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Dust Extinction in the Diffuse Interstellar Medium

Dust Extinction in the Diffuse Interstellar Medium

Dust extinction measurements provide important constraints on the size ,composition, shape, and abundance of dust grains and an empirical model to account of the effects of extinction on astrophysical objects. For decades our understanding of dust grains was strongly biased by measurements in our Galaxy and the ultraviolet (UV). The UV bias is due to the extensive spectroscopic observations taken with the IUE satellite revealing the details of the 2175 A bump, far-UV rise, and underlying extinction continuum. I will discuss the results of a dedicated effort to expand our spectroscopic measurements of dust extinction to the far-UV, optical, near-infrared, and mid-infrared wavelength regimes. This work has revealed new optical extinction features, enabled the first combined combined study of UV and MIR extinction features, shown the possible presence of ice in the diffuse interstellar medium, and revealed an intriguing correlation between UV extinction and molecular hydrogen. Building on these works, a new R(V) dependent extinction relationship at spectroscopic resolution from 912 A to 32 microns has been determined. Moving out of our Galaxy, in progress work shows that the 2175 A bump is rare in an expanded sample of UV extinction curves and M31 and M33 show UV extinction curves quite similar to those seen our our Galaxy. Finally, prospects for future work especially with HST and JWST will be presented.

Karl Gordon

April 19, 2023

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  1. Dust Extinction in the Diffuse Interstellar Medium Karl D. Gordon,

    Astronomer STScI, Baltimore, MD, USA Cardiff University School of Physics & Astronomy 19 Apr 2023 [email protected] @astrodon.social karllark@github “Have Dust – Will Study” Slides on speakerdeck
  2. Summary • Extinction measurements are fundamental to understanding dust •

    Spectroscopic Extinction from FUV to MIR in Milky Way – Overall quite smooth – Few broad features including some new ones – One intriguing correlation, comforting non-correlations – One R(V) extinction relationship for all wavelengths • Beyond Milky Way (if there is time) – Mostly like the the Milky Way, but not all • Future w/ HST and JWST – More galaxies, more environments, larger samples
  3. Extinction • Critical clues to dust grain abundance, size, composition,

    and shape • Clues in the continuum and features • Straightforward measurement • Focus on spectroscopic measurements • Biased by my views, not comprehensive
  4. Not Covered • Dust scattering (e.g., albedo, scattering phase function)

    • Dust emission • Atomic composition of dust (e.g., depletions) • Dust Polarization • Dust grain models • All important, but not the focus of this talk
  5. Extinction (not Attenuation) • Extinction – Absorption and scattering out

    of line-of-sight – Specific to a point source dust – Proportional to dust grain properties • Attenuation – scattering into the line-of-sight – Varying extinction to stars – Applies to galaxies, circumstellar dust, etc.
  6. Basic measurement is the color “excess” versus wavelength (in magnitudes

    as we are astronomers) Normalize so measurements with different dust columns can be compared A(V) determined by extrapolating to infinite wavelength e.g.,
  7. Wavelength axis variations Allow full FUV-MIR extinction to be seen

    Wavelength scale is proportional to energy Emphasizes UV wavelengths Versus λ Versus 1/λ UV Opt NIR MIR
  8. Diffuse vs Dense Dust • Diffuse sightlines – Most detailed

    extinction studies – Generally A(V) values less than a few – No 3.0 μm H2 0 ice feature • Dense sightlines – Generally studied in the NIR/MIR only – Have 3.0 μm H2 0 ice feature Decleir et al. (2022, ApJ, 930, 15) H20 Ice
  9. 2175 A Bump width → strong variation center → almost

    no variation Fitzpatrick & Massa (1986, ApJ, 307, 286)
  10. Pre-FUSE ORFEUS: Sasseen et al. 2002, ApJ, 566, 267 Voyager:

    Snow et al. 1990, ApJ, 399, L23 See also: Buss et al. 1994, France et al. 2004; Lewis et al. 2005
  11. FUV (& all UV) extinction smooth Residuals due to stellar

    or ISM HI FUV rise consistent with feature peaking at ~800 Å
  12. FUV rise extinction component & H2 column Very strong correlation

    & goes through zero!!! Grain responsible for FUV rise and H2 co-spatial Not the case for the 2175 Å bump
  13. Pre-Hubble • Continuum measured mainly via photometry (until 2019) •

    Except for Orion dust – Cadelli & Clayton (1988, AJ, 95, 516) • Very Broad Structure with low resolution spectra – Next slide
  14. Very Broad Structure = Deviations from linear w/ 1/lambda Whiteoak

    1966, ApJ, 144, 305 Hayes 1973, IAUS, 53, 83
  15. Broad Optical Features (origin unknown) Long known Very Broad Structure

    Explained! Massa, Fitzpatrick, Gordon, et al. 2020, ApJ, 891, 67 Centers: 4370, 4870, & 6300 Å Widths: ~10% Two blue correlate w/ 2175 Å
  16. NIR/MIR Extinction • Continuum measured via photometry (until 2021) •

    Features measured spectroscopically – Narrow wavelength ranges or towards “complicated” stars – 3.0 μm ice – 3.4 μm hydrocarbon – 10/20 μm silicate features
  17. • Many ice features • 3.0 μm H2 0 the

    strongest • Requires A(V) > 3 Boogert, Gerakines, & Whittet
  18. 3.0 μm ice feature measured in the diffuse average at

    A(ice)/A(V) = 0.0019 +/- 0.007 Not a significant detection, but intriguing at the level predicted by Potapov et al. (2021) if ice is present in shadowed pits in silicate grains
  19. Variations and 1st Direct Comparison of UV and MIR Strong

    variation Not correlated with grain size Silicates not correlated with 2175 A bump
  20. Fitting a Line New penultimate technique* • Fully accounts for

    correlated x & y uncertainties • Allows for different correlations for different points • Line integral for each point *The authors await knowledge of what paper to cite where this was first shown (likely from decades ago)
  21. MW Extinction Summary • Spectroscopic from FUV to MIR –

    FUV smooth & consistent with feature peaking ~800 Å – H2 correlates with FUV rise → co-spatial! – New optical features → unknown origin – NIR powerlaw & may contain H2 0 ice – MIR versus UV features → 2175 Å not due to silicates – R(V) dependent extinction relationship → one relationship for all wavelengths
  22. SMC Extinction Curves Milky Way-like! (2175 Å bump) 4 similar

    curves are found in the star forming bar of the SMC! Ha image of the SMC Gordon & Clayton (1998, ApJ, 500, 816) Gordon et al. (2003, ApJ, 594, 279) STIS
  23. SMC AzV 456 SMC Bar LMC2 LMC General MW Quiescent

    Processed Continuum of Properties Gordon et al. (2003, ApJ, 594, 279) Gordon et al. (2016, ApJ, 826, 104)
  24. M31/M33 Extinction • All w/ HST photometry • Similar to

    MW • Radial, metallicity variations • 1st measurements in M33 Petia Yanchulov Merica-Jones Clayton, G et al., in prep Yanchulov M-J, P. et al., in prep
  25. Beyond the MW Summary • Large Magellanic Cloud (½ solar)

    – Most of LMC similar to MW – Differences in LMC2 Supershell (near 30 Dor) • Small Magellanic Cloud (1/5 solar) – Most very different from MW (very steep, no bump) – Small fraction, closer to MW (flatter w/ bump) • M31 (~solar) and M33 (½ solar) – Similar to MW (preliminary)
  26. HST • More galaxies – Investigate low metallicity galaxies •

    SMC unique? – Expand samples in large galaxies • Milky Way R(V) dependent extinction relationship works? • High/low R(V) – Probe extremes of dust size, curvature in R(V) relationship?
  27. JWST • Two cycle 1 programs (PIs: Decleir & Zeegers)

    – 21 Milky Way sightlines from 2.4-28 μm – Continuum and 3.4, 10, & 20 μm features • Map features photometrically • LMC/SMC continuum & silicate measurements • Other galaxies (sensitivity?)
  28. Summary • Extinction measurements are fundamental to understanding dust •

    Spectroscopic Extinction from FUV to MIR in Milky Way – Overall quite smooth, few broad features, DIBs (dust?) only narrow features – New broad features found in the optical – Intriguing correlations (or not) between features and gas tracers (esp. H2 ) – One R(V) extinction relationship for all wavelengths • Beyond Milky Way – Most of LMC, small fraction of SMC, M31, & M33 → similar to Milky Way – SMC & LMC2 Supershell region → weaker/non-existent bumps, stronger UV slope • Future w/ HST and JWST – More galaxies, more environments, larger samples