Detailed Line Profile Simulations of the λ6614 diffuse interstellar band

A synthetic DIB spectrum as seen toward HD 183143. Each white line represents a known DIB. The inset shows two strong DIBs that are now known to be caused by C60+ . (Figure provided by Jan Cami) Detailed Line Profile Simulations of the λ6614 diffuse interstellar band •The DIB mystery Charmi Bhatt ([email protected]), Department of Physics and Astronomy Co-authors: Jan Cami, Peter J. Sarre, Harold Linnartz and the EDIBLES collaboration The diffuse interstellar bands (DIBs) are: •are more than 500 absorption features that are seen ubiquitously from the near-UV to the near-IR [1]. • are caused by interstellar molecules, most likely large carbonaceous species. • have been unidentified since their first discovery over 100 years ago [2] – except for C60+ that is responsible for 2 strong and 3 weak DIBs in the near-IR [3]. References [1] Cami, J. & Cox, N. L. J., eds. 2014, IAU Symposium, Vol. 297, The Diffuse Interstellar Bands [2] Heger, M. L. 1922, Lick Observatory Bulletin, 10, 146 [3] Campbell, E. K., Holz, M., Gerlich, D., & Maier, J. P. 2015, Nature, 523, 322 Here I focus on the DIB at 6614 Å which has a triple peak structure. The goal is to simulate the detailed band structure of the λ6614 DIB, including its variations across different single-cloud sightlines, to determine size (and physical environment) of the unidentified interstellar molecule. We used a LevenbergMarquardt algorithm to determine molecular parameters that best match variations in λ6614 profiles, allowing for different line widths and rotational temperatures per sightline. We modeled 12 single-cloud sightlines and found that λ6614 profiles can be reproduced by a ~54C disc-like molecule with a rotational temperature of ~85K. Best fit models (in red) compared to observations (in black) are shown here. •Summary of Results The key new finding is that the sightline-to-sightline variations in λ6614 profiles are driven by linewidth.  Moreover, the linewidth of λ6614 carrier correlates with that of CH+, indicating that they might be co-spatial.  We developed rotational contour model to simulate molecular absorption by planar PAH-like molecules. The detailed profile shapes change markedly for different molecular size and temperature. The colored profiles shown below are models while observed profile is shown in black.  We measured the KI and CH+ linewidths of along these sightlines, finding an average of 1.4 ± 0.3 km s−1 for KI; and is 3.2 ± 0.3 km s−1 for CH+. We find correlation coefficient of 0.66 between CH+ and 6614 DIB carrier linewidths. Download the research article