The Optical to Infrared $0.6-5.3\,{\rm \mu m}$ Dust Extinction Law of the Milky Way with JWST NIRSpec: Westerlund 2

Abstract

The interstellar extinction law is important for interpreting observations and inferring the properties of interstellar dust grains. Based on the 993 prism/CLEAR spectra from the James Webb Space Telescope (JWST), we investigate $0.6-5.3\,{\rm \mu m}$ interstellar dust extinction law. We propose a pair method to obtain the reddening curves based only on JWST observed spectra. Most of the high extinction sources are toward the young star cluster Westerlund 2. The infrared (IR) $1.0-5.3\,{\rm \mu m}$ reddening curves agree with the power-law $A_\lambda \propto \lambda^{-\alpha}$ well. We determine an average value of $\alpha=1.98\pm0.15$, which is consistent with the average value of the Galaxy. We find that $\alpha$ may be variable and independent of $R_{\rm V}$. With the derived $\alpha$, we convert the reddening curves into the extinction curves and establish the non-parameterized $\alpha$-dependent extinction curves in the wavelength range of $0.6-5.3\,{\rm \mu m}$. At $\lambda<1\,{\rm \mu m}$, the derived extinction law is not well described by the parameterized power-law type curve. Our non-parameterized $\alpha$-dependent extinction curves are suitable for the extinction correction of JWST-based photometry and spectra measurements at $0.6-5.3\,{\rm \mu m}$. We also provide the extinction coefficients for the JWST NIRCam bandpasses with different $\alpha$.