The Enigmatic Diffuse Interstellar Bands: A Reservoir of Organic Material Ben McCall Department of Chemistry and Department of Astronomy University of Illinois at Urbana-Champaign Apache Point Observatory DIB Collaboration: Meredith Drosback (Colorado), Scott Friedman (STScI), Lew Hobbs (Yerkes), Ben McCall (UIUC), Takeshi Oka (Chicago), Brian Rachford (ERAU), Ted Snow (Colorado), Paule Sonnentrucker (JHU), Julie Thorburn (Yerkes), Dan Welty (Chicago), Don York (Chicago)

Astro biology Precursors in diffuse clouds? Precursors in dense clouds? Precursors in disks? Delivery to planets? Production of pre-biotic molecules on planets Formation of life ^ pre

Dense vs. Diffuse Clouds Diffuse clouds: H ↔ H 2 C  C + n(H 2 ) ~ 10 1 –10 3 cm -3 T ~ 50 K  Persei Photo: Jose Fernandez Garcia Dense molecular clouds: H  H 2 C  CO n(H 2 ) ~ 10 4 –10 6 cm -3 T ~ 20 K Pound ApJ 493, L113 (1998)

Molecules in Diffuse Clouds H· 3×10 9 M = 6×10 42 g ~ 3×10 66 atoms Optical: OH~ 1×10 59 CH + ~ 8×10 58 C 2 ~ 5×10 58 CN~ 7×10 57 CH~ 5×10 57 C 3 ~ 4×10 57 NH~ 2×10 57 mm-wave: C 2 H~ 3×10 58 H 2 CO~ 1×10 58 HCN~ 8×10 57 CS~ 5×10 57 HCO + ~ 5×10 57 C 3 H 2 ~ 2×10 57 HNC~ 1×10 57 HOC + ~ 8×10 55 Based on abundances from Snow & McCall, ARAA 44, 367 (2006) DIBs ~ ? Ultraviolet: H 2 ~ 1×10 66 CO~ 2×10 61 HD~ 1×10 60 N 2 ~ 9×10 58 HCl~ 6×10 56 Infrared: H 3 + ~ 2×10 59

Discovery of the DIBs B. J. McCall, in preparation 5780, 5797 seen as unidentified bands –  Per,  Leo (Mary Lea Heger, Lick, 1919) Broad (“diffuse”) Possibly “stationary”

Interstellar Origin P. W. Merrill, ApJ 83, 126 (1936) P. W. Merrill, PASP 483, 179 (1936)

DIBs as Radicals or Ions?

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. 2008

Mostly in the visible –4175–8763 Å None known in UV Few in near-infrared –9577, 9632, 11797, Å

Examples: Lorentzian Profiles Snow, Zukowski, & Massey, ApJ 578, 877 (2002) 0.38 ps

Examples: Smooth Profiles Krelowski & Schmidt, ApJ 477, 209 (1997) M. Drosback, Ph.D. Thesis (2006)

Examples: Fine Structure Galazutdinov et al., MNRAS 345, 365 (2003)

Examples: Molecular Structure? Kerr et al., MNRAS 283, L105 (1996) Sarre et al., MNRAS 277, L41 (1995) Galazutdinov et al., MNRAS 345, 365 (2003)

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 (115 reddened) Measurements & analysis still underway

Echelle Image K H D A Fraunhofer Lines H: Ca II ~ 3968 Å K: Ca II ~ 3934 Å D: Na I ~ 5890 Å A: O 2 ~ 7650 Å

Stellar Spectra A B C D F KH HD E B-V = 0.84 Cyg OB2 #12 E B-V = 3.2

Stellar Spectra 4428 CH 5780, 5797

Overview of Preliminary Results Spectral atlas of DIBs –for comparison to laboratory spectra –Version 1: HD –Version 2: Four reddened stars DIB correlations –between DIBs & other species → chemistry, environment of carriers –among DIBs → spectra of carriers

DIB Spectral Atlas: Version 1 Initial target: HD –heavily reddened: E B-V =1.11 –early spectral type: ~B0V –fairly bright: V=7.94 –member of open cluster Trumpler 37 in Cepheus OB2 association Abundant C 2 Champion of C 3 Adamkovics, Blake, & McCall, ApJ 595, 235 (2003)

Spectroscopic Binary! 10 Lac avg Night 1 Night 2 Night 3 Night 4 Night 5

Great Test for DIBs DIB count: 267 confirmed, 113 new, 380 total! ApJ in press (see

DIB Spectral Atlas: Version 2 Four heavily reddened stars: StarSp. TypeE B-V N(C 2 ) HD B0V ×10 12 Cyg OB2 5O7f ×10 12 HD O8e ×10 12 HD B7Iae1.27< 6×10 12

Sample of Spectra reddened unreddened previous work DIBs

Statistics & Status Issues still to address: –defining the continuum –blends of DIBs with each other Complete atlas ~late 2008? Criterion (2 stars) Criterion (4 stars) New DIBs Confirmed DIBs Total DIBs 10 σ5 σ σ ― σ4 σ

DIB Correlations: H & H 2 r=0.944 (linear fit, w/o outliers) r=0.589 (linear fit) λ5780 well correlated with H [a la Herbig ApJ 407, 142 (1993)] York et al., in preparation no correlation with H 2

The “C 2 DIBs” First set of DIBs known to be correlated with a known species! Thorburn et al, ApJ 584, 339 (2003)

Correlations Among DIBs 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

Correlation Plots A B Star #1 Star #2 Star #3 Intensity Wavelength Strength of DIB A Strength of DIB B Fixed ratio

Example: Bad Correlation r=0.55

Example: Good Correlation r=0.985 measurement errors could be causing deviations?

Statistics of Correlations 58 strong DIBs Pairs of DIBs observed in >40 stars 1218 pairs Generally well correlated Few very good correlations –118 with r > 0.90 –19 with r > 0.95

Why So Few Perfect Correlations? Assumption of a common ground state bad? –energetically accessible excited states? spin-orbit splitting? –if linear molecules low lying vibrationally excited states? –if very large molecules “Vertical” transitions? –intense origin band –weaker vibronic bands correlations could be seen with weaker bands? Lots of individual molecular carriers?

The Road to a Solution Laboratory spectroscopy is essential! Blind laboratory searches unlikely to work ~10 7 organic molecules known on Earth ~ stable molecules of weight < 750 containing only C, H, N, O, S Observational constraints & progress are also essential! Computational chemistry will play an important role Close collaborations needed! Great astrobiological significance!

NASA Laboratory Astrophysics Acknowledgments NSF Divisions of Chemistry & Astronomy ACS Petroleum Research Fund Dreyfus New Faculty Award Packard Fellowship Air Force Young Investigator Award Cottrell Scholarship

Infrared spectra of ions important in: –astrochemistry –atmospheric chemistry –propulsion/combustion SCRIBES: Sensitive Cooled Resolved Ion BEam Spectroscopy Advertisement Optical spectra → DIBs ?

Evaluation of Proposed DIB Carriers Need a laboratory spectrum –gas phase (avoid matrix shifts) –rotationally resolved (or profile resolved) Need to be able to simulate spectrum –interstellar temperatures, excitation conditions DIB, simulated spectra must match exactly –central wavelength & profile –relative intensities & correlation –all laboratory bands present McCall et al., ApJ 559, L49 (2001) C7-C7- l-C 3 H 2 - McCall et al., ApJ 567, L145 (2002)

Carbon Chains as DIB Carriers? Some DIBs correlated with C 2 C 3 widely observed in diffuse clouds –J. P. Maier 2001 But, search for C 4, C 5 unsuccessful so far Conclusions: –Need high abundance, or –Large oscillator strength –Maier, Walker, & Bohlender [ApJ 602, 286 (2004)]: Potential carbon chain DIB carriers must have >15 carbon atoms C 2n+1 (n=7-15); HC n H (n>40); C 2n (n>10); C n H; HC n H + ; C n - –No lab spectra of long chains; very little of cations Ádaámkovics, Blake, &McCall, ApJ 595, 235 (2003)

PAHs as DIB Carriers? Polycyclic Aromatic Hydrocarbons –proposed by Leger & d’Hendecourt and by van der Zwet & Allamandola in 1985 Would expect complex mixture –ionization stages (cation, neutral, anion?) –hydrogenation states So far, no spectroscopic match with DIBs Cation transitions observed so far in gas- phase are too broad! Still no convincing evidence

Fullerenes as DIB Carriers? Expected to form in carbon star outflows IP(C 60 ) = 7.6 eV –Ionized in diffuse clouds C 60 + in Ne matrix –two bands near 9600 Å Detection claimed in HD Need gas-phase spectrum! –Experiment in preparation Fulara, Jakobi, & Maier Chem. Phys. Lett. 211, 227 (1993) Foing & Ehrenfreund A&A 319, L59 (1997) C 60 + Ne matrix HD

C 2 DIBs toward HD Unusual sightline with only diffuse cloud “core” –outer layers stripped away by shock (?) –DIBs undetected [Snow et al. ApJ 573, 670 (2002)] Recent Keck observations (higher S/N) –Classical DIBs (e.g. λ5780) very weak –C 2 DIBs among the few DIBs observed C 2 DIBs evidently form in denser (molecular) regions Ádámkovics, Blake, & McCall ApJ 625, 857 (2005)

DIB "Families" Look for sets of DIBs in which all correlation coefficients are high λ6196 λ5780 λ6204 λ6284 λ5487 λ λ4963 λ4727 λ4984 λ blend

More on λ6613 & λ6196 Observed r=0.985 Assume perfection Add Gaussian noise 1000 M.C. trials Double the noise 1000 M.C. trials Statistically OK if we underestimated errors r=0.996 r=0.985 Can observed scatter be due to measurement errors?

Measurement Errors Is doubling the error estimate reasonable? Error sources include: –finite S/N –unexpected structure HD –interfering DIBs? –continuum placement Agreement not always perfect! –CH + A-X band Hard to say; but it’s certainly a very good correlation! r=0.985