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The Enigmatic Diffuse Interstellar Bands: A Reservoir of Organic Material

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 :

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The Enigmatic Diffuse Interstellar Bands: A Reservoir of Organic Material

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  1. 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)

  2. pre 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

  3. Dense vs. Diffuse Clouds  Persei Dense molecular clouds: • H  H2 • C  CO • n(H2) ~ 104–106 cm-3 • T ~ 20 K Diffuse clouds: • H ↔ H2 • C  C+ • n(H2) ~ 101–103 cm-3 • T ~ 50 K Pound ApJ 493, L113 (1998) Photo: Jose Fernandez Garcia

  4. Molecules in Diffuse Clouds H· 3×109 M = 6×1042 g ~ 3×1066 atoms mm-wave: C2H ~ 3×1058 H2CO ~ 1×1058 HCN ~ 8×1057 CS ~ 5×1057 HCO+ ~ 5×1057 C3H2 ~ 2×1057 HNC ~ 1×1057 HOC+ ~ 8×1055 Optical: OH ~ 1×1059 CH+ ~ 8×1058 C2 ~ 5×1058 CN ~ 7×1057 CH ~ 5×1057 C3 ~ 4×1057 NH ~ 2×1057 Ultraviolet: H2 ~ 1×1066 CO ~ 2×1061 HD ~ 1×1060 N2 ~ 9×1058 HCl ~ 6×1056 Infrared: H3+ ~ 2×1059 DIBs ~ 1058 ? Based on abundances from Snow & McCall, ARAA 44, 367 (2006)

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

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

  7. DIBs as Radicals or Ions?

  8. A Growing Problem Tuairisg et al. 2000 Jenniskens & Desert 1994 Hobbs et al. 2008 Merrill & Wilson 1960 Merrill & Wilson 1938 Herbig 1988 Herbig 1975 Herbig 1966 Heger 1919

  9. Mostly in the visible • 4175–8763 Å • None known in UV • Few in near-infrared • 9577, 9632, 11797, 13175 Å

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

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

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

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

  14. 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

  15. Echelle Image Fraunhofer Lines H: Ca II ~ 3968 Å K: Ca II ~ 3934 Å D: Na I ~ 5890 Å A: O2 ~ 7650 Å K H D A

  16. Stellar Spectra C HD 50064 EB-V = 0.84 F B D A KH Cyg OB2 #12 EB-V = 3.2

  17. Stellar Spectra 4428 5780, 5797 CH

  18. Overview of Preliminary Results • Spectral atlas of DIBs • for comparison to laboratory spectra • Version 1: HD 204827 • Version 2: Four reddened stars • DIB correlations • between DIBs & other species • chemistry, environment of carriers • among DIBs • spectra of carriers

  19. DIB Spectral Atlas: Version 1 • Initial target: HD 204827 • heavily reddened: EB-V=1.11 • early spectral type: ~B0V • fairly bright: V=7.94 • member of open cluster Trumpler 37 • in Cepheus OB2 association • Abundant C2 • Champion of C3 Adamkovics, Blake, & McCall, ApJ 595, 235 (2003)

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

  21. Great Test for DIBs • DIB count: 267 confirmed, 113 new, 380 total! • ApJ in press (see http://dibdata.org)

  22. DIB Spectral Atlas: Version 2 Four heavily reddened stars:

  23. Sample of Spectra reddened previous work DIBs unreddened

  24. Statistics & Status • Issues still to address: • defining the continuum • blends of DIBs with each other • Complete atlas ~late 2008? 8 σ 4 σ 151 291 442 ― 284 350 634 4 σ

  25. DIB Correlations: H & H2 no correlation with H2 λ5780 well correlated with H [a la Herbig ApJ 407, 142 (1993)] r=0.944 (linear fit, w/o outliers) r=0.589 (linear fit) York et al., in preparation

  26. The “C2 DIBs” • First set of DIBs known to be correlated with a known species! Thorburn et al, ApJ 584, 339 (2003)

  27. Correlations Among DIBs A v=0 X v=0 • 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

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

  29. Example: Bad Correlation r=0.55

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

  31. 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

  32. 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?

  33. The Road to a Solution • Laboratory spectroscopy is essential! • Blind laboratory searches unlikely to work ~107 organic molecules known on Earth ~10200 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!

  34. Acknowledgments ACS Petroleum Research Fund Dreyfus New Faculty Award NASA Laboratory Astrophysics NSF Divisions of Chemistry & Astronomy Packard Fellowship Air Force Young Investigator Award Cottrell Scholarship http://astrochemistry.uiuc.edu

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

  36. 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 C7- McCall et al., ApJ 559, L49 (2001) l-C3H2- McCall et al., ApJ 567, L145 (2002)

  37. Carbon Chains as DIB Carriers? • Some DIBs correlated with C2 • C3 widely observed in diffuse clouds • J. P. Maier 2001 • But, search for C4, C5 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 • C2n+1 (n=7-15); HCnH (n>40); C2n (n>10); CnH; HCnH+; Cn- • No lab spectra of long chains; very little of cations Ádaámkovics, Blake, &McCall, ApJ 595, 235 (2003)

  38. 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

  39. Fullerenes as DIB Carriers? HD 183143 C60+ Ne matrix • Expected to form in carbon star outflows • IP(C60) = 7.6 eV • Ionized in diffuse clouds • C60+ in Ne matrix • two bands near 9600 Å • Detection claimed in HD 183143 • Need gas-phase spectrum! • Experiment in preparation Fulara, Jakobi, & Maier Chem. Phys. Lett. 211, 227 (1993) Foing & Ehrenfreund A&A 319, L59 (1997)

  40. C2 DIBs toward HD 62542 • 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 • C2 DIBs among the few DIBs observed • C2 DIBs evidently form in denser (molecular) regions Ádámkovics, Blake, & McCall ApJ 625, 857 (2005)

  41. DIB "Families" λ4727 λ4963 0.94 0.90 0.93 0.93 0.86 0.92 λ5418 λ4984 • Look for sets of DIBs in which all correlation coefficients are high λ6196 0.99 0.97 blend λ5780 λ6613 0.96 0.96 0.96 0.97 0.94 0.96 0.90 0.97 0.94 0.96 λ6204 λ5487 0.92 0.93 0.98 blend λ6284

  42. More on λ6613 & λ6196 Can observed scatter be due to measurement errors? • 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=0.985 r=0.996

  43. Measurement Errors • Is doubling the error estimate reasonable? • Error sources include: • finite S/N • unexpected structure • interfering DIBs? • continuum placement HD 281159 • Agreement not always perfect! • CH+ A-X band • Hard to say; but it’s certainly a very good correlation! r=0.985

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