Transcript α Centauri: a double star - University of Canterbury

Can we find Earth-mass planets
orbiting our nearest star,
α Centauri?
John Hearnshaw,
University of Canterbury,
Christchurch, NZ
Humboldt Symposium
University of Otago
Dunedin
29 January 2010
Harlow Shapley (1885-1972)
Harvard College Observatory
“Millions of planetary systems must exist.
Whatever the method of origin, planets
may be the common heritage of all stars …
“Our kind of chemistry, the chemistry of our Sun, our Earth,
is the common chemistry of the universe…
“On some of these planets is there actually life? Or is that
biochemical operation strangely limited to our planet? …
“Is life thus restricted? Of course not. We are not alone.”
Harlow Shapley in a lecture on
‘Religion in an age of science’ at
Vanderbilt University, spring 1958.
The masses of planets
Jupiter mass planets: 1 MJ = 10-3 MSun
1
1
Earth-mass planets: 1 ME =
MJ =
MSun
300
300,000
1 A.U. = 1 astronomical unit = Earth-Sun distance
= 150 million km.
• No Earth-mass planets have yet been detected, but
a few planets slightly bigger than the Earth have been
found.
• Mainly they are much closer than the Earth-Sun
distance (= 1 AU) so very hot.
Three ways of finding Earth-like planets
1. The Doppler method: periodic radial-velocity
variation of a star, detected spectroscopically
• 346 planets orbiting 294 stars discovered since 1995
• Most are Jupiter-mass objects (~300 MEarth )
• Lowest mass is Gliese 581e (mass ≥ 1.94 MEarth) at 0.03 AU
• Note also Gliese 581 d (mass ≥ 7.1 MEarth) at 0.22 AU
(Mayor et al. June 2009)
Radial-velocity (Doppler) method
HD114762: original data of
Latham et al. (small dots),
and more precise data from
McDonald (Cochran et al.)
(large dots).
mp sin i (mJ)
a (AU)
P (days)
e
= 11.0
=
0.3
= 84.03
= 0.334
51 Pegasi: Marcy and Butler
0.47
0.05
4.22
0.00
Three ways of finding Earth-like planets
2. Transits of planets across disk of a star
• 62 planets detected by precise photometry
• First was HD209458 (Charbonneau et al. 2000)
• Typical change in light ~ 10-4 to 10-5
• Smallest is CoRoT-7b: size = 1.7 REarth; mass = 4.8 MEarth
a = 0.017 AU; P = 0.85 d (20.5 h) (Leger et al. Aug 2009)
Finding planets by the transit method
The principle of planet detection by the transit technique.
Jupiter would cause a fall in brightness of the Sun by
~1% if it was in a transit event for a distant observer.
KEPLER (NASA)
• Launch 5 March 2009
• Search for planetary transits
• Monitor >100,000 stars for 4 yr
continuously and simultaneously
• Stars are brighter than mV = 14
• Sensitivity to Earth-size planets as
well as gas giants
Kepler satellite
KEPLER: planet sensitivity region
and the habitable zone
Kepler search area
as function of stellar
mass and orbital
semi-major axis.
Three ways of finding Earth-like planets
3. Microlensing
• Nine planets found, first in 2004
• Lowest mass objects:
(i) OGLE-05-BLG-390Lb, mass = 5.4 MEarth
at 2.1 AU (projected on sky plane)
(ii) MOA-07-BLG-192Lb, mass = 3.2 MEarth
at 0.62 AU
Microlensing light curves for two lens stars with
low mass planets
Left: light curve of
OGLE-05-BLG-390
Below: light curve of
MOA-07-BLG-192
α Centauri: position in the sky
Right ascension: A: 14h 39m 36.4951s B: … 35.0803s
Declination:
A: -60° 50′ 02.308 B: … 13.761″
Hence: culminates at midnight in early May
In NZ (lat 44°S), lower culmination at altitude 15° (Nov).
α Centauri: name and brightness
Names: α Centauri, Rigil Kent, Rigil Kentaurus, Toliman
Distance: d = 1.34 parsecs = 4.37 light-yr ~ 40 × 1012 km
Brightness:
When viewed by naked eye as a single star, V = –0.27,
α Centauri is the 3rd brightest star in the sky.
α CentauriA (V = +0.01) is 4th brightest after Sirius,
Canopus and Arcturus.
α CentauriB (V = 1.33) is 21st brightest in visual
apparent magnitude.
α Centauri: a double star
• α Centauri is a double star with star A similar to the Sun
and star B a little cooler than the Sun
• Orbital elements:
Period P = 79.91 yr
Eccentricity e = 0.52
Semi-major axis a = 23.4 A.U. (separation varies between
Inclination i = 79°
11.2 and 35.6 AU)
α Centauri: a double star
Angular separation of stars varies from 2 to 22 arc sec
2008:
8.3
2009:
7.5
2016:
4.0
max separation:
1995, 2075
closest approach:
1955, 2035
α Centauri: the stellar components
apparent mag mV
luminosity L
1.6
mass M
radius R
temperature Teff (K)
(of surface)
age (billions of yr)
A
+0.01
B
1.33
0.45
1.10
1.227
5790
0.91
0.865
5260
6.52±0.3
6.52±0.3
relative sizes of α Cen
components and the Sun
Stability of planetary orbits
in α Cen AB
Wiegert & Holman found stable orbits inside 2.34
AU, but unstable 3 to 70 AU from each star,
provided i = 0° (coplanar with binary orbit).
Planets in inclined orbital planes are much
less stable.
Beyond 75 AU from the barycentre, stable orbits
are again possible.
Habitable zone planets
The habitable zone of a star is the zone
where a planet can have liquid water.
More precisely, the continuously
habitable zone about a star
is the zone in which an Earth-like planet
will undergo neither a runaway greenhouse
effect in the early stages of its history nor
runaway glaciation after it develops an oxidizing atmosphere.
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)
Habitable zones
The relative sizes of habitable zones around four of the
nearest stars. Sirius A, alpha Centauri A and B, and Proxima
Centauri. At this scale, the habitable zone around the
red dwarf Proxima is so small that it is only about the size of
the full stop at the end of this sentence.
Habitable zone planets
for α Centauri A and B
Can planets form in the α Cen system?
An example of planet
formation in a circumstellar disk
around α CenB. The disk is initially
populated by 600 lunar-mass
planetary embryos in nearly circular
orbits. The radius of each circle is
proportional to the size of the object.
After 200 Myr four planets have
formed. One planet has about
the mass of Mercury and is at a =
0.2 AU, two 0.6 ME planets form at
a = 0.7 and a = 1.8 AU, and a 1.8 ME
planet forms at a = 1.09 AU.
Javiera Guedes et al ApJ 679, 1582 (2008)
The detection of planets
by the Doppler method
 2G 
K 

 P 
 29.8
1
m p sin i
 0.094
m p sin i
3
M a
(M   m p )
2
.
3
1
1  e2
(m/s) (mp in Jupiter masses)
m p sin i
(m/s) (mp in Earth masses)
M a
K = velocity amplitude of star’s ‘wobble’ caused by planet
mp = mass of planet in Jupiter or Earth masses
M* = stellar mass in solar mass units
a = size of planetary orbit in AU
i = inclination of orbit to line of sight (i = 90º is edge on)
α Centauri: Existing upper mass limits for planets
The study of M. Endl et al. (2001) looked for periodic
RV variations in α CenA and B, and found no planets.
Typical velocity precision ~ 10 m/s.
For α CenA and α CenB : No Jupiter-mass planets were
detected.
Conclusion: There are no Jupiters in α Centauri!
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
for 1 ME and 10 ME planets in orbits of different size, a.
α Cen A
Earth
a (AU)
0.05
0.1
0.4
0.6
1.0
2.0
3.0
1 ME
K (m/s)
0.39
0.28
0.14
0.11
0.09
0.06
0.05
α Cen B
super-Earth
10 ME
K (m/s)
3.92
2.77
1.38
1.13
0.88
0.62
0.51
Earth
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
super-Earth
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
Sample spectra of α Cen B through I2 cell showing
thousands of fine I2 lines superimposed on stellar spectrum
Recorded by JBH at Mt John 2009 Jan 24
Hercules
High Efficiency and Resolution Canterbury
University Large Echelle Spectrograph
Hercules in the spectrograph room at MJ
December 2006
Two spectra of ζ TrA (F9 V)
showing a 15 km/s shift in
the stellar lines
Two spectra of ζ TrA (F9 V)
showing a 15 km/s shift in
the stellar lines
ζ TrA: a test spectroscopic binary
Spectral Instruments
4k × 4k CCD camera
on Hercules Dec 2006
Latest developments in
the Hercules instrument
• Tests of an iodine vapour cell (S. Barnes, M. Endl)
The cell was placed at the Cassegrain focus just before
the fibre entrance. I2 absorption lines superimposed on
stellar spectrum act as a precise wavelength calibration.
Cell length 15 cm
Iodine vapour temperature 50.0 ± 0.1 °C.
2009 April data for α Cen A showing a
precision of 2.68 m/s from 963 observations
using iodine cell
Why observe α Centauri from
Mt John Observatory New Zealand?
• We have a high resolution spectrograph able to
deliver 1 m/s precision on late-type star velocities.
• We have a 1-m telescope with enough time available
for an intensive observing program over several years.
• We are the only observatory in the world able to
observe α Centauri all year, even in November and
December when α Cen passes through lower culmination
(altitude ~ 15°). In Chile, Australia, S Africa the lower
culmination is at altitude 0° and the observing season
is 9 to 10 months through large air mass.
What do we need to do to detect
Earth-mass planets?
For K ~ 1 m/s, about 300 spectra at S/N 300:1 over
about 3 years, with σ ~ 2.5 m/s to detect super-Earths.
For K ~ 0.1 m/s, about 30,000 spectra (or more)
at S/N 300:1 over about 3 years, with σ ~ 2.5 m/s to detect
Earths-mass planets.
Typical exposure times for this S/N, using R = 70,000:
α Cen A: ~40 s
α Cen B: ~ 2 min in typical 2 arc s seeing.
α Centauri program: progress in last year
From 2008 August to 2009 October we have acquired
Hercules spectra with an iodine cell as follows:
• α Centauri A:
6536 spectra
• α Centauri B:
3359 spectra
Observers: Kilmartin, Hearnshaw and Barnes.
S/N ratio ~ 300:1
Our aim is to increase the rate of acquisition of spectra
in 2010.
How precisely do the positions of Doppler
line shifts need to be measured?
• Hercules pixel size = 15 μm ≡ 1.2 km/s
• A precision of 1 m/s requires measuring line position
to 1 in 300 million.
• A 1 m/s velocity shift ≡ a line displacement of ~ 12 nm
(~ 10–3 pixels!) on the CCD detector.
• A 10 cm/s shift ≡ 1.2 nm ~ 10-4 pixels
~ 5 × diameter of a Si atom in the CCD chip.
Semi-empirical RV data for α Cen A.
Based on actual spectra taken April 2009, but (a) reproduced over
a 4-yr period; (b) with an 8 cm/s 370-d period signal added; (c)
binned into 17 equally spaced bins (each of about 3 weeks)
RV simulation on α Cen A to find
a one Earth-mass planet at 1 A.U.
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!
Can we send a space probe to α Centauri
to confirm the existence of a planet?
Answer: Yes, may be!
• If we can travel at 0.1c (30,000 km/s), the journey would
take about 50 years.
• To reach that speed, we need to
accelerate at 0.04 m/s2 for 25 years,
and then decelerate for another 25 yr.
• To do that we need a light sail driven
by radiation pressure (sunlight or lasers)
• Sail area needed ~25 km2
• Technology may be available in 50 yr
from now. Arrival at α Cen ~ 100 yr from
now. Return of first images 4.3 yr after
that.
α and β Centauri
The End