Transcript L1527anion.ppt

Modeling Anions in L1527
Nanase Harada
Eric Herbst
The Ohio State University
Molecular Spectroscopy Meeting 2008
Columbus OH
Overview
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L1527 and its Chemistry
Species and Reactions in Gas-Phase Anion Model
Results from Anion Model
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Comparison with Observation
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Anion-to-Neutral Ratio
Carbon Chain Abundances
Variation of Initial Conditions
Conclusion
L1527 and its Chemistry
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Low-mass star-forming region with a class 0/I Protostar
T=30K
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Unsaturated carbon-chain molecules were detected
(Sakai et al. 2008)
C4H, C4H2, HC7N, HC9N, etc…
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Possible scenarios
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Evaporation of precursor methane from grains
Cold remnants
Detection of two negative ions
C4H-, C6H-
Anion Model in OSU Gas Phase Code
In addition to osu_01_2007 gas phase code,
 Families of CnH-, Cn-, and C3N Radiative electron attachment (Cn + e- -> Cn- + hν )
 Photodetachment (Cn- + hν -> Cn + e-)
 Dissociative attachment
(HNCCC + e- -> C3N- + H)
 Associative detachment (Cn- + C -> Cn+1 + e-)
 Anion-neutral reactions ( CnH- + O -> Cn-1H- + CO)
 Mutual neutralization (CnH- + Na+ -> CnH + Na)
Initial Conditions
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Start mostly in molecular
form
Lower initial metal
abundances –depletion
to grains based on the
warm-up model (Hassel
et al. 2008)
T=30K, n0=106cm-3
species
ni/nH
Species
ni/nH
H2
0.5
HCN
3x10-9
He
6x10-2
HNC
3x10-9
CO
5x10-5
H2 S
1x10-9
CH4
3x10-6
NH3
1x10-8
N2
1x10-5
S
1x10-8
H3 +
6x10-11
S+
1x10-
Fe+
1x10-11
HCO+
2x10-9
11
Results: Anion Abundance,
Anion-to-Neutral Ratio
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A factor of few difference for
[C6H-]/[C6H]
A few orders of magnitude
difference for [C4H-]/[C4H]
Anion abundance can be even
higher than electron abundance
Results : Carbon-Chain Abundances
Solid lines – Model with Anions
Dotted lines – Model without Anions
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Higher abundances for
longer carbon-chain after
105yrs.
With anions, less
abundance for CnH, more
abundance for CnH2,
HCnN
Results : Reactions
Without Anions
C6
C6H2
C6H
C6Hm+
With Anions
C6
C6-
C6H
C7H
C6H-
C6H2
C6H2+
HC7N
Comparison to the Observation
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19 out of 23 species within an order of
magnitude, 14 species within factor of 3
agreement
Large disagreement for C4H-, order of
magnitude difference for C6H- at the optimal
time
Confidence level method (Garrod et al. 2006)
i  erfc(
| log( X i )  log( X i ,obs ) |
2
)
κ=0.63 – factor 3
κ=0.32 – factor 5
Our result has the average of slightly better than factor of 3 agreement.
Variation of Initial Abundances
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CH4 abundance
Initial
CH4
Confidence
Level
Optimal
Time (yrs)
Number of
Species in
Agreement
3(-07)
0.601
8800
18
1(-06)
0.642
6500
18
3(-06)
0.652
4800
18
1(-05)
0.652
4300
19
3(-05)
0.647
4300
19
This agrees with methane evaporation scenario.
Conclusion
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The observed species are reproduced with best
agreement around 5x103 yr.
The anion model causes very high synthetic
power.
Anion-to-neutral ratios are higher than the
observation. The radiative electron attachment
rate should be re-examined.
The anion model also needs more investigation
(extension of model, calculation of rates, etc).
Acknowledgement
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I would like to thank…
Nami Sakai and Satoshi Yamamoto
George Hassel
All of you