Transcript Prezentace aplikace PowerPoint

Spectra of meteors and meteor
trains
Jiří Borovička
Department of Interplanetary Matter
Meteor
photograph
All-sky
image
Kouřim bolide
(– 13 mag)
Bolide – 18 mag
Double-station video meteor
Meteor speeds
11 – 73 km/s
Meteor heights
Faint meteors: 110 – 80 km
Fireballs: 200 – 20 km
HIGH RESOLUTION
PHOTOGRAPHIC SPECTRA
OF FIREBALLS
Battery of six photographic grating
cameras with rotating shutter in Ondřejov
Example of a
photographic
prism spectrum
of a bright
Perseid meteor
Detail of the prism spectrum
Example of photographic grating
spectrum of a slow sporadic fireball
zero
order
first order
second order
detail of grating spectrum
Detail of a Perseid spectrum
almost head-on meteor
blue part shown (3700–4600 Å)
Radiative transfer in spectral lines
Assuming thermal equilibrium
Emission curve of growth
Model assumptions
• The radiation originates in a finite slab of gas
(plasma) with a cross section P
• Atomic level population is described by the
Boltzmann law for an excitation temperature T
• Self-absorption is taken into account (the gas is
not optically thin)
Free parameters
•
•
•
•
Excitation temperature, T
Column densities of observable atoms, Nj
Meteor cross-section, P
Damping constant, 
Total number of Fe atoms
1E+23
Number of Fe atoms
EN 270200
1E+22
light curve
F
1E+21
2.00
2.20
2.40
Time [s]
2.60
2.80
Temperature
5000
Temperature [K]
EN 270200
4500
4000
light curve
F
3500
2.00
2.20
2.40
Time [s]
2.60
2.80
Cross-section
3E+6
Cross section [cm2]
EN 270200
2E+6
1E+6
light curve
F
0E+0
2.00
2.20
2.40
Time [s]
2.60
2.80
Electron density
1E+14
Electron density [cm-3]
EN 270200
1E+13
1E+12
light curve
F
1E+11
2.00
2.20
2.40
Time [s]
2.60
2.80
Two components in meteor
spectra
• The spectra can be explained by the
superposition of two components with
different temperatures
• The main component, T = 4500 K
- present in all spectra
- temperature does not depend on velocity!
- originates from a relaxed vapor cloud
near and behind the meteoroid
• The second component, T = 10 000 K
- present in bright and fast meteors
(vapor lines – air lines present also in
faint fast meteors)
- temperature does not depend on
velocity (or only slightly)
- originates from a transition zone in the
front of the vapor cloud
- typical lines: Ca II, Mg II, Si II
Two components
Example of a Perseid fireball
Determination of elemental
abundances
•
•
•
•
Estimation of electron density
Use of Saha equation
Determine ionization degree
Recompute neutral atom abundances to total
abundances
Estimation of electron density
1. From meteor size and atom column
densities + neutrality condition
2. From CaII/CaI ratio (if the high
temperature component is absent)
3. By combining both components
podivat se podrobneji !
Electron density from atom densities
Abundances in meteor vapors
Log (ratio to CI abundance)
normalized to Mg
1.0
0.0
asteroidal
Geminids
-1.0
Taurid
low
cometary
Fe/Mg
volatile
depletion
in Geminids
-2.0
Perseids
1P/ Halley
incomplete
evaporation
-3.0
Leonids
Al
Ca
Cr ??
Ni
Mg
Fe
Si
--> increasing volatility
Cr
Mn
Na
Incomplete evaporation
Abundances along the trajectory
EN 270200
Mg
1E+0
Element/Fe ratio
Na
1E-1
1E-2
Cr
1E-3
Ca
Al
1E-4
2.00
2.20
2.40
Time [s]
spike
2.60
2.80
Ca/Fe model evaporation
Schaefer &
Fegley (2005)
LOW RESOLUTION VIDEO
SPECTRA OF METEORS
Spectral and direct cameras in Ondřejov
LEONID METEOR SPECTRUM
November 18, 2001 10:24:14 UT Mt. Lemmon
Meteor magnitude: –1.5
frame 21P
height 109 km
IR end
O
[O] 557nm
Na
Mg
blue end
O
Mg
Na
O
Mg
Na
O
Mg
Na
O
Mg
Na
O
Mg
Na
h=109 km
h=101.5 km
O
Mg
Na
h= 98.5 km
Mg
O
h=117 km
Na
O
Meteor spectral classes
Fe I - 15
Mainstream
Normal
Na poor
Fe poor
Enhanced Na
Other
Irons
30
40
20
15
Na free
Na rich
Mg I - 2
Na I - 1
“All-wavelength” spectrum
From Carbary et al. (2003)
SPECTRA OF METEOR
TRAINS
Three phases of train evolution
1. Initial rapid decay of intensity, dominated
by atomic line emission (the afterglow)
2. Atomic emissions persisting for about 30
seconds (the line phase)
3. Continuous emission emerging about 20 s
after train formation and persisting for
minutes (the continuum phase)
The meteor and afterglow spectrum
METEOR
• Contains high
excitation/ionization
lines: Ca+, Mg+, Si+, Fe+,
H (10,000 K component)
• Contains high excitation
atmospheric lines: N, O
AFTERGLOW
• Contains low excitation
semi-forbidden
(intercombination) lines:
*Fe, *Mg, *Ca
• Contains forbidden green
oxygen line
COMMON: low excitation allowed transitions: Na, Fe
Afterglow explanation
• The line decay rate is proportional to the
excitation potential
• Rapid cooling of gas under non-equilibrium
conditions
• Low electron density causes non-Boltzmann
level populations
Afterglow “physics”
Line intensity: I ~ hv Ni Aij
Level population from statistical equilibrium:
radiative deexcitation
+
collision deexcitation
=
collisional excitation
~ Ni Ai
~ NiCi 0  Ni neQi
~ N0C0i ~ N0neQi e
 Ei / kT
Afterglow level populations
 Ei / kT
N 0e
Ni ~
Ai
1
neQi
1
(neQ  10 s )
5
Train initial cooling
5000
Temperature [K]
4000
1999 train
(Borovicka & Jenniskens 2000)
3000
2000
2001 Train 1
1000
0
0.0
0.5
1.0
Time [s]
1.5
2.0
The spectrum in the line phase
The spectrum in the line phase (2)
The spectrum in the line phase (3)
LINE PHASE
AFTERGLOW
• The Mg line at 517 nm
of medium excitation
(5 eV) is strong and
persisting
• Mg lines of even
higher excitation are
present and persisting
• Lines of medium
excitation are much
fainter than low
excitation lines and
decay much more
rapidly
Different spectra, different physical mechanisms
What is the physical mechanism
behind the line radiation?
• A mechanism to populate high levels (up to
7 eV) needed
• Thermal collisions absolutely insufficient
because of low temperature
• Chemical reaction are not so exothermal
• Recombination suggested though previously
discarded (Cook & Hawkins 1956)
Recombination “physics”
radiative deexcitation
+
collision deexcitation
=
collisional excitation
+
direct recombination &
downward cascade
~ Ni Ai
~ NiCi 0  Ni neQi
~0
(negligible)
~ ne N  (T , Ei ) 

 ne N 0 (T )e

 Ei / D
empirical factor
Level populations for
recombination
ne N  0 (T )e
Ni ~
Ai  neQi

neQ  10 s
4
1
 Ei / D
D ~ 0.84 eV  9800 K /k
Instrumental intensity
Fitting the spectrum with the
recombination formula
- computed
Mg
- observed
Na
*Fe *Mg
*Fe
*Fe,
Ca
Fe
Mg
Mg
4000
4500
*Ca
5000
Na
5500
Wavelength [A]
Na
6000
6500
Transition to the continuum
phase
• Animation of train 6
• Time 24 – 60 s
The continuum phase
What causes the continuum?
• The continuum is probably produced by
molecular emissions excited by chemical
reactions
• We need to identify the molecules
• Various sources suggested:
– FeO (Jenniskens et al. 2000)
– NO2 (Borovicka & Jenniskens 2000)
– OH (Clemensha et al. 2001) for IR radiation
Comparison with laboratory FeO
TRAIN 6
observed
(40 - 60 s)
laboratory FeO
(Jenniskens et. al. 2000)
not calibrated
5000
5500
6000
Wavelength [A]
6500
7000
Comments on identifications
• FeO is likely present but does not explain
all radiation
• FeO bands are not well pronounced and the
observed radiation is stronger in red and
near-infrared (a ~750 nm maximum?)
• Possible additional contributors:
OH, NO2, CaO
Conclusion
Three phases of Leonid train evolution:
1. Afterglow
=
cooling phase
2. Line phase
=
recombination
3. Continuum phase =
chemiluminescence
All phases are relatively well separated in time