
preprocessing methods and improve target
selection for missions such as COROT,
Eddington and Kepler. By studying the power
spectrum of the solar irradiance data from
the VIRGO instrument on board SOHO, a
low-frequency component was identified,
well correlated with chromospheric activity
indicators, which can be filtered out using a
high-pass filter, but higher frequency granulation-related components remain a problem.
Filters based on knowledge of the
shape of the transit promise to curb
this problem but must be tested to
the limit. Future work will therefore
concentrate on producing realistic
simulated light curves for stars with
a range of ages, masses and luminosities, and on the potential use of
colour information.
The final talk of the day was by Richard
Nelson (Queen Mary, University of London)
who spoke about recent theoretical work on the
formation of terrestrial and gas-giant planets,
and summarized the likely impact of missions
such as Eddington and Kepler on planet formation theories. Computer simulations of terrestrial planet formation performed by George
Wetherill and co-workers provide modeldependent predications for the likely distribution of planetary masses, semi-major axes,
eccentricities, and orbital inclinations. The
detection of terrestrial planets via transit
searches could be compared with these model
predictions to constrain the initial conditions of
planet formation. In addition, Eddington and
Kepler will detect many giant planets and provide information about their orbital period distribution. This distribution may be used to
constrain models of planetary migration via
disc–planet interaction. Eddington and Kepler
will be able to detect planets in binary systems,
an area that is only just beginning to receive
significant theoretical attention. ●
“While Eddington and Kepler search
for ‘hot and habitable Earths’,
ground-based hourly monitoring
of ~1000 galactic microlensing
events can find ‘cool Earths’.”
Extrasolar planet hunting
The afternoon session was chaired by Ian
Roxburgh and covered the science of extrasolar
planets, as examined by Eddington. The first
talk was by Alan Penny (Rutherford Appleton
Laboratory), who spoke on the prospects for
planet finding. Some 3% of nearby stars have
Jupiter-mass objects orbiting within 3 AU, and
theory points to the formation of terrestrialmass planets. Eddington would be able to detect
planets of 0.8 Earth mass with orbits of less
than a year, and determine their frequency of
occurrence and distribution of masses, orbital
radii and inclinations and the presence of massive moons. In the main target field the stars
would be at some 500–1000 pc, and Eddington
would find many thousands of Jupiters, some
2000 of terrestrial mass, including a few hundred in the habitable zone, and a few tens like
the Earth. Additional planet data would come
from asteroseismology. Secondary eclipses and
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Eddington will find ~104 “hot Jupiters”, ~102
“hot Earths”, and ~10 “habitable Earths”. The
1% abundance of “hot Jupiters” is known from
Doppler wobble studies, but the abundance of
Earths is currently unknown. While Eddington
and Kepler search for “hot and habitable
Earths”, ground-based hourly monitoring of
~1000 galactic microlensing events can find
“cool Earths”. If both experiments are done, we
will be able to interpolate safely to estimate the
abundance and hence distances to the nearest
habitable Earths.
William Borucki (NASA Ames Research
Center), Kepler Principal Investigator, compared the Kepler and Eddington missions.
These are spaceborne photometric missions
with similar apertures, which are both capable
of finding Earth-size extrasolar planets and
both can detect p-mode oscillations in stars. The
Kepler mission is optimized to find Earth-size
planets in the habitability zone of Sun-like stars
and does asteroseismology only as incidental
science. The Eddington mission appears to be
optimized for asteroseismology. The Kepler
design provides a very large field of view, a low
measurement cadence, a heliocentric orbit, and
a long mission duration. The demand for a large
field-of-view results in a Schmidt design with a
massive corrector. However, the use of the corrector allows a 105 square degree field of view
Alan Penny, Rutherford Appleton Laboratory,
[email protected] (with thanks to all the
speakers who wrote the reports on their talks).
The Eddington website is sci.esa.int/home/
eddington/index.cfm.
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