Line Profiles of the Diffuse Interstellar Bands M

Line Profiles of the Diffuse Interstellar Bands M. M. Drosback, T. P. Snow, J. A. Thorburn, L. M. Hobbs, D. E. Welty, D. G. York, B. J. Mc. Call, P. Sonnentrucker, S. D. Friedman, and B. L. Rachford International Symposium on Molecular Spectroscopy June 23, 2005

Outline § Introduction to the Diffuse Interstellar Bands (DIBs) § Line Profile Study & Methodology § Analysis of 4 DIBs in progress § Results & Conclusions M. Drosback June 23, 2005

Diffuse Interstellar Bands § First observed around 1921 by Mary Lea Heger, two features noted § Merrill’s studies in 1934 showed DIBs to be of interstellar origin § Proposed carriers: molecules, solid-state (dust), ions § Seen in spectra of reddened stars, throughout visible portion of spectrum § About 700 features known today M. Drosback June 23, 2005

DIB Database § Echelle Spectrograph (R~ 38, 000) at the Astrophysical Research Consortium 3. 5 -m telescope at Apache Point Observatory § Goal: S/N » 1000 at 5780 Å § Currently completed observations of nearly 200 stars M. Drosback June 23, 2005

Line Profile Analysis § Goal is to measure line profiles of DIBs to characterize physics of transitions § Assume that all stellar photospheric lines act to decrease intensity; underlying profile is intrinsic to DIB § Without removing stellar features, fit the upper envelope of the absorption by eye M. Drosback June 23, 2005

4428 Å : An Example DIB M. Drosback June 23, 2005

Fitting Functions § To a single DIB in all lines of sight, fit three profiles: �Gaussian �Lorentzian �Drude (solid state absorption profile; similar to Lorentzian function, but slightly asymmentric) M. Drosback June 23, 2005

Fitting Functions M. Drosback June 23, 2005

4428 Å DIB § Broadest of the DIBs § First published observation by Merrill (1936) § Little work done focusing on the profile over the years § Snow et al. (2002) studied 35 lines of sight toward Cyg OB 2 association; found the profile to be invariant to within their errors and best fit by a Lorentzian M. Drosback June 23, 2005

4428 Å Sample fits M. Drosback June 23, 2005

Results for 4428 Å DIB § 52 lines of sight § Lorentzian and Drude are better fits than Gaussian § Drude profile is unlikely candidate due to lack of correlation between measured FWHM and c § Best fit parameters: � c = 4428. 5 ± 0. 8 Å �FWHM = 18. 3 ± 2. 0 Å�� M. Drosback June 23, 2005

6284 Å DIB § May have been observed as early as 1930 by Merrill § Suggested by Mc. Call et al. (2001) also to have Lorentzian profile similar to 4428 Å DIB M. Drosback June 23, 2005

6284 Å Sample Fits M. Drosback June 23, 2005

Results for 6284 Å DIB § 113 lines of sight § Like 4428 Å DIB, Lorentzian and Drude functions represented best fit § Drude profile eliminated based on same argument as for 4428 DIB Å § Best fit parameters: � c = 6284. 0 ± 0. 2 Å�� �FWHM = 3. 9 ± 0. 2 Å M. Drosback June 23, 2005

4762 Å Sample Fits M. Drosback June 23, 2005

4762 Å Results § 131 lines of sight § Results of this study are inconclusive; none of the fits are consistently better than the others § Individual line of sight stellar modeling could produce more definitive results M. Drosback June 23, 2005

6196 Å Sample Fits M. Drosback June 23, 2005

6196 Å Results § 141 Lines of sight § Gaussian appears to be best fit to this data set § Best fit parameters: � c = 6196. 0 ± 0. 1 Å �FWHM = 0. 47 ± 0. 03 Å M. Drosback June 23, 2005

Summary § Of the 4 DIBs in the study: � 2 were determined to have Lorentzian lineshapes � 1 was best fit by a Gaussian lineshape � 1 failed to yield conclusive results § This method will not work for all DIBs, particularly those in regions crowded with stellar features and those with internal structure M. Drosback June 23, 2005

Lorentzian Implications § Carrier is molecular § Can calculate lifetime of upper state of transition �lifetime = 3. 6 x 10 -13 s for 4428 Å �lifetime = 1. 7 x 10 -12 s for 6284 Å § Short lifetimes could be indicative of internal energy conversion in large molecule M. Drosback June 23, 2005

Future Work § Continue to pursue more DIB profiles with this type of analysis § Fit and remove stellar features in crowded areas around DIBs of interest to more obviously reveal intrinsic DIB profiles M. Drosback June 23, 2005