Transcript ASU Geology - Arizona State University

Meteoritic Constraints on
Astrophysical Models of
Star and Planet Formation
Steve Desch, Arizona State University
Star Formation
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Chondrites: Leftover crumbs
from solar system formation
Cross section of
Carraweena (L3.9)
MATRIX GRAINS
CHONDRULES
CAIs
CAIs: The first minerals formed in
the solar system
CAIs contain many minerals that
are the first to condense out of a
solar nebula gas (Grossman 1972):
•melilite: Ca(Al,Mg)(Si,Al)2O7,
•hibonite: Ca2(AlTi)24O35,
•anorthite: CaAl2Si2O8,
•pyroxene: (FeMg)SiO3
McSween 1999
CAIs
Fluffy Type A:
•Not as large as other CAIs (< 1 mm)
•Most abundant (about 1% of all CCs and OCs, 2% of Allende)
•aggregations of small, zoned spheroids with spinel at their cores
and mantles of melilite (Wark & Lovering 1977)
•Group II Rare Earth Element patterns show ultrarefractory
component depleted (Tanaka & Masuda 1973); that component
apparently concentrated in nuggets like those recently found
(Hiyagon et al 2003)
•Formed (condensed?) in hot environment: 1400 K < T < 1800 K
Compact Type A:
•Same compositions as fluffy type A, but were melted
CAIs
Types B and C:
•Larger (up to cm-size)
•Very abundant in CVs (6-10%
of volume), nonexistent in
others
•Clearly melted after formation
•Type B CAI cooling rates
constrained from chemical
zoning in melilite: 0.5 - 50 K/hr
(Stolper & Paque 1986; Jones et al
2000)
•At one time contained 26Al,
41Ca, 10Be, etc.
CAIs
FUN inclusions:
•“Fractionation and Unknown Nuclear effects”
•Very rare (only 6)
•Large mass-dependent fractionations in O, Mg, Si:
apparently were severely heated and evaporated
•Are anomalous in certain neutron-rich nuclei: 48Ca, 50Ti
•Contain evidence they once contained 10Be
•Contain no evidence they ever contained 26Al, 41Ca, etc.
Short-Lived Radionuclides
• CAIs contained live short-lived radionuclides:
41Ca (t
1/2 = 0.1 Myr) (Srinivasan et al. 1994)
36Cl (t
1/2 = 0.3 Myr) (Murty et al. 1997)
26Al (t
1/2 = 0.7 Myr) (Lee et al. 1976)
60Fe (t
1/2 = 1.5 Myr) (Tachibana & Huss 2003)
10Be (t
1/2 = 1.5 Myr) (McKeegan et al. 2000)
53Mn (t
1/2 = 3.7 Myr) (Birck & Allegre 1985)
• These half-lives are so short, the radionuclides must have been
created shortly before, or during, solar system formation
• CAIs with evidence for 26Al all have remarkably uniform ratio
26Al/27Al = 5 x 10-5: they all formed within ~105 years of each other
10Be/9Be
Ratios
CAIs formed with
10Be/9Be = 9 x 10-4
10Be
decays to 10B
with t1/2=1.5 Myr
Natural 10B/11B level
Excess 10B correlates
with amount of Be:
this 10B is from the
decay of 10Be
Slope gives initial 10Be/9Be ratio
McKeegan et al. (2000)
10Be/9Be
•
Ratios
10Be
has been found in
every CAI looked at, at
levels consistent with
10Be/9Be = 9 x 10-4
• 10Be is present even if
other radionuclides such
as 26Al, 41Ca are not, in
FUN inclusions and
hibonites (Marhas et al. 2002;
MacPherson et al. 2003)
10Be
Table from Desch et al. (2004)
has a different origin than 26Al, 41Ca, etc.
(Marhas et al. 2002): Could it be Galactic Cosmic Rays?
Collapse of Cloud Cores: Observations
• Stars form in parts of molecular
clouds that have gravitationally
collapsed, dragged in magnetic
field lines
• Even the Orion Nebula must
have gone through this stage
(Schleuning 1998)
•
1.3 pc
10Be
GCRs follow magnetic
field lines, are trapped when
column densities first exceed
~ 10-2 g cm-2 (before first stars)
Side view:
Schleuning (1998)
Collapse of Cloud Cores: Calculations
• Numerical simulations show how
magnetic fields and gas densities
vary with time in collapsing
molecular cloud core
(Desch & Mouschovias 2001)
• We calculate rates at which 10Be
GCRs are trapped, and 10Be is
produced by spallation
• First stars form << 1 Myr after t=0
1.5 pc
Desch &
Mouschovias (2001)
10Be
in a Collapsing Cloud Core
Trapped 10Be GCRs
Total
•
CAI ratio
9.5 x 10-4
10Be
produced by GCR protons
spalling CNO nuclei in gas
10Be/9Be
ratio = CAI ratio
as first stars form!
• All of the 10Be in CAIs is
attributable to GCRs:
80% from 10Be GCRs
trapped in cloud core
20% produced by spallation
reactions
• 10Be/9Be ratio does indeed
peak when column densities
exceed ~ 10-2 g cm-2
Supernova Injection of Radionuclides
• We attribute 10Be to trapped 10Be Galactic cosmic rays
• A type II supernova is the most likely source of all the other
radionuclides: 41Ca, 36Cl, 26Al, 60Fe produced in proportions
seen in meteorites (Meyer & Clayton 2000; Meyer et al. 2003)
• We do not claim that a supernova triggered the collapse of the
solar system’s cloud core
• We claim the solar nebula already existed and CAIs were
forming when the supernova ejecta entered the solar system
(Sahijpal & Goswami 1998): “Late injection”
• FUN inclusions are CAIs that formed before 26Al, 41Ca, 60Fe,
and anomalous 48Ca and 50Ti were injected by supernova
The Sun’s Star-Formation Environment
• 80% of Sunlike stars
form near a star
massive enough to
supernova (Adams &
•Ionization fronts probably
triggered star formation in
Eagle Nebula
Laughlin 2001)
• Before massive star
goes supernova it
ionizes, heats, and
“photoevaporates”
surrounding gas
Hester et al (1996)
Evaporating
gaseous
globules: new
solar systems
The Sun’s Star-Formation Environment
• After EGG stage, solar
system emerges into H II
region as a “proplyd”
• Disk resides in H II region
for ~105 yr until O star(s)
supernova
• Disk intercepts supernova
ejecta with radionuclides
• Proplyds in Orion will
acquire 26Al/27Al ~ 5 x 10-5
when 1 Ori C supernovas
Protoplanetary Disks
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HH30: Watson, Stapelfeldt,
Krist & Burrows (2000)
PPDs:
•Protoplanetary disks are accretion disks
•Angular momentum is transported outward,
mass moves inward
outflow
•Angular momentum transport probably due to
magnetohydrodynamic turbulence (Desch 2004)
Mass accretes onto star through disk
PPDs:
•As mass moves inward, gravitational energy is
released, mostly at midplane
•Temperatures highest at midplane, lowest at
surfaces
•Heat flux drives convection (Bell et al 1997)
•Gas rises, cools in convection cells, rocks condense
Evidence for Condensation
T = 1270 K
Metallic Fe
condenses
FeMg silicates
T = 1370 K
Refractory minerals
condense as rising
gas cools
T = 1770 K
All vapor
Z
Simon et al (2002)
More Evidence for Condensation
•FeNi metal condenses as gas moves from
T=1370 K to 1270 K
•Ni zoning reproduced if condensation
takes a few weeks, as in a convection cell
model (Meibom, Desch et al 2000)
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Meibom et al (1999, 2000)
PPDs:
•Convection repeatedly moves material through
hot midplane, evaporates most silicates
•Only most refractory minerals grow (CAIs)
•Convection and turbulence disperse CAIs
widely (Cuzzi et al 2003a,b, 2004)
•This stage requires dM/dt > 10-6 Msol/yr,
ends after ~105 yr (Bell et al 2000)
PPDs:
•After few x 105 years, magnetohydrodynamic
turbulence occurs only in surface layers
•Temperatures are everywhere much cooler, FeMg
silicates form at midplane (chondrules)
•Accretion is unsteady with time, leads to shocks
•Shocks melt CAIs and chondrules, cool at rates ~
50 K/hr (Desch & Connolly 2002)
Conclusions
CAI radionuclides constrain setting of solar system formation:
• 10Be attributable to trapped 10Be Galactic cosmic rays
• Other radionuclides (26Al, 41Ca, esp. 60Fe) injected by supernova
• Injection occurred after first CAIs (FUN inclusions) formed
• Implicates formation in H II region like Orion or Eagle Nebula
CAIs constrain disk temperatures, dynamics, timescales…
• CAI mineralogy implicates hot (> 1400 K) protoplanetary disk
• Condensates implicate convection
• Requires high mass accretion rates through disk > 10-6 Msol/yr,
attainable only for ~ 105 yr
• Convection, turbulence will then widely disperse CAIs