Modeling Linear Molecules as Carriers of the 5797 and 6614 Å Diffuse Interstellar Bands Jane Huang, Takeshi Oka 69 th International Symposium on Molecular Spectroscopy June 16, 2014 University 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: 2 Π  2 Π ▫λ6614: 2 Δ  2 Π 2 Π  2 Π schematic

Model inputs and assumptions AssumptionsImportant variables Based on original Her 36 models: ▫Linear molecule ▫μ = 4 Debye (permanent dipole moment) ▫T k = 100 K (kinetic temperature) ▫C = 1.0 x s -1 (collision rate) ▫Δt = 50 ps (spontaneous emission lifetime) ▫Ground and excited state rotational constants ▫T r (radiative temperature) ▫Origins and relative intensities of spin-orbit components

Assessing molecular size constraints

B (MHz)BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity R Å Å Modeling the 5797 Å DIB 2Π  2Π2Π  2Π

B (MHz)BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity Å Å2.7 B (MHz) BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity R Å Å Modeling the 6614 Å DIB 2Δ2Π2Δ2Π

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 T r due to infrared pumping from Her 36 SE) (Oka et al. 2013)

B (MHz) BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity R Å Å Δ2Π2Δ2Π Modeling the anomalous 6614 Å DIB

B (MHz)BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity R Å Å The anomalous 5797 Å DIB, model 1 2Π  2Π2Π  2Π

B (MHz) BT r (K)λ 0, Ω=3/2λ 0, Ω=1/2Spin-orbit component relative intensity R , Å Å The anomalous 5797 Å DIB, model 2 2Π  2Π2Π  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 atoms Composition of interstellar clouds places additional constraints on make-up of carrier candidates: C, N, O (possibly S, Si) i.e. HC 5 N HC 4 NC +, HC 4 NC - SiC 5 +, SiC 5 - C 5 S +, C 5 S – Some other molecules fitting these criteria have been studied and rejected as carriers (C 6 H, NC 4 N + )

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, 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

References Araki, M., et al. 2004, ApJ, 616, 1301 Ball, C. D., McCarthy, M. C., & Thaddeus, P. 2000, ApJL, 529, 61 Galazutdinov, G. A., Lo Curto, G., & Krełowski, J. 2008, MNRAS, 386, 2003 Kerr, T.H., Hibbins, R.E., Miles, J.R. et al., 1996, MNRAS, 283, L105 Kerr et al., T.H., Hibbins, R.E., Fossey, S.J. et al, ApJ, 1998, 495, 941 Kovacs, I. Rotational Structure in the Spectra of Diatomic Molecules, 1st ed.; Academiai Kiado, 1969 Maier, J. P., Walker, G. A. H., Bohlender, D. A. et al., ApJ, 2011, 726, 41 Oka, T., Welty, D., Johnson, S. et al, 2013, ApJ, 773, 42

Acknowledgements Donald York Dan Welty Sean 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.