Living with a Red Dwarf - Center for Space and Habitability (CSH)

Transcription

Exoclimes 2014, Davos
Living with a Red Dwarf
Raymond T. Pierrehumbert
The University of Chicago
1
The H-R diagram
2
What makes a star shine?
• Fusion of light elements into heavy elements
• Hydrogen is by far the most abundant fuel in the Universe
• Main sequence stars burn H into He
• 90% of stars are main sequence stars
• Stars do not evolve along the Main Sequence.
they enter the Main Sequence when they start fusing H,
and leave it when H fuel is exhausted
3
The mass-luminosity relation
Luminosity: The power output of a star
For stars burning H to He,
L∼

M 4
M 2.3
ifM > .4M
ifM < .4M
→ Low-mass stars are dimmer
4
Relation between color, luminosity and mass
• For any radiating body, dominant wavelength is
inversely proportional to temperature
• Cooler = more red. Hotter = more blue
• Relevant temperature for star is ”surface” (photosphere) temperature,
where light escapes from
5
Relation between color, luminosity and mass
L = 4πr2σT 4, so T 4 ∼ L/r(M )2
r(M ) ∼ M α, (e.g. α = 1
3 if mean density were constant). Then:
T ∼

M 1− 12 α
M

.575− 1
2α
ifM > .4M
ifM < .4M
→ Low-mass stars smaller, cooler and redder
6
How long does a main sequence star shine?
e
p
p
n
He
4.3 x 10-12 joules
p
n p
E = 6.4 x 1014 joules/(kg H)
p
p
e
Total conversion lifetime t∞ = M · E/L
100 billion years for Sun,
– vs 10 billion years actual main sequence lifetime for a Sunlike star
7
To burn H, you have to mix it into the core
Helium “ash”
accumulates here
e Photosp
tiv ive re
iat
Conve
c
Rad
H H H
Fusion
H He
re
he n
gio
High Mass Star
H
He
He
H H
Fusion
He H
He H
Low Mass Star
8
From mass-luminosity relation, total conversion lifetime is:

t∞,( M )−3
M
t∞ = E·M
=
L
4.7t∞,( M )−1.3
M
ifM > .4M
ifM < .4M
e.g. 2.2 trillion years for M = .3M (like GJ581)
M stars are extremely long-lived, evolve slowly
9
M stars in the Universe
Commonly estimated that 70% of stars
(76% of main-sequence stars) are M-dwarfs
But our ability to do a complete census of dim stars is limited,
even in our own galaxy.
10
M stars in our neighborhood
1
0.6
54%
0.4
The Sun
Fraction of stars
0.8
0.2
0
2000
3000
4000
5000
6000
7000
8000
9000
10000
Temperature (K)
Gliese catalog of the ∼ 3, 803 stars within 25 parsecs (82 light years)
11
46 of the 71 stars in the 50 nearest systems are M stars
12
Our closest neighbor is an M-star
A
B
α Centauri
400
Nept
une
orbi
ts
Proxima Centauri
T = 3042 K
L = .0017Lsun
13
Radial velocity surveys
e.g. HARPS (Chile)
14
Transit method
15
Ground based transit surveys
MEarth
WASP/Super-WASP
16
Kepler: Space-based transit survey
17
Do planets form around M-stars?
Planets with known mass
100
Too Cold
Too Hot
1
Gas Giants
(no surface)
0.1
0.01
0.001
0.0001
0.0001
0.001
0.01
0.1
1
10
Solar Energy Received (Relative to Earth)
100
Lose
Atmosphere
Planet Mass (in Jupiters)
10
18
Do planets form around M-stars?
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:;<
:;:<
:;::<
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19
Red dwarfs are redder than the Sun...
1.2
irradiance (ADLEO)
Planck3390K
Irradiance (Sun)
Planck5780K
Normalized irradiance
1
0.8
0.6
0.4
0.2
0
1
2
3
Wavelength (microns)
4
5
20
... but not so red as the name implies
21
Consequences: Not as much blue to make the sky blue
Atmosphere reflects
much energy back to space
Atmosphere reflects
little energy back to space
All other things being equal, M-dwarf planets absorb more of the incident
stellar radiation
22
But more of the stellar radiation is deposited high in the atmosphere,
less at the ground and in deep atmosphere
23
Implies weaker convection, shallower troposphere
absorption here
T(z
)
T(z
)
absorption here
G star (Earthlike case)
M-star planet
24
Consequences:Ice and snow are not as ”white” as on Earth
Planck 3000K
Planck 6000K
-1
10
6
1
10
4
0.8
100
0.6
1
0.4
0.01
0.2
Normalized stellar spectrum
Extinction coeff (cm )
water ice absorption
0
0.0001
0.1
1
10
100
Wavelength (microns)
1000
25
Reduced reflectivity contrast between ice/snow and ocean
makes it harder for a planet to freeze over and turn into a Snowball
Shields, et al Astrobiology 2013, computed with 300ppm CO2
26
But this doesn’t affect the outer edge of the habitable zone, because the
dense atmospheres needed to make a planet habitable there make the
planetary reflectivity nearly independent of the surface characteristics.
For dim stars, habitable zone planets are in close orbits
L~ = RL2
(L in units of Solar luminosity, R = orbital dist. in a.u.)
L~ = 1 → R =
1
2
L
= .44M 1.15
(M measured in Solar masses)
Orbital period (Earth years): P = R1.5/M .5 = .39M 1.22
e.g for Gliese 581:
M = .3 → R = .11 a.u. P = .09 years = 33 Earth days
27
Strong tidal stresses slow the planets rotation
e
c
or
centrifugal
gravity
,
ity
av d
gr war
g
n
on d i
str ulle
p
f
y
l
t
vi rd uga
a
gr twa trif
ak ou cen
e
w led y
b
l
Pu
Stress: M/r3 = 11.7M −2.45
(e.g. 1 for Earth, 17.25 for Mercury, 223 for the HZ of GJ581)
For circular orbit, end-state is tide-locked
28
Tide-locked planets in circular orbits have
permanent dayside and nightside
Substellar point (local noon, where star is directly overhead)
is geographically fixed
29
Slow rotation → weak Coriolis force
→ weak temperature gradients in the atmosphere
i.e. atmosphere doesn’t condense out onto the nightside
30
But there can be strong temperature gradients near the surface,
31
... and even ice formation if the planet has an ocean
32
But if the orbit is elliptical ...
• Variation in orbital angular speed with distance from star means that
exact tide-locked states are no longer possible.
• Quasi-synchronous states, where substellar point
rocks back and forth a bit
• Low-order spin-orbit resonances.
(e.g. 3 days per two years, as for Mercury
33
So, neighborhood of M-dwarfs is great real estate,
but there’s a catch ...
M-star rotation and deep convection make strong magnetic fields,
promote flaring, activity
→ Strong extreme ultraviolet and X-ray emission
(relative to luminosity)
34
Ultraviolet (UV) is important because ...
• Very shortwave ultraviolet (EUV) and X-rays are absorbed high up in
the atmosphere, and heat it to the point where the atmosphere can
escape to space.
• i.e. it’s the rocket fuel that brings molecules up to escape velocity and
can launch atmosphere out of the gravity well.
• Shorter wave ultraviolet drives photochemistry, and can break up
heavy molecules into lighter components that escape more easily.
• Low mass stars can take a half billion years to enter the main sequence, and UV/X-ray luminosity is further elevated throughout this
time.
• But as M stars age on the main sequence, they can quiet down
If planets can regenerate an atmosphere later, habitability could be
recovered. (but no easy way to regenerate a nitrogen atmosphere).
• More on all this in Lecture 2
35
Summary
• There are likely to be many planets in the habitable zone of M stars
• With an atmosphere, they would have unusual seasonal/diurnal cycles, shallower tropospheres with weak convection.
• Weak horizontal temperature gradients aloft, monsoonal circulations
with most of rainfall and warm waters under the substellar point
• ... but none of this is a threat to habitability
• The main question is whether any of these planets formed with, and
retained volatiles (atmosphere, ocean).
• But we know some are all atmosphere!
(cf. GJ1214b, more in Lecture 3)
• Essential next step is a catalog of which M star planets
have retained an atmosphere
36

Living with a Red Dwarf - Center for Space and Habitability (CSH)

Transcription

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