Far-Ultraviolet to Mid-Infrared Dust Extinction Curves in the Milky Way, LMC, and SMC

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Karl D. Gordon, Astronomer STScI, Baltimore, MD, USA Visiting Prof, Ghent University Ghent University 29 Apr 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

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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

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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.

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Extinction vs Attenuation Github: karllark/dust_attenuation Github: karllark/dust_extinction

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Pair Method Github: karllark/measure_extinction OB stars ideal targets Luminous at all wavelengths Fairly simple atmospheres

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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.,

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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

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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

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Our Galaxy

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Stecher 1969, ApJ, 157, 125

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International Ultraviolet Explorer 18 years! 100,000 UV spectra!

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Witt, Bohlin, & Stecher 1984, 279, 698

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2175 A Bump width → strong variation center → almost no variation Fitzpatrick & Massa (1986, ApJ, 307, 286)

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Fitzpatrick & Massa 1988, ApJ, 328, 734

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Cardelli, Clayton, & Mathis 1988, ApJ, 329L, 33; 1989, ApJ, 345, 245

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Valencic, Clayton, & Gordon 2004, ApJ, 616, 912

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Far Ultraviolet Spectroscopic Explorer 905 to 1195 Å

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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

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Gordon, Cartledge, & Clayton 2009, ApJ, 705, 1320 See also: Sofia et al. 2005

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FUV (& all UV) extinction smooth Residuals due to stellar or ISM HI FUV rise consistent with feature peaking at ~800 Å

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ApJ, 944, 33 Dries Van De Putte

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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

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Hubble Space Telescope 912 Å to 2.5 μm

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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

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Very Broad Structure = Deviations from linear w/ 1/lambda Whiteoak 1966, ApJ, 144, 305 Hayes 1973, IAUS, 53, 83

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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

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ISS features as strong as Si features

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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

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● Many ice features ● 3.0 μm H2 0 the strongest ● Requires A(V) > 3 Boogert, Gerakines, & Whittet

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3.4 μm Hydrocarbon grains Pendelton & Allamandola (2002, ApJS, 138, 75)

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10 & 20 μm Silicate Features Chiar & Tielens (2006, ApJ, 637, 774)

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Crystalline Silicate Fraction ~0.2% (or zero) Kemper, Viernd, & Tielens (2004, ApJ, 609, 826) Amorphous Crystalline

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NASA’s Infrared Telescope Facility 0.8 to 20 μm

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Marjorie Decleir

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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

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Spitzer Space Telescope 3 to 160 μm

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Variations and 1st Direct Comparison of UV and MIR Strong variation Not correlated with grain size Silicates not correlated with 2175 A bump

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Putting all the Wavelengths Together

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Cardelli, Clayton, & Mathis 1988, ApJ, 329L, 33; 1989, ApJ, 345, 245

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ApJ, 2023, 950, 86 Based on spectroscopy at all wavelengths

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Average Variation with R(V)/grain size

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Average Variation with R(V)/grain size Intermediate Scale Structure (ISS) features (Massa et al. 2020) size dependent

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Significant Deviations from Literature Curves at Select R(V)

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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

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Beyond Milky Way

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Clayton & Martin 1985, ApJ, 288, 558 Add figure showing 30 Dor/LMC2

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Misselt et al. (1999, ApJ, 515, 128)

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Spatial Distribution Misselt et al. (1999, ApJ, 515, 128)

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Prevot et al. 1984, A&A, 132, 389

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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)

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ApJ, submitted (favorable referee report received)

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Bump and Bumpless sightlines throughout galaxy

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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)

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Family of Curves = Order in the Chaos

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Hints at Origin

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N(HI)/A(V) traces metallicity and dust formation & destruction

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UV vs MIR Carbonaceous Features

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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

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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

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Thanks ● Hiring in Fall – postdoc position, PDRs, extinction curves, dust, ... ● Interested in joint PhD between UGhent and STScI? – Talk to me ● Interested in jobs at STScI – Talk to me

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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

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Slide from Dries Van De Puttte

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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)

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Clayton et al. 2015, ApJ, 815, 14

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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

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Dust Grain Modeling: DGFit ● Calculate how new observations affect our understanding of dust grains – Bigger/smaller grains, new materials, ... ● Quantitatively compare different grain materials – Bayesian statistics may help (Bayes Factors) ● Add additional observational constraints – E.g., albedo and scattering phase function asymmetry

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MW Average

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SMC Average

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MEAD Measuring Extinction and Abundances of Dust ● Cycle 1 JWST program ● PI: Marjorie Decleir (+3 Co-Is) ● 9 sightlines towards OB stars – A(V) from 1.2 – 2.5 mag – R(V) from 2.5 – 5, f(H2) from 0.1 – 0.65 – Preexisting HST program, PI: M. Decleir ● Measure atomic dust abundances via UV spectra of ISM gas absorption lines (depletions) – Existing IUE spectra, optical/NIR photometry ● MIRI MRS + NIRCam grism observations – Spectra from 2.5 – 4 µm & 5 – 28 µm ● Data status – NIRCam grism: all data taken – MIRI MRS: 6/9 sightlines (just!) taken

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WISCI Webb Investigation of Silicates, Carbon, and Ices ● Cycle 1 JWST program ● PI: Sascha Zeegers (+20 Co-Is) ● 12 sightlines towards OB stars – A(V) from 4.6 – 8.1 mag – Followup HST program, PI: Zeegers ● Optical for all, UV for half ● MIRI MRS + NIRCam grism observations – Complete spectrum from 2.5 – 28 µm ● Data status – NIRCam grism: all data taken – MIRI MRS: 8/12 sightlines taken Sascha Zeegers ESA

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NIRCam+MIRI spectra WISCI MEAD RJ = Rayleigh-Jeans Units = λ2 F(ν)

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JWST 10 µm Silicate Feature WISCI MEAD Models A(V) = 5 Models A(V) = 1.5 Si-O Si-O Huα Pfα/Huβ Huα Pfα/Huβ

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JWST NIR Carbonaceous Features WISCI MEAD Models A(V) = 5 Models A(V) = 1.5 C-H (aliphatic) C-H (aromatic) C-H (aliphatic) C-H (aromatic) Pfδ Pfε Pfγ Pfδ Pfε Pfγ

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JWST MIR Carbonaceous Features I WISCI MEAD Models A(V) = 5 Models A(V) = 1.5 C=C C=O C=C=C? C-H? C=C C=O C=C=C? C-H? C=C=C C=O: carbonyl C=C: aromatic/olefinic C-H: aromatic/aliphatic Huγ Huγ