Detailed Line Profile Simulations of the λ6614 Diffuse Interstellar Band

Fig 4: Best-fit models compared to the EDIBLES observations. Each sightline is characterized by its own environmental parameters, but the molecular parameters are the same across all sightlines. Note that the variations in linewidth are much larger compared to variations in temperature. 1. The Diffuse Interstellar Bands (DIBs) •are more than 600 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 +whichis responsible for 2strong and 3 weak DIBs in the near-IR [3]. Some DIBs show profiles with double or triple sub-peaks which resemble unresolved rotational contours of large molecules. 2. Rotational Contour Simulations Detailed Line Profile Simulations of the λ6614 Diffuse Interstellar Band Charmi Bhatt (She/Her), Western University, London, Ontario, Canada. Collaborators: Jan Cami, Peter J. Sarre, Harold Linnartz, and the EDIBLES collaboration Our goal is to simulate the detailed band structure of the 6614 DIB, including its variations across different single-cloud sightlines, with a single rotational contour model for molecular species with an oblate symmetry. 3. Sightline-to-Sightline Variations We used a Levenberg-Marquardt 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. smaller larger cooler hotter References [1] Cami, J. & Cox, N. L. J., eds.2014,IAU Symposium, Vol.297,The Diffuse Interstellar Bands [6] MacIsaac, H., Cami, J., Cox, N. L. J., et al. 2022, A&A, 662, A24 [4] Kerr, T. H., Hibbins, R. E., Miles, J. R., et al. 1996, Monthly Notices of the Royal Astronomical Society, 283, L105 [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 [5]Cami, J., Salama, F., et al.2004, The Astrophysical Journal, 611, L113 We focus here on the λ6614 DIB: •Note a clear triple peak sub-structure. •Early studies suggest carriers are: •14-30 C atom ring molecules or >40 C atom PAHs [4]. •Variations in peak separations interpreted as changes in rotational temperature [5,6]. 3. Apply Boltzmann population distribution to lower state. Multiply Boltzmann factors with intrinsic line strengths. wavenumber Intensity 4. Convolve linelist with gaussian intrinsic line profile and do basic radiative transfer to turn into absorption spectrum. 1. Calculate rotational level structure as function of quantum numbers J and K and apply selection rules to make a line list. Energy (J,K) K = 0 K = 1 K = 2 K = 3 K = 4 K = 5 Here’s how we simulate these bands in 4 major steps: 2. Transition frequencies = Energy differences. Intrinsic line strengths = Hönl-London factors. The key new finding of this paper 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 measured the KI and CH+linewidths (bvalues) 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. Sightline-to-sightline variations in the λ6614 DIB profile are driven by linewidth. -A surprising connection to CH+ cbhat[email protected]a Fig 2: λ6614 shows a clear triple peak substructure (Cami et al. 2004) Fig 3: The detailed profile shapes change markedly for different molecular size and rotational temperature