Modeling Linear Molecules as Carriers of the 5797 and 6614 Å Diffuse Interstellar Bands Jane Huang,...
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Transcript of Modeling Linear Molecules as Carriers of the 5797 and 6614 Å Diffuse Interstellar Bands Jane Huang,...
Modeling Linear Molecules as Carriers of the5797 and 6614 Å Diffuse Interstellar Bands
Jane Huang, Takeshi Oka
69th International Symposium on Molecular SpectroscopyJune 16, 2014University of Illinois, Urbana-Champaign
• DIBs are broad absorption features observed in spectra of hundreds of stars
• Discovered in early 1900s, but carriers remain unidentified• Most often attributed to electronic transitions of gas phase
molecules in interstellar medium• Hypothesized carriers include carbon chains or polycyclic
aromatic hydrocarbons • Identifying carriers will allow for more detailed studies of the
interstellar medium
The spectroscopic mystery of diffuse interstellar bands
Motivation: The Anomalous Herschel 36 DIBs
Oka, T., Welty, D. E., & Johnson, S. et al. 2013, ApJ, 773, 42
• Her 36 near Her 36 SE, an IR source
• Models of Her 36 DIBs indicated that “extended tails toward red” could be produced by electronic transitions of polar linear molecules if IR pumping occurred
The λ5797 DIB
Kerr, T. H., Hibbins, R. E., Fossey, S. J., Miles, J. R., & Sarre, P. J. 1998, ApJ, 495, 941
The λ6614 DIB
Galazutdinov, G. A., Lo Curto, G., & Krełowski, J. 2008, MNRAS, 386, 2003
Modeling spin-orbit effects
•Assumed Hund’s case (a): Ω, Λ, ∑ are good quantum numbers (line intensities calculated from Kovacs 1969)
•Hypothesized transitions:▫λ5797: 2Π2Π▫λ6614: 2Δ2Π 2Π2Π schematic
Model inputs and assumptionsAssumptions Important variables
Based on original Her 36 models:▫ Linear molecule ▫ μ = 4 Debye (permanent
dipole moment)▫ Tk = 100 K (kinetic
temperature)▫ C = 1.0 x 10-7 s-1
(collision rate)▫ Δt = 50 ps (spontaneous
emission lifetime)
▫ Ground and excited state rotational constants
▫ Tr (radiative temperature)
▫ Origins and relative intensities of spin-orbit components
Assessing molecular size constraints
B (MHz) B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
R
1250 0.99 2.73 5797.0 Å 5797.2 Å 2.75 115000
Modeling the5797 ÅDIB
2Π 2Π
B (MHz) B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
1400 0.98 2.73 5797.0 Å 5797.3 Å 2.7
B (MHz)
B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
R
1130 0.944 5.57 6613.2 Å 6613.62 Å 1.58 115000
Modeling the 6614 Å DIB
2Δ2Π
Re-examining the Her 36 DIBs
It is important to reconcile models for “typical” DIBs (such as those of 20 Aql) with the anomalous Her 36 DIBs (hypothesized high Tr due to infrared pumping from Her 36 SE) (Oka et al. 2013)
B (MHz)
B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
R
1130 0.944 60 6613.2 Å 6613.62 Å 1.58 48000
2Δ2Π
Modeling the anomalous 6614 Å DIB
B (MHz) B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
R
1250 0.99 25 5797.15 Å 5797.35 Å 2.75 48000
The anomalous 5797 Å DIB, model 1
2Π 2Π
B (MHz)
B Tr (K) λ0 , Ω=3/2 λ0 , Ω=1/2 Spin-orbit component relative intensity
R
1250 0.99 2.73, 25 5797.15 Å 5797.35 Å 2.75 48000
The anomalous 5797 Å DIB, model 2
2Π 2Π
Implied characteristics of carriers•Large B (>1000 MHz) implies relatively
small carriers (i.e., no more than 6 carbons or similarly heavy atoms)
•Caveat: ▫Maier et al. (2011) have argued linear
molecules had to be >10 carbons in order to have sufficient oscillator strength to produce observed DIBs
▫Maier & collaborators have obtained electronic spectra ruling out a number of smaller carbon chains as carriers
Examples of possible open-shell, linear molecules w/ 6 heavy atomsComposition of interstellar clouds places additional constraints on make-up of carrier candidates: C, N, O (possibly S, Si)
i.e.
• HC5N
• HC4NC+, HC4NC -
• SiC5+, SiC5
-
• C5S +, C5S –
Some other molecules fitting these criteria have been studied and rejected as carriers (C6H, NC4N+)
Ball, C. D., McCarthy, M. C., & Thaddeus, P. 2000, ApJL, 529, 61
• Band at 4429 Å, suggested to be due to near-prolate top (Araki, M., et al. 2004, ApJ, 616, 1301)
• Similar molecule may account for 5797 DIB
• Best fit: Planar oblate symmetric top, 14-30 carbon atoms (Kerr, T.H., Hibbins, R.E., Miles, J.R. et al., 1996, MNRAS, 283, L105)
• Such a carrier would not be affected by differences in radiative temperature (Oka et al. 2013)
Some alternative interpretations
Conclusions• Spin-orbit splitting may explain fine
structure observed in λ5797 and λ6614 DIBs• Model provides plausible fits to λ6614
spectra, although other models have also achieved good fits
• λ5797 simulation fit is not as close, but fewer alternatives have been posited
• Polar linear molecules warrant further investigation as carriers of the λ5797 and λ6614 DIBs
ReferencesAraki, M., et al. 2004, ApJ, 616, 1301Ball, C. D., McCarthy, M. C., & Thaddeus, P. 2000, ApJL, 529,
61Galazutdinov, G. A., Lo Curto, G., & Krełowski, J.
2008, MNRAS, 386, 2003Kerr, T.H., Hibbins, R.E., Miles, J.R. et al., 1996, MNRAS, 283,
L105Kerr et al., T.H., Hibbins, R.E., Fossey, S.J. et al, ApJ, 1998,
495, 941Kovacs, I. Rotational Structure in the Spectra of Diatomic
Molecules, 1st ed.; Academiai Kiado, 1969Maier, J. P., Walker, G. A. H., Bohlender, D. A. et al., ApJ, 2011,
726, 41Oka, T., Welty, D., Johnson, S. et al, 2013, ApJ, 773, 42
Acknowledgements
Donald YorkDan WeltySean Johnson
This presentation used data obtained from the ESO Science Archive Facility, based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 078.C-0403, 077.B-0348, 079.D-0564, 081.D-2008, and 083.D-0589.