Transcript α Cen A + iodine cell spectrum - Department of Physics and Astronomy

Can we find Earth-mass habitable
planets orbiting our nearest star,
α Centauri?
John Hearnshaw,
An Earth-mass planet
in the habitable zone of
α Centauri A, with star
B at a distance of
about 20 AU. (Artist’s
impression).
Dept. of Physics and Astronomy
University of Canterbury
collaborators: Stuart Barnes (AAO, Australia) and Mike Endl (Austin, Texas)
Three ways of finding Earth-like planets
Habitable zone planets
1. The Doppler method: periodic radial-velocity
The habitable zone of a star is the zone
where a planet can have liquid water.
right: Artist’s
impression
of habitable
zone Earth-like
planet in the
α Cen system
The challenge of detecting
Earth-mass planets
Earth-mass planets require velocity
precision of ~ 1 m/s. The table gives
velocity amplitudes of α Cen A and B
caused by 1 ME and 10 ME planets in
orbits of different size, a.
α Cen A
a (AU)
0.05
0.1
0.4
0.6
1.0
2.0
3.0
2. Transits of planets across disk of a star
For α Centauri A: habitable zone 1.1 – 1.3 AU
(1 from A)
For α Centauri B: habitable zone 0.5 – 0.9 AU
(0.6 from B)
1-m Mt John telescope and Hercules
fibre-fed échelle spectrograph
1 ME
K (m/s)
0.39
0.28
0.14
0.11
0.09
0.06
0.05
10 ME
K (m/s)
3.92
2.77
1.38
1.13
0.88
0.62
0.51
Right: McLellan 1-m
telescope MJUO
α Centauri (left) and β Centauri (right)
α Cen B
P (d)
3.88
10.99
87.9
161.5
347.5
982.8
1805.
1 ME
K (m/s)
0.43
0.30
0.15
0.12
0.10
0.07
0.05
10 ME
K (m/s)
4.26
3.01
1.51
1.23
0.95
0.67
0.55
P (d)
4.23
11.95
95.6
175.6
377.9
1069.
1964.
α Cen A + iodine cell spectrum:
2009 Jan 22
Hercules optical layout
3. Gravitational microlensing
Left: Hercules 4k × 4k
CCD camera; above
Hercules vacuum tank
All three are poised for success in the
next few years. All use latest cutting-edge
technology.
Stable planetary orbits must be within 2 or 3 A.U. of
each star and coplanar with the binary star orbit, i = 79°.
α Centauri data
R.A.: A: 14h 39m 36.4951s B: … 35.0803s
Dec.: A: -60° 50′ 02.308
B: … 13.761″
parallax p = 0.75
distance d = 1.34 parsecs = 4.37 light-yr
α Centauri is a double star (G2V + K1V)
Orbital elements:
Period P = 79.91 yr
Eccentricity e = 0.52
Semi-major axis a = 23.4 A.U.
Inclination i = 79°
Separation of stars 11.2 to 35.6 AU
Angular separation of stars varies 2 to 22
2008: 8.3
2009: 7.5
2016: 4.0
apastron: 1995, 2075
periastron: 1955, 2035
left: α Centauri
true and
projected orbits
Can planets form in the α Cen system?
apparent mag mV:
spectral type:
absolute mag MV:
luminosity L:
mass M:
radius R:
temperature Teff (K):
colour index (B-V):
age (Gyr):
A
+0.01
G2V
4.37
1.6
1.10
1.227
5790
0.69
6.52±0.3
B
1.33
K1V
5.71
0.45
0.91
0.865
5260
0.90
6.52±0.3
Why observe α Centauri from Mt
John Observatory New Zealand?
Sample spectra of α Cen B through I2
cell showing thousands of fine I2 lines
superimposed on stellar spectrum
Recorded at Mt John 2009 Jan 24
Iodine cell velocity precision ~2.5 m/s
Results from planet formation simulations
by Guedes et al. for α CenB. All simulations yield 1 to 4 Earth-mass planets of
which 42% lie inside the star’s habitable
zone (dashed lines). The planetary configuration of the solar system is shown for
reference.
Starting conditions: N lunar-mass bodies
in a disk with 1/a surface density.
More data for the
α Cen triple system
A fibre scrambler for Hercules
stabilizes the fibre illumination at
the exit inside Hercules. This should
allow ~2 m/s velocity precision,
even without I2 cell.
• We have a 1-m telescope available
for an intensive observing program
over several years.
• We can observe α Centauri all year,
even in November and December
when α Cen passes through lower
culmination (altitude ~ 15°). .
Above: early trials with I2, May-Jun 2007.
Left: 963
spectra of
α Cen A, in
2009 Apr
show 2.68
m/s precision
Above: α Cen at lower culmination
RV simulation on α Cen A to find
a one Earth-mass planet at 1 A.U.
Proxima
13.1var
M5Ve
15.53
5.1 × 10–5
0.12
0.12
3240
1.81
• We have a high resolution spectrograph able to deliver ~1 m/s
precision.
above: α Centauri sizes
α Cen A is 23% larger than Sun and a
little hotter. B is cooler than Sun and
less than half Sun’s luminosity. Note
that B is 20% less massive than A.
The simulation assumed 11,500 spectra
per year each with σ = 3 m/s. The planet
induces a signal with K = 8 cm/s, P = 370
d. The power spectrum shows this planet
is easily detectable, even after 2 years!
The data used were 963 actual Hercules
data recorded April 2009, which were
used to generate 46000 simulated
observations over 4 years.