Far-Ultraviolet to Mid-Infrared Dust Extinction

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, existing and new work shows that the 2175 A bump is rare in an expanded sample of SMC UV extinction curves. Combining the Galaxy, LMC, and SMC extinction curves finds that there are trends in extinction curve shape with gas-to-dust ratio.

Talk given at Chalmers Univ in Gothenberg, Sweden

Transcript

1. Karl D. Gordon, Astronomer STScI, Baltimore, MD, USA Visiting Prof, Ghent University Chalmers Univ Gothenberg, Sweden 4 Dec 2024 kgordon@stsci.edu karllark@github “Have Dust – Will Study” Slides on speakerdeck Far-Ultraviolet to Mid-Infrared Dust Extinction Curves in the Milky Way, LMC, and SMC
2. Ask me about Dust and ... Brewing Woodworking 3 postdoc position on stellar populations, carbonaceous/PAH features, and UV spectroscopy in the ISM*@ST group
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. 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.
5. Extinction vs Attenuation Github: karllark/dust_attenuation Github: karllark/dust_extinction
6. Pair Method Github: karllark/measure_extinction OB stars ideal targets Luminous at all wavelengths Fairly simple atmospheres
7. 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.,
8. 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
9. 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
10. Our Galaxy
11. Stecher 1969, ApJ, 157, 125
12. International Ultraviolet Explorer 18 years! 100,000 UV spectra!
14. Witt, Bohlin, & Stecher 1984, 279, 698
15. 2175 A Bump width → strong variation center → almost no variation Fitzpatrick & Massa (1986, ApJ, 307, 286)
16. Wolff, Clayton, Kim, Martin, & Anderson
17. Fitzpatrick & Massa 1988, ApJ, 328, 734
19. Cardelli, Clayton, & Mathis 1988, ApJ, 329L, 33; 1989, ApJ, 345, 245
20. Valencic, Clayton, & Gordon 2004, ApJ, 616, 912
21. Far Ultraviolet Spectroscopic Explorer 905 to 1195 Å
22. 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
23. Gordon, Cartledge, & Clayton 2009, ApJ, 705, 1320 See also: Sofia et al. 2005
24. FUV (& all UV) extinction smooth Residuals due to stellar or ISM HI FUV rise consistent with feature peaking at ~800 Å
25. 2023, ApJ, 944, 33 Dries Van De Putte
26. 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
27. Hubble Space Telescope 912 Å to 2.5 μm
28. Pre-Hubble • Optical 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
29. Very Broad Structure = Deviations from linear w/ 1/lambda Whiteoak 1966, ApJ, 144, 305 Hayes 1973, IAUS, 53, 83
31. Broad Optical Features (origin unknown) Long known Very Broad Structure Explained! Massa, Fitzpatrick, & Gordon 2020, ApJ, 891, 67 Centers: 4370, 4870, & 6300 Å Widths: ~10% Two blue correlate w/ 2175 Å
32. ISS features nearly as strong as Si features
33. 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
34. • Many ice features • 3.0 μm H2 0 the strongest • Requires A(V) > 3 Boogert, Gerakines, & Whittet
35. 3.4 μm Hydrocarbon grains Pendelton & Allamandola (2002, ApJS, 138, 75)
36. 10 & 20 μm Silicate Features Chiar & Tielens (2006, ApJ, 637, 774)
37. Crystalline Silicate Fraction ~0.2% (or zero) Kemper, Viernd, & Tielens (2004, ApJ, 609, 826) Amorphous Crystalline
38. NASA’s Infrared Telescope Facility 0.8 to 20 μm
39. Marjorie Decleir
40. 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
41. Spitzer Space Telescope 3 to 160 μm
44. Variations and 1st Direct Comparison of UV and MIR Strong variation Not correlated with grain size Silicates not correlated with 2175 A bump
45. Putting all the Wavelengths Together
46. Cardelli, Clayton, & Mathis 1988, ApJ, 329L, 33; 1989, ApJ, 345, 245
47. ApJ, 2023, 950, 86 Based on spectroscopy at all wavelengths
48. Average Variation with R(V)/grain size
49. Average Variation with R(V)/grain size Intermediate Scale Structure (ISS) features (Massa et al. 2020) size dependent
50. Significant Deviations from Literature Curves at Select R(V)
51. 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
52. Beyond Milky Way
53. Clayton & Martin 1985, ApJ, 288, 558 Add figure showing 30 Dor/LMC2
54. Misselt et al. (1999, ApJ, 515, 128)
55. Spatial Distribution Misselt et al. (1999, ApJ, 515, 128)
56. LMC (& SMC) Averages
57. Prevot et al. 1984, A&A, 132, 389
58. 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 Small region Lots of variation! Maiz Apellaniz & Rubio (2012, A&A, 541, 54)
60. Bump and Bumpless sightlines throughout galaxy
61. Averages
63. 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)
64. Family of Curves = Order in the Chaos
65. Hints at Origin Gordon et al. (2003, ApJ, 594, 279)
66. N(HI)/A(V) traces metallicity and dust formation & destruction
67. UV vs MIR Carbonaceous Features
68. Future • HST programs – M31/M33 UV extinction (PI: Clayton) • Extinction beyond the MW/LMC/SMC – MW expanded high/low R(V) sample (PI: Decleir) • Dust at the extremes • JWST programs – WISCI (PI: Zeegers) & MEAD (PI: Decleir) • NIR/MIR MW extinction continuum and features – LMC/SMC MIR spectra (PI: Gordon) • 1st measure of 10 um silicate feature
69. dust_extinction
70. 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 – Larger variation than MW – Intriguing correlations including with gas-to-dust [N(HI)/A(V)] • Future bright w/ HST and JWST
71. Thanks