Duncan Wright

The Hunt for Habitable-Zone Planets
with CYCLOPS on the AAT
D.J. Wright and C. G. Tinney
Department of Astrophysics, University of New South Wales, Sydney, NSW, 2052
focused purely on the characterization of known planets with well-constrained orbits. That
Abstract
is, a future TPF -style instrument could likely be realized sooner if the census of low-mass
Background
planets in the habitable zones around nearby stars was known.
The most viable candidate stars for finding Habitable zone planets using the radial velocity
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concept for a space telescope that would indirectly detect exoplanets through the
stars the
flux from the star is much lower than for higher mass stars and hence the habitable
astrometric perturbation of their host stars. The requisite astrometric precision for detecting
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defined
radii range
over
which liquidThe
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earth planets
aroundimaged
M Dwarf
stars using
CYCLOPS.
Finding Earth-like planets orbiting in the ‘habitable zone’ of other stars is one of the major
goals of modern astronomy. Detecting Earth-like planets, in Earth-like orbits around Sunlike stars is currently beyond reach of any facility except Kepler (and is problematic even
then). What is possible now, is the detection of Earth-like planets in habitable-zone orbits
around low-mass stars using the radial velocity technique. Early indications from both
Doppler planet searches and Kepler are that rocky planets around low-mass stars (<1 M¤)
are common (20-30%). We are initiating a search, sensitive to low-mass planets orbiting in
the habitable zones of M4-M6 dwarf stars. This search will utilize a new high-precision arclamp wavelength calibration method developed for the CYCLOPS fibre-feed to the UCLES
spectrograph.
Table 1 – Orbital Properties of a 5MEarth Habitable Zone Planet Orbiting an M Dwarf
Host M Dwarf
Semi-major Axis
Period
Doppler Amplitude
Mass (M¤)
Sp. Type
Inner
Outer
Inner
Outer
Inner (m s-1)
Outer (m s-1)
0.1
M6
0.021 AU
0.061 AU
3.5 d
18 d
9.8
5.7
0.3
M3
0.09 AU
0.21 AU
4.5 d
70 d
4.4
1.8
Expected RV Amplitudes
Searching for low mass (rocky) planets around the lowest mass stars (late M Dwarfs) has two
very significant advantages over larger stars:
1 – The Doppler signature of any resident planets scales inversely with the mass of the star.
M4-M6 dwarfs are 0.3-0.1 M¤ hence Doppler amplitudes of planets are 3-10 times larger
than for the same planet around a solar-type star.
2 – M Dwarfs have low luminosity, so the lower flux they deliver to a planetary surface
means that the orbital radius corresponding to the location of the habitable zone is much
smaller. This makes the Doppler amplitudes larger again (e.g. Fig. 1).
Figure 2.1 The theoretical continuous (> 5 Gyr) habitable zone as a function of stellar mass (shaded region).
different shading areas correspond to various assumptions on cloudiness and the efficiency of the
FigureThe
1:
The theoretical habitable zone (shaded region) as a function of stellar mass (from
runaway greenhouse. The dotted lines delineate the extreme outer edges of the habitable zone. Planets
Selsis inward
et al. of2007).
Venus,
Earth,
andlocked
Mars
arethan
indicated
for Earth,
the Sun.
Theareorbital
the dashed
line become
tidally
in less
1 Gyr. Venus,
and Mars
indicatedpositions
for
the Sun. sizes
The orbital
positions
relative
sizes ofdiscovered
the first three planets
discovered
orbiting
M dwarf GJ
and relative
of the
firstand
three
planets
orbiting
the 0.3
M¤ the
M-dwarf
GJ 581
581 (Udry et al. 2007) are also shown. The corresponding orbital periods of habitable-zone planets around
(Udry stars
et al.
shown
for
inward
dashed
with2007)
masses are
between
0.1 and
0.2comparison.
M! are between Planets
approximately
4.5 andof
70 the
d. These
planetsline
havebecome
a
probability
transiting
for atmospheric
with upcoming
tidallyhigh
locked
in of
less
thanand
1 would
Gyr be
– excellent
thoughtargets
planets
with an studies
atmosphere
are facilities
nonetheless
like JWST. Figure from Selsis et al. 2007.
Table 1 demonstrates why M-dwarfs make such compelling targets for habitable zone
searches – such a planet will orbit a 0.3 M¤ star in periods of just 4.5 to 70d. For an even
smaller 0.1M¤ star they orbit in just 3.5-18d. These short orbital periods make such planets
far more detectable – a 5MEarth planet orbiting a M-dwarf with mass between 0.1-0.3M¤
delivers a Doppler amplitudes of 1.8-9.8 m s-1.
expected to host regions capable of supporting liquid water (e.g. Wordsworth et al. 2011).
The second track for studying habitable planets endorsed by the 2010 Decadal Report is
focused on the opportunity offered by low-mass M dwarfs. In contrast to solar-type stars,
UCLES + CYCLOPS on the AAT
CYCLOPS is a single object Integral field unit made up of 15 close-packed 0.6” hexagonal fibres (only 12 functioning) that are arranged to have a ~2.5”
diameter on the sky (see Figure 2). The fibres are reformatted to make a pseudo slit that injects light into UCLES at resolution ~70000. Each of the 12 fibres
produces it’s own spectrum that is extracted.
To be capable of detecting the <10 m s-1 Doppler amplitudes expected from habitable zone planets around M Dwarfs we need to be able to calibrate the
UCLES spectrograph to < 2 m s-1. This is possible with CYCLOPS due to the tremendous amount of position information available when we take a
calibration Thorium-Argon (ThAr) image because of the 12 independent fibre spectra present. By extracting all the ThAr spectra from a reference image
and those from another ThAr image (Figure 3-a) we can measure the changes in position of the emission lines precisely by fitting a Gaussian to the crosscorrelation of a section of the two spectra (Figure 3-b and c). By doing this for different sections of ThAr spectra over the image wecan build up information
on the spatial dependence of the positions of the ThAr lines (Figure 3-d). Finally the spatial dependence of the ThAr line shifts can be fitted with a
polynomial surface of order 3-by-3, an examination of the residuals (e.g. Figure 3-e) presents a generalised error distribution (an exponential power
function), for the example in Figure 3 this indicates an overall precision of 0.86 m s-1. A similar process is used to find the RV change of a stellar
observation, though there is insufficient room on this poster for a detailed explanation.
250
2.5
0.4
500
Figure 2: The CYCLOPS fibre bundle on-sky
layout.
3.45
200
ref. ThAr
0.35
3.4
Stellar Calib. ThAr
400
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Pixel
400
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0.25
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Crossïcorrelation value
350
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300
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3.25
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(a)  A gray-scale ThAr image. Each
fibre produces it’s own separately
extracted ThAr spectrum.
0
800
100
3.05
1500
50
1000
150
50
0.5
100
900
3.3
3.1
0.1
800
200
3.35
# of pieces of ThAr spectrum
450
Extracted ThAr shift (pixel)
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ï15
1200
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(b) Section of 400 pixels from one
fibre spectrum from a reference
ThAr (red) and a stellar calibration
ThAr (blue).
1000
Pixel
crossïcorrelation shift (pixel)
Position (extracted pixel)
0
(c) Cross-correlation of ThAr
spectrum pieces (blue +’s) and
Gaussian fit (red).
2000
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0
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Residual ThAr position (km sï1)
Pixel
(d) 3D plot of shifts from crosscorrelations of pieces all over the
image (blue +’s) and a polynomial
surface fit (red +’s).
(e) Histogram of the residuals to
the 3D fit in m s-1.
Figure 3: Figures a – e demonstrate how to calibrate the CYCLOPS + UCLES spectrum to high precision using a Thorium-Argon emission arc lamp. In this example we achieve an overall precision of <1ms-1.
Working in the Infra-red
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The M Dwarf stars that will be observed are nearby, but
are intrinsically faint. Most M dwarfs are V > 9 mag.
They emit most of their flux in the Infra-red and tend to
be ~2 magnitudes brighter in the I band. The M Dwarfs
are cool, and so have a molecular absorption ‘forest’ in
the infra red which can provide a large amount of RV
information. To try to maximise our RV precision we can
observe with UCLES+CYCLOPS in the Infra-red,
however the ThAr lamp used for calibration has several
bright Argon lines in that wavelength region that make
calibration difficult (see Figure 4 left). To try to overcome
this obstacle we have tried other calibration lamps
including Thorium-Krypton and Thorium-Xenon.
Example images are shown alongside the ThAr images in
Figure 4. It is clear the Thorium-Xenon lamp will offer
superior infra-red calibration and so shall be used
throughout our M Dwarf planet search.
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Figure 4: Left: A 120s Thorium-Argon arc lamp exposure. Middle: The same for a Thorium-Krypton lamp. Right: The same for a Thorium-Xenon
lamp. All images are on the sam color-scale shown on the right. Note the saturating lines in the top of the left and middle images, the top is the infra-red
part of the spectrum and these saturating lines make calibration difficult.