Transcript No Slide Title

Microlensing Studies in Crowded Fields
Craig Mackay,
Institute of Astronomy, University of Cambridge.
7 May 2014: Cambridge Exoplanets Group
Introduction and Outline
• We have very little idea about the intrinsic mass function of
objects formed when planetary systems come into being.
• Virtually all detected Exoplanets are much larger than the
Earth orbiting relatively close to the host star, so estimates of
the numbers of Earth size planets have large error bars.
• Essentially the only technique that allows us to detect
significant numbers of Earth size planets is gravitational
microlensing.
7 May 2014: Cambridge Exoplanets Group
Introduction and Outline
• Will start by summarising the constraints we must work with
in order to detect good numbers of Earth-size planets.
• We need to maximise the number of potential targets and
develop the right technologies to observe them reliably and
consistently.
• Will describe an instrument that may well be able to deliver
the performance that we need relatively easily, relatively
quickly and without being impossibly expensive!
• Finally will look at some advanced technologies that could
give even better performance, and discuss the problems of
operating such an instrument.
7 May 2014: Cambridge Exoplanets Group
What is the problem we have?
• Gravitational microlensing is the only technique that appears to
have the potential to detect good numbers of Earth-size planets.
• These events are rare so that large numbers of stars need to be
observed. We use Baade’s window in the bulge of the Milky Way.
• Only main sequence stars will interest us: giant stars are too big to
give the microlensing effect we seek.
What is the problem we have?
• This means looking at stars with I>18 mag.
• The brighter giant stars (~14-16 mag)
make large parts difficult to observe.
• This is part of Baade’s Window
with HST.
• About 0.6% of the area shown!
•
From Holtzman et al,
ApJ, 115,1946 (1998)
• In the range I ~ 18-22mag
(MI=3-7), star surface
densities are ~1/sq arcsec.
• Fainter stars also lens well.
Ten times as many to I<24.5
• Over a 30x30 arcmin field
there are ~few million stars.
• With better resolution we can
see these stars more easily.
• Ground-based seeing is much
poorer so large amounts of
time are needed to observe a
field.
What is the problem we
have?
•
From Holtzman et al, ApJ,
115,1946 (1998)
7 May 2014: Cambridge Exoplanets Group
• These images are of the
bulge, microlensing event
MACHO-96-BLG-5.
• The difference in image
quality is dramatic.
• The top images are
ground-based with 1 arcsec
seeing.
• Most current surveys use
even poorer images.
• The lower images are from
HST (left-hand is single
frame, right-hand is
multiframe drizzled).
What is the problem we
have?
•
From Bennett, arXiv:
astro-ph/0404075v1, 2004
7 May 2014: Cambridge Exoplanets Group
The Photon Budget Is Also a Problem
• On a 4 m telescope, a star with I~22 will give 5-20 detected
photons per second depending on band pass (achromatic).
• Sharper PSF gives a much fainter background per star image.
• A 2% (10%) planet lensing
effect will require an SNR of
150(30) which will take 40
(1.5) minutes neglecting sky
background (can’t do this).
• SNR much better than this
because lensing brightens
source, often by large factor.
• This is only possible if we
have much better seeing than
~1 arcsec.
And We Need Good Time Resolution!
• We cannot assume that high speed, high resolution follow-up will
be available elsewhere.
• The microlensing signature from an earth-mass event may only
last for a few hours or less.
• We need to be able to carry out accurate photometry every few
minutes in order to get the most information from the event.
• That photometry must be done at the full resolution of the
instrument to avoid confusion/contamination in crowded fields.
•
From Bennett,
arXiv:0911.2703v2 (2010)
12 February 2013: AstroDoha_2013
Sensitivity to Earth and Moon mass objects!
• OGLE-2005-BLG-390 was a star ~9.6 Rsun and planet of ~5Mearth.
• An Earth mass planet at the same location around that giant star
would have given a ~3% deviation.
• A Moon mass object around a Sun like star would have given ~1%
deviation lasting over only one hour.
• This kind of deviation will be relatively easy to detect with a large
ground-based telescope.
12 February 2013: AstroDoha_2013
Achieving Higher Resolution
• How do we achieve this?
• HST time is far too valuable and the instruments have much
too small a field of view.
• Space projects cost $zillions, and take for ever!
• Ground-based survey telescopes have relatively poor point
spread functions so take much longer to survey a given
number of targets.
• Adaptive optics can give excellent resolution but only over a
tiny field of view (a few arcseconds in the visible).
• The only method that can routinely give Hubble resolution
from the ground is Lucky Imaging.
7 May 2014: Cambridge Exoplanets Group
Lucky Imaging in the Visible.
• Technique originally suggested by Hufnagel (1966) and
developed by Fried (1978).
• Images taken fast enough to freeze the motion due to turbulence.
• On a 2.5 m telescope (the NOT) in I band, on a good site (LPO),
under typical conditions 10-30% of images are ~ diffraction
limited at 20 frames per sec.
• The best images are selected and combined to give a neardiffraction limited image.
• The isoplanatic patch size is much larger than with AO, typically
~60-120 arcsec rather than ~3-5 arcsec diameter.
• The key enabling technology are electron multiplying CCDs.
7 May 2014: Cambridge Exoplanets Group
The Einstein Cross
• The image on the left is from the Hubble Space Telescope Advanced Camera for Surveys
(ACS) while the image on the right is the lucky image taken on the NOT in July 2009 through
significant amounts of dust.
• The central slightly fuzzy object is the core of the nearby Zwicky galaxy, ZW 2237+030
that gives four gravitationally lensed images of a distant quasar at redshift of 1.7
7 May 2014: Cambridge Exoplanets Group
Amateurs Are Enthusiastic about Lucky Imaging
• Image of the
International Space
Station, with Space
Shuttle Atlantis &
Soyuz, June 2007.
• Resolution ~20 cm at
an altitude of 330 km
altitude, or ~ 0.12
arcsec.
• Downward looking
resolution is much
better, ~20 milliarcsecs
or ~ 2 cm.
7 May 2014: Cambridge Exoplanets Group
GravityCam
• The EMCCDs used for Lucky Imaging are relatively small:
1024 x 1024 pixels typically although bigger ones could be
made if funds allowed it.
• The CCDs cannot be butted but could be mosaiced to cover a
large field of view.
• Working with astronomers in the UK and Switzerland we
have been looking at an instrument that would use lucky
imaging for weak gravitational shear studies.
• It turns out that the instrument is almost exactly what might
be needed for gravitational microlensing detection of Earthlike planets.
7 May 2014: Cambridge Exoplanets Group
GravityCam
• The two applications of the same instrument would be highly
complimentary since microlensing is done towards the
Galactic Centre and weak gravitational shear is done as far
away from the Galactic
• The next problem is large telescope characteristics and their
availability on a top quality site.
• Most telescopes > 2.5 m diameter have small fields of view.
• The NTT (3.6 m at La Silla in Chile) has excellent optical
performance and a field of view of 30 arcminutes diameter.
• We have looked at how such an instrument might look and
how it would perform.
7 May 2014: Cambridge Exoplanets Group
• The focal plane is covered with
an array of independent
EMCCDs each using lucky
imaging.
• 100 of these EMCCDs would
cover the focal plane with 70
milliarcsec pixels.
• An entire field of 30 arcminute
diameter would be covered in
six pointings.
• With 10 minutes on each
pointing a 30`x30`field would
be fully covered in ~ one hour.
• Other strategies are possible.
GravityCam
7 May 2014: Cambridge Exoplanets Group
• The instrument would mount directly on
the Naysmith focus of the NTT.
• The NTT has excellent optical
performance and space needed for such
an instrument.
GravityCam
12 February 2013: AstroDoha_2013
GravityCam
• The lucky images would not be diffraction limited, but deliver
images of 0.2-0.35 arcseconds FWHM using at least 50% of the
images and often up to 100%, seeing dependent.
• Would allow ~20 times as many stars to be observed.
• Detailed simulations of the performance have been carried out.
• Optically very simple: atmospheric dispersion corrector then
directly onto focal plane array of CCDs.
• We have already used a simpler version of our LuckyCam very
successfully on the NTT.
• Virtually all the technologies needed have already been
demonstrated and the cost need not be excessive ($4-8 million).
• Should not take too long to build (3 years: software main
challenge).
7 May 2014: Cambridge Exoplanets Group
GravityCam: Predicted Performance
• In a field such as Baade's Window, a single pointing of
GravityCam will see approximately 0.5x106 stars with I<22.
• With six pointings, around 3x106 stars in one hour.
• The I=22 stars will have ~10,000 photons detected in 10 min,
so with SNR ~ 80-100 after sky background correction.
• With an optical depth of ~2.4x10-6 (MACHO & OGLE
estimates) we expect to see a few new events every hour.
• Can survey at least six fields (so ~2x107 stars) per night so
should be able to detect ~20-50 new microlensing events/night.
• The planetary detection probabilities from Bennett & Rhie
(ApJ,472,660,1996) suggest we should detect a good number
of Earth size planets.
7 May 2014: Cambridge Exoplanets Group
GravityCam: Advanced Technologies
• There are some advantages in using faster
frame rate detectors if they were available.
• A new generation of solid-state detectors,
scientific CMOS (sCMOS) are beginning to
be used, with over 100 Hz frame rate.
• With massively parallel outputs and a
buttable package, much higher focal plane
coverage is possible.
• Could double DQE, and increase filling
factor from 0.17 to about 0.75.
• Increase star imaging rates x6 or so.
• Upgrade would cost ~$2-3 million, but take
2-3 years to design detectors.
7 May 2014: Cambridge Exoplanets Group
GravityCam: Advanced Technologies
• Data management will be interesting!
• Each EMCCD will generate approximately
50 MB per second (at 25 Hz frame rate),
and each sCMOS could produce over 400
MB per second (at 100 Hz frame rate).
• Total 150 Tb (CMOS 1200 Tb) per night!
• This would need dedicated processing
hardware such as the Nvidia GPUs with
direct data transfer.
• The lucky imaging process could be
significantly improved using techniques
described by Bramich (MNRAS,
1210.2926, Difference Image Analysis).
• Nvidia Kepler core
PCI-e card.
• ~4 Tflops.
• 250 GB/sec memory
bandwidth.
• 2688 CPU cores.
7 May 2014: Cambridge Exoplanets Group
Other applications of GravityCam.
• Study dark matter via weak lensing regime in galaxy clusters
(high spatial resolution of distortions, wide field coverage of
clusters).
• Asteroseismology with many bright stars in one pointing.
• Variable stars in crowded fields (galactic bulge, clusters,
Magellanic clouds), including sensitivity to very short period
variability.
• Wide field multi-filter high-resolution imaging for many
purposes (e.g. stellar populations).
• Occultation of stars by small solar system bodies. Access to the
small end of the Kuiper Belt size distribution and first detection
of true Oort cloud objects via serendipitous occultations.
7 May 2014: Cambridge Exoplanets Group
Conclusions.
• Lucky Imaging offers a relatively inexpensive way of
improving the seeing on ground-based telescopes by a factor of
3-5 (9-25 by area) under most conditions on a good site.
• It allows the observation of much higher density star fields, and
to fainter limits.
• Main sequence stars towards the Galactic Centre may then be
surveyed in very large numbers.
• GravityCam provides a model of a relatively simple instrument
development programme.
• By observing ~2x107 (I<22) and seeing lensing at A>10 on
~2x108 (I<24.5) stars per night should allows new Earth size
planet to be detected much more frequently.
7 May 2014: Cambridge Exoplanets Group
Lucky Imaging Group
Institute of Astronomy
University of Cambridge, UK
[email protected]
7 May 2014: Cambridge Exoplanets Group