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.