Transcript East Valley Astronomy Club

New Developments in the
Formation of the Solar System
Steve Desch
Arizona State University
Outline
What was the solar nebula like? How
massive was it, how large was it, how did it
evolve, what about its surroundings?
First, the old story: large, low-mass disks
in a low-mass star-forming region like Taurus.
New evidence for a smaller but more
massive disk, from planets and meteorites.
New evidence for a birth in a stellar
cluster,
much more like the Orion
Nebula.
The “Old” Story
Not long ago,
astronomers
considered the
Taurus Molecular Cloud to
be a typical
star-forming
environment...
The “Old” Story
...and the solar
systems that
formed there to
be typical.
Accretion
disks in
Taurus
100 - 1000 AU
HST WFPC2/NICMOS
But Taurus is not
typical. It’s just
close.
Taurus = 130 pc
Orion = 450 pc
Sco-Cen= 450pc
The “Old” Story
Only 10-30% of
Sun-like stars
form in regions
like Taurus (Lada
& Lada 2003).
Most form in
regions like the
Orion Nebula
that contain at
least one really
massive star.
Disks forming in these environments are much smaller
than in Taurus-like regions (< 50 AU), shaped by
photoevaporation by ultraviolet radiation, by stellar
winds, by stellar close encounters, and by supernovae.
1Ori C:
imminent
supernova
~0.2pc
disks
Are these the type of
environments in which our
Solar System formed?
Meteorites are one way we can find out!
Cross section of Carraweena
(L3.9) ordinary chondrite
MATRIX GRAINS
1 mm
CHONDRULES
CAIs
Chondrules are millimeter-sized, ferromagnesian
melt droplets.
Their textures are reproduced only if they
temperatures > 1575 C, for minutes, then cooled
over a matter of hours.
Chondrules probably formed when they were hit
by shock waves in the gas of the solar nebula.
Temperature and cooling rates of a
chondrule passing through a 7 km/s
shock
Desch & Connolly
(2002)
Melting by nebular shocks is consistent with all known
properties of chondrules, but only if the gas density is
~ 1 x 10-9 g cm-3
About 5% of chondrules are compound chondrules, stuck
together while molten (Ciesla et al. 2004).
Assuming relative velocities < 100 m/s (!), there must have
been > 10 chondrules per cubic meter (Gooding & Keil 1981).
Chondrules lost K, Mg,
Fe and Si while melted,
but no isotopic fractionation is measured.
Implies rock vapor
stayed in the vicinity of
chondrules: need > 10
chondrules per cubic
meter, in clouds > 1000
km across (Cuzzi &
Alexander 2006)
Chondrule densities nc ~ 10 m-3 are high!
nc = (c / gas) gas
4/3 s ac3
= C (0.005) (10-9 g cm-3) (gas / 10-9 g cm-3)
4/3 (2.5 g cm-3) (300 m)3
nc = 0.015 C (gas / 1 x 10-9 g cm-3) m-3
Chondrules probably were concentrated,
but probably not by factors C > 200.
The upshot: chondrules had to form in a region
with gas density ~ 1 x 10-9 g cm-3, greater than
had been thought possible at 2 or 3 AU.
Minimum Mass
Solar Nebula
E
V
Gas surface density
is estimated using
“Minimum Mass
Solar Nebula”
(Weidenschilling
1977)
If a disk thickness is
estimated, the gas
density is derived.
J
M
S
M
U
N
Problems with the Minimum Mass Solar Nebula
This model yields densities that are too low to melt
chondrules by shocks, by a factor of about 5 (i.e.,
gas / 1 x 10-9 g cm-3 at 2-3 AU).
A bigger problem: Jupiter, Saturn, Uranus &
Neptune can’t accrete H2 & He gas until their
rocky cores reach about 10 Earth masses in size.
No models of planet growth predict such a large
planet can form in < 10 Myr at 5.2 AU, unless the
density of the solar nebula is at least 5 x the
minimum mass solar nebula.
Even then, growth of Uranus and Neptune at 19
AU and 30 AU impossible in < 5 Gyr.
Enter the Solution: the Nice Model
According to the `Nice Model’, the Giant Planets did not
form where we find them today!
Here’s where they started:
Jupiter = 5.45 AU, 12.7 yr (5.2 AU, 11.9 yr)
Saturn = 8.18 AU, 23.4 yr (9.6 AU, 29.5 yr)
Neptune = 11.5 AU, 40 yr
(30.1 AU, 165 yr)
Uranus = 14.2 AU, 54 yr
(19.2 AU, 84 yr)
+ 35 Earth Mass Disk of ‘Planetesimals’ = 16 - 30 AU
The Nice Model
Uranus scattered planetesimals inward, which
gave `gravity assists’ to the planets
The Nice Model
Jupiter moved inward, Saturn moved out, until they
reached a 2:1 resonance (after about 700 Myr)... Solar
System went chaotic, driving Neptune and Uranus out.
A close encounter between Uranus and Neptune
led to them switching places!
The Nice Model
Continued migration rapidly
depletes planetesimal disk,
sending some in toward Earth.
Many planetesimals are lost, some scattered into
the Oort cloud, and many are scattered into the
modern-day Kuiper Belt
The Nice Model
Neptune stops migrating when
the number of planetesimals it
can scatter gets too low.
Pluto &
Kuiper Belt
This happens 10-20 Myr after Jupiter & Saturn went
chaotic (700 Myr after Solar System birth).
Solar System has been stable ever since (for 3.9 Gyr).
Planets start on
circular, coplanar
orbits, but end up on
slightly eccentric,
inclined orbits.
Gomes et
al. (2005)
Nice Model explains why the giant planets have the orbits
they do, and why the Late Heavy Bombardment of the
inner solar system occurred (and the structure and size of
the Kuiper Belt, and Jupiter’s Trojan asteroids, etc., etc.).
If true, the planets formed closer to the Sun, which
speeds up their formation, but still not < 10 Myr.
However, if the planets formed closer together, the
Minimum Mass Solar Nebula must be wrong!
The planets were spread out from 5-15 AU, not 5-30 AU.
One quarter the area = 4 x denser!!
Desch
(2007)
Amazing
conformance
of diverse
data to a
single trend!
But density
falls off very
steeply with
radius, too
steeply to be
an accretion
disk...
Not possible to have
a steady-state disk
with mass moving in
and have that profile...
But it’s a prediction
(Desch 2007) if the
disk is being
photoevaporated and
gas is steadily moving
out while the planets
form!
Photoevaporating disks in the
Orion Nebula (HST)
The steady-state decretion
disk solution of Desch
(2007) is very favorable for
planet growth... all four giant
planets form in < 10 Myr!
Outward flow of mass also
explains how CAIs ended
up in Comet 81P/Wild 2,
sampled by Stardust
(Zolensky et al. 2006)
Speaking of
CAIs.....
In the first few Myr after its birth, the
Solar System contained live radioactive
elements with half-lives ~ 1 Myr.
26Mg
24Mg
27Al
26Mg
24Mg
=
26Mg
24Mg
+
/ 24Mg
26Al
27Al
x
27Al
24Mg
SLR
Half-life
41Ca
0.1 Myr
41Ca/40Ca
= 1.4 x 10-8
36Cl
0.3 Myr
36Cl/35Cl
= 3 x 10-6
26Al
0.7 Myr
26Al/27Al
= 5 x 10-5
60Fe
1.5 Myr
60Fe/56Fe
=5x
10Be
1.5 Myr
10Be/9Be
= 9 x 10-4
53Mn
3.7 Myr
53Mn/55Mn
= 1.4 x 10-5
107Pd
6.5 Myr
107Pd/108Pd
= 2 x 10-5
182Hf/180Hf
= 2 x 10-4
129I/127I
= 1 x 10-4
182Hf
129I
9 Myr
15.7 Myr
Initial Abundance
10-7
McKeegan &
Davis (2003)
probably from
a nearby
supernova
definitely a
supernova!
unique
source
probably part of
the inter-stellar
cloud gas from
which Solar
System formed.
Cosmic Ray Origin of 10Be
total
trapped
meteoritic
10Be/9Be
spallation
Desch et
al. (2004)
Schleuning (1998)
10Be
Galactic Cosmic Rays must have been trapped in our
molecular cloud core. Trapped GCRs match the meteoritic
At least a
third, and probably all, of the 10Be in the early
Solar System attributable to cosmic rays!
10Be/9Be.
Uncertainties are factors of 2-3 total.
Supernova Origin for 26Al, 60Fe, etc.
~0.2pc
disks
1Ori C:
imminent
G353.2+0.9 H II
region in NGC 6357
(Healy et al. 2004;
Hester & Desch 2005)
Pismis 24-1 (O3 If*),
Pismis 24-17 (O3 IIIf*)
and Wolf-Rayet stars
(Massey et al. 2001)
~ 0.4 pc
These stars will
supernova in < 1 Myr
What will happen to those disks?
Protoplanetary disks ~ 0.3 pc from a supernova (1051 erg) are not
destroyed! (Chevalier 2000; Ouellette et al. 2005; Ouellette et al. 2007)
Ouellette et al. (2007)
Gas is not mixed in well,
but dust grains are!
(Ouellette et al. 2007)
Short-Lived Radionuclides
Injection of radioactive grains directly into protoplanetary
disk supplies just enough 60Fe!
Iron likely in form of dust grains: gas-phase Fe disappeared
from SN 1987A ejecta at same time (2 years post-explosion)
that 10-3 M of dust formed (Colgan et al 1994)
Mass of 60Fe ejected by 25 M supernova ~ 8 x 10-6 M
(Woosley & Weaver 1995)
Fraction intercepted by 30 AU radius disk at 0.3 pc away ~
(30 AU)2 / 4 (0.3 pc)2 ~ 6 x 10-8
Mixed with 0.01 M of solar composition material,
60Fe
/ 56Fe ~ 1 x 10-6
One supernova can also inject the other short-lived radionuclides
with observed abundances (Ellinger et al. 2007, in preparation)
Conclusions
Our Solar System grew up in a tough neighborhood!
Models of the formation of chondrules in meteorites tell us the
disk was very dense and did not spread out much.
New models of the “minimum mass solar nebula” show the
nebula really was very dense, but this structure is only
understood if the disk was photoevaporating because of a
nearby massive star.
Meteorites show the Solar System contained live 60Fe, almost
certainly ejected by a nearby massive star that went
supernova.
The Sun formed in a region much more like the Orion Nebula
than the Taurus molecular cloud!