The Nearly Perfect Correlation between the Diffuse Interstellar Bands λλ and Ben McCall Department of Chemistry and Department of Astronomy University of Illinois at Urbana-Champaign Collaborators: Meredith M. Drosback (Virginia), Julie Thorburn Dahlstrom (Carthage College), Don York (Chicago), Scott Friedman (STScI), Lew Hobbs (Yerkes), Brian Rachford (Embry Riddle), Ted Snow (Colorado), Paule Sonnentrucker (STScI), Dan Welty (Illinois)

Discovery of the DIBs 5780, 5797 seen as unidentified bands –  Per,  Leo (Mary Lea Heger, Lick, 1919) Broad (“diffuse”) “Stationary” (interstellar)

A Growing Problem Heger 1919 Merrill & Wilson 1938 Merrill & Wilson 1960 Herbig 1966 Herbig 1975 Herbig 1988 Jenniskens & Desert 1994 Tuairisg et al Hobbs et al Hobbs et al Greatest unsolved mystery in spectroscopy!

The APO DIB Survey Apache Point Observatory 3.5-meter 3,600–10,200 Å ; /  ~ 37,500 (8 km/s) 119 nights, from Jan 1999 to Jan 2003 S/N 5780Å) > 500 for 160 stars (114 reddened) Measurements & analysis still very much underway

Search for a Common Carrier Assumptions: –gas phase molecules –DIBs are vibronic bands –low temperature carriers all in v=0 –relative intensities fixed Franck-Condon factors independent of T, n Method: –look for DIBs with tight correlations in intensity Prospect: –identify vibronic spectrum of single carrier –spacings may suggest ID X v=0 A

DIB Correlations r=0.55 r=0.986

Statistics of Correlations 1218 pairs of DIBs observed in >40 stars 58 DIBs included Histogram of r Few very good correlations –19 with r > 0.95 Most strong DIBs have distinct carriers Still much work to do, especially on weaker bands!

Example APO DIB Spectra

Correlation 114 Sightlines f H2 = 2.6×10 -6 – 0.76 E B-V = 0.02 – % O, 68% B, 3% A K I components: 1 – 17

Ordinary Least Squares Assume a relationship y=α+βx Minimize sum of squared residuals Compute Pearson’s correlation coefficient α = -5.0±2.2, β = 3.96±0.06 r = 0.986, r 2 = % of variance in explained by Problems with least squares: –asymmetric treatment of two variables –ignores any knowledge of uncertainties

Error Estimates Statistical errors → easy to calculate from rms of “continuum” nearby Systematic errors → larger, harder to quantify –we don’t know the bandshape (limits of integration) –we don’t know the background (continuum) –there could be overlapping transitions (we know at least one!) –we adopt rms uncertainty in continuum fit (~10× the statistical errors) twice the rms continuum shift

Maximum Likelihood Functional Relationship Used to compare different analytical techniques Equivalent to: –“heteroscedastic errors-in-variables model” –FITEXY from Numerical Recipes Assume functional relationship v i = α+βu i –v i, u i “true” values, contaminated by errors → y i, x i –errors independent, normal, stdev’s σ xi & σ yi –minimize the quantity:

MFLR Results Expect ΣS i 2 /(N-2) ~1; we get 3.35 Chi-square probability function [p-value or Q(χ 2 |ν)] = 3.9× !! This is the probability that observed sum-of- squares would exceed this value based on chance alone, if underlying model is correct. Either not a perfect relationship, or we’ve underestimated our errors. What if true errors are twice our estimates? ΣS i 2 /(N-2) = 0.84, p-value = 0.89 perfect linear relationship!

Comparison with Other Correlations r=0.821 r=0.953 (w/o outliers) CH + A-X 1-0 R(0) CH + A-X 0-0 R(0) r=0.985

Two Possibilities A B C Galazutdinov et al. A&A 384, 215 (2002) Ueda & Shimanouchi JMS 28, 350 (1968) λλ and have the same carrier –first ID of two DIBs from same molecule –ratio of Franck-Condon factors ~1:4 –excited state vibrational spacing cm -1 –search for other (weak) DIBs from this carrier! –need to explain differences in width & shape λλ and don’t share a carrier –two molecules are amazingly well correlated –best correlation ever between molecules –what kind of chemical pathways can maintain abundance ratio so constant, over such a wide range of conditions?

Future Work Needed 1)More thorough investigation of potential error sources → better estimates of uncertainties? 2)Search for some parameter that correlates with residuals → clues to interfering lines? 3)Observations with higher S/N, resolving power → help resolve interfering lines 4)Theoretical explanation of how two vibronic bands could (or could not) produce such different profiles → plausibility/disproof of conclusion of common carrier

Reasonable correlation with dust extinction –but “level off” at high A V → diffuse clouds only? –for a long time, solid state carriers favored Several characteristics argue against dust: –constancy of –lack of emission –fine structure! Present consensus: –gas-phase molecules –probably large –likely carbon-based –reservoir of organic material Greatest unsolved mystery in spectroscopy! What are the DIBs? Sarre et al., MNRAS 277, L41 (1995)