Transcript DrosbackOhioState2005.ppt

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. McCall, 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 OB2 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 McCall 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