EX5. P98. EPSC2013-331
Exoplanet detection capability of microlensing observations
Sergei Ipatov1, Keith Horne2, Khalid Alsubai3, Dan Bramich4, Martin Dominik2,*, Markus Hundertmark2,
2
5
6
6
Christine Liebig , Colin Snodgrass , Rachel Street , Yiannis Tsapras
1) Alsubai Est. for Scientific Studies, Doha, Qatar; 2) Univ. of St. Andrews, St. Andrews, Scotland, United Kingdom; 3) Qatar Foundation, Doha, Qatar; 4) European Southern Observatory, Garching bei
München, Germany; 5) Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany; 6) Las Cumbres Observatory Global Telescope Network, Santa Barbara, USA. * Royal Society
University Research Fellow. †supported by Qatar National Research Fund (QNRF), member of Qatar Foundation (grant NPRP 09-476-1-078)
Abstract
We summarize the status of a computer simulator for microlens planet surveys. The simulator generates
synthetic light curves of microlensing events observed with specified networks of telescopes over specified
periods of time. The main purpose is to assess the impact on planet detection capabilities of different
observing strategies, and different telescope resources, and to quantify the planet detection efficiency of
our actual observing network, so that we can use the observations to constrain planet abundance
distributions.
We have developed models for sky brightness and seeing, calibrated by fitting to data from the OGLE
survey and RoboNet observations in 2011. Time intervals during which events are observable are identified
by accounting for positions of the Sun, the Moon and other restrictions on telescope pointing. Simulated
observations are then generated for an algorithm that adjusts target priorities in real time with the aim of
maximizing planet detection zone area summed over all the available events. The exoplanet detection
capability of microlensing observations is compared for several telescopes.
Maximizing planet detection zone area
Seeing (in arcsec) vs. air mass. FTS observations of 39 events. A thick straight line is based on χ2 optimization (y= so +
s1(x-1), so =1.334, s1=0.519). Thinner straight lines differ from this line by +/- Ϭ (Ϭ=0.367). Non-straight lines show
mean and median values (the line for the mean value is thicker).
Microlensing is unique in its sensitivity to wider-orbit (i.e. cool) planetary-mass bodies.
Based on the approach presented in [1], at each time step for different events we calculate the detection
zone area and the probability of detection of an exoplanet. The event with a maximum probability at a time
step is chosen for observations.
We define the ‘detection zone’ as the region on the lens plane (x,y) where the light curve anomaly
δ(t,x,y,q) is large enough to be detected by the observations (q is the ratio of the planet to that of the star).
The photometric S/N (signal to noise) ratio and hence the area w of an isolated planet detection zone
scales as the square root of the exposure time : S/N = (Δt /τ) 1/2 , w = g Δt 1/2 .
Here τ is the exposure time required to reach S/N=1. The 'goodness' gi of an available target depends on
the target's brightness and magnification, the telescope and detector characteristics, and observing
conditions (air mass, sky brightness, seeing).
See [1] for details. [1] Horne K., Snodgrass C., Tsapras Y., MNRAS, 2009, v. 396, 2087-2102.
Sky brightness residuals vs. solar elevation
The influence of solar elevation θSun on sky brightness began to play a role at θSun>-14o, and was considerable at
θSun>-7o. For example, if we consider only FTS observations with the Moon below the horizon, then sky brightness
residual sbr can be up to -3 mag at -8o<θSun<-7o, sbr>-1 mag at θSun<-8o, and sbr>-0.4 mag at θSun<-14o.
Light curves for events selected for observations
Telescopes (with numbers Nt from 1 to
13) considered:
•1. 2m FTS - Faulkes Telescope South - Siding
Springs, Australia.
•2. 2m FTN - Faulkes Telescope North Haleakela, Hawaii.
•3. 2m LT - Liverpool Telescope - La Palma,
Canary Islands.
•4. 1.3m OGLE - The Optical Gravitational
Lensing Experiment - Las Campanas, Chile
•5-7. Three 1m CTIO - Cerro Tololo InterAmerican Observatory in Chile .
•8. 1m MDO - McDonald observatory in Texas.
•9-11. Three 1m SAAO - South African
Astronomical Observatory.
•12-13. Two 1m SSO - Siding Spring
Observatory near Coonabarabran, New South
Wales, Australia.
Time intervals for events selected for observations
with OGLE (at actual times of peaks of light curves).
Considered events: 1110001-111562.
Comparison of the efficiency of telescopes for microlensing observations
Our simulator suggests what events it is better to observe at specific time intervals with a specific telescope in order
to increase the probability of finding new exoplanets using microlensing observations. For estimates of the
probability, for best events we considered wsum=∑ gi[(Δt+tdone)1/2-tdone1/2] (where Δt=2tslew for an event observed at a
current time, and Δt=tslew and tdone=0 for other events) and rwsumt=(wsum/wsumOGLE)×(tsum/tsumOGLE), where tsum is the total
time during considered time interval when it is possible to observe at least one event. For best events we also calculated
wsumo=∑gi[(ts+tdone)1/2-tdone1/2] and rwsumto=(wsumo/wsumoOGLE)×(tsum/tsumOGLE) with ts=20 s. The value of tsum/tsumOGLE is
about 0.65, 0.55, and 0.52 for FTN, LT, and MDO, respectively. For other considered sites, it is close to 1.
In our calculations, rwsumt and rwsumto for observations with a 1-m telescope (located at CTIO, SAAO, SSO, or MDO)
equipped with the Sinistro CCD, and with a 2-m telescope (FTS, FTN, or LT) were mainly about 0.8-1.2 and 1.4-2.2 of
that for OGLE, respectively. The above ratio of probabilities is different for different considered events, and can be
outside the above intervals (see the plot below). The ratio of wsum for FTS and SSO located at the same site usually was
about 2.
For the SBIG CCD, the values of wsum (and wsumo) were smaller by a factor of ~1.2 than those for the Sinistro CCD.
In the case when 1-m telescopes located at the same site observe different events at the same time, the values of wsum
usually are relatively close for different telescopes if there are no high light curve peaks during (or close to) the
considered time intervals, and wsum can differ much for the telescopes in the case of the high peaks. In the latter case, it
may be better to observe the same event with all telescopes, even though in this case a detection zone area grows as a
square root of an effective area of telescopes (i.e., sqrt of the number of 1-m telescopes used). The value of wsum was
typically proportional to the diameter of the mirror. The difference in wsum is about 5% if for SSO we use the values of
Isky(0) and the dependence of seeing vs. air mass as those for OGLE, compared to those for FTS.
The values of wsum obtained at our approach are close to those obtained for the choice of events selected for
observations according to [3] Dominik et al., Astron. Nachr, 2010, v. 331, No 7, 671-691.
If only events with the peak value of the magnification Amax>50 (4% of events) are observed, then the ratio r50 of the
value of wsum for such observations to the value of wsum for the observations allowed for all 1562 events can differ by an
order of magnitude for different considered time intervals. For example, for OGLE r50 equals 0.07 and 0.99 for 5-day
intervals starting from April 22 and August 1, 2011, respectively; for 100-day intervals, r50 is 0.63 and 0.83,
respectively.
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•Observations analyzed for construction of sky model:
•For studies of sky brightness for FTS, FTN, and LT, we considered those events observed in 2011 for which
.dat files are greater than 1 kbt (i.e., the minimum number of lightcurve data points is greater than 15): FTS 39 events; FTN - 19 events, LT – 20 events. For OGLE we considered 20 events (110251-110270).
•Calculations of Isky(0) (sky brightness at zenith) and the coefficients (k1 and ko) presented
in the tables and on the plots were based on χ2 optimization of the straight line fit (y=k1·x+ko, χ2=∑[(yi-k1·xiko)/σi]2, σi2 is variance). The value of Isky(0) was chosen in such a way that the sum of squares of differences
between observational and model sky brightness magnitudes were minimum in the case when the Moon is
below the horizon. The used sky model was based mainly on [2] K. Krisciunas & B. Schaefer, PASP, 1991,
v. 103, 1033-1039.
Dependences of seeing on air mass and values of sky brightness at zenith obtained based
on analysis of observations
Values Isky(0) of sky brightness at zenith
(I magnitude per square arcsec) for an
extinction coefficient extmag=0.05 (for
extmag equal to 0 and 0.1, values of Isky(0)
differed by less than 0.3%). Seeing
(FWHM in arcsec) vs. airmass (χ2
approximation):
seeing= so +s1×(airmass-1)
Light curves (with error bars) for events selected for
observations with OGLE (at actual times of peaks of
light curves). Considered events: 110001-111562.
Telescope FTS
FTN
LT
OGLE
19.0
18.7
19.6
18.1
so
1.33
0.68
1.35
1.33
s1
0.52
0.21
0.42
0.29
sigma
0.37
0.21
0.50
0.25
Isky(0)
.
Sky brightness (mag) vs. air mass for the Moon below the horizon. Different points are for OGLE observations
of 20 different events. The lines are for the χ2 optimization (b=b1·a+bo) with different bo (different values for
different events) and the same b1. The most solid line is for the model for which bo is the same for all events. For
the Moon below the horizon, the values of bo (which characterize sky zenith brightness near different events)
differ typically by not more that 1 mag.
Files with the poster can be found on http://star-www.st-and.ac.uk/~si8/epsc2013.ppt (also .pdf,
this 1-page poster) and http://star-www.st-and.ac.uk/~si8/epsc2013sl.ppt (several A4 pages with
a longer version). See also http://arxiv.org/abs/1308.6159. Contact: siipatov@hotmail.com.
The values of rwsumt=(wsum/wsumOGLE)/(tsum/tsumOGLE), which
characterize the exoplanet detection capability of observations (in
the case of 1562 events available for observations), vs. the number
Nt of a telescope in the case when 1 m telescopes (equipped with
the Sinistro CCD) located at the same site observe different events
at the same time. Black or red crests and ellipses are for the 100day time interval beginning from April 22 and August 1, 2011,
respectively. The signs for calculations with actual values of t0 (the
time corresponding to the peak of a light curve) and
with random values of t0 (t0 = RNDM∙(tmx+2tE )-tE+to, where tE is the
time scale equal to the ratio of the angular Einstein radius to the
relative proper motion, RNDM is a random value between 0 and 1,
tmx is the duration of the considered time interval, to is the
beginning of the interval) are black greater and red smaller,
respectively. Green crests are for actual values of t0 and 90-day
interval beginning from April 22, 2011. Small signs are for nonpriority telescopes. For random values of t0, the number of light
curve peaks was greater.