Milky Way Optical and Mid-Infrared Extinction Curves

Interstellar dust extinction in the optical and mid-infrared (MIR)
wavelength ranges is characterized by an overall decreasing extinction
with increasing wavelength with superimposed broad extinction
features. I will present work focused on measuring the spectroscopic
dust extinction in these two wavelength ranges. The first study gives
the first truly spectroscopic measurement of the diffuse ISM MIR
extinction based on a sample of sightlines observed by the Spitzer
Space Telescope. This new measurement provides the full 5-40 micron
average extinction curve including profiles for the the two silicate
grain diagnostic features at 10 and 20 microns. The second study
uses HST/STIS spectra to perform the first systematic spectroscopic
study of optical extinction in the Milky Way. This study has revealed
that the Very Broad Structure in optical extinction is due to three
broad extinction features, two of which have strengths that
intriguingly correlate with the 2175 A extinction feature. The
results of these two studies provide new constraints on dust grain
properties and empirical averages useful for modeling and/or
correcting for the effects of dust extinction on background sources.

Transcript

1. Milky Way Optical and Mid-IR Spectroscopic Extinction Curves Karl D. Gordon STScI, Baltimore, MD USA HotSci 17 July 2019 ISM@ST member kgordon@stsci.edu @karllark2000 karllark@github “Have Dust – Will Study” Slides on SpeakerDeck
2. Summary • Spectroscopic diffuse ISM extinction measurements – Mid-IR and optical (joining UV) • Silicate features 10 & 20 micron – Do not correlate w/ 2175 A or R(V) • New optical features (4370, 4870, & 6300 A) – Two blue correlate w/ 2175 A • NIR spectroscopic next – IRTF SpeX data taken, analysis planned
3. Why Extinction? • Pillar in constraining dust grain properties Weingartner & Draine (2003)
4. Why Extinction? Jones et al. (2013)
5. Extinction (not Attenuation) • Extinction – Absorption and scattering out of the line-of-sight – Specific to a point source behind a screen of dust • (dust not too near the point source) – Extinction proportional to dust grain properties • Attenuation includes information not present in extinction – scattering into the line-of-sight – Star/dust geometry – Applies to galaxies, regions of galaxies, stars with circumstellar dust
6. Pair Method Github: karllark/measure_extinction
7. Extinction vs Attenuation Github: karllark/dust_attenuation Github: karllark/dust_extinction
8. Mid-Infrared Extinction
9. Past MIR measurements • Selection (but representative of literature) • Usually high A(V) sightlines
10. Spitzer program • Two programs (PIs: Gordon & Misselt) – IRS spectra (5-40 um) – IRAC/IRS Blue/MIPS photometry (pesky slit losses) • Classical pair method • Stars with measured UV extinction curves – Compare with UV properties • Spectral reduction challenging – Multiple techniques used → convergence! Gordon, Misselt et al. 2019, goal submission
12. IR Spectra (standards)
13. IR Spectra
14. IR Spectra Too Windy
15. E(λ – V) Curves Github: karllark/measure_extinction
16. E(lambda-V) → A(lambda)/A(V) • Usually done via ~1.1 E(K-V) • More accuracy needed • Fit Pei (1992) functional form
17. E(lambda-V) → A(lambda)/A(V) • Usually done via ~1.1 E(K-V) • More accuracy needed • Fit Pei (1992) functional form
18. E(λ – V) Curves Github: karllark/measure_extinction
23. Zasowski et al. 2009, ApJ, 707, 510
24. No Correlations(!)
25. Optical Extinction
26. Optical Spectra • Spectroscopic! • Hubble/STIS • OB stars • Fit w/ Tlusty models Fitzpatrick, Massa, Gordon, et al. 2019, almost submitted
27. Optical Extinction • Spectroscopic! • Hubble/STIS • Very Broad Structure – Known since 1970s – Deviations from a line – Not DIBs Fitzpatrick, Massa, Gordon, et al. 2019, almost submitted
28. R(V) Dependence - F20 Model (dust_extinction python package) Fitzpatrick, Massa, Gordon, et al. 2019, almost submitted Github: karllark/dust_extinction
29. Comparison to Previous (all from photometry)
30. 3 Drudes + polynomial Massa, Fitzpatrick, Gordon, et al. 2019, almost submitted Centers: 4370, 4870, & 6300 A Widths: ~10% Two blue correlate w/ 2175 A
31. Near-Infrared Extinction
32. NIR Extinction • Spectroscopic! • NASA IRTF – SpeX instrument • Spectra taken, analysis soon – Publish before JWST launch
33. EWASS 2019: C. Jäger Talk
34. Dorschner et al. 1995, A&A, 300, 503 Gordon et al. 2000, ApJ, 544, 859 Olivine Silicates Fe Feature @ ~1.5 micron? Emission spectrum from reflection nebula NGC 7023 NW filament
35. Summary • Spectroscopic diffuse ISM extinction measurements – Mid-IR and optical (joining UV) • Silicate features 10 & 20 micron – Do not correlate w/ 2175 A or R(V) • New optical features (4370, 4870, & 6300 A) – Two blue correlate w/ 2175 A • NIR spectroscopic next – IRTF SpeX data in hand Github: karllark/dust_extinction
36. Thanks
37. Simple Attenuation Example Github: karllark/dust_attenuation