SMEI: A CURRENTLY OPERATIONAL ALL SKY EXTRASOLAR PLANET

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SMEI A CURRENTLY OPERATIONAL ALL SKY EXTRASOLAR
PLANET TRANSIT SEARCH MISSION
Steve Spreckley Ian Stevens
We are currently working with data from a
spaceborne all sky photometric monitor to try
and detect planetary transits around nearby
stars. Using an instrument with an effective
cadence of 100 minutes and a total mission
lifetime of 3 years (2 of which have already been
completed) we are able to build up lightcurves of
over 34,000 stars with I brighter than 8th mag.
Early results suggest that we will be able to
achieve the required photometric accuracy to
detect transits for over 70 of these
lightcurves, i.e., for 23,000 stars.
The Instrument
Unique data set
We are using the Solar Mass Ejection Imager, a
space based all sky photometric satellite in an
800km Sun Synchronous polar orbit to obtain high
precision photometry of over 34,000 stars with I
lt 8. The instrument, designed to observe CMEs
from the Sun, consists of three CCD devices that
each view a field that is 60o x 3o and are
arranged on the Coriolis spacecraft (shown below
left, with the cameras circled in red) in such a
way that they essentially view an arc of the sky
170o x 3o in total which sweeps over almost the
entire sky in a single orbit. The imaging system
has a cadence of four seconds but for a given
star we combine the images from a single orbit to
produce a system with an effective exposure time
of 16 to 20 seconds and a cadence of 100 minutes.

• A number of key features of the SMEI mission
  offer us a number of unique opportunities in
  terms of both stellar photometry and in terms of
  planetary transit detection
• 100 sky coverage
• 3 year mission lifetime
• Near continuous coverage of a single star for
  over 200 days per year
• A broad CCD response peaking in the R band,
  ideal for observing G, K, M stars

These features enable us to produce very long
timescale lightcurves in which we can monitor
known stellar phenomena, and eclipsing binary
systems, but also find new long period phenomena,
new eclipsing binaries, and very importantly it
offers us an opportunity to find transiting
extrasolar planets with moderate orbital periods.
Left The Coriolis spacecraft with the SMEI
cameras highlighted. Right A series of
consecutive SMEI frames taken every 4 seconds,
showing stars traverse the CCD. Below An all sky
image produced by SMEI.
Moderate period transiting planets
There are an increasing number of unanswered
questions in the field of extrasolar planets, one
of which is whether there exists a period valley
in the 10 100 day period regime (e.g. Udry et
al., 2003). Depending on how a period
distribution plot is binned one can argue that
the paucity of planets in this region is a real
phenomenon, or is just due to a convenient choice
of bin size. Another question lies in where the
transition in the apparent high mass planet
regime in long period orbits to the low mass
planets in short period orbits takes place.
With SMEI we may be able to detect transiting
planets in the 10 100 day period regime, which
is crucial to answering these questions.
Searching for periodic signals in SMEI data
Our preliminary lightcurves are encouraging, and
suggest we could be able to detect transit like
features in around 23,000 lightcurves. Early
lightcurve production has focussed on the very
brightest stars in the sky e.g. Canopus. In the
preliminary data the Canopus lightcurve exhibits
a noise level of ? 0.006 mags, well below the
required noise threshold for Hot Jupiter
transiting planets to be found. We estimate that
in order for the signal of a transiting Hot
Jupiter to be detected we require a signal to
noise ratio (where the signal is the transit
depth) greater than 0.75. A Hot Jupiter planet
causes a dip of typically 0.01 mags and therefore
the noise level needs to be lower than 0.013
magnitudes.
The main goal of the project is to find
transiting planet signatures, but a large number
of eclipsing binary systems, and other periodic
signals in lightcurves will also be found.
Preliminary lightcurves from SMEI include those
of the Lambda Tau and Algol eclipsing binary
systems. We aim to produce a pipeline that
automatically characterises different types of
periodic phenomena, and as well as studying
these, we can then remove long period trends from
lightcurves that may contain transits.
Algol
Canopus
Lambda Tau
Based on the success of box fitting methods for
searching for planetary transit signals we
decided to try such a method in our search. In
order to test the effectiveness of such period
searching algorithms for this project we
initially used synthetic data consisting of white
Gaussian noise with a transit signal implanted
within it and we varied the period and duration
of the transit signal in different lightcurves.
However, now the first lightcurves are being
produced we are implanting transit signatures
into them and analyzing the outcome. For example
we implanted a 0.01 magnitude variation at a
period of 3 days in the Canopus lightcurve and
used an algorithm based on the method described
in Kovacs (2002) to search for periodic signals.
The resulting power spectrum is shown left, and
there is a clear detection peak at the correct
period, therefore we will use a box fitting
method when we perform the full scale analysis of
the lightcurves.
The phase folded lightcurves of the eclipsing
binary systems Lambda Tau (above left) and Algol
(above right) clearly display the primary dip in
both cases and also the secondary dip is visible
in both but is more clear in the Lambda Tau
lightcurve . A view of a portion of the Algol
light curve over a single eclipse (right) reveals
that over a short timescale the photometry is
very stable. This suggests that with the use of
relative photometry, and with improved photometry
techniques overall, the point to point noise in
our light curves can be reduced significantly.
Future Work includes

• Characterize star PSF as a function of colour
  and magnitude
• Devise a more effective background removal
  system
• Fully automate data reduction pipeline

References
Davies S.R., 1990, MNRAS, 244, 93
Using an L-statistic test (Davies, 1990) to
search for periodic signals in the lightcure of
Lambda Tau, provides a confident detection of the
correct binary period (3.95 days) of the system
(right). This is one method that may be used in
the automated characterisation of lightcurves
before we search them for transiting planet
signals.
Kovacs G., Zucker S., and Mazeh T., 2002, AA,
391, 369
Udry S., Mayor M., and Santos N.C., 2003, AA,
407, 369